# MatrixSSL Developer Guide

MatrixSSL 4.4.0

December 2021

Copyright © Rambus Inc 2021. All Rights Reserved.

# Overview

This guidance is a general SSL/TLS overview and a MatrixSSL specific integration reference for adding SSL/TLS security into an application.

This document is primarily intended for the software developer performing MatrixSSL integration into their custom application but is also a useful reference for anybody wishing to learn more about MatrixSSL or the SSL/TLS protocol in general.

For additional information on the APIs discussed here please see the MatrixSSL APIs reference manual included in the product package.

## The TLS protocol

TLS (Transport Layer Security) is network security protocol that provides data confidentiality, integrity and peer authentication. Residing between the transport and application layers, TLS is independent of the underlying transport protocol, but is typically stacked on top of TCP. Application layer protocols can be transparently layered on top of TLS.

The earlier name for TLS was SSL (Secure Sockets Layer). Despite the difference in acronym, TLS 1.0 is simply version 3.1 of SSL. There are no practical security differences between the protocols, and only minor differences in how they are implemented. It was felt that ‘Transport Layer Security’ was a more appropriate name than ‘Secure Sockets Layer’ going forward beyond SSL 3.0. In this documentation, the term SSL is used generically to mean SSL/TLS, and TLS is used to indicate specifically the TLS protocol. SSL 2.0 is deprecated and not supported. MatrixSSL supports SSL 3.0, TLS 1.0, TLS 1.1, TLS 1.2 and TLS 1.3 protocols. In addition, the DTLS protocol is based closely on TLS 1.1 and beyond. In this document, we typically use TLS to refer to both TLS and SSL 3.0.

All versions of TLS are based on a two-layered model. The handshake subprotocol is responsible for negotiating cryptographic parameters and symmetric keys for application data protection. The handshake protocol also provides peer authentication using public key cryptography. The record subprotocol divides the handshake messages and application data into records, protecting their confidentiality and integrity using the negotiated keys. In TLS 1.2 and below, record protection is enabled at the end of a successful handshake; in TLS 1.3, large parts of the handshake itself are protected as well.

## Supported RFCs

The following TLS RFCs are implemented by MatrixSSL.

- RFC 3749 Transport Layer Security Protocol Compression Methods: Supported. Disabled by default due to security issues. See CRIME.

- RFC 4162 Addition of SEED Cipher Suites to Transport Layer Security (TLS): Supported. Disabled by default at compile time.

RFC 4279 Pre-Shared Key Ciphersuites for Transport Layer Security (TLS)

  Supported:
  * `TLS_DHE_PSK_WITH_AES_256_CBC_SHA`,
  * `TLS_DHE_PSK_WITH_AES_128_CBC_SHA`,
  * `TLS_PSK_WITH_AES_256_CBC_SHA`,
  * `TLS_PSK_WITH_AES_128_CBC_SHA`.

RFC 8422 Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS) Versions 1.2 and Earlier

  Supported:
  * `TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA`,
  * `TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA`,
  * `TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA`,
  * `TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA`,
  * `TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA`,
  * `TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA`,
  * `TLS_ECDH_RSA_WITH_AES_256_CBC_SHA`,
  * `TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA`,
  * `TLS_ECDH_RSA_WITH_AES_128_CBC_SHA`.

  Supported Elliptic Curves:
  * `secp192r1`,
  * `secp224r1`,
  * `secp256r1`,
  * `secp384r1`,
  * `secp521r1`,
  * `x25519` (in TLS 1.3)

  Supported Point Formats: `uncompressed`.

RFC5077 Transport Layer Security (TLS) Session Resumption without Server-Side State

  Supported (Session Tickets).

RFC5246 The Transport Layer Security (TLS) Protocol Version 1.2.

  Supported, including TLS 1.1 and 1.0.

RFC 5288 AES Galois Counter Mode (GCM) Cipher Suites for TLS

  Supported:
  `TLS_RSA_WITH_AES_256_GCM_SHA384`
  `TLS_RSA_WITH_AES_128_GCM_SHA256`

RFC 5289 TLS Elliptic Curve Cipher Suites with SHA-256/384 and AES Galois Counter Mode (GCM)

  Supported:
  `TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384`
  `TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384`
  `TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384`
  `TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256`
  `TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256`
  `TLS_ECDH_ECDSA_WITH_AES_256_GCM_SHA384`
  `TLS_ECDH_RSA_WITH_AES_256_GCM_SHA384`
  `TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256`
  `TLS_ECDH_RSA_WITH_AES_128_GCM_SHA256`

RFC 5430 Suite B Profile for Transport Layer Security (TLS)

  Supported via compile time configuration.

- RFC 5469 DES and IDEA Cipher Suites for Transport Layer Security (TLS). These ciphers are removed per spec:

> …the single-DES cipher suites SHOULD NOT be
> implemented by TLS libraries …the IDEA cipher suite
> SHOULD NOT be implemented by TLS libraries and SHOULD
> be removed from existing implementations.

RFC 5487 Pre-Shared Key Cipher Suites for TLS with SHA-256/384 and AES Galois Counter Mode

  Supported:
  `TLS_PSK_WITH_AES_256_CBC_SHA384`
  `TLS_PSK_WITH_AES_128_CBC_SHA256`

RFC 5746 Transport Layer Security (TLS) Renegotiation Indication Extension

  Supported. Extension required by compile time default.

RFC 6066 Transport Layer Security (TLS) Extensions: Extension Definitions

  `server_name` Server Name Indication Supported
  `max_fragment_length` Supported
  `client_certificate_url` Unsupported (denial of service risk)
  `trusted_ca_keys` Unsupported
  `truncated_hmac` Supported
  `status_request` OCSP Supported

RFC 6176 Prohibiting Secure Sockets Layer (SSL) Version 2.0

  Supported. SSL 2.0 (including ClientHello) deprecated.

RFC 6347 Datagram Transport Layer Security Version 1.2

  Supported, including DTLS 1.0.

RFC 7027 Elliptic Curve Cryptography (ECC) Brainpool Curves for Transport Layer Security (TLS)

  Supported Curves:
  `brainpoolP224r1`, `brainpoolP256r1`, `brainpoolP384r1`, `brainpoolP512r1`

RFC 7301 Transport Layer Security (TLS) Application-Layer Protocol Negotiation Extension

  Supported

RFC 7457 Summarizing Known Attacks on Transport Layer Security (TLS) and Datagram TLS (DTLS)

  Supported. See Security Considerations below.

RFC 7465 Prohibiting RC4 Cipher Suites

  Supported. RC4 ciphers are disabled by default at compile time.

RFC 7507 TLS Fallback Signaling Cipher Suite Value (SCSV) for Preventing Protocol Downgrade Attacks

  Supported

RFC 7525 Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)

  Supported. See Security Considerations below.

RFC 7568 Deprecating Secure Sockets Layer Version 3.0

  Supported. SSL 3.0 is disabled by default at compile time.

RFC 7627 Transport Layer Security (TLS) Session Hash and Extended Master Secret Extension

  Supported

RFC 7748 Elliptic Curves for Security

  X25519 supported.

RFC 8032 Edwards-Curve Digital Signature Algorithm (EdDSA)

  Ed25519 supported.

RFC 8410 Algorithm Identifiers for Ed25519, Ed448, X25519, and X448 for Use in the Internet X.509 Public Key Infrastructure

  Ed25519 and X25519 supported.

RFC 8174 ChaCha20 and Poly1305 for IETF Protocols.

  Supported:
  `TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256`
  `TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256`

RFC 8446 The Transport Layer Security (TLS) Protocol Version 1.3

  Supported

- draft-ietf-dice-profile-17 TLS/DTLS Profiles for the Internet of Things. Supported via compile time configuration.

draft-ietf-tls-falsestart Transport Layer Security (TLS) False Start

  Supported. Disabled by default due to security concerns – see False Start Weakness.

## Currently Unsupported RFCs

The following “Proposed Standard RFCs” for TLS are not currently supported. Numerous “Experimental” and “Informational” RFCs are not listed here.

- RFC 2712 Addition of Kerberos Cipher Suites to Transport Layer Security (TLS)

- RFC 4785 Pre-Shared Key (PSK) Ciphersuites with NULL Encryption for Transport Layer Security (TLS)

- RFC 5705 Keying Material Exporters for Transport Layer Security (TLS)

- RFC 5929 Channel Bindings for TLS

- RFC 5932 Camellia Cipher Suites for TLS

- RFC 6520 Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) Heartbeat Extension

- RFC 6655 AES-CCM Cipher Suites for Transport Layer Security (TLS)

- RFC 6961 The Transport Layer Security (TLS) Multiple Certificate Status Request Extension

- RFC 7250 Using Raw Public Keys in Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)

- RFC 7366 Encrypt-then-MAC for Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)

- RFC 7633 X.509v3 Transport Layer Security (TLS) Feature Extension

- RFC 7685 A Transport Layer Security (TLS) ClientHello Padding Extension

## Supported Cipher Suites

Note that not all suites are enabled by default.

Supported TLS 1.3 suites:

    TLS_AES_128_GCM_SHA256
    TLS_AES_256_GCM_SHA384
    TLS_CHACHA20_POLY1305_SHA256

Supported SSL 3.0, TLS 1.0, 1.1 and 1.2 suites in priority order:

    TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384
    TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
    TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256
    TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256
    TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384
    TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384
    TLS_DHE_RSA_WITH_AES_256_GCM_SHA384
    TLS_DHE_RSA_WITH_AES_256_CBC_SHA256
    TLS_DHE_RSA_WITH_AES_128_CBC_SHA256
    TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256
    TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
    TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256
    TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256
    TLS_DHE_RSA_WITH_AES_256_CBC_SHA
    TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA
    TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA
    TLS_DHE_RSA_WITH_AES_128_CBC_SHA
    TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA
    TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA
    SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA
    TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA
    TLS_DHE_PSK_WITH_AES_256_CBC_SHA
    TLS_DHE_PSK_WITH_AES_128_CBC_SHA
    TLS_RSA_WITH_AES_256_GCM_SHA384
    TLS_ECDH_ECDSA_WITH_AES_256_GCM_SHA384
    TLS_ECDH_RSA_WITH_AES_256_GCM_SHA384
    TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA384
    TLS_ECDH_RSA_WITH_AES_256_CBC_SHA384
    TLS_RSA_WITH_AES_256_CBC_SHA256
    TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256
    TLS_RSA_WITH_AES_128_GCM_SHA256
    TLS_ECDH_RSA_WITH_AES_128_GCM_SHA256
    TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA256
    TLS_RSA_WITH_AES_128_CBC_SHA256
    TLS_ECDH_RSA_WITH_AES_128_CBC_SHA256
    TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA
    TLS_RSA_WITH_AES_256_CBC_SHA
    TLS_ECDH_RSA_WITH_AES_256_CBC_SHA
    TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA
    TLS_RSA_WITH_AES_128_CBC_SHA
    TLS_ECDH_RSA_WITH_AES_128_CBC_SHA
    SSL_RSA_WITH_3DES_EDE_CBC_SHA
    TLS_PSK_WITH_AES_256_CBC_SHA384
    TLS_PSK_WITH_AES_128_CBC_SHA256
    TLS_PSK_WITH_AES_256_CBC_SHA
    TLS_PSK_WITH_AES_128_CBC_SHA

Deprecated, disabled by default:

    SSL_RSA_WITH_RC4_128_SHA
    TLS_RSA_WITH_SEED_CBC_SHA
    TLS_RSA_WITH_IDEA_CBC_SHA
    SSL_RSA_WITH_RC4_128_MD5
    TLS_RSA_WITH_NULL_SHA256
    SSL_RSA_WITH_NULL_SHA
    SSL_RSA_WITH_NULL_MD5
    TLS_DH_anon_WITH_AES_256_CBC_SHA
    TLS_DH_anon_WITH_AES_128_CBC_SHA
    SSL_DH_anon_WITH_3DES_EDE_CBC_SHA
    SSL_DH_anon_WITH_RC4_128_MD5

# Security considerations

Prior to working directly with the MatrixSSL library there are some critical TLS security concepts that application integrators should be familiar with.

## Protocol version

Although TLS 1.0 and above can be considered secure if configured correctly, several weaknesses have been discovered in some versions and cipher combinations. For an overview, see:
RFC7457 Summarizing Known Attacks on Transport Layer Security (TLS) and Datagram TLS (DTLS)

Currently, TLS 1.3 is considered the most secure variant of the protocol. TLS 1.3 has been designed from the ground up to be resistant to all known attacks that affected earlier protocol versions. Theoretically sound and formally verified by cryptographers, it also endeavours to prevent future attacks.

At the time of writing, most TLS implementations support TLS 1.2. In addition, many implementations are adopting TLS 1.3. It is therefore currently recommended to support both TLS 1.3 and 1.2, and only support TLS 1.1 and below if necessary for compatibility reasons.

## Timing and Power Attacks

While attempts are made for constant-time operation, the MatrixSSL crypto library is not explicitly designed to be resilient to every type of timing, cache and power attack. If these are a concern, MatrixSSL TLS protocol library has support for hardware crypto, DPA resistant tokens, and hardened software crypto implementations. Also, the commercially sold MatrixSSL FIPS Edition uses a more advanced crypto library that provides constant-time implementations of many algorithms.

## Known attacks against TLS

All of the issues discovered below are mitigated by default in MatrixSSL. Additionally, SSL 3.0 and weak ciphers and key strengths are disabled by default in MatrixSSL to reduce version downgrade attacks and the padding oracle attack (POODLE). TLS 1.0 is also disabled by default at compile time in an effort to move protocol support forward.

BEAST
  `USE_BEAST_WORKAROUND` enabled by default for SSL 3.0 and TLS 1.0. Some implementations of TLS are not compatible with this workaround.
  TLS 1.1 and above are not vulnerable to this attack. By default TLS 1.1 is the minimum compiled-in version for MatrixSSL.
BERserk
  MatrixSSL was tested against this PKCS#1 v1.5 RSA parsing bug and is not vulnerable.
Bleichenbacher Type Attacks
  Private RSA key information can be leaked if libraries aren’t careful in their implementation of RSA PKCS#1 padding. MatrixSSL has been analyzed as secure by internal and 3rd party security researchers. Additionally, the more secure RSA-PSS signatures are supported, however TLS 1.2 and below allows only PKCS#1v1.5 signatures. Also, MatrixSSL has not been found to be vulnerable to the recent Bleichenbacher attack called ROBOT.
CRIME, BREACH
  `USE_ZLIB_COMPRESSION` disabled and deprecated by default.
  Application code should not compress frequently used headers.
DROWN
  SSLv2 and export ciphers are not part of the MatrixSSL codebase so this attack cannot be applied.
FREAK
  MatrixSSL has never supported weak, export grade ciphers.
Heartbleed
  MatrixSSL does not support the Heartbeat extension.
Lenstra Type Attacks
  Private RSA key information can be leaked if a hardware error or memory overrun occurs on the private key, or on intermediate results of the RSA signature operation. MatrixSSL verifies all RSA private key signatures before they are transmitted, so these rareerrors will be caught before they can be exploited.
Logjam
  `USE_DH` is disabled by default. In addition, `MIN_DH_BITS` can be increased from the default 1024 bits to reduce the feasibility of this attack. Custom Diffie-Hellman parameters are loaded by the API `pkcs3ParseDhParam`.
LUCKY13
  Internal blinding for block cipher padding automatically applied.
  Stream and AEAD ciphers are not affected.
POODLE
  SSL 3.0 disabled by default with `DISABLE_SSL3` since version 3.3.1 as per RFC 7568
  TLS 1.0 and above are not affected.
Renegotiation Attacks
  `REQUIRE_SECURE_REHANDSHAKES` enabled by default, as per RFC 5746.
  `TLS_FALLBACK_SCSV` always enabled as per RFC7507.
SMACK
  MatrixSSL was tested against state machine attacks where are messages are missing or out of order and is not vulnerable. MatrixSSL tracks both the current state and the expected state of the state machine against incoming handshake messages.
SWEET32
  3DES ciphersuites disabled by default in the recommended pre-defined configurations
False Start Weakness
  `ENABLE_FALSE_START` disabled and deprecated by default.
RC4 Weakness
  `USE_SSL_RSA_WITH_RC4_128` disabled and deprecated by default. If enabled, internal code limits the number of bytes RC4 will encode. RFC 7465 proposes to remove these suites from TLS.
MD5 MAC Weakness, SLOTH
  `USE_MD5` and `USE_SSL_RSA_WITH_RC4_128_MD5` cipher disabled by default.
  All other MD5 based ciphers disabled by default.
MD5 Certificate Weakness
  `ENABLE_MD5_SIGNED_CERTS` disabled by default.
SHA1 Certificate Weakness
  `ENABLE_SHA1_SIGNED_CERTS` can be disabled. Many certificates still use SHA1, so enabling may introduce compatibility issues with certain hosts.
TLS 1.3 early (0 RTT) data weakness
  Data sent as early data in TLS 1.3 is not forward secret and vulnerable against replay attacks, where the attacker intercepts a piece of early data and reuses it in a subsequent connection with the same server. It is therefore not recommended to send sensitive data (such as HTTP passwords) as early data. MatrixSSL does not send any early data by default. Early data encryption must be separately invoked by the client program as described in the MatrixSSL APIs manual.

## Cryptographic components

### Ciphersuites

The strength of the secure communications is primarily determined by the choice of ciphersuites that will be supported. A ciphersuite determines how two peers progress through an SSL handshake as well as how the final application data will be encrypted over the secure connection.

The TLS 1.0, 1.1 and 1.2 ciphersuites have four components: the key exchange, authentication, encryption algorithms and digest hash. TLS 1.3 ciphersuites only indicate the encryption and the hash algorithms; in TLS 1.3 the authentication and key exchange algorithms are negotiated separately during the handshake.

Note that only TLS 1.3 ciphersuites can only be used with TLS 1.3, and it is not possible to use TLS 1.3 ciphersuites in TLS 1.2 and below. TLS 1.2, on the other hand, does support most of the earlier TLS 1.0, 1.1 and 1.2 ciphersuites (excluding those that use the IDEA or single DES symmetric ciphers), but some ciphersuites are not allowed in TLS 1.1 and below. As a rule of thumb, ciphersuites containing the component GCM or ending with _SHA_256 or _SHA_384 are not allowed in TLS 1.1 and below. It is up to the library user to choose a valid combination of protocol versions and ciphersuites.

#### Examples

TLS 1.3 ciphersuites:

Cipher Suite | Encryption | Handshake hash
--- | --- | ---
`TLS_AES_128_GCM_SHA256` | AES-GCM-128 | SHA256
`TLS_AES_256_GCM_SHA384` | AES-GCM-256 | SHA384
`TLS_CHACHA20_POLY1305_SHA256` | CHACHA20-POLY1305 | SHA256

Here is a selection of cipher suites that illustrate how to identify the four components of a pre-TLS 1.3 ciphersuite.

Cipher Suite | Key Exchange | Auth Type | Encryption | MAC
--- | --- | --- | --- | ---
`SSL_RSA_WITH_3DES_EDE_CBC_SHA` | RSA | RSA | 3DES | SHA1
`SSL_DH_anon_WITH_RC4_128_MD5` | DH | Anon | RC4-128 | MD5
`TLS_RSA_WITH_AES_128_CBC_SHA` | RSA | RSA | AES-CBC-128 | SHA1
`TLS_DHE_RSA_WITH_AES_256_CBC_SHA` | DHE | RSA | AES-CBC-256 | SHA1
`TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA` | ECDHE | RSA | AES-CBC-128 | SHA1
`TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA` | ECDHE | ECDSA | AES-CBC-256 | SHA1
`TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384` | ECDH | ECDSA | AES-GCM-256 | GMAC
`TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256` | ECDH | ECDSA | ChaCha20 | Poly1305

### Key exchange

Key exchange refers to how the peers agree upon a common symmetric key that will be used to encrypt data after handshaking is complete. The two common key exchange algorithms are RSA and Diffie-Hellman (DH or ECDH).

There are two variants of Diffie-Hellman key exchange: the traditional finite field (DH) and elliptic variants (ECDH) curve . Currently, Diffie-Hellman is used almost exclusively in ephemeral mode (DHE or ECDHE) in which new private key pairs are generated for each connection to allow perfect forward secrecy. Forward secrecy means the compromise of the long-term authentication key does not allow attackers to decrypt all future connections. The trade-off for the ephemeral mode is a slower handshake as key generation is a relatively processor-intensive operation.

Both standard and elliptic curve Diffie-Hellman requires the TLS peers to agree on common group parameters to use. In elliptic curve Diffie-Hellman, the groups parameters are widely standardized and referred to by names such as secp256r1 and x25519. All the standardized elliptic curve groups supported by MatrixSSL are considered secure. TLS 1.2 and below allows the server to choose arbitrary DHE parameters, leaving it up to the server administrator to choose a secure group. This is a serious disadvantage of standard Diffie-Hellman and for this reason, ECDHE should be preferred when possible. In TLS 1.3 standard Diffie-Hellman has been made more secure by allowing only a few standardized DHE groups such as ffhde2048 and ffdhe3072.

In RSA key exchange (also called RSA key transport), the client creates a random bit string called the premaster secret, encrypts it using the server’s public RSA key and sends the ciphertext to the server. RSA key exchange has the major disadvantage that it does not provide forward secrecy and is forbidden in TLS 1.3.

The most common key exchange algorithms in TLS 1.3 are ECDHE using the X25519 and NIST P-256 (secp256r1) elliptic curves. X25519 is typically faster than secp256r1.

### Authentication

Authentication algorithms specify how the peers will prove their identities to each other. Authentication options within cipher suites are RSA, DSA, Elliptic Curve DSA (ECDSA), Pre-shared Key (PSK), or anonymous if no authentication is required.

RSA has the unique property that it can be used for both key exchange and authentication. When RSA key exchange is used, no signatures are needed, since only the server has the private key that allows decrypting the premaster secret sent by the client. RSA has become one of the most widely used authentication algorithms and is still the dominant signature algorithm in X.509 certificate chains. RSA key strengths of between 1024 and 2048 bits are the most common. Before TLS 1.3, the most common RSA signature format was PKCS #1.5. In TLS 1.3, because of Bleichenbacher attacks against the format, PKCS #1.5 signatures are no longer allowed in the handshake in TLS 1.3; instead, a more secure variant called RSASSA-PSS is used. However, TLS 1.3 still allows PKCS #1.5 signatures in X.509 certificates.

When DHE or ECDHE key exchange is used, the authentication algorithm refers to the signature algorithm used for signing the CertificateVerify handshake message (TLS 1.3) or the key exchange parameters (TLS 1.0, 1.1 and 1.2). The peer that is to be authenticated produces the signature using its long-term private key. The other party can verify the signature using the public key of the peer. The signature can be verified as correct The most common authentication algorithms in TLS 1.3 are ECDSA with the P-256 and P-384 curves, RSASSA-PSS and Ed25519.

In PSK key exchange, both parties have already agreed upon a common secret to use before starting the handshake. Thus, each peer authenticates itself implicitly by proving that it possesses the correct PSK.

#### Public key infrastructure

Authentication relies on a public key infrastructure (PKI) to tie the identity of the peer to its public key. This is typically accomplished via X.509 certificates (see RFC 5280). A trusted third party called a Certificate Authority (CA) certifies that public key X belongs to identity Y by producing a certificate that includes both the public key X and an encoding of the identity Y and by signing the certificate with its private key. The CA itself can be certified by another CA, leading to a chain of certificates that ends in a self-signed root certificate of CA that both parties (hopefully) can trust. These root certificates must be stored on disk or in memory by the TLS client or server that wishes to authenticate its peer.

Sometimes the private key of a trusted CA gets compromised, the CA itself turns out to be malevolent or is tricked by an attacker to issue a questionable certificate. To prevent fraudulent certificates from being accepted as valid, a certificate blacklisting system such as certificate revocation lists (CRLs) or online certificate status protocol (OCSP) are used in typical TLS setups.

#### Mutual authentication

By default in SSL, it is the server that is authenticated by a client. It is easiest to remember this when thinking about purchasing a product online with a credit card over an HTTPS (SSL) connection. The client Web browser must authenticate the server in order to be confident the credit card information is being sent to a trusted source. This is referred to as one-way authentication or server authentication and is performed as part of all standard SSL connections (unless, of course, a cipher suite with an authentication type of anonymous has been agreed upon).

However, in some use-case scenarios the user may require that both peers authenticate each other. This is referred to as mutual authentication or client authentication. If the project requires client authentication there is an additional set of key material that must be used to support it as described in the next section.

### Symmetric encryption

The encryption component of the cipher suite identifies which symmetric cipher is to be used when exchanging data at the completion of the handshake. There are two main categories of symmetric ciphers: block ciphers and stream ciphers. A block cipher uses the secret key and a counter (the initial value, often called a nonce or IV) to encrypt each block of the plaintext separately. The AES cipher has a block size of 128 bits. The disadvantage of block ciphers is that they can handle only full blocks; if the plaintext is not divisible by the block size, some padding must be added to the plaintext before encryption. A stream cipher derives a stream of pseudorandom bits from the secret key and the initial value, and XORs these bits with the plaintext to produce the ciphertext.

Block ciphers are always used together with a mode of operation that adds depencies between ciphertexts (such that it is not possible to decrypt each ciphertext block separately). One of the most common modes of operation is CBC (cipher block chaining). The CBC mode is forbidden in TLS 1.3 because it is hard to use it together with integrity checking. Authenticated encryption modes such as GCM offer a built-in integrity check in addition to secrecy. The GCM (galois counter mode) actually converts the block cipher to a stream cipher, also removing the need to use padding, which can result in exploitable side-channel attacks on the decryption side if not carefully implemented. ChaCha20-Poly1305 is a stream cipher that provides authentication encryption.

AES is typically recommended for new implementations, because it is widely supported and is the most likely cipher to have hardware acceleration support. For software-only implementations, ChaCha20 is usually the fastest.

### Hash algorithm

The digest hash is the choice of checksum algorithm used to confirm the integrity of exchanged data. In TLS 1.3, the hash algorithm specified by the ciphersuite name is used in the running handshake hash that verifies the integrity of handshake messages. In TLS 1.2 and below, it is used as a part of hash-based message authentication (HMAC) and determines the HMAC algorithm to use, such as HMAC-SHA-256.

The ciphersuite hash algorithm is also used during key derivation. In TLS 1.3 it used as the hash algorithm in hash-based key derivation (HKDF). TLS 1.2 and below use a custom pseudorandom function (PRF) for key derivation, which involves a HMAC invocation using a HMAC algorithm corresponding to the ciphersuite hash. TLS 1.1 and below always use a combination of MD5 and SHA in the PRF.

### X.509 certificates

With a cipher suite and authentication mode chosen, the user will need to obtain or generate the necessary key material for supporting the authentication and key exchange mechanisms. X.509 (see RFC 5280) is the standard for how public keys are stored in certificate files, while private keys are typically stored in algorithm-specific formats using DER or PEM encoding (see RFC 5958, RFC 5915, RFC 8017, etc.)

The peer that is being authenticated must have a private key and a public certificate. The peer performing the authentication must have the Certificate Authority (CA) certificate that was used to issue the public certificate. In the standard one-way authentication scenario this means the server will load a private key and certificate while the client will load the CA file.

If client authentication is needed the mirror image of CA, certificate, and private key files must also be used. This chart shows which files clients and server must load when using a standard RSA based cipher suite such as `TLS_RSA_WITH_AES_256_GCM_SHA384`.

Authentication Mode | Server Key Files | Client Key Files
--- | --- | ---
One-way server authentication | RSA server certificate file and corresponding RSA private key file. | Certificate Authority certificate file that issued the server certificate
Additions for client authentication | Certificate Authority certificate file that issued the client certificate | RSA client certificate file and corresopnding RSA private key file.

#### Certificate Validation and Authentication

As described above, the mapping of peer identities to their public keys is certified by certificate authorities (CAs), leading to certificate chains.

Example Equifax GeoTrust trusted root certificate loaded by a MatrixSSL client with `matrixSslLoadRsaKeys`.

    **Subject: C=US, O=Equifax, OU=Equifax Secure Certificate Authority**
    *Authority KeyId*:48:E6:68:F9:2B:D2:B2:95... [Valid, self-signed OK for root]
    *Subject KeyId*: 48:E6:68:F9:2B:D2:B2:95...
    *Basic Constraints*: critical CA:TRUE [Valid, this certificate can sign others]
    *Key Usage*: critical Certificate Sign, CRL Sign [Valid, able to sign certificates]
    *Validity*: (Aug 22 16:41:51 1998 GMT to Aug 22 16:41:51 2018 GMT) [Valid]

Certificate chain sent to a MatrixSSL client during SSL handshake Certificate message by remote server https://www.google.com.

    **C=US, O=GeoTrust Inc., CN=GeoTrust Global CA**
    *Issuer*: C=US, O=Equifax, OU=Equifax Secure Certificate Authority
    [Valid, matches a loaded trusted root subject]
    *Authority KeyId*: 48:E6:68:F9:2B:D2:B2:95... [Valid, matches the Issuer Subject KeyId]
    *Subject KeyId*: C0:7A:98:68:8D:89:FB:AB...
    *Basic Constraints*: critical CA:TRUE [Valid, this certificate can sign others]
    *Key Usage*: critical Certificate Sign, CRL Sign [Valid, able to sign certificates]
    *Validity*: (May 21 04:00:00 2002 GMT to Aug 21 04:00:00 2018 GMT) [Valid]
    >>**C=US, O=Google Inc, CN=Google Internet Authority G2**
    *Issuer*: C=US, O=GeoTrust Inc., CN=GeoTrust Global CA [Valid, matches parent subject]
    *Authority KeyId*: C0:7A:98:68:8D:89:FB:AB... [Valid, matches the Issuer Subject KeyId]
    *Subject KeyId*: 4A:DD:06:16:1B:BC:F6:68...
    *Validity*: (Apr  5 15:15:55 2013 GMT to Apr  4 15:15:55 2015 GMT) [Valid]
    *Basic Constraints*: critical CA:TRUE, pathlen:0
    [Valid, this certificate can sign others and the signed certificate is not also a CA]
    *Key Usage*: critical Certificate Sign, CRL Sign [Valid, able to sign certificates]
    *Version*: 3 [Valid]
    >>>**C=US, ST=California, L=Mountain View, O=Google Inc, CN=*.google.com**
    *Issuer*: C=US, O=Google Inc, CN=Google Internet Authority G2 [Valid, matches parent subject]
    *X509v3 Basic Constraints*: critical CA:FALSE [Valid, this is a leaf cert]
    *Extended Key Usage*: TLS Web Server Authentication, TLS Web Client Authentication [Valid]
    *X509v3 Subject Alternative Name*: DNS:*.google.com, DNS:*.android.com...
    [Valid, matches expected DNS name]
    *Validity*: (Mar 12 09:53:40 2014 GMT to Jun 10 00:00:00 2014 GMT) [Valid]

#### MatrixSSL internal certificate validation

In MatrixSSL, certificate chain validation is a two-step process. First, MatrixSSL performs its own internal validation. After that, the application program is allowed to perform its own custom validation using a certificate validation callback function. Certificate chain validation is a complex topic and the certificates are allowed to contain numerous extensions that can affect the validation result. For this reason, MatrixSSL’s internal validation can only perform a subset of all possible validation checks. If more thorough validation is needed, this must be performed in the application callback.

The following table lists the most important checks performed by default in the MatrixSSL internal validation.

X.509 Field | Validation Performed
--- | ---
Version | Must be a version 3 certificate.
Serial | Used for lookup in a CRL, if `USE_CRL` defined.
Signature Algorithm | RSA or ECDSA algorithms. Should be SHA256, SHA384 or SHA512. SHA1 enabled by default with `ENABLE_SHA1_SIGNED_CERTS`. MD2 and MD5 support for RSA signatures is disabled by default by `ENABLE_MD5_SIGNED_CERTS`.
Issuer | In a chain, issuer must match the subject of the immediate (following) parent certificate. Self-signed certificates (`Issuer` == `Subject`) are allowed as loaded root certificates, but not as part of a chain. Common name must contain only printable characters.
Validity | Current date must be within notBefore and notAfter range on all certs in the chain. Time is not currently validated. On platforms without a date function, the range check is always flagged as failed and must be handled by the Certificate Validation Callback. For platforms without a date API, validation must be done within the user Certificate Validation Callback.
Subject | Common name must contain only printable characters. Common name will be validated via full match to expectedName, if provided in `matrixSslNewClientSession`. Partial match not allowed. Wildcard match is allowed for the first segment of a DNS name. More complex validation must be done within the user Certificate Validation Callback.
Subject Public Key Info | RSA and ECC keys supported. RSA public key modulus must be at least `MIN_RSA_SIZE` bits. ECC public key must be at least `MIN_ECC_SIZE` bits.
Signature | The hash of the certificate contents must match the hash that is signed by the `Issuer Public Key`.
Basic Constraints | For Root or intermediate certs, must be marked `Critical` with `CA:TRUE`. Path Length constraints are validated.
Key Usage | For Root or intermediate certs, must be marked for use as `CertificateSign`. For CRL checks, `CrlSign` flag must be set.
Extended Key Usage | If marked Critical, must have `TLS Web Server Authentication` or `TLS Web Client Authentication` set in the leaf certificate.
Subject Alternative Name | If an expectedName is specified in `matrixSslNewClientSession` and does not match `Subject Common Name`, or any printable `Subject Alternative Name` of type `DNS`, `Email` or `IP`, validation will fail.
Authority Key Identifier | If specified, the direct Issuer of the certificate must have a defined, matching `Subject Key Identifier`.
Subject Key Identifier | If specified, any direct children of the Issuer must have a defined, matching `Authority Key Identifier`.
CRL Distribution Points | If USE_CRL is defined, `matrixSslGetCRL` will download the CRL files from each URI type distribution point provided for each trusted root certificate (Note: not intermediate certificates).
CRL Validation | CRL file must be signed by certificate with `CrlSign` Basic constraints. MD5 signatures not supported by default.
Unknown Extensions | Unknown extensions are ignored, unless flagged as `Critical`. Validation will fail for any Critical extension unrecognized by MatrixSSL.

#### Generating certificates

In MatrixSSL Commercial Edition, Certificate Authority root and child certificates can be created using the provided command-line tools and API. For more information, please consult the Key and Cert Generation Utilities and MatrixSSL Certificates and Certificate Revocation Lists documents.

### MatrixSSL supported algorithms and recommendations

Key exchange algorithms supported by MatrixSSL:

Algorithm | Key Size | Ok? | Typical Risks
--- | --- | --- | ---
RSA | < 1024 | No | Weak. Below MIN_RSA_SIZE connections will be refused.
RSA | 1024 | No | In wide usage. Recommended to not use going forward
RSA | > 1024 | Yes | Recommend at least 2048 bit keys, per NIST and FIPS.
DH | < 1024 | No | Weak. Below `MIN_DH_SIZE` connections will be refused.
DH | 1024 | No | In wide usage. Recommended to not use going forward
DH | > 1024 | Yes | Recommend at least 2048 bit DH group per NIST and FIPS.
DHE | > 1024 | Yes | See chart below. Ephemeral cipher suites provide perfect forward secrecy, and are generally the strongest available, although they are also the slowest performing for key exchange.
ECDHE | >= 192 | Yes | See chart below. Ephemeral cipher suites provide perfect forward secrecy, and are generally the strongest available, although they are also the slowest performing for key exchange. 224 and greater required by FIPS.
ECDH | >= 192 | Yes | 192 bit DH group and above is currently assumed secure. Smaller groups are not supported in MatrixSSL. Below `MIN_ECC_SIZE` connections will be refused. 224 and greater required by FIPS.
PSK | >= 128 | Yes | Pre-shared Key ciphers rely on offline key agreement. They avoid any weaknesses of Key Exchange Algorithms, however, it is not easy to change keys once they are installed when used as session keys. When PSK is used only for authentication (`DHE_PSK` and `ECDHE_PSK` cipher suites), the session encryption keys are generated each connection.

Authentication algorithms supported by MatrixSSL:

Suite Type | Auth | Exchange | Ok? | Typical Risks
--- | --- | --- | --- | ---
`RSA_WITH_NULL` | RSA | none | No | No encryption. Authentication via RSA. Typically used for debugging connections only.
`DH_anon` | none | Diffie-Hellman | No | No Authentication. Key exchange only. If this ciphersuite is used, authentication MUST be done by direct comparison of remote DH key ID to trusted key ID, similar to SSH authentication. If DH key ID authentication is done, this is similar in strength to DHE_PSK ciphers, although the keys exchanged are not ephemeral. Authentication to a trusted key ID can mitigate many attacks related to X.509 PKI infrastructure.
`PSK` | Pre-shared Key | Pre-shared Key | Yes | Pre-shared Keys can be used for authentication, since the same secret must be shared between client and server. `DHE_PSK` suites use PSK only for authentication, while PSK suites use PSK for authentication and session keys. PSK keys are difficult to change in the field, however authentication with PSK can mitigate many attacks related to X.509 PKI infrastructure.
`RSA` | RSA | RSA | Yes | The most commonly used authentication method. Supported by X.509 PKI infrastructure. Additional security can be had by directly comparing `RSA` key IDs to trusted Key Ids (similar to Certificate Pinning). Usually faster than ECC based authentication. RSASSA-PSS signature format is regarded safe than the PKCS #1.5, although correctly implemented PKCS #1.5 is still secure.
`DHE_RSA` | RSA | Diffie-Hellman Ephemeral | Yes | RSA for authentication, Ephemeral DH for key exchange. Provides perfect forward secrecy.
`DHE_PSK` | PSK | Diffie-Hellman Ephemeral | Yes | PSK for authentication, Ephemeral DH for key exchange. Does not rely on X.509.
`ECDH_ECDSA` | ECC DSA | ECC Diffie-Hellman | Yes | ECC DSA for authentication, ECC for key exchange. Most commonly used in embedded devices supporting hardware based ECC support.
`ECDH_RSA` | RSA | ECC Diffie-Hellman | Yes | ECC key exchange, with RSA authentication. Uses widely deployed X.509 RSA certificate infrastructure, but ECC for key exchange. Not often deployed due to the implementation having to support ECC and RSA.
`ECDHE_ECDSA` | ECC DSA | ECC Diffie-Hellman Ephemeral | Yes | ECC key exchange with ephemeral keys, ECC DSA authentication. Most commonly used in embedded devices supporting hardware based ECC support.
`ECDHE_RSA` | RSA | ECC Diffie-Hellman Ephemeral | Yes | Ephemeral counterpart to ECDH_RSA.

Symmetric encryption algorithms supported by MatrixSSL:

Algorithm | Ok? | Typical Risks
--- | --- | ---
RC4 | No | Several known weaknesses. Can be OK for small amounts of data. All RC4 ciphersuites disabled by default.
3DES | No | Theoretical weaknesses due to key strength. AES typically a better candidate. 3DES ciphersuites disabled by default.
SEED | No | Standard usage only in Korea. AES is a better candidate. Disabled by default.
IDEA | No | Disabled by default.
AES-CBC | Yes | AES-256 preferred over AES-192 and AES-128. Lucky13 Attack mitigated internally, but all block ciphers are vulnerable.
AES-GCM | Yes | AES-256 preferred over AES-192 and AES-128. Not vulnerable to Lucky13 Attack. Without hardware acceleration, can be slower than AES-SHA. Risk that an as-yet undiscovered AES attack will compromise both encryption and record validation.
ChaCha20 | Yes | Stream cipher with 256 bit equivalent security. Supported as per RFC 8174

Hash algorithms supported by MatrixSSL:

Algorithm | Ok? | Typical Risks
--- | --- | ---
MD2 | No | Known weak. Used only for legacy certificate signatures. `USE_MD2` disabled by default.
MD4 | No | Known weak. Used only for legacy certificate signatures. `USE_MD4` disabled by default.
MD5 | No | Proven attacks. SSL 3.0 through TLS 1.1 require MD5 in combination with SHA-1 for their internal protocol (and therefore are at least as strong as SHA-1). TLS 1.2 does not require MD5. All MD5 based cipher suites disabled by default. `ENABLE_MD5_SIGNED_CERTS` disabled by default.
SHA-1 | Deprecate | SHA-1 is still widely deployed despite recent collision attacks. Only TLS 1.2 and newly issued certificates using SHA-2 are able to remove SHA-1 completely from the TLS protocol.
SHA-256 | Yes | Assumed secure.
SHA-384 | Yes | Assumed secure.
SHA-512 | Yes | Assumed secure.
Poly1305 | Yes | Supported as per RFC 8174

# Build-time configuration

MatrixSSL contains a set of optional features that are configurable at compile time. This allows the user to remove unneeded functionality to reduce code size footprint and disable potentially insecure features. Each of these options are pre-processor defines that can be disabled by simply commenting out the #define in the header files or by using the -D compile flag during build. APIs with dependencies on optional features are highlighted in the Define Dependencies sub-section in the API documentation for that function.

Not all configurable options are listed below. See comments directly in configuration header files for more fine-tuning.

## Protocol and Performance

`USE_CLIENT_SIDE_SSL`
  matrixsslConfig.h - Enables client side SSL support
`USE_SERVER_SIDE_SSL`
  matrixsslConfig.h - Enables server side SSL support
`USE_TLS_1_2_AND_ABOVE`
`USE_TLS_1_1_AND_ABOVE`
`USE_TLS_1_0_AND_ABOVE`
  Enable one of these settings to specify which versions are compiled in. Clients or servers can select between compiled in versions at runtime if desired as decribed in the Application integration flow chapter. Defaults to TLS 1.1 and above.
`DISABLE_TLS_1_3`
  If a minimal footprint build is required, it is possible to disable TLS 1.3 separately.
`USE_DTLS`
  Support DTLS connections (TLS over UDP) in addition to TLS. DTLS version support is based on the underlying level of TLS support.
`SSL_SESSION_TABLE_SIZE`
  matrixsslConfig.h – Applicable to servers only. The size of the session resumption table for caching session identifiers. Old entries will be overwritten when size is reached
`SSL_SESSION_ENTRY_LIFE`
  matrixsslConfig.h – Applicable to servers only. The time in seconds that a session identifier will be valid in the session table. A value of 0 will disable SSL resumption. Also applies to the lifetime of Stateless Session Tickets, below.
`USE_STATELESS_SESSION_TICKETS`
  matrixsslConfig.h – Enable stateless session tickets as defined in RFC 5077
`USE_TLS_1_3_RESUMPTION`
  matrixsslConfig.h - Enable resumed handshakes in TLS 1.3. Requires USE_STATELESS_SESSION_TICKETS.
`USE_REHANDSHAKING`
  matrixsslConfig.h - Enable secure rehandshaking as defined in RFC 5746.
  Legacy (insecure) rehandshaking is no longer supported.
  Disabled by default. Rehandshaking is not allowed in TLS 1.3.
`REQUESTED_MAX_PLAINTEXT_RECORD_LEN`
  matrixsslConfig.h – Enable the “max_fragment_length” TLS extension defined in RFC 4366. Value of #define determines fragment length (server may reject)
`USE_CLIENT_AUTH`
  matrixsslConfig.h - Enables two-way(mutual) authentication
`USE_EXT_CERTIFICATE_VERIFY_SIGNING`
  matrixsslConfig.h - Enables client authentication using an external module. See the External Module Integration document for details.
`SERVER_CAN_SEND_EMPTY_CERT_REQUEST`
  matrixsslConfig.h – A client authentication feature. Allows the server to send an empty CertificateRequest message if no CA files have been loaded
`SERVER_WILL_ACCEPT_EMPTY_CLIENT_CERT_MSG`
  matrixsslConfig.h – A client authentication feature. Allows the server to ‘downgrade’ a client authentication handshake to a standard handshake if client does not provide a certificate
`USE_ALPN`
  matrixsslConfig.h - Enable Application Level Protocol Negotiation. Also must be enabled via runtime option for new client sessions.
`USE_TRUSTED_CA_INDICATION`
  matrixsslConfig.h - Enable the Trusted CA Indication extension defined in RFC 6066.
`USE_OCSP`
`USE_OCSP_MUST_STAPLE`
  cryptoConfig.h and matrixsslConfig.h respectively. Enable OCSP and require OCSP stapling.
`USE_X509`
  cryptoConfig.h - Enables basic support for X.509 certificates.
`USE_CERT_PARSE`
  cryptoConfig.h - Enables X.509 certificate parsing.
`USE_FULL_CERT_PARSE`
  cryptoConfig.h - Enables the parsing of some additional certificate extensions, such as nameConstraints and authorityInfoAccess.
`USE_CERT_GEN`
  cryptoConfig.h - (MatrixSSL Commercial Edition only:) Enables X.509 certificate generation.
`ENABLE_MD5_SIGNED_CERTS`
`ENABLE_SHA1_SIGNED_CERTS`
  cryptoConfig.h – Support MD5 or SHA1 signature algorithm in X.509 certificates and Certificate Revocation Lists.
`ALWAYS_KEEP_CERT_DER`
  cryptoConfig.h - When parsing certificates, always also retain the unparsed DER data in the psX509Cert_t structure.
`USE_CRL`
  cryptoConfig.h - Enable Certificate Revocation List APIs.
`ALLOW_CRL_ISSUERS_WITHOUT_KEYUSAGE`
  cryptoConfig.h - Compatibility option. Allows CRL authentication to succeed when signer CA’s cert does not have the keyUsage extension.
`USE_FIPS_CRYPTO`
`USE_CL_CRYPTO`
  cryptoConfig.h - (MatrixSSL FIPS Edition only:) Enable using the FIPS 140-2 validated SafeZone CL/FIPSLib 1.2 as the cryptographic library in MatrixSSL. For more information on FIPS 140-2 specific configuration options, please consult the MatrixSSL with CL Library document, included with the MatrixSSL FIPS Edition.
`USE_CMS`
  cryptoConfig.h - (MatrixSSL Commercial Edition only:) Enable support for Cryptographic Messaging Syntax (CMS).
`USE_RSA`
`USE_ECC`
`USE_DH`
  cryptoConfig.h - Enable RSA, ECC and DH support, respectively.
`USE_X25519`
  cryptoConfig.h - Enable ECDHE key exchange with the X25519 curve in TLS 1.3.
`USE_ED25519`
  cryptoConfig.h - Enable the Ed25519 signature algorithm in TLS 1.3.
`USE_DSA_VERIFY`
  cryptoConfig.h - (MatrixSSL FIPS Edition only:) Enable verification of DSA signatures in certificate validation.
`MIN_RSA_SIZE`
`MIN_DH_SIZE`
`MIN_ECC_SIZE`
  cryptoConfig.h – The minimum size in bits that MatrixSSL will accept for key exchange for each algorithm. Prevents weak keys from being used or downgraded to.
`USE_PRIVATE_KEY_PARSING`
  cryptoConfig.h - Enables X.509 private key parsing
`USE_PKCS5`
  cryptoConfig.h - Enables the parsing of password protected private keys
`USE_PKCS8`
  cryptoConfig.h - Enables the parsing of PKCS#8 formatted private keys
`USE_PKCS12`
  cryptoConfig.h - Enables the parsing of PKCS#12 formatted certificate and key material
`USE_BEAST_WORKAROUND`
  matrixssllib.h – Enabled by default. See code comments in file.
`ENABLE_FALSE_START`
  matrixsslConfig.h - Disabled by default. See code comments in file.
`NO_ECC_EPHEMERAL_CACHE`
  matrixsslConfig.h - By default, MatrixSSL caches the ECDHE keys it generates and re-uses the cached keys for connections within a certain time frame and a certain usage count. This improves performance of ECDHE suites, and is in line with the configuration current web browsers. Enabling NO_ECC_EPHEMERAL_CACHE disables the key caching and forces new ECDHE keys to be created for every TLS connection. Note that caching ECDHE keys is against some standards such as NIST SP 800-56A, and disallowed in the FIPS 140-2 mode of operation. NO_ECC_EPHEMERAL_CACHE is disabled by default.
`ECC_EPHEMERAL_CACHE_SECONDS`
`ECC_EPHEMERAL_CACHE_USAGE`
  matrixssllib.h - The maximum time period and usage count of a cached ECDHE key. After these limits have been exceeded, the key will be removed from the cache.
`USE_1024_KEY_SPEED_OPTIMIZATIONS`
`USE_2048_KEY_SPEED_OPTIMIZATIONS`
`PS_PUBKEY_OPTIMIZE_FOR_SMALLER_RAM`
`PS_PUBKEY_OPTIMIZE_FOR_FASTER_SPEED`
  crypto/layer/layer.h - RSA and Diffie-Hellman speed vs. runtime memory tradeoff. By default these will be enabled if the compiler is invoked with optimization that is not for size (eg. `-O1 to -O3`). They will be disabled for `-O0` and `-Os`.
`PS_AES_IMPROVE_PERF_INCREASE_CODESIZE`
`PS_3DES_IMPROVE_PERF_INCREASE_CODESIZE`
`PS_MD5_IMPROVE_PERF_INCREASE_CODESIZE`
`PS_SHA1_IMPROVE_PERF_INCREASE_CODESIZE`
  crypto/layer/layer.h - Optionally enable for selected algorithms to improve performance at the cost of increased binary code size. By default these will be enabled if the compiler is invoked with optimization that is not for size (eg. `-O1 to -O3`). They will be disabled for `-O0` and `-Os`.
`MATRIX_USE_FILE_SYSTEM`
  Determined automatically in core/osdep.h. Enables keys to be read from a filesystem, in addition to in-memory keys.
`USE_MULTITHREADING`
  coreConfig.h: Enable if using MatrixSSL with a multithreading client or server, where session cache may be shared between threads simultaneously.

## Public Key Math Assembly Optimizations

Optimizing assembly code for low level math operations is available for many common processor architectures. The files pstm_montgomery_reduce.c, pstm_mul_comba.c, and pstm_sqr_comba.c in the crypto/math directory implement the available assembly optimizations. These following defines are set in the osdep.h header file by detecting the platform. These should be set accordingly when porting to an unsupported platform.

`PSTM_X86`
  32-bit x86 processor
`PSTM_X86_64`
  64-bit x86 processor
`PSTM_ARM`
  ARMv4 or greater processor
`PSTM_MIPS`
  32 bit MIPS processor
`PSTM_PPC`
  32 bit PowerPC processor
None of the above
  Standard C code implementation

## Debug Configuration

MatrixSSL contains a set of optional debug features that are configurable at compile time. Each of these options are pre-processor defines that can be disabled by simply commenting out the #define in the specified header files.

`HALT_ON_PS_ERROR`
  coreConfig.h - Enables the osdepBreak platform function whenever a psError trace function is called. Helpful in debug environments.
`USE_CORE_ERROR`
`USE_CORE_ASSERT`
`USE_CORE_TRACE`
  coreConfig.h - Enables the psTraceCore family of APIs that display function-level messages in the core module. Disabling these can reduce static code size significantly, as the trace strings will not be included in the final binary.
`USE_CRYPTO_TRACE`
  cryptoConfig.h - Enables the psTraceCrypto family of APIs that display function-level messages in the crypto module.
`USE_SSL_HANDSHAKE_MSG_TRACE`
  matrixsslConfig.h - Enables TLS level debug messages. The logged information includes error messages as well as indications to which handshake messages and extensions are currently being created or parsed.
`USE_SSL_INFORMATIONAL_TRACE`
  matrixsslConfig.h - Enables TLS level trace messages. The logged information includes a summary of the contents of handshake messages and extensions as well as additional protocol flow information. Requires `USE_SSL_HANDSHAKE_MSG_TRACE`.
`USE_DTLS_DEBUG_TRACE`
  matrixsslConfig.h - Enables DTLS-specific trace messages for troubleshooting connection problems.
`DTLS_SEND_RECORDS_INDIVIDUALLY`
  matrixsslConfig.h - If enabled, each DTLS handshake message will be returned individually when matrixDtlsGetOutdata is called. When left disabled, the default behaviour of matrixDtlsGetOutdata is to return as much data as possible that fits within the maximum PMTU.

## Minimum Firmware Configuration

MatrixSSL can be built to a minimum size using TLS 1.2, PSK cipher with AES128 and SHA256. If interoperability with OpenSSL is desired, then a few changes are necessary, since the USE_TLS_PSK_WITH_AES_128_CBC_SHA256 ciphersuite is not implemented by OpenSSL (as of 1.0.2d). In this case, `USE_SHA1` and `USE_HMAC_SHA1` must also be defined and the cipher suite changed to
`USE_TLS_PSK_WITH_AES_256_CBC_SHA` or
`USE_TLS_PSK_WITH_AES_128_CBC_SHA`.

To enable minimal configuration, all options in core/coreConfig.h, crypto/cryptoConfig.h and matrixssl/matrixsslConfig.h should be commented out, except for the following:

coreConfig.h
  Optional: USE_MATRIX_MEMORY_MANAGEMENT
  Optional: Disable `USE_CORE_ERROR` and `USE_CORE_ASSERT`
cryptoConfig.h
  `USE_AES_CBC`, `USE_SHA256`, `USE_HMAC`, `USE_HMAC_SHA256`
  Optional: `__AES__` block to enable AESNI on Intel platforms.
  Optional: For OpenSSL compatibility, also enable `USE_SHA1` and `USE_HMAC_SHA1`
matrixsslConfig.h
  `USE_TLS_PSK_WITH_AES_128_CBC_SHA256`
  `USE_TLS_1_2_AND_ABOVE`, `USE_CLIENT_SIDE_SSL` and/or `USE_SERVER_SIDE_SSL`
  Optional: `SSL_DEFAULT_IN_BUF_SIZE`, `SSL_DEFAULT_OUT_BUF_SIZE` set to 1500 for reduced RAM footprint.
  Optional for Server: `SSL_SESSION_TABLE_SIZE` as low as 1 for reduced RAM footprint.
  Optional: `USE_DTLS`
  Optional: For OpenSSL compatibility, enable:
  `USE_TLS_PSK_WITH_AES_256_CBC_SHA` or
  `USE_TLS_PSK_WITH_AES_128_CBC_SHA`

The pre-defined `psk` configuration is closest to the minimal configuration. When either server or client side is removed, it achieves code size of around 60KB on ARM Thumb 2. It is possible to get much further below this number by tweaking the configuration manually. When making minimal footprint builds, it is important to use linker garbage collection so that the linker automatically removes unnecessary code. Please refer your linker manual for details. If linker gargage collection is not working properly, it may be necessary to “ifdef out” some unneeded functionality, especially from the core library.

## Example configurations

MatrixSSL ships with a few example configurations, which are described here. MatrixSSL FIPS edition contains additional example configurations; these are described in the MatrixCrypto CL document included in that distribution.

Configuration | Comment
--- | ---
default | The default configuration
noecc | Disables ECC support
rsaonly | Disables ECC and DH support
tls | The recommended configuration for TLS 1.2 and below
tls13 | The recommended configuration for TLS 1.3 and below
psk | The minimal PSK configuration described in the last subsection, except that both server- and client side support are enabled.

These configurations can be applied with the commands make default-config, make noecc-config, etc. Applying a new configuration will override the existing ./core/coreConfig.h, ./crypto/cryptoConfig.h, and ./matrixssl/matrixsslConfig.h files. After applying the configuration, these files can be further adapted per specific needs and the MatrixSSL can be compiled as usual, via the make commands described in the Getting Started document.

In MatrixSSL FIPS Edition the names of the pre-defined configurations are prefixed with either “fips” or “nonfips” to indicate whether they comply with FIPS 140-2 and NIST requirements.

# Application integration flow

MatrixSSL is a C code library that provides a security layer for client and server applications allowing them to securely communicate with other SSL enabled peers. MatrixSSL is transport agnostic and can just as easily integrate with an HTTP server as it could with a device communicating through a serial port. For simplicity, this guidance will assume a socket-based implementation for all its examples unless otherwise noted.

The term application in this document refers to the peer (client or server) application the MatrixSSL library is being integrated into.

This section will detail the specific points in the application life cycle where MatrixSSL should be integrated. In general, MatrixSSL APIs are used for initialization/cleanup, when new secure connections are being established (handshaking), and when encrypting/decrypting messages exchanged with peers.

Refer to the MatrixSSL API document to get familiar with the interface to the library and with the example code to see how they are used at implementation. Follow the guidelines below when using these APIs to integrate MatrixSSL into an application.

## The ssl_t Structure

The `ssl_t` structure holds the state and keys for each client or server connection as well as buffers for encoding and decoding SSL data. The buffers are dynamically managed internally to make the integration with existing non-secure software easier. SSL is a record based protocol, and the internal buffer management makes a better ‘impedance match’ with classic stream based protocols. For example, data may be read from a socket, but if a full SSL record has not been received, no data is available for the caller to process. This partial record is held within the `ssl_t` buffer. The MatrixSSL API is also designed so there are no buffer copies, and the caller is able to read and write network data directly into the SSL buffers, providing a very low memory overhead per session.

## Initialization

### Library initialization

MatrixSSL must be initialized as part of the application initialization with a call to `matrixSslOpen`. This function sets up the internal structures needed by the library.

### Loading keys and certificates

After initializing the MatrixSSL library, the typical application will subsequently load the key material to use in TLS. Keys and certificates can be loaded both from disk and from binary data embedded within the application.

The first step is to create a new `sslKeys_t` object with the `matrixSslNewKeys` function. This function allocates storage for the keys to be loaded. The user then calls `matrixSslLoadKeys` or `matrixSslLoadKeysMem` to load key material from a file or from memory, respectively. These functions attempt to deduce the type of the key from the key material itself. It is also possible to call algorithm-specific loading functions such as `matrixSslLoadRsaKeys` and `matrixSslLoadEcKeys`; these allow a bit smaller code footprint. Each key loading API takes as an argument the private key, the server or client certificate and the trusted CA certificates. Several trusted CA certificates often need to be loaded to support multiple peers having different certificate chains. This can be done by concatenating the CA certificates into a single PEM file or into a single DER octet stream.

The populated `sslKeys_t` structure will be used as an input parameter to `matrixSslNewClientSession` or `matrixSslNewServerSession`.

Once the application is done with the keys, the associated memory is freed with a call to `matrixSslDeleteKeys`.

#### DH parameters

If the application wishes to use standard finite field Diffie-Hellman in TLS 1.2 and below, it must load the DH group parameters separately using `matrixSslLoadDhParams`. In TLS 1.3, the DH parameters to use are specified via session options, as described below. Elliptic curve Diffie-Hellman parameters to use are specified via session options in both TLS 1.3 and below.

#### Pre-shared keys

Pre-shared keys can be loaded with `matrixSslLoadPsk` and `matrixSslLoadTls13Psk`. TLS 1.3 PSKs have some as an additional requirement that the hash algorithm that was used to derive the PSK must be the same as the ciphersuite hash algorithm to be used in the PSK handshake. Also, in TLS 1.3, the length of the pre-shared key itself must be equal to the length of the ciphersuite hash.

PSKs can be created during an initial (non-PSK) handshake via so-called session tickets. PSKs can also be agreed externally, in which case no previous TLS handshake is needed between the parties.

#### Dynamic key and certificate selection

It is not always possible to know the suitable key material to use in advance. For example, in client authentication, the server typically sends a list of acceptable CAs in the CertificateRequest protocol message, and may reject all client certificates that have not been issued by one of the trusted CAs. Another example is server authentication when multiple servers are hosted at a single IP address and the client indicates the server it wishes to communicate with using the Server Name Indication (SNI) ClientHello extension. In such cases, it may be necessary to load multiple key-cert pairs or postpone the loading until after all the requirements (such as the CA list) are known.

##### Loading multiple keys

The `matrixSslLoadKeys` and `matrixSslLoadKeysMem` functions can be called several times to load multiple key-cert pairs (identities). MatrixSSL will then decide during the handshake which of the loaded identitities to use, based on the negotiated parameters such as the ciphersuite, authentication algorithm and the list of trusted CAs sent by the peer.

##### Postponed key loading

It is also possible to postpone the loading of keys and certificates and only load them on demand during the handshake. This allows e.g. the server to its key and cert pair depending on which server_name the client supplied in the server name indication (SNI) extension, which is extremely useful in virtual server settings, where multiple servers are hosted on the same IP address.

There are several mechanisms for postponed key loading:

- Server key and certificate can be loaded in the SNI callback, which is called during the handshake, before ciphersuite selection, if the SNI extension has been received from the client. Please consult the entry for `matrixSslRegisterSNICallback` in the `MatrixSSL APIs` document for details.

- When the build-time option USE_EXT_CLIENT_CERT_KEY_LOADING is enabled in matrixsslConfig.h, one of the top-level MatrixSSL APIs (`matrixSslReceivedData`) will return a special value (PS_PENDING). When the client application sees this, it calls matrixSslNeedClientCert to confirm that a client certifiate is needed, clears the existing key material in `ssl->keys` (if any) and loads new keys with e.g. `matrixSslLoadKeys` and assigns them to the `ssl_t` struct with `matrixSslSetClientIdentity`. The client programs then informs the TLS state machine of a key change with `matrixSslClientCertUpdated` and calls `matrixSslReceivedData` again with the same parameters to continue the handshake with the new client key-cert pair. See the `External Module Integration` manual (included in `MatrixSSL Commercial Edition`) for details. Please note that feature currently only works with TLS 1.2 and below.

- External client authentication. It is also possible to not load the client private key into MatrixSSL at all by using the external client authentication feature. This feature can be used to implement client authentication using an external secure hardware token. See the `External Module Integration` manual (included in `MatrixSSL Commercial Edition`) for details.

## Creating a Session

The next MatrixSSL integration point in the application is when a new session is starting. In the case of a client, this is whenever it chooses to begin one because TLS is a client-initiated protocol (like HTTP). In the case of a server, a new session should be started when the server accepts an incoming connection from a client on a secure port. In a socket based application, this would typically happen when the accept socket call returns with a valid incoming socket. The application sets up a new session with the API `matrixSslNewClientSession` or `matrixSslNewServerSession`. The returned `ssl_t` context will become the input parameter for all public APIs that act at a per-session level.

The required input parameters to the session creation APIs differ based on whether the application is assuming a server or client role. Both require a populated keys structure (discussed in the previous section) but a client can also nominate a specific cipher suite or session ID when starting a session. The ciphers that the server will accept are determined at compile time.

### Certificate validation callback

The client should also always nominate a certificate callback function during `matrixSslNewClientSession`. This callback function will be invoked mid-handshake to allow the user to inspect the key material, date and other certificate information sent from the server. For detailed information on this callback function, see the API documentation for the Certificate Validation Callback Function section.

The server may also choose to nominate a certificate callback function if client authentication is desired. The MatrixSSL library must have been compiled with `USE_CLIENT_AUTH` defined in order to use this parameter in the `matrixSslNewServerSession` function.

### Configuring session resumption

For clients wishing to quickly (and securely) reconnect to a server that it has recently connected to, there is an optional sessionId parameter that may be used to initiate a faster resumed handshake (the cpu intensive public key exchange is omitted). To use the session parameter, a client should allocate a `sslSessionId_t` structure with `matrixSslNewSessionId` and pass it to `matrixSslNewClientSession` during the initial connection with the server. Over the course of the session negotiation, the MatrixSSL library will populate that structure behind-the-scenes so that during the next connection the same sessionId parameter address can be used to initiate the resumed session.

The same `sslSessionId_t` struct is used for TLS 1.3 session resumption and both session id and ticket based resumption in TLS 1.2 and below.

A server that wishes to support resumption with TLS 1.3 or ticket based resumption in TLS 1.2, must load a symmetric encryption key that it will use to encrypt the session information (a so-called session ticket). The key can be loaded using the `matrixSslLoadSessionTicketKeys` API. The session ticket stored on the client side and is fully opaque to the client, so clients do not need to load session ticket keys.

### Session options

The final argument to `matrixSslNewClientSession` and `matrixSslNewServerSession` is a session options struct (`sslSessOpts_t`). Before modifying the options, the application program should create a new `sslSessOpts_t` and memset it to zero. A zero value ensures that a default value is used if the option is not modified.

#### Run-time protocol version configuration

The most important session option is which of the compiled-in protocol versions to support. It is possible to either select a version range with matrixSslSessOptsSetClientVersionRange and matrixSslSessOptsSetServerVersionRange, or simply list (in order of priority) the versions to support with matrixSslSessOptsSetClientTlsVersions and matrixSslSessOptsSetServerTlsVersions. The selected versions must be have been enabled in the compile-time configuration.

Which version actually gets selected for the connection depends on both the client and server preferences. In TLS 1.3, the client advertises the versions it supports in the supported_versions ClientHello extension. The server selects its preferred version from the client’s list. In TLS 1.2 and below, the client can advertise only a single version. The server can then either select that version or propose a lower version (i.e. propose a version downgrade) in its ServerHello. Based on its configured list of enabled versions, the client then either accepts or rejects the downgrade.

By default, all compiled-in versions are supported and the most recent version is given the highest priority.

#### Selecting key exchange groups

In TLS 1.3, the application must choose the Diffie-Hellman group to use in the (EC)DHE key exchange. The possibilities are finite field Diffie-Hellman groups (ffdhe2048 and ffdhe3072), elliptic curve groups (secp256r1, secp384r1, secp521r1 and x25519). The selection can be performed using the `matrixSslSessOptsSetKeyExGroups` API. Please consult the `MatrixSSL APIs` manual for details.

In TLS 1.2 and below, the negotiated ciphersuite defines whether finite field or elliptic curve Diffie-Hellman is used. The finite field DHE parameters must be loaded from disk using the `matrixSslLoadDhParams` API. Which of the compiled-in elliptic curve groups to support is specified using the `ecFlags` member of `sslSessOpts_t`. Again, please refer to the `MatrixSSL APIs` for details. In future versions, it is likely that in future MatrixSSL versions, the `matrixSslSessOptsSetKeyExGroups` API will be expanded to also support TLS 1.2 and below.

#### Selecting signature algorithms

In TLS 1.3 and TLS 1.2, the signature algorithms to support for authentication signatures can be selected at run time. The `matrixSslSessOptsSetSigAlgs` API takes in a priority-ordered list of algorithms. There are a few restrictions. The Ed25519 signature algorithm is supported only in TLS 1.3. The older PKCS #1.5 format RSA signatures are only supported in TLS 1.2.

In TLS 1.3, the signature algorithms to support in certificates can be configured separately with the `matrixSslSessOptsSetSigAlgs` API.

## Handshaking

During client session initialization with `matrixSslNewClientSession` the SSL handshake message `CLIENT_HELLO` is encoded to the internal outgoing buffer. The client now needs to send this message to the server over a communication channel.
The sequence of events that should always be used to transmit pending handshake data is as follows:

1. The user calls `matrixSslGetOutdata` to retrieve the encoded data and number of bytes to be sent.
2. The user sends the number of bytes indicated from the out data buffer pointer to the peer.
3. The user calls `matrixSslSentData` with the actual number of bytes that were sent.
4. If more data remains (bytes sent < bytes to be sent), repeat the above 3 steps when the transport layer is ready to send again.

When the server receives notice that a client is starting a new session the `matrixSslNewServerSession` API is invoked and the incoming data is retrieved and processed.
The sequence of events that should always be used when expecting handshake data from a peer is as follows:

1. The application calls `matrixSslGetReadbuf` to retrieve a pointer to available buffer space in the `ssl_t` structure.
2. The application reads (or copies) incoming data into that buffer.
3. The application calls `matrixSslReceivedData` to process the data.
4. The application examines the return code from `matrixSslReceivedData` to determine the next step

All incoming messages should be copied into the provided buffer and passed to `matrixSslReceivedData`, which processes the message and drives the handshake through the built-in SSLv3 or TLS state machine. The parameters include the SSL context and the number of bytes that have been received. The return code from `matrixSslReceivedData` tells the application what the message was and how it is to be handled:

`MATRIXSSL_REQUEST_SEND`
  Success. The processing of the received data resulted in an SSL response message that needs to be sent to the peer. If this return code is hit the user should call `matrixSslGetOutdata` to retrieve the encoded outgoing data.
`MATRIXSSL_REQUEST_RECV`
  Success. More data must be received and this function must be called again. User must first call `matrixSslGetReadbuf` again to receive the updated buffer pointer and length to where the remaining data should be read into.
`MATRIXSSL_HANDSHAKE_COMPLETE`
  Success. The SSL handshake is complete. This return code is returned to client side implementation during a full handshake after parsing the `FINISHED` message from the server. It is possible for a server to receive this value if a resumed handshake is being performed where the client sends the final `FINISHED` message.
`MATRIXSSL_RECEIVED_ALERT`
  Success. The data that was processed was an SSL alert message. In this case, the ptbuf pointer will be two bytes (ptLen will be 2) in which the first byte will be the alert level and the second byte will be the alert description. After examining the alert, the user must call `matrixSslProcessedData` to indicate the alert was processed and the data may be internally discarded.
`MATRIXSSL_APP_DATA`
  Success. The data that was processed was application data that the user should process. In this return code case the ptbuf and ptLen output parameters will be valid. The user may process the data directly from ptbuf or copy it aside for later processing. After handling the data the user must call `matrixSslProcessedData` to indicate the plain text data may be internally discarded
`PS_SUCCESS`
  Success. This return code will be returned if the bytes parameter is 0 and there is no remaining internal data to process. This could be useful as a polling mechanism to confirm the internal buffer is empty. One real life use-case for this method of invocation is when dealing with a Google Chrome browser that uses False Start.
`< 0`
  Failure. See API documentation

## Communicating Securely With Peers

### Encrypting Data

Once the handshake is complete, the application wishing to encrypt data that will be sent to the peer has the choice between two encoding options.

#### Encrypting TLS 1.3 early data

In TLS 1.3, the client is allowed to send some early (or 0 RTT) application immediately after its first handshake message, provided that the parties already share a PSK. In other words, early data can be send only during a resumed handshake. To send early data, the client must first check the session ticket it received from the server earlier, to see how early data the server is willing to accept. This can be done using the `matrixSslGetMaxEarlyData` API. After this, the client performs regular application data encryption, as described below. The difference is that the invocation to the application data encryption APIs takes place after the call to `matrixSslNewClientSession`, but before the client sends any data (including handshake messages) over the wire. For details on how to use early data, see the MatrixSSL APIs manual.

#### In-Situ Encryption

An in-situ encryption occurs when the outputted cipher text overwrites the plain text during the encoding process. In this case, the user will retrieve an allocated buffer from the MatrixSSL library, populate the buffer with the desired plaintext, and then notify the library that the plaintext is ready to be encoded. The API steps for the in-situ method are as follows:

1. The application first determines the length of the plaintext that needs to be sent
2. The application calls `matrixSslGetWritebuf` with that length to retrieve a pointer to an internally allocated buffer.
3. The application writes the plaintext into the buffer and then calls `matrixSslEncodeWritebuf` to encrypt the plaintext.
4. The application calls `matrixSslGetOutdata` to retrieve the encoded data and length to be sent (SSL always adds some overhead to the message size).
5. The application sends the out data buffer contents to the peer.
6. The application calls `matrixSslSentData` with the number of bytes that were actually sent

#### User provided plaintext data location

The alternative to in-situ encryption is to allow the user to provide the location and length of the plaintext data that needs to be encoded. In this case, the encrypted data is still written to the internal MatrixSSL out data buffer but the user provided plaintext data is left untouched. The API steps for this method are as follows:

1. The user passes the plaintext and length to `matrixSslEncodeToOutdata`.
2. The application calls `matrixSslGetOutdata` to retrieve the encoded data and length to be sent (SSL always adds some overhead to the message size).
3. The application sends the out data buffer contents to the peer.
4. The application calls `matrixSslSentData` with the # of bytes that were actually sent.

### Decrypting Data

The sequence of events that should always be used when expecting application data from a peer is as follows:

1. The application calls `matrixSslGetReadbuf` to retrieve an allocated buffer.
2. The application copies the incoming data into that buffer.
3. The application calls `matrixSslReceivedData` to process the data.
4. The application confirms the return code from `matrixSslReceivedData` is `MATRIXSSL_APP_DATA` and parses `ptLen` bytes of the returned plain text.
5. If the return code does not indicate application data, handle the return code as described in the handshaking section above.
6. The application calls `matrixSslProcessedData` to inform the library it is finished with the plaintext and checks to see if there are additional records in the buffer to process.

### Decrypting TLS 1.3 early data

If the server receives early data from the client, `matrixSslReceivedData` returns MATRIXSSL_APP_DATA before it returns MATRIXSSL_HANDSHAKE_COMPLETE. If the server detects MATRIXSSL_APP_DATA before MATRIXSSL_HANDSHAKE_COMPLETE, it must decrypt the early data as described above.

## Ending a Session

When the application receives notice that the session is complete or has determined itself that the session is complete, it should notify the other side, close the socket and delete the session. Calling `matrixSslEncodeClosureAlert` and `matrixSslDeleteSession` will perform this step.

A call to `matrixSslEncodeClosureAlert` is an optional step that will encode an alert message to pass along to the other side to inform them to close the session cleanly. The closure alert buffer is retrieved and sent using the same `matrixSslGetOutdata` then `matrixSslSentData` mechanism that all outgoing data uses. Since the connection is being closed, the application shouldn’t block indefinitely on sending the closure alert.

## Closing the Library

At application exit the MatrixSSL library should be un-initialized with a call to `matrixSslClose`. If the application has called `matrixSsNewKeys` as part of the initialization process and kept its keys in memory it should call `matrixSslDeleteKeys` before calling `matrixSslClose`. Also, any existing SSL sessions should be freed by calling `matrixSslDeleteSession` before calling `matrixSslClose`.

Example implementations of MatrixSSL client and server applications integration can be found in the apps subdirectory of the distribution package.

# TLS handshaking

The core of TLS security is the handshake protocol that allows two peers to authenticate and negotiate symmetric encryption keys. A handshake is defined by the specific sequence of SSL messages that are exchanged between the client and server. A collection of messages being sent from one peer to another is called a flight.

## TLS 1.3

### Basic handshake

The basic TLS 1.3 handshake contains three flights.

In all TLS version, negotiation of session parameters such as protocol version, ciphersuite, key exchange group and signature algorithms proceeds in a “client proposes, server selects” fashion.

The ClientHello message contains a priority-ordered list of the ciphersuites the client is willing to support, a random nonce and a set of extensions.

The most important extension is key_share, which contains the client’s (EC)DHE public value. The supported_versions contains the protocol versions supported by the client in order of priority. This extension allows using a TLS 1.3 ClientHello message to negotiate TLS 1.2 or below with older servers. The client’s preferences for key exchange groups (e.g. secp256r1, x25519) and signature algorithms (e.g. rsa_pss_sha256) are listed in supported_groups and signature_algorithms, respectively.

The server responds with the IDs of the chosen ciphersuite, protocol version, key exchange group and signature algorithm. These are contained in the ServerHello message and its extensions. The ServerHello also contains the server’s part of the key exchange (the key_share extension). At this point, handshake keys can be negotiated. In the same flight, the server sends its first encrypted handshake message, EncryptedExtensions. The server then sends the Certificate message containing the server’s public key. The CertificateVerify message contains a signature that proves that the server is in possession of the corresponding private key. A successful validation of this signature authenticates the server. The Finished message contains a message authentication code (MAC), computed over the hashed transcript of the handshake messages exchanged thus far.

The client then responds with its own Finished message.

After successful validation of the MACs in the Finished messages, the parties switch to application data encryption keys.

The handshake messages in parenthesis are only send when the server requires that the client must authenticate itself. The server indicates this by sending a CertificateRequest message, and the client must respond with its own Certificate and CertificateVerify messages.

Note that the server is allowed to send a NewSessionTicket message at any time after receiving the client’s Finished message.

Client Notes:

- The client is the first to send and the last to receive. Therefore, a MatrixSSL implementation of a client must be testing for the `MATRIXSSL_HANDSHAKE_COMPLETE` return code from `matrixSslReceivedData` to determine when application data is ready to be encrypted and sent to the server. When a client wishes to begin a standard handshake, `matrixSslNewClientSession` will be called with an empty sessionId.

- The client is the first to send and the last to receive. Therefore, a MatrixSSL implementation of a client must be testing for the `MATRIXSSL_HANDSHAKE_COMPLETE` return code from `matrixSslReceivedData` to determine when application data is ready to be encrypted and sent to the server. This does not apply to early data, that the client can send at any time during the handshake, provided that it shares a PSK with the server.

- In order to participate in a client authentication handshake, the client must have loaded a Certificate Authority file during the call to `matrixSslLoadRsaKeys`.

Server Notes:

- To prepare for a client authentication handshake the server must nominate a certificate and private key during the call to `matrixSslLoadRsaKeys`. The actual determination of whether or not to perform a client authentication handshake is made when nominating a certificate callback parameter when invoking `matrixSslNewServerSession`. If the callback is provided, a client authentication handshake will be requested. (This note applies to both TLS 1.3 and below).

### Resumed handshake

Once an initial (full) handshake has completeted, both parties derive a PSK from the agreed secret keying material. Typically, the server does not store store the PSK; the server sends the connection parameters and the PSK value in encrypted form in the NewSessionTicket message. The client stores the ticket (along with the self-derived PSK) either in memory or on disk. Tickets have a maximum lifetime of seven days. When the client wishes to resume an earlier session, it must choose the same ciphersuite and in the previous session and send the encrypted ticket in the pre_shared_key ClientHello extension. The server, in possession of the correct session ticket key, decrypts the ticket to retrieve the PSK. Both parties then use the PSK to bootstrap the key derivation process.

The advantage of a resumed hanshake is that no authentication messages (Certificate, CertificateVerify) need to be transmitted. The parties already authenticated each other during the initial handshake, and sharing a common PSK is an implicit proof that their identities have not changed. The resumed handshake has only two flights compared to the full handshake.

Client Notes:

- The client initiates a session resumption handshake by reusing the same `sessionId_t` structure from a previously connected session when calling `matrixSslNewClientSession`. This structure is automatically filled during the initial handshake and also contains the PSK. On-disk storage (serialization) of the `sessionId_t` struct and its contents must be performed by the client program.

Server Notes:

- To enable the session to be resumed, the server must have loaded session ticket keys with `matrixSslLoadSessionKeys`.

### Handshake using externally established PSKs

A variant of the resumed handshake is a handshake using externally established PSKs. In this case, the peers have agreed upon a common PSK using some off-band method. No initial handshake or session ticket needs to take place. The PSK-based handshake also needs to authentication messages. It is also possible to combine the PSK with (EC)DHE key to provide forward secrecry; this is called psk_dhe.

Notes:

The externally established PSK must be loaded with `matrixSslLoadTls13Psk`.

### Early (0 RTT) data

Another advantage of resumed and PSK-based handshake is that the client is allowed to send encrypted application data already in its first flight, before receiving any messages from the server. Such application data is called early or 0 RTT data. The client is allowed to transmit early data until the handshake has completed and application data keys can be activated. Early data has the disadvantage that it is not forward secret and suspectible to replay attacks.

## TLS 1.2 and below

### Standard Handshake

In TLS 1.2 and below, the standard handshake contains four flights (this is in contrast to the three flights of a standard TLS 1.3 handshake).

Client Notes:

- The client is the first to send and the last to receive. Therefore, a MatrixSSL implementation of a client must be testing for the `MATRIXSSL_HANDSHAKE_COMPLETE` return code from `matrixSslReceivedData` to determine when application data is ready to be encrypted and sent to the server. When a client wishes to begin a standard handshake, `matrixSslNewClientSession` will be called with an empty sessionId.

### Client Authentication Handshake

The client authentication handshake allows a two-way authentication. There are four flights in the client authentication handshake.

Client Notes:

- In order to participate in a client authentication handshake, the client must have loaded a Certificate Authority file during the call to `matrixSslLoadRsaKeys`.

Server Notes:

- To prepare for a client authentication handshake the server must nominate a certificate and private key during the call to `matrixSslLoadRsaKeys`. The actual determination of whether or not to perform a client authentication handshake is made when nominating a certificate callback parameter when invoking `matrixSslNewServerSession`. If the callback is provided, a client authentication handshake will be requested. (This note applies to both TLS 1.3 and below).

### Session Resumption Handshake

Session resumption enables a previously connected client to quickly resume a session with a server. Session resumption is much faster than other handshake types because public key authentication is not performed (authentication is implicit since both sides will be using secret information from the previous connection). This handshake types has three flights.

Client Notes:
The client is the first and the last to send data. Therefore, a MatrixSSL implementation of a client must be testing for the `MATRIXSSL_HANDSHAKE_COMPLETE` return code from `matrixSslSentData` to determine when application data is ready to be encrypted and sent to the server.
The client initiates a session resumption handshake by reusing the same `sessionId_t` structure from a previously connected session when calling `matrixSslNewClientSession`.

Server Notes:
The MatrixSSL server will cache a `SSL_SESSION_TABLE_SIZE` number of session IDs for resumption. The length of time a session ID will remain in the case is determined by `SSL_SESSION_ENTRY_LIFE`. Also, the server sends the `FINISHED` message first in this case, which is different from the standard handshake.

### Other Handshakes

Other cipher suites can require variations on the handshake flights. PSK cipher suites do not use any key exchange. DSA cipher suites do not use certificates, and DH/DHE/ECDH/ECDHE cipher suites may or may not use certificates for authentication.

### Re-Handshakes

A re-handshake is a handshake over a currently connected SSL session. A re-handshake may take the form of a standard handshake, a client authentication handshake, or a resumed handshake. Either the client or server may initiate a re-handshake.

> Re-handshaking is not often used and can be the source of cross protocol attacks and implementation bugs. MatrixSSL by default disables the `USE_REHANDSHAKING` option at compile time to reduce code size and complexity.

The `matrixSslEncodeRehandshake` API is used to initiate a re-handshake. The three most common reasons for initiating re-handshakes are:

1. Re-key the symmetric cryptographic material
   Re-keying the symmetric keys adds an extra level of security for applications that require the connection be open for long periods of time or transferring large amounts of data. Periodic changes to the keys can discourage hackers who are mounting timing attacks on a connection.

2. Perform a client authentication handshake
   A scenario may arise in which the server requires that the data being exchanged is only allowed for a client whose certificate has been authenticated, but the original negotiation took place without client authentication. In order to do a client authenticated re-handshake the server must call matrixSslEncodeRehandshake with a certificate callback parameter.

3. Change cipher spec
   The cipher suite may be changed on a connected session using a re-handshake if needed. The client must call matrixSslEncodeRehandshake with the new cipherSpec.

### Disable Re-Handshaking At Runtime

Global disabling of re-handshakes can be controlled at compile time using the `USE_REHANDSHAKING` define but sometimes a per-session control of the feature is required. In these cases, the `matrixSslDisableRehandshakes` and `matrixSslReEnableRehandshakes` APIs are used.

### The Re-Handshake Credit Mechanism

The re-handshake feature has been used at the entry point in a couple TLS attacks. In an effort to combat these attacks, MatrixSSL has incorporated a mechanism that prevents a peer from continually re-handshaking. This “re-handshake credit” mechanism is simply a count of how often the MatrixSSL enabled application will allow a peer to request a re-handshake before sending the `NO_RENEGOTIATION` alert. The default number of credits is set using the `DEFAULT_RH_CREDITS` define in matrixssllib.h. The shipped default is 1.

In order to allow real-life conditions of re-handshakes, a single credit will be added after transmitting a given number of application data bytes. The default count of bytes that have to be sent before gaining a credit is set using the `BYTES_BEFORE_RH_CREDIT` define in matrixssllib.h. The shipped default is 20MB.

# Optional features

This section describes some of the optional SSL handshake features. Additional details can be found in the API documentation for the specific functions that are referenced here.

## Stateless Session Ticket Resumption

RFC 5077 defines an alternative method to the standard server-cached session ID mechanism. The stateless ticket mechanism allows the server to send an encrypted session ticket to the client that the client can use in a later connection to speed up the handshake process. The server does not have to store a large number of session ID entries when this stateless mechanism is used.

Servers and Clients: The feature is made available with the USE_STATELESS_SESSION_TICKETS define in matrixsslConfig.h.

Clients: Clients that wish to use the stateless session resumption mechanism must set the ticketResumption member of the `sslSessOpts_t` structure to 1 when calling `matrixSslNewClientSession`.
With that session option set, the client only has to use the standard session resumption API, `matrixSslNewSessionId`, to complete the use of the feature. If a server does not support stateless session tickets, the standard resumption mechanism will still work.

Servers: The server must load at least one session ticket key using `matrixSslLoadSessionTicketKeys` to enable the feature. A user callback can optionally be registered that will be called each time a session ticket is received from a client. The callback will indicate to the user whether or not the server already has the correct ticket key cached. The callback can be used to locate a ticket key or to void the ticket and revert to a full handshake. The `matrixSslSetSessionTicketCallback` API is used to register this function.

The MatrixSSL implementation for resumption does not renew the session ticket as described in section 3.1 of the RFC (Figure 2). If the ticket is valid, the server progresses with the standard resumed handshake without a `NewSessionTicket` handshake message. If the server is unable to decrypt the session ticket, a full handshake will take place and a new session ticket will be issued. The MatrixSSL library also handles the expiration of a session ticket based on the value of the `SSL_SESSION_ENTRY_LIFE` in matrixsslConfig.h.

## Server Name Indication Extension

RFC 6066 defines a TLS hello extension to allow the client to send the name of the server it is trying to securely connect to. This allows “virtual” servers to locate the correct server with the expected key material to complete the connection.

Servers: Server applications should register the SNI callback using `matrixSslRegisterSNICallback`. This function must be called immediately after `matrixSslNewServerSession` before the first incoming flight from the client is processed. The callback will be invoked during the processing of the `CLIENT_HELLO` message if the client has included the SNI extension. The callback will use the incoming hostname to locate the correct key material and return them in the `sslKeys_t` structure format.

Clients: Clients must include the SNI extension in the `CLIENT_HELLO` message. The utility function `matrixSslCreateSNIext` is provided to help format the extension given a hostname and hostname length. Once the extension format has been created it will be loaded into the `tlsExtension_t` structure with the `matrixSslLoadHelloExtension` API (`matrixSslNewHelloExtension` must first be called). The `tlsExtension_t` type is then passed to `matrixSslNewClientSession` to complete the client side SNI integration.

## Extended Master Secret

The “extended master secret” as specified in RFC 7627 is an important security feature for TLS implementations that use session resumption. The extended master secret feature associates the internal TLS master secret directly to the connection context to prevent man-in-the-middle attacks during session resumption. One such attack is a synchronizing triple handshake as described in Triple Handshakes and Cookie Cutters: Breaking and Fixing Authentication over TLS.

This feature is always enabled by default in both MatrixSSL clients and servers. The peer agreement mechanism is the `CLIENT_HELLO` and `SERVER_HELLO` `extended_master_secret` extension.

Clients: A client will always include the `extended_master_secret` extension when creating the `CLIENT_HELLO` message. If the server replies with an `extended_master_secret`, the upgraded master secret generation will be used. If the server does not reply with an `extended_master_secret`, the standard master secret generation will be used for the connection.

A client may REQUIRE that a server support the `extended_master_secret` feature by setting the `extendedMasterSecret` member of `sslSessOpts_t` to 1. The `sslSessOpts_t` structure is passed to `matrixSslNewClientSession` when starting a TLS session. If `extendedMasterSecret` is set, the client will send a fatal `handshake_failure` alert to the server if the `extended_master_secret` extension is not included in the `SERVER_HELLO`.

Servers: A server will always reply with the `extended_master_secret` extension if the client includes it in the `CLIENT_HELLO` message.

A server may require that a client support the `extended_master_secret` feature by setting the `extendedMasterSecret` member of `sslSessOpts_t` to 1. The `sslSessOpts_t` structure is passed to `matrixSslNewServerSession` when starting a TLS session. If `extendedMasterSecret` is set, the server will send a fatal `handshake_failure` alert to the client if the `extended_master_secret` extension is not included in the `CLIENT_HELLO`.

When creating the session resumption information (either the standard session table or the stateless session ticket) the server will flag whether the extended master secret was used for the initial connection. When a client attempts session resumption, the `CLIENT_HELLO` must include the `extended_master_secret` extension if it was used in the initial connection. Likewise, if the initial connection did not use the `extended_master_secret` the session resumption `CLIENT_HELLO` must also exclude that extension. If there is a mismatch, the server will not allow the session resumption and a full handshake will occur instead.

## Maximum Fragment Length

RFC 6066 defines a TLS extension for negotiating a smaller maximum message size. The default maximum is 16KB (and can’t be set larger). The only allowed sizes that may be negotiated are 512, 1024, 2048, or 4096 bytes. The client requests the feature in a `CLIENT_HELLO` extension and if the server agrees to the new maximum fragment length it will acknowledge that in the `SERVER_HELLO` reply.

Clients: To request a smaller maximum fragment length the user sets the maxFragLen member of the `sslSessOpts_t *` options parameter to 512, 1024, 2048, or 4096 when calling `matrixSslNewClientSession`. The server is free to deny the request.

Servers: Servers will agree to the maximum fragment length request by default. To disable the feature for a session, the user may set the `maxFragLen` member of the `sslSessOpts_t *` options parameter to -1 when calling `matrixSslNewServerSession`.

## Truncated HMAC

RFC 6066 defines a TLS extension for negotiating an HMAC length of 10 bytes. The client requests the feature in a `CLIENT_HELLO` extension and if the server agrees to the truncation it will acknowledge that in the `SERVER_HELLO` reply.

Clients: To request a truncated HMAC session the user sets the `truncHmac` member of the `sslSessOpts_t *`options parameter to PS_TRUE when calling `matrixSslNewClientSession`. The server is free to deny the request.

Servers: Servers will agree to HMAC truncation by default. To disable the feature for a session, the user may set the `truncHmac` member of the `sslSessOpts_t *` options parameter to -1 when calling `matrixSslNewServerSession`.

## Application Layer Protocol Negotiation Extension

RFC 7301 defines a TLS hello extension that enables servers and client to agree on the protocol that will be used after the TLS handshake is complete. The idea is to embed the negotiation in the TLS handshake to save any round trips that might be needed to negotiate the protocol after the handshake. The extension works the same as any extension by the client sending a list of protocols it wishes to use in the `CLIENT_HELLO` and the server replying with an extension in the `SERVER_HELLO`. The trade-off for negotiating the protocol during the handshake is that both MatrixSSL servers and clients must be prepared to intervene in the middle of the handshake process via registered callback functions.

Servers and Clients: The ALPN extension APIs will be available only if the `USE_ALPN` define in matrixsslConfig.h is enabled at compile-time. The define `MAX_PROTO_EXT` is the maximum number of protocols that can be expected in the list of protocols. The default is 8 and can be found in matrixssllib.h.

Servers: Servers that wish to process ALPN extensions sent from a client must call the `matrixSslRegisterALPNCallback` function immediately after the session is created with `matrixSslNewServerSession`. The timing of the registration is important so that the callback can be associated with the proper session context before the first handshake message from the client is passed to `matrixSslReceivedData`.

The server ALPN callback that is registered with `matrixSslRegisterALPNCallback` must have a prototype of:

    void ALPN_callback(void *ssl, short protoCount, char *proto[MAX_PROTO_EXT], int32_t protoLen[MAX_PROTO_EXT], int32_t *index)

`ssl`
  parameter is the session context and may be typecast to an `ssl_t *` type if access is required.
`protoCount`
  is the number of protocols that the client has sent in the `CLIENT_HELLO` extension. It is the count of the number of array entries in the `proto` and `protoLen` parameters to follow.
`proto`
  parameter is the priority-ordered list of string protocol names the client wants to communicate with following the TLS handshake. The `protoLen` parameter holds the string lengths of the `proto` counterpart parameter for each protocol.
`index`
  parameter is an output that the callback logic will assign based on the desired action:

- The index of the proto array member the server has agreed to use. The index is the zero-based index to the array so a return value of 0 will indicate the first protocol in the list. This selection will result in the server including its own ALPN extension in the `SERVER_HELLO` message with the chosen protocol.
- A negative value assigned to index indicates the server is not willing to communicate using any of the protocols. A fatal “`no_application_protocol`” alert will be sent to the client and the handshake will terminate.
- If the callback does not assign any value to the outgoing parameter, the server will not take any action. That is, neither a reply ALPN extension nor an alert will be sent to the client and the handshake will continue normally.

Clients: To support this feature, clients must be able to generate the ALPN extension and also receive the server reply.

To generate the ALPN extension, the API `matrixSslCreateALPNext` is used in conjunction with the `matrixSslNewHelloExtension` or `matrixSslLoadHelloExtension` framework.

The `matrixSslCreateALPNext` API accepts an array of `unsigned char *` string values (array length of `MAX_PROTO_EXT`) along with a companion array that hold the string lengths for the protocol list. The function will format the protocols into the specified ALPN extension format and return that to the caller in the output parameters. Once the extension has been created the client must load the extension using the `matrixSslLoadHelloExtension` API (`matrixSslNewHelloExtension` must have been called as well). Finally, the extension must be passed to `matrixSslNewClientSession` in the extensions parameter. Here is what the ALPN extension creation and session start might look like:

    tlsExtension_t * extension;
    unsigned char   *alpn[MAX_PROTO_EXT];
    int32_t         alpnLen[MAX_PROTO_EXT];

    matrixSslNewHelloExtension(&extension);

    alpn[0] = psMalloc(NULL, strlen("http/1.0"));
    memcpy(alpn[0], "http/1.0", strlen("http/1.0"));
    alpnLen[0] = strlen("http/1.0");
    alpn[1] = psMalloc(NULL, strlen("http/1.1"));
    memcpy(alpn[1], "http/1.1", strlen("http/1.1"));
    alpnLen[1] = strlen("http/1.1");

    matrixSslCreateALPNext(NULL, 2, alpn, alpnLen, &ext, &extLen);
    matrixSslLoadHelloExtension(extension, ext, extLen, EXT_ALPN);

    psFree(alpn[0]);
    psFree(alpn[1]);

    matrixSslNewClientSession(&ssl, keys, sid, g_cipher, g_ciphers, certCb, g_ip, extension, extensionCb, &options);

    matrixSslDeleteHelloExtension(extension);

To receive the server reply to the ALPN extension the client must register an extension callback routine using the `extCb` parameter when calling `matrixSslNewClientSession`. The callback will be invoked with the ALPN extension ID of `EXT_ALPN` (16) with a format of a single byte length followed by the protocol string value the server has agreed to.

See the example in `apps/ssl/client.c` for full implementation details.

## TLS Fallback SCSV

RFC 7507 defines a method to prevent protocol downgrade attacks. Such an attack is illustrated below:

MatrixSSL does not automatically do such version fallback, but client software using MatrixSSL may choose to do this for compatibility with unknown servers. In this case, the `TLS_FALLBACK_SCSV` option flag MUST be set for each connection attempt at a lower protocol version. This will mitigate the attack as follows:

The client indicates that the lower version being requested is due to a previous response from the server that it was not supported. If the server sees this flag, but supports a higher version, it will recognize that something is amiss and return an alert, rather than a valid `SERVER_HELLO`.

> Note that if the attacker attempted to remove the `TLS_FALLBACK_SCSV` indication from the message, the tampering would be detected by the server and client later, as part of the `FINISHED` message handshake hash validation. The indication is specified by the RFC as a special ciphersuite value, rather than a TLS extension for maximum compatibility.

Clients
If the client connection is being made at a lower protocol level because the server indicated it did not support the desired level, the `fallbackScsv` member of the `sslSessOpts_t *`options parameter must be set to `PS_TRUE` when calling `matrixSslNewClientSession` to prevent this type of attack.

Servers
Servers will evaluate the FALLBACK_SCSV indication automatically as per RFC:

> If TLS_FALLBACK_SCSV appears in ClientHello.cipher_suites and the highest protocol version supported by the server is higher than the version indicated in ClientHello.client_version, the server MUST respond with a fatal inappropriate_fallback alert

## Online Certificate Status Protocol (OCSP)

The Online Certificate Status Protocol (OCSP) is an alternative to the Certificate Revocation List (CRL) mechanism for performing certificate revocation tests on server keys. TLS integrates with OCSP in a mechanism known as OCSP stapling. This feature allows the client to request that the server provide a time-stamped OCSP response when presenting the X.509 certificate during the TLS handshake. The primary goal for this method is to allow resource constrained clients to perform certificate revocation tests without having to communicate with an OCSP Responder themselves. The general process is illustrated below. The MatrixSSL also
supports OCSP request generation, to do revocation tests on certificates without TLS server supporting OCSP stapling.

OCSP stapling is specified in Section 8 (Certificate Status Request) of the TLS Extensions RFC 6066. The `USE_OCSP` define in cryptoConfig.h must be enabled for these features to be available.

Clients
A client application can request OCSP stapling by setting the OCSPstapling member of the `sslSessOpts_t` structure when invoking `matrixSslNewClientSession`. This flag will trigger the creation of the Certificate Status Request extension in the `CLIENT_HELLO` message. The resulting `status_request` extension will not specify any responder identification hints or request extensions. This indicates that the server is free to provide whatever OCSP response is relevant to its identity certificate.

In order to validate the signature of provided OCSP response, the client will have to verify the Certificate Authority of the OCSP responder. There are two places the MatrixSSL library will search for this CA file. The first place the library will look is in the CA material that is loaded in the standard `matrixSslLoadRsaKeys` (or `matrixSslLoadEcKeys`) API. If the CA file is not located in this pre-loaded key material, the library will next look to the server’s certificate chain. In practice, many TLS servers that implement OCSP stapling will create a certificate chain in which the parent certificate of the primary identity certificate also acts as the OCSP responder. At the time of the OCSP validation test, the `CERTIFICATE` message will have already been processed and validated. If the client has confirmed the server to have a valid chain of trust, it is appropriate to trust that same certificate chain to provide the OSCP response. If the client is unable to locate the CA file for the public key of the OCSP responder, the handshake will fail.

In order to validate the time stamp of the OCSP response the client library will invoke the `checkOCSPtimestamp` function in crypto/keyformat/x509.c. The default time window for accepting an OCSP response is 1 week and can be changed using the `OCSP_VALID_TIME_WINDOW` define in cryptolib.h.

The OCSP stapling specification does not have guidance on how a client should behave if a server does not provide a `CERTIFICATE_STATUS` message when requested. The `USE_OCSP_MUST_STAPLE` define in matrixsslConfig.h is included to allow the client application to require that the server provide the message. If `USE_OCSP_MUST_STAPLE` is enabled and the client has requested `CERTIFICATE_STATUS`, the handshake will abort if the server does not provide one.

Servers
A server application wishing to support OCSP stapling must communicate out of band with an OCSP responder to periodically update the signed OCSP response, as defined in RFC 6960. The response is loaded into MatrixSSL by calling `matrixSslLoadOCSPResponse`. This function takes a fully formed `OCSPResponse` ASN.1 buffer as defined in RFC6960 section 4.2 and loads it into the provided `sslKeys_t` structure. Whenever the server application gets a new OCSP response, the same `matrixSslLoadOCSPResponse` API can be called to update the `sslKeys_t` structure.

When a client sends the `status_request` extension the server will look to see if an OCSP response is available in the `sslKeys_t` structure and reply with a `status_request` extension and the `CERTIFICATE_STATUS` message that holds the OCSP response for the client to validate.

### Configuring OCSP Feature for Use

The OCSP functionality depends on definitions in `cryptoConfig.h`. Full read/write functionality and server features requires following defines:

    #define USE_X509
    #define USE_CERT_PARSE
    #define USE_FULL_CERT_PARSE
    #define USE_CRL
    #define USE_OCSP
    #define USE_OCSP_MUST_STAPLE

The standard configurations like `default` on MatrixSSL FIPS and Commercial releases have already neccessary features enabled.

### Invoking OCSP Manually

#### OCSP Example Application

OCSP use has example application `ocsp.c` in `apps/crypto` subdirectory of MatrixSSL. The compiled application will be known as `matrixOCSP`.
This application can be used to create OCSP requests and get responses. The easiest way to study OCSP functionality is to use and examine this application. The application provides usage instructions when invoked without parameters. Some of most interesting options include `-request`and `-reply` which allow storing requests and responses for examination with various other tools supporting OCSP and/or ASN.1 DER/BER.
The most MatrixSSL supported functions are used by this program.

The most of functionality of `ocsp.c` program is found in `OCSPRequestAndResponseTest()` function, and this function is most useful to study for creating OCSP requests and validating OCSP responses.

#### Creating OCSP request data

In MatrixSSL, function `matrixSslWriteOCSPRequestExt()` is used to create OCSP requests.

The function is invoked as follows:

        if (matrixSslWriteOCSPRequestExt(NULL,
                                         subject,
                                         issuer,
                                         &request,
                                         &requestLen,
                                         &info) != PS_SUCCESS) {
            /* Error handling */
            return PS_FAILURE;
    }

The memory pool is usually passed in as NULL. The certificates `subject` (certificate to check) and `issuer` (for issuer of certificate to check needs to be provided). New memory will be allocated for request and the pointer of request will be written to `request`, and the amount of memory allocated will be written to `requestLen`.

`info` is used to provide options for OCSP request. These options are discussed below. The memory allocated for OCSP request shall be freed after the request
is no longer needed.

##### Providing Nonce Extension

MatrixSSL provides nonce for OCSP nonce extension via `info.flags` flag `MATRIXSSL_WRITE_OCSP_REQUEST_FLAG_NONCE`. The OCSP nonce extension provide unique nonce value, which can be used to the request is replied with a new (rather than cached) OCSP response.

##### Providing name for requestor (requestorName)

The MatrixSSL allows providing name for OCSP requestor. The name is represented using X.509 GeneralName. matrixSslWriteOCSPRequestInfoSetRequestorId() function is provided to help encoding the OCSP requestor name in suitable format for OCSP request. Note: This option is relatively rarely used and some OCSP Responders will reject requests with RequestorName.

##### Providing list of requests (requestList)

If `matrixSslWriteOCSPRequestInfo_t` flag `MATRIXSSL_WRITE_OCSP_REQUEST_FLAG_CERT_LIST` is specified,
then OCSP request is made concerning a list of certificates linked (via `subject->next`). This is a convenient way to allow for less network traffic when working with the certificates from the same issuer.

Note: Some OCSP responders only support single request, and will reject requests concerning multiple certificates.

#### Interacting with OCSP server

The most OCSP servers use HTTP protocol. MatrixSSL provides `psUrlInteract()` function, to create HTTP request and get HTTP response. When OCSP is used in SSL/TLS context, the most commonly `subject` certificate will contain AuthorityInfoAccess information regarding what server will provide OCSP information. It is accessed via `authorityInfoAccess` field of the Certificate structure `psX509Cert_t`.

You can see `getOCSPResponse()` function in `ocsp.c` for a concrete example on how to interact with the server.

To use HTTP protocol, it is necessary to know the correct URL. The URL is in practice commonly specified in AuthorityInfoAccess extensions of X.509 certificate. The following function will extract authority information from X.509 certificate.

    static char *getAuthorityInfoOCSP_URI(const psX509Cert_t *subject)
    {
        char *url = NULL;
        x509authorityInfoAccess_t *authInfo;
        authInfo = subject->extensions.authorityInfoAccess;

        /* Find the first AuthorityInfoAccess extension data with
           OCSP information. */
        while(authInfo != NULL) {
            if (authInfo->ocsp && authInfo->ocspLen > 0) {
                url = calloc(1, authInfo->ocspLen + 1);
                if (url) {
                    memcpy(url, authInfo->ocsp, authInfo->ocspLen);
                }
                break;
            }
            authInfo = authInfo->next;
        }
        return url;
    }

The interaction will result in getting an HTTP error code or OCSP response. The OCSP response can be analyzed and verified using the functions described in the next chapter.

#### Working with OCSP response

Working with OCSP response actually consists of four phases:

- Obtaining and checking issuer certificate
- Parsing
- Checking of time-stamps
- Validation of response

The functions support both OCSP responses just retrieved as response to a constructed OCSP request and cached OCSP requests. One type of cache OCSP response often used in SSL/TLS is the ones used in OCSP stapling.

#### Obtaining and checking issuer certificate

To validate OCSP response, an issuer certificate is used. Many organizations use the same certificates to sign OCSP responses than are used to validate certificates themselves. In this case, the issuer certificate has been obtained and validated already as part of X.509 certificate validation. In case, OCSP responses have been signed with other keys designated for that purpose, then they shall be validated just like any other X.509 intermediate certificate, e.g. using `psX509ParseCert()` and `psX509AuthenticateCert()`.

The certificate of keys intended to be used for OCSP may have been marked for that purpose. This can be checked e.g. with the following code:

    bool gotEKU_OCSP = false;
    bool gotKU_OCSP = false;
    const x509v3extensions_t *ext = &issuer->extensions;
    /* Specific flag for OCSP. */
    if ((ext->ekuFlags & EXT_KEY_USAGE_OCSP_SIGNING) > 0)
        gotEKU_OCSP = true;
    /* No OCSP specific flag, use generic digital signatures flag. */
    if ((ext->keyUsageFlags & KEY_USAGE_DIGITAL_SIGNATURE) > 0)
        gotKY_OCSP = true;

Only if both key usage and extended key usage flags are found in the certificate, the key can be considered to be marked for OCSP usage. However, many of the keys used for OCSP do not have this purpose marked in their certificate, and the most important validity check for OCSP signers remains ensuring that they have been signed by (one of) the CA root(s).

#### Parsing OCSP response

OCSP response can be parsed using function `parseOCSPResponse()`. Parsing does not validate the response for correctness, but translates the response to internal form (`mOCSPResponse_t`), which allows easier working with the response.

#### Checking time-stamps

The times in OCSP response are validated with `checkOCSPResponseDates()` function. The function will check the times in the OCSP response with respect to the current time and provide `struct tm` field representing various times (ProducedAt, thisUpdate, nextUpdate) within OCSP response. If OCSP response has timed out the function will return `PS_TIMEOUT_FAIL`.

The time validation function allows some clock skew between server and client. The default value is provided by `PS_OCSP_TIME_LINGER`, it is two minutes. It is possible to provide different values depending on how well the clocks are expected to be synchronized.

If OCSP response is found to be no longer valid or there is another issue in timestamps, then the application should not accept the OCSP response or process it further.

#### Validating OCSP response

The purpose of OCSP is to check if a certificate is revoked. `validateOCSPResponse_ex()` function is (finally) provided for this purpose. The function supports many different arguments. The options are used to pass in optional arguments.

    opts.knownFlag = &known;
    opts.revocationFlag = &revocated;
    opts.nonceMatch = &nonceOk;
    opts.revocationTime = &revocationTime;
    opts.revocationReason = &revocationReason;
    opts.request = request;
    opts.requestLen = requestLen;
    if (validateOCSPResponse_ex(NULL,
                                issuer,
                                subject,
                                &response,
                                &opts) != PS_SUCCESS) {
        /* Error handling */
        return PS_FAILURE;
    }

The memory pool is usually passed in as NULL. The certificates `subject` (certificate to check) and `issuer` (for expected issuer of certificate needs to be provided). `response` is the response to work with. This function returns PS_SUCCESS only when validation has been deemed success, and the certificate has not been found to be revoked.

In the simplest usage of this function, only the return code is checked. The PS_SUCCESS return code is only gotten if certificate is not revoked and the response has been successfully validated.

`opts` is used to provide options for OCSP response parsing. These options will provide more information regarding successful and unsuccessful invocation of `validateOCSPResponse_ex()`. The options are discussed below.

##### Revocation status and reason

Flag pointed by `opts.knownFlag` will be set to true (1) if OCSP response contained information regarding this certificate (`subject`). Responses not containing information regarding the certificate should be dismissed.
If certificate has been revoked, flag pointed by `opts.revocationFlag` will be set to true (1). If available, variables pointed by `opts.revocationReason` and `opts.revocationTime` and will be set to indicate the revocation reason and time respectively. In OCSP protocol, revocation reasons are indicated by numbers between 1 and 10, the same codes used by CRL. (See function `mapRevocationReason()` in `ocsp.c` for mapping the numbers to various reasons).

##### Nonce validation

For validating OCSP nonce, it is necessary to provide the OCSP request as pointer and length pair to `validateOCSPResponse_ex()` function, using `opts.request` and `opts.requestLen`. If the same nonce is found in OCSP request and response, that is considered a match and flag pointed to by `opts.nonceMatch` is set to true (1).

Currently, OCSP nonce is not supported by many of the OCSP servers deployed in practice, and therefore, it is recommended to not rely on OCSP nonce feature to be provided by third party OCSP servers.

## Client Authentication using an External Security Token

MatrixSSL allows the TLS client to authenticate itself using an external security token. The external client authentication feature allows the client-side private key operation (i.e. the signing of the handshake_messages hash in the CertificateVerify handshake message) to be offloaded from MatrixSSL to an external module. Please consult the `External Client Authentication` section in MatrixSSL External Module Integration manual for details on how to use this feature.

# Deprecated Features

The features in this section are minimally supported and should only be used in cases where they are explicitly required for compatibility. Please be aware of any security implications of these features before enabling them.

## EAP_FAST Mode

EAP-FAST is an aging, proprietary EAP method that uses TLS to bootstrap a higher level EAP authentication method. It is supported by MatrixSSL for the client (EAP Supplicant) side only. Unlike EAP-TLS or EAP-TTLS, EAP-FAST requires modifications to the TLS protocol and therefore must be explicitly enabled with both a compile time define `USE_EAP_FAST` and a client side API `matrixSslSetSessionIdEapFast` in matrixssllib.h.

EAP-FAST requires a Protected Access Credential (PAC) to be provisioned between the peers out-of-band (see RFC5422 Dynamic Provisioning Using Flexible Authentication via Secure Tunneling Extensible Authentication Protocol (EAP-FAST). Like the `TLS_PSK` cipher suites, this PAC consists of a secret key (pac-key) and a unique id associated with the key, both shared between the peers. However, the `TLS_PSK` ciphers are not used for this mechanism directly.

The PAC is exchanged between the peers by the client sending the Stateless Session Ticket Resumption specification: the `CLIENT_HELLO SessionTicket Extension`. However, a PAC explicitly cannot be received by a client in the corresponding `NewSessionTicket` handshake message from the server, and must be provisioned out-of-band. Unfortunately this requires alteration of the standard TLS state machine logic.

    sslSessionId_t  *sid;
    ssl_t           *ssl;
    ...
    rc = matrixSslNewSessionId(&sid, NULL);
    matrixSslSetSessionIdEapFast(sid, pac_key, pac_opaque, pac_opaque_len);
    rc = matrixSslNewClientSession(&ssl, keys, sid, ...);
    ...
    /* When TLS session is successfully negotiated */
    rc = matrixSslGetEapFastSKS(ssl, session_key_seed);
    ...
    matrixSslDeleteSession(ssl);
    matrixSslDeleteSessionId(sid);

## ZLIB Compression

The TLS specification specifies a mechanism for peers to agree on an algorithm to compress data before being encrypted. Although the feature is not widely adopted and is deprecated due to the ‘CRIME’ attack, there is limited support in MatrixSSL for zlib compression for implementations that are sensitive to throughput.

To enable the feature, enable `USE_ZLIB_COMPRESSION` in matrixssllib.h. It will also be necessary to edit the development environment to link with a zlib library. For a standard GCC POSIX environment this should simply mean including `–lz` in the linker flags.

The built-in support for this feature is limited. The feature only supports the internal compression and decompression of the `FINISHED` handshake message for initial handshakes. This means re-handshaking is not supported and that the application MUST compress and decompress application data manually.

On the application data sending side:
After a successful handshake with `USE_ZLIB_COMPRESSION` enabled, the user should call `matrixSslIsSessionCompressionOn` to test whether that mode has been successfully negotiated. If `PS_TRUE`, the user must manually zlib deflate any application data before calling the Matrix encryption functions. Do not compress more plaintext data in a single record than the maximum allowed record size to remain compatible with 3rd party SSL implementations.

On the application data receiving side:
Applications must test for `MATRIXSSL_APP_DATA_COMPRESSED` as the return code from `matrixSslReceivedData`. If found, the data must be zlib inflated to obtain the plaintext data

## MatrixSSL Statistics Framework

Implementations that wish to capture counts of SSL events can tap into the `MATRIXSSL_STATS` framework by enabling `USE_MATRIXSSL_STATS` during the compile. The mechanism is a very simple callback that can be registered to record whatever specific SSL event the user wants. The default set of events capture the following:

- `CLIENT_HELLO` count (sent for clients and received for servers)

- `SERVER_HELLO` count (sent for servers and received for clients)

- Alerts sent

- Resumed handshake count

- Failed resumed handshake count

- Number of

- application data bytes received

- Number of application data bytes sent

To add an event to the framework the user must:

1. Add a unique ID to the list of existing stats in matrixsslApi.h
2. Add the call to `matrixsslUpdateStat` in the appropriate place in the MatrixSSL library