具體的一個MD5實現
　　/*
　　* md5 -- compute and check MD5 message digest.
　　* this version only can calculate the char string.
　　*
　　* MD5 (Message-Digest algorithm 5) is a widely used, partially
　　* insecure cryptographic hash function with a 128-bit hash value.
　　*
　　* Author: redraiment
　　* Date: Aug 27, 2008
　　* Version: 0.1.6
　　*/
　　#include <stdlib.h>
　　#include <string.h>
　　#include <stdio.h>
　　#include <math.h>
　　#define SINGLE_ONE_BIT 0x80
　　#define BLOCK_SIZE 512
　　#define MOD_SIZE 448
　　#define APP_SIZE 64
　　#define BITS 8
　　// MD5 Chaining Variable
　　#define A 0x67452301UL
　　#define B 0xEFCDAB89UL
　　#define C 0x98BADCFEUL
　　#define D 0x10325476UL
　　// Creating own types
　　#ifdef UINT64
　　# undef UINT64
　　#endif
　　#ifdef UINT32
　　# undef UINT32
　　#endif
　　typedef unsigned long long UINT64;
　　typedef unsigned long UINT32;
　　typedef unsigned char UINT8;
　　typedef struct
　　{
　　char * message;
　　UINT64 length;
　　}STRING;
　　const UINT32 X[4][2] = {{0, 1}, {1, 5}, {5, 3}, {0, 7}};
　　// Constants for MD5 transform routine.
　　const UINT32 S[4][4] = {
　　{ 7, 12, 17, 22 },
　　{ 5, 9, 14, 20 },
　　{ 4, 11, 16, 23 },
　　{ 6, 10, 15, 21 }
　　};
　　// F, G, H and I are basic MD5 functions.
　　UINT32 F( UINT32 X, UINT32 Y, UINT32 Z )
　　{
　　return ( X & Y ) | ( ~X & Z );
　　}
　　UINT32 G( UINT32 X, UINT32 Y, UINT32 Z )
　　{
　　return ( X & Z ) | ( Y & ~Z );
　　}
　　UINT32 H( UINT32 X, UINT32 Y, UINT32 Z )
　　{
　　return X ^ Y ^ Z;
　　}
　　UINT32 I( UINT32 X, UINT32 Y, UINT32 Z )
　　{
　　return Y ^ ( X | ~Z );
　　}
　　// rotates x left s bits.
　　UINT32 rotate_left( UINT32 x, UINT32 s )
　　{
　　return ( x << s ) | ( x >> ( 32 - s ) );
　　}
　　// Pre-processin
　　UINT32 count_padding_bits ( UINT32 length )
　　{
　　UINT32 div = length * BITS / BLOCK_SIZE;
　　UINT32 mod = length * BITS % BLOCK_SIZE;
　　UINT32 c_bits;
　　if ( mod == 0 )
　　c_bits = MOD_SIZE;
　　else
　　c_bits = ( MOD_SIZE + BLOCK_SIZE - mod ) % BLOCK_SIZE;
　　return c_bits / BITS;
　　}
　　STRING append_padding_bits ( char * argv )
　　{
　　UINT32 msg_length = strlen ( argv );
　　UINT32 bit_length = count_padding_bits ( msg_length );
　　UINT64 app_length = msg_length * BITS;
　　STRING string;
　　string.message = (char *)malloc(msg_length + bit_length + APP_SIZE / BITS);
　　// Save message
　　strncpy ( string.message, argv, msg_length );
　　// Pad out to mod 64.
　　memset ( string.message + msg_length, 0, bit_length );
　　string.message [ msg_length ] = SINGLE_ONE_BIT;
　　// Append length (before padding).
　　memmove ( string.message + msg_length + bit_length, (char *)&app_length, sizeof( UINT64 ) );
　　string.length = msg_length + bit_length + sizeof( UINT64 );
　　return string;
　　}
　　int main ( int argc, char *argv[] )
　　{
　　STRING string;
　　UINT32 w[16];
　　UINT32 chain[4];
　　UINT32 state[4];
　　UINT8 r[16];
　　UINT32 ( *auxi[ 4 ])( UINT32, UINT32, UINT32 ) = { F, G, H, I };
　　int roundIdx;
　　int argIdx;
　　int sIdx;
　　int wIdx;
　　int i;
　　int j;
　　if ( argc < 2 )
　　{
　　fprintf ( stderr, "usage: %s string ...\n", argv[ 0 ] );
　　return EXIT_FAILURE;
　　}
　　for ( argIdx = 1; argIdx < argc; argIdx++ )
　　{
　　string = append_padding_bits ( argv[ argIdx ] );
　　// MD5 initialization.
　　chain[0] = A;
　　chain[1] = B;
　　chain[2] = C;
　　chain[3] = D;
　　for ( j = 0; j < string.length; j += BLOCK_SIZE / BITS)
　　{
　　memmove ( (char *)w, string.message + j, BLOCK_SIZE / BITS );
　　memmove ( state, chain, sizeof(chain) );
　　for ( roundIdx = 0; roundIdx < 4; roundIdx++ )
　　{
　　wIdx = X[ roundIdx ][ 0 ];
　　sIdx = 0;
　　for ( i = 0; i < 16; i++ )
　　{
　　// FF, GG, HH, and II transformations for rounds 1, 2, 3, and 4.
　　// Rotation is separate from addition to prevent recomputation.
　　state[sIdx] = state [ (sIdx + 1) % 4 ] +
　　rotate_left ( state[sIdx] +
　　( *auxi[ roundIdx ] )
　　( state[(sIdx+1) % 4], state[(sIdx+2) % 4], state[(sIdx+3) % 4]) +
　　w[ wIdx ] +
　　(UINT32)floor( (1ULL << 32) * fabs(sin( roundIdx * 16 + i + 1 )) ),
　　S[ roundIdx ][ i % 4 ]);
　　sIdx = ( sIdx + 3 ) % 4;
　　wIdx = ( wIdx + X[ roundIdx ][ 1 ] ) & 0xF;
　　}
　　}
　　chain[ 0 ] += state[ 0 ];
　　chain[ 1 ] += state[ 1 ];
　　chain[ 2 ] += state[ 2 ];
　　chain[ 3 ] += state[ 3 ];
　　}
　　memmove ( r + 0, (char *)&chain[0], sizeof(UINT32) );
　　memmove ( r + 4, (char *)&chain[1], sizeof(UINT32) );
　　memmove ( r + 8, (char *)&chain[2], sizeof(UINT32) );
　　memmove ( r + 12, (char *)&chain[3], sizeof(UINT32) );
　　for ( i = 0; i < 16; i++ )
　　printf ( "%02x", r[i] );
　　putchar ( '\n' );
　　}
　　return EXIT_SUCCESS;
　　}
　　/* 以上程序可以在任意一款支持ANSI C的編譯器上編譯通過 */