Ciphers page

There are many different ways of encrypting information that exist. Some of the simplest types of cipher, have been described in this page. I have created a few executable (windows 95 onwards) which help the process of deciphering information. As you may have guessed, this page is still under heavy development, although not active development at the moment - as soon as I have worked on it, it will be updated! However, all information up is accurate - if you discover any inaccuracies in the software / text / dead-links then email me please!

My team has even got recognition by the local paper! Click here to see the article, posted in the Argus,17/1/04, page3

Different types if cipher

Straight Substitution Ciphers
This is where a cipher-text alphabet is used instead of the standard A-Z. e.g.:
ORIGINAL   :
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
CIPHER-TEXT: K E Y W O R D F G H I J L M N P Q S T U V X Z A B C

As you can see in the example above, each letter takes on another letter. The above uses the keyword 'KEYWORD', but any keyword can be used. The only limitations to a keyword is that if there are any repeating letters, the repeats must be removed. Once the keyword has been entered, the rest of the alphabet must be filled in behind it.

Examples of straight substitution ciphers are: Keyword, Ceaser, Affine shift.

Straight substitution ciphers are very weak as far as ciphers go, as it is quite easy to crack using frequency analysis (see below). However, if they are combined with an encryption method such as anagram / transposition cipher, they are then made quite difficult (when decrypting, you won't get the original cipher-text straight out, therefore making it seem a little confusing at first.
/software/Subst Cipher.exe

Caesar Ciphers
These are the most simplest type of cipher to encode /decode. These are the type where you take the original letter, and 'rotate' it by a number

Example: (rot 1)
ORIGINAL   :
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
CIPHER-TEXT: B C D E F G H I J K L M N O P Q R S T U V W X Y Z A

(i.e. A becomes B, B becomes C
à moving each letter 1 forwards, hence rot 1)

 VDKK CNMD ENQ CDBQXOSHMF SGHR EHQRS BHOGDQ. HSR MNS SGZS GZQC QDZKKX HR HS?!

The traditional way of solving this cipher (by paper) would be with a wheel, but in this modern age of computers, I have made this application, where you enter the cipher, and enter the rotation amount (unfortunately it only accepts positive rotation values… rot -1 = rot 25). You can download this here:
/software/Ceasar Cipher.exe

Affine Shift Ciphers
This type of cipher is one of the most hardest to encrypt by hand, but as with all straight substitution ciphers, it can be cracked easily with frequency analysis.
How they work: Each letter is assigned a number (A=1 à Z=26). Then to encrypt it, you choose a multiplier and adder values (take x as the letter to encrypt) C = (Mx + a ) % 26
For those who don't understand % (same as MOD), this is where you divide by the number, and the remainder is the result returned - this returns a number that can be put back in the alphabet again.
However, you cannot encrypt for all multiplier values, as they can return the same letter for different letters put in. (e.g. 0, all even #s, 13...) Download link below:
/software/Affine Shift.exe

Analysing Ciphers

Letter Frequency Analysis
If a very long piece of text is required for decryption, then this program is your best bet. By counting the occurrences of each letter in the text, a probability can be made, and the ratios can then be compared with a standard 'English' piece of text, and you can then be fairly certain of which each letter corresponds to. Obviously, if the cipher is a ceasar, then it would just be easier to try each rotation, and see if one works...!
/cipher/letter frequencies.txt
/software/Text Frequency Analyser.exe

Common single letters
If the structure of the cipher has been kept, then it is quite likely, that a single letter on its own, will be either an A or an I (assuming standard english).
TBC
Diagraph analysis
TBC
Triagraph analysis
TBC

Deciphering is a very new concept to me, but having background knowledge in programming, creating the software was easy. This year at college I joined the cipher club, which involves different techniques for creating ciphers etc. Although the Southampton cipher challenge is over now, I shall still try to create a few more pieces of software, although the last two ciphers will require some quite complex array handling skills, as they use the ADFGVX cipher, being two stage.

Useful Cipher links

http://www.cipher.maths.soton.ac.uk          - Cipher Challenge, run by Southampton university
http://www.simonsingh.net                          - Simon Singh's website

If you have any comments / criticisms etc that you would like to tell me about on this subject, then please email me ASAP (!!)

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Last updated 29 June 2004
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