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(PHP 4 >= 4.0.2, PHP 5)
mcrypt_decrypt — Decrypts crypttext with given parameters
Decrypts the data and returns the unencrypted data.
One of the MCRYPT_ciphername constants, or the name of the algorithm as string.
The key with which the data was encrypted. If it's smaller than the required keysize, it is padded with '\0'.
The data that will be decrypted with the given cipher and mode. If the size of the data is not n * blocksize, the data will be padded with '\0'.
One of the MCRYPT_MODE_modename constants, or one of the following strings: "ecb", "cbc", "cfb", "ofb", "nofb" or "stream".
The iv parameter is used for the initialization in CBC, CFB, OFB modes, and in some algorithms in STREAM mode. If you do not supply an IV, while it is needed for an algorithm, the function issues a warning and uses an IV with all its bytes set to '\0'.
Returns the decrypted data as a string.
To remove PKCS7 padding:
<?php
$decrypted = mdecrypt_generic($td, base64_decode($enc_auth_token));
$dec_s = strlen($decrypted);
$padding = ord($decrypted[$dec_s-1]);
$decrypted = substr($decrypted, 0, -$padding);
?>
kooktroop's example (09-Jul-2004) may be a bit misleading. Using a random IV for encryption, then another random IV for decryption? If you run it, it works!
That's because ECB doesn't use an IV. For modes that do use one you need the same one for encryption and decryption.
For example, Rijndael-256 (aka AES-256) and CFB:
<?php
$text = "The quick brown fox jumps over the lazy dog.";
$key = "haseveryletter";
$iv = mcrypt_create_iv(mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_CFB), MCRYPT_RAND);
$encrypted = mcrypt_encrypt(MCRYPT_RIJNDAEL_256, $key, $text, MCRYPT_MODE_CFB, $iv);
// create a new IV
$newiv = mcrypt_create_iv(mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_CFB), MCRYPT_RAND);
$corrupt = mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $key, $encrypted, MCRYPT_MODE_CFB, $newiv);
echo $corrupt; // [binary junk...]he lazy dog
$good = mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $key, $encrypted, MCRYPT_MODE_CFB, $iv);
echo $good; // The quick brown fox jumps over the lazy dog
?>
Rijndael can sync up on each block so only the first one (here, 256 bits = 32 characters long) will be corrupted. Not necessarily true for other algorithms...
I was wrong about ColdFusion always padding with EOT. By default, ColdFusion uses PKCS#5 padding. See the comments on http://us3.php.net/manual/en/ref.mcrypt.php for pkcs5_(un)pad functions
If you happen to be decrypting something encrypted in ColdFusion, you'll discover that its encrypt function apparently pads the plaintext with ASCII 4, the "end of transmission" character.
Building on eddiec's code, you can remove both nulls and EOTs with this:
<?php
$retval = mcrypt_decrypt( ...etc ...);
$retval = rtrim($retval, "\0\4"); // trim ONLY the nulls and EOTs at the END
?>
Since the returned data seems to be still padded with extra characters, you can get *only* the original data that was encrypted by str_replace()'ing the \x0 characters.
<?php
$decryptedData = str_replace("\x0", '', $encryptedData);
?>
It appears that mcrypt_decrypt pads the *RETURN STRING* with nulls ('\0') to fill out to n * blocksize. For old C-programmers, like myself, it is easy to believe the string ends at the first null. In PHP it does not:
strlen("abc\0\0") returns 5 and *NOT* 3
strcmp("abc", "abc\0\0") returns -2 and *NOT* 0
I learned this lesson painfully when I passed a string returned from mycrypt_decrypt into a NuSoap message, which happily passed the nulls along to the receiver, who couldn't figure out what I was talking about.
My solution was:
<?
$retval = mcrypt_decrypt( ...etc ...);
$retval = rtrim($retval, "\0"); // trim ONLY the nulls at the END
?>
Following on from the mcrypt_encrypt() example:
<?php
$text = "boggles the inivisble monkey will rule the world";
$key = "This is a very secret key";
$iv_size = mcrypt_get_iv_size(MCRYPT_XTEA, MCRYPT_MODE_ECB);
$iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);
echo strlen($text) . "<br>";
$enc = mcrypt_encrypt(MCRYPT_XTEA, $key, $text, MCRYPT_MODE_ECB, $iv);
echo strlen($enc) . "<br>";
$iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);
$key = "This is a very secret key";
$text = "Meet me at 11 o'clock behind the monument.";
echo strlen($text) . "<br>";
$crypttext = mcrypt_decrypt(MCRYPT_XTEA, $key, $enc, MCRYPT_MODE_ECB, $iv);
echo "$crypttext<br>";
?>
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