Technology explained: What is encryption?
22 January 2016 Last updated at 22:35 GMT
Encryption is a way of scrambling computer data so it can only be read by the people you want.
It is an essential part of online shopping and banking, and can make emails and instant messages illegible if they are intercepted.
Encryption has been in the news recently. Some governments want service providers to build backdoors into their encryption systems that would let law enforcement access suspects' data.
The BBC's Chris Foxx explains how one type of encryption - public key encryption - can be useful in secure communications.