pki_gcrypt: Add a little more ASN1 infrastructure

* src/pki_gcrypt.c (ASN1_OCTET_STRING): New macro.
(ASN1_OBJECT_IDENTIFIER): Likewise.
(asn1_check_tag): New function.

Signed-off-by: Justus Winter <justus@g10code.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/pki_gcrypt.c

@@ -50,6 +50,8 @@
 #define MAX_PASSPHRASE_SIZE 1024
 #define ASN1_INTEGER 2
 #define ASN1_BIT_STRING 3
+#define ASN1_OCTET_STRING 4
+#define ASN1_OBJECT_IDENTIFIER 6
 #define ASN1_SEQUENCE 48
 #define PKCS5_SALT_LEN 8
 
@@ -228,6 +230,19 @@ static int asn1_check_sequence(ssh_buffer buffer) {
   return 1;
 }
 
+static int asn1_check_tag(ssh_buffer buffer, unsigned char tag) {
+    unsigned char tmp;
+    uint32_t len;
+
+    len = ssh_buffer_get_data(buffer, &tmp, 1);
+    if (len == 0 || tmp != tag) {
+        return 0;
+    }
+
+    (void) asn1_get_len(buffer);
+    return 1;
+}
+
 static int passphrase_to_key(char *data, unsigned int datalen,
     unsigned char *salt, unsigned char *key, unsigned int keylen) {
   MD5CTX md;