X-Git-Url: https://www.bearssl.org/gitweb//home/git/?p=BearSSL;a=blobdiff_plain;f=src%2Fec%2Fecdsa_atr.c;h=3a11226e49446667136e5825d9a171649f01aced;hp=91a1d0cc9be6d47723411fe2267a602068c14a03;hb=6dd8c51ba7e8ca106ede7ff58b5c507042bbf6eb;hpb=3210f38e0491b39aec1ef419cb4114e9483089fb

diff --git a/src/ec/ecdsa_atr.c b/src/ec/ecdsa_atr.c
index 91a1d0c..3a11226 100644
--- a/src/ec/ecdsa_atr.c
+++ b/src/ec/ecdsa_atr.c
@@ -33,6 +33,11 @@ br_ecdsa_asn1_to_raw(void *sig, size_t sig_len)
 	 * deviations to DER with regards to minimality of encoding of
 	 * lengths and integer values. These deviations are still
 	 * unambiguous.
+	 *
+	 * Signature format is a SEQUENCE of two INTEGER values. We
+	 * support only integers of less than 127 bytes each (signed
+	 * encoding) so the resulting raw signature will have length
+	 * at most 254 bytes.
 	 */
 
 	unsigned char *buf, *r, *s;
@@ -43,9 +48,20 @@ br_ecdsa_asn1_to_raw(void *sig, size_t sig_len)
 	if (sig_len < 8) {
 		return 0;
 	}
+
+	/*
+	 * First byte is SEQUENCE tag.
+	 */
 	if (buf[0] != 0x30) {
 		return 0;
 	}
+
+	/*
+	 * The SEQUENCE length will be encoded over one or two bytes. We
+	 * limit the total SEQUENCE contents to 255 bytes, because it
+	 * makes things simpler; this is enough for subgroup orders up
+	 * to 999 bits.
+	 */
 	zlen = buf[1];
 	if (zlen > 0x80) {
 		if (zlen != 0x81) {
@@ -62,6 +78,10 @@ br_ecdsa_asn1_to_raw(void *sig, size_t sig_len)
 		}
 		off = 2;
 	}
+
+	/*
+	 * First INTEGER (r).
+	 */
 	if (buf[off ++] != 0x02) {
 		return 0;
 	}
@@ -71,6 +91,10 @@ br_ecdsa_asn1_to_raw(void *sig, size_t sig_len)
 	}
 	r = buf + off;
 	off += rlen;
+
+	/*
+	 * Second INTEGER (s).
+	 */
 	if (off + 2 > sig_len) {
 		return 0;
 	}
@@ -83,6 +107,9 @@ br_ecdsa_asn1_to_raw(void *sig, size_t sig_len)
 	}
 	s = buf + off;
 
+	/*
+	 * Removing leading zeros from r and s.
+	 */
 	while (rlen > 0 && *r == 0) {
 		rlen --;
 		r ++;
@@ -92,6 +119,11 @@ br_ecdsa_asn1_to_raw(void *sig, size_t sig_len)
 		s ++;
 	}
 
+	/*
+	 * Compute common length for the two integers, then copy integers
+	 * into the temporary buffer, and finally copy it back over the
+	 * signature buffer.
+	 */
 	zlen = rlen > slen ? rlen : slen;
 	sig_len = zlen << 1;
 	memset(tmp, 0, sig_len);