Question

I need to decrypt a message encrypted using RSA but I only have the Modulus and the Private Exponent as Base64 strings.

My approach was to create a RSAParameter object and import it using the RSACryptoProvider.ImportParameters method:

RSAParameters decryptParameters = new RSAParameters();

// D is the private exponent parameter
decryptParameters.D = Convert.FromBase64String(d);

// n is the Modulus
decryptParameters.Modulus = Convert.FromBase64String(n);

RSACryptoServiceProvider decrypter = new RSACryptoServiceProvider();
try
{
     decrypter.ImportParameters(decryptParameters);
}
catch (Exception ex)
{
    MessageBox.Show(ex.Message);
}

The problem is that I allways get a “Bad Data” CryptographycException.

What am I doing wrong?

Have I to initialize other values in the RSAParameters structure? Which ones? Theoretically, the Private Key used to decrypt messages should be (n, d) being n the Modulus parameter but the implementation seems to need some more info.

Answer 1

In theory you should only need the modulus and private exponent to decrypt. However API wise, because they use an optimization based on chinese remainder theorem, you need the original primes too, and with those primes also you need to precompute dP,dQ and Qinv.