Unable to decrypt my encrypted string in a substitution cipher - php

Question

I am developing my own encryption and decryption algorithms. My encryption function is working fine, but my decryption function is not working properly. The following are my functions:
<?php
function encrypt($os,$mode=64,$pattern="tyv9xXa2iUEMhZLD6YlF4BOjg8AdJz0nVHKPRTpb5smfQ1WwroIkqcN3eSG7Cu"){
$ns="";
for($i=0;$i<strlen($os);$i++){
$ns.=$pattern[(strpos($pattern,$os[$i])+$mode)%strlen($pattern)];
}
return $ns;
}
function decrypt($os,$mode=64,$pattern="tyv9xXa2iUEMhZLD6YlF4BOjg8AdJz0nVHKPRTpb5smfQ1WwroIkqcN3eSG7Cu"){
$ns="";
for($i=0;$i<strlen($os);$i++){
$ns.=$pattern[abs(strpos($pattern,$os[$i])-$mode)%strlen($pattern)];
}
return $ns;
}
echo decrypt(encrypt("abcde"));
?>
My expected output is: abcde
But the output returned is: ejUPa

Answer 1

The encryption works in this way:
$new_char_index = ($char_index + $shift) % $alphabet_length;
where the modulo % handles the wrap around so that the new index is still in the alphabet range. This works well for positive numbers, but doesn't work like you would expect it to for negative numbers. Here are some examples:
echo -3 % 7; // -3
echo -11 % 7; // -4
That is why simply changing + for - doesn't work:
$new_char_index = ($char_index - $shift) % $alphabet_length;
This can lead to negative numbers. You can't access arrays with negative numbers in PHP, but you could do that in Python.
The easy fix is to make sure that the resulting index is always a positive numbers. You've tried that with abs(), but the problem is that this doesn't correctly wrap around from the other side of the alphabet, but only removes the sign which is wrong.
An easy fix is adding the alphabet length before the modulo operation in order to get a positive number:
$new_char_index = ($char_index - $shift + $alphabet_length) % $alphabet_length;
This works, because $alphabet_length % $alphabet_length == 0. It wraps to the correct position of the alphabet.
Now you only have to make sure that $shift is already in the alphabet range:
$shift = $shift % $alphabet_length;
One final improvement: you can use the same function for encryption and decryption, because the $enc_shift == -$dec_shift and the last formula should give you work for both.

Answer 2

This is not encryption. This is a variation on a Caeser cipher. Simply put, you should never implement your own encryption (unless you know exactly what you're doing...). This would do for obfuscation and nothing more.
As for the code itself, I suspect its an order of operations issue. Simply replacing a + with a - won't reverse the operator precedence in the encrypt() function. A handy generic string rotation function you could adapt is in the comments of this php documentation page.
If you want encryption there are many good articles about solid encryption; this is my personal opinion of a good starting point.

Answer 3

Here is a solution that works with every $mode and $pattern size.
You have to notice that you can only "encrypt" chars that are contained in $pattern.
<?php
function encrypt($os,$mode=64,$pattern=" tyv9xXa2iUEMhZLD6YlF4BOjg8AdJz0nVHKPRTpb5smfQ1WwroIkqcN3eSG7Cu"){
$patternLength = strlen($pattern);
if($mode < 0) {
$mode = ($patternLength - $mode) % $patternLength;
}
if($mode >= $patternLength) {
$mode %= $patternLength;
}
$ns="";
for($i=0;$i<strlen($os);$i++){
$ns.=$pattern[(strpos($pattern,$os[$i])+$mode)%strlen($pattern)];
}
return $ns;
}
function decrypt($os,$mode=64,$pattern=" tyv9xXa2iUEMhZLD6YlF4BOjg8AdJz0nVHKPRTpb5smfQ1WwroIkqcN3eSG7Cu"){
$patternLength = strlen($pattern);
if($mode < 0) {
$mode = ($patternLength - $mode) % $patternLength;
}
if($mode >= $patternLength) {
$mode %= $patternLength;
}
$ns="";
for($i=0;$i<strlen($os);$i++){
$pos = strpos($pattern,$os[$i]);
if($pos >= $mode ) {
$ns .= $pattern[$pos - $mode];
} else {
// $pos - $mode is negative so we need + sign
$ns .= $pattern[$patternLength + ($pos - $mode)];
}
}
return $ns;
}
To test this, you could do something like that:
$failed = false;
for($mode = -128; $mode <= 128; $mode++) {
// include all possible chars in the test to see if encryption and
// decryption works for all.
$allChars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789 ";
if(decrypt(encrypt($allChars, $mode), $mode) != $allChars) {
echo "mode ".$mode." failed<br>";
$failed = true;
};
}
if(!$failed) {
echo "success";
}

Answer 4

Fixed it:
$ns.=$pattern[abs(strpos($pattern,$os[$i]))-$mode%strlen($pattern)];
In your decrypt ^^.
Just a matter of bracket placing.
Full fixed code:
function encrypt($os,$mode=64,$pattern="tyv9xXa2iUEMhZLD6YlF4BOjg8AdJz0nVHKPRTpb5smfQ1WwroIkqcN3eSG7Cu"){
$ns="";
for($i=0;$i<strlen($os);$i++){
$ns.=$pattern[(strpos($pattern,$os[$i])+$mode)%strlen($pattern)];
}
return $ns;
}
function decrypt($os,$mode=64,$pattern="tyv9xXa2iUEMhZLD6YlF4BOjg8AdJz0nVHKPRTpb5smfQ1WwroIkqcN3eSG7Cu"){
$ns="";
for($i=0;$i<strlen($os);$i++){
$ns.=$pattern[abs(strpos($pattern,$os[$i]))-$mode%strlen($pattern)];
}
return $ns;
}
echo decrypt(encrypt("abcde"));
Hopefully you can understand the difference yourself.
And now to fix the issue with spaces:
$pattern=" tyv9xXa2iUEMhZLD6YlF4BOjg8AdJz0nVHKPRTpb5smfQ1WwroIkqcN3eSG7Cu"
Take note of the space at the beginning of the string.
An additional note due to comments...
Should you wish to change $mode to let's say 62, you would need to shorten $pattern appropriately.
e.g.
function encrypt($os,$mode=62,$pattern=" tyv9xXa2iUEMhZLD6YlF4BOjg8AdJz0nVHKPRTpb5smfQ1WwroIkqcN3eSG7"){
$ns="";
for($i=0;$i<strlen($os);$i++){
$ns.=$pattern[(strpos($pattern,$os[$i])+$mode)%strlen($pattern)];
}
return $ns;
}
function decrypt($os,$mode=62,$pattern=" tyv9xXa2iUEMhZLD6YlF4BOjg8AdJz0nVHKPRTpb5smfQ1WwroIkqcN3eSG7"){
$ns="";
for($i=0;$i<strlen($os);$i++){
$ns.=$pattern[abs(strpos($pattern,$os[$i]))-$mode%strlen($pattern)];
}
return $ns;
}
echo decrypt(encrypt("abcde"));
Works just fine, notice $pattern is now missing the last 2 characters.