The book teaches about private keys, session keys, public keys, digital signatures, and numerous cryptography protocols. Each protocol includes a scenario where the cast of characters exchange messages in ways that explain the protocol. These discussions are invaluable to someone who is evaluating encryption methods to select one that best matches a particular set of requirements. The association between public-key encryption and digital signatures is interesting. Each user of a public-key cryptosystem publishes a public key.
Correspondents use the public key to encrypt messages to the user.
An associated private key, known only to the user, decrypts the message. Everyone can send encrypted messages but only the intended receiver can decrypt them. Inversely, a message encrypted with the private key can be decrypted only with the associated public key. Receivers know that a broadcast message originated with a particular user because only that user's public key successfully decrypts the message. The encryption becomes, in effect, a digital signature.