# `crypt` — Function to check Unix passwords

Source code: Lib/crypt.py

Deprecated since version 3.11, will be removed in version 3.13: The `crypt` module is deprecated (see PEP 594 for details and alternatives). The `hashlib` module is a potential replacement for certain use cases. The passlib package can replace all use cases of this module.

This module implements an interface to the crypt(3) routine, which is a one-way hash function based upon a modified DES algorithm; see the Unix man page for further details. Possible uses include storing hashed passwords so you can check passwords without storing the actual password, or attempting to crack Unix passwords with a dictionary.

Notice that the behavior of this module depends on the actual implementation of the crypt(3) routine in the running system. Therefore, any extensions available on the current implementation will also be available on this module.

Availability: Unix, not VxWorks.

Availability: not Emscripten, not WASI.

This module does not work or is not available on WebAssembly platforms `wasm32-emscripten` and `wasm32-wasi`. See WebAssembly platforms for more information.

## Hashing Methods

Added in version 3.3.

The `crypt` module defines the list of hashing methods (not all methods are available on all platforms):

crypt.METHOD_SHA512

  A Modular Crypt Format method with 16 character salt and 86 character hash based on the SHA-512 hash function. This is the strongest method.

crypt.METHOD_SHA256

  Another Modular Crypt Format method with 16 character salt and 43 character hash based on the SHA-256 hash function.

crypt.METHOD_BLOWFISH

  Another Modular Crypt Format method with 22 character salt and 31 character hash based on the Blowfish cipher.

  Added in version 3.7.

crypt.METHOD_MD5

  Another Modular Crypt Format method with 8 character salt and 22 character hash based on the MD5 hash function.

crypt.METHOD_CRYPT

  The traditional method with a 2 character salt and 13 characters of hash. This is the weakest method.

## Module Attributes

Added in version 3.3.

crypt.methods

  A list of available password hashing algorithms, as `crypt.METHOD_*` objects. This list is sorted from strongest to weakest.

## Module Functions

The `crypt` module defines the following functions:

crypt.crypt(word, salt=None)

  word will usually be a user’s password as typed at a prompt or in a graphical interface. The optional salt is either a string as returned from `mksalt()`, one of the `crypt.METHOD_*` values (though not all may be available on all platforms), or a full encrypted password including salt, as returned by this function. If salt is not provided, the strongest method available in `methods` will be used.

  Checking a password is usually done by passing the plain-text password as word and the full results of a previous `crypt()` call, which should be the same as the results of this call.

  salt (either a random 2 or 16 character string, possibly prefixed with `$digit$` to indicate the method) which will be used to perturb the encryption algorithm. The characters in salt must be in the set `[./a-zA-Z0-9]`, with the exception of Modular Crypt Format which prefixes a `$digit$`.

  Returns the hashed password as a string, which will be composed of characters from the same alphabet as the salt.

  Since a few crypt(3) extensions allow different values, with different sizes in the salt, it is recommended to use the full crypted password as salt when checking for a password.

  Changed in version 3.3: Accept `crypt.METHOD_*` values in addition to strings for salt.

crypt.mksalt(method=None, *, rounds=None)

  Return a randomly generated salt of the specified method. If no method is given, the strongest method available in `methods` is used.

  The return value is a string suitable for passing as the salt argument to `crypt()`.

  rounds specifies the number of rounds for `METHOD_SHA256`, `METHOD_SHA512` and `METHOD_BLOWFISH`. For `METHOD_SHA256` and `METHOD_SHA512` it must be an integer between `1000` and `999_999_999`, the default is `5000`. For `METHOD_BLOWFISH` it must be a power of two between `16` (24) and `2_147_483_648` (231), the default is `4096` (212).

  Added in version 3.3.

  Changed in version 3.7: Added the rounds parameter.

## Examples

A simple example illustrating typical use (a constant-time comparison operation is needed to limit exposure to timing attacks. `hmac.compare_digest()` is suitable for this purpose):

    import pwd
    import crypt
    import getpass
    from hmac import compare_digest as compare_hash

    def login():
        username = input('Python login: ')
        cryptedpasswd = pwd.getpwnam(username)[1]
        if cryptedpasswd:
            if cryptedpasswd == 'x' or cryptedpasswd == '*':
                raise ValueError('no support for shadow passwords')
            cleartext = getpass.getpass()
            return compare_hash(crypt.crypt(cleartext, cryptedpasswd), cryptedpasswd)
        else:
            return True

To generate a hash of a password using the strongest available method and check it against the original:

    import crypt
    from hmac import compare_digest as compare_hash

    hashed = crypt.crypt(plaintext)
    if not compare_hash(hashed, crypt.crypt(plaintext, hashed)):
        raise ValueError("hashed version doesn't validate against original")