A short book in the "Web Security Topics" series, by the well-known authors Nigel and Jenny Chapman. Web applications are often entrusted with sensitive data which must be protected in transit between the Web browser and server to prevent its interception. Networks, especially wireless networks, are susceptible to eavesdropping, and precautions must be taken to ensure that it is not possible to read or interfere with data in the event of interception. Care must also be taken that data goes to its intended destination and is not waylaid en route. Written for professional and student Web developers, this little book provides a clear, non-mathematical introduction to the essentials of cryptography and to the protocols used for securing communication on the World Wide Web, specifically TLS/SSL and HTTPS. The examples focus especially on the requirements of small e-commerce sites. Short working programs written in JavaScript/Node.js are provided throughout the book and via the companion site websecuritytopics.info. Topics covered include the encryption and decryption of data, the principles underlying ciphers such as DES and AES, message authentication using SHA and HMAC, the use of OpenSSL for encryption and hash computation, public key cryptography and certificates and digital signatures. The TLS/SSL protocol is described in some detail together with its use in HTTPS, the secure version of HTTP used to safeguard sensitive Web sites. Clear key points provide useful summaries at the end of each section, and technical terms are defined in a 14-page glossary. A mail-in form at the back of the book enables every purchaser to claim a free PDF copy from the publisher.