MTH6115: Cryptography

Cipher Challenge 2011: Part 2

Rules for the decryptions

DEADLINE: Tuesday 22nd March 2011, 11am. (This is an extension.)
Extended yet again: The new deadline is on Monday 28th March 2011, 2:35am (note that this is during the night).
Please note that you’ll also have deadlines for regular coursework on 14th March and 21st March.

These rules are different from what they have been in previous years, so please read them carefully.

You may submit solutions (by e-mail, as a plain text file, please) to UP TO THREE ciphers (except your own cipher, of course). Only the FIRST THREE you submit will be counted.

Solutions should be sent to me J.N.Bray [@ you know the rest]. As before, no Microsoft Word, Richtext, PDF, etc. Plain text only. (On Microsoft Windows the Notepad application produces the nearest approximation to plain text.) Also, please remember to press ENTER (also called RETURN) at the end of your files.

You should send all your solutions at once, if possible, and solutions to different ciphers should be in different attachments. In the main body of your e-mail, and at the start of each attachment state your SOLVER CODE (something like 42 BRA: I will e-mail those to you individually), your FULL NAME and your STUDENT NUMBER. In each attachment, state the number of the cipher you are trying to crack, and in the main part of your e-mail state all ciphers you have attempted to crack.

In order to be counted, your solution must contain a FULL EXPLANATION of the method of cryptanalysis. This means a step-by-step guide to each deduction (or guess) you make, and why. This would particularly apply if say nulls have been used, or the letter frequencies are slightly strange.

If you use a computer program, it must be one you’ve written yourself, and the program must be submitted along with your solution. You must also fully document the program to say what each bit is doing.

The deadline for submission is 11am, Tuesday 22nd March 2011.

Please play fair and do not help anyone else to break your own cipher. I expect to be able to spot most cases of collusion and will punish them appropriately.

Marking scheme

You will receive an overall mark out of 100 for this part of the cipher challenge. This counts 5% of your overall mark for the module.

The marks allocated to breaking each cipher will be of the form Cδnα, where n people break that cipher and C, δ and α are real numbers, with C, δ > 0 and α < 0. The numbers C and α are cipher-independent, while the ‘difficulty’ score δ might depend on cipher. Probably C = 60 and α = −1, and currently δ = 1 for all ciphers; but these may be subject to some adjustment if I feel the current parameters give an unfair distribution of marks. I will probably only adjust δ if I feel that certain ciphers are producing absurd marks. For example, with the current values of C, δ and α, if 30 people submit solutions to the same cipher, they will score just 2 marks each for it.

There will be a cash prize for the person who breaks the most difficult cipher (in the opinion of the judges).

The Ciphers

There are about 10 ciphers missing from the ones below, awaiting the outcomes of enquiries from their creators. A couple of others are missing at present, and some of the ciphers below I regard as being invalid. (All of the invalid ones are hard to crack.)

Select a cipher from those below. For various reasons it is possible that a “harder” is easier to crack than an “easier” one, so don’t give up on the “harder” ones. More details to follow at some point.

“Easier” Ciphers

Contains all the substitution ciphers, and Vigenère ciphers with shortish keywords. The only ciphers in this section likely to cause problems are substitution ciphers where the plaintext comes from a text such as Gadsby or A void.

Cipher 3, Cipher 15, Cipher 17, Cipher 18, Cipher 21, Cipher 22, Cipher 23, Cipher 25, Cipher 28, Cipher 30, Cipher 38, Cipher 39, Cipher 47, Cipher 49, Cipher 50, Cipher 52, Cipher 61, Cipher 69, Cipher 71, Cipher 74, Cipher 75, Cipher 92.

“Harder” Ciphers

Note: Some of the ciphers in this part are not-so-hard, and actually easier than those I have part in the “Easier” section, and a couple I have placed in here on merely formal grounds really ought to be in the “Easier” section.

Cipher 2, Cipher 4, Cipher 7, Cipher 10, Cipher 11, Cipher 29, Cipher 36, Cipher 37, Cipher 40, Cipher 45, Cipher 46, Cipher 48, Cipher 51, Cipher 55, Cipher 59, Cipher 62, Cipher 64, Cipher 65, Cipher 67, Cipher 68, Cipher 70, Cipher 72, Cipher 78, Cipher 79, Cipher 80, Cipher 83, Cipher 86, Cipher 87, Cipher 89, Cipher 90, Cipher 95.

Select a cipher from above.

Plain-text files may be in preparation, if you feel these would be helpful.
MRC This page is maintained by Dr John N. Bray.

Last updated: 21st March 2011.