5.2 The AVENUE encryption scheme
5.2.1 Problem
AVENUE is the programming language of ArcView, a popular mapping software produced by ESRI of Redlands,CA. ArcView stores AVENUE 'scripts' (user-authored programs) as part of a simple text files (known as a 'project') with the extension 'apr'. These files are not proprietary and can be viewed with any text editor. The user has the option of encrypting these scripts with an algorithm by which, according to ESRI, "is an irreversible action which lets you build a project with customizations that are hidden from other users... ...Once encrypted, scripts cannot be restored to text for further development or debugging." But in reality, AVENUE uses a code that is literally thousands of years old to encrypt scripts (the Vigenere cipher), and its weaknesses are well known.
To encode using this cipher, a keyword must be chosen, such as "ARCVIEW". This is written above the plaintext repeatedly :
Code:
ARCVIEWARCVIEWARCVIEWARCVIEWA
Plaintext: MEETATMIDNIGHTBYTHEOLDOAKTREE
To encode the plaintext, one simply adds the position of the cipher letter in the alphabet to the position of the plaintext letter, using wraparound if necessary. So in the above example, the first coded letter would be 'A' (1st in alphabet) + 'M' (14th in alphabet) = 'N'(15th in alphabet) To decrypt the Vigenere cipher, you run this process in reverse. The keyphrase for Arcview is not a word but a 8-bit block of numeric shift values {-5,8,1,-2,7,4,-5,8}. These values are repeatedly cycled through the
ASCII values of the encrypted script text to reproduce the sourcecode. A flaw of this algorithm is that long strings of identical characters repeat themselves in the encrypted text; notice for example the string of asterisks in this segment of an unencrypted and encrypted script. It was this exact pattern led to the decryption of the algorithm.
There are some caveats. All line feeds (
ASCII Character 10) and Carriage Returns (
ASCII Character 13) are ignored. Clearly this is an effort on behalf of the programmers to make the .apr files portable between different platforms (IBM, MAC, UNIX), as they all use different end-of-line signatures. The first and last double-quotes are ignored, these being the ones that surround the encrypted text. The forward slash '\' indicates that the next character is to be taken literally, although the initial forward slash is ignored. This is used for such characters as the double-quote and forward slash itself that can stand for plaintext characters in the encrypted text.
Zitat
Baum-Darstellung
