An effective method for encrypting a license file?

Question

For a web application, I would like to create a simple but effective licensing system. In C#, this is a little difficult, since my decryption method could be viewed by anyone wi

Answer 1

Wolfwyrd's answer is excellent, but I just wanted to offer a muich simpler version of Wolfwyrd's GetPublicKeyFromAssembly method (which uses a lot of helper methods from the library Wolfwyrd graciously supplied).

In this version, this is all the code you need:

/// 

/// Extracts an RSA public key from a signed assembly
/// 
/// Signed assembly to extract the key from
/// RSA Crypto Service Provider initialised with the public key from the assembly
private RSACryptoServiceProvider GetPublicKeyFromAssembly(Assembly assembly)
{
    // Extract public key - note that public key stored in assembly has an extra 3 headers
    // (12 bytes) at the front that are not part of a CAPI public key blob, so they must be removed
    byte[] rawPublicKeyData = assembly.GetName().GetPublicKey();

    int extraHeadersLen = 12;
    int bytesToRead = rawPublicKeyData.Length - extraHeadersLen;
    byte[] publicKeyData = new byte[bytesToRead];
    Buffer.BlockCopy(rawPublicKeyData, extraHeadersLen, publicKeyData, 0, bytesToRead);

    RSACryptoServiceProvider publicKey = new RSACryptoServiceProvider();
    publicKey.ImportCspBlob(publicKeyData);

    return publicKey;
}