哈希值是一段数据唯一且极其紧凑的数值表示形式。如果散列一段明文而且哪怕只更改该段落的一个字母，随后的哈希都将产生不同的值。要找到散列为同一个值的两个不同的输入，在计算上是不可能的，所以数据的哈希值可以检验数据的完整性。

通常来说，用户通过Hash算法对目标信息生成一段特定长度的唯一的Hash值，却不能通过这个Hash值重新获得目标信息。因此Hash算法常用在不可还原的密码存储、信息完整性校验等。

(Hash)算法,就是我们在数学中所学的，散列函数。它是一种单向运算,只有加密,没有解密。hash的这种特点，使得它可用于许多操作，包括身份验证和数字签名。或叫做消息摘要。

常见的Hash算法有MD2、MD4、MD5、HAVAL、SHA，(HMAC）

本文中提到的算法，都是当前在用的主流算法。非常非常常见。

插楼，准备上代码

#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 static void MD5Transform PROTO_LIST ((UINT4 [4], unsigned char [64]));
static void Encode PROTO_LIST
((unsigned char *, UINT4 *, unsigned int));
static void Decode PROTO_LIST
((UINT4 *, unsigned char *, unsigned int));
static void MD5_memcpy PROTO_LIST ((POINTER, POINTER, unsigned int));
static void MD5_memset PROTO_LIST ((POINTER, int, unsigned int)); static unsigned char PADDING[64] = {
0x80, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
};
#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 ROTATE_LEFT(x, n) (((x) << (n)) | ((x) >> (32-(n))))
#define FF(a, b, c, d, x, s, ac) { \
(a) += F ((b), (c), (d)) + (x) + (UINT4)(ac); \
(a) = ROTATE_LEFT ((a), (s)); \
(a) += (b); \
}
#define GG(a, b, c, d, x, s, ac) { \
(a) += G ((b), (c), (d)) + (x) + (UINT4)(ac); \
(a) = ROTATE_LEFT ((a), (s)); \
(a) += (b); \
}
#define HH(a, b, c, d, x, s, ac) { \
(a) += H ((b), (c), (d)) + (x) + (UINT4)(ac); \
(a) = ROTATE_LEFT ((a), (s)); \
(a) += (b); \
}
#define II(a, b, c, d, x, s, ac) { \
(a) += I ((b), (c), (d)) + (x) + (UINT4)(ac); \
(a) = ROTATE_LEFT ((a), (s)); \
(a) += (b); \
}
void MD5Init (context)
MD5_CTX *context;
{
context->count[0] = context->count[1] = 0;
context->state[0] = 0x67452301;
context->state[1] = 0xefcdab89;
context->state[2] = 0x98badcfe;
context->state[3] = 0x10325476;
}
void MD5Update (context, input, inputLen)
MD5_CTX *context;
unsigned char *input;
unsigned int inputLen;
{
unsigned int i, index, partLen;
index = (unsigned int)((context->count[0] >> 3) & 0x3F);
if ((context->count[0] += ((UINT4)inputLen << 3))
< ((UINT4)inputLen << 3))
context->count[1]++;
context->count[1] += ((UINT4)inputLen >> 29); partLen = 64 - index;
if (inputLen >= partLen) {
MD5_memcpy
((POINTER)&context->buffer[index], (POINTER)input, partLen);
MD5Transform (context->state, context->buffer); for (i = partLen; i + 63 < inputLen; i += 64)
MD5Transform (context->state, &input[i]); index = 0;
}
else
i = 0;
MD5_memcpy
((POINTER)&context->buffer[index], (POINTER)&input[i],
inputLen-i);
}
void MD5Final (digest, context)
I

unsigned char digest[16];
MD5_CTX *context;
{
unsigned char bits[8];
unsigned int index, padLen;
Encode (bits, context->count, 8);
index = (unsigned int)((context->count[0] >> 3) & 0x3f);
padLen = (index < 56) ? (56 - index) : (120 - index);
MD5Update (context, PADDING, padLen);
MD5Update (context, bits, 8);
Encode (digest, context->state, 16);
MD5_memset ((POINTER)context, 0, sizeof (*context));
}
static void MD5Transform (state, block)
UINT4 state[4];
unsigned char block[64];
{
UINT4 a = state[0], b = state[1], c = state[2], d = state[3], x[16]; Decode (x, block, 64);
FF (a, b, c, d, x[ 0], S11, 0xd76aa478);
FF (d, a, b, c, x[ 1], S12, 0xe8c7b756);
FF (c, d, a, b, x[ 2], S13, 0x242070db);
FF (b, c, d, a, x[ 3], S14, 0xc1bdceee);
FF (a, b, c, d, x[ 4], S11, 0xf57c0faf);
FF (d, a, b, c, x[ 5], S12, 0x4787c62a);
FF (c, d, a, b, x[ 6], S13, 0xa8304613);
FF (b, c, d, a, x[ 7], S14, 0xfd469501);
FF (a, b, c, d, x[ 8], S11, 0x698098d8);
FF (d, a, b, c, x[ 9], S12, 0x8b44f7af);
FF (c, d, a, b, x[10], S13, 0xffff5bb1);
FF (b, c, d, a, x[11], S14, 0x895cd7be);
FF (a, b, c, d, x[12], S11, 0x6b901122);
FF (d, a, b, c, x[13], S12, 0xfd987193);
FF (c, d, a, b, x[14], S13, 0xa679438e);
FF (b, c, d, a, x[15], S14, 0x49b40821);
GG (a, b, c, d, x[ 1], S21, 0xf61e2562);
GG (d, a, b, c, x[ 6], S22, 0xc040b340);
G***, a, b, x[11], S23, 0x265e5a51);
GG (b, c, d, a, x[ 0], S24, 0xe9b6c7aa);
GG (a, b, c, d, x[ 5], S21, 0xd62f105d);
GG (d, a, b, c, x[10], S22, 0x2441453);
G***, a, b, x[15], S23, 0xd8a1e681);
GG (b, c, d, a, x[ 4], S24, 0xe7d3fbc8);
GG (a, b, c, d, x[ 9], S21, 0x21e1cde6);
GG (d, a, b, c, x[14], S22, 0xc33707d6);
G***, a, b, x[ 3], S23, 0xf4d50d87);
GG (b, c, d, a, x[ 8], S24, 0x455a14ed);
GG (a, b, c, d, x[13], S21, 0xa9e3e905);
GG (d, a, b, c, x[ 2], S22, 0xfcefa3f8);
G***, a, b, x[ 7], S23, 0x676f02d9);
GG (b, c, d, a, x[12], S24, 0x8d2a4c8a);
HH (a, b, c, d, x[ 5], S31, 0xfffa3942);
HH (d, a, b, c, x[ 8], S32, 0x8771f681);
HH (c, d, a, b, x[11], S33, 0x6d9d6122);
HH (b, c, d, a, x[14], S34, 0xfde5380c);
HH (a, b, c, d, x[ 1], S31, 0xa4beea44);
HH (d, a, b, c, x[ 4], S32, 0x4bdecfa9);
HH (c, d, a, b, x[ 7], S33, 0xf6bb4b60);
HH (b, c, d, a, x[10], S34, 0xbebfbc70);
HH (a, b, c, d, x[13], S31, 0x289b7ec6);
HH (d, a, b, c, x[ 0], S32, 0xeaa127fa);
HH (c, d, a, b, x[ 3], S33, 0xd4ef3085);
HH (b, c, d, a, x[ 6], S34, 0x4881d05);
HH (a, b, c, d, x[ 9], S31, 0xd9d4d039);
HH (d, a, b, c, x[12], S32, 0xe6db99e5);
HH (c, d, a, b, x[15], S33, 0x1fa27cf8);
HH (b, c, d, a, x[ 2], S34, 0xc4ac5665);
II (a, b, c, d, x[ 0], S41, 0xf4292244);
II (d, a, b, c, x[ 7], S42, 0x432aff97);
II (c, d, a, b, x[14], S43, 0xab9423a7);
II (b, c, d, a, x[ 5], S44, 0xfc93a039);
II (a, b, c, d, x[12], S41, 0x655b59c3);
II (d, a, b, c, x[ 3], S42, 0x8f0ccc92);
II (c, d, a, b, x[10], S43, 0xffeff47d);
II (b, c, d, a, x[ 1], S44, 0x85845dd1);
II (a, b, c, d, x[ 8], S41, 0x6fa87e4f);
II (d, a, b, c, x[15], S42, 0xfe2ce6e0);
II (c, d, a, b, x[ 6], S43, 0xa3014314);
II (b, c, d, a, x[13], S44, 0x4e0811a1);
II (a, b, c, d, x[ 4], S41, 0xf7537e82);
II (d, a, b, c, x[11], S42, 0xbd3af235);
II (c, d, a, b, x[ 2], S43, 0x2ad7d2bb);
II (b, c, d, a, x[ 9], S44, 0xeb86d391);
state[0] += a;
state[1] += b;
state[2] += c;
state[3] += d;
MD5_memset ((POINTER)x, 0, sizeof (x));
}
static void Encode (output, input, len)
unsigned char *output;
UINT4 *input;
unsigned int len;
{
unsigned int i, j; for (i = 0, j = 0; j < len; i++, j += 4) {
output[j] = (unsigned char)(input[i] & 0xff);
output[j+1] = (unsigned char)((input[i] >> 8) & 0xff);
output[j+2] = (unsigned char)((input[i] >> 16) & 0xff);
output[j+3] = (unsigned char)((input[i] >> 24) & 0xff);
}
}
static void Decode (output, input, len)
UINT4 *output;
unsigned char *input;
unsigned int len;
{
unsigned int i, j; for (i = 0, j = 0; j < len; i++, j += 4)
output[i] = ((UINT4)input[j]) | (((UINT4)input[j+1]) << 8) |
(((UINT4)input[j+2]) << 16) | (((UINT4)input[j+3]) << 24);
}
POINTER output;
POINTER input;
unsigned int len;
{
unsigned int i; for (i = 0; i < len; i++)
output[i] = input[i];
}
static void MD5_memset (output, value, len)
POINTER output;
int value;
unsigned int len;
{
unsigned int i; for (i = 0; i < len; i++)
((char *)output)[i] = (char)value;
}

以上是，MD5 算法源代码，C语言版本。

神。。。。

代码是你写的？好厉害

国密的有源码吗？

有diffie hellman算法的c语言代码吗？

学习了，

大佬，想问一下0基础学按键精灵要多久能写自由幻想跑家族的脚本