I'm trying to get up to speed on how to get some code that uses OpenSSL for cryptography, to play nice with another program that I'm writing in C#, using the Microsoft cryptography providers available in .NET.

More to the point, I'm trying to have the C# program verify an RSA message signature generated by the OpenSSL code. The code that generates the signature looks something like this:

// Code in C, using the OpenSSL RSA implementation

char msgToSign[] = "Hello World";     // the message to be signed
char signature[RSA_size(rsa)];        // buffer that will hold signature
int slen = 0;                         // will contain signature size

// rsa is an OpenSSL RSA context, that's loaded with the public/private key pair

memset(signature, 0, sizeof(signature));

RSA_sign(NID_sha1
      , (unsigned char*)msgToSign
      , strlen(msgToSign)
      , signature
      , &slen
      , rsa);

// now signature contains the message signature
//  and can be verified using the RSA_verify counterpart
// .. I would like to verify the signature in C#

In C#, I would do the following:

• import the other side's public key into an RSACryptoServiceProvider object
• receive the message and it's signature
• try to verify the signature

I've got the first two parts working (I've verified that the public key is loading properly because I managed to send an RSA encrypted text from the C# code to the OpenSSL code in C and successfully have it decrypted)

In order to verify the signature in C#, I've tried using the: VerifySignature method of the RSACryptoServiceProvider but that didn't work. And digging around the internet I was only able to find some vague information pointing out that .NET uses a different method for generating the signature than OpenSSL does. So, does anybody know how to accomplish this?