Can you convert the output of php crypt() to valid MD5?

Question

I have some strings that have been encrypted using the PHP function crypt().

The outputs look something like this:

$1$Vf/.4.1.$CgCo33ebiHVuFhpwS.kMI0
$1$84..vD4.$Ps1PdaLWRoaiWDKCfjLyV1
$1$or1.RY4.$v3xo04v1yfB7JxDj1sC/J/

While I believe crypt() is using the MD5 algorithm, the outputs are not valid MD5 hashes.

Is there a way of converting the produced hashes into valid MD5 hashes (16-byte hex values)?

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Update:

Thanks for the replies so answers so far. I'm pretty sure the crypt function used is using some sort of MD5 algorithm. What I'm looking to do is convert the ouput that I have into an MD5 hash that looks something like the following:

9e107d9d372bb6826bd81d3542a419d6  
e4d909c290d0fb1ca068ffaddf22cbd0  
d41d8cd98f00b204e9800998ecf8427e

(taken from Wikipedia)

Is there a way of converting from the hashes I have to ones like the above?

Answer 1

OK, so maybe this answer is a year late, but I'll give it a shot. In your own answer, you note that crypt() is using the FreeBSD MD5, which also does some interesting transformations on the salt before running the hash, so the result of what I'm about to give you will never quite match up with the results of a call to md5(). That said, the only difference between the output you are seeing and the format you are used to is that the output you are seeing is encoded as follows

$1$        # this indicates that it is MD5
Vf/.4.1.   # these eight characters are the significant portion of the salt
$          # this character is technically part of the salt, but it is ignored
CgCo33eb   # the last 22 characters are the actual hash
iHVuFhpw   # they are base64 encoded (to be printable) using crypt's alphabet
S.kMI0     # floor(22 * 6 / 8) = 16 (the length in bytes of a raw MD5 hash)

To my knowledge, the alphabet used by crypt looks like this:

./0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz

So, with all of this borne in mind, here is how you can convert the 22 character crypt-base64 hash into a 32 character base16 (hexadecimal) hash:

First, you need something to convert the base64 (with custom alphabet) into a raw 16-byte MD5 hash.

define('CRYPT_ALPHA','./0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz');
/**
 * Decodes a base64 string based on the alphabet set in constant CRYPT_ALPHA
 * Uses string functions rather than binary transformations, because said
 * transformations aren't really much faster in PHP
 * @params string $str  The string to decode
 * @return string       The raw output, which may include unprintable characters
 */
function base64_decode_ex($str) {
    // set up the array to feed numerical data using characters as keys
    $alpha = array_flip(str_split(CRYPT_ALPHA));
    // split the input into single-character (6 bit) chunks
    $bitArray = str_split($str);
    $decodedStr = '';
    foreach ($bitArray as &$bits) {
        if ($bits == '$') { // $ indicates the end of the string, to stop processing here
            break;
        }
        if (!isset($alpha[$bits])) { // if we encounter a character not in the alphabet
            return false;            // then break execution, the string is invalid
        }
        // decbin will only return significant digits, so use sprintf to pad to 6 bits
        $decodedStr .= sprintf('%06s', decbin($alpha[$bits]));
    }
    // there can be up to 6 unused bits at the end of a string, so discard them
    $decodedStr = substr($decodedStr, 0, strlen($decodedStr) - (strlen($decodedStr) % 8));
    $byteArray = str_split($decodedStr, 8);
    foreach ($byteArray as &$byte) {
        $byte = chr(bindec($byte));
    }
    return join($byteArray);
}

Now that you've got the raw data, you'll need a method to convert it to the base-16 format you're expecting, which couldn't be easier.

/**
 * Takes an input in base 256 and encodes it to base 16 using the Hex alphabet
 * This function will not be commented.  For more info:
 * @see http://php.net/str-split
 * @see http://php.net/sprintf
 *
 * @param string $str   The value to convert
 * @return string       The base 16 rendering
 */
function base16_encode($str) {
    $byteArray = str_split($str);
    foreach ($byteArray as &$byte) {
        $byte = sprintf('%02x', ord($byte));
    }
    return join($byteArray);
}

Finally, since the output of crypt includes a lot of data we don't need (and, in fact, cannot use) for this process, a short and sweet function to not only tie these two together but to allow for direct input of output from crypt.

/**
 * Takes a 22 byte crypt-base-64 hash and converts it to base 16
 * If the input is longer than 22 chars (e.g., the entire output of crypt()),
 * then this function will strip all but the last 22.  Fails if under 22 chars
 *
 * @param string $hash  The hash to convert
 * @param string        The equivalent base16 hash (therefore a number)
 */
function md5_b64tob16($hash) {
    if (strlen($hash) < 22) {
        return false;
    }
    if (strlen($hash) > 22) {
        $hash = substr($hash,-22);
    }
    return base16_encode(base64_decode_ex($hash));
}

Given these functions, the base16 representation of your three examples are:

3ac3b4145aa7b9387a46dd7c780c1850
6f80dba665e27749ae88f58eaef5fe84
ec5f74086ec3fab34957d3ef0f838154

Of course, it is important to remember that they were always valid, just formatted differently.

Answer 2

$1$ indeed means that this is a MD5 hash, but crypt generates a random salt. This is why you find a different MD5 value. If you include the generated salt you will find the same result.

The salt is base64 encoded in the output, as the hash.

The algorithm used is a system wide parameter. Generally this is MD5, you are right.