Invert function: base 38 conversion
Question
Can you tell me how the invert function for the following PHP function is?
<?php
function id2secure($old_number) {
$alphabet_en = '1357902468acegikmoqsuwybdfhjlnprtvxz-_';
$new_number = '';
while ($old_number > 0) {
$rest = $old_number%38;
if ($rest >= 38) { return FALSE; }
$new_number .= $alphabet_en[$rest];
$old_number = floor($old_number/38);
}
$new_number = strrev($new_number);
return $new_number;
}
echo id2secure(172293);
?>
Thank you very much in advance!
Solution
This is secure :) Took me a few minutes to crack it. Here you go,
function secure2id($new_number) {
$alphabet_en = '1357902468acegikmoqsuwybdfhjlnprtvxz';
$old_number = 0;
$new_number = strrev($new_number);
$len=strlen($new_number);
$n=0;
$base=1;
while($n<$len){
$c=$new_number[$n];
$index = strpos($alphabet_en, $c);
if ($index === false)
break;
$old_number += $base * $index;
$base *= 38;
$n++;
}
return $old_number;
}
OTHER TIPS
Haven't tested this code, but it might work:
<?php function secure2id($sr)
{
$s = strrev($sr);
$alpha = '1357902468acegikmoqsuwybdfhjlnprtvxz';
$alpha2num = array();
$n = strlen($alpha);
for($i = 0; $i < $n; $i++)
{
$alpha2num[$alpha[$i]] = $i;
}
$rez = 0;
$n = strlen($s);
$b = 1;
for($i = 0; $i < $n; $i++)
{
$rez += $b * $alpha2num[$s[$i]];
$b *= 38;
}
return $rez;
} ?>
`
Are you asking how to convert base 38 to base 10? The numerical algorithm is this:
- Let N be the new number in base 10. Set N to zero to start with.
- Let X be the original number in base 38.
- Multiply N by 38.
- Let D be the most significant (leftmost) digit of X.
- Let T be the value of D in base 10.
- Add T to N.
- Remove D from X (so the number X is now 1 digit shorter).
- If X is empty, goto 10.
- Goto 3.
- Return N, which is now the fully converted base 10 number.
Now that you understand the math, it should be fairly straightforward to convert these steps to a PHP function.
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