FipsSHA256SignatureDescription.cs

/*
 * FipsSHA256SignatureDescription.cs
 * 
 * Copyright (c) 2013-2018, Sustainsys and contributors. All rights reserved.
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
 * MA 02110-1301  USA
 */

// This is a slightly modified variant of the following file:
// https://github.com/Sustainsys/Saml2/blob/master/Sustainsys.Saml2/ManagedSha256SignatureDescription.cs

namespace YOUR_PROJECT_NAMESPACE
{
    using System;
    using System.Security.Cryptography;

    /// <summary>
    /// Crypto description for a managed FIPS-compliant implementation of SHA256 signatures.
    /// </summary>
    public class FipsSHA256SignatureDescription : SignatureDescription
    {
        private readonly string sha256ImplementationName;

        public FipsSHA256SignatureDescription()
        {
            // Create FIPS-compliant SHA256 algorithm when FIPS is enforced:
            this.sha256ImplementationName = CryptoConfig.AllowOnlyFipsAlgorithms
                ? typeof(SHA256Cng).FullName
                : typeof(SHA256Managed).FullName;

            KeyAlgorithm = typeof(RSACryptoServiceProvider).FullName;
            DigestAlgorithm = sha256ImplementationName;
        }

        public override AsymmetricSignatureDeformatter CreateDeformatter(AsymmetricAlgorithm key)
        {
            if (key == null)
            {
                throw new ArgumentNullException(nameof(key));
            }

            var df = new RSAPKCS1SignatureDeformatter(key);
            df.SetHashAlgorithm(this.sha256ImplementationName);
            return df;
        }

        public override AsymmetricSignatureFormatter CreateFormatter(AsymmetricAlgorithm key)
        {
            if (key == null)
            {
                throw new ArgumentNullException(nameof(key));
            }

            var provider = GetSha256EnabledRSACryptoServiceProvider((RSACryptoServiceProvider)key);

            var formatter = new RSAPKCS1SignatureFormatter(provider);
            formatter.SetHashAlgorithm(this.sha256ImplementationName);
            return formatter;
        }

        //
        // Taken from here:
        // https://github.com/Sustainsys/Saml2/blob/master/Sustainsys.Saml2/Internal/CryptographyExtensions.cs#L63
        //
        private static RSACryptoServiceProvider GetSha256EnabledRSACryptoServiceProvider(
            RSACryptoServiceProvider original)
        {
            // The provider is probably using the default ProviderType. That's
            // a problem, because it doesn't support SHA256. Let's do some
            // black magic and create a new provider of a type that supports
            // SHA256 without the user ever knowing we fix this. This is what
            // is done in X509AsymmetricKey.GetSignatureFormatter if
            // http://www.w3.org/2001/04/xmldsig-more#rsa-sha256 isn't
            // a known algorithm, so users kind of expect this to be handled
            // for them magically.

            var cspParams = new CspParameters();
            cspParams.ProviderType = 24; //PROV_RSA_AES
            cspParams.KeyContainerName = original.CspKeyContainerInfo.KeyContainerName;
            cspParams.KeyNumber = (int)original.CspKeyContainerInfo.KeyNumber;
            SetMachineKeyFlag(original, cspParams);

            cspParams.Flags |= CspProviderFlags.UseExistingKey;

            return new RSACryptoServiceProvider(cspParams);
        }

        private static void SetMachineKeyFlag(RSACryptoServiceProvider provider, CspParameters cspParams)
        {
            if (provider.CspKeyContainerInfo.MachineKeyStore)
            {
                cspParams.Flags = CspProviderFlags.UseMachineKeyStore;
            }
        }
    }
}