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PEP 247 -- API for Cryptographic Hash Functions

PEP:247
Title:API for Cryptographic Hash Functions
Author:A.M. Kuchling <amk at amk.ca>
Status:Final
Type:Informational
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.

Source: https://github.com/python/peps/blob/master/pep-0247.txt