National Cipher Challenge - PowerPoint Presentation

National Cipher Challenge A beginner’s guide to codes and ciphersPart 5, the keyword cipher

While mathematics can provide some very powerful ciphers, the two we have seen are not very strong. The problem is the keyspace. There are only 26 keys for the Caesar shift cipher and only 312 keys for the affine shift.

A long time ago the spies invented a much better system. It had to be:Strong – there had to be a lot of different keys. Memorable and easy to use – spies work under pressure and they don’t want to make mistakes.

The keyword cipher The key can be any word or name, or even phrase that you choose. You write it down, missing out repeated letters. Then complete the alphabet in order starting with the first missing letter. A B C D E F G H I J K L M E X A M P L N O Q R S T U N O P Q R S T U V W X Y Z V W Y Z B C D F G H I J K

Now we just encrypt the message one letter at a time. The submarine will pick up the agent on Friday dop cfxuebqvp hqtt yqas fy dop enpvd wv lbqmej

Leaving the word structure intact like this does weaken the cipher as we saw in the earlier lessons. We can try guessing that DOP stands for THE . dop cfxuebqvp hqtt yqas fy dop enpvT wv lbqmej A B C D E F G H I J K L M P O N O P Q R S T U V W X Y Z D

But that really doesn ’t get us very far! A B C D E F G H I J K L M ? ???P??O?????NOPQRSTUVWXYZ??????D????P? THE cfxuebqv E hqtt yqas fy THE en E v T wv lbqmej

A CRIB can be enormous help. A B C D E F G H I J K L M ? ? ??P??O?????NOPQRSTUVWXYZ??????D?????? THE cfxuebqv E hqtt yqas fy THE enEvT wv lbqmej If we are expecting a message about a submarine for example we might look for that.

Submarine hunter A B C D E F G H I J K L M E X POQUNOPQRSTUVWXYZVBCDFTHE cfxuebqvE hqtt yqas fy SUBMARINE?THE en E v T wv lbqmej

Submarine hunter A B C D E F G H I J K L M E X POQUNOPQRSTUVWXYZVBCDFDOP cfxuebqvP hqtt yqas fyTHE SUBMARINE I I U DOP enPvD wv lbqmejTHE A ENT N RI A

This is looking good! A B C D E F G H I J K L M E X POQUNOPQRSTUVWXYZVBCDFDOP cfxuebqvP hqtt yqas fyTHE SUBMARINE I I U DOP enPvD wv lbqmejTHE A ENT N RI A

This is looking better than good! A B C D E F G H I J K L M E X PNOQUNOPQRSTUVWXYZVBCDFDOP cfxuebqvP hqtt yqas fyTHE SUBMARINE I I U DOP enPvD wv lbqmejTHE AGENT N RI A

This is looking excellent! A B C D E F G H I J K L M E X PNOQRSTUNOPQRSTUVWXYZVBCDFDOP cfxuebqvP hqtt yqas fy THE SUBMARINE I I U DOP enPvD wv lbqmejTHE AGENT N RI A

Wow we are good at this! A B C D E F G H I J K L M E X PNOQRSTUNOPQRSTUVWXYZVBCDFDOP cfxuebqvP hqtt yqas fy THE SUBMARINE ILL I K U DOP enPvD wv lbqmejTHE AGENT N RI A

This agent is as good as caught! A B C D E F G H I J K L M E X PNOQRSTUNOPQRSTUVWXYZVBCDFGHDOP cfxuebqvP hqtt yqas fy THE SUBMARINE ILL I K U DOP enPvD wv lbqmejTHE AGENT N RI A

Hmm, getting trickier now! A B C D E F G H I J K L M E X PNOQRSTUNOPQRSTUVWXYZVBCDFGHDOP cfxuebqvP hqtt yqas fy THE SUBMARINE WILL I K U DOP enPvD wv lbqmejTHE AGENT N RI A

Ah that’s better! A B C D E F G H I J K L M E X APNOQRSTUNOPQRSTUVWXYZVYBCDFGHDOP cfxuebqvP hqtt yqas fy THE SUBMARINE WILL PICK UP DOP enPvD wv lbqmejTHE AGENT N RI A

Got you now! A B C D E F G H I J K L M E X APNOQRSTUNOPQRSTUVWXYZVWYBCDFGHDOP cfxuebqvP hqtt yqas fyTHE SUBMARINE WILL PICK UP DOP enPvD wv lbqmejTHE AGENT ON RI A

In the bag! A B C D E F G H I J K L M E X AMPLNOQRSTUNOPQRSTUVWXYZVWYBCDFGHJDOP cfxuebqvP hqtt yqas fyTHE SUBMARINE WILL PICK UP DOP enPvD wv lbqmejTHE AGENT ON FRIDAY

We are the best! A B C D E F G H I J K L M E X AMPLNOQRSTUNOPQRSTUVWXYZVWYZBCDFGHIJK DOP cfxuebqvP hqtt yqas fyTHE SUBMARINE WILL PICK UP DOP enPvD wv lbqmej THE AGENT ON FRIDAY

Uh oh!