RSASSA-PSS without parameters using SHA-256 .Net 4.5 support

Question

I'm trying to use System.Security.Cryptography (targeted framework .NET 4.5) to create xml digital signatures, so far I managed to create and verify signatures using the following scheme : RSA PKCS#1 v1.5 and SHA-256: http://www.w3.org/2001/04/xmldsig-more#rsa-sha256

However, I'm not able to use the following scheme: ‘RSASSA-PSS without parameters using SHA-256’ [RFC6931]: http://www.w3.org/2007/05/xmldsig-more#sha256-rsa-MGF1

The error being displayed is clear "SignatureDescription could not be created for the signature algorithm supplied."

For 'RSA PKCS#1 v1.5 and SHA-256' I added the following public class as its signature:

   public class RSAPKCS1SHA256SignatureDescription : SignatureDescription
   {
        public RSAPKCS1SHA256SignatureDescription()
        {
            base.KeyAlgorithm = "System.Security.Cryptography.RSACryptoServiceProvider";
            base.DigestAlgorithm = "System.Security.Cryptography.SHA256Managed";
            base.FormatterAlgorithm = "System.Security.Cryptography.RSAPKCS1SignatureFormatter";
            base.DeformatterAlgorithm = "System.Security.Cryptography.RSAPKCS1SignatureDeformatter";
        }

        public override AsymmetricSignatureDeformatter CreateDeformatter(AsymmetricAlgorithm key)
        {
            AsymmetricSignatureDeformatter asymmetricSignatureDeformatter = (AsymmetricSignatureDeformatter)
            CryptoConfig.CreateFromName(base.DeformatterAlgorithm);
            asymmetricSignatureDeformatter.SetKey(key);
            asymmetricSignatureDeformatter.SetHashAlgorithm("SHA256");
            return asymmetricSignatureDeformatter;
        }
    }

However, I have no clue whether ‘RSASSA-PSS without parameters using SHA-256’ is supported by .Net 4.5 and if so how to set its signature definition.

I would be really thankful if anyone had similar experience and can provide some help.

Answer 1

I have finally figured it out. The trick is to register the algorithm that the XML signature specifies, in my case "http://www.w3.org/2007/05/xmldsig-more#sha256-rsa-MGF1". Register it to a custom class, that uses a custom signature formatter and deformatter, which apply RSASignaturePadding.Pss.

Here is an implementation where all you have to do is call RsaSsaPss.RegisterSha256RsaMgf1() once, such as from a static constructor in your client. Then, SignedXml.CheckSignature() and SignedXml.ComputeSignature() automatically work for any XML signature that specifies this algorithm.

Tested on Core 2.1 and Framework 4.7.1:

/// <summary>
/// Contains classes for using RSA-SSA-PSS, as well as methods for registering them.
/// Registering such types adds support for them to SignedXml.
/// </summary>
public class RsaSsaPss
{
    /// <summary>
    /// Registers an implementation for "http://www.w3.org/2007/05/xmldsig-more#sha256-rsa-MGF1".
    /// </summary>
    public static void RegisterSha256RsaMgf1()
    {
        CryptoConfig.AddAlgorithm(typeof(RsaPssSha256SignatureDescription), "http://www.w3.org/2007/05/xmldsig-more#sha256-rsa-MGF1");
    }

    // Can add further registrations here...

    public class RsaPssSha256SignatureDescription : SignatureDescription
    {
        public RsaPssSha256SignatureDescription()
        {
            using (var rsa = RSA.Create())
            {
                this.KeyAlgorithm = rsa.GetType().AssemblyQualifiedName; // Does not like a simple algorithm name, but wants a type name (AssembyQualifiedName in Core)
            }
            this.DigestAlgorithm = "SHA256"; // Somehow wants a simple algorithm name
            this.FormatterAlgorithm = typeof(RsaPssSignatureFormatter).FullName;
            this.DeformatterAlgorithm = typeof(RsaPssSignatureDeformatter).FullName;
        }

        public override AsymmetricSignatureFormatter CreateFormatter(AsymmetricAlgorithm key)
        {
            var signatureFormatter = new RsaPssSignatureFormatter();
            signatureFormatter.SetKey(key);
            signatureFormatter.SetHashAlgorithm(this.DigestAlgorithm);
            return signatureFormatter;
        }

        public override AsymmetricSignatureDeformatter CreateDeformatter(AsymmetricAlgorithm key)
        {
            var signatureDeformatter = new RsaPssSignatureDeformatter();
            signatureDeformatter.SetKey(key);
            signatureDeformatter.SetHashAlgorithm(this.DigestAlgorithm);
            return signatureDeformatter;
        }

        public class RsaPssSignatureFormatter : AsymmetricSignatureFormatter
        {
            private RSA Key { get; set; }
            private string HashAlgorithmName { get; set; }

            public override void SetKey(AsymmetricAlgorithm key)
            {
                this.Key = (RSA)key;
            }

            public override void SetHashAlgorithm(string strName)
            {
                // Verify the name
                Oid.FromFriendlyName(strName, OidGroup.HashAlgorithm);

                this.HashAlgorithmName = strName;
            }

            public override byte[] CreateSignature(byte[] rgbHash)
            {
                return this.Key.SignHash(rgbHash, new HashAlgorithmName(this.HashAlgorithmName), RSASignaturePadding.Pss);
            }
        }

        public class RsaPssSignatureDeformatter : AsymmetricSignatureDeformatter
        {
            private RSA Key { get; set; }
            private string HashAlgorithmName { get; set; }

            public override void SetKey(AsymmetricAlgorithm key)
            {
                this.Key = (RSA)key;
            }

            public override void SetHashAlgorithm(string strName)
            {
                // Verify the name
                Oid.FromFriendlyName(strName, OidGroup.HashAlgorithm);

                this.HashAlgorithmName = strName;
            }

            public override bool VerifySignature(byte[] rgbHash, byte[] rgbSignature)
            {
                return this.Key.VerifyHash(rgbHash, rgbSignature, new HashAlgorithmName(this.HashAlgorithmName), RSASignaturePadding.Pss);
            }
        }
    }
}