libcruft-crypto/block/speck.cpp

/*
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 *
 * Copyright 2018 Danny Robson <danny@nerdcruft.net>
 */

#include "speck.hpp"

#include <cruft/util/bitwise.hpp>

using cruft::crypto::block::speck;


///////////////////////////////////////////////////////////////////////////////
/// The round function for the encryption direction.
///
/// Mutates the x and y values.
///
/// \param x  first data word
/// \param y  second data word
/// \param k  key word
static void
round_enc (u64 &x, u64 &y, u64 const &k)
{
    x  = cruft::rotater (x, 8);
    x += y;

    x ^= k;

    y  = cruft::rotatel (y, 3);
    y ^= x;
}


///----------------------------------------------------------------------------
/// The round function for the decryption direction
///
/// Mutates the x and y values.
///
/// \param x
/// \param y
/// \param k
static void
round_dec (u64 &x, u64 &y, u64 const &k)
{
    y ^= x;
    y  = cruft::rotater (y, 3);

    x ^= k;

    x -= y;
    x  = cruft::rotatel (x, 8);
}


///////////////////////////////////////////////////////////////////////////////
/// Initialise the key schedule
speck::speck (std::array<u64,2> _key)
{
    auto &x = _key[0];
    auto &y = _key[1];

    // Allocate a space to store the schedule as it's computed
    std::array<u64,32> schedule;
    schedule[0] = y;

    // Initialise the key to be indices 0..31
    std::iota (std::begin (m_key), std::end (m_key), 0);

    // 'encrypt' the key using the index schedule
    for (int i = 0; i < 31; ++i) {
        round_enc (x, y, m_key[i]);
        schedule[i + 1] = y;
    }

    // Copy the computed schedule back out
    m_key = schedule;
}


///////////////////////////////////////////////////////////////////////////////
speck::block_t
speck::encrypt (block_t plaintext) const
{
    u64 &x = plaintext[0];
    u64 &y = plaintext[1];

    for (int i = 0; i < 32; i++)
        round_enc (x, y, m_key[i]);

    return { x, y };
}


//-----------------------------------------------------------------------------
std::array<u64,2>
speck::decrypt (block_t plaintext) const
{
    u64 &x = plaintext[0];
    u64 &y = plaintext[1];

    for (int i = 0; i < 32; i++)
        round_dec (x, y, m_key[31 - i]);

    return { x, y };
}