mcrypt_encrypt garbled text - php

Question

First, excuse the question, it might be a simple problem, but I have troubles understanding the encryption methods..
I'm using the following functions to encrypt / decrypt:
private function encodemc($value,$skey){
if(!$value){return false;}
$skey = substr($skey, 2, 4);
$text = $value;
$iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_ECB);
$iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);
$crypttext = mcrypt_encrypt(MCRYPT_RIJNDAEL_256, $skey, $text, MCRYPT_MODE_ECB, $iv);
return trim($this->safe_encode($crypttext)); // safe_encode adds another encoding using `base64_encode`
}
private function decodemc($value,$skey){
if(!$value){return false;}
$skey = substr($skey, 2, 4);
$crypttext = $this->safe_decode($value);
$iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_ECB);
$iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);
$decrypttext = mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $skey, $crypttext, MCRYPT_MODE_ECB, $iv);
return trim($decrypttext);
}
The $key looks like this: 570c45546dwq45gjk191.
I pass the value to be encrypted to the first function, then I save it to the db, and then I retrieve it from the db, I decrypt it and show it as html text.
The problem is that some text does not get decrypted/encrypted right, and it displays in the html page as if it was in the wrong text encoding.
The weird part is that out of 10 items, only 2 or 3 are garbled, depending on the key.
In addition, sometimes only a portion of the string is garbled.
I've found out that what causes the garbling are some random letters. For instance, when using the above key the letter S breaks the code and the text gets garbled.
So then I've applied substr($skey, 2, 4); to the key to see if anything changed. Turns out that if I change the key the characters that break the code are different.
But even with a key of lenght == 1 the problem persists.
Any idea on what's the problem?
EDIT:
Here the rest of the code.
private function safe_encode($string) {
$data = base64_encode($string);
$data = str_replace(array('+','/','='),array('-','_',''),$data);
return $data;
}
private function safe_decode($string) {
$data = str_replace(array('-','_'),array('+','/'),$string);
$mod4 = strlen($data) % 4;
if ($mod4) {
$data .= substr('====', $mod4);
}
return stripslashes(base64_decode($data));
}

Answer 1

That seems like a very weird issue.
I do not have a definite answer for you, but here are a couple of things you can try.
Encrypt/Decrypt without going to the database. If the issues goes away, then there is a problem with the database handling the characters output though the encryption function.
Trim the value before you encrypt it. If the issues goes away, then the trim after encryption is wrong.
Remove the safe_encode and safe_decode code. If the issue goes away, then these functions are adding/removing important things.
It is important that you do all the above in one go, as there might be multiple issues. If the above steps remove the issues, then reintroduce them one at a time to identify which is causing the problem.