CascadiaPHP 2024


(PHP 4, PHP 5, PHP 7, PHP 8)

cryptOne-way string hashing


This function is not (yet) binary safe!


crypt(#[\SensitiveParameter] string $string, string $salt): string

crypt() will return a hashed string using the standard Unix DES-based algorithm or alternative algorithms. password_verify() is compatible with crypt(). Therefore, password hashes created by crypt() can be used with password_verify().

Prior to PHP 8.0.0, the salt parameter was optional. However, crypt() creates a weak hash without the salt, and raises 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.

The hash type is triggered by the salt argument. 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 string, so longer strings that start with the same eight characters will generate the same result (when the same salt is used).

The following hash types are supported:

  • 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 characters of iteration count and 4 characters of salt. Each of these 4-character strings encode 24 bits, 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 algorithm and must be in range 04-31, values outside this range will cause crypt() to fail. "$2x$" hashes are potentially weak; "$2a$" hashes are compatible and mitigate this weakness. For new hashes, "$2y$" should be used.
  • 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.



The string to be hashed.


Using the CRYPT_BLOWFISH algorithm, will result in the string parameter being truncated to a maximum length of 72 bytes.


A 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.


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 provides hash_equals() for this purpose.


Version Description
8.0.0 The salt is no longer optional.


Example #1 crypt() examples

= 'rasmuslerdorf';
$hashed_password = '$6$rounds=1000000$NJy4rIPjpOaU$0ACEYGg/aKCY3v8O8AfyiO7CTfZQ8/W231Qfh2tRLmfdvFD6XfHk12u6hMr9cYIA4hnpjLNSTRtUwYr9km9Ij/';

// Validate an existing crypt() hash in a way that is compatible with non-PHP software.
if (hash_equals($hashed_password, crypt($user_input, $hashed_password))) {
"Password verified!";


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
  • The Unix man page for your crypt function for more information

add a note

User Contributed Notes 8 notes

bob dot orr at mailinator dot com
9 years ago
The #2 comment on this comments page (as of Feb 2015) is 9 years old and recommends phpass. I have independently security audited this product and, while it continues to be recommended for password security, it is actually insecure and should NOT be used. It hasn't seen any updates in years (still at v0.3) and there are more recent alternatives such as using the newer built-in PHP password_hash() function that are much better. Everyone, please take a few moments to confirm what I'm saying is accurate (i.e. review the phpass code for yourself) and then click the down arrow to sink the phpass comment to the bottom. You'll be increasing security across the Internet by doing so.

For those who want details: md5() with microtime() are a fallback position within the source code of phpass. Instead of terminating, it continues to execute code. The author's intentions of trying to work everywhere are admirable but, when it comes to application security, that stance actually backfires. The only correct answer in a security context is to terminate the application rather than fallback to a weak position that can potentially be exploited (usually by forcing that weaker position to happen).
Marten Jacobs
10 years ago
As I understand it, blowfish is generally seen a secure hashing algorithm, even for enterprise use (correct me if I'm wrong). Because of this, I created functions to create and check secure password hashes using this algorithm, and using the (also deemed cryptographically secure) openssl_random_pseudo_bytes function to generate the salt.

* Generate a secure hash for a given password. The cost is passed
* to the blowfish algorithm. Check the PHP manual page for crypt to
* find more information about this setting.
function generate_hash($password, $cost=11){
/* To generate the salt, first generate enough random bytes. Because
* base64 returns one character for each 6 bits, the we should generate
* at least 22*6/8=16.5 bytes, so we generate 17. Then we get the first
* 22 base64 characters
/* As blowfish takes a salt with the alphabet ./A-Za-z0-9 we have to
* replace any '+' in the base64 string with '.'. We don't have to do
* anything about the '=', as this only occurs when the b64 string is
* padded, which is always after the first 22 characters.
/* Next, create a string that will be passed to crypt, containing all
* of the settings, separated by dollar signs
"2y", //select the most secure version of blowfish (>=PHP 5.3.7)
str_pad($cost,2,"0",STR_PAD_LEFT), //add the cost in two digits
$salt //add the salt

//now do the actual hashing
return crypt($password,$param);

* Check the password against a hash generated by the generate_hash
* function.
function validate_pw($password, $hash){
/* Regenerating the with an available hash as the options parameter should
* produce the same hash if the same password is passed.
return crypt($password, $hash)==$hash;
kaminski at istori dot com
13 years ago
Here is an expression to generate pseudorandom salt for the CRYPT_BLOWFISH hash type:

<?php $salt = substr(str_replace('+', '.', base64_encode(pack('N4', mt_rand(), mt_rand(), mt_rand(), mt_rand()))), 0, 22); ?>

It is intended for use on systems where mt_getrandmax() == 2147483647.

The salt created will be 128 bits in length, padded to 132 bits and then expressed in 22 base64 characters. (CRYPT_BLOWFISH only uses 128 bits for the salt, even though there are 132 bits in 22 base64 characters. If you examine the CRYPT_BLOWFISH input and output, you can see that it ignores the last four bits on input, and sets them to zero on output.)

Note that the high-order bits of the four 32-bit dwords returned by mt_rand() will always be zero (since mt_getrandmax == 2^31), so only 124 of the 128 bits will be pseudorandom. I found that acceptable for my application.
steve at tobtu dot com
11 years ago
To generate salt use mcrypt_create_iv() not mt_rand() because no matter how many times you call mt_rand() it will only have at most 32 bits of entropy. Which you will start seeing salt collisions after about 2^16 users. mt_rand() is seeded poorly so it should happen sooner.

For bcrypt this will actually generate a 128 bit salt:
<?php $salt = strtr(base64_encode(mcrypt_create_iv(16, MCRYPT_DEV_URANDOM)), '+', '.'); ?>

*** Bike shed ***
The last character in the 22 character salt is 2 bits.
base64_encode() will have these four character "AQgw"
bcrypt will have these four character ".Oeu"

You don't need to do a full translate because they "round" to different characters:
echo crypt('', '$2y$05$.....................A') . "\n";
echo crypt('', '$2y$05$.....................Q') . "\n";
echo crypt('', '$2y$05$.....................g') . "\n";
echo crypt('', '$2y$05$.....................w') . "\n";

jette at nerdgirl dot dk
11 years ago
The crypt() function cant handle plus signs correctly. So if for example you are using crypt in a login function, use urlencode on the password first to make sure that the login procedure can handle any character:

= '12+#æ345';
$pass = urlencode($user_input));
$pass_crypt = crypt($pass);

if (
$pass_crypt == crypt($pass, $pass_crypt)) {
"Success! Valid password";
} else {
"Invalid password";
6 years ago
steve at tobtu dot com was right 4 years ago, but now mcrypt_create_iv() (and bcrypt in general) is deprecated!

Use random_bytes() instead:

= base64_encode(random_bytes(16));
6 years ago
While the documentation says that crypt will fail for DES if the salt is invalid, this turns out to not be the case.

The crypt function will accept any string of two characters or more for DES as long as it doesn't match the pattern for any other hashing schema. The remaining characters will be ignored.
ian+php dot net at eiloart dot ocm
10 years ago
If you're stuck with CRYPT_EXT_DES, then you'll want to pick a number of iterations: the 2nd-5th characters of the "salt".

My experimentation suggests that the 5th character is the most significant. A '.' is a zero and 'Z' is the highest value. Using all dots will create an error: all passwords will be encrypted to the same value.

Here are some encryption timings (in seconds) that I obtained, with five different iteration counts over the same salt, and the same password, on a quad core 2.66GHz Intel Xeon machine.

_1111 time: 0.15666794776917
_J9.Z time: 1.8860530853271
_J9.. time: 0.00015401840209961
_...Z time: 1.9095730781555
_ZZZZ time: 1.9124970436096
_...A time: 0.61211705207825

I think a half a second is reasonable for an application, but for the back end authentication? I'm not so sure: there's a significant risk of overloading the back end if we're getting lots of authentication requests.
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