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.