MD5（Message-Digest Algorithm 5）是一种广泛使用的密码散列函数，由Ron Rivest在1991年设计。MD5算法可以接收任意长度的输入信息，将其处理成一个128位的散列值，这个散列值通常用32个十六进制数表示。由于其简单易用，MD5在数据完整性校验、密码存储等领域得到了广泛应用。

本文将深入探讨MD5算法的原理，并通过C语言源码实现，帮助读者更好地理解MD5算法的内部机制。

一、MD5算法原理

MD5算法采用了分块处理数据的方式，将输入信息分割成512位的块进行处理。MD5算法的流程可以概括为以下几个步骤：

1.初始化：设置四个初始值A、B、C、D，分别对应于一个32位的整数，它们的值由MD5算法定义。

2.处理信息：将输入信息分块处理后，对每个块执行一系列的运算，包括异或操作、循环左移操作、非线性函数等。

3.输出结果：将处理后的结果与初始值进行合并，经过一系列运算后得到最终的散列值。

二、MD5算法C源码实现

下面是MD5算法的C语言源码实现，包含了初始化、处理信息、输出结果等步骤：

`c

include <stdio.h>

include <stdlib.h>

define MD5_SIZE 16

define BLOCK_SIZE 512

typedef struct { unsigned int count[2]; unsigned int state[4]; unsigned char buffer[64]; } MD5_CTX;

void MD5Init(MD5CTX *ctx) { ctx->count[0] = 0; ctx->count[1] = 0; ctx->state[0] = 0x67452301; ctx->state[1] = 0xEFCDAB89; ctx->state[2] = 0x98BADCFE; ctx->state[3] = 0x10325476; }

void MD5Update(MD5CTX ctx, unsigned char input, unsigned int inputLen) { unsigned int i, index, partLen; index = (unsigned int)(ctx->count[0] >> 3) & 0x3F; partLen = 64 - index; if (inputLen >= partLen) { memcpy(&ctx->buffer[index], input, partLen); MD5ProcessBlock(ctx, ctx->buffer); for (i = partLen; i + 63 < inputLen; i += 64) { MD5ProcessBlock(ctx, &input[i]); } index = 0; } else { partLen = inputLen; } memcpy(&ctx->buffer[index], input, partLen); ctx->count[0] += (unsigned long)inputLen << 3; }

void MD5Final(unsigned char *output, MD5CTX *ctx) { unsigned int index, padLen; index = (unsigned int)(ctx->count[0] >> 3) & 0x3F; padLen = (index < 56) ? (56 - index) : (120 - index); MD5Update(ctx, "\x80", 1); while (padLen > 56) { MD5Update(ctx, "\0", 1); padLen--; } MD5Update(ctx, "\0\0\0\0", 8); MD5Update(ctx, &ctx->state[0], 16); memcpy(output, &ctx->state[0], 16); }

void MD5ProcessBlock(MD5CTX ctx, unsigned char block) { unsigned int a, b, c, d, x[16], i; for (i = 0; i < 16; i++) { x[i] = (unsigned int)(block[i * 4] << 24) | (unsigned int)(block[i * 4 + 1] << 16) | (unsigned int)(block[i * 4 + 2] << 8) | (unsigned int)block[i * 4 + 3]; } a = ctx->state[0]; b = ctx->state[1]; c = ctx->state[2]; d = ctx->state[3]; MD5F(a, b, c, d, x[ 0], 7, -680876936); MD5F(d, a, b, c, x[ 1], 12, -389564586); MD5F(c, d, a, b, x[ 2], 17, 606105819); MD5F(b, c, d, a, x[ 3], 22, -1044525330); MD5F(a, b, c, d, x[ 4], 7, -176418897); MD5F(d, a, b, c, x[ 5], 12, 1200080426); MD5F(c, d, a, b, x[ 6], 17, -1473231341); MD5F(b, c, d, a, x[ 7], 22, -45705983); MD5F(a, b, c, d, x[ 8], 7, 1770035416); MD5F(d, a, b, c, x[ 9], 12, -1958414417); MD5F(c, d, a, b, x[10], 17, -42063); MD5F(b, c, d, a, x[11], 22, -1990404162); MD5F(a, b, c, d, x[12], 7, 1804603682); MD5F(d, a, b, c, x[13], 12, -40341101); MD5F(c, d, a, b, x[14], 17, -1502002290); MD5F(b, c, d, a, x[15], 22, 1236535329); ctx->state[0] += a; ctx->state[1] += b; ctx->state[2] += c; ctx->state[3] += d; }

void MD5_F(unsigned int x, unsigned int y, unsigned int z, unsigned int w, unsigned int aa, unsigned int bb, unsigned int cc) { x = x + ((bb & cc) | (~bb & dd)) + aa + w; x = (x << bb) | (x >> (32 - bb)); x = x + aa; }

void MD5_Transform(unsigned int x[16], unsigned int a, unsigned int b, unsigned int c, unsigned int d, unsigned int aa, unsigned int bb, unsigned int cc, unsigned int dd) { unsigned int aa1, bb1, cc1, dd1; aa1 = aa + ((bb & cc) | (~bb & dd)) + x[ 0] + dd; aa1 = (aa1 << bb) | (aa1 >> (32 - bb)); aa1 = aa1 + aa; bb1 = bb + ((cc & dd) | (~cc & aa)) + x[ 1] + aa1; bb1 = (bb1 << cc) | (bb1 >> (32 - cc)); bb1 = bb1 + bb; cc1 = cc + ((dd & aa) | (~dd & bb)) + x[ 2] + bb1; cc1 = (cc1 << dd) | (cc1 >> (32 - dd)); cc1 = cc1 + cc; dd1 = dd + ((aa & bb) | (~aa & cc)) + x[ 3] + cc1; dd1 = (dd1 << aa) | (dd1 >> (32 - aa)); dd1 = dd1 + dd; aa = aa1; bb = bb1; cc = cc1; dd = dd1; }

void MD5_Transform2(unsigned int x[16], unsigned int a, unsigned int b, unsigned int c, unsigned int d, unsigned int aa, unsigned int bb, unsigned int cc, unsigned int dd) { unsigned int aa1, bb1, cc1, dd1; aa1 = aa + ((bb & cc) | (~bb & dd)) + x[ 4] + dd; aa1 = (aa1 << bb) | (aa1 >> (32 - bb)); aa1 = aa1 + aa; bb1 = bb + ((cc & dd) | (~cc & aa)) + x[ 5] + aa1; bb1 = (bb1 << cc) | (bb1 >> (32 - cc)); bb1 = bb1 + bb; cc1 = cc + ((dd & aa) | (~dd & bb)) + x[ 6] + bb1; cc1 = (cc1 << dd) | (cc1 >> (32 - dd)); cc1 = cc1 + cc; dd1 = dd + ((aa & bb) | (~aa & cc)) + x[ 7] + cc1; dd1 = (dd1 << aa) | (dd1 >> (32 - aa)); dd1 = dd1 + dd; aa = aa1; bb = bb1; cc = cc1; dd = dd1; }

void MD5_Transform4(unsigned int x[16], unsigned int a, unsigned int b, unsigned int c, unsigned int d, unsigned int aa, unsigned int bb, unsigned int cc, unsigned int dd) { unsigned int aa1, bb1, cc1, dd1; aa1 = aa + ((bb & cc) | (~bb & dd)) + x[ 8] + dd; aa1 = (aa1 << bb) | (aa1 >> (32 - bb)); aa1 = aa1 + aa; bb1 = bb + ((cc & dd) | (~cc & aa)) + x[ 9] + aa1; bb1 = (bb1 << cc) | (bb1 >> (32 - cc)); bb1 = bb1 + bb; cc1 = cc + ((dd & aa) | (~dd & bb)) + x[10] + bb1; cc1 = (cc1 << dd) | (cc1 >> (32 - dd)); cc1 = cc1 + cc; dd1 = dd + ((aa & bb) | (~aa & cc)) + x[11] + cc1; dd1 = (dd1 << aa) | (dd1 >> (32 - aa)); dd1 = dd1 + dd; aa = aa1; bb = bb1; cc = cc1; dd = dd1; }

void MD5_Transform5(unsigned int x[16], unsigned int a, unsigned int b, unsigned int c, unsigned int d, unsigned int aa, unsigned int bb, unsigned int cc, unsigned int dd) { unsigned int aa1, bb1, cc1, dd1; aa1 = aa + ((bb & cc) | (~bb & dd)) + x[12] + dd; aa1 = (aa1 << bb) | (aa1 >> (32 - bb)); aa1 = aa1 + aa; bb1 = bb + ((cc & dd) | (~cc & aa)) + x[13] + aa1; bb1 = (bb1 << cc) | (bb1 >> (32 - cc)); bb1 = bb1 + bb; cc1 = cc + ((dd & aa) | (~dd & bb)) + x[14] + bb1; cc1 = (cc1 << dd) | (cc1 >> (32 - dd)); cc1 = cc1 + cc; dd1 = dd + ((aa & bb) | (~aa & cc)) + x[15] + cc1; dd1 = (dd1 << aa) | (dd1 >> (32 - aa)); dd1 = dd1 + dd; aa = aa1; bb = bb1; cc = cc1; dd = dd1; }

void MD5ProcessMessage(MD5CTX ctx) { unsigned int i; unsigned int index, partLen; unsigned int words = malloc(16 sizeof(unsigned int)); unsigned int x = malloc(16 sizeof(unsigned int)); unsigned char block = malloc(64 * sizeof(unsigned char)); index = (unsigned int)(ctx->count[0] >> 3) & 0x3F; partLen = 64 - index; memcpy(&block[index], &ctx->buffer, partLen); MD5ProcessBlock(ctx, block); for (i = 0; i < 14; i++) { memcpy(&block, &ctx->buffer, 64); MD5ProcessBlock(ctx, block); } for (i = 0; i < 16; i++) { words[i] = 0; } memcpy(words, &ctx->state, 16 * sizeof(unsigned int)); for (i = 0; i < 16; i++) { x[i] = 0; } for (i = 0; i < 16; i++) { x[i] = words[i]; } for (i = 0; i < 16; i++) { x[i] = x[i] + (unsigned int)(ctx->count[0] >> 3) + (unsigned int)(ctx->count[1] << 29); } MD5Transform(x, ctx->state[0], ctx->state[1], ctx->state[2], ctx->state[3], 0xd76aa478, 0xe8c7b756, 0x242070db, 0xc1bdceee); MD5Transform5(x, ctx->state[0], ctx->state[1], ctx->state[2], ctx->state[3], 0xf57c0faf, 0x4787c62a, 0xa8304613, 0xfd469501); MD5Transform5(x, ctx->state[0], ctx->state[1], ctx->state[2], ctx->state[3], 0x698098d8, 0x8b44f7af, 0xffff5bb1, 0x895cd7be); MD5Transform5(x, ctx->state[0], ctx->state[1], ctx->state[2], ctx->state[3], 0x6b901122, 0xfd987193, 0xa679438e, 0x49b40821); MD5Transform4(x, ctx->state[0], ctx->state[1], ctx->state[2], ctx->state[3], 0xfde5380c, 0xa4beea44, 0x4bdecfa9, 0xf6bb4b60); MD5Transform4(x, ctx->state[0], ctx->state[1], ctx->state[2], ctx->state[3], 0xbebfbc70, 0x289b7ec6, 0xeaa127fa, 0xd4ef3085); MD5Transform4(x, ctx->state[0], ctx->state[1], ctx->state[2], ctx->state[3], 0x04881d05, 0xd9d4d039, 0xe6db99e5, 0x1fa27cf8); MD5Transform4(x, ctx->state[0], ctx->state[1], ctx->state[2], ctx->state[3], 0xc4ac5665, 0xf4292244, 0x432aff97, 0xab9423a7); MD5Transform3(x, ctx->state[0], ctx->state[1], ctx->state[2], ctx->state[3], 0xfc93a039, 0x655b59c3, 0x8f0ccc92, 0xffeff47d); MD5Transform3(x, ctx->state[0], ctx->state[1], ctx->state[2], ctx->state[3], 0x85845dd1, 0x6fa87e4f, 0xfe2ce6e0, 0xa3014314); MD5Transform3(x, ctx->state[0], ctx->state[1], ctx->state[2], ctx->state[3], 0x4e0811a1, 0xf7537e82, 0xbd3af235, 0x2ad7d2bb); MD5Transform3(x, ctx->state[0], ctx->state[1], ctx->state[2], ctx->state[3], 0xeb86d391, 0xc67178f2, 0xe3725b38, 0xdfbe4ba0); for (i = 0; i < 16; i++) { ctx->state[i] += x[i]; } free(words); free(x); free(block); }

void MD5(string input, unsigned char *output) { MD5CTX ctx; unsigned char *inputCopy = malloc(strlen(input) + 1); strcpy(inputCopy, input); MD5Init(&ctx); MD5Update(&ctx, inputCopy, strlen(input)); MD5Final(output, &ctx); free(inputCopy); }

int main() { char input[] = "The quick brown fox jumps over the lazy dog"; unsigned char output[MD5SIZE]; MD5(input, output); printf("MD5: "); for (int i = 0; i < MD5SIZE; i++) { printf("%02x", output[i]); } printf("\n"); return 0; } `

三、总结

通过本文的介绍，我们了解了MD5算法的原理和C语言源码实现。MD5算法在处理数据完整性校验、密码存储等领域具有广泛的应用。在实际应用中