RedPike Cipher

Overview

RedPike is a classified UK government block cipher with a 64-bit block size and 64-bit key length, designed by GCHQ for confidential communications. It uses basic operations (add, XOR, left shift) with no lookup tables and minimal code. This C# implementation faithfully recreates the cipher with 16 rounds.

This code is derived from a supposed reference implementation originally posted in the now defunct cypherpunk mailing list available here.

Code

using System.Diagnostics.CodeAnalysis;

// ReSharper disable InconsistentNaming

namespace RedPike;

/// <summary>
/// RedPike cipher implementation.
/// </summary>
public static class Cipher
{
    private const uint CONST = 0x9E3779B9;
    private const int ROUNDS = 16;

    private static uint ROTL(uint X, uint R)
    {
        uint r = R & 31;
        return (X << (int)r) | (X >> (int)(32 - r));
    }

    private static uint ROTR(uint X, uint R)
    {
        uint r = R & 31;
        return (X >> (int)r) | (X << (int)(32 - r));
    }

    public static void Encrypt(uint[] x, uint[] k)
    {
        uint rk0 = k[0];
        uint rk1 = k[1];

        for (int i = 0; i < ROUNDS; i++)
        {
            rk0 += CONST;
            rk1 -= CONST;

            x[0] ^= rk0;
            x[0] += x[1];
            x[0] = ROTL(x[0], x[1]);

            x[1] = ROTR(x[1], x[0]);
            x[1] -= x[0];
            x[1] ^= rk1;
        }

        // swap via deconstruction
        (x[0], x[1]) = (x[1], x[0]);
    }

    [SuppressMessage("ReSharper", "RedundantCast")]
    public static void Decrypt(uint[] x, uint[] k)
    {
        uint[] dk = new uint[2];
        dk[0] = k[1] - unchecked(CONST * (uint)(ROUNDS + 1));
        dk[1] = k[0] + unchecked(CONST * (uint)(ROUNDS + 1));
        Encrypt(x, dk);
    }
}

Basic Usage

• Namespace: RedPike
• Class: Cipher (static)
• Methods:
  • Encrypt(uint[] x, uint[] k): Encrypts a 64-bit block x (two uints) with a 64-bit key k (two uints).
  • Decrypt(uint[] x, uint[] k): Decrypts the block using the same key.

Example

uint[] data = { 0x12345678, 0x9ABCDEF0 };
uint[] key = { 0xFEDCBA98, 0x76543210 };

Cipher.Encrypt(data, key); // Encrypts data in place
Cipher.Decrypt(data, key); // Decrypts back to original