PHP mcrypt_encrypt/mcrypt_decrypt issue, returns a different value

Question

I was looking for an answer but could not find it here. Please excuse me if this question was already asked.

I have a simple code encrypting and decrypting a string, strings look the same, but when comparing them using == they do not appear to be the same, so hashes are different as well..

Here is my code:

$oppa = "rompish";
$opp_enc = mcrypt_encrypt(MCRYPT_RIJNDAEL_256, "key", $oppa, MCRYPT_MODE_ECB);
$opp_dec = mcrypt_decrypt(MCRYPT_RIJNDAEL_256, "key", $opp_enc, MCRYPT_MODE_ECB);

echo $oppa."<br />".$opp_dec."<br />";

if ($oppa == $opp_dec) echo "YAY"; else echo "NOPE";

On the page:

rompish rompish NOPE

Please tell me what I am doing wrong.

Thank you!

AES always encrypts things in blocks of 16 bytes. Apparently mcrypt_encrypt pads the string out with zero bytes until it is a multiple of 16. mcrypt_decrypt dutifully decrypts this but lacks the information to remove the padding. And you are fooling yourself because the displayed values look the same even though oppa_dec actually ends with 9 zero bytes. Use a sensible padding scheme instead. — President James K. Polk
– President James K. Polk, Commented Mar 10, 2013 at 2:23
That's an answer, not a comment, GregS (make it so or I'll post it including your name at the end :P) — Maarten Bodewes
– Maarten Bodewes, Commented Mar 10, 2013 at 23:06

Community · Accepted Answer · 2017-05-23 10:27:00Z

2

AES always encrypts things in blocks of 16 bytes. Apparently mcrypt_encrypt pads the string out with zero bytes until it is a multiple of 16. mcrypt_decrypt dutifully decrypts this but lacks the information to remove the padding. And you are fooling yourself because the displayed values look the same even though oppa_dec actually ends with 9 zero bytes. Use a sensible padding scheme instead. – GregS

To remove these null characters, you can use the rtrim function. After running the decrypted output through that it should be equal.

edited May 23, 2017 at 10:27

CommunityBot

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answered Mar 12, 2013 at 2:17

jmc734

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