[crypto] Add concept of authentication tag to cipher algorithms

Some ciphers (such as GCM) support the concept of a tag that can be
used to authenticate the encrypted data.  Add a cipher method for
generating an authentication tag.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/crypto/arc4.c

@@ -119,8 +119,10 @@ struct cipher_algorithm arc4_algorithm = {
 	.name = "ARC4",
 	.ctxsize = ARC4_CTX_SIZE,
 	.blocksize = 1,
+	.authsize = 0,
 	.setkey = arc4_setkey,
 	.setiv = cipher_null_setiv,
 	.encrypt = arc4_xor,
 	.decrypt = arc4_xor,
+	.auth = cipher_null_auth,
 };