PEP: 247 Title: API for Cryptographic Hash Functions Version: $Revision$
Last-Modified: $Date$ Author: A.M. Kuchling <amk@amk.ca> Status: Final
Type: Informational Content-Type: text/x-rst Created: 23-Mar-2001
Post-History: 20-Sep-2001

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])    (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 strings 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.

  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 a single 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.

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 string 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 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:

    >>> from Crypto.Hash import MD5
    >>> m = MD5.new()
    >>> m.digest_size
    16
    >>> m.update('abc')
    >>> m.digest()
    '\x90\x01P\x98<\xd2O\xb0\xd6\x96?}(\xe1\x7fr'
    >>> m.hexdigest()
    '900150983cd24fb0d6963f7d28e17f72'
    >>> MD5.new('abc').digest()
    '\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

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.

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.