This textbook describes the main techniques and features of contemporary cryptography, but does so using secondary school mathematics so that the concepts discussed can be understood by non-mathematicians. The topics addressed include block ciphers, stream ciphers, public key encryption, digital signatures, cryptographic protocols, elliptic curve cryptography, theoretical security, blockchain and cryptocurrencies, issues concerning random numbers, and steganography. The key results discussed in each chapter are mathematically proven, and the methods are described in sufficient detail to enable their computational implementation. Exercises are provided.