# Detailed explanation and implementation of MD5 algorithm (C language)

Posted by ziong on Wed, 23 Feb 2022 11:07:18 +0100

See the column collection for the detailed explanation and implementation of other modern cryptography algorithms~

### MD5 algorithm

• Algorithm process

(i) Message filling

First, fill in the message so that its length is 64 bits less than the integer multiple of 512 (these 64 bits are used to record the original data length). The filled content consists of a 1 and subsequent 0. It must be filled, even if it is satisfied at the beginning.

(ii) grouping

Divide the filled message into L groups, each group has 512 bits, that is, a 64 byte group, and then represent each group as 16 32 bits (4 bytes).

(iii) buffer initialization

That is, four magic numbers are generated for the initial value of cyclic calculation, namely:

A=01234567(0x67452301)

B=89ABCDEF(0xEFCDAB89)

D=76543210(0x10325476)

(iv) iteration (key and difficult points)

First, determine the logic function. There are four logic functions FF, GG, HH and II. Each function has seven inputs: GA, B, C, D, x, s and Ti

Where A, B, C and D are magic numbers, X is A 32-bit (4-byte) data, and s and Ti are constants. Where ti is A pseudo-random process used to eliminate the regularity of input data, and S is the number of bits that move the string A to the left in each round.

The value of the first parameter is changed by and or not or, addition and shift operations.

The four functions run 16 times in a group in a certain order. The FF,GG,HH,II and functions run 16 times respectively.

In one-step iteration, B, C and D are brought into A basic logic function for calculation, so that the result is added to A, and then the result is added to Ti. The obtained result is shifted to the left by S bit, added with B and transmitted to B as A new B. The original value of B is unchanged and transmitted to C, C is transmitted to D, and D is transmitted to A to start the next iteration. By analogy, after 16 rounds of iteration, complete the iteration of the first group, get four updated magic numbers, add them to the input of the first round respectively, get four new magic numbers A ', B', C ', D', and input them as four standard magic numbers of the next round into the Hmd5 processing of the next group.

(v) Output

After all 16 groups of iterations are completed, four magic numbers A64, B64, C64 and d64 will be obtained, and they will be arranged in order and displayed in hexadecimal, which is the final MD5 result, i.e. summary.

• Goal of algorithm design

Diffusion: let every bit in the ciphertext be affected by many bits in the plaintext;

Confusion: it is to make the statistical relationship between the ciphertext and the key as complex as possible, so that even if the opponent obtains some statistical characteristics about the ciphertext, it can not infer the key.

Avalanche effect: any slight change in key or plaintext should cause drastic changes in ciphertext. If one bit in the input changes, at least half of the HASH value will change.

Collision resistance: find any (M1,M2), make H(M1)=H(M2) computationally infeasible, and prevent birthday attacks.

• main parameter

The generated summary value is 128 bits.

Advantages: it can not only ensure the integrity of the message, but also make encryption and decryption not bring too much burden to the communication system.

### code implementation

```#include <stdio.h>
#include <stdlib.h>
#include <memory.h>
#include "md5.h"

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};

void MD5Init(MD5_CTX* context)
{
context->count = 0;
context->count = 0;
context->state = 0x67452301;
context->state = 0xEFCDAB89;
context->state = 0x10325476;
}
void MD5Update(MD5_CTX* context, unsigned char* input, unsigned int inputlen)
{
unsigned int i = 0, index = 0, partlen = 0;
index = (context->count >> 3) & 0x3F;
partlen = 64 - index;
context->count += inputlen << 3;
if(context->count < (inputlen << 3))
context->count++;
context->count += inputlen >> 29;

if(inputlen >= partlen)
{
memcpy(&context->buffer[index], input, partlen);
MD5Transform(context->state, context->buffer);
for(i = partlen; i + 64 <= inputlen; i += 64)
MD5Transform(context->state, &input[i]);
index = 0;
}
else
{
i = 0;
}
memcpy(&context->buffer[index], &input[i], inputlen - i);
}
void MD5Final(MD5_CTX* context, unsigned char digest)
{
unsigned int index = 0, padlen = 0;
unsigned char bits;
index = (context->count >> 3) & 0x3F;
padlen = (index < 56) ? (56 - index) : (120 - index);
MD5Encode(bits, context->count, 8);
MD5Update(context, bits, 8);
MD5Encode(digest, context->state, 16);
}
void MD5Encode(unsigned char* output, unsigned int* input, unsigned int len)
{
unsigned int i = 0, j = 0;
while(j < len)
{
output[j] = input[i] & 0xFF;
output[j + 1] = (input[i] >> 8) & 0xFF;
output[j + 2] = (input[i] >> 16) & 0xFF;
output[j + 3] = (input[i] >> 24) & 0xFF;
i++;
j += 4;
}
}
void MD5Decode(unsigned int* output, unsigned char* input, unsigned int len)
{
unsigned int i = 0, j = 0;
while(j < len)
{
output[i] = (input[j]) |
(input[j + 1] << 8) |
(input[j + 2] << 16) |
(input[j + 3] << 24);
i++;
j += 4;
}
}
void MD5Transform(unsigned int state, unsigned char block)
{
unsigned int a = state;
unsigned int b = state;
unsigned int c = state;
unsigned int d = state;
unsigned int x;
MD5Decode(x, block, 64);
FF(a, b, c, d, x, 7, 0xd76aa478); /* 1 */
FF(d, a, b, c, x, 12, 0xe8c7b756); /* 2 */
FF(c, d, a, b, x, 17, 0x242070db); /* 3 */
FF(b, c, d, a, x, 22, 0xc1bdceee); /* 4 */
FF(a, b, c, d, x, 7, 0xf57c0faf); /* 5 */
FF(d, a, b, c, x, 12, 0x4787c62a); /* 6 */
FF(c, d, a, b, x, 17, 0xa8304613); /* 7 */
FF(b, c, d, a, x, 22, 0xfd469501); /* 8 */
FF(a, b, c, d, x, 7, 0x698098d8); /* 9 */
FF(d, a, b, c, x, 12, 0x8b44f7af); /* 10 */
FF(c, d, a, b, x, 17, 0xffff5bb1); /* 11 */
FF(b, c, d, a, x, 22, 0x895cd7be); /* 12 */
FF(a, b, c, d, x, 7, 0x6b901122); /* 13 */
FF(d, a, b, c, x, 12, 0xfd987193); /* 14 */
FF(c, d, a, b, x, 17, 0xa679438e); /* 15 */
FF(b, c, d, a, x, 22, 0x49b40821); /* 16 */

/* Round 2 */
GG(a, b, c, d, x, 5, 0xf61e2562); /* 17 */
GG(d, a, b, c, x, 9, 0xc040b340); /* 18 */
GG(c, d, a, b, x, 14, 0x265e5a51); /* 19 */
GG(b, c, d, a, x, 20, 0xe9b6c7aa); /* 20 */
GG(a, b, c, d, x, 5, 0xd62f105d); /* 21 */
GG(d, a, b, c, x, 9, 0x2441453); /* 22 */
GG(c, d, a, b, x, 14, 0xd8a1e681); /* 23 */
GG(b, c, d, a, x, 20, 0xe7d3fbc8); /* 24 */
GG(a, b, c, d, x, 5, 0x21e1cde6); /* 25 */
GG(d, a, b, c, x, 9, 0xc33707d6); /* 26 */
GG(c, d, a, b, x, 14, 0xf4d50d87); /* 27 */
GG(b, c, d, a, x, 20, 0x455a14ed); /* 28 */
GG(a, b, c, d, x, 5, 0xa9e3e905); /* 29 */
GG(d, a, b, c, x, 9, 0xfcefa3f8); /* 30 */
GG(c, d, a, b, x, 14, 0x676f02d9); /* 31 */
GG(b, c, d, a, x, 20, 0x8d2a4c8a); /* 32 */

/* Round 3 */
HH(a, b, c, d, x, 4, 0xfffa3942); /* 33 */
HH(d, a, b, c, x, 11, 0x8771f681); /* 34 */
HH(c, d, a, b, x, 16, 0x6d9d6122); /* 35 */
HH(b, c, d, a, x, 23, 0xfde5380c); /* 36 */
HH(a, b, c, d, x, 4, 0xa4beea44); /* 37 */
HH(d, a, b, c, x, 11, 0x4bdecfa9); /* 38 */
HH(c, d, a, b, x, 16, 0xf6bb4b60); /* 39 */
HH(b, c, d, a, x, 23, 0xbebfbc70); /* 40 */
HH(a, b, c, d, x, 4, 0x289b7ec6); /* 41 */
HH(d, a, b, c, x, 11, 0xeaa127fa); /* 42 */
HH(c, d, a, b, x, 16, 0xd4ef3085); /* 43 */
HH(b, c, d, a, x, 23, 0x4881d05); /* 44 */
HH(a, b, c, d, x, 4, 0xd9d4d039); /* 45 */
HH(d, a, b, c, x, 11, 0xe6db99e5); /* 46 */
HH(c, d, a, b, x, 16, 0x1fa27cf8); /* 47 */
HH(b, c, d, a, x, 23, 0xc4ac5665); /* 48 */

/* Round 4 */
II(a, b, c, d, x, 6, 0xf4292244); /* 49 */
II(d, a, b, c, x, 10, 0x432aff97); /* 50 */
II(c, d, a, b, x, 15, 0xab9423a7); /* 51 */
II(b, c, d, a, x, 21, 0xfc93a039); /* 52 */
II(a, b, c, d, x, 6, 0x655b59c3); /* 53 */
II(d, a, b, c, x, 10, 0x8f0ccc92); /* 54 */
II(c, d, a, b, x, 15, 0xffeff47d); /* 55 */
II(b, c, d, a, x, 21, 0x85845dd1); /* 56 */
II(a, b, c, d, x, 6, 0x6fa87e4f); /* 57 */
II(d, a, b, c, x, 10, 0xfe2ce6e0); /* 58 */
II(c, d, a, b, x, 15, 0xa3014314); /* 59 */
II(b, c, d, a, x, 21, 0x4e0811a1); /* 60 */
II(a, b, c, d, x, 6, 0xf7537e82); /* 61 */
II(d, a, b, c, x, 10, 0xbd3af235); /* 62 */
II(c, d, a, b, x, 15, 0x2ad7d2bb); /* 63 */
II(b, c, d, a, x, 21, 0xeb86d391); /* 64 */
state += a;
state += b;
state += c;
state += d;
}

int main(int argc, char* argv[])
{
int i;
unsigned char decrypt;
MD5_CTX md5;
MD5Init(&md5);
MD5Update(&md5, encrypt, strlen((char*) encrypt));
MD5Final(&md5, decrypt);
printf("Before encryption:%s\n After encryption:", encrypt);
for(i = 0; i < 16; i++)
{
printf("%02x", decrypt[i]);
}

getchar();

return 0;
}```

Topics: C Algorithm cryptology