Vigenère

Vigenère cypher library for XSLT

Introduction

This is an XSLT library which enables you to encrypt plain text or decrypt cypher text
using the Vigenère cypher. It’s
almost completely useless for security purposes. However this cypher turns up relatively
frequently in the books I’ve been reading lately, so I wrote this library for fun and
convenience.

The name Vigenère properly includes a grave accent. I’ve omitted the accent in file
names, namespaces, namespace prefixes and repository names, to avoid possible problems
with accented characters. I know, it shouldn’t be a problem in this day and age, but
still…

Prerequisites

An XSLT processor compatible with XSLT 1.0 or 1.1 and
eXSLT. It works with XSLTProc.

Vigenere.xsl

This provides two functions:

• vigenere:decrypt(code, cypher_text) decrypts the cypher_text using the key code provided.
• vigenere:encrypt(code, plain_text) encrypts the plain_text using the key code provided.

The namespace URI for these functions is urnfunction. You need to declare
this namespace in any XSLT stylesheets that use this library.

The libraries make use of a Tabula recta. I
could have used modular arithmetic to perform the encryption and decryption, but I found
this method easier to visualise.

The plain_text and the cypher_text must only contain lower case alphabetical
characters with no accents; i.e. no numbers, spaces or punctuation. This is not enforced
by the library at the moment. If this condition is not met, the library will generate
incorrect results.

Demo.xsl

This stylesheet demonstrates how the library works. It reads a source file in XML Java
Properties format.
(The choice of this format is not particularly significant; it’s just a handy,
established XML format for storing key / value pairs.)

The source file must contain a code entry containing the key code, and any number of
plain entries containing plain text and / or cypher entries containing cypher text.
Wherever the stylesheet finds a cypher entry, it decrypts the cypher text; wherever the
stylesheet finds a plain entry, it encrypts the plain text.

I’ve also provided a few example files.

Examples

You can use the files in the examples folder to try out different key codes and crack
the cypher:

xsltproc Demo.xsl examples/Feel_the_Fear.xml

The files in the spoilers folder already contain the correct key codes, so don’t peek
if you want to solve them yourself!

xsltproc Demo.xsl spoilers/Red_Shift.xml

Note

It’s possible to use the decryption operation on plain text, giving a different cypher
text to what you would get using the encryption operation. Cypher text encrypted in this
way must be decrypted using the encryption operation. An example of this is shown in the
file Red_Shift.xml. The cypher text in this source file is stored in the plain entry;
this forces the demo stylesheet to decrypt it using the encryption operation.