c++实现MD5算法实现代码

实现MD5算法的代码可以分成以下几个步骤：

1. 将数据填充到512位的块中（padding it），满足mod 512 = 448。

2. 将数据块分成16个32位的字，每个字称为W。

3. 初始化4个32位寄存器A、B、C、D，用于存储最终的结果。

4. 对每一个数据块进行四轮的处理，每轮处理16次，通过位运算来更新结果寄存器。

5. 所有数据块处理完后，将A、B、C、D四个寄存器按照顺序拼接起来，输出MD5结果。

下面是c++实现MD5的示例代码：

#include <iostream>
#include <iomanip>
#include <sstream>
#include <cstring>

using namespace std;

unsigned long int shift_amounts[64] =
{
    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
};

unsigned long int constants[64] =
{
    0xd76aa478, 0xe8c7b756, 0x242070db, 0xc1bdceee,
    0xf57c0faf, 0x4787c62a, 0xa8304613, 0xfd469501,
    0x698098d8, 0x8b44f7af, 0xffff5bb1, 0x895cd7be,
    0x6b901122, 0xfd987193, 0xa679438e, 0x49b40821,
    0xf61e2562, 0xc040b340, 0x265e5a51, 0xe9b6c7aa,
    0xd62f105d, 0x2441453, 0xd8a1e681, 0xe7d3fbc8,
    0x21e1cde6, 0xc33707d6, 0xf4d50d87, 0x455a14ed,
    0xa9e3e905, 0xfcefa3f8, 0x676f02d9, 0x8d2a4c8a,
    0xfffa3942, 0x8771f681, 0x6d9d6122, 0xfde5380c,
    0xa4beea44, 0x4bdecfa9, 0xf6bb4b60, 0xbebfbc70,
    0x289b7ec6, 0xeaa127fa, 0xd4ef3085, 0x4881d05,
    0xd9d4d039, 0xe6db99e5, 0x1fa27cf8, 0xc4ac5665,
    0xf4292244, 0x432aff97, 0xab9423a7, 0xfc93a039,
    0x655b59c3, 0x8f0ccc92, 0xffeff47d, 0x85845dd1,
    0x6fa87e4f, 0xfe2ce6e0, 0xa3014314, 0x4e0811a1,
    0xf7537e82, 0xbd3af235, 0x2ad7d2bb, 0xeb86d391
};

string md5(const string input) {

    unsigned long int h0 = 0x67452301;
    unsigned long int h1 = 0xEFCDAB89;
    unsigned long int h2 = 0x98BADCFE;
    unsigned long int h3 = 0x10325476;

    unsigned long int a, b, c, d, f, g, temp;
    unsigned long int *chunk;
    unsigned long int w[16];
    unsigned int i, j;

    // Pre-processing
    int original_length = input.size();
    int padded_length = (((original_length + 8) / 64) + 1) * 64;
    chunk = new unsigned long int[padded_length];
    memcpy(chunk, input.c_str(), original_length);

    chunk[original_length] = 0x80;

    for (i = original_length + 1; i < padded_length - 8; ++i) {
        chunk[i] = 0;
    }

    long int bit_length = 8 * original_length;
    memcpy(chunk + padded_length - 8, &bit_length, 4);

    // Main loop
    for (i = 0; i < padded_length; i += 64) {

        a = h0;
        b = h1;
        c = h2;
        d = h3;

        // Load chunk into w
        for (j = 0; j < 16; ++j) {
            w[j] = chunk[i + j];
        }

        // Main loop
        for (j = 0; j < 64; ++j) {

            if (j < 16) {
                f = (b & c) | ((~b) & d);
                g = j;
            }
            else if (j < 32) {
                f = (d & b) | ((~d) & c);
                g = (5 * j + 1) % 16;
            }
            else if (j < 48) {
                f = b ^ c ^ d;
                g = (3 * j + 5) % 16;
            }
            else {
                f = c ^ (b | (~d));
                g = (7 * j) % 16;
            }

            temp = d;
            d = c;
            c = b;

            b = b + ((a + f + constants[j] + w[g]) << shift_amounts[j]);
            a = temp;
        }

        h0 += a;
        h1 += b;
        h2 += c;
        h3 += d;
    }

    // Cleanup
    ostringstream result;
    result << hex << setw(8) << setfill('0') << h0
        << hex << setw(8) << setfill('0') << h1
        << hex << setw(8) << setfill('0') << h2
        << hex << setw(8) << setfill('0') << h3;
    string md5_hash = result.str();
    delete[] chunk;

    return md5_hash;
}

int main()
{
    string input1 = "hello world";
    string md5_hash1 = md5(input1);
    cout << "MD5 hash of \"" << input1 << "\" is: " << md5_hash1 << endl;

    string input2 = "MD5 implementation in C++";
    string md5_hash2 = md5(input2);
    cout << "MD5 hash of \"" << input2 << "\" is: " << md5_hash2 << endl;
    return 0;
}

上述代码中定义了常量数组shift_amounts和constants，分别用于MD5的左移位数和常数。

代码中还定义了函数md5，输入一个字符串，输出该字符串的MD5哈希值。实现代码中同时包含了两个示例，分别对字符串“hello world”和“MD5 implementation in C++”进行了MD5哈希。