174 lines
6.2 KiB
Plaintext
174 lines
6.2 KiB
Plaintext
PEP: 247
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Title: API for Cryptographic Hash Functions
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Version: $Revision$
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Last-Modified: $Date$
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Author: A.M. Kuchling <amk@amk.ca>
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Status: Final
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Type: Informational
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Created: 23-Mar-2001
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Post-History: 20-Sep-2001
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Abstract
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There are several different modules available that implement
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cryptographic hashing algorithms such as MD5 or SHA. This
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document specifies a standard API for such algorithms, to make it
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easier to switch between different implementations.
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Specification
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All hashing modules should present the same interface. Additional
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methods or variables can be added, but those described in this
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document should always be present.
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Hash function modules define one function:
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new([string]) (unkeyed hashes)
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new([key] , [string]) (keyed hashes)
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Create a new hashing object and return it. The first form is
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for hashes that are unkeyed, such as MD5 or SHA. For keyed
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hashes such as HMAC, 'key' is a required parameter containing
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a string giving the key to use. In both cases, the optional
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'string' parameter, if supplied, will be immediately hashed
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into the object's starting state, as if obj.update(string) was
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called.
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After creating a hashing object, arbitrary strings can be fed
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into the object using its update() method, and the hash value
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can be obtained at any time by calling the object's digest()
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method.
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Arbitrary additional keyword arguments can be added to this
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function, but if they're not supplied, sensible default values
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should be used. For example, 'rounds' and 'digest_size'
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keywords could be added for a hash function which supports a
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variable number of rounds and several different output sizes,
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and they should default to values believed to be secure.
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Hash function modules define one variable:
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digest_size
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An integer value; the size of the digest produced by the
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hashing objects created by this module, measured in bytes.
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You could also obtain this value by creating a sample object
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and accessing its 'digest_size' attribute, but it can be
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convenient to have this value available from the module.
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Hashes with a variable output size will set this variable to
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None.
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Hashing objects require a single attribute:
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digest_size
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This attribute is identical to the module-level digest_size
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variable, measuring the size of the digest produced by the
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hashing object, measured in bytes. If the hash has a variable
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output size, this output size must be chosen when the hashing
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object is created, and this attribute must contain the
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selected size. Therefore, None is *not* a legal value for this
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attribute.
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Hashing objects require the following methods:
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copy()
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Return a separate copy of this hashing object. An update to
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this copy won't affect the original object.
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digest()
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Return the hash value of this hashing object as a string
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containing 8-bit data. The object is not altered in any way
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by this function; you can continue updating the object after
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calling this function.
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hexdigest()
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Return the hash value of this hashing object as a string
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containing hexadecimal digits. Lowercase letters should be used
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for the digits 'a' through 'f'. Like the .digest() method, this
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method mustn't alter the object.
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update(string)
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Hash 'string' into the current state of the hashing object.
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update() can be called any number of times during a hashing
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object's lifetime.
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Hashing modules can define additional module-level functions or
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object methods and still be compliant with this specification.
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Here's an example, using a module named 'MD5':
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>>> from Crypto.Hash import MD5
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>>> m = MD5.new()
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>>> m.digest_size
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16
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>>> m.update('abc')
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>>> m.digest()
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'\x90\x01P\x98<\xd2O\xb0\xd6\x96?}(\xe1\x7fr'
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>>> m.hexdigest()
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'900150983cd24fb0d6963f7d28e17f72'
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>>> MD5.new('abc').digest()
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'\x90\x01P\x98<\xd2O\xb0\xd6\x96?}(\xe1\x7fr'
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Rationale
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The digest size is measured in bytes, not bits, even though hash
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algorithm sizes are usually quoted in bits; MD5 is a 128-bit
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algorithm and not a 16-byte one, for example. This is because, in
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the sample code I looked at, the length in bytes is often needed
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(to seek ahead or behind in a file; to compute the length of an
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output string) while the length in bits is rarely used.
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Therefore, the burden will fall on the few people actually needing
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the size in bits, who will have to multiply digest_size by 8.
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It's been suggested that the update() method would be better named
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append(). However, that method is really causing the current
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state of the hashing object to be updated, and update() is already
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used by the md5 and sha modules included with Python, so it seems
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simplest to leave the name update() alone.
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The order of the constructor's arguments for keyed hashes was a
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sticky issue. It wasn't clear whether the key should come first
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or second. It's a required parameter, and the usual convention is
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to place required parameters first, but that also means that the
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'string' parameter moves from the first position to the second.
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It would be possible to get confused and pass a single argument to
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a keyed hash, thinking that you're passing an initial string to an
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unkeyed hash, but it doesn't seem worth making the interface
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for keyed hashes more obscure to avoid this potential error.
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Changes
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2001-09-17: Renamed clear() to reset(); added digest_size attribute
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to objects; added .hexdigest() method.
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2001-09-20: Removed reset() method completely.
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2001-09-28: Set digest_size to None for variable-size hashes.
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Acknowledgements
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Thanks to Aahz, Andrew Archibald, Rich Salz, Itamar
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Shtull-Trauring, and the readers of the python-crypto list for
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their comments on this PEP.
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Copyright
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This document has been placed in the public domain.
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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End:
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