はじめに

コンパイル時に, ハッシュ値計算や暗号化したいことがあると思います. 検索すればいくらでもコードが出てきますが, ライセンス問題悩みたくないため、書き起こしました.

クライアント・サーバ間の認証・暗号化は設定を間違えなければ, 安全が確立された技術ですが, 配布するアプリケーションのコードやデータを保護することは難しいです. このコンパイル時のハッシュ値計算や暗号化は延命策の一つです.

実装

SHA-256とChaCha20ですが, やり方を覚えてしまえばいくらでも別のアルゴリズムを実装できるでしょう.

SHA-256

SHA-256

#ifndef INC_CRYPT_H_
#define INC_CRYPT_H_
/*
# License
This software is distributed under two licenses, choose whichever you like.

## MIT License
Copyright (c) 2025 Takuro Sakai

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

## Public Domain
This is free and unencumbered software released into the public domain.

Anyone is free to copy, modify, publish, use, compile, sell, or
distribute this software, either in source code form or as a compiled
binary, for any purpose, commercial or non-commercial, and by any
means.

In jurisdictions that recognize copyright laws, the author or authors
of this software dedicate any and all copyright interest in the
software to the public domain. We make this dedication for the benefit
of the public at large and to the detriment of our heirs and
successors. We intend this dedication to be an overt act of
relinquishment in perpetuity of all present and future rights to this
software under copyright law.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
OTHER DEALINGS IN THE SOFTWARE.

For more information, please refer to <http://unlicense.org>
*/
namespace cthash
{
    struct sha256_state
    {
        //Round constants
        static constexpr uint32_t K[64] = {
            0x428a2f98UL, 0x71374491UL, 0xb5c0fbcfUL, 0xe9b5dba5UL, 0x3956c25bUL,
            0x59f111f1UL, 0x923f82a4UL, 0xab1c5ed5UL, 0xd807aa98UL, 0x12835b01UL,
            0x243185beUL, 0x550c7dc3UL, 0x72be5d74UL, 0x80deb1feUL, 0x9bdc06a7UL,
            0xc19bf174UL, 0xe49b69c1UL, 0xefbe4786UL, 0x0fc19dc6UL, 0x240ca1ccUL,
            0x2de92c6fUL, 0x4a7484aaUL, 0x5cb0a9dcUL, 0x76f988daUL, 0x983e5152UL,
            0xa831c66dUL, 0xb00327c8UL, 0xbf597fc7UL, 0xc6e00bf3UL, 0xd5a79147UL,
            0x06ca6351UL, 0x14292967UL, 0x27b70a85UL, 0x2e1b2138UL, 0x4d2c6dfcUL,
            0x53380d13UL, 0x650a7354UL, 0x766a0abbUL, 0x81c2c92eUL, 0x92722c85UL,
            0xa2bfe8a1UL, 0xa81a664bUL, 0xc24b8b70UL, 0xc76c51a3UL, 0xd192e819UL,
            0xd6990624UL, 0xf40e3585UL, 0x106aa070UL, 0x19a4c116UL, 0x1e376c08UL,
            0x2748774cUL, 0x34b0bcb5UL, 0x391c0cb3UL, 0x4ed8aa4aUL, 0x5b9cca4fUL,
            0x682e6ff3UL, 0x748f82eeUL, 0x78a5636fUL, 0x84c87814UL, 0x8cc70208UL,
            0x90befffaUL, 0xa4506cebUL, 0xbef9a3f7UL, 0xc67178f2UL
        };
        uint64_t length_;
        uint64_t bitlen_;
        uint32_t state_[8];
        uint8_t data_[64];

        static constexpr uint32_t choose(uint32_t e, uint32_t f, uint32_t g)
        {
            return (e & f) ^ (~e & g);
        }

        static constexpr uint32_t majority(uint32_t a, uint32_t b, uint32_t c)
        {
            return (a & (b | c)) | (b & c);
        }

        static constexpr uint32_t rotr(uint32_t x, uint32_t n)
        {
            return (x >> n) | (x << (32 - n));
        }

        static constexpr uint32_t sigma0(uint32_t x)
        {
            return rotr(x, 7) ^ rotr(x, 18) ^ (x >> 3);
        }

        static constexpr uint32_t sigma1(uint32_t x)
        {
            return rotr(x, 17) ^ rotr(x, 19) ^ (x >> 10);
        }

        constexpr void compress()
        {
            uint32_t m[64];
            // Copy block into first 16 words m[0..15] of the message schedule array
            for (int32_t i = 0, j = 0; i < 16; ++i, j += 4) {
                m[i] = (data_[j] << 24) | (data_[j + 1] << 16) | (data_[j + 2] << 8) | (data_[j + 3]);
            }

            // Extend the first 16 words into the remaining 48 words m[16..63] of the message schedule array
            for (int32_t i = 16; i < 64; ++i) {
                m[i] = sigma1(m[i - 2]) + m[i - 7] + sigma0(m[i - 15]) + m[i - 16];
            }

            uint32_t state[8];
            for (int32_t i = 0; i < 8; ++i) {
                state[i] = state_[i];
            }

            for (int32_t i = 0; i < 64; ++i) {
                uint32_t s1 = rotr(state[4], 6) ^ rotr(state[4], 11) ^ rotr(state[4], 25);
                uint32_t ch = choose(state[4], state[5], state[6]);
                uint32_t temp1 = m[i] + K[i] + state[7] + ch + s1;
                uint32_t s0 = rotr(state[0], 2) ^ rotr(state[0], 13) ^ rotr(state[0], 22);
                uint32_t maj = majority(state[0], state[1], state[2]);
                uint32_t temp2 = s0 + maj;

                state[7] = state[6];
                state[6] = state[5];
                state[5] = state[4];
                state[4] = state[3] + temp1;
                state[3] = state[2];
                state[2] = state[1];
                state[1] = state[0];
                state[0] = temp1 + temp2;
            }

            // Add the compressed chunk to the current hash value
            for (int32_t i = 0; i < 8; ++i) {
                state_[i] += state[i];
            }
        }

        constexpr void init()
        {
            length_ = 0;
            bitlen_ = 0;
            state_[0] = 0x6A09E667UL;
            state_[1] = 0xBB67AE85UL;
            state_[2] = 0x3C6EF372UL;
            state_[3] = 0xA54FF53AUL;
            state_[4] = 0x510E527FUL;
            state_[5] = 0x9B05688CUL;
            state_[6] = 0x1F83D9ABUL;
            state_[7] = 0x5BE0CD19UL;
        }

        constexpr void update(size_t size, const uint8_t* src)
        {
            const size_t block_size = 64;
            const uint8_t* data = src;

            for (size_t i = 0; i < size; ++i) {
                data_[length_++] = data[i];
                if (block_size == length_) {
                    compress();
                    // End of the block
                    bitlen_ += 8 * block_size;
                    length_ = 0;
                }
            }
        }

        constexpr void memset(uint8_t* mem, uint8_t x, size_t size)
        {
            for (size_t i = 0; i < size; ++i) {
                mem[i] = x;
            }
        }

        constexpr std::array<uint8_t, 32> finalize()
        {
            uint64_t i = length_;
            uint8_t end = length_ < 56 ? 56 : 64;

            data_[i++] = 0x80; // Append a bit 1
            while (i < end) {
                data_[i++] = 0x00; // Pad with zeros
            }

            if (56 <= length_) {
                compress();
                sha256_state::memset(data_, 0, 56);
            }

            // Append to the padding the total message's length in bits and compress.
            bitlen_ += length_ * 8;
            data_[63] = static_cast<uint8_t>(bitlen_);
            data_[62] = static_cast<uint8_t>(bitlen_ >> 8);
            data_[61] = static_cast<uint8_t>(bitlen_ >> 16);
            data_[60] = static_cast<uint8_t>(bitlen_ >> 24);
            data_[59] = static_cast<uint8_t>(bitlen_ >> 32);
            data_[58] = static_cast<uint8_t>(bitlen_ >> 40);
            data_[57] = static_cast<uint8_t>(bitlen_ >> 48);
            data_[56] = static_cast<uint8_t>(bitlen_ >> 56);
            compress();

            // Produces hash in the big-endian order
            std::array<uint8_t, 32> hash;
            for (uint8_t i = 0; i < 4; ++i) {
                for (uint8_t j = 0; j < 8; ++j) {
                    hash[i + (j * 4)] = (state_[j] >> (24 - i * 8)) & 0x000000ffU;
                }
            }
            return hash;
        }
    };

    template<size_t N>
    constexpr std::array<uint8_t, 32> sha256_string(const uint8_t(&src)[N])
    {
        static_assert(0 < N);
        sha256_state state;
        state.init();
        // Do not include the null terminator in the hash
        state.update(N - 1, src);
        return state.finalize();
    }

    template<size_t N>
    constexpr std::array<uint8_t, 32> sha256_data(const uint8_t(&src)[N])
    {
        static_assert(0 < N);
        sha256_state state;
        state.init();
        state.update(N, src);
        return state.finalize();
    }
}
#endif //INC_CRYPT_H_

ChaCha20

ChaCha20

#ifndef INC_CRYPT_H_
#define INC_CRYPT_H_
/*
# License
This software is distributed under two licenses, choose whichever you like.

## MIT License
Copyright (c) 2025 Takuro Sakai

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

## Public Domain
This is free and unencumbered software released into the public domain.

Anyone is free to copy, modify, publish, use, compile, sell, or
distribute this software, either in source code form or as a compiled
binary, for any purpose, commercial or non-commercial, and by any
means.

In jurisdictions that recognize copyright laws, the author or authors
of this software dedicate any and all copyright interest in the
software to the public domain. We make this dedication for the benefit
of the public at large and to the detriment of our heirs and
successors. We intend this dedication to be an overt act of
relinquishment in perpetuity of all present and future rights to this
software under copyright law.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
OTHER DEALINGS IN THE SOFTWARE.

For more information, please refer to <http://unlicense.org>
*/
namespace cthash
{
#define CHACHA_ROTL(a, b) (((a) << (b)) | ((a) >> (32 - (b))))
#define CHACHA_QR(a, b, c, d) (             \
    a += b, d ^= a, d = CHACHA_ROTL(d, 16), \
    c += d, b ^= c, b = CHACHA_ROTL(b, 12), \
    a += b, d ^= a, d = CHACHA_ROTL(d, 8),  \
    c += d, b ^= c, b = CHACHA_ROTL(b, 7))

    constexpr void chacha20_block(uint32_t out[16], const uint32_t in[16])
    {
        uint32_t x[16];
        for (int32_t i = 0; i < 16; ++i)
        {
            x[i] = in[i];
        }
        for (int32_t i = 0; i < 10; ++i)
        {
            // Odd round
            CHACHA_QR(x[0], x[4], x[8], x[12]);  // Column 0
            CHACHA_QR(x[1], x[5], x[9], x[13]);  // Column 1
            CHACHA_QR(x[2], x[6], x[10], x[14]); // Column 2
            CHACHA_QR(x[3], x[7], x[11], x[15]); // Column 3
            // Even round
            CHACHA_QR(x[0], x[5], x[10], x[15]); // Diagonal 0
            CHACHA_QR(x[1], x[6], x[11], x[12]); // Diagonal 1
            CHACHA_QR(x[2], x[7], x[8], x[13]);  // Diagonal 2
            CHACHA_QR(x[3], x[4], x[9], x[14]);  // Diagonal 3
        }
        for (int32_t i = 0; i < 16; ++i)
        {
            out[i] = x[i] + in[i];
        }
    }

    constexpr uint64_t to_uint64(uint32_t low, uint32_t high)
    {
        return (static_cast<uint64_t>(high) << 32) | static_cast<uint64_t>(low);
    }

    constexpr void to_uint32(uint32_t& low, uint32_t& high, uint64_t x)
    {
        low = static_cast<uint32_t>(x & 0xFFFFFFFFUL);
        high = static_cast<uint32_t>((x >> 32) & 0xFFFFFFFFUL);
    }

    constexpr void chacha20_encrypt(uint32_t input[16], uint32_t size, uint8_t* message)
    {
        uint32_t output[16] = {};
        uint8_t* p = message;
        uint64_t counter = to_uint64(input[12], input[13]);
        for (;;)
        {
            chacha20_block(output, input);
            counter += 1;
            if (size <= 64)
            {
                uint8_t* pb = p;
                uint32_t* po = output;
                uint32_t blocks = size>>2;
                uint32_t count = blocks<<2;
                uint32_t remain = size - count;

                for (uint32_t i=0,j=0; j < count; ++i,j+=4)
                {
                    pb[j+0] = pb[j+0] ^ ((po[i]>>0)&0xFFU);
                    pb[j+1] = pb[j+1] ^ ((po[i]>>8)&0xFFU);
                    pb[j+2] = pb[j+2] ^ ((po[i]>>16)&0xFFU);
                    pb[j+3] = pb[j+3] ^ ((po[i]>>24)&0xFFU);
                }

                switch(remain){
                case 1:{
                    pb[count + 0] = pb[count + 0] ^ ((po[blocks] >> 0) & 0xFFU);
                }
                    break;
                case 2:{
                    pb[count + 0] = pb[count + 0] ^ ((po[blocks] >> 0) & 0xFFU);
                    pb[count + 1] = pb[count + 1] ^ ((po[blocks] >> 8) & 0xFFU);
                }
                    break;
                case 3:{
                    pb[count + 0] = pb[count + 0] ^ ((po[blocks] >> 0) & 0xFFU);
                    pb[count + 1] = pb[count + 1] ^ ((po[blocks] >> 8) & 0xFFU);
                    pb[count + 2] = pb[count + 2] ^ ((po[blocks] >> 16) & 0xFFU);
                }
                    break;
                }
                break;
            }
            else
            {
                for (uint32_t i=0,j=0; i < 16; ++i,j+=4)
                {
                    p[j+0] ^= ((output[i]>>0)&0xFFU);
                    p[j+1] ^= ((output[i]>>8)&0xFFU);
                    p[j+2] ^= ((output[i]>>16)&0xFFU);
                    p[j+3] ^= ((output[i]>>24)&0xFFU);
                }
                size -= 64;
                p += (16*4);
            }
        }
        to_uint32(input[12], input[13], counter);
    }

    constexpr void chacha20_encrypt(const std::array<uint8_t, 32>& key, uint32_t size, uint8_t* message)
    {
        static constexpr uint32_t iv_32[4] = {0x61707865U, 0x3320646eU, 0x79622d32U, 0x6b206574U};
        static constexpr uint32_t nonce[2] = { 0x55783743U, 0x421d5279U };

        uint32_t input[16];

        for (int32_t i = 0; i < 4; ++i)
        {
            input[i] = iv_32[i];
        }

        for (int32_t i = 0, j = 0; i < 8; ++i,j+=4)
        {
            input[4 + i] = (key[j+0]<<24) | (key[j+1]<<16) | (key[j+2]<<8) | (key[j+3]);
        }

        input[12] = 0;
        input[13] = 0;
        input[14] = nonce[0];
        input[15] = nonce[1];
        chacha20_encrypt(input, size, message);
    }

    template<size_t N>
    constexpr std::array<uint8_t, N> chacha20_encrypt(const std::array<uint8_t, 32>& key, const std::array<uint8_t, N>& src)
    {
        std::array<uint8_t, N> message = src;
        chacha20_encrypt(key, N, &message[0]);
        return message;
    }

    template<size_t N>
    constexpr std::array<uint8_t, N> chacha20_encrypt(const std::array<uint8_t, 32>& key, const uint8_t(&src)[N])
    {
        static constexpr uint32_t iv_32[4] = {0x61707865U, 0x3320646eU, 0x79622d32U, 0x6b206574U};
        static constexpr uint32_t nonce[2] = { 0x55783743U, 0x421d5279U };

        uint32_t input[16];

        for (int32_t i = 0; i < 4; ++i)
        {
            input[i] = iv_32[i];
        }

        for (int32_t i = 0, j = 0; i < 8; ++i,j+=4)
        {
            input[4 + i] = (key[j+0]<<24) | (key[j+1]<<16) | (key[j+2]<<8) | (key[j+3]);
        }

        input[12] = 0;
        input[13] = 0;
        input[14] = nonce[0];
        input[15] = nonce[1];
        std::array<uint8_t, N> message;
        std::copy(src, src + N, message.begin());
        chacha20_encrypt(input, N, &message[0]);
        return message;
    }
}
#endif //INC_CRYPT_H_

使い方

#include <iostream>
#include "cthash.h"

static constexpr std::array<uint8_t, 32> key = cthash::sha256_string((const uint8_t[])"Hello World!");
static constexpr auto encrypted = cthash::chacha20_encrypt(key, (const uint8_t[])"Hello World!");

int main(void)
{
    std::cout << key << std::endl;
    std::cout << "encrypted: " << encrypted << std::endl;
    auto decrypted = cthash::chacha20_encrypt(key, encrypted);
    std::cout << "decrypted: " << decrypted << std::endl;
    return 0;
}

まとめ

使い方が不細工になってしまっているので, C++26が一般的になった時には, キャストを入れて使いやすくしましょう P2738R1（PDF注意）.