python-peps/peps/pep-0452.rst

PEP: 452
Title: API for Cryptographic Hash Functions v2.0
Version: $Revision$
Last-Modified: $Date$
Author: A.M. Kuchling <amk@amk.ca>, Christian Heimes <christian@python.org>
Status: Final
Type: Informational
Content-Type: text/x-rst
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 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
   ``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 refers 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.


.. _issue 18532: http://bugs.python.org/issue18532

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