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:mod:`hashlib` --- Secure hashes and message digests
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====================================================
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:synopsis: Secure hash and message digest algorithms.
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.. moduleauthor:: Gregory P. Smith <greg@krypto.org>
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.. sectionauthor:: Gregory P. Smith <greg@krypto.org>
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single: message digest, MD5
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single: secure hash algorithm, SHA1, SHA224, SHA256, SHA384, SHA512
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**Source code:** :source:`Lib/hashlib.py`
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This module implements a common interface to many different secure hash and
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message digest algorithms. Included are the FIPS secure hash algorithms SHA1,
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SHA224, SHA256, SHA384, and SHA512 (defined in FIPS 180-2) as well as RSA's MD5
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algorithm (defined in Internet :rfc:`1321`). The terms "secure hash" and
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"message digest" are interchangeable. Older algorithms were called message
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digests. The modern term is secure hash.
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If you want the adler32 or crc32 hash functions, they are available in
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the :mod:`zlib` module.
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Some algorithms have known hash collision weaknesses, refer to the "See
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also" section at the end.
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There is one constructor method named for each type of :dfn:`hash`. All return
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a hash object with the same simple interface. For example: use :func:`sha1` to
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create a SHA1 hash object. You can now feed this object with :term:`bytes-like
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object`\ s (normally :class:`bytes`) using the :meth:`update` method.
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At any point you can ask it for the :dfn:`digest` of the
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concatenation of the data fed to it so far using the :meth:`digest` or
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:meth:`hexdigest` methods.
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For better multithreading performance, the Python :term:`GIL` is released for
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data larger than 2047 bytes at object creation or on update.
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Feeding string objects into :meth:`update` is not supported, as hashes work
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on bytes, not on characters.
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.. index:: single: OpenSSL; (use in module hashlib)
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Constructors for hash algorithms that are always present in this module are
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:func:`md5`, :func:`sha1`, :func:`sha224`, :func:`sha256`, :func:`sha384`,
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:func:`sha512`, :func:`sha3_224`, :func:`sha3_256`, :func:`sha3_384`, and
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:func:`sha3_512`. Additional algorithms may also be available depending upon
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the OpenSSL library that Python uses on your platform.
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.. versionchanged:: 3.4
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Add sha3 family of hash algorithms.
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For example, to obtain the digest of the byte string ``b'Nobody inspects the
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spammish repetition'``::
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>>> m.update(b"Nobody inspects")
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>>> m.update(b" the spammish repetition")
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b'\xbbd\x9c\x83\xdd\x1e\xa5\xc9\xd9\xde\xc9\xa1\x8d\xf0\xff\xe9'
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>>> hashlib.sha224(b"Nobody inspects the spammish repetition").hexdigest()
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'a4337bc45a8fc544c03f52dc550cd6e1e87021bc896588bd79e901e2'
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.. function:: new(name[, data])
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Is a generic constructor that takes the string name of the desired
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algorithm as its first parameter. It also exists to allow access to the
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above listed hashes as well as any other algorithms that your OpenSSL
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library may offer. The named constructors are much faster than :func:`new`
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and should be preferred.
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Using :func:`new` with an algorithm provided by OpenSSL:
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>>> h = hashlib.new('ripemd160')
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>>> h.update(b"Nobody inspects the spammish repetition")
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'cc4a5ce1b3df48aec5d22d1f16b894a0b894eccc'
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Hashlib provides the following constant attributes:
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.. data:: algorithms_guaranteed
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Contains the names of the hash algorithms guaranteed to be supported
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by this module on all platforms.
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.. versionadded:: 3.2
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.. data:: algorithms_available
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Contains the names of the hash algorithms that are available
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in the running Python interpreter. These names will be recognized
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when passed to :func:`new`. :attr:`algorithms_guaranteed`
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will always be a subset. Duplicate algorithms with different
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name formats may appear in this set (thanks to OpenSSL).
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.. versionadded:: 3.2
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The following values are provided as constant attributes of the hash objects
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returned by the constructors:
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.. data:: hash.digest_size
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The size of the resulting hash in bytes.
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.. data:: hash.block_size
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The internal block size of the hash algorithm in bytes.
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A hash object has the following attributes:
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.. attribute:: hash.name
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The canonical name of this hash, always lowercase and always suitable as a
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parameter to :func:`new` to create another hash of this type.
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.. versionchanged:: 3.4
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The name attribute has been present in CPython since its inception, but
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until Python 3.4 was not formally specified, so may not exist on some
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A hash object has the following methods:
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.. method:: hash.update(arg)
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Update the hash object with the object *arg*, which must be interpretable as
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a buffer of bytes. Repeated calls are equivalent to a single call with the
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concatenation of all the arguments: ``m.update(a); m.update(b)`` is
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equivalent to ``m.update(a+b)``.
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.. versionchanged:: 3.1
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The Python GIL is released to allow other threads to run while hash
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updates on data larger than 2047 bytes is taking place when using hash
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algorithms supplied by OpenSSL.
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.. method:: hash.digest()
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Return the digest of the data passed to the :meth:`update` method so far.
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This is a bytes object of size :attr:`digest_size` which may contain bytes in
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the whole range from 0 to 255.
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.. method:: hash.hexdigest()
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Like :meth:`digest` except the digest is returned as a string object of
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double length, containing only hexadecimal digits. This may be used to
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exchange the value safely in email or other non-binary environments.
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.. method:: hash.copy()
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Return a copy ("clone") of the hash object. This can be used to efficiently
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compute the digests of data sharing a common initial substring.
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Key Derivation Function
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-----------------------
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Key derivation and key stretching algorithms are designed for secure password
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hashing. Naive algorithms such as ``sha1(password)`` are not resistant
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against brute-force attacks. A good password hashing function must be tunable,
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slow and include a salt.
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.. function:: pbkdf2_hmac(name, password, salt, rounds, dklen=None)
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The function provides PKCS#5 password-based key derivation function 2. It
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uses HMAC as pseudorandom function.
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The string *name* is the desired name of the hash digest algorithm for
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HMAC, e.g. 'sha1' or 'sha256'. *password* and *salt* are interpreted as
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buffers of bytes. Applications and libraries should limit *password* to
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a sensible value (e.g. 1024). *salt* should be about 16 or more bytes from
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a proper source, e.g. :func:`os.urandom`.
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The number of *rounds* should be chosen based on the hash algorithm and
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computing power. As of 2013 a value of at least 100,000 rounds of SHA-256
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*dklen* is the length of the derived key. If *dklen* is ``None`` then the
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digest size of the hash algorithm *name* is used, e.g. 64 for SHA-512.
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>>> import hashlib, binascii
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>>> dk = hashlib.pbkdf2_hmac('sha256', b'password', b'salt', 100000)
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>>> binascii.hexlify(dk)
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b'0394a2ede332c9a13eb82e9b24631604c31df978b4e2f0fbd2c549944f9d79a5'
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.. versionadded:: 3.4
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.. note:: A fast implementation of *pbkdf2_hmac* is available with OpenSSL.
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The Python implementation uses an inline version of :mod:`hmac`. It is
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about three times slower and doesn't release the GIL.
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A module to generate message authentication codes using hashes.
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Another way to encode binary hashes for non-binary environments.
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http://csrc.nist.gov/publications/fips/fips180-2/fips180-2.pdf
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The FIPS 180-2 publication on Secure Hash Algorithms.
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http://en.wikipedia.org/wiki/Cryptographic_hash_function#Cryptographic_hash_algorithms
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Wikipedia article with information on which algorithms have known issues and
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what that means regarding their use.
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http://www.ietf.org/rfc/rfc2898.txt
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PKCS #5: Password-Based Cryptography Specification Version 2.0