由于网络上传了很多关于C++实现md5加密的类，至于那个是原创，我不敢妄加猜测，只是这里我声明我是转载的，并支持原创。

对于md5加密算法，我提供两文件：

#ifndef MD5_H 
#define MD5_H #include <string> 
#include <fstream> /* Type define */
typedef unsigned char byte; 
typedef unsigned int uint32; using std::string; 
using std::ifstream; /* MD5 declaration. */
class MD5 { 
public: MD5(); MD5(const void* input, size_t length); MD5(const string& str); MD5(ifstream& in); void update(const void* input, size_t length); void update(const string& str); void update(ifstream& in); const byte* digest(); string toString(); void reset(); string ToMD5(const string& str);//如此，只需调用该函数便完成加密过程private: void update(const byte* input, size_t length); void final(); void transform(const byte block[64]); void encode(const uint32* input, byte* output, size_t length); void decode(const byte* input, uint32* output, size_t length); string bytesToHexString(const byte* input, size_t length); /* class uncopyable */MD5(const MD5&); MD5& operator=(const MD5&); private: uint32 _state[4]; /* state (ABCD) */uint32 _count[2]; /* number of bits, modulo 2^64 (low-order word first) */byte _buffer[64]; /* input buffer */byte _digest[16]; /* message digest */bool _finished; /* calculate finished ? */static const byte PADDING[64]; /* padding for calculate */static const char HEX[16]; enum { BUFFER_SIZE = 1024 }; 
}; #endif /*MD5_H*/

#include "md5.h" 
#include "string.h"
const string MD5_Key="wow";
using namespace std; /* Constants for MD5Transform routine. */
#define S11 7 
#define S12 12 
#define S13 17 
#define S14 22 
#define S21 5 
#define S22 9 
#define S23 14 
#define S24 20 
#define S31 4 
#define S32 11 
#define S33 16 
#define S34 23 
#define S41 6 
#define S42 10 
#define S43 15 
#define S44 21 /* F, G, H and I are basic MD5 functions. 
*/
#define F(x, y, z) (((x) & (y)) | ((~x) & (z))) 
#define G(x, y, z) (((x) & (z)) | ((y) & (~z))) 
#define H(x, y, z) ((x) ^ (y) ^ (z)) 
#define I(x, y, z) ((y) ^ ((x) | (~z))) /* ROTATE_LEFT rotates x left n bits. 
*/
#define ROTATE_LEFT(x, n) (((x) << (n)) | ((x) >> (32-(n)))) /* FF, GG, HH, and II transformations for rounds 1, 2, 3, and 4. 
Rotation is separate from addition to prevent recomputation. 
*/
#define FF(a, b, c, d, x, s, ac) { (a) += F ((b), (c), (d)) + (x) + ac; (a) = ROTATE_LEFT ((a), (s)); (a) += (b); } 
#define GG(a, b, c, d, x, s, ac) { (a) += G ((b), (c), (d)) + (x) + ac; (a) = ROTATE_LEFT ((a), (s)); (a) += (b); } 
#define HH(a, b, c, d, x, s, ac) { (a) += H ((b), (c), (d)) + (x) + ac; (a) = ROTATE_LEFT ((a), (s)); (a) += (b); } 
#define II(a, b, c, d, x, s, ac) { (a) += I ((b), (c), (d)) + (x) + ac; (a) = ROTATE_LEFT ((a), (s)); (a) += (b); } const byte MD5::PADDING[64] = { 0x80 }; 
const char MD5::HEX[16] = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'
}; /* Default construct. */
MD5::MD5() { reset(); 
} /* Construct a MD5 object with a input buffer. */
MD5::MD5(const void* input, size_t length) { reset(); update(input, length); 
} /* Construct a MD5 object with a string. */
MD5::MD5(const string& str) { reset(); update(str); 
} /* Construct a MD5 object with a file. */
MD5::MD5(ifstream& in) { reset(); update(in); 
} /* Return the message-digest */
const byte* MD5::digest() { if (!_finished) { _finished = true; final(); } return _digest; 
} /* Reset the calculate state */
void MD5::reset() { _finished = false; /* reset number of bits. */_count[0] = _count[1] = 0; /* Load magic initialization constants. */_state[0] = 0x67452301; _state[1] = 0xefcdab89; _state[2] = 0x98badcfe; _state[3] = 0x10325476; 
} /* Updating the context with a input buffer. */
void MD5::update(const void* input, size_t length) { update((const byte*)input, length); 
} /* Updating the context with a string. */
void MD5::update(const string& str) { update((const byte*)str.c_str(), str.length()); 
} /* Updating the context with a file. */
void MD5::update(ifstream& in) { if (!in) { return; } std::streamsize length; char buffer[BUFFER_SIZE]; while (!in.eof()) { in.read(buffer, BUFFER_SIZE); length = in.gcount(); if (length > 0) { update(buffer, length); } } in.close(); 
} /* MD5 block update operation. Continues an MD5 message-digest 
operation, processing another message block, and updating the 
context. 
*/
void MD5::update(const byte* input, size_t length) { uint32 i, index, partLen; _finished = false; /* Compute number of bytes mod 64 */index = (uint32)((_count[0] >> 3) & 0x3f); /* update number of bits */if ((_count[0] += ((uint32)length << 3)) < ((uint32)length << 3)) { ++_count[1]; } _count[1] += ((uint32)length >> 29); partLen = 64 - index; /* transform as many times as possible. */if (length >= partLen) { memcpy(&_buffer[index], input, partLen); transform(_buffer); for (i = partLen; i + 63 < length; i += 64) { transform(&input[i]); } index = 0; } else { i = 0; } /* Buffer remaining input */memcpy(&_buffer[index], &input[i], length - i); 
} /* MD5 finalization. Ends an MD5 message-_digest operation, writing the 
the message _digest and zeroizing the context. 
*/
void MD5::final() { byte bits[8]; uint32 oldState[4]; uint32 oldCount[2]; uint32 index, padLen; /* Save current state and count. */memcpy(oldState, _state, 16); memcpy(oldCount, _count, 8); /* Save number of bits */encode(_count, bits, 8); /* Pad out to 56 mod 64. */index = (uint32)((_count[0] >> 3) & 0x3f); padLen = (index < 56) ? (56 - index) : (120 - index); update(PADDING, padLen); /* Append length (before padding) */update(bits, 8); /* Store state in digest */encode(_state, _digest, 16); /* Restore current state and count. */memcpy(_state, oldState, 16); memcpy(_count, oldCount, 8); 
} /* MD5 basic transformation. Transforms _state based on block. */
void MD5::transform(const byte block[64]) { uint32 a = _state[0], b = _state[1], c = _state[2], d = _state[3], x[16]; decode(block, x, 64); /* Round 1 */FF (a, b, c, d, x[ 0], S11, 0xd76aa478); /* 1 */FF (d, a, b, c, x[ 1], S12, 0xe8c7b756); /* 2 */FF (c, d, a, b, x[ 2], S13, 0x242070db); /* 3 */FF (b, c, d, a, x[ 3], S14, 0xc1bdceee); /* 4 */FF (a, b, c, d, x[ 4], S11, 0xf57c0faf); /* 5 */FF (d, a, b, c, x[ 5], S12, 0x4787c62a); /* 6 */FF (c, d, a, b, x[ 6], S13, 0xa8304613); /* 7 */FF (b, c, d, a, x[ 7], S14, 0xfd469501); /* 8 */FF (a, b, c, d, x[ 8], S11, 0x698098d8); /* 9 */FF (d, a, b, c, x[ 9], S12, 0x8b44f7af); /* 10 */FF (c, d, a, b, x[10], S13, 0xffff5bb1); /* 11 */FF (b, c, d, a, x[11], S14, 0x895cd7be); /* 12 */FF (a, b, c, d, x[12], S11, 0x6b901122); /* 13 */FF (d, a, b, c, x[13], S12, 0xfd987193); /* 14 */FF (c, d, a, b, x[14], S13, 0xa679438e); /* 15 */FF (b, c, d, a, x[15], S14, 0x49b40821); /* 16 *//* Round 2 */GG (a, b, c, d, x[ 1], S21, 0xf61e2562); /* 17 */GG (d, a, b, c, x[ 6], S22, 0xc040b340); /* 18 */GG (c, d, a, b, x[11], S23, 0x265e5a51); /* 19 */GG (b, c, d, a, x[ 0], S24, 0xe9b6c7aa); /* 20 */GG (a, b, c, d, x[ 5], S21, 0xd62f105d); /* 21 */GG (d, a, b, c, x[10], S22, 0x2441453); /* 22 */GG (c, d, a, b, x[15], S23, 0xd8a1e681); /* 23 */GG (b, c, d, a, x[ 4], S24, 0xe7d3fbc8); /* 24 */GG (a, b, c, d, x[ 9], S21, 0x21e1cde6); /* 25 */GG (d, a, b, c, x[14], S22, 0xc33707d6); /* 26 */GG (c, d, a, b, x[ 3], S23, 0xf4d50d87); /* 27 */GG (b, c, d, a, x[ 8], S24, 0x455a14ed); /* 28 */GG (a, b, c, d, x[13], S21, 0xa9e3e905); /* 29 */GG (d, a, b, c, x[ 2], S22, 0xfcefa3f8); /* 30 */GG (c, d, a, b, x[ 7], S23, 0x676f02d9); /* 31 */GG (b, c, d, a, x[12], S24, 0x8d2a4c8a); /* 32 *//* Round 3 */HH (a, b, c, d, x[ 5], S31, 0xfffa3942); /* 33 */HH (d, a, b, c, x[ 8], S32, 0x8771f681); /* 34 */HH (c, d, a, b, x[11], S33, 0x6d9d6122); /* 35 */HH (b, c, d, a, x[14], S34, 0xfde5380c); /* 36 */HH (a, b, c, d, x[ 1], S31, 0xa4beea44); /* 37 */HH (d, a, b, c, x[ 4], S32, 0x4bdecfa9); /* 38 */HH (c, d, a, b, x[ 7], S33, 0xf6bb4b60); /* 39 */HH (b, c, d, a, x[10], S34, 0xbebfbc70); /* 40 */HH (a, b, c, d, x[13], S31, 0x289b7ec6); /* 41 */HH (d, a, b, c, x[ 0], S32, 0xeaa127fa); /* 42 */HH (c, d, a, b, x[ 3], S33, 0xd4ef3085); /* 43 */HH (b, c, d, a, x[ 6], S34, 0x4881d05); /* 44 */HH (a, b, c, d, x[ 9], S31, 0xd9d4d039); /* 45 */HH (d, a, b, c, x[12], S32, 0xe6db99e5); /* 46 */HH (c, d, a, b, x[15], S33, 0x1fa27cf8); /* 47 */HH (b, c, d, a, x[ 2], S34, 0xc4ac5665); /* 48 *//* Round 4 */II (a, b, c, d, x[ 0], S41, 0xf4292244); /* 49 */II (d, a, b, c, x[ 7], S42, 0x432aff97); /* 50 */II (c, d, a, b, x[14], S43, 0xab9423a7); /* 51 */II (b, c, d, a, x[ 5], S44, 0xfc93a039); /* 52 */II (a, b, c, d, x[12], S41, 0x655b59c3); /* 53 */II (d, a, b, c, x[ 3], S42, 0x8f0ccc92); /* 54 */II (c, d, a, b, x[10], S43, 0xffeff47d); /* 55 */II (b, c, d, a, x[ 1], S44, 0x85845dd1); /* 56 */II (a, b, c, d, x[ 8], S41, 0x6fa87e4f); /* 57 */II (d, a, b, c, x[15], S42, 0xfe2ce6e0); /* 58 */II (c, d, a, b, x[ 6], S43, 0xa3014314); /* 59 */II (b, c, d, a, x[13], S44, 0x4e0811a1); /* 60 */II (a, b, c, d, x[ 4], S41, 0xf7537e82); /* 61 */II (d, a, b, c, x[11], S42, 0xbd3af235); /* 62 */II (c, d, a, b, x[ 2], S43, 0x2ad7d2bb); /* 63 */II (b, c, d, a, x[ 9], S44, 0xeb86d391); /* 64 */_state[0] += a; _state[1] += b; _state[2] += c; _state[3] += d; 
} /* Encodes input (ulong) into output (byte). Assumes length is 
a multiple of 4. 
*/
void MD5::encode(const uint32* input, byte* output, size_t length) { for (size_t i = 0, j = 0; j < length; ++i, j += 4) { output[j]= (byte)(input[i] & 0xff); output[j + 1] = (byte)((input[i] >> 8) & 0xff); output[j + 2] = (byte)((input[i] >> 16) & 0xff); output[j + 3] = (byte)((input[i] >> 24) & 0xff); } 
} /* Decodes input (byte) into output (ulong). Assumes length is 
a multiple of 4. 
*/
void MD5::decode(const byte* input, uint32* output, size_t length) { for (size_t i = 0, j = 0; j < length; ++i, j += 4) { output[i] = ((uint32)input[j]) | (((uint32)input[j + 1]) << 8) | (((uint32)input[j + 2]) << 16) | (((uint32)input[j + 3]) << 24); } 
} /* Convert byte array to hex string. */
string MD5::bytesToHexString(const byte* input, size_t length) { string str; str.reserve(length << 1); for (size_t i = 0; i < length; ++i) { int t = input[i]; int a = t / 16; int b = t % 16; str.append(1, HEX[a]); str.append(1, HEX[b]); } return str; 
} /* Convert digest to string value */
string MD5::toString() { return bytesToHexString(digest(), 16); 
} string MD5::ToMD5(const string& str) //MD5加密，其中密钥key=xyf，密钥需加在加密之前
{const string str_word=str+MD5_Key;reset();update(str_word);return toString();
}

那么如何调用呢？

MD5 md5;                 //定义MD5的类
md5.update("xxxxxx");//因为update函数只接收string类型，所以使用getbuffer()函数转换CString为string
char*p = md5.toString().c_str();  

不过值得注意的是你如果还要计算另一个字符串的md5值，那么你得调用md5.reset()方法重置