1 files changed, 27 insertions, 0 deletions
diff --git a/scan_asn1BITSTRING.c b/scan_asn1BITSTRING.c
new file mode 100644
index 0000000..b42901a
--- /dev/null
+++ b/scan_asn1BITSTRING.c
@@ -0,0 +1,27 @@
+#include "asn1.h"
+
+size_t scan_asn1BITSTRING(const char* src,const char* max,const char** s,size_t* l) {
+  size_t tmp;
+  unsigned long tag;
+  enum asn1_tagclass tc;
+  enum asn1_tagtype tt;
+  if ((tmp=scan_asn1string(src,max,&tc,&tt,&tag,s,l)))
+    if (tc==UNIVERSAL && tt==PRIMITIVE && tag==BIT_STRING) {
+      unsigned char lastbyte;
+      if (*l==0 ||	/* length must be at least 1 because for bit strings, the first octet contains the number of unused bits in the last octet */
+	  (unsigned char)(**s)>7)	/* the number of unused bits in the last octet must not be negative and can be at most 7 */
+	return 0;
+      /* these are DER checks */
+      /* can't have unused bits if the length is 0 */
+      if (*l==1 && **s)
+	return 0;
+      /* now check if the unused bits are 0 */
+      lastbyte=(*s)[*l+1];
+      if (lastbyte & (0xff >> (8-**s)))
+	return 0;
+      *l=(*l-1)*8-(unsigned char)(**s);
+      ++*s;
+      return tmp;
+    }
+  return 0;
+}