15.1. hashlib — Secure hashes and message digests

Source code: Lib/hashlib.py

This module implements a common interface to many different secure hash and message digest algorithms. Included are the FIPS secure hash algorithms SHA1, SHA224, SHA256, SHA384, and SHA512 (defined in FIPS 180-2) as well as RSA’s MD5 algorithm (defined in Internet RFC 1321). The terms “secure hash” and “message digest” are interchangeable. Older algorithms were called message digests. The modern term is secure hash.


If you want the adler32 or crc32 hash functions, they are available in the zlib module.


Some algorithms have known hash collision weaknesses, refer to the “See also” section at the end.

15.1.1. Hash algorithms

There is one constructor method named for each type of hash. All return a hash object with the same simple interface. For example: use sha1() to create a SHA1 hash object. You can now feed this object with bytes-like objects (normally bytes) using the update() method. At any point you can ask it for the digest of the concatenation of the data fed to it so far using the digest() or hexdigest() methods.


For better multithreading performance, the Python GIL is released for data larger than 2047 bytes at object creation or on update.


Feeding string objects into update() is not supported, as hashes work on bytes, not on characters.

Constructors for hash algorithms that are always present in this module are md5(), sha1(), sha224(), sha256(), sha384(), and sha512(). Additional algorithms may also be available depending upon the OpenSSL library that Python uses on your platform.

For example, to obtain the digest of the byte string b'Nobody inspects the spammish repetition':

>>> import hashlib
>>> m = hashlib.md5()
>>> m.update(b"Nobody inspects")
>>> m.update(b" the spammish repetition")
>>> m.digest()
>>> m.digest_size
>>> m.block_size

More condensed:

>>> hashlib.sha224(b"Nobody inspects the spammish repetition").hexdigest()
hashlib.new(name[, data])

Is a generic constructor that takes the string name of the desired algorithm as its first parameter. It also exists to allow access to the above listed hashes as well as any other algorithms that your OpenSSL library may offer. The named constructors are much faster than new() and should be preferred.

Using new() with an algorithm provided by OpenSSL:

>>> h = hashlib.new('ripemd160')
>>> h.update(b"Nobody inspects the spammish repetition")
>>> h.hexdigest()

Hashlib provides the following constant attributes:


A set containing the names of the hash algorithms guaranteed to be supported by this module on all platforms.

New in version 3.2.


A set containing the names of the hash algorithms that are available in the running Python interpreter. These names will be recognized when passed to new(). algorithms_guaranteed will always be a subset. The same algorithm may appear multiple times in this set under different names (thanks to OpenSSL).

New in version 3.2.

The following values are provided as constant attributes of the hash objects returned by the constructors:


The size of the resulting hash in bytes.


The internal block size of the hash algorithm in bytes.

A hash object has the following attributes:


The canonical name of this hash, always lowercase and always suitable as a parameter to new() to create another hash of this type.

Changed in version 3.4: The name attribute has been present in CPython since its inception, but until Python 3.4 was not formally specified, so may not exist on some platforms.

A hash object has the following methods:


Update the hash object with the object arg, which must be interpretable as a buffer of bytes. Repeated calls are equivalent to a single call with the concatenation of all the arguments: m.update(a); m.update(b) is equivalent to m.update(a+b).

Changed in version 3.1: The Python GIL is released to allow other threads to run while hash updates on data larger than 2047 bytes is taking place when using hash algorithms supplied by OpenSSL.


Return the digest of the data passed to the update() method so far. This is a bytes object of size digest_size which may contain bytes in the whole range from 0 to 255.


Like digest() except the digest is returned as a string object of double length, containing only hexadecimal digits. This may be used to exchange the value safely in email or other non-binary environments.


Return a copy (“clone”) of the hash object. This can be used to efficiently compute the digests of data sharing a common initial substring.

15.1.2. Key derivation

Key derivation and key stretching algorithms are designed for secure password hashing. Naive algorithms such as sha1(password) are not resistant against brute-force attacks. A good password hashing function must be tunable, slow, and include a salt.

hashlib.pbkdf2_hmac(name, password, salt, rounds, dklen=None)

The function provides PKCS#5 password-based key derivation function 2. It uses HMAC as pseudorandom function.

The string name is the desired name of the hash digest algorithm for HMAC, e.g. ‘sha1’ or ‘sha256’. password and salt are interpreted as buffers of bytes. Applications and libraries should limit password to a sensible value (e.g. 1024). salt should be about 16 or more bytes from a proper source, e.g. os.urandom().

The number of rounds should be chosen based on the hash algorithm and computing power. As of 2013, at least 100,000 rounds of SHA-256 is suggested.

dklen is the length of the derived key. If dklen is None then the digest size of the hash algorithm name is used, e.g. 64 for SHA-512.

>>> import hashlib, binascii
>>> dk = hashlib.pbkdf2_hmac('sha256', b'password', b'salt', 100000)
>>> binascii.hexlify(dk)

New in version 3.4.


A fast implementation of pbkdf2_hmac is available with OpenSSL. The Python implementation uses an inline version of hmac. It is about three times slower and doesn’t release the GIL.

See also

Module hmac
A module to generate message authentication codes using hashes.
Module base64
Another way to encode binary hashes for non-binary environments.
The FIPS 180-2 publication on Secure Hash Algorithms.
Wikipedia article with information on which algorithms have known issues and what that means regarding their use.
PKCS #5: Password-Based Cryptography Specification Version 2.0