127 lines
4.7 KiB
Plaintext
127 lines
4.7 KiB
Plaintext
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=pod
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=head1 NAME
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CMS_verify, CMS_get0_signers - verify a CMS SignedData structure
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=head1 SYNOPSIS
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#include <openssl/cms.h>
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int CMS_verify(CMS_ContentInfo *cms, STACK_OF(X509) *certs, X509_STORE *store, BIO *indata, BIO *out, unsigned int flags);
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STACK_OF(X509) *CMS_get0_signers(CMS_ContentInfo *cms);
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=head1 DESCRIPTION
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CMS_verify() verifies a CMS SignedData structure. B<cms> is the CMS_ContentInfo
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structure to verify. B<certs> is a set of certificates in which to search for
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the signing certificate(s). B<store> is a trusted certificate store used for
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chain verification. B<indata> is the detached content if the content is not
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present in B<cms>. The content is written to B<out> if it is not NULL.
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B<flags> is an optional set of flags, which can be used to modify the verify
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operation.
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CMS_get0_signers() retrieves the signing certificate(s) from B<cms>, it must
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be called after a successful CMS_verify() operation.
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=head1 VERIFY PROCESS
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Normally the verify process proceeds as follows.
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Initially some sanity checks are performed on B<cms>. The type of B<cms> must
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be SignedData. There must be at least one signature on the data and if
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the content is detached B<indata> cannot be B<NULL>.
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An attempt is made to locate all the signing certificate(s), first looking in
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the B<certs> parameter (if it is not NULL) and then looking in any
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certificates contained in the B<cms> structure itself. If any signing
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certificate cannot be located the operation fails.
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Each signing certificate is chain verified using the B<smimesign> purpose and
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the supplied trusted certificate store. Any internal certificates in the message
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are used as untrusted CAs. If CRL checking is enabled in B<store> any internal
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CRLs are used in addition to attempting to look them up in B<store>. If any
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chain verify fails an error code is returned.
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Finally the signed content is read (and written to B<out> is it is not NULL)
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and the signature's checked.
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If all signature's verify correctly then the function is successful.
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Any of the following flags (ored together) can be passed in the B<flags>
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parameter to change the default verify behaviour.
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If B<CMS_NOINTERN> is set the certificates in the message itself are not
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searched when locating the signing certificate(s). This means that all the
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signing certificates must be in the B<certs> parameter.
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If B<CMS_NOCRL> is set and CRL checking is enabled in B<store> then any
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CRLs in the message itself are ignored.
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If the B<CMS_TEXT> flag is set MIME headers for type B<text/plain> are deleted
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from the content. If the content is not of type B<text/plain> then an error is
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returned.
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If B<CMS_NO_SIGNER_CERT_VERIFY> is set the signing certificates are not
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verified.
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If B<CMS_NO_ATTR_VERIFY> is set the signed attributes signature is not
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verified.
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If B<CMS_NO_CONTENT_VERIFY> is set then the content digest is not checked.
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=head1 NOTES
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One application of B<CMS_NOINTERN> is to only accept messages signed by
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a small number of certificates. The acceptable certificates would be passed
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in the B<certs> parameter. In this case if the signer is not one of the
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certificates supplied in B<certs> then the verify will fail because the
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signer cannot be found.
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In some cases the standard techniques for looking up and validating
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certificates are not appropriate: for example an application may wish to
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lookup certificates in a database or perform customised verification. This
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can be achieved by setting and verifying the signers certificates manually
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using the signed data utility functions.
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Care should be taken when modifying the default verify behaviour, for example
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setting B<CMS_NO_CONTENT_VERIFY> will totally disable all content verification
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and any modified content will be considered valid. This combination is however
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useful if one merely wishes to write the content to B<out> and its validity
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is not considered important.
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Chain verification should arguably be performed using the signing time rather
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than the current time. However since the signing time is supplied by the
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signer it cannot be trusted without additional evidence (such as a trusted
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timestamp).
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=head1 RETURN VALUES
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CMS_verify() returns 1 for a successful verification and zero if an error
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occurred.
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CMS_get0_signers() returns all signers or NULL if an error occurred.
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The error can be obtained from L<ERR_get_error(3)|ERR_get_error(3)>
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=head1 BUGS
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The trusted certificate store is not searched for the signing certificate,
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this is primarily due to the inadequacies of the current B<X509_STORE>
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functionality.
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The lack of single pass processing means that the signed content must all
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be held in memory if it is not detached.
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=head1 SEE ALSO
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L<ERR_get_error(3)|ERR_get_error(3)>, L<CMS_sign(3)|CMS_sign(3)>
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=head1 HISTORY
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CMS_verify() was added to OpenSSL 0.9.8
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=cut
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