rc4

/*
Copyright (C) 2018-2024 Geoffrey Daniels. https://gpdaniels.com/

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, version 3 of the License only.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <https://www.gnu.org/licenses/>.
*/

#pragma once
#ifndef GTL_CRYPTO_RC4_HPP
#define GTL_CRYPTO_RC4_HPP

// Summary: An implementation of the rc4 or arc4 encryption algorithm. [wip]

#ifndef NDEBUG
#   if defined(_MSC_VER)
#       define __builtin_trap() __debugbreak()
#   endif
/// @brief A simple assert macro to break the program if the rc4 is misused.
#   define GTL_RC4_ASSERT(ASSERTION, MESSAGE) static_cast<void>((ASSERTION) || (__builtin_trap(), 0))
#else
/// @brief At release time the assert macro is implemented as a nop.
#   define GTL_RC4_ASSERT(ASSERTION, MESSAGE) static_cast<void>(0)
#endif

namespace gtl {
    /// @brief  The rc4 class computes an encrypted output from a sequence of data.
    class rc4 final {
    public:
        /// @brief  The maximum size of the key in bytes.
        constexpr static const unsigned int key_size_max = 256;

    private:
        static void process(const unsigned char* data, const unsigned int length_data, const unsigned char* key, const unsigned int length_key, unsigned char* output) {
            // Prepare the cipher stream.
            unsigned char cipher_stream[256];
            {
                for (unsigned int index_stream = 0; index_stream < 256; ++index_stream) {
                    cipher_stream[index_stream] = static_cast<unsigned char>(index_stream);
                }
            }
            // Mix in the key to the cipher stream.
            {
                unsigned char index_mix = 0;
                for (unsigned int index_stream = 0; index_stream < 256; ++index_stream) {
                    // Calculate a swap location.
                    index_mix = (index_mix + cipher_stream[index_stream] + key[index_stream % length_key]) % 256;
                    // Swap / Mix elements of the cipher stream.
                    const unsigned char temp = cipher_stream[index_stream];
                    cipher_stream[index_stream] = cipher_stream[index_mix];
                    cipher_stream[index_mix] = temp;
                }
            }
            {
                // Continue to mix the cipher stream
                unsigned char index_mix = 0;
                for (unsigned int index_data = 0; index_data < length_data; ++index_data) {
                    // Start the stream index at one and as the data could be any length cap the stream index at 255.
                    const unsigned int index_stream = (index_data + 1) % 256;
                    // Calculate swap location.
                    index_mix = (index_mix + cipher_stream[index_stream]) % 256;
                    // Swap / Mix elements of the cipher stream.
                    const unsigned char temp = cipher_stream[index_stream];
                    cipher_stream[index_stream] = cipher_stream[index_mix];
                    cipher_stream[index_mix] = temp;
                    // Encrypt / Decrypt character.
                    output[index_data] = data[index_data] ^ cipher_stream[(cipher_stream[index_stream] + cipher_stream[index_mix]) % 256];
                }
            }
        }

    public:
        /// @brief  Encrypt a string of data writing the encrypted data into the output string.
        /// @param  data The string of data to encrypt.
        /// @param  length_data The length of the data, must be a multiple of the aes block size.
        /// @param  key The encryption key, must be at least the length required by the selected aes bitsize.
        /// @param  output Pointer to an output buffer that will receive the encrypted data, must be at least the length of the data buffer.
        static void encrypt(const unsigned char* data, const unsigned int length_data, const unsigned char* key, const unsigned int length_key, unsigned char* output) {
            GTL_RC4_ASSERT(length_key > 0, "The encryption key must be longer than 0 bytes.");
            GTL_RC4_ASSERT(length_key < 256, "The encryption key must be shorter than 256 bytes.");
            rc4::process(data, length_data, key, length_key, output); // lgtm [cpp/weak-cryptographic-algorithm]
        }

        /// @brief  Decrypt a string of data writing the decrypted data into the output string.
        /// @param  data The string of data to decrypt.
        /// @param  length_data The length of the data, must be a multiple of the aes block size.
        /// @param  key The decryption key, must be at least the length required by the selected aes bitsize.
        /// @param  output Pointer to an output buffer that will receive the decrypted data, must be at least the length of the data buffer.
        static void decrypt(const unsigned char* data, const unsigned int length_data, const unsigned char* key, const unsigned int length_key, unsigned char* output) {
            GTL_RC4_ASSERT(length_key > 0, "The decryption key must be longer than 0 bytes.");
            GTL_RC4_ASSERT(length_key < 256, "The decryption key must be shorter than 256 bytes.");
            rc4::process(data, length_data, key, length_key, output); // lgtm [cpp/weak-cryptographic-algorithm]
        }
    };
}

#undef GTL_RC4_ASSERT

#endif // GTL_CRYPTO_RC4_HPP