CODEWITHSUNDEEP
c#.netcryptography
Patrick M
Patrick M

Reputation: 93

RSACryptoServiceProvider.SignHash with SHA256 returns 128 bytes instead of 32?

I'm new to cryptography, so perhaps I'm confused on what should happen here. I start with a 32 byte message digest and attempt to sign it using my private key. The output of RSA.SignHash is a 128 bytes. I am expecting 32. 

    static private byte[] RSAHashAndSignData(byte[] data, RSAParameters privateKey)
    {
        byte[] signedHash;
        //Create a new instance of RSACryptoServiceProvider.
        using (RSACryptoServiceProvider RSA = new RSACryptoServiceProvider())
        {
            //Import the public key 
            RSA.ImportParameters(privateKey);

            //Encrypt the passed byte array
            var sha = SHA256Managed.Create();
            var digest = sha.ComputeHash(data);
            signedHash = RSA.SignHash(digest, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
        }

        return signedHash;
    }

Upvotes: 2

Views: 2517

Answers (1)

vcsjones
vcsjones

Reputation: 141638

I am expecting 32.

That is an incorrect assumption. Signing does not produce a size equal to the size of the input.

Using PKCS#1 padding, using a 1024-bit RSA key will produce as 128-byte signature. The size of a signature is related to the size of the modulus of the signing key.

Upvotes: 5

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