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PEP: 452
Title: API for Cryptographic Hash Functions v2.0
Version: 0cc8684e72ab
Last-Modified:  2013-08-19 18:30:13 +0200 (Mon, 19 Aug 2013)
Author: A.M. Kuchling <amk at amk.ca>, Christian Heimes <christian at python.org>
Status: Draft
Type: Informational
Created: 15-Aug-2013
Post-History: 
Replaces: 247

Abstract

    There are several different modules available that implement
    cryptographic hashing algorithms such as MD5 or SHA.  This
    document specifies a standard API for such algorithms, to make it
    easier to switch between different implementations.


Specification

    All hashing modules should present the same interface.  Additional
    methods or variables can be added, but those described in this
    document should always be present.

    Hash function modules define one function:

    new([string])            (unkeyed hashes)
    new(key, [string], [digestmod])    (keyed hashes)

        Create a new hashing object and return it.  The first form is
        for hashes that are unkeyed, such as MD5 or SHA.  For keyed
        hashes such as HMAC, 'key' is a required parameter containing
        a string giving the key to use.  In both cases, the optional
        'string' parameter, if supplied, will be immediately hashed
        into the object's starting state, as if obj.update(string) was
        called.

        After creating a hashing object, arbitrary bytes can be fed
        into the object using its update() method, and the hash value
        can be obtained at any time by calling the object's digest()
        method.

        Although the parameter is called 'string', hashing objects operate
        on 8-bit data only. Both 'key' and 'string' must be a bytes-like
        object (bytes, bytearray...). A hashing object may support
        one-dimensional, contiguous buffers as argument, too. Text
        (unicode) is no longer supported in Python 3.x. Python 2.x
        implementations may take ASCII-only unicode as argument, but
        portable code should not rely on the feature.

        Arbitrary additional keyword arguments can be added to this
        function, but if they're not supplied, sensible default values
        should be used.  For example, 'rounds' and 'digest_size'
        keywords could be added for a hash function which supports a
        variable number of rounds and several different output sizes,
        and they should default to values believed to be secure.

    Hash function modules define one variable:

    digest_size

        An integer value; the size of the digest produced by the
        hashing objects created by this module, measured in bytes.
        You could also obtain this value by creating a sample object
        and accessing its 'digest_size' attribute, but it can be
        convenient to have this value available from the module.
        Hashes with a variable output size will set this variable to
        None.

    Hashing objects require the following attribute:

    digest_size

        This attribute is identical to the module-level digest_size
        variable, measuring the size of the digest produced by the
        hashing object, measured in bytes.  If the hash has a variable
        output size, this output size must be chosen when the hashing
        object is created, and this attribute must contain the
        selected size.  Therefore None is *not* a legal value for this
        attribute.

    block_size

        An integer value or ``NotImplemented``; the internal block size
        of the hash algorithm in bytes. The block size is used by the
        HMAC module to pad the secret key to digest_size or to hash the
        secret key if it is longer than digest_size. If no HMAC
        algorithm is standardized for the the hash algorithm, return
        ``NotImplemented`` instead.

    name

        A text string value; the canonical, lowercase name of the hashing
        algorithm. The name should be a suitable parameter for
        :func:`hashlib.new`.

    Hashing objects require the following methods:

    copy()

        Return a separate copy of this hashing object.  An update to
        this copy won't affect the original object.

    digest()

        Return the hash value of this hashing object as a bytes
        containing 8-bit data.  The object is not altered in any way
        by this function; you can continue updating the object after
        calling this function.

    hexdigest()

        Return the hash value of this hashing object as a string
        containing hexadecimal digits.  Lowercase letters should be used
        for the digits 'a' through 'f'.  Like the .digest() method, this
        method mustn't alter the object.

    update(string)

        Hash bytes-like 'string' into the current state of the hashing
        object. update() can be called any number of times during a
        hashing object's lifetime.

    Hashing modules can define additional module-level functions or
    object methods and still be compliant with this specification.

    Here's an example, using a module named 'MD5':

        >>> import hashlib
        >>> from Crypto.Hash import MD5
        >>> m = MD5.new()
        >>> isinstance(m, hashlib.CryptoHash)
        True
        >>> m.name
        'md5'
        >>> m.digest_size
        16
        >>> m.block_size
        64
        >>> m.update(b'abc')
        >>> m.digest()
        b'\x90\x01P\x98<\xd2O\xb0\xd6\x96?}(\xe1\x7fr'
        >>> m.hexdigest()
        '900150983cd24fb0d6963f7d28e17f72'
        >>> MD5.new(b'abc').digest()
        b'\x90\x01P\x98<\xd2O\xb0\xd6\x96?}(\xe1\x7fr'


Rationale

    The digest size is measured in bytes, not bits, even though hash
    algorithm sizes are usually quoted in bits; MD5 is a 128-bit
    algorithm and not a 16-byte one, for example.  This is because, in
    the sample code I looked at, the length in bytes is often needed
    (to seek ahead or behind in a file; to compute the length of an
    output string) while the length in bits is rarely used.
    Therefore, the burden will fall on the few people actually needing
    the size in bits, who will have to multiply digest_size by 8.

    It's been suggested that the update() method would be better named
    append().  However, that method is really causing the current
    state of the hashing object to be updated, and update() is already
    used by the md5 and sha modules included with Python, so it seems
    simplest to leave the name update() alone.

    The order of the constructor's arguments for keyed hashes was a
    sticky issue.  It wasn't clear whether the key should come first
    or second.  It's a required parameter, and the usual convention is
    to place required parameters first, but that also means that the
    'string' parameter moves from the first position to the second.
    It would be possible to get confused and pass a single argument to
    a keyed hash, thinking that you're passing an initial string to an
    unkeyed hash, but it doesn't seem worth making the interface
    for keyed hashes more obscure to avoid this potential error.


Changes from Version 1.0 to Version 2.0

    Version 2.0 of API for Cryptographic Hash Functions clarifies some
    aspects of the API and brings it up-to-date. It also formalized aspects
    that were already de-facto standards and provided by most
    implementations.

    Version 2.0 introduces the following new attributes:

    name

        The name property was made mandatory by :issue:`18532`.

    block_size

        The new version also specifies that the return value
        ``NotImplemented`` prevents HMAC support.

    Version 2.0 takes the separation of binary and text data in Python
    3.0 into account. The 'string' argument to new() and update() as
    well as the 'key' argument must be bytes-like objects. On Python
    2.x a hashing object may also support ASCII-only unicode. The actual
    name of argument is not changed as it is part of the public API.
    Code may depend on the fact that the argument is called 'string'.


Recommended names for common hashing algorithms

    algorithm       variant         recommended name
    ----------      ---------       ----------------
    MD5                             md5
    RIPEMD-160                      ripemd160
    SHA-1                           sha1
    SHA-2           SHA-224         sha224
                    SHA-256         sha256
                    SHA-384         sha384
                    SHA-512         sha512
    SHA-3           SHA-3-224       sha3_224
                    SHA-3-256       sha3_256
                    SHA-3-384       sha3_384
                    SHA-3-512       sha3_512
    WHIRLPOOL                       whirlpool


Changes

    2001-09-17: Renamed clear() to reset(); added digest_size attribute
                to objects; added .hexdigest() method.
    2001-09-20: Removed reset() method completely.
    2001-09-28: Set digest_size to None for variable-size hashes.
    2013-08-15: Added block_size and name attributes; clarified that
               'string' actually referes to bytes-like objects.


Acknowledgements

    Thanks to Aahz, Andrew Archibald, Rich Salz, Itamar
    Shtull-Trauring, and the readers of the python-crypto list for
    their comments on this PEP.


Copyright

    This document has been placed in the public domain.