Abstract

This document specifies a standard API for secret-key block encryption algorithms such as DES or Rijndael, making it easier to switch between different algorithms and implementations.

Introduction

Encryption algorithms transform their input data (called plaintext) in some way that is dependent on a variable key, producing ciphertext. The transformation can easily be reversed if and only if one knows the key. The key is a sequence of bits chosen from some very large space of possible keys. There are two classes of encryption algorithms: block ciphers and stream ciphers.

Block ciphers encrypt multibyte inputs of a fixed size (frequently 8 or 16 bytes long), and can be operated in various feedback modes. The feedback modes supported in this specification are:

These modes are to be implemented as described in NIST publication SP 800-38A [1]. Descriptions of the first three feedback modes can also be found in Bruce Schneier’s book Applied Cryptography [2].

(The numeric value 4 is reserved for MODE_PGP, a variant of CFB described in RFC 2440: “OpenPGP Message Format”. This mode isn’t considered important enough to make it worth requiring it for all block encryption ciphers, though supporting it is a nice extra feature.)

In a strict formal sense, stream ciphers encrypt data bit-by-bit; practically, stream ciphers work on a character-by-character basis. This PEP only aims at specifying an interface for block ciphers, though stream ciphers can support the interface described here by fixing ‘block_size’ to 1. Feedback modes also don’t make sense for stream ciphers, so the only reasonable feedback mode would be ECB mode.

Specification

Encryption modules can add additional functions, methods, and attributes beyond those described in this PEP, but all of the features described in this PEP must be present for a module to claim compliance with it.

Secret-key encryption modules should define one function:

new(key, mode, [IV], **kwargs)

Returns a ciphering object, using the secret key contained in the string ‘key’, and using the feedback mode ‘mode’, which must be one of the constants from the table above.

If ‘mode’ is MODE_CBC or MODE_CFB, ‘IV’ must be provided and must be a string of the same length as the block size. Not providing a value of ‘IV’ will result in a ValueError exception being raised.

Depending on the algorithm, a module may support additional keyword arguments to this function. Some keyword arguments are specified by this PEP, and modules are free to add additional keyword arguments. If a value isn’t provided for a given keyword, a secure default value should be used. For example, if an algorithm has a selectable number of rounds between 1 and 16, and 1-round encryption is insecure and 8-round encryption is believed secure, the default value for ‘rounds’ should be 8 or more. (Module implementors can choose a very slow but secure value, too, such as 16 in this example. This decision is left up to the implementor.)

The following table lists keyword arguments defined by this PEP:

The Counter feedback mode requires a sequence of input blocks, called counters, that are used to produce the output. When ‘mode’ is MODE_CTR, the ‘counter’ keyword argument must be provided, and its value must be a callable object, such as a function or method. Successive calls to this callable object must return a sequence of strings that are of the length ‘block_size’ and that never repeats. (Appendix B of the NIST publication gives a way to generate such a sequence, but that’s beyond the scope of this PEP.)

The CFB mode operates on segments of the plaintext and ciphertext that are ‘segment_size’ bits long. Therefore, when using this mode, the input and output strings must be a multiple of ‘segment_size’ bits in length. ‘segment_size’ must be an integer between 1 and block_size*8, inclusive. (The factor of 8 comes from ‘block_size’ being measured in bytes and not in bits). The default value for this parameter should be block_size*8. Implementors are allowed to constrain ‘segment_size’ to be a multiple of 8 for simplicity, but they’re encouraged to support arbitrary values for generality.

Secret-key encryption modules should define two variables:

• block_size

  An integer value; the size of the blocks encrypted by this module, measured in bytes. For all feedback modes, the length of strings passed to the encrypt() and decrypt() must be a multiple of the block size.

• key_size

  An integer value; the size of the keys required by this module, measured in bytes. If key_size is None, then the algorithm accepts variable-length keys. This may mean the module accepts keys of any random length, or that there are a few different possible lengths, e.g. 16, 24, or 32 bytes. You cannot pass a key of length 0 (that is, the null string ‘’) as a variable-length key.

Cipher objects should have two attributes:

• block_size

  An integer value equal to the size of the blocks encrypted by this object. For algorithms with a variable block size, this value is equal to the block size selected for this object.

• IV

  Contains the initial value which will be used to start a cipher feedback mode; it will always be a string exactly one block in length. After encrypting or decrypting a string, this value is updated to reflect the modified feedback text. It is read-only, and cannot be assigned a new value.

Cipher objects require the following methods:

• decrypt(string)

  Decrypts ‘string’, using the key-dependent data in the object and with the appropriate feedback mode. The string’s length must be an exact multiple of the algorithm’s block size or, in CFB mode, of the segment size. Returns a string containing the plaintext.

• encrypt(string)

  Encrypts a non-empty string, using the key-dependent data in the object, and with the appropriate feedback mode. The string’s length must be an exact multiple of the algorithm’s block size or, in CFB mode, of the segment size. Returns a string containing the ciphertext.

Here’s an example, using a module named ‘DES’:

>>> import DES
>>> obj = DES.new('abcdefgh', DES.MODE_ECB)
>>> plaintext = "Guido van Rossum is a space alien."
>>> len(plaintext)
34
>>> obj.encrypt(plaintext)
Traceback (innermost last):
  File "<stdin>", line 1, in ?
ValueError: Strings for DES must be a multiple of 8 in length
>>> ciphertext = obj.encrypt(plain+'XXXXXX')   # Add padding
>>> ciphertext
'\021,\343Nq\214DY\337T\342pA\372\255\311s\210\363,\300j\330\250\312\347\342I\3215w\03561\303dgb/\006'
>>> obj.decrypt(ciphertext)
'Guido van Rossum is a space alien.XXXXXX'

References

[1]

NIST publication SP 800-38A, “Recommendation for Block Cipher Modes of Operation” (http://csrc.nist.gov/encryption/modes/)

[2]

Applied Cryptography

Changes

2002-04: Removed references to stream ciphers; retitled PEP; prefixed feedback mode constants with MODE_; removed PGP feedback mode; added CTR and OFB feedback modes; clarified where numbers are measured in bytes and where in bits.

2002-09: Clarified the discussion of key length by using “variable-length keys” instead of “arbitrary-length”.

Acknowledgements

Thanks to the readers of the python-crypto list for their comments on this PEP.

Copyright

This document has been placed in the public domain.