diff --git a/src/lib/signature.c b/src/lib/signature.c
index 48ecba48d7..949d680b91 100644
--- a/src/lib/signature.c
+++ b/src/lib/signature.c
@@ -301,6 +301,10 @@ rsa_verify(pgp_hash_alg_t type,
 		return 0;
 	}
 
+	if (hashbuf_from_sig[0] != 0 || hashbuf_from_sig[1] != 1) {
+		return 0;
+	}
+
 	switch (type) {
 	case PGP_HASH_MD5:
 		prefix = prefix_md5;