Add CBS_peek_asn1_tag.

Intended to make parsing ASN.1 structures with OPTIONAL elements easier. (Just
attempting to parse the next tag doesn't distinguish between a malformed CBS
which has now been partially advanced and an optional tag mismatch.)

Change-Id: Idceb3dfd6ec028e87e1bc5aaddcec177b0c32150
Reviewed-on: https://boringssl-review.googlesource.com/1995
Reviewed-by: Adam Langley <agl@google.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/crypto/bytestring/bytestring_test.c b/crypto/bytestring/bytestring_test.c
index e4afccd..f30179d 100644
--- a/crypto/bytestring/bytestring_test.c
+++ b/crypto/bytestring/bytestring_test.c
@@ -109,6 +109,10 @@ static int test_get_asn1(void) {
   CBS data, contents;
 
   CBS_init(&data, kData1, sizeof(kData1));
+  if (CBS_peek_asn1_tag(&data, 0x1) ||
+      !CBS_peek_asn1_tag(&data, 0x30)) {
+    return 0;
+  }
   if (!CBS_get_asn1(&data, &contents, 0x30) ||
       CBS_len(&contents) != 2 ||
       memcmp(CBS_data(&contents), "\x01\x02", 2) != 0) {
@@ -145,6 +149,11 @@ static int test_get_asn1(void) {
     return 0;
   }
 
+  CBS_init(&data, NULL, 0);
+  if (CBS_peek_asn1_tag(&data, 0x30)) {
+    return 0;
+  }
+
   return 1;
 }