MD5 script in PHP

Question

As an exercise, I'm attempting to implement MD5 in PHP. I know PHP has a built-in function for this but I would like to read, run and study a working source. I found this script which works great when appropriate sections are uncommented (to enable message padding) and arrays are formatted accordingly (for compatibility for my version of PHP.) However the hash produced - despite being of correct length - is not MD5. For example, the MD5 hash for a zero length string should be:

d41d8cd98f00b204e9800998ecf8427e

but the hash returned by the script for the same is:

85bd946a585af9fd3fb9eda68707c1d8

I've tried other strings but there is no correlation. I've been studying MD5 so have a reasonable knowledge of how it works. I've been interrogating the script but it seems legitimate. I guess I'm giving a shout out to another up for the challenge of discovering why this script isn't returning MD5.

I ran it and it spit out a slew of `undefined index`es so I gave up right there. — AbraCadaver, Oct 18 '16 at 20:47
Hi, unfortunately I'm running an old version - 5.3.28 however, it should still return a genuine MD5 hash. Don't suppose you'd be able to tell me if this script returns a genuine MD5 hash in 7? — James, Oct 18 '16 at 20:50
Have you looked for other open source MD5 projects? Since it's just for education, and it's not exactly a niche algorithm, my guess is you could find something else that works out of the box that you can learn from — alexanderbird, Oct 18 '16 at 21:11
@alexanderbird Yes. There's no lack of information about MD5 and I've read Rivest's original memo. Transposing from another language hasn't solved the mystery of this script either. Surprisingly I've yet to find one source that follows through with 1 example - that would be helpful. — James, Oct 18 '16 at 22:19
Could you clarify: how did you choose this script? And if someone else has a different (working) example as an alternative, would that be enough, or are you determined to understand this specific script? — alexanderbird, Oct 18 '16 at 22:32
@alexanderbird I use PHP and this script seemed to be well structured, easy to read and elegant. Finding another PHP script of MD5 would be useful however, following a deeper understanding, I'd still be keen to discover what appears to be amiss with that script on github. — James, Oct 19 '16 at 00:16

score 2 · Accepted Answer · edited Oct 25 '16 at 14:03

This script wasn't returning genuine MD5 hashes because there lay an issue with the rotation function.

PHP currently doesn't offer native bitwise rotation function(s). Nevertheless, bitwise rotation can be achieved by combining a left shift result and a right shift result. However, during right bit shifts on negative signed integers the sign bit is shifted in for sign-preservation of the operand; this has unwanted consequences and is overcome using a bit mask.

Where $x is input value and $c is number of bits to shift. All values are 32-bit.

Original Code:

`return ($x << $c) | ($x >> (32 - $c));`

New Code:

if($x < 0){
    return ($x << $c) | abs( ((pow(2, $c)) * -1) - ($x >> (32 - $c)));
} else { 
    return ($x << $c) |                            ($x >> (32 - $c)) ;
}

I've deliberately left multiple spaces on line 4 to contrast with line 2 to show formula similarity and presence (or absence) of the bit mask.

Ternary formatted:

 return ($x < 0) ? (($x << $c) | abs( ((pow(2, $c)) * -1) - 
        ($x >> (32 - $c)))) : (($x << $c) | ($x >> (32 - $c)));

MD5 script in PHP

1 Answers1