One-way string hashing

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String Functions
PHP Manual

# crypt

(PHP 4, PHP 5)

crypt — One-way string hashing

### Description

string crypt ( string `$str` [, string `$salt` ] )

crypt() will return a hashed string using the standard Unix DES-based algorithm or alternative algorithms that may be available on the system.

The `salt` parameter is optional. However, crypt() creates a weak password without the `salt`. PHP 5.6 or later raise an E_NOTICE error without it. Make sure to specify a strong enough salt for better security.

password_hash() uses a strong hash, generates a strong salt, and applies proper rounds automatically. password_hash() is a simple crypt() wrapper and compatible with existing password hashes. Use of password_hash() is encouraged.

Some operating systems support more than one type of hash. In fact, sometimes the standard DES-based algorithm is replaced by an MD5-based algorithm. The hash type is triggered by the salt argument. Prior to 5.3, PHP would determine the available algorithms at install-time based on the system's crypt(). If no salt is provided, PHP will auto-generate either a standard two character (DES) salt, or a twelve character (MD5), depending on the availability of MD5 crypt(). PHP sets a constant named `CRYPT_SALT_LENGTH` which indicates the longest valid salt allowed by the available hashes.

The standard DES-based crypt() returns the salt as the first two characters of the output. It also only uses the first eight characters of `str`, so longer strings that start with the same eight characters will generate the same result (when the same salt is used).

On systems where the crypt() function supports multiple hash types, the following constants are set to 0 or 1 depending on whether the given type is available:

- `CRYPT_STD_DES` - Standard DES-based hash with a two character salt from the alphabet "./0-9A-Za-z". Using invalid characters in the salt will cause crypt() to fail.
- `CRYPT_EXT_DES` - Extended DES-based hash. The "salt" is a 9-character string consisting of an underscore followed by 4 bytes of iteration count and 4 bytes of salt. These are encoded as printable characters, 6 bits per character, least significant character first. The values 0 to 63 are encoded as "./0-9A-Za-z". Using invalid characters in the salt will cause crypt() to fail.
- `CRYPT_MD5` - MD5 hashing with a twelve character salt starting with $1$
- `CRYPT_BLOWFISH` - Blowfish hashing with a salt as follows: "$2a$", "$2x$" or "$2y$", a two digit cost parameter, "$", and 22 characters from the alphabet "./0-9A-Za-z". Using characters outside of this range in the salt will cause crypt() to return a zero-length string. The two digit cost parameter is the base-2 logarithm of the iteration count for the underlying Blowfish-based hashing algorithmeter and must be in range 04-31, values outside this range will cause crypt() to fail. Versions of PHP before 5.3.7 only support "$2a$" as the salt prefix: PHP 5.3.7 introduced the new prefixes to fix a security weakness in the Blowfish implementation. Please refer to » this document for full details of the security fix, but to summarise, developers targeting only PHP 5.3.7 and later should use "$2y$" in preference to "$2a$".
- `CRYPT_SHA256` - SHA-256 hash with a sixteen character salt prefixed with $5$. If the salt string starts with 'rounds=<N>$', the numeric value of N is used to indicate how many times the hashing loop should be executed, much like the cost parameter on Blowfish. The default number of rounds is 5000, there is a minimum of 1000 and a maximum of 999,999,999. Any selection of N outside this range will be truncated to the nearest limit.
- `CRYPT_SHA512` - SHA-512 hash with a sixteen character salt prefixed with $6$. If the salt string starts with 'rounds=<N>$', the numeric value of N is used to indicate how many times the hashing loop should be executed, much like the cost parameter on Blowfish. The default number of rounds is 5000, there is a minimum of 1000 and a maximum of 999,999,999. Any selection of N outside this range will be truncated to the nearest limit.

> Note:

> As of PHP 5.3.0, PHP contains its own implementation and will use that if the system lacks of support for one or more of the algorithms.

### Parameters

`str`

  The string to be hashed.

  Caution

  Using the `CRYPT_BLOWFISH` algorithm, will result in the `str` parameter being truncated to a maximum length of 72 characters.

`salt`

  An optional salt string to base the hashing on. If not provided, the behaviour is defined by the algorithm implementation and can lead to unexpected results.

### Return Values

Returns the hashed string or a string that is shorter than 13 characters and is guaranteed to differ from the salt on failure.

Warning

When validating passwords, a string comparison function that isn't vulnerable to timing attacks should be used to compare the output of crypt() to the previously known hash. PHP 5.6 onwards provides hash_equals() for this purpose.

### Changelog

Version | Description
--- | ---
5.6.5 | When the failure string "*0" is given as the `salt`, "*1" will now be returned for consistency with other crypt implementations. Prior to this version, PHP 5.6 would incorrectly return a DES hash.
5.6.0 | Raise E_NOTICE security warning if `salt` is omitted.
5.5.21 | When the failure string "*0" is given as the `salt`, "*1" will now be returned for consistency with other crypt implementations. Prior to this version, PHP 5.5 (and earlier branches) would incorrectly return a DES hash.
5.3.7 | Added $2x$ and $2y$ Blowfish modes to deal with potential high-bit attacks.
5.3.2 | Added SHA-256 and SHA-512 crypt based on Ulrich Drepper's » implementation.
5.3.2 | Fixed Blowfish behaviour on invalid rounds to return "failure" string ("*0" or "*1"), instead of falling back to DES.
5.3.0 | PHP now contains its own implementation for the MD5 crypt, Standard DES, Extended DES and the Blowfish algorithms and will use that if the system lacks of support for one or more of the algorithms.

### Examples

Example #1 crypt() examples

`<?php
$hashed_password = crypt('mypassword'); // let the salt be automatically generated
/* You should pass the entire results of crypt() as the salt for comparing a
password, to avoid problems when different hashing algorithms are used. (As
it says above, standard DES-based password hashing uses a 2-character salt,
but MD5-based hashing uses 12.) */
if (hash_equals($hashed_password, crypt($user_input, $hashed_password))) {
echo "Password verified!";
}
?>`

Example #2 Using crypt() with htpasswd

`<?php
// Set the password
$password = 'mypassword';
// Get the hash, letting the salt be automatically generated
$hash = crypt($password);
?>`

Example #3 Using crypt() with different hash types

`<?php
/* These salts are examples only, and should not be used verbatim in your code.
You should generate a distinct, correctly-formatted salt for each password.
*/
if (CRYPT_STD_DES == 1) {
echo 'Standard DES: ' . crypt('rasmuslerdorf', 'rl') . "\n";
}
if (CRYPT_EXT_DES == 1) {
echo 'Extended DES: ' . crypt('rasmuslerdorf', '_J9..rasm') . "\n";
}
if (CRYPT_MD5 == 1) {
echo 'MD5: ' . crypt('rasmuslerdorf', '$1$rasmusle$') . "\n";
}
if (CRYPT_BLOWFISH == 1) {
echo 'Blowfish: ' . crypt('rasmuslerdorf', '$2a$07$usesomesillystringforsalt$') . "\n";
}
if (CRYPT_SHA256 == 1) {
echo 'SHA-256: ' . crypt('rasmuslerdorf', '$5$rounds=5000$usesomesillystringforsalt$') . "\n";
}
if (CRYPT_SHA512 == 1) {
echo 'SHA-512: ' . crypt('rasmuslerdorf', '$6$rounds=5000$usesomesillystringforsalt$') . "\n";
}
?>`

The above example will output something similar to:

    Standard DES: rl.3StKT.4T8M
    Extended DES: _J9..rasmBYk8r9AiWNc
    MD5:          $1$rasmusle$rISCgZzpwk3UhDidwXvin0
    Blowfish:     $2a$07$usesomesillystringfore2uDLvp1Ii2e./U9C8sBjqp8I90dH6hi
    SHA-256:      $5$rounds=5000$usesomesillystri$KqJWpanXZHKq2BOB43TSaYhEWsQ1Lr5QNyPCDH/Tp.6
    SHA-512:      $6$rounds=5000$usesomesillystri$D4IrlXatmP7rx3P3InaxBeoomnAihCKRVQP22JZ6EY47Wc6BkroIuUUBOov1i.S5KPgErtP/EN5mcO.ChWQW21

### Notes

> Note: There is no decrypt function, since crypt() uses a one-way algorithm.

### See Also

- hash_equals() - Timing attack safe string comparison
- password_hash() - Creates a password hash
- md5() - Calculate the md5 hash of a string
- The Mcrypt extension
- The Unix man page for your crypt function for more information

crc32
echo
String Functions
PHP Manual