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>
diff --git a/include/openssl/bytestring.h b/include/openssl/bytestring.h
index 1af20e8..acaba8e 100644
--- a/include/openssl/bytestring.h
+++ b/include/openssl/bytestring.h
@@ -141,6 +141,13 @@
* ASN.1 header bytes too. */
OPENSSL_EXPORT int CBS_get_asn1_element(CBS *cbs, CBS *out, unsigned tag_value);
+/* CBS_peek_asn1_tag looks ahead at the next ASN.1 tag and returns one
+ * if the next ASN.1 element on |cbs| would have tag |tag_value|. If
+ * |cbs| is empty or the tag does not match, it returns zero. Note: if
+ * it returns one, CBS_get_asn1 may still fail if the rest of the
+ * element is malformed. */
+OPENSSL_EXPORT int CBS_peek_asn1_tag(const CBS *cbs, unsigned tag_value);
+
/* CBS_get_any_asn1_element sets |*out| to contain the next ASN.1 element from
* |*cbs| (including header bytes) and advances |*cbs|. It sets |*out_tag| to
* the tag number and |*out_header_len| to the length of the ASN.1 header. If
