/// <summary>
/// Derives an RSA from a username and password
/// </summary>
/// <param name="username">Username</param>
/// <param name="password">Password</param>
/// <returns>The derived private and public key</returns>
internal static RsaKeyPair DeriveRsaKey(byte[] username, byte[] password)
{
const int primeSize = RsaPublicKeySize / 2;
Console.WriteLine("DeriveRsaKey: deriving key...");
var derived = PBKDF2(password, username, RsaPublicKeySize);
var one = new byte[primeSize];
var two = new byte[primeSize];
Array.Copy(derived, 0, one, 0, primeSize);
Array.Copy(derived, primeSize, two, 0, primeSize);
Console.WriteLine("DeriveRsaKey: finding prime 1...");
var p = new BigInt(one).NextPrimeGMP();
Console.WriteLine("DeriveRsaKey: finding prime 2...");
var q = new BigInt(two).NextPrimeGMP();
Console.WriteLine("DeriveRsaKey: creating key pair...");
var n = p * q;
var φn = (p - 1) * (q - 1);
var e = new BigInt(RsaExponent);
var d = e.InvertMod(φn);
Console.WriteLine("DeriveRsaKey: done");
return new RsaKeyPair(n.ToByteArray(), d.ToByteArray());
}
