VIGENERE(1) General Commands Manual VIGENERE(1)
NAME
vigenere, kasiski – polyalphabetic cipher encryption and attack utilities
SYNOPSIS
vigenere [-de] key
kasiski [-ep]
DESCRIPTION
The vigenere utility implements encryption and decryption using the
Vigenere polyalphabetic shift cipher on a given alphabetic key. The
kasiski utility implements a method of cryptanalysis of such cipher based
on Kasiski examination and frequence analysis.
vigenere uses the key given as argument to encrypt or decrypt the text
read from standard input. The result is written into standard output.
Both the key argument and the standard input must contain plain
alphabetic characters (ie, uppercase or lowercase latin letters) encoded
in ASCII; any other character is ignored and is output as it is, without
being encrypted or decrypted.
The options for vigenere are as follows:
-d Perform decryption
-e Perform encryption (this is the default if no option is given).
kasiski reads a ciphertext from standard input and tries to guess the key
used to encrypt it with vigenere. The guessed key is written into
standard output. The read ciphertext must have been generated by
vigenere on either an English plaintext message or a Portuguese plaintext
message (only the frequency of letters in those languages are known).
The options for kasiski are as follows:
-e Use letter frequency in the English language for the frequency
analysis. (this is the default if no option is given).
-p Use letter frequency in the Portuguese language for the frequency
analysis.
EXAMPLES
Consider the following message text in the file sample.txt:
I'd just like to interject for a moment. What you're referring to as
Linux, is in fact, GNU/Linux, or as I've recently taken to calling it,
GNU plus Linux. Linux is not an operating system unto itself, but
rather another free component of a fully functioning GNU system made
useful by the GNU corelibs, shell utilities and vital system components
comprising a full OS as defined by POSIX.
Many computer users run a modified version of the GNU system every day,
without realizing it. Through a peculiar turn of events, the version of
GNU which is widely used today is often called "Linux", and many of its
users are not aware that it is basically the GNU system, developed by
the GNU Project.
There really is a Linux, and these people are using it, but it is just a
part of the system they use. Linux is the kernel: the program in the
system that allocates the machine's resources to the other programs that
you run. The kernel is an essential part of an operating system, but
useless by itself; it can only function in the context of a complete
operating system. Linux is normally used in combination with the GNU
operating system: the whole system is basically GNU with Linux added, or
GNU/Linux. All the so-called "Linux" distributions are really
distributions of GNU/Linux.
The following example encrypts the message: using the string "stallman"
as key.
$ <sample.txt vigenere -e stallman
A'w jfdf lvcx tz tztrjcene roe s foxpzt. [...]
The following example encrypts and then decrypts the same message using
the same key.
$ <sample.txt vigenere -e stallman | vigenere -d stallman
I'd just like to interject for a moment. [...]
The following example encrypts the same message using the same key and
then tries to guess the key used to encrypt it.
$ <sample.txt vigenere -e stallman | kasiski -e
stallman
HISTORY
The vigenere and kasiski utilities are the product of a project for 2022
2nd semester's course of Cyber Security of the Computer Science
department of the University of Brasilia, Brasil.
BUGS
kasiski is not perfect. It may not guess the key for relly short
messages, messages with more coincidental than accidental trigram
repetitions, messages with no trigram repetition at all, or messages with
a letter frequency very different from the ones common in the English or
Portuguese languages.
Short keys may be correctly guessed, but repeated a few number of times
(which are also valid. In the Vigenere cipher, a key is equivalent to its
repetition).
$ <sample.txt vigenere -e gnu | kasiski -e
gnugnugnugnu
Mid-size to long keys whose length is a prime number are probably guessed
wrong:
$ <sample.txt vigenere -e installgentoo | kasiski -e
inwtallgentooihstaalhpntooinstallkentdoinstallgenguo
Fiddling with the THRESHOLD value in the code can sometimes make kasiski
guess the right key.
kasiski also uses lots of memory. Mostly because of its trigram
hashtable with no collision.
AUTHORS
Lucas de Sena ⟨lucas at seninha dot org⟩
OpenBSD 7.2 December 9, 2022 OpenBSD 7.2