How to sign a JWT using RS256 with RSA private key

Question

I am using the jose-jwt library and want to create a signed JWT in C# using the RS256 algorithm for encryption. I have no experience with cryptography, so please excuse my ignorance. I see the following example in the docs:

var payload = new Dictionary<string, object>()
{
    { "sub", "[email protected]" },
    { "exp", 1300819380 }
};

var privateKey=new X509Certificate2("my-key.p12", "password", X509KeyStorageFlags.Exportable | X509KeyStorageFlags.MachineKeySet).PrivateKey as RSACryptoServiceProvider;

string token=Jose.JWT.Encode(payload, privateKey, JwsAlgorithm.RS256);

which shows the use of a p12 file, but how do I use an RSA key file of the form below? I am looking at the docs for X509Certificate2, but I see no option for RSA private keys. It appears to only accept PKCS7, which I understand to be public keys.

-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----

Finally, what is the difference between the two options listed in the docs, and how do I choose between the two?

-------------------------- OPTION 1 --------------------------

RS-* and PS-* family

CLR:

RS256, RS384, RS512 and PS256, PS384, PS512 signatures require RSACryptoServiceProvider (usually private) key of corresponding length. CSP need to be forced to use Microsoft Enhanced RSA and AES Cryptographic Provider. Which usually can be done be re-importing RSAParameters. See http://clrsecurity.codeplex.com/discussions/243156 for details.

-------------------------- OPTION 2 --------------------------

CORECLR: RS256, RS384, RS512 signatures require RSA (usually private) key of corresponding length.

Answer 1

I know this post is old, but it took me forever to figure this out, so I thought I would share.

To test I created RSA keys using OpenSSL:

openssl genrsa -out privateKey.pem 512
openssl rsa -in privateKey.pem -pubout -out publicKey.pem

You will need the following 2 nuget packages:

1. https://github.com/dvsekhvalnov/jose-jwt
2. http://www.bouncycastle.org/csharp/

Test Code

public static void Test()
{
        string publicKey = File.ReadAllText(@"W:\Dev\Temp\rsa_keys\publicKey.pem");
        string privateKey = File.ReadAllText(@"W:\Dev\Temp\rsa_keys\privateKey.pem");
        
        var claims = new List<Claim>();
        claims.Add(new Claim("claim1", "value1"));
        claims.Add(new Claim("claim2", "value2"));
        claims.Add(new Claim("claim3", "value3"));

        var token = CreateToken(claims, privateKey);
        var payload = DecodeToken(token, publicKey);
}

Create Token

public static string CreateToken(List<Claim> claims, string privateRsaKey)
{
     RSAParameters rsaParams;
     using (var tr = new StringReader(privateRsaKey))
     {
          var pemReader = new PemReader(tr);
          var keyPair = pemReader.ReadObject() as AsymmetricCipherKeyPair;
          if (keyPair == null)
          {
               throw new Exception("Could not read RSA private key");
          } 
          var privateRsaParams = keyPair.Private as RsaPrivateCrtKeyParameters;
          rsaParams = DotNetUtilities.ToRSAParameters(privateRsaParams);
     }
     using (RSACryptoServiceProvider rsa = new RSACryptoServiceProvider())
     {
          rsa.ImportParameters(rsaParams);
          Dictionary<string, object> payload = claims.ToDictionary(k => k.Type, v => (object)v.Value);
          return Jose.JWT.Encode(payload, rsa, Jose.JwsAlgorithm.RS256);
     }
}

Decode Token

public static string DecodeToken(string token, string publicRsaKey)
{
     RSAParameters rsaParams;

     using (var tr = new StringReader(publicRsaKey))
     {
          var pemReader = new PemReader(tr);
          var publicKeyParams = pemReader.ReadObject() as RsaKeyParameters;
          if (publicKeyParams == null)
          {
               throw new Exception("Could not read RSA public key");
          }
          rsaParams = DotNetUtilities.ToRSAParameters(publicKeyParams);
     }
     using (RSACryptoServiceProvider rsa = new RSACryptoServiceProvider())
     {
          rsa.ImportParameters(rsaParams);
          // This will throw if the signature is invalid
          return Jose.JWT.Decode(token, rsa, Jose.JwsAlgorithm.RS256);  
     }
}

I found https://jwt.io/ a great resource to test your tokens