c语言编写Sha256算法,用在16位MCU上

在16位MCU上实现SHA256算法需要一定的技术和计算资源。以下是一个基于C语言的SHA256算法示例代码：

#include <stdint.h>
#include <string.h>

#define SHA256_BLOCK_SIZE 32

typedef struct {
    uint32_t total[2];
    uint32_t state[8];
    uint8_t buffer[64];
} sha256_ctx;

static void sha256_transform(sha256_ctx *ctx, const uint8_t *data) {
    uint32_t state[8], a, b, c, d, e, f, g, h, t1, t2, m[64];
    int i, j;

    memcpy(state, ctx->state, sizeof(state));

    for (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]);
    }

    for (; i < 64; ++i) {
        m[i] = SHA256_F4(m[i - 2]) + m[i - 7] + SHA256_F3(m[i - 15]) + m[i - 16];
    }

    for (i = 0; i < 64; ++i) {
        t1 = h + SHA256_F2(e) + SHA256_CH(e, f, g) + SHA256_K[i] + m[i];
        t2 = SHA256_F1(a) + SHA256_MAJ(a, b, c);
        h = g;
        g = f;
        f = e;
        e = d + t1;
        d = c;
        c = b;
        b = a;
        a = t1 + t2;
    }

    for (i = 0; i < 8; ++i) {
        ctx->state[i] += state[i];
    }
}

void sha256_init(sha256_ctx *ctx) {
    ctx->total[0] = 0;
    ctx->total[1] = 0;

    ctx->state[0] = 0x6a09e667;
    ctx->state[1] = 0xbb67ae85;
    ctx->state[2] = 0x3c6ef372;
    ctx->state[3] = 0xa54ff53a;
    ctx->state[4] = 0x510e527f;
    ctx->state[5] = 0x9b05688c;
    ctx->state[6] = 0x1f83d9ab;
    ctx->state[7] = 0x5be0cd19;
}

void sha256_update(sha256_ctx *ctx, const uint8_t *data, uint32_t len) {
    uint32_t i, index;

    index = (ctx->total[0] >> 3) & 0x3f;

    ctx->total[0] += len << 3;
    if (ctx->total[0] < (len << 3)) {
        ctx->total[1]++;
    }
    ctx->total[1] += len >> 29;

    for (i = 0; i < len; ++i) {
        ctx->buffer[index++] = data[i];
        if (index == 64) {
            sha256_transform(ctx, ctx->buffer);
            index = 0;
        }
    }
}

void sha256_final(sha256_ctx *ctx, uint8_t *digest) {
    uint32_t i, index, padlen;
    uint8_t bits[8];

    index = (ctx->total[0] >> 3) & 0x3f;
    padlen = (index < 56) ? (56 - index) : (120 - index);

    for (i = 0; i < 8; ++i) {
        bits[i] = (uint8_t) (ctx->total[i >> 2] >> ((3 - (i & 3)) << 3));
    }

    sha256_update(ctx, (uint8_t *) "\x80", 1);
    while (index != 56) {
        sha256_update(ctx, (uint8_t *) "\0", 1);
        index = (index + 1) & 0x3f;
    }
    sha256_update(ctx, bits, sizeof(bits));

    for (i = 0; i < SHA256_BLOCK_SIZE; ++i) {
        digest[i] = (uint8_t) (ctx->state[i >> 2] >> ((3 - (i & 3)) << 3));
    }

    memset(ctx, 0, sizeof(*ctx));
}

其中，`sha256_init`、`sha256_update`和`sha256_final`分别用于初始化算法上下文、更新输入数据和计算哈希值。SHA256算法的具体实现可以参考NIST SP 800-57 Part 1 Revision 4中的相关规范。在这里，我们假设16位MCU的处理器支持32位整数运算，并且有足够的RAM存储算法上下文和输入数据。