C++ 原生实现 MD5 哈希算法¶
一段不使用外部库的 MD5 实现。代码涵盖了填充、分块、四轮 64 步压缩以及小端序输出等完整流程。逻辑基本正确，但存在一处内存泄漏和若干类型安全问题。
原始代码问题¶
内存泄漏¶
add() 函数内 new unsigned int[num*16] 分配了动态数组，返回给 getMD5() 使用后从未 delete[] 释放。每次调用 getMD5() 都会泄露一块堆内存。
changeHex 参数类型¶
函数签名 changeHex(int a) 接受有符号整数，内部对 a 做位移和取模运算。当哈希中间值高位为 1 时（即 unsigned int 下 ≥ 0x80000000），int 将其解释为负数，右移操作行为由实现定义，且 % 取模对负数结果不直观。应改为 unsigned int。
全局状态变量¶
atemp、btemp、ctemp、dtemp 和 strlength 定义在全局作用域，导致函数不可重入——同时计算两个字符串的 MD5 会互相覆盖中间状态。对于单线程演示程序影响不大，但应知晓这一限制。
shift 宏的边界隐患¶
当 n == 0 时，右移 32 位在 32 位无符号整数上是未定义行为。MD5 规范中旋转移位量均不为 0 或 32，实际不会触发，但作为通用位旋转宏不够健壮。
修正后代码¶
#include <iostream>
#include <string>
using namespace std;
#define shift(x, n) (((x) << (n)) | ((x) >> (32-(n))))
#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)))
#define A 0x67452301
#define B 0xefcdab89
#define C 0x98badcfe
#define D 0x10325476
unsigned int strlength;
unsigned int atemp;
unsigned int btemp;
unsigned int ctemp;
unsigned int dtemp;
const unsigned int k[] = {
0xd76aa478,0xe8c7b756,0x242070db,0xc1bdceee,
0xf57c0faf,0x4787c62a,0xa8304613,0xfd469501,0x698098d8,
0x8b44f7af,0xffff5bb1,0x895cd7be,0x6b901122,0xfd987193,
0xa679438e,0x49b40821,0xf61e2562,0xc040b340,0x265e5a51,
0xe9b6c7aa,0xd62f105d,0x02441453,0xd8a1e681,0xe7d3fbc8,
0x21e1cde6,0xc33707d6,0xf4d50d87,0x455a14ed,0xa9e3e905,
0xfcefa3f8,0x676f02d9,0x8d2a4c8a,0xfffa3942,0x8771f681,
0x6d9d6122,0xfde5380c,0xa4beea44,0x4bdecfa9,0xf6bb4b60,
0xbebfbc70,0x289b7ec6,0xeaa127fa,0xd4ef3085,0x04881d05,
0xd9d4d039,0xe6db99e5,0x1fa27cf8,0xc4ac5665,0xf4292244,
0x432aff97,0xab9423a7,0xfc93a039,0x655b59c3,0x8f0ccc92,
0xffeff47d,0x85845dd1,0x6fa87e4f,0xfe2ce6e0,0xa3014314,
0x4e0811a1,0xf7537e82,0xbd3af235,0x2ad7d2bb,0xeb86d391};
const unsigned int s[] = {7,12,17,22,7,12,17,22,7,12,17,22,7,
12,17,22,5,9,14,20,5,9,14,20,5,9,14,20,5,9,14,20,
4,11,16,23,4,11,16,23,4,11,16,23,4,11,16,23,6,10,
15,21,6,10,15,21,6,10,15,21,6,10,15,21};
const char str16[] = "0123456789abcdef";
void mainLoop(unsigned int M[])
{
unsigned int f, g;
unsigned int a = atemp;
unsigned int b = btemp;
unsigned int c = ctemp;
unsigned int d = dtemp;
for (unsigned int i = 0; i < 64; i++)
{
if (i < 16) {
f = F(b, c, d);
g = i;
} else if (i < 32) {
f = G(b, c, d);
g = (5 * i + 1) % 16;
} else if (i < 48) {
f = H(b, c, d);
g = (3 * i + 5) % 16;
} else {
f = I(b, c, d);
g = (7 * i) % 16;
}
unsigned int tmp = d;
d = c;
c = b;
b = b + shift((a + f + k[i] + M[g]), s[i]);
a = tmp;
}
atemp = a + atemp;
btemp = b + btemp;
ctemp = c + ctemp;
dtemp = d + dtemp;
}
unsigned int* add(string str)
{
unsigned int num = ((str.length() + 8) / 64) + 1;
unsigned int *strByte = new unsigned int[num * 16];
strlength = num * 16;
for (unsigned int i = 0; i < num * 16; i++)
strByte[i] = 0;
for (unsigned int i = 0; i < str.length(); i++)
{
strByte[i >> 2] |= (str[i]) << ((i % 4) * 8);
}
strByte[str.length() >> 2] |= 0x80 << (((str.length() % 4)) * 8);
strByte[num * 16 - 2] = str.length() * 8;
return strByte;
}
string changeHex(unsigned int a) // 修正：int 改为 unsigned int
{
unsigned int b; // 修正：int 改为 unsigned int
string str1;
string str = "";
for (int i = 0; i < 4; i++)
{
str1 = "";
b = ((a >> i * 8) % (1 << 8)) & 0xff;
for (int j = 0; j < 2; j++)
{
str1.insert(0, 1, str16[b % 16]);
b = b / 16;
}
str += str1;
}
return str;
}
string getMD5(string source)
{
atemp = A;
btemp = B;
ctemp = C;
dtemp = D;
unsigned int *strByte = add(source);
for (unsigned int i = 0; i < strlength / 16; i++)
{
unsigned int num[16];
for (unsigned int j = 0; j < 16; j++)
num[j] = strByte[i * 16 + j];
mainLoop(num);
}
string result = changeHex(atemp)
.append(changeHex(btemp))
.append(changeHex(ctemp))
.append(changeHex(dtemp));
delete[] strByte; // 新增：释放动态分配的内存
return result;
}
int main()
{
string ss;
// cin >> ss;
string s = getMD5("abc");
cout << s;
return 0;
}
改动集中在两处：changeHex 的参数和局部变量从 int 改为 unsigned int，消除有符号整数位移的歧义；getMD5 末尾补充 delete[] strByte，修复内存泄漏。
已知测试向量 MD5("abc") 应输出 900150983cd24fb0d6963f7d28e17f72。