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.

To allow multithreading, the Python GIL is released while computing a hash supplied more than 2047 bytes of data at once in its constructor or .update method.

Constructors for hash algorithms that are always present in this module are md5(), sha1(), sha224(), sha256(), sha384(), sha512(), sha3_224(), sha3_256(), sha3_384(), sha3_512(), shake_128(), shake_256(), blake2b(), and blake2s(). These correspond to algorithms_guaranteed.

Any of these may nonetheless be missing or blocked in unusual environments, such as a rare “FIPS compliant” build of Python or when OpenSSL’s “FIPS mode” is configured to exclude some algorithms from its default provider. Calling the constructor of an algorithm that is unavailable raises ValueError.

Additional algorithms may also be available if your Python distribution’s hashlib was linked against a build of OpenSSL that provides others. Others are not guaranteed available on all installations and will only be accessible by name via new(). See algorithms_available.

Usage¶

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.hexdigest()
'031edd7d41651593c5fe5c006fa5752b37fddff7bc4e843aa6af0c950f4b9406'

More condensed:

>>> hashlib.sha256(b"Nobody inspects the spammish repetition").hexdigest()
'031edd7d41651593c5fe5c006fa5752b37fddff7bc4e843aa6af0c950f4b9406'

Constructors¶

hashlib.new(name, [data, ]*, usedforsecurity=True)¶

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.

Using new() with an algorithm name:

>>> h = hashlib.new('sha256')
>>> h.update(b"Nobody inspects the spammish repetition")
>>> h.hexdigest()
'031edd7d41651593c5fe5c006fa5752b37fddff7bc4e843aa6af0c950f4b9406'

hashlib.md5([data, ]*, usedforsecurity=True)¶

hashlib.sha1([data, ]*, usedforsecurity=True)¶

hashlib.sha224([data, ]*, usedforsecurity=True)¶

hashlib.sha256([data, ]*, usedforsecurity=True)¶

hashlib.sha384([data, ]*, usedforsecurity=True)¶

hashlib.sha512([data, ]*, usedforsecurity=True)¶

hashlib.sha3_224([data, ]*, usedforsecurity=True)¶

hashlib.sha3_256([data, ]*, usedforsecurity=True)¶

hashlib.sha3_384([data, ]*, usedforsecurity=True)¶

hashlib.sha3_512([data, ]*, usedforsecurity=True)¶

Named constructors such as these are faster than passing an algorithm name to new().

Attributes¶

Hashlib provides the following constant module 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.

Added 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).

Added in version 3.2.

Hash Objects¶

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(data)¶

Update the hash object with the bytes-like object. 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).

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¶

hashlib.shake_128([data, ]*, usedforsecurity=True)¶

hashlib.shake_256([data, ]*, usedforsecurity=True)¶

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)¶

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 in email or other non-binary environments.

Example use:

>>> h = hashlib.shake_256(b'Nobody inspects the spammish repetition')
>>> h.hexdigest(20)
'44709d6fcb83d92a76dcb0b668c98e1b1d3dafe7'

File hashing¶

The hashlib module provides a helper function for efficient hashing of a file or file-like object.

hashlib.file_digest(fileobj, digest, /)¶

Return a digest object that has been updated with contents of file object.

fileobj must be a file-like object opened for reading in binary mode. It accepts file objects from builtin open(), BytesIO instances, SocketIO objects from socket.socket.makefile(), and similar. fileobj must be opened in blocking mode, otherwise a BlockingIOError may be raised.

The function may bypass Python’s I/O and use the file descriptor from fileno() directly. fileobj must be assumed to be in an unknown state after this function returns or raises. It is up to the caller to close fileobj.

digest must either be a hash algorithm name as a str, a hash constructor, or a callable that returns a hash object.

Example:

>>> import io, hashlib, hmac
>>> with open("library/hashlib.rst", "rb") as f:
...     digest = hashlib.file_digest(f, "sha256")
...
>>> digest.hexdigest()
'...'

>>> buf = io.BytesIO(b"somedata")
>>> mac1 = hmac.HMAC(b"key", digestmod=hashlib.sha512)
>>> digest = hashlib.file_digest(buf, lambda: mac1)

>>> digest is mac1
True
>>> mac2 = hmac.HMAC(b"key", b"somedata", digestmod=hashlib.sha512)
>>> mac1.digest() == mac2.digest()
True

Added in version 3.11.

Changed in version 3.14: Now raises a BlockingIOError if the file is opened in non-blocking mode. Previously, spurious null bytes were added to the digest.

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 2022, hundreds of thousands of iterations of SHA-256 are suggested. For rationale as to why and how to choose what is best for your application, read Appendix A.2.2 of NIST-SP-800-132. The answers on the stackexchange pbkdf2 iterations question explain in detail.

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

>>> from hashlib import pbkdf2_hmac
>>> our_app_iters = 500_000  # Application specific, read above.
>>> dk = pbkdf2_hmac('sha256', b'password', b'bad salt' * 2, our_app_iters)
>>> dk.hex()
'15530bba69924174860db778f2c6f8104d3aaf9d26241840c8c4a641c8d000a9'

Function only available when Python is compiled with OpenSSL.

Added in version 3.4.

Changed in version 3.12: Function now only available when Python is built with OpenSSL. The slow pure Python implementation has been removed.

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 MiB). dklen is the length of the derived key in bytes.

Added 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, usedforsecurity=True)¶

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, usedforsecurity=True)¶

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 bytes-like object. It can be passed only as positional argument.

• 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).

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

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

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

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

>>> 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

>>> 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'

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

>>> 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

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

>>> 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

>>> 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'

>>> 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=

  10
 /  \
00  01

>>> 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'