介紹

MD4是一種資訊摘要演算法，由麻省理工學院教Ronald Rivest於1990年開發完成，演算法根據輸入的數值計算128位的摘要資訊，用於檢測資訊的完整性。該演算法影響了後來其他的密碼演算法的設計,如MD5、sha-1和RIPEMD演算法。

安全性

1991年，Den Boer和Bosselaers發表論文揭示了MD4演算法的脆弱性。1995年， Hans Dobbertin第一次利用演算法的漏洞全方位攻擊該演算法，僅僅在幾秒鐘內該便被破解。2004年，該演算法再次遭遇挑戰，研究人員發現一種有效的攻擊該演算法的方式，包括其後的MD4/MD5/SHA-1/RIPEMD 家族的摘要演算法均存在此漏洞。現在，正向計算hash值和反向推斷原值的耗時幾乎相同，因此在2011年，RFC6150中闡明該演算法已經被淘汰。（From Wikipedia）

程式碼實現（openssl原始碼修改）

MD4.h 標頭檔案中定義了計算hash值所需的結構體Md4，程式碼實現如下：

#define min(a,b) (((a) < (b)) ? (a) : (b))
/************************************************************************/
/*  Global Struct Define Section
/************************************************************************/

enum {
    MD4_BLOCK_SIZE = 64,
 

    MD4_DIGEST_SIZE = 16,
    MD4_PAD_SIZE = 56
};

typedef struct Md4 {
    unsigned int  buffLen;   /* in bytes          */
    unsigned int  loLen;     /* length in bytes   */
    unsigned int  hiLen;     /* length in bytes   */
    unsigned int  digest[MD4_DIGEST_SIZE / sizeof(unsigned int)];
    unsigned int  buffer[MD4_BLOCK_SIZE / sizeof(unsigned int)];
 

} Md4;

MD4.c main函式以此呼叫以下函式計算摘要值：
- initMd4(md4);
- md4Update(md4, buffer, strlen(buffer));
- md4Final(md4, md4Code);
程式碼實現如下，最終計算“abcdefghijklmnopqrstuvwxyz”的MD4碼為:
“d79e1c308aa5bbcdeea8ed63df412da9”

void main()
{
    unsigned char buffer[] = "abcdefghijklmnopqrstuvwxyz";
    unsigned char md4Code[MD4_DIGEST_SIZE ] = {0};
    Md4 *md4 = 0;
    md4 = (struct Md4*)malloc(sizeof(struct Md4));                                  
    memset(md4, 0, sizeof(md4));

    initMd4(md4);
    md4Update(md4, buffer, strlen(buffer));
    md4Final(md4, md4Code);

    free(md4);
}

void initMd4(Md4* md4)
{
    md4->digest[0] = 0x67452301L;
    md4->digest[1] = 0xefcdab89L;
    md4->digest[2] = 0x98badcfeL;
    md4->digest[3] = 0x10325476L;

    md4->buffLen = 0;
    md4->loLen = 0;
    md4->hiLen = 0;
}


static void AddLength(Md4* md4, unsigned int len)
{
    unsigned int tmp = md4->loLen;
    if ((md4->loLen += len) < tmp)
        md4->hiLen++;                       /* carry low to high */
}

static unsigned int rotlFixed(unsigned int x, unsigned int y)
{
    return y ? _lrotl(x, y) : x;
}

static void Transform(Md4* md4)
{
#define F(x, y, z) ((z) ^ ((x) & ((y) ^ (z))))
#define G(x, y, z) (((x) & (y)) | ((x) & (z)) | ((y) & (z)))
#define H(x, y, z) ((x) ^ (y) ^ (z))

    /* Copy context->state[] to working vars  */
    unsigned int A = md4->digest[0];
    unsigned int B = md4->digest[1];
    unsigned int C = md4->digest[2];
    unsigned int D = md4->digest[3];

#define function(a,b,c,d,k,s) a=rotlFixed(a+F(b,c,d)+md4->buffer[k],s);
    function(A, B, C, D, 0, 3);
    function(D, A, B, C, 1, 7);
    function(C, D, A, B, 2, 11);
    function(B, C, D, A, 3, 19);
    function(A, B, C, D, 4, 3);
    function(D, A, B, C, 5, 7);
    function(C, D, A, B, 6, 11);
    function(B, C, D, A, 7, 19);
    function(A, B, C, D, 8, 3);
    function(D, A, B, C, 9, 7);
    function(C, D, A, B, 10, 11);
    function(B, C, D, A, 11, 19);
    function(A, B, C, D, 12, 3);
    function(D, A, B, C, 13, 7);
    function(C, D, A, B, 14, 11);
    function(B, C, D, A, 15, 19);

#undef function   
#define function(a,b,c,d,k,s) \
    a=rotlFixed(a+G(b,c,d)+md4->buffer[k]+0x5a827999,s);

    function(A, B, C, D, 0, 3);
    function(D, A, B, C, 4, 5);
    function(C, D, A, B, 8, 9);
    function(B, C, D, A, 12, 13);
    function(A, B, C, D, 1, 3);
    function(D, A, B, C, 5, 5);
    function(C, D, A, B, 9, 9);
    function(B, C, D, A, 13, 13);
    function(A, B, C, D, 2, 3);
    function(D, A, B, C, 6, 5);
    function(C, D, A, B, 10, 9);
    function(B, C, D, A, 14, 13);
    function(A, B, C, D, 3, 3);
    function(D, A, B, C, 7, 5);
    function(C, D, A, B, 11, 9);
    function(B, C, D, A, 15, 13);

#undef function  
#define function(a,b,c,d,k,s) \
    a=rotlFixed(a+H(b,c,d)+md4->buffer[k]+0x6ed9eba1,s);

    function(A, B, C, D, 0, 3);
    function(D, A, B, C, 8, 9);
    function(C, D, A, B, 4, 11);
    function(B, C, D, A, 12, 15);
    function(A, B, C, D, 2, 3);
    function(D, A, B, C, 10, 9);
    function(C, D, A, B, 6, 11);
    function(B, C, D, A, 14, 15);
    function(A, B, C, D, 1, 3);
    function(D, A, B, C, 9, 9);
    function(C, D, A, B, 5, 11);
    function(B, C, D, A, 13, 15);
    function(A, B, C, D, 3, 3);
    function(D, A, B, C, 11, 9);
    function(C, D, A, B, 7, 11);
    function(B, C, D, A, 15, 15);

    /* Add the working vars back into digest state[]  */
    md4->digest[0] += A;
    md4->digest[1] += B;
    md4->digest[2] += C;
    md4->digest[3] += D;
}


void md4Update(Md4* md4, const unsigned char* data, unsigned int len)
{
    /* do block size increments */
    unsigned char* local = (unsigned char*)md4->buffer;

    while (len) {
        unsigned int add = min(len, MD4_BLOCK_SIZE - md4->buffLen);
        memcpy(&local[md4->buffLen], data, add);

        md4->buffLen += add;
        data += add;
        len -= add;

        if (md4->buffLen == MD4_BLOCK_SIZE) {
            Transform(md4);
            AddLength(md4, MD4_BLOCK_SIZE);
            md4->buffLen = 0;
        }
    }
}

void md4Final(Md4* md4, unsigned char* hash)
{
    unsigned char* local = (unsigned char*)md4->buffer;

    AddLength(md4, md4->buffLen);               /* before adding pads */

    local[md4->buffLen++] = 0x80;  /* add 1 */

                                   /* pad with zeros */
    if (md4->buffLen > MD4_PAD_SIZE) {
        memset(&local[md4->buffLen], 0, MD4_BLOCK_SIZE - md4->buffLen);
        md4->buffLen += MD4_BLOCK_SIZE - md4->buffLen;

        Transform(md4);
        md4->buffLen = 0;
    }
    memset(&local[md4->buffLen], 0, MD4_PAD_SIZE - md4->buffLen);

    /* put lengths in bits */
    md4->hiLen = (md4->loLen >> (8 * sizeof(md4->loLen) - 3)) +
        (md4->hiLen << 3);
    md4->loLen = md4->loLen << 3;


    /* ! length ordering dependent on digest endian type ! */
    memcpy(&local[MD4_PAD_SIZE], &md4->loLen, sizeof(unsigned int));
    memcpy(&local[MD4_PAD_SIZE + sizeof(unsigned int)], &md4->hiLen, sizeof(unsigned int));

    Transform(md4);

    memcpy(hash, md4->digest, MD4_DIGEST_SIZE);

    initMd4(md4);  /* reset state */
}