As part of my recent IDS project, I will be chronicling my ‘learning journey’ in the subject of cryptography and cryptoanalysis through a series of posts, each detailing a brief summary of what I have learned and/or accomplished.

Today I began reading the Code Book, an excellent summary of the history of cryptoanalysis. I have already read this book for pleasure, but this is the first time I am reading it for the purposes of research.

The Code Book covers a variety of different encryption methods and how they were implemented, beginning with simple substitution ciphers. Substitution ciphers were ciphers in which one letter was swapped from another letter based on a key, which is just a simple table of letters. For example:

A – Q,

B – M,

C – G,

D – I,

And so on, with each letter mapping to another letter. For example, the message CDAB would be encrypted as GIQM.

Subsequently, the Code Book explains frequency analysis, a technique used to decrypt substitution ciphers without knowledge of the key. It is quite complicated, but it essentially involves analyzing the most common letters of the encrypted text and comparing them to the most common letters of whatever language it’s in, in order to deduce the key.

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