A cryptanalyst analyzes and tries to break unknown codes and ciphers
without knowing the "key" or the encoding method used.
The problem is that for most people the subject seems awfully esoteric - codes and ciphers
are what spies do, and are out of the range of concerns for ordinary people.
Mysterious Messages: A History of Codes and Ciphers
offers a fine survey of history's most dangerous secrets and ciphers that readers can use themselves.
The mysterious codes and ciphers
Zoe attempts to solve will fascinate the reader.
United States diplomatic codes and ciphers