MD5算法的RFC网址：http://www.faqs.org/rfcs/rfc1321.html
word表示32-bit
byte表示8-bit
X_i表示Xi
X^i表示Xi
X<<<s表示左循环移位
not(X)表示~X
XvY表示X|Y
XY表示X&Y
假设有消息m，则b = m.getBytes().length * 8
if ((b % 512) >= 448) {
padding (512 – b % 512 + 448) bits
} else if ((b % 512) < 448) {
padding (448 – b % 512) bits
}
padding规则是一个1，n个0。使padding后的消息长度为448加上512的倍数。(0 <= n <= 512)
将b用64-bit表示，如果b > 264，则只取低64 bits，然后按照低位优先的原则将64 bits填充到上述结果后面，使的消息长度为512的倍数。该消息用M[0...N-1]表示。其中M[i]是一个word。
word A: 01 23 45 67
word B: 89 ab cd ef
word C: fe dc ba 98
word D: 76 54 32 10
即
A = 0×67452301
B = 0xefcdab89
C = 0x98badcfe
D = 0×10325476
4个辅助函数
f(x,y,z) = (x & y) | ((~x) & z)
g(x,y,z) = (x & z) | (y & (~z))
h(x,y,z) = x ^ y ^ z
i(x,y,z) = y ^ (x | (~z))
表T[1...64]，其中T[i] = 4294967296 * abs(sin(i))的整数部分，i的单位是弧度。
for (int i = 0; i < N % 16; i++) {
for (int j = 0; j < 16; j++) {
X[j] = M[i * 16 + j];
}
AA = A;
BB = B;
CC = c;
DD = d;
/* Round 1 */
/*
* Let [abcd k s i] denote the operation
* a = b + ((a + F(b, c, d) + X[k] + T[i]) <<< s.
*/
[ABCD 0 7 1] [DABC 1 12 2] [CDAB 2 17 3] [BCDA 3 22 4]
[ABCD 4 7 5] [DABC 5 12 6] [CDAB 6 17 7] [BCDA 7 22 8]
[ABCD 8 7 9] [DABC 9 12 10] [CDAB 10 17 11] [BCDA 11 22 12]
[ABCD 12 7 13] [DABC 13 12 14] [CDAB 14 17 15] [BCDA 15 22 16]
/* Round 2 */
/*
* Let [abcd k s i] denote the operation
* a = b + ((a + G(b,c,d) + X[k] + T[i]) <<< s).
*/
[ABCD 1 5 17] [DABC 6 9 18] [CDAB 11 14 19] [BCDA 0 20 20]
[ABCD 5 5 21] [DABC 10 9 22] [CDAB 15 14 23] [BCDA 4 20 24]
[ABCD 9 5 25] [DABC 14 9 26] [CDAB 3 14 27] [BCDA 8 20 28]
[ABCD 13 5 29] [DABC 2 9 30] [CDAB 7 14 31] [BCDA 12 20 32]
/* Round 3 */
/*
* Let [abcd k s i] denote the operation
* a = b + ((a + H(b,c,d) + X[k] + T[i]) <<< s.
*/
[ABCD 5 4 33] [DABC 8 11 34] [CDAB 11 16 35] [BCDA 14 23 36]
[ABCD 1 4 37] [DABC 4 11 38] [CDAB 7 16 39] [BCDA 10 23 40]
[ABCD 13 4 41] [DABC 0 11 42] [CDAB 3 16 43] [BCDA 6 23 44]
[ABCD 9 4 45] [DABC 12 11 46] [CDAB 15 16 47] [BCDA 2 23 48]
/* Round 4 */
/*
* Let [abcd k s i] denote the operation
* a = b + ((a + I(b,c,d) + X[k] + T[i]) <<< s).
*/
[ABCD 0 6 49] [DABC 7 10 50] [CDAB 14 15 51] [BCDA 5 21 52]
[ABCD 12 6 53] [DABC 3 10 54] [CDAB 10 15 55] [BCDA 1 21 56]
[ABCD 8 6 57] [DABC 15 10 58] [CDAB 6 15 59] [BCDA 13 21 58]
[ABCD 4 6 61] [DABC 11 10 62] [CDAB 2 15 63] [BCDA 9 21 64]
/*
* Then perform the following additions. (That is increment each
* of the four registers by the value it had before this block
* was started.)
*/
A = A + AA
B = B + BB
C = C + CC
D = D + DD
}
依照低位优先的原则输出A、B、C和D
测试数据
md5("") = d41d8cd98f00b204e9800998ecf8427e
md5("a") = 0cc175b9c0f1b6a831c399e269772661
md5("abc") = 900150983cd24fb0d6963f7d28e17f72
md5("message digest") = f96b697d7cb7938d525a2f31aaf161d0
md5("abcdefghijklmnopqrstuvwxyz") = c3fcd3d76192e4007dfb496cca67e13b
md5("abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz0123456789") =
d174ab98d277d9f5a5611c2c9f419d9f
md5("12345678901234567890123456789012345678901234567890123456789012345678901234567890") =
57edf4a22be3c955ac49da2e2107b67a
转自：http://sangua.com/md5-algorithm-description-and-the-realization-of-java.html 含java源码