PHP: mcrypt_decrypt - Manual

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mcrypt_ecb mcrypt_create_iv

Last updated: Fri, 26 Jan 2007

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# mcrypt_decrypt

(PHP 4 >= 4.0.2, PHP 5)

mcrypt_decrypt -- Decrypts crypttext with given parameters

## Description

string mcrypt_decrypt ( string cipher, string key, string data, string mode [, string iv] )

mcrypt_decrypt() decrypts the data and returns the unencrypted data.

`Cipher` is one of the MCRYPT_ciphername constants of the name of the algorithm as string.

`Key` is the key with which the data is encrypted. If it's smaller that the required keysize, it is padded with '`\0`'.

`Data` is 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`'.

`Mode` is one of the MCRYPT_MODE_modename constants of one of "ecb", "cbc", "cfb", "ofb", "nofb" or "stream".

The `IV` parameter is used for the initialisation 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 bytes set to '`\0`'.

add a note User Contributed Notes
mcrypt_decrypt
eddiec at stararcher dot com
13-Jul-2005 06:26
`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
?>`
kooktroop at gmail dot com
09-Jul-2004 03:13
`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>";
?>`
add a note
mcrypt_ecb mcrypt_create_iv

Last updated: Fri, 26 Jan 2007
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