mcrypt_create_iv
(PHP 4, PHP 5)
mcrypt_create_iv — Create an initialization vector (IV) from a random source
Description
string mcrypt_create_iv
( int $size
[, int $source
] )
Parameter
size
determines the size of the IV,
parameter source
(defaults to random value)
specifies the source of the IV.
The source can be MCRYPT_RAND (system random
number generator), MCRYPT_DEV_RANDOM (read
data from /dev/random) and
MCRYPT_DEV_URANDOM (read data from
/dev/urandom). MCRYPT_RAND
is the only one supported on Windows because Windows (of course)
doesn't have /dev/random or
/dev/urandom.
Note:
When using MCRYPT_RAND, remember to call
srand() before
mcrypt_create_iv() to initialize the random
number generator; it is not seeded automatically like
rand() is.
Example #1 mcrypt_create_iv() example
<?php
$size = mcrypt_get_iv_size(MCRYPT_CAST_256, MCRYPT_MODE_CFB);
$iv = mcrypt_create_iv($size, MCRYPT_DEV_RANDOM);
?>
The IV is only meant to give an alternative seed to the encryption
routines. This IV does not need to be secret at all, though it can be
desirable. You even can send it along with your ciphertext without
losing security.
More information can be found at » http://www.ciphersbyritter.com/GLOSSARY.HTM#IV, » http://fn2.freenet.edmonton.ab.ca/~jsavard/crypto/co0409.htm and in chapter 9.3 of
Applied Cryptography by Schneier (ISBN 0-471-11709-9) for a discussion of
this topic.
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