# Finding the encryption key for a cipher given the plaintext and the ciphertext

The given cipher text is this:

CWT VIFTDBOTBC NDINIAMRA GTDT DTYTQCTS PK CWT RMPIED ANIZTAOMB MA XDDTRTFMBC CI CWT NDIPRTOA CWMC CWT RXPDMDK ATDFXQT XA UMQXBV

Using frequency analysis and some trial and error I have the plaintext:

THE GOVERNMENT PROPOSALS WERE REJECTED BY THE LABOUR SPOKESMAN AS IRRELEVANT TO THE PROBLEMS THAT THE LIBRARY SERVICE IS FACING

I have been told that this ciphertext has been obtained using a mono-alphabetic (substitution) cipher that is not an affine cipher. However I can't seem to find the key, I have tried using a 2-by-2 hill cipher and a Caesar Cipher but neither work. There is no common link I can find between the plain text alphabet and the Cipher text alphabet below:

a b c d e f g h i j k l m n o p r s t u v w y
m p q s t u v w x y z r o b i n d a c e f g k


where q,x,z have no known ciphertext encryption.

The cipher alphabet seems to be based on the keyword "robin hood" then q would map to h ,and we remove letters we already used (so the double o disappears), where you start half-way the alphabet (a bit non-standard) at l, and go circular. After the keyword letters are used up you start at the start of the alphabet (that is the acefg part). So x would map to j, q to h, z to l.

For the keyword idea, also see wikipedia. This is quite standard, though not very good, as the permutations tend to get recognisable sequences in alphabetic order ,which can be abused for solving them.

• I would suggest considering "start at L" to be part of the key. It's not exactly halway anyway. – Henning Makholm Feb 9 '17 at 20:01
• @HenningMakholm sure, "robin hood" and "start at l" would be a (the) way to rederive the full permutation key. – Henno Brandsma Feb 9 '17 at 20:04