LibCrypto: Add early exit to SECP.verify if signature is invalid

Author: https://github.com/stelar7
Commit: 54a3ffcd42
Pull-request: https://github.com/SerenityOS/serenity/pull/23737
Reviewed-by: https://github.com/ADKaster ✅
Reviewed-by: https://github.com/trflynn89

Userland/Libraries/LibCrypto/Curves/SECPxxxr1.h

@@ -186,6 +186,11 @@ public:
         auto r_bigint = TRY(asn1_decoder.read<Crypto::UnsignedBigInteger>(Crypto::ASN1::Class::Universal, Crypto::ASN1::Kind::Integer));
         auto s_bigint = TRY(asn1_decoder.read<Crypto::UnsignedBigInteger>(Crypto::ASN1::Class::Universal, Crypto::ASN1::Kind::Integer));
 
+        size_t expected_word_count = KEY_BIT_SIZE / 32;
+        if (r_bigint.length() < expected_word_count || s_bigint.length() < expected_word_count) {
+            return false;
+        }
+
         StorageType r = 0u;
         StorageType s = 0u;
         for (size_t i = 0; i < (KEY_BIT_SIZE / 32); i++) {