CODEWITHSUNDEEP
opensslm2cryptoverifycrypto++
Bastien Auneau
Bastien Auneau

Reputation: 41

verify data signature generated with openssl, using crypto++

I have a server, running under python, signing a message sha256 digest using m2crypto I use a public and private RSA key generated by openssl CLI. On the server side everythgin is OK
Python code :

privateKey = M2Crypto.RSA.load_key(sys.argv[2])
signedDigest = privateKey.sign(digest, 'sha256')

I double check that signature is good :

pubKey = M2Crypto.RSA.load_pub_key("key.pub.pem")
if pubKey.verify(digest, signedDigest, 'sha256') (etc....)

I store the signed sha256 digest in a file and send it with the original message to the client.
On the client side, running under c++ vc6, I load the signed sha256 digest (as binary), and the message that was signed. The aim is now to verify the message , together with the signed sha256. I have cryptopp as static link, and I know it works fine, because I can compute sha256, and compare with sha256 from python having same result. Here is the code :

RSA::PublicKey pubKey;
pubKey.Load( FileSource(LicenseControl::pubKeyPath, true));
RSASS< PKCS1v15, SHA >::Verifier verifier(pubKey);
//shaDigest is newly computed sha256, signatureByte is the signature of the message received from the server
result = verifier.VerifyMessage( shaDigest, CryptoPP::SHA256::DIGESTSIZE, signatureByte, 512);

This compiles and run, but always return false. To ensure that signature is valid, I have verified it using directly openssl CLI (not through m2crypto python wrapper) :

openssl dgst -sha256 -verify key.pub.pem -signature sign original_file
Verified OK

This confirms that signed sha256 digest is ok, and that it can be used to verify message successfully using the public key. I am aware of DER and PEM format (using PEM for openssl, DER for cryptopp). So I believe the public key is correct. Now my problem is How to use cryptopp library to verify the signature ??? I have been through the doc, but after days on it, it still looks like chinese to me. I hav tried thing like

RSASS< PSSR, SHA >::Verifier verifier(pubKey);

using PSSR to encrypt in python code, but no luck... I am now considering to only decrypt with public key the signed sha256 digest and compare it myself to the newly sha256 digest computed from the receive file. But even that simple, I hevn't found in the doc... Any idea how to use verifier properly ?
How to decrypt using public key ? in case previous question can not be solved

Upvotes: 4

Views: 3188

Answers (1)

Jack Lloyd
Jack Lloyd

Reputation: 8405

Two issues here I think:

First, SHA in RSASS< PKCS1v15, SHA > means SHA-1, not SHA_256. You'd want SHA256 here instead.

Also, VerifyMessage takes the entire message, not just a hash - the hash is computed internally for you. So right now when you're trying to verify the message, you're actually (as far as Crypto++ is concerned) trying to verify SHA-1(SHA-256(msg)), so naturally it fails. Pass the entire actual message instead, skipping your extra SHA-256 computation.

Upvotes: 1

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