Cipher challenge Spring 2008

Rules for the decryptions

EXTENDED DEADLINE (by popular request): TUESDAY 4th MARCH, 10am.

These rules are different from last year, so please read them carefully.

You may submit solutions (by email, 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 to ciphers 1--49 should be sent to P.Keevash (at qmul.ac.uk), and solutions to ciphers 50-99 should be sent to R.A.Wilson (at qmul.ac.uk). As before, no attachments please. And definitely no Microsoft Word documents!

In order to be counted, your solution must contain a full explanation of the method of cryptanalysis.

If you use a computer program, it must be one you've written yourself, and the program must be submitted along with your solution.

The deadline for submission is noon, Thursday 28th February

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 20 for this part of the cipher challenge. This counts 7.5% of your overall mark for the module.

There is a fixed number of marks available for each cipher, and these marks will be shared out equally between the correct solutions to that cipher. (Probably 50 marks per cipher, but this may go up or down if I feel it gives an unfair distribution of marks. Thus if 25 people submit a solution to the same cipher, they will get 2 marks each.)

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


We have received a total of 99 ciphers, the majority of which we have been able to check, and in some cases correct. They have been anonymised and randomised for fairness.

I have put up the remaining ciphers without checking them, if the encryption method is too complicated, or not clear from the explanation submitted.

There are about 20 or 30 ciphers which can be broken by the methods discussed in the course, e.g. substitution, Vigenere with relatively short keys, etc.

Don't be put off by numbers: these are sometimes only a superficial disguise. Similarly a simple transposition may also be used as a superficial disguise.

First steps: how big is the alphabet? Is the frequency analysis flat or spiky? Is there any sign of null characters or homophones being used? Can you spot any repeated digraphs or trigraphs? On this evidence, what type of cipher is it likely to be?

Many people have used a combination of a Vigenere cipher and a substitution cipher. If you are ambitious, why don't you have a go at some of these?

Good luck!

Cipher 50, Cipher 15, Cipher 35, Cipher 39, Cipher 87, Cipher 41, Cipher 33, Cipher 23,
Cipher 14, Cipher 45, Cipher 11, Cipher 1, Cipher 6, Cipher 34, Cipher 63, Cipher 83,
Cipher 80, Cipher 21, Cipher 26, Cipher 37, Cipher 73, Cipher 4, Cipher 20, Cipher 68,
Cipher 28, Cipher 49, Cipher 22, Cipher 8, Cipher 18, Cipher 55, Cipher 31, Cipher 43,
Cipher 7, Cipher 74, Cipher 84, Cipher 2, Cipher 10, Cipher 64, Cipher 65, Cipher 53,
Cipher 19, Cipher 16, Cipher 70, Cipher 54, Cipher 42, Cipher 78, Cipher 86, Cipher 59,
Cipher 36, Cipher 58, Cipher 77, Cipher 12, Cipher 69, Cipher 5, Cipher 52, Cipher 62,
Cipher 24, Cipher 27, Cipher 38, Cipher 47, Cipher 67, Cipher 46, Cipher 48, Cipher 61,
Cipher 29, Cipher 85, Cipher 56, Cipher 13, Cipher 57, Cipher 25, Cipher 72, Cipher 17,
Cipher 40, Cipher 71, Cipher 30, Cipher 75, Cipher 3, Cipher 66, Cipher 82, Cipher 40,
Cipher 63, Cipher 12, Cipher 44, Cipher 76, Cipher 11, Cipher 14, Cipher 73

Select a cipher from above

Robert A. Wilson
Created 19 February 2007
Updated 26 February 2008