PKCS11Interop Hash with SHA256 and Sign with RSA in two steps

Question

I have two applications, one that calculates the SHA-256 hash of a document and the other which makes the RSA signing. Trying different things i came to the conclusion that making CKM_SHA256 and then making CKM_RSA_PKCS give a different result than just making the CKM_SHA256_RSA_PKCS of the document itself.

So my question is, what is the difference between this two implementations? What information is added to the hash in CKM_SHA256_RSA_PKCS mechanism that turns in a complete different signature?

Answer 1

Mechanims CKM_SHA256_RSA_PKCS does following things:

1. Computes SHA256 hash of the data just like CKM_SHA256 does
2. Constructs DER encoded DigestInfo structure defined in RFC 8017
3. Signs DigestInfo structure with private key just like CKM_RSA_PKCS does

Several approaches are possible when it comes to construction of DER encoded DigestInfo structure:

1. In Pkcs11Admin application I did use BouncyCastle library:

public static byte[] CreateDigestInfo(byte[] hash, string hashOid)
{
    DerObjectIdentifier derObjectIdentifier = new DerObjectIdentifier(hashOid);
    AlgorithmIdentifier algorithmIdentifier = new AlgorithmIdentifier(derObjectIdentifier, null);
    DigestInfo digestInfo = new DigestInfo(algorithmIdentifier, hash);
    return digestInfo.GetDerEncoded();
}

2. In Pkcs11Interop.X509Store library I did use precomputed arrays:

/// <summary>
/// Creates DER encoded PKCS#1 DigestInfo structure defined in RFC 8017
/// </summary>
/// <param name="hash">Hash value</param>
/// <param name="hashAlgorithm">Hash algorithm</param>
/// <returns>DER encoded PKCS#1 DigestInfo structure or null</returns>
private static byte[] CreatePkcs1DigestInfo(byte[] hash, HashAlgorithmName hashAlgorithm)
{
    if (hash == null || hash.Length == 0)
        throw new ArgumentNullException(nameof(hash));

    byte[] pkcs1DigestInfo = null;

    if (hashAlgorithm == HashAlgorithmName.MD5)
    {
        if (hash.Length != 16)
            throw new ArgumentException("Invalid lenght of hash value");

        pkcs1DigestInfo = new byte[] { 0x30, 0x20, 0x30, 0x0C, 0x06, 0x08, 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05, 0x05, 0x00, 0x04, 0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 };
        Array.Copy(hash, 0, pkcs1DigestInfo, pkcs1DigestInfo.Length - hash.Length, hash.Length);
    }
    else if (hashAlgorithm == HashAlgorithmName.SHA1)
    {
        if (hash.Length != 20)
            throw new ArgumentException("Invalid lenght of hash value");

        pkcs1DigestInfo = new byte[] { 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2B, 0x0E, 0x03, 0x02, 0x1A, 0x05, 0x00, 0x04, 0x14, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 };
        Array.Copy(hash, 0, pkcs1DigestInfo, pkcs1DigestInfo.Length - hash.Length, hash.Length);
    }
    else if (hashAlgorithm == HashAlgorithmName.SHA256)
    {
        if (hash.Length != 32)
            throw new ArgumentException("Invalid lenght of hash value");

        pkcs1DigestInfo = new byte[] { 0x30, 0x31, 0x30, 0x0D, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05, 0x00, 0x04, 0x20, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 };
        Array.Copy(hash, 0, pkcs1DigestInfo, pkcs1DigestInfo.Length - hash.Length, hash.Length);
    }
    else if (hashAlgorithm == HashAlgorithmName.SHA384)
    {
        if (hash.Length != 48)
            throw new ArgumentException("Invalid lenght of hash value");

        pkcs1DigestInfo = new byte[] { 0x30, 0x41, 0x30, 0x0D, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02, 0x05, 0x00, 0x04, 0x30, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 };
        Array.Copy(hash, 0, pkcs1DigestInfo, pkcs1DigestInfo.Length - hash.Length, hash.Length);
    }
    else if (hashAlgorithm == HashAlgorithmName.SHA512)
    {
        if (hash.Length != 64)
            throw new ArgumentException("Invalid lenght of hash value");

        pkcs1DigestInfo = new byte[] { 0x30, 0x51, 0x30, 0x0D, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03, 0x05, 0x00, 0x04, 0x40, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 };
        Array.Copy(hash, 0, pkcs1DigestInfo, pkcs1DigestInfo.Length - hash.Length, hash.Length);
    }

    return pkcs1DigestInfo;
}