This book explains compactly, without theoretical superstructure and with as little mathematical formalism as possible, the essential concepts in the encryption of messages and data worthy of protection. The focus is on the description of the historically and practically important cipher, signature and authentication methods. Both symmetric encryption and public-key ciphers are discussed. In each case, the strategies used to attack and attempt to crack encryption are also discussed. Special emphasis is placed on the practical use of ciphers, especially in the everyday environment. The book is suitable for working groups at STEM schools and STEM teacher training, for introductory courses at universities as well as for interested students and adults.