Export CBS_get_any_asn1_element.

Useful for parsing things when you don't know what tag to expect (e.g. a
CHOICE). Also allow its operands to be NULL, so that it can be used to skip
arbitrary elements.

Delete CBS_ASN1_ANY which wasn't doing anything.

Change-Id: I56413e68b4f2e13860ea3e55373d5830713d7e5c
Reviewed-on: https://boringssl-review.googlesource.com/1661
Reviewed-by: Adam Langley <agl@google.com>

crypto/bytestring/cbs.c

@@ -162,6 +162,12 @@ int CBS_get_any_asn1_element(CBS *cbs, CBS *out, unsigned *out_tag,
                              size_t *out_header_len) {
   uint8_t tag, length_byte;
   CBS header = *cbs;
+  CBS throwaway;
+
+  if (out == NULL) {
+    out = &throwaway;
+  }
+
   if (!CBS_get_u8(&header, &tag) ||
       !CBS_get_u8(&header, &length_byte)) {
     return 0;
@@ -172,13 +178,17 @@ int CBS_get_any_asn1_element(CBS *cbs, CBS *out, unsigned *out_tag,
     return 0;
   }
 
-  *out_tag = tag;
+  if (out_tag != NULL) {
+    *out_tag = tag;
+  }
 
   size_t len;
   if ((length_byte & 0x80) == 0) {
     /* Short form length. */
     len = ((size_t) length_byte) + 2;
-    *out_header_len = 2;
+    if (out_header_len != NULL) {
+      *out_header_len = 2;
+    }
   } else {
     /* Long form length. */
     const size_t num_bytes = length_byte & 0x7f;
@@ -210,7 +220,9 @@ int CBS_get_any_asn1_element(CBS *cbs, CBS *out, unsigned *out_tag,
       return 0;
     }
     len += 2 + num_bytes;
-    *out_header_len = 2 + num_bytes;
+    if (out_header_len != NULL) {
+      *out_header_len = 2 + num_bytes;
+    }
   }
 
   return CBS_get_bytes(cbs, out, len);

crypto/bytestring/internal.h

@@ -22,15 +22,6 @@ extern "C" {
 #endif
 
 
-/* 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
- * the element has indefinite length then |*out| will only contain the header.
- *
- * Tag numbers greater than 31 are not supported. */
-int CBS_get_any_asn1_element(CBS *cbs, CBS *out, unsigned *out_tag,
-                             size_t *out_header_len);
-
 /* CBS_asn1_ber_to_der reads an ASN.1 structure from |in|. If it finds
  * indefinite-length elements then it attempts to convert the BER data to DER
  * and sets |*out| and |*out_length| to describe a malloced buffer containing

include/openssl/bytestring.h

@@ -128,8 +128,6 @@ OPENSSL_EXPORT int CBS_get_u24_length_prefixed(CBS *cbs, CBS *out);
 #define CBS_ASN1_CONSTRUCTED 0x20
 #define CBS_ASN1_CONTEXT_SPECIFIC 0x80
 
-#define CBS_ASN1_ANY 0x10000
-
 /* CBS_get_asn1 sets |*out| to the contents of DER-encoded, ASN.1 element (not
  * including tag and length bytes) and advances |cbs| over it. The ASN.1
  * element must match |tag_value|. It returns one on success and zero
@@ -142,6 +140,18 @@ OPENSSL_EXPORT int CBS_get_asn1(CBS *cbs, CBS *out, unsigned tag_value);
  * ASN.1 header bytes too. */
 OPENSSL_EXPORT int CBS_get_asn1_element(CBS *cbs, CBS *out, 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
+ * the element has indefinite length then |*out| will only contain the
+ * header. Each of |out|, |out_tag|, and |out_header_len| may be NULL to ignore
+ * the value.
+ *
+ * Tag numbers greater than 31 are not supported. */
+OPENSSL_EXPORT int CBS_get_any_asn1_element(CBS *cbs, CBS *out,
+                                            unsigned *out_tag,
+                                            size_t *out_header_len);
+
 /* CBS_get_asn1_uint64 gets an ASN.1 INTEGER from |cbs| using |CBS_get_asn1|
  * and sets |*out| to its value. It returns one on success and zero on error,
  * where error includes the integer being negative, or too large to represent