Sign data with MD5WithRSA from .Pem/.Pkcs8 keyfile in C#

Question

Hi, I've got the following code sample in Java, and I need to re-enact it in C#:

PKCS8EncodedKeySpec privKeySpec = new PKCS8EncodedKeySpec(pkcs8PrivateKey);
KeyFactory keyFactory = KeyFactory.getInstance("RSA");
PrivateKey privKey = keyFactory.generatePrivate(privKeySpec);
Signature sign = Signature.getInstance("MD5withRSA");

sign.initSign(privKey);
sign.update(data);
byte[] signature = sign.sign();

Is it possible with the standard .Net Crypto API, or should I use BouncyCastle? Thanks,

b.

Answer 1

This SO question answers the PKCS#8 part of your code. The rest of the .NET RSA classes are a bizarre jumble of partially overlapping classes that are very difficult to fathom. It certainly appears that signature support is in either of the RSACryptoServiceProvider and/or RSAPKCS1SignatureFormatter classes.

Answer 2

Disclaimer: I know Java and cryptography, but my knowledge of C# and .NET is very limited. I am writing here only under the influence of my Google-fu skills.

Assuming that you could decode a PKCS#8-encoded RSA private key, then, from what I read on MSDN, the rest of the code should look like this:

byte[] hv = MD5.Create().ComputeHash(data);
RSACryptoServiceProvider rsp = new RSACryptoServiceProvider();
RSAParameters rsp = new RSAParameters();
// here fill rsp fields by decoding pkcs8PrivateKey
rsp.ImportParameters(key);
RSAPKCS1SignatureFormatter rf = new RSAPKCS1SignatureFormatter(rsp);
rf.SetHashAlgorithm("MD5");
byte[] signature = rf.CreateSignature(hv);

The relevant classes are in the System.Security.Cryptography namespace.

As for the PKCS#8 key blob decoding (i.e. filling in the rsp fields), I found this page which describes a command-line utility in C# which can perform that job. The source code is provided and is a single C# file. From what I read in it, that code decodes the PKCS#8 file "manually"; indirectly, this should mean that raw .NET (2.0) does not have facilities for PKCS#8 key file decoding (otherwise the author of that tool would not have went to the trouble of implementing such decoding). For your task at hand, you could scavenge from that source file the parts that you need, skipping anything about PEM and symmetric encryption; your entry point would be the DecodePrivateKeyInfo() function, which apparently expects a DER-encoded unencrypted PKCS#8 file, just like Java's PKCS8EncodedKeySpec.

Answer 3

I am running into a very similar problem trying to create a native C# tool for packing Chrome extensions (using SHA1, not MD5, but that's not a big difference). I believe I have tried literally every possible solution for .Net: System.Security.Cryptography, BouncyCastle, OpenSSL.Net and Chilkat RSA.

The best solution is probably Chilkat; their interface is the cleanest and most straightforward, it's well-supported and well-documented, and there are a million examples. For instance, here's some code using their library that does something very close to what you want: http://www.example-code.com/csharp/rsa_signPkcs8.asp. However, it's not free (though $150 is not unreasonable, seeing as I have burned 2 days trying to figure this out, and I make a bit more than $75 a day!).

As a free alternative, JavaScience offers up a number of crypto utilities in source form for multiple languages (including C#/.Net) at http://www.jensign.com/JavaScience/cryptoutils/index.html. The one that's most salient to what you are trying to do is opensslkey (http://www.jensign.com/opensslkey/index.html), which will let you generate a RSACryptoServiceProvider from a .pem file. You can then use that provider to sign your code:

        string pemContents = new StreamReader("pkcs8privatekey.pem").ReadToEnd();
        var der = opensslkey.DecodePkcs8PrivateKey(pemContents);
        RSACryptoServiceProvider rsa = opensslkey.DecodePrivateKeyInfo(der);

        signature = rsa.SignData(data, new MD5CryptoServiceProvider());

Answer 4

Another way is to use CNG (Cryptography Next Generation), along with the Security.Cryptography DLL from CodePlex

Then you can write:

byte[] dataToSign = Encoding.UTF8.GetBytes("Data to sign");
using (CngKey signingKey = CngKey.Import(pkcs8PrivateKey, CngKeyBlobFormat.Pkcs8PrivateBlob))
  using (RSACng rsa = new RSACng(signingKey))
  {
    rsa.SignatureHashAlgorithm = CngAlgorithm.MD5;
    return rsa.SignData(dataToSign);
  }