CODEWITHSUNDEEP
phpbittorrentinfo-hash
miyuru
miyuru

Reputation: 1201

How to convert bit torrent info hash from Base 32 into Base 16

I have a Base32 info hash. e.g. IXE2K3JMCPUZWTW3YQZZOIB5XD6KZIEQ, and I need to convert it to base16.

How can I do it with PHP?

My code looks like this:

$hash32=strtolower($hash32);
echo $hash32; // shows - IXE2K3JMCPUZWTW3YQZZOIB5XD6KZIEQ
$hash32=sha1($hash32);
$hash16=base_convert($hash32, 32, 16);
echo "</br>";
echo $hash16 // shows - 3ee5e7325a282c56fe2011125e0492f6ffbcd467

In my code the 16 based info hash is not valid..

The valid Info hash is 45C9A56D2C13E99B4EDBC43397203DB8FCACA090

How can I get a valid info hash?

Thanks

Upvotes: 3

Views: 1547

Answers (1)

Romain
Romain

Reputation: 6430

I give you an answer with voluntarily omitting gmp_strval(gmp_init(strtoupper($hash32), 32), 16); which only works with GMP installed in your server.

function convBase($numberInput, $fromBaseInput, $toBaseInput)
{
    if ($fromBaseInput==$toBaseInput) return $numberInput;
    $fromBase = str_split($fromBaseInput,1);
    $toBase = str_split($toBaseInput,1);
    $number = str_split($numberInput,1);
    $fromLen=strlen($fromBaseInput);
    $toLen=strlen($toBaseInput);
    $numberLen=strlen($numberInput);
    $retval='';
    if ($toBaseInput == '0123456789')
    {
        $retval=0;
        for ($i = 1;$i <= $numberLen; $i++)
            $retval = bcadd($retval, bcmul(array_search($number[$i-1], $fromBase),bcpow($fromLen,$numberLen-$i)));
        return $retval;
    }
    if ($fromBaseInput != '0123456789')
        $base10=convBase($numberInput, $fromBaseInput, '0123456789');
    else
        $base10 = $numberInput;
    if ($base10<strlen($toBaseInput))
        return $toBase[$base10];
    while($base10 != '0')
    {
        $retval = $toBase[bcmod($base10,$toLen)].$retval;
        $base10 = bcdiv($base10,$toLen,0);
    }
    return $retval;
}

This function, found here, "converts an arbitrarily large number from any base to any base". You only need to convert from base 32 to base 16, hence:

base 32 alphabet is: ABCDEFGHIJKLMNOPQRSTUVWXYZ234567

$hash32='IXE2K3JMCPUZWTW3YQZZOIB5XD6KZIEQ';
$hash16=convBase($hash32, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ234567', '0123456789ABCDEF');
//$hash16='45C9A56D2C13E99B4EDBC43397203DB8FCACA090'

The problem here was that base_convert mishandle large numbers.

Upvotes: 5

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