Question

I am trying to implement a basic MD5 algorithm in c++. I am mainly following the wiki page’s algorithm which seems pretty straight forward.
However, I’ve really been battling for some time now. I am not getting the required hash value for an empty string. I have even gone so far as to see what is happening at each of the 64 rounds but to no avail. This is what I have done so far. The code is not optimized or written particularly well but I have just hacked it together so see if I was on the right track.

#include "md5.h"
#include <iostream>
#include <iomanip>
#include <windows.h>

using namespace std;

unsigned int rotateL(unsigned int x, int n)
{
    return ((x) << n) | ((x) >> (32 - n));

}

int main()
{
//The required hash value using existing md5 algorithm

MD5 md5 ;
cout << "empty str is:" << endl;
puts( md5.digestString( "" ) ) ;


//Initialize Vars
unsigned int a = 0x67452301;
unsigned int b = 0xefcdab89;
unsigned int c = 0x98badcfe;
unsigned int d = 0x10325476;

//Make a copy of above
unsigned int A = a;
unsigned int B = b;
unsigned int C = c;
unsigned int D = d;


//Assuming an empty string ""
//The message would be appended with 1 and then padded with 0's
//so m = 10000....until 512 bits
//is this true?
unsigned int m[16] = {0x80000000, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
    , 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00};

int r[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 int k[64] = { 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 };


// Do an ititial Round just for testing...
unsigned int f = 0;
cout << "Doing round " << dec << 1 << endl;
f = (b & c) | ((~b) & d);   //round 1
cout << "f is " << hex << f << endl;
unsigned int temp = d;
d = c;
c = b;
int i = 0;
int g = 0;
cout << "k is " << hex << k[0] << endl;
cout << "r is " << dec << r[0] << endl;
b = b^(rotateL( a ^ f ^ k[0] ^ m[0] ,r[0]));
a = temp;

cout << "a is " << hex << a << endl;
cout << "b is " << hex << b << endl;
cout << "c is " << hex << c << endl;
cout << "d is " << hex << d << endl;

for (int i=1;i<16;i++){
    cout << "Doing round " << dec << i+1 << endl;
    f = (b & c) | ((~b) & d);   //round 1
    g = i;
    cout << "f is " << hex << f << endl;
    //res = d & b | ~d & c; //round 2

    unsigned int temp = d;
    d = c;
    c = b;
    cout << "k is " << hex << k[i] << endl;
    cout << "r is " << dec << r[i] << endl;
    b = b^(rotateL( a ^ f ^ k[i] ^ m[g] ,r[i]));
    a = temp;

    cout << "a is " << hex << a << endl;
    cout << "b is " << hex << b << endl;
    cout << "c is " << hex << c << endl;
    cout << "d is " << hex << d << endl;
    system("PAUSE");
}

for (int i=16;i<32;i++){
    cout << "Doing round " << dec << i+1 << endl;
    f = d & b | ~d & c; //round 2
    cout << "f is " << hex << f << endl;

    g = ((5*i)+1)%16;

    unsigned int temp = d;
    d = c;
    c = b;
    cout << "k is " << hex << k[i] << endl;
    cout << "r is " << dec << r[i] << endl;
    b = b^(rotateL( a ^ f ^ k[i] ^ m[g] ,r[i]));
    a = temp;

    cout << "a is " << hex << a << endl;
    cout << "b is " << hex << b << endl;
    cout << "c is " << hex << c << endl;
    cout << "d is " << hex << d << endl;
    system("PAUSE");
}


for (int i=32;i<48;i++){
    cout << "Doing round " << dec << i+1 << endl;
    f = b ^ c ^ d;
    cout << "f is " << hex << f << endl;

    g = ((3*i)+5)%16;

    unsigned int temp = d;
    d = c;
    c = b;
    cout << "k is " << hex << k[i] << endl;
    cout << "r is " << dec << r[i] << endl;
    b = b^(rotateL( a ^ f ^ k[i] ^ m[g] ,r[i]));
    a = temp;

    cout << "a is " << hex << a << endl;
    cout << "b is " << hex << b << endl;
    cout << "c is " << hex << c << endl;
    cout << "d is " << hex << d << endl;
    system("PAUSE");
}

for (int i=48;i<64;i++){
    cout << "Doing round " << dec << i+1 << endl;
    f = c ^ (b | ~ d);
    cout << "f is " << hex << f << endl;

    g = ((7*i))%16;

    unsigned int temp = d;
    d = c;
    c = b;
    cout << "k is " << hex << k[i] << endl;
    cout << "r is " << dec << r[i] << endl;
    b = b^(rotateL( a ^ f ^ k[i] ^ m[g] ,r[i]));
    a = temp;

    cout << "a is " << hex << a << endl;
    cout << "b is " << hex << b << endl;
    cout << "c is " << hex << c << endl;
    cout << "d is " << hex << d << endl;
    system("PAUSE");
}

A = A ^ a;
B = B ^ b;
C = C ^ c;
D = D ^ d;

cout << "Finally: " << endl;
cout << "A is " << hex << A << endl;
cout << "B is " << hex << B << endl;
cout << "D is " << hex << C << endl;
cout << "D is " << hex << D << endl;

//The above would now be appended but they 
//do not equal to the required hash! 
//So clearly it is wrong??


return 0;

}

I have a feeling that perhaps I am doing something wrong at the padding stage but I can’t quite figure it out. Any help would be greatly appreciated!

Answer 1

This might be one of your issues:

for (int i=1;i<16;i++){

I think “i” needs to be initialized to 0, instead of 1. Because the wikipedia code shows it like this:

//Main loop:
    for i from 0 to 63
        if 0 ≤ i ≤ 15 then
            f := (b and c) or ((not b) and d)
            g := i

Also, when you update “b” on every round, you implement it as follows:

b = b^(rotateL( a ^ f ^ k[i] ^ m[g] ,r[i]));

But the reference code on the wikipedia page implements it like this:

b := b + leftrotate((a + f + k[i] + w[g]) , r[i])

Notice that the reference code just adds all those values together, you are using the xor (^) operator.

I suspect there are other issues, so you may need to double check your implementation with the reference code.