Test of being false does not work with unserialize - php

Question

I want to decrypt a string when submitting form after entering an input string to be decrypted :
static function decrypt($data) {
$key = 'AVtr34EN';
$td = mcrypt_module_open(MCRYPT_DES,"",MCRYPT_MODE_ECB,"");
$iv = mcrypt_create_iv(mcrypt_enc_get_iv_size($td), MCRYPT_RAND);
mcrypt_generic_init($td,$key,$iv);
$data = mdecrypt_generic($td, base64_decode($data));
mcrypt_generic_deinit($td);
if (substr($data,0,1) != '!')
return false;
$data = substr($data,1,strlen($data)-1);
return unserialize($data);
}
When I call this function provided with an argument value of :
zeL1smxhfIEMWCews6vb1Y8yFa5tHbB3b489X0R3QtA=
then I get nothing !
I made a test if ( utils::decrypt($data_) === false ) but the program does not enter in the block of the if !
So how to treat this case ?

Answer 1

You give the string zeL1smxhfIEMWCews6vb1Y8yFa5tHbB3b489X0R3QtA= and using this andsandboxing the PHP it works exactly as expected:
static function decrypt($data) {
$key = 'AVtr34EN';
$td = mcrypt_module_open(MCRYPT_DES,"",MCRYPT_MODE_ECB,"");
$iv = mcrypt_create_iv(mcrypt_enc_get_iv_size($td), MCRYPT_RAND);
mcrypt_generic_init($td,$key,$iv);
$data = mdecrypt_generic($td, base64_decode($data));
mcrypt_generic_deinit($td);
//$data = "!s:20:"2�Y:H���6|1|2|1|2|4";���"
if (substr($data,0,1) != '!')
return false;
// If will never be reached in this case,
// as data first character does equal "!"
$data = substr($data,1,strlen($data)-1);
return unserialize($data); //"2�Y:H���6|1|2|1|2|4"
}
Please clarify your question. changing the inpt string to "FDHFDHeh" (or whatever) correctly returns false showing the IF statement is fired.
NOTE:
You are using an input $data variable but then also generating a new $data variable in the function, this is bad practise to have two different things with the same name, and judging by comments this is causing me, if not you, confusion.
Also you can simplify the substr as simply $data = substr($data,1); as because you're starting at character 1, the string length with be strlen($data)-1.
Recommended you rewrite:
static function decrypt($data) {
$key = 'AVtr34EN';
$td = mcrypt_module_open(MCRYPT_DES,"",MCRYPT_MODE_ECB,"");
$iv = mcrypt_create_iv(mcrypt_enc_get_iv_size($td), MCRYPT_RAND);
mcrypt_generic_init($td,$key,$iv);
$decryptData = mdecrypt_generic($td, base64_decode($data));
mcrypt_generic_deinit($td);
if (substr($decryptData,0,1) !== '!'){
//re: comments below:
//using !== makes it a strict type comparison
return false;
}
$outputData = substr($decryptData,1);
return unserialize($data);
}
It may also be worthwhile using the mb_string functions.
You can also rewrite the string comparison using arrays (as a string is an array of letters) so, replace if(substr(...)) with:
if ($decryptData[0] !== '!')
return false;
Which means "IF the first character of variable $decryptData is not "!" then return false.