基于Emscripten的WebAssembly轻量JWT编码及HmacSHA256实现咨询

嘿，作为C语言底子不算厚的前端开发者，你已经搞定了URL安全Base64的Wasm移植，现在要补全HmacSHA256来做轻量化的JWT仅编码模块——这个思路太对了，轻量化Wasm刚好适配前端对包体积敏感的场景！下面我给你一套能直接落地的方案，尽量用简洁的C代码，适配Emscripten编译，还能和你已有的代码无缝配合。

1. 先理清楚HmacSHA256的核心逻辑（简化版） #

HMAC-SHA256本质是用密钥给原始数据做带密钥的SHA256哈希。为了保证Wasm体积最小，我们不用引入重型库（比如OpenSSL），而是用精简的手写SHA256实现，再套上HMAC的逻辑。

2. 可直接复用的C代码实现 #

下面的代码包含了精简版SHA256、HmacSHA256、你已有的URL安全Base64，以及最终的JWT生成函数：

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

// 精简版SHA256核心定义
#define SHA256_BLOCK_SIZE 32
#define SHA256_CHUNK_SIZE 64

typedef struct {
    uint8_t data[SHA256_CHUNK_SIZE];
    uint32_t datalen;
    uint64_t bitlen;
    uint32_t state[8];
} SHA256_CTX;

void sha256_init(SHA256_CTX *ctx);
void sha256_update(SHA256_CTX *ctx, const uint8_t data[], size_t len);
void sha256_final(SHA256_CTX *ctx, uint8_t hash[]);

// HMAC-SHA256实现
void hmac_sha256(const uint8_t *key, size_t key_len, const uint8_t *msg, size_t msg_len, uint8_t *out) {
    SHA256_CTX ctx;
    uint8_t k_ipad[SHA256_CHUNK_SIZE] = {0};
    uint8_t k_opad[SHA256_CHUNK_SIZE] = {0};
    uint8_t temp_hash[SHA256_BLOCK_SIZE];

    // 处理过长密钥：先哈希成32字节
    if (key_len > SHA256_CHUNK_SIZE) {
        sha256_init(&ctx);
        sha256_update(&ctx, key, key_len);
        sha256_final(&ctx, k_ipad);
        memcpy(k_opad, k_ipad, SHA256_BLOCK_SIZE);
    } else {
        memcpy(k_ipad, key, key_len);
        memcpy(k_opad, key, key_len);
    }

    // 生成ipad和opad
    for (int i = 0; i < SHA256_CHUNK_SIZE; i++) {
        k_ipad[i] ^= 0x36;
        k_opad[i] ^= 0x5c;
    }

    // 第一步：哈希(ipad + 消息)
    sha256_init(&ctx);
    sha256_update(&ctx, k_ipad, SHA256_CHUNK_SIZE);
    sha256_update(&ctx, msg, msg_len);
    sha256_final(&ctx, temp_hash);

    // 第二步：哈希(opad + 第一步哈希结果)
    sha256_init(&ctx);
    sha256_update(&ctx, k_opad, SHA256_CHUNK_SIZE);
    sha256_update(&ctx, temp_hash, SHA256_BLOCK_SIZE);
    sha256_final(&ctx, out);
}

// SHA256底层实现（精简版，无需修改）
static const uint32_t k[64] = {
    0x428a2f98,0x71374491,0xb5c0fbcf,0xe9b5dba5,0x3956c25b,0x59f111f1,0x923f82a4,0xab1c5ed5,
    0xd807aa98,0x12835b01,0x243185be,0x550c7dc3,0x72be5d74,0x80deb1fe,0x9bdc06a7,0xc19bf174,
    0xe49b69c1,0xefbe4786,0x0fc19dc6,0x240ca1cc,0x2de92c6f,0x4a7484aa,0x5cb0a9dc,0x76f988da,
    0x983e5152,0xa831c66d,0xb00327c8,0xbf597fc7,0xc6e00bf3,0xd5a79147,0x06ca6351,0x14292967,
    0x27b70a85,0x2e1b2138,0x4d2c6dfc,0x53380d13,0x650a7354,0x766a0abb,0x81c2c92e,0x92722c85,
    0xa2bfe8a1,0xa81a664b,0xc24b8b70,0xc76c51a3,0xd192e819,0xd6990624,0xf40e3585,0x106aa070,
    0x19a4c116,0x1e376c08,0x2748774c,0x34b0bcb5,0x391c0cb3,0x4ed8aa4a,0x5b9cca4f,0x682e6ff3,
    0x748f82ee,0x78a5636f,0x84c87814,0x8cc70208,0x90befffa,0xa4506ceb,0xbef9a3f7,0xc67178f2
};

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

static void sha256_transform(SHA256_CTX *ctx, const uint8_t data[]) {
    uint32_t a, b, c, d, e, f, g, h, i, j, t1, t2, m[64];

    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] = rotate_right(m[i-15], 7) ^ rotate_right(m[i-15], 18) ^ (m[i-15] >> 3) ^ m[i-2] ^ rotate_right(m[i-2], 17) ^ rotate_right(m[i-2], 19) ^ (m[i-2] >> 10);

    a = ctx->state[0];
    b = ctx->state[1];
    c = ctx->state[2];
    d = ctx->state[3];
    e = ctx->state[4];
    f = ctx->state[5];
    g = ctx->state[6];
    h = ctx->state[7];

    for (i = 0; i < 64; ++i) {
        t1 = h + rotate_right(e, 6) ^ rotate_right(e, 11) ^ rotate_right(e, 25) ^ (f & (e ^ g)) ^ g ^ k[i] ^ m[i];
        t2 = rotate_right(a, 2) ^ rotate_right(a, 13) ^ rotate_right(a, 22) ^ ((a & b) | (a & c) | (b & c));
        h = g;
        g = f;
        f = e;
        e = d + t1;
        d = c;
        c = b;
        b = a;
        a = t1 + t2;
    }

    ctx->state[0] += a;
    ctx->state[1] += b;
    ctx->state[2] += c;
    ctx->state[3] += d;
    ctx->state[4] += e;
    ctx->state[5] += f;
    ctx->state[6] += g;
    ctx->state[7] += h;
}

void sha256_init(SHA256_CTX *ctx) {
    ctx->datalen = 0;
    ctx->bitlen = 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[], size_t len) {
    for (size_t i = 0; i < len; ++i) {
        ctx->data[ctx->datalen] = data[i];
        ctx->datalen++;
        if (ctx->datalen == SHA256_CHUNK_SIZE) {
            sha256_transform(ctx, ctx->data);
            ctx->bitlen += 512;
            ctx->datalen = 0;
        }
    }
}

void sha256_final(SHA256_CTX *ctx, uint8_t hash[]) {
    uint32_t i = ctx->datalen;

    // 添加填充位
    if (ctx->datalen < 56) {
        ctx->data[i++] = 0x80;
        while (i < 56)
            ctx->data[i++] = 0x00;
    } else {
        ctx->data[i++] = 0x80;
        while (i < SHA256_CHUNK_SIZE)
            ctx->data[i++] = 0x00;
        sha256_transform(ctx, ctx->data);
        memset(ctx->data, 0, 56);
    }

    // 添加长度（大端序）
    ctx->bitlen += ctx->datalen * 8;
    ctx->data[63] = ctx->bitlen;
    ctx->data[62] = ctx->bitlen >> 8;
    ctx->data[61] = ctx->bitlen >> 16;
    ctx->data[60] = ctx->bitlen >> 24;
    ctx->data[59] = ctx->bitlen >> 32;
    ctx->data[58] = ctx->bitlen >> 40;
    ctx->data[57] = ctx->bitlen >> 48;
    ctx->data[56] = ctx->bitlen >> 56;
    sha256_transform(ctx, ctx->data);

    // 输出哈希结果
    for (i = 0; i < 8; ++i) {
        hash[i*4] = (ctx->state[i] >> 24) & 0x000000FF;
        hash[i*4+1] = (ctx->state[i] >> 16) & 0x000000FF;
        hash[i*4+2] = (ctx->state[i] >> 8) & 0x000000FF;
        hash[i*4+3] = ctx->state[i] & 0x000000FF;
    }
}

// 你的URL安全Base64编码器（补全了逻辑）
char _keyStr[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_=";
char ret_val[200];

char* encode(char *data) {
    int len = strlen(data);
    int i = 0;
    int j = 0;
    while(i < len) {
        char chr1 = (i < len) ? data[i++] : 0;
        char chr2 = (i < len) ? data[i++] : 0;
        char chr3 = (i < len) ? data[i++] : 0;

        uint8_t enc1 = chr1 >> 2;
        uint8_t enc2 = ((chr1 & 0x3) << 4) | (chr2 >> 4);
        uint8_t enc3 = ((chr2 & 0xF) << 2) | (chr3 >> 6);
        uint8_t enc4 = chr3 & 0x3F;

        ret_val[j++] = _keyStr[enc1];
        ret_val[j++] = _keyStr[enc2];
        ret_val[j++] = (i <= len) ? _keyStr[enc3] : '=';
        ret_val[j++] = (i <= len+1) ? _keyStr[enc4] : '=';
    }
    ret_val[j] = '\0';
    return ret_val;
}

// JWT仅编码核心函数
char* generate_jwt(const char* header, const char* payload, const char* secret) {
    static char jwt_buf[512];
    char combined[256];
    uint8_t hmac_result[32];
    char hmac_b64[45]; // URL-safe Base64 of 32-byte hash is 44 chars + null

    // 拼接Base64编码后的header和payload
    snprintf(combined, sizeof(combined), "%s.%s", encode((char*)header), encode((char*)payload));

    // 计算HMAC-SHA256签名
    hmac_sha256((const uint8_t*)secret, strlen(secret), (const uint8_t*)combined, strlen(combined), hmac_result);

    // 把二进制签名转成URL安全Base64
    int idx = 0;
    for (int i = 0; i < 32; i += 3) {
        uint32_t triple = (hmac_result[i] << 16) | ((i+1 <32) ? hmac_result[i+1] <<8 :0) | ((i+2 <32) ? hmac_result[i+2] :0);
        hmac_b64[idx++] = _keyStr[(triple >> 18) & 0x3F];
        hmac_b64[idx++] = _keyStr[(triple >> 12) & 0x3F];
        hmac_b64[idx++] = (i+1 <32) ? _keyStr[(triple >>6) &0x3F] : '=';
        hmac_b64[idx++] = (i+2 <32) ? _keyStr[triple &0x3F] : '=';
    }
    hmac_b64[idx] = '\0';

    // 拼接成最终JWT
    snprintf(jwt_buf, sizeof(jwt_buf), "%s.%s", combined, hmac_b64);
    return jwt_buf;
}

3. Emscripten编译配置（极致轻量化） #

为了让Wasm体积最小，编译时一定要加这些参数：

emcc jwt-hmac.c -o jwt-hmac.js -Os -s EXPORTED_FUNCTIONS='["_generate_jwt", "_encode"]' -s EXPORTED_RUNTIME_METHODS='["ccall", "cwrap"]' -s NO_FILESYSTEM=1 -s NO_DYNAMIC_EXECUTION=1

关键参数说明：

• -Os：优先优化体积，比-O2更适合轻量化场景
• EXPORTED_FUNCTIONS：导出需要在JS中调用的C函数
• NO_FILESYSTEM/NO_DYNAMIC_EXECUTION：禁用不需要的功能，进一步缩小体积

4. JavaScript中调用Wasm的示例 #

// 加载Wasm模块
Module.onRuntimeInitialized = () => {
    // 定义J