mcrypt_create_iv
(PHP 4, PHP 5)
mcrypt_create_iv — Create an initialization vector (IV) from a random source
Description
mcrypt_create_iv() is used to create an IV.
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.
Замечание: 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.
Пример #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.
- PHP Руководство
- Функции по категориям
- Индекс функций
- Справочник функций
- Криптографические расширения
- Mcrypt
- mcrypt_cbc
- mcrypt_cfb
- mcrypt_create_iv
- mcrypt_decrypt
- mcrypt_ecb
- mcrypt_enc_get_algorithms_name
- mcrypt_enc_get_block_size
- mcrypt_enc_get_iv_size
- mcrypt_enc_get_key_size
- mcrypt_enc_get_modes_name
- mcrypt_enc_get_supported_key_sizes
- mcrypt_enc_is_block_algorithm_mode
- mcrypt_enc_is_block_algorithm
- mcrypt_enc_is_block_mode
- mcrypt_enc_self_test
- mcrypt_encrypt
- mcrypt_generic_deinit
- mcrypt_generic_end
- mcrypt_generic_init
- mcrypt_generic
- mcrypt_get_block_size
- mcrypt_get_cipher_name
- mcrypt_get_iv_size
- mcrypt_get_key_size
- mcrypt_list_algorithms
- mcrypt_list_modes
- mcrypt_module_close
- mcrypt_module_get_algo_block_size
- mcrypt_module_get_algo_key_size
- mcrypt_module_get_supported_key_sizes
- mcrypt_module_is_block_algorithm_mode
- mcrypt_module_is_block_algorithm
- mcrypt_module_is_block_mode
- mcrypt_module_open
- mcrypt_module_self_test
- mcrypt_ofb
- mdecrypt_generic
Коментарии
>First, the IV should be random and variable. The whole >point of it is to ensure that the same plaintext does not >encrypt to the same ciphertext every time. You most >certainly do lose security if the IV is constant or public.
Wrong, Wrong WRONG! The initialization vector is ALLOWED to be PUBLIC! It is generally sent along with the ciphertext, UNENCRYPTED.
>The ciphertext should be E(IV | plaintext, key)
Wrong again! The initialization vector is NOT prepended to the plaintext before encryption. The IV is used to seed the feedback system! (which is why you don't need one in ECB mode - there is no feedback)
>Second, the IV should not be part of the decryption >parameters at all. You should be able to decrypt the cipher >text, throw away the initial vector at the front w/o even >reading it, and have your plaintext:
Nope. You need to seed the feedback mechanism during decryption to the SAME state as it was seeded during encryption. This means using the SAME IV!
In relation to all of the crypto "advice" seen here, my suggestion is that you ignore most of it. Some of it is good, some of it is bad, but most of it skips the critical issues.
I had hoped to write out a nice long explanation, but PHP's commenting system tells me my essay is too long. Instead I will say this:
You should use CBC, with a randomly chosen IV that is unique per key, and you should transmit that IV in the clear along with your ciphertext. You should also perform an authenticity check of that entire data blob, using something like HMAC-SHA256, with another independent key.
Here's the full-text of what I was going to write: http://pastebin.com/sN6buivY
If you're interested in this stuff, or just want more information, check out the Wikipedia articles around block cipher modes, block ciphers, HMAC, etc.
I also suggest reading Practical Cryptography by Bruce Schneier, as well as Cryptography Engineering by Niels Ferguson, both of which are very easy-to-digest books on practical cryptography.