Simple explanation of Keyword Ciphers?

[Anonymous]

I have been trying for the past hour to work out how to use Keyword Ciphers, especially when in blocks of letters instead of whole words. Can somebody explain how to find the keyword very simply as I’m not that clever? Frequency analysis works but not as well when letters are not grouped as words!

[Anonymous]

Have you looked at the sixth lesson PowerPoint on the resources page?

Resources

[Anonymous]

Letter frequency analysis should be exactly the same no matter the groupings of the letters.

If you want to crack a keyword cipher using frequency analysis, you’d do s by observing the most common letter in the ciphertext, i.e. G. We know that E is the most common letter in the English language, so we assume that G translates to E. Then, let’s say the second most common letter in your ciphertext is K. The second most common letter in English would be T, so K -> T etc.

[Anonymous]

The principle is simple. For a monoalphabetic substitution cipher (one letter in plaintext always maps to its CIPHERTEXT equivalent letter) a keyword is a convenient way of remembering the jumbled up letters that are the CIPHERTEXT alphabet.

The keyword is used to determine the letter mappings of the CIPHERTEXT alphabet to the plaintext alphabet. Any repeats of letters in the word are removed, then the CIPHERTEXT alphabet is generated with the remaining unused letters.

However, there are many ways to interpret that:

Method 1: Use remaining (unused) letters in alphabetic order starting at “A” (or the first unused letter in the alphabet if “A” is in the keyword) as described in Wikipedia.

e.g Keyword KRYPTOS the first unused letter in the alphabet is “A” so…
Plaintext: 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
CIPHERTEXT: K R Y P T O S A B C D E F G H I J L M N Q U V W X Z

Method 2: Use remaining (unused) letters in alphabetic order starting at the last letter of the keyword. This is the method described by Prof. Graham A. Niblo’s A beginner’s guide to codebreaking (Version 1.1b 1st October 2016) on this site.

e.g. Keyword SIMPSONS the second “S” is ignored (repeat letter) and the last letter of the key is then “N” and the next unused letter is “Q” (“O” and “P” are already in the keyword) so…
plaintext: 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
CIPHERTEXT: S I M P O N Q R T U V W X Y Z A B C D E F G H J K L

Method 3: Use remaining (unused) letters in alphabetic order starting at the first unused letter after the most advanced letter in the keyword as used in the National Cipher challenge 2016.

e.g. Keyword WESTON. The most advanced letter is “W” so start using up the unused letters with XYZABC…
plaintext: 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
CIPHERTEXT: W E S T O N X Y Z A B C D F G H I J K L M P Q R U V

NOTE: This is the most commonly used method in recent National Cipher Challenges.

Methods 4++ Use the remaining (unused letters) in a predetermined sequence, for example place them in a grid and read off by column, or in a spiral, or backwards rows, or any other method that the sender and receiver both know.

[Anonymous]

Re-reading the original question, Lebosi asked how to find the keyword.

1. Try a few cribs (informed guesses) or…
2. Start your cryptanalysis using the usual methods for a substitution cipher (analysis of letter, bigram, trigram and double letter frequencies, sociability etc.) As you proceed the keyword might reveal itself, or…
3. Complete the deciphering of the ciphertext and then compare the plaintext alphabe with the ciphertext mappings. Any keyword will then be evident.

[Author]

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