CryptoPP::ed25519::Verifier shows different result from libsignal

Question

I'm trying to implement curve25519 verification with CryptoPP. I tried the libsignal library first, witch shows correct result. Then I tried the same data with CryptoPP, but shows wrong result.

Here is the code using libsignal to verify a signature:

string pub = str2hex("0504f05568cc7a16fa9b4bc1c9d9294a80b7727e349365f855031a180bf0f80910");
ec_public_key* pub_key;
curve_decode_point(&pub_key, (uint8_t*)pub.data(), pub.size(), 0);

string message = str2hex("05f1fd491d63f1860bdaf3f9b0eb46c2494b7f184a32d9e6c859a421ad284f4307");
string signature = str2hex("5e525df3360ea62281efe8fb9e183521105bb3d9ba8ad43be9fac9d87dd216a6ea9e64099f6f05fbcd6e5a39ab239aad8c1e03d27a1378e4bcbf8937eac4300a");

int ret = curve_verify_signature(
    pub_key, 
    (uint8_t*)message.data(), message.size(), 
    (uint8_t*)signature.data(), signature.size()
    );

cout << "ret: " << ret << endl; // shows 1 (correct)

The result is 1 which means corrent. Please note libsignal requires the pub_key begin with a byte 0x05(key-type), not for CryptoPP here.

The code using CryptoPP :

string pub = str2hex("04f05568cc7a16fa9b4bc1c9d9294a80b7727e349365f855031a180bf0f80910");

string message = str2hex("05f1fd491d63f1860bdaf3f9b0eb46c2494b7f184a32d9e6c859a421ad284f4307");
string signature = str2hex("5e525df3360ea62281efe8fb9e183521105bb3d9ba8ad43be9fac9d87dd216a6ea9e64099f6f05fbcd6e5a39ab239aad8c1e03d27a1378e4bcbf8937eac4300a");

ed25519::Verifier verifier((uint8_t*)pub.data());
bool ret = verifier.VerifyMessage(
    (uint8_t*)message.data(), message.size(), 
    (uint8_t*)signature.data(), signature.size()
    );
cout << "ret: " << ret << endl; // shows 0 (wrong)

It shows 0, what's wrong with the code?

Answer 1

libsignal has a customized implementation on curve25519_verification:

• curve25519 public key is converted to ed25519 public key (see here )
• the verification is done using ed25519 signature verification algorithm (see here )

If you try to verify the signature using the converted ed25519 public key with CryptoPP, the verification would fail. That's because the sign and edd25519_verify operations are also slightly adjusted in libsignal.