Cryptanalysis is the fine art of taking what we don’t know and converting it into something we do. In this case, taking an encrypted message and converting into a non-encrypted one, plaintext. To discover meaning from an encrypted message there has to be an understanding of what method of encryption was used. And so we are left with two problems, discovering how a message was encrypted and discovering what it’s unencrypted value is.

There are two types of encryption, classical and modern. Classical encryption is the substitution and mixing of characters, effectively a character is encrypted to another character. This ‘jumbling’ up makes the message undecipherable even though all the characters are ‘readable’.

Modern encryption takes an input message, and using number theory and its ilk, converts it into a digital message. The form of encryption can work on individual characters or multiple characters.

In a word, modern cryptanalysis is tough, classical is easy. The ‘ease’ of decryption for classic encryption makes it unsuitable for modern use, still, it is a ‘fun’ thing to investigate it and multiple articles and books are provided.

An American giant in the field of classical cryptanalysis was William Fredrick Friedman. He and his group were responsible for, for example, decrypting the Japanese declaration of war on the United States in World War II, and delivering the decrypted message to Franklin Delano Roosevelt before the Japanese ambassador could deliver it. His rank in the cryptanalysis world, and his writing extensive enough, to be included as a separate topic with some of his publications.

Modern Cryptanalysis Books & Papers
Cryptanalysis: a study of ciphers and their solution
A Self-study Course in Block-Cipher Cryptanalysis
A Tutorial on Linear and Differential Cryptanalysis
Algorithmic Cryptanalysis
Algorithmic Cryptanalysis Companion Web Site
An introduction to cryptography and cryptanalysis
Block Ciphers and Cryptanalysis
Cryptanalysis and Design of Stream Ciphers
Cryptanalysis of RSA Using Algebraic and Lattice Methods
Differential Cryptanalysis
Differential Cryptanalysis of the Data Encryption Standard
Differential Cryptanalysis of DES-like Cryptosystems
Efficient Cryptanalysis of Homophonic Substitution Ciphers
The Applications of Genetic Algorithms in Cryptanalysis
Classical Cryptanalysis Books & Papers
Amazon cryptanalysis books
Army Cryptanalysis Manual: FM 32-40-2
Crypto Corner: Worksheet and Teachers Notes
Classical Cryptanalysis Techniques
Cryptography of Multiplicative Ciphers
Cryptography of the Vigenere Cipher
Classical Cryptography
How we Cracked the Code Book Ciphers
Hybrid Vigenere-Columnar Cipher
History of Cryptography
Index of Coincidence
Integral Cryptanalysis on Full MISTY1
Secret Codes & Number Games
The Mathematics of Encryption: An Elementary Introduction
The Mathematics of Encryption: An Elementary Introduction
William Frederick Friedman
Published book Listing
Military Cryptanalysis Part I: Monoalphabetic Substitution Systems
Military Cryptanalysis II: Simpler Varieties of Polyalphabetic Substitution Systems
Military Cryptanalysis Part III: Simpler Varieties of Aperiodic Substitution Systems
Military Cryptanalysis IV: Transposition and Fractionating Systems