The Code Book by Simon Singh

The Code Book is a short and approachable introduction to cryptography. On 200 pages it goes through Caesar and Vigenere cipher through enigma all the way to Diffie-Hellman key exchange and RSA public key cryptography. It’s a good book if these topics are new to you, but even with basic knowledge of cryptography and mathematics it falls short and the stories from the history on their own doesn’t make it worthwhile read. For a summary of this book, see:

• The Imitation Game (2014 movie)
• RSA
• Diffie-Hellman
• Elliptic-curve cryptography

Or go more in-depth with the details of the Signal protocol:

• The X3DH Key Agreement Protocol
• The Double Ratchet Algorithm
• The Sesame Algorithm: Session Management for Asynchronous Message Encryption