diff --git a/scan/scan_asn1dertag.c b/scan/scan_asn1dertag.c
index 7c5e7c1..a003d14 100644
--- a/scan/scan_asn1dertag.c
+++ b/scan/scan_asn1dertag.c
@@ -2,25 +2,10 @@
 
 size_t scan_asn1dertag(const char* src,size_t len,unsigned long long* length) {
   size_t n;
-  unsigned long long l;
-  unsigned int bits=0;
-  if (len==0) return 0;
-  {
-    unsigned int k=src[0]&0x7f;
-    if (!(src[0]&0x80)) {
-      *length=k;
-      return 1;
-    }
-    if (!(l=k)) return 0;	// non-minimal encoding
-    while (k) {	// count bits in leading byte
-      ++bits;
-      k>>=1;
-    }
-    bits=sizeof(l)*8-bits;
-  }
-  for (n=1; n<len; ++n) {
-    if (bits<7) return 0;
-    bits-=7;
+  unsigned long long l=0;
+  if (len==0 || (unsigned char)src[0]==0x80) return 0;	// catch non-minimal encoding
+  for (n=0; n<len; ++n) {
+    if (l>>(sizeof(l)*8-7)) return 0;	// catch integer overflow
     l=(l<<7) | (src[n]&0x7f);
     if (!(src[n]&0x80)) {
       *length=l;
@@ -29,3 +14,4 @@ size_t scan_asn1dertag(const char* src,size_t len,unsigned long long* length) {
   }
   return 0;
 }
+