libcruft-crypto/stream/rc4.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 2015 Danny Robson <danny@nerdcruft.net>
 */

#include "rc4.hpp"

#include <algorithm>
#include <numeric>

using cruft::crypto::stream::RC4;


///////////////////////////////////////////////////////////////////////////////
RC4::RC4 (const uint8_t *restrict key, size_t len):
    x (0),
    y (0)
{
    std::iota (S.begin (), S.end (), 0);

    for (size_t i = 0, j = 0; i < 256; ++i) {
        j = (j + S[i] + key[i % len]) % 256;
        std::swap (S[i], S[j]);
    }
}


//-----------------------------------------------------------------------------
void
RC4::discard (size_t len)
{
    while (len--)
        get ();
}


//-----------------------------------------------------------------------------
void
RC4::generate (uint8_t *restrict dst, size_t len)
{
    std::generate_n (dst, len, [this] (void) { return get (); });
}


//-----------------------------------------------------------------------------
uint8_t
RC4::get (void)
{
    x = (x +    1) % 256;
    y = (y + S[x]) % 256;

    std::swap (S[x], S[y]);

    return S[(S[x] + S[y]) % 256];
}