SRP6Utilities.cs :  » PDF » iTextSharp » Org » BouncyCastle » Crypto » Agreement » Srp » C# / CSharp Open Source

using System;

using Org.BouncyCastle.Math;
using Org.BouncyCastle.Security;
using Org.BouncyCastle.Utilities;

namespace Org.BouncyCastle.Crypto.Agreement.Srp{
  public class Srp6Utilities
  {
    public static BigInteger CalculateK(IDigest digest, BigInteger N, BigInteger g)
    {
      return HashPaddedPair(digest, N, N, g);
    }

      public static BigInteger CalculateU(IDigest digest, BigInteger N, BigInteger A, BigInteger B)
      {
        return HashPaddedPair(digest, N, A, B);
      }

    public static BigInteger CalculateX(IDigest digest, BigInteger N, byte[] salt, byte[] identity, byte[] password)
      {
          byte[] output = new byte[digest.GetDigestSize()];

          digest.BlockUpdate(identity, 0, identity.Length);
          digest.Update((byte)':');
          digest.BlockUpdate(password, 0, password.Length);
          digest.DoFinal(output, 0);

          digest.BlockUpdate(salt, 0, salt.Length);
          digest.BlockUpdate(output, 0, output.Length);
          digest.DoFinal(output, 0);

          return new BigInteger(1, output).Mod(N);
      }

    public static BigInteger GeneratePrivateValue(IDigest digest, BigInteger N, BigInteger g, SecureRandom random)
      {
      int minBits = System.Math.Min(256, N.BitLength / 2);
          BigInteger min = BigInteger.One.ShiftLeft(minBits - 1);
          BigInteger max = N.Subtract(BigInteger.One);

          return BigIntegers.CreateRandomInRange(min, max, random);
      }

    public static BigInteger ValidatePublicValue(BigInteger N, BigInteger val)
    {
        val = val.Mod(N);

          // Check that val % N != 0
          if (val.Equals(BigInteger.Zero))
              throw new CryptoException("Invalid public value: 0");

        return val;
    }

    private static BigInteger HashPaddedPair(IDigest digest, BigInteger N, BigInteger n1, BigInteger n2)
    {
        int padLength = (N.BitLength + 7) / 8;

        byte[] n1_bytes = GetPadded(n1, padLength);
        byte[] n2_bytes = GetPadded(n2, padLength);

          digest.BlockUpdate(n1_bytes, 0, n1_bytes.Length);
          digest.BlockUpdate(n2_bytes, 0, n2_bytes.Length);

          byte[] output = new byte[digest.GetDigestSize()];
          digest.DoFinal(output, 0);

          return new BigInteger(1, output).Mod(N);
    }

    private static byte[] GetPadded(BigInteger n, int length)
    {
      byte[] bs = BigIntegers.AsUnsignedByteArray(n);
      if (bs.Length < length)
      {
        byte[] tmp = new byte[length];
        Array.Copy(bs, 0, tmp, length - bs.Length, bs.Length);
        bs = tmp;
      }
      return bs;
    }
  }
}