transmission/libtransmission/tr-arc4.h

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// This file Copyright © 2021-2022 Mike Gelfand
// It may be used under the 3-clause BSD (SPDX: BSD-3-Clause).
// License text can be found in the licenses/ folder.
#pragma once
#include <array>
#include <cstddef> // size_t
#include <cstdint> // uint8_t
/**
* This is a tiny and reusable implementation of alleged RC4 cipher.
* https://en.wikipedia.org/wiki/RC4
*
* The use of RC4 is declining due to security concerns.
* Popular cryptographic libraries have deprecated or removed it:
*
* - OpenSSL: disabled by default in 1.1, moved to "legacy" in 3.0
* - WolfSSL (CyaSSL): disabled by default in 3.4.6
* - MbedTLS (PolarSSL): removed in 3.0
*
* Nonetheless it's still used in BitTorrent Protocol Encryption
* https://en.wikipedia.org/wiki/BitTorrent_protocol_encryption,
* so this header file provides an implementation.
*/
class tr_arc4
{
public:
constexpr tr_arc4(void const* key, size_t key_length)
{
for (size_t i = 0; i < 256; ++i)
{
s_[i] = static_cast<uint8_t>(i);
}
for (size_t i = 0, j = 0; i < 256; ++i)
{
j = static_cast<uint8_t>(j + s_[i] + ((uint8_t const*)key)[i % key_length]);
arc4_swap(i, j);
}
}
constexpr void process(void const* src_data, void* dst_data, size_t data_length)
{
for (size_t i = 0; i < data_length; ++i)
{
((uint8_t*)dst_data)[i] = ((uint8_t const*)src_data)[i] ^ arc4_next();
}
}
constexpr void discard(size_t length)
{
while (length-- > 0)
{
arc4_next();
}
}
private:
constexpr void arc4_swap(size_t i, size_t j)
{
auto const tmp = s_[i];
s_[i] = s_[j];
s_[j] = tmp;
}
constexpr uint8_t arc4_next()
{
i_ += 1;
j_ += s_[i_];
arc4_swap(i_, j_);
return s_[static_cast<uint8_t>(s_[i_] + s_[j_])];
}
uint8_t i_ = 0;
uint8_t j_ = 0;
std::array<uint8_t, 256> s_ = {};
};