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Python 3.6.5 Documentation > "hashlib" — Secure hashes and message digests

"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.

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

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

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
"sha256()" to create a SHA-256 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.

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

Note: 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 "sha1()", "sha224()", "sha256()", "sha384()", "sha512()",
"blake2b()", and "blake2s()". "md5()" is normally available as well,
though it may be missing if you are using a rare “FIPS compliant”
build of Python. Additional algorithms may also be available depending
upon the OpenSSL library that Python uses on your platform. On most
platforms the "sha3_224()", "sha3_256()", "sha3_384()", "sha3_512()",
"shake_128()", "shake_256()" are also available.

New in version 3.6: SHA3 (Keccak) and SHAKE constructors "sha3_224()",
"sha3_256()", "sha3_384()", "sha3_512()", "shake_128()",
"shake_256()".

New in version 3.6: "blake2b()" and "blake2s()" were added.

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

>>> import hashlib
>>> m = hashlib.sha256()
>>> m.update(b"Nobody inspects")
>>> m.update(b" the spammish repetition")
>>> m.digest()
b'\x03\x1e\xdd}Ae\x15\x93\xc5\xfe\\\x00o\xa5u+7\xfd\xdf\xf7\xbcN\x84:\xa6\xaf\x0c\x95\x0fK\x94\x06'
>>> m.digest_size
32
>>> m.block_size
64

More condensed:

>>> hashlib.sha224(b"Nobody inspects the spammish repetition").hexdigest()
'a4337bc45a8fc544c03f52dc550cd6e1e87021bc896588bd79e901e2'

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()
'cc4a5ce1b3df48aec5d22d1f16b894a0b894eccc'

Hashlib provides the following constant attributes:

hashlib.algorithms_guaranteed

A set containing the names of the hash algorithms guaranteed to be
supported by this module on all platforms. Note that ‘md5’ is in
this list despite some upstream vendors offering an odd “FIPS
compliant” Python build that excludes it.

New in version 3.2.

hashlib.algorithms_available

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:

hash.digest_size

The size of the resulting hash in bytes.

hash.block_size

The internal block size of the hash algorithm in bytes.

A hash object has the following attributes:

hash.name

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:

hash.update(arg)

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.

hash.digest()

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.

hash.hexdigest()

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.

hash.copy()

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

SHAKE variable length digests
=============================

The "shake_128()" and "shake_256()" algorithms provide variable length
digests with length_in_bits//2 up to 128 or 256 bits of security. As
such, their digest methods require a length. Maximum length is not
limited by the SHAKE algorithm.

shake.digest(length)

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

shake.hexdigest(length)

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(hash_name, password, salt, iterations, dklen=None)

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

The string *hash_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 length (e.g. 1024). *salt*
should be about 16 or more bytes from a proper source, e.g.
"os.urandom()".

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

*dklen* is the length of the derived key. If *dklen* is "None" then
the digest size of the hash algorithm *hash_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)
b'0394a2ede332c9a13eb82e9b24631604c31df978b4e2f0fbd2c549944f9d79a5'

New in version 3.4.

Note: 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.

hashlib.scrypt(password, *, salt, n, r, p, maxmem=0, dklen=64)

The function provides scrypt password-based key derivation function
as defined in **RFC 7914**.

*password* and *salt* must be bytes-like objects. Applications and
libraries should limit *password* to a sensible length (e.g. 1024).
*salt* should be about 16 or more bytes from a proper source, e.g.
"os.urandom()".

*n* is the CPU/Memory cost factor, *r* the block size, *p*
parallelization factor and *maxmem* limits memory (OpenSSL 1.1.0
defaults to 32 MB). *dklen* is the length of the derived key.

Availability: OpenSSL 1.1+

New in version 3.6.

BLAKE2
======

BLAKE2 is a cryptographic hash function defined in RFC-7693 that comes
in two flavors:

* **BLAKE2b**, optimized for 64-bit platforms and produces digests
of any size between 1 and 64 bytes,

* **BLAKE2s**, optimized for 8- to 32-bit platforms and produces
digests of any size between 1 and 32 bytes.

BLAKE2 supports **keyed mode** (a faster and simpler replacement for
HMAC), **salted hashing**, **personalization**, and **tree hashing**.

Hash objects from this module follow the API of standard library’s
"hashlib" objects.

Creating hash objects
---------------------

New hash objects are created by calling constructor functions:

hashlib.blake2b(data=b'', digest_size=64, key=b'', salt=b'', person=b'', fanout=1, depth=1, leaf_size=0, node_offset=0, node_depth=0, inner_size=0, last_node=False)

hashlib.blake2s(data=b'', digest_size=32, key=b'', salt=b'', person=b'', fanout=1, depth=1, leaf_size=0, node_offset=0, node_depth=0, inner_size=0, last_node=False)

These functions return the corresponding hash objects for calculating
BLAKE2b or BLAKE2s. They optionally take these general parameters:

* *data*: initial chunk of data to hash, which must be interpretable
as buffer of bytes.

* *digest_size*: size of output digest in bytes.

* *key*: key for keyed hashing (up to 64 bytes for BLAKE2b, up to 32
bytes for BLAKE2s).

* *salt*: salt for randomized hashing (up to 16 bytes for BLAKE2b,
up to 8 bytes for BLAKE2s).

* *person*: personalization string (up to 16 bytes for BLAKE2b, up
to 8 bytes for BLAKE2s).

The following table shows limits for general parameters (in bytes):

+---------+-------------+----------+-----------+-------------+
| Hash | digest_size | len(key) | len(salt) | len(person) |
+=========+=============+==========+===========+=============+
| BLAKE2b | 64 | 64 | 16 | 16 |
+---------+-------------+----------+-----------+-------------+
| BLAKE2s | 32 | 32 | 8 | 8 |
+---------+-------------+----------+-----------+-------------+

Note: BLAKE2 specification defines constant lengths for salt and
personalization parameters, however, for convenience, this
implementation accepts byte strings of any size up to the specified
length. If the length of the parameter is less than specified, it is
padded with zeros, thus, for example, "b'salt'" and "b'salt\x00'" is
the same value. (This is not the case for *key*.)

These sizes are available as module constants described below.

Constructor functions also accept the following tree hashing
parameters:

* *fanout*: fanout (0 to 255, 0 if unlimited, 1 in sequential mode).

* *depth*: maximal depth of tree (1 to 255, 255 if unlimited, 1 in
sequential mode).

* *leaf_size*: maximal byte length of leaf (0 to 2**32-1, 0 if
unlimited or in sequential mode).

* *node_offset*: node offset (0 to 2**64-1 for BLAKE2b, 0 to 2**48-1
for BLAKE2s, 0 for the first, leftmost, leaf, or in sequential
mode).

* *node_depth*: node depth (0 to 255, 0 for leaves, or in sequential
mode).

* *inner_size*: inner digest size (0 to 64 for BLAKE2b, 0 to 32 for
BLAKE2s, 0 in sequential mode).

* *last_node*: boolean indicating whether the processed node is the
last one (*False* for sequential mode).

[image: Explanation of tree mode parameters.][image]

See section 2.10 in BLAKE2 specification for comprehensive review of
tree hashing.

Constants
---------

blake2b.SALT_SIZE

blake2s.SALT_SIZE

Salt length (maximum length accepted by constructors).

blake2b.PERSON_SIZE

blake2s.PERSON_SIZE

Personalization string length (maximum length accepted by
constructors).

blake2b.MAX_KEY_SIZE

blake2s.MAX_KEY_SIZE

Maximum key size.

blake2b.MAX_DIGEST_SIZE

blake2s.MAX_DIGEST_SIZE

Maximum digest size that the hash function can output.

Examples
--------

Simple hashing
~~~~~~~~~~~~~~

To calculate hash of some data, you should first construct a hash
object by calling the appropriate constructor function ("blake2b()" or
"blake2s()"), then update it with the data by calling "update()" on
the object, and, finally, get the digest out of the object by calling
"digest()" (or "hexdigest()" for hex-encoded string).

>>> from hashlib import blake2b
>>> h = blake2b()
>>> h.update(b'Hello world')
>>> h.hexdigest()
'6ff843ba685842aa82031d3f53c48b66326df7639a63d128974c5c14f31a0f33343a8c65551134ed1ae0f2b0dd2bb495dc81039e3eeb0aa1bb0388bbeac29183'

As a shortcut, you can pass the first chunk of data to update directly
to the constructor as the first argument (or as *data* keyword
argument):

>>> from hashlib import blake2b
>>> blake2b(b'Hello world').hexdigest()
'6ff843ba685842aa82031d3f53c48b66326df7639a63d128974c5c14f31a0f33343a8c65551134ed1ae0f2b0dd2bb495dc81039e3eeb0aa1bb0388bbeac29183'

You can call "hash.update()" as many times as you need to iteratively
update the hash:

>>> from hashlib import blake2b
>>> items = [b'Hello', b' ', b'world']
>>> h = blake2b()
>>> for item in items:
... h.update(item)
>>> h.hexdigest()
'6ff843ba685842aa82031d3f53c48b66326df7639a63d128974c5c14f31a0f33343a8c65551134ed1ae0f2b0dd2bb495dc81039e3eeb0aa1bb0388bbeac29183'

Using different digest sizes
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

BLAKE2 has configurable size of digests up to 64 bytes for BLAKE2b and
up to 32 bytes for BLAKE2s. For example, to replace SHA-1 with BLAKE2b
without changing the size of output, we can tell BLAKE2b to produce
20-byte digests:

>>> from hashlib import blake2b
>>> h = blake2b(digest_size=20)
>>> h.update(b'Replacing SHA1 with the more secure function')
>>> h.hexdigest()
'd24f26cf8de66472d58d4e1b1774b4c9158b1f4c'
>>> h.digest_size
20
>>> len(h.digest())
20

Hash objects with different digest sizes have completely different
outputs (shorter hashes are *not* prefixes of longer hashes); BLAKE2b
and BLAKE2s produce different outputs even if the output length is the
same:

>>> from hashlib import blake2b, blake2s
>>> blake2b(digest_size=10).hexdigest()
'6fa1d8fcfd719046d762'
>>> blake2b(digest_size=11).hexdigest()
'eb6ec15daf9546254f0809'
>>> blake2s(digest_size=10).hexdigest()
'1bf21a98c78a1c376ae9'
>>> blake2s(digest_size=11).hexdigest()
'567004bf96e4a25773ebf4'

Keyed hashing
~~~~~~~~~~~~~

Keyed hashing can be used for authentication as a faster and simpler
replacement for Hash-based message authentication code (HMAC). BLAKE2
can be securely used in prefix-MAC mode thanks to the
indifferentiability property inherited from BLAKE.

This example shows how to get a (hex-encoded) 128-bit authentication
code for message "b'message data'" with key "b'pseudorandom key'":

>>> from hashlib import blake2b
>>> h = blake2b(key=b'pseudorandom key', digest_size=16)
>>> h.update(b'message data')
>>> h.hexdigest()
'3d363ff7401e02026f4a4687d4863ced'

As a practical example, a web application can symmetrically sign
cookies sent to users and later verify them to make sure they weren’t
tampered with:

>>> from hashlib import blake2b
>>> from hmac import compare_digest
>>>
>>> SECRET_KEY = b'pseudorandomly generated server secret key'
>>> AUTH_SIZE = 16
>>>
>>> def sign(cookie):
... h = blake2b(digest_size=AUTH_SIZE, key=SECRET_KEY)
... h.update(cookie)
... return h.hexdigest().encode('utf-8')
>>>
>>> def verify(cookie, sig):
... good_sig = sign(cookie)
... return compare_digest(good_sig, sig)
>>>
>>> cookie = b'user-alice'
>>> sig = sign(cookie)
>>> print("{0},{1}".format(cookie.decode('utf-8'), sig))
user-alice,b'43b3c982cf697e0c5ab22172d1ca7421'
>>> verify(cookie, sig)
True
>>> verify(b'user-bob', sig)
False
>>> verify(cookie, b'0102030405060708090a0b0c0d0e0f00')
False

Even though there’s a native keyed hashing mode, BLAKE2 can, of
course, be used in HMAC construction with "hmac" module:

>>> import hmac, hashlib
>>> m = hmac.new(b'secret key', digestmod=hashlib.blake2s)
>>> m.update(b'message')
>>> m.hexdigest()
'e3c8102868d28b5ff85fc35dda07329970d1a01e273c37481326fe0c861c8142'

Randomized hashing
~~~~~~~~~~~~~~~~~~

By setting *salt* parameter users can introduce randomization to the
hash function. Randomized hashing is useful for protecting against
collision attacks on the hash function used in digital signatures.

Randomized hashing is designed for situations where one party, the
message preparer, generates all or part of a message to be signed
by a second party, the message signer. If the message preparer is
able to find cryptographic hash function collisions (i.e., two
messages producing the same hash value), then she might prepare
meaningful versions of the message that would produce the same hash
value and digital signature, but with different results (e.g.,
transferring $1,000,000 to an account, rather than $10).
Cryptographic hash functions have been designed with collision
resistance as a major goal, but the current concentration on
attacking cryptographic hash functions may result in a given
cryptographic hash function providing less collision resistance
than expected. Randomized hashing offers the signer additional
protection by reducing the likelihood that a preparer can generate
two or more messages that ultimately yield the same hash value
during the digital signature generation process — even if it is
practical to find collisions for the hash function. However, the
use of randomized hashing may reduce the amount of security
provided by a digital signature when all portions of the message
are prepared by the signer.

(NIST SP-800-106 “Randomized Hashing for Digital Signatures”)

In BLAKE2 the salt is processed as a one-time input to the hash
function during initialization, rather than as an input to each
compression function.

Warning: *Salted hashing* (or just hashing) with BLAKE2 or any other
general- purpose cryptographic hash function, such as SHA-256, is
not suitable for hashing passwords. See BLAKE2 FAQ for more
information.

>>> import os
>>> from hashlib import blake2b
>>> msg = b'some message'
>>> # Calculate the first hash with a random salt.
>>> salt1 = os.urandom(blake2b.SALT_SIZE)
>>> h1 = blake2b(salt=salt1)
>>> h1.update(msg)
>>> # Calculate the second hash with a different random salt.
>>> salt2 = os.urandom(blake2b.SALT_SIZE)
>>> h2 = blake2b(salt=salt2)
>>> h2.update(msg)
>>> # The digests are different.
>>> h1.digest() != h2.digest()
True

Personalization
~~~~~~~~~~~~~~~

Sometimes it is useful to force hash function to produce different
digests for the same input for different purposes. Quoting the authors
of the Skein hash function:

We recommend that all application designers seriously consider
doing this; we have seen many protocols where a hash that is
computed in one part of the protocol can be used in an entirely
different part because two hash computations were done on similar
or related data, and the attacker can force the application to make
the hash inputs the same. Personalizing each hash function used in
the protocol summarily stops this type of attack.

(The Skein Hash Function Family, p. 21)

BLAKE2 can be personalized by passing bytes to the *person* argument:

>>> from hashlib import blake2b
>>> FILES_HASH_PERSON = b'MyApp Files Hash'
>>> BLOCK_HASH_PERSON = b'MyApp Block Hash'
>>> h = blake2b(digest_size=32, person=FILES_HASH_PERSON)
>>> h.update(b'the same content')
>>> h.hexdigest()
'20d9cd024d4fb086aae819a1432dd2466de12947831b75c5a30cf2676095d3b4'
>>> h = blake2b(digest_size=32, person=BLOCK_HASH_PERSON)
>>> h.update(b'the same content')
>>> h.hexdigest()
'cf68fb5761b9c44e7878bfb2c4c9aea52264a80b75005e65619778de59f383a3'

Personalization together with the keyed mode can also be used to
derive different keys from a single one.

>>> from hashlib import blake2s
>>> from base64 import b64decode, b64encode
>>> orig_key = b64decode(b'Rm5EPJai72qcK3RGBpW3vPNfZy5OZothY+kHY6h21KM=')
>>> enc_key = blake2s(key=orig_key, person=b'kEncrypt').digest()
>>> mac_key = blake2s(key=orig_key, person=b'kMAC').digest()
>>> print(b64encode(enc_key).decode('utf-8'))
rbPb15S/Z9t+agffno5wuhB77VbRi6F9Iv2qIxU7WHw=
>>> print(b64encode(mac_key).decode('utf-8'))
G9GtHFE1YluXY1zWPlYk1e/nWfu0WSEb0KRcjhDeP/o=

Tree mode
~~~~~~~~~

Here’s an example of hashing a minimal tree with two leaf nodes:

10
/ \
00 01

This example uses 64-byte internal digests, and returns the 32-byte
final digest:

>>> from hashlib import blake2b
>>>
>>> FANOUT = 2
>>> DEPTH = 2
>>> LEAF_SIZE = 4096
>>> INNER_SIZE = 64
>>>
>>> buf = bytearray(6000)
>>>
>>> # Left leaf
... h00 = blake2b(buf[0:LEAF_SIZE], fanout=FANOUT, depth=DEPTH,
... leaf_size=LEAF_SIZE, inner_size=INNER_SIZE,
... node_offset=0, node_depth=0, last_node=False)
>>> # Right leaf
... h01 = blake2b(buf[LEAF_SIZE:], fanout=FANOUT, depth=DEPTH,
... leaf_size=LEAF_SIZE, inner_size=INNER_SIZE,
... node_offset=1, node_depth=0, last_node=True)
>>> # Root node
... h10 = blake2b(digest_size=32, fanout=FANOUT, depth=DEPTH,
... leaf_size=LEAF_SIZE, inner_size=INNER_SIZE,
... node_offset=0, node_depth=1, last_node=True)
>>> h10.update(h00.digest())
>>> h10.update(h01.digest())
>>> h10.hexdigest()
'3ad2a9b37c6070e374c7a8c508fe20ca86b6ed54e286e93a0318e95e881db5aa'

Credits
-------

BLAKE2 was designed by *Jean-Philippe Aumasson*, *Samuel Neves*,
*Zooko Wilcox-O’Hearn*, and *Christian Winnerlein* based on SHA-3
finalist BLAKE created by *Jean-Philippe Aumasson*, *Luca Henzen*,
*Willi Meier*, and *Raphael C.-W. Phan*.

It uses core algorithm from ChaCha cipher designed by *Daniel J.
Bernstein*.

The stdlib implementation is based on pyblake2 module. It was written
by *Dmitry Chestnykh* based on C implementation written by *Samuel
Neves*. The documentation was copied from pyblake2 and written by
*Dmitry Chestnykh*.

The C code was partly rewritten for Python by *Christian Heimes*.

The following public domain dedication applies for both C hash
function implementation, extension code, and this documentation:

To the extent possible under law, the author(s) have dedicated all
copyright and related and neighboring rights to this software to
the public domain worldwide. This software is distributed without
any warranty.

You should have received a copy of the CC0 Public Domain Dedication
along with this software. If not, see
http://creativecommons.org/publicdomain/zero/1.0/.

The following people have helped with development or contributed their
changes to the project and the public domain according to the Creative
Commons Public Domain Dedication 1.0 Universal:

* *Alexandr Sokolovskiy*