基本介紹

HMAC(散列消息身份驗證碼: Hashed Message Authentication Code)

它不是散列函數，而是采用散列函數（MD5 or 或SHA）與共享密鑰一起使用的消息身份驗證機制。

詳細見 RFC 2104

使用場景

服務端生成key，傳給客戶端；
客戶端使用key將帳號和密碼做HMAC，生成一串散列值，傳給服務端；
服務端使用key和數據庫中用戶和密碼做HMAC計算散列值，比對來自客戶端的散列值。

按照散列函數的不同，可以有如下實現。

Hmac_MD5，Hmac_sha1，Hmac_sha224，Hmac_sha256，Hmac_sha384，Hmac_sha512。

Hmac_MD5：

/**
 * MD5(Key XOR opad, MD5(Key XOR ipad, text))
 *
 * where K is an n byte key
 * ipad is the byte 0x36 repeated 64 times
 * opad is the byte 0x5c repeated 64 times
 * and text is the data being protected(maybe user or password).
 */

    memcpy( k_ipad, key, key_len);
    memcpy( k_opad, key, key_len);

    /* XOR key with ipad and opad values */
    for (i = 0; i < KEY_IOPAD_SIZE; i++) {
        k_ipad[i] ^= 0x36;
        k_opad[i] ^= 0x5c;
    }

    // perform inner MD5
    MD5Init(&context);                    /* init context for 1st pass */
    MD5Update(&context, k_ipad, KEY_IOPAD_SIZE);      /* start with inner pad */
    MD5Update(&context, (unsigned char*)text, text_len); /* then text of datagram */
    MD5Final(hmac, &context);             /* finish up 1st pass */

    // perform outer MD5
    MD5Init(&context);                   /* init context for 2nd pass */
    MD5Update(&context, k_opad, KEY_IOPAD_SIZE);     /* start with outer pad */
    MD5Update(&context, hmac, MD5_DIGEST_SIZE);     /* then results of 1st hash */
    MD5Final(hmac, &context);          /* finish up 2nd pass */

Hmac_sha1：

    /**
     * SHA(Key XOR opad, SHA(Key XOR ipad, text))
     *
     * where K is an n byte key
     * ipad is the byte 0x36 repeated 64 times
     * opad is the byte 0x5c repeated 64 times
     * and text is the data being protected(maybe user or password).
     */

    memcpy(k_ipad, key, key_len);
    memcpy(k_opad, key, key_len);
    /* XOR key with ipad and opad values */
    for (i = 0; i < KEY_IOPAD_SIZE; i++) {
        k_ipad[i] ^= 0x36;
        k_opad[i] ^= 0x5c;
    }

    // perform inner SHA
    SHA_Init(&context);                    /* init context for 1st pass */
    SHA_Bytes(&context, k_ipad, KEY_IOPAD_SIZE);      /* start with inner pad */
    SHA_Bytes(&context, text, text_len);   /* then text of datagram */
    SHA_Final(&context, hmac);             /* finish up 1st pass */

    // perform outer SHA
    SHA_Init(&context);                   /* init context for 2nd pass */
    SHA_Bytes(&context, k_opad, KEY_IOPAD_SIZE);     /* start with outer pad */
    SHA_Bytes(&context, hmac, SHA1_DIGEST_SIZE);     /* then results of 1st hash */
    SHA_Final(&context, hmac);          /* finish up 2nd pass */

Hmac_sha224，mac_sha256 和Hmac_sh啊類似，把SHA換成SHA224或SHA256即可，注意 ipad和opad的長度為64.

Hmac_sha384：Hmac_sha512和Hmac_sha384類似，把SHA384換成SHA512即可，注意 ipad和opad的長度為128.

    /**
     * SHA384(Key XOR opad, SHA(Key XOR ipad, text))
     *
     * where K is an n byte key
     * ipad is the byte 0x36 repeated 128 times
     * opad is the byte 0x5c repeated 128 times
     * and text is the data being protected(maybe user or password).
     */

    memcpy(k_ipad, key, key_len);
    memcpy(k_opad, key, key_len);
    /* XOR key with ipad and opad values */
    for (i = 0; i < KEY_IOPAD_SIZE128; i++) {
        k_ipad[i] ^= 0x36;
        k_opad[i] ^= 0x5c;
    }

    // perform inner SHA384
    SHA384_Init(&context);                    /* init context for 1st pass */
    SHA384_Bytes(&context, k_ipad, KEY_IOPAD_SIZE128);      /* start with inner pad */
    SHA384_Bytes(&context, text, text_len); /* then text of datagram */
    SHA384_Final(&context, hmac);             /* finish up 1st pass */

    // perform outer SHA384
    SHA384_Init(&context);                   /* init context for 2nd pass */
    SHA384_Bytes(&context, k_opad, KEY_IOPAD_SIZE128);     /* start with outer pad */
    SHA384_Bytes(&context, hmac, SHA384_DIGEST_SIZE);     /* then results of 1st hash */
    SHA384_Final(&context, hmac);          /* finish up 2nd pass */

　　

C implement at github:https://github.com/mygityf/cipher/blob/master/cipher/hmac.c

Done.

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