diff options
author | Guido van Rossum <guido@python.org> | 1994-08-08 12:30:22 (GMT) |
---|---|---|
committer | Guido van Rossum <guido@python.org> | 1994-08-08 12:30:22 (GMT) |
commit | 16d6e7109deb1bcfd8a860cb60c16c02a0ef183b (patch) | |
tree | 81624359068cca2b8476d0894c8cd28788d0762e /Doc/librotor.tex | |
parent | 4b4c664d2e93279c8d749da027000453f9e2cd46 (diff) | |
download | cpython-16d6e7109deb1bcfd8a860cb60c16c02a0ef183b.zip cpython-16d6e7109deb1bcfd8a860cb60c16c02a0ef183b.tar.gz cpython-16d6e7109deb1bcfd8a860cb60c16c02a0ef183b.tar.bz2 |
Lots of small corrections by Andrew Kuchling (plus all new rotor docs)
Diffstat (limited to 'Doc/librotor.tex')
-rw-r--r-- | Doc/librotor.tex | 105 |
1 files changed, 102 insertions, 3 deletions
diff --git a/Doc/librotor.tex b/Doc/librotor.tex index 657d2ac..b931333 100644 --- a/Doc/librotor.tex +++ b/Doc/librotor.tex @@ -1,6 +1,105 @@ \section{Built-in module \sectcode{rotor}} \bimodindex{rotor} -This module implements a rotor-based encryption algorithm, contributed -by Lance Ellinghouse. Currently no further documentation is available ---- you are kindly advised to read the source... +This module implements a rotor-based encryption algorithm, contributed by +Lance Ellinghouse. The design is derived from the Enigma device, a machine +used during World War II to encipher messages. A rotor is simply a +permutation. For example, if the character `A' is the origin of the rotor, +then a given rotor might map `A' to `L', `B' to `Z', `C' to `G', and so on. +To encrypt, we choose several different rotors, and set the origins of the +rotors to known positions; their initial position is the ciphering key. To +encipher a character, we permute the original character by the first rotor, +and then apply the second rotor's permutation to the result. We continue +until we've applied all the rotors; the resulting character is our +ciphertext. We then change the origin of the final rotor by one position, +from `A' to `B'; if the final rotor has made a complete revolution, then we +rotate the next-to-last rotor by one position, and apply the same procedure +recursively. In other words, after enciphering one character, we advance +the rotors in the same fashion as a car's odometer. Decoding works in the +same way, except we reverse the permutations and apply them in the opposite +order. +\index{Ellinghouse, Lance} +\indexii{Enigma}{cipher} + +The available functions in this module are: + +\renewcommand{\indexsubitem}{(in module rotor)} +\begin{funcdesc}{newrotor}{key\optional{\, numrotors}} +Returns a rotor object. \var{key} is a string containing the encryption key +for the object; it can contain arbitrary binary data. The key will be used +to randomly generate the rotor permutations and their initial positions. +\var{numrotors} is the number of rotor permutations in the returned object; +if it is omitted, a default value of 6 will be used. +\end{funcdesc} + +Rotor objects have the following methods: + +\renewcommand{\indexsubitem}{(rotor method)} +\begin{funcdesc}{setkey}{} +Resets the rotor to its initial state. +\end{funcdesc} + +\begin{funcdesc}{encrypt}{plaintext} +Resets the rotor object to its initial state and encrypts \var{plaintext}, +returning a string containing the ciphertext. The ciphertext is always the +same length as the original plaintext. +\end{funcdesc} + +\begin{funcdesc}{encryptmore}{plaintext} +Encrypts \var{plaintext} without resetting the rotor object, and returns a +string containing the ciphertext. +\end{funcdesc} + +\begin{funcdesc}{decrypt}{ciphertext} +Resets the rotor object to its initial state and decrypts \var{ciphertext}, +returning a string containing the ciphertext. The plaintext string will +always be the same length as the ciphertext. +\end{funcdesc} + +\begin{funcdesc}{decryptmore}{ciphertext} +Decrypts \var{ciphertext} without resetting the rotor object, and returns a +string containing the ciphertext. +\end{funcdesc} + +An example usage: +\bcode\begin{verbatim} +>>> import rotor +>>> rt = rotor.newrotor('key', 12) +>>> rt.encrypt('bar') +'\2534\363' +>>> rt.encryptmore('bar') +'\357\375$' +>>> rt.encrypt('bar') +'\2534\363' +>>> rt.decrypt('\2534\363') +'bar' +>>> rt.decryptmore('\357\375$') +'bar' +>>> rt.decrypt('\357\375$') +'l(\315' +>>> del rt +\end{verbatim}\ecode + +The module's code is not an exact simulation of the original Enigma device; +it implements the rotor encryption scheme differently from the original. The +most important difference is that in the original Enigma, there were only 5 +or 6 different rotors in existence, and they were applied twice to each +character; the cipher key was the order in which they were placed in the +machine. The Python rotor module uses the supplied key to initialize a +random number generator; the rotor permutations and their initial positions +are then randomly generated. The original device only enciphered the +letters of the alphabet, while this module can handle any 8-bit binary data; +it also produces binary output. This module can also operate with an +arbitrary number of rotors. + +The original Enigma cipher was broken in 1944. % XXX: Is this right? +The version implemented here is probably a good deal more difficult to crack +(especially if you use many rotors), but it won't be impossible for +a truly skilful and determined attacker to break the cipher. So if you want +to keep the NSA out of your files, this rotor cipher may well be unsafe, but +for discouraging casual snooping through your files, it will probably be +just fine, and may be somewhat safer than using the Unix \file{crypt} +command. +\index{National Security Agency}\index{crypt(1)} +% XXX How were Unix commands represented in the docs? + |