php – crypt function


(PHP 3, PHP 4, PHP 5)
crypt — One-way string encryption (hashing)
string crypt ( string str [, string salt] )

crypt() will return an encrypted string using the standard Unix DES-based encryption algorithm or alternative algorithms that may be available on the system. Arguments are a string to be encrypted and an optional salt string to base the encryption on. See the Unix man page for your crypt function for more information.

If the salt argument is not provided, one will be randomly generated by PHP each time you call this function.

Some operating systems support more than one type of encryption. In fact, sometimes the standard DES-based encryption is replaced by an MD5-based encryption algorithm. The encryption type is triggered by the salt argument. At install time, PHP determines the capabilities of the crypt function and will accept salts for other encryption types. If no salt is provided, PHP will auto-generate a standard two character salt by default, unless the default encryption type on the system is MD5, in which case a random MD5-compatible salt is generated. PHP sets a constant named CRYPT_SALT_LENGTH which tells you whether a regular two character salt applies to your system or the longer twelve character salt is applicable.

If you are using the supplied salt, you should be aware that the salt is generated once. If you are calling this function repeatedly, this may impact both appearance and security.

The standard DES-based encryption 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 encryption types, the following constants are set to 0 or 1 depending on whether the given type is available:


CRYPT_STD_DES – Standard DES-based encryption with a two character salt

CRYPT_EXT_DES – Extended DES-based encryption with a nine character salt

CRYPT_MD5 – MD5 encryption with a twelve character salt starting with $1$

CRYPT_BLOWFISH – Blowfish encryption with a sixteen character salt starting with $2$ or $2a$

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

Example 1. crypt() examples
$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 (crypt($user_input, $password) == $password) {
echo “Password verified!”;


  1. guillaume badin said

    Excuse me but when I execute crypt I can’t use CRYPT_BLOWFISH and CRYPT_EXT_DES because they don’t pass the TEST.
    Do you know how can I install them or just give me a way to do that
    This is very important for a current project

    I’ m under ubuntu-linux feisty-fawn

  2. Tim McCormack said

    I have the same question as the above commenter. Namely, where does blowfish capability come from?

    (I’m on Ubuntu as well.)

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