PEP: 272 Title: API for Secret-Key Encryption Algorithms Version: $Revision$ Author: A.M. Kuchling Status: Draft Type: Informational Created: 18-Sep-2001 Post-History: Abstract This document specifies a standard API for secret-key encryption algorithms such as DES or Rijndael, making it easier to switch between different algorithms and implementations. The API is intended to be suitable for both block and stream ciphers. 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: Number Constant Description 1 MODE_ECB Electronic Code Book 2 MODE_CBC Cipher Block Chaining 3 MODE_CFB Cipher FeedBack 5 MODE_OFB Output Feedback 6 MODE_CTR Counter These modes are to be implemented as described in NIST publication SP-800A [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" [3]. 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. Stream ciphers can support the API described here by using fixing the block length at 1; this is how block and stream ciphers could be distinguished. The only feedback mode available for stream ciphers is 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: Keyword Meaning counter Callable object that returns counter blocks (see below; CTR mode only) rounds Number of rounds of encryption to use segment_size Size of data and ciphertext segments, measured in bits (see below; CFB mode only) 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. For stream ciphers, block_size will be 1. 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 arbitrary-length keys. You cannot pass a key of length 0 (that is, the null string '') as such 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. For stream ciphers, the string can be of any length. 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 "", 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 [3] RFC2440: "OpenPGP Message Format" (http://rfc2440.x42.com, http://www.faqs.org/rfcs/rfc2440.html) Copyright This document has been placed in the public domain. Local Variables: mode: indented-text indent-tabs-mode: nil End: