When communicating over a two-way radio, it is important to understand that anyone can listen to the messages communicated over the channels if they are on the same channel. This is may not be acceptable when the device is used by security agencies and other private users who wish to keep their conversations private and inaccessible to unauthorized persons.
This problem can be eliminated by electronically encrypting conversations before transmitting them and then decrypting the conversations at the receivers end, therefore ensuring their privacy. This encryption capability is provided in many high-end commercially available radios.
Generally, a group of two-way radios to be used together are subjected to a configuration set up wherein the encryption and decryption codes that are to be used during transmission and receiving of the data are programmed into the devices. This may involve putting the radios in a programming mode so they can "talk" to each other to learn the codes needed to decrypt a message.
Anyone intercepting a decrypted message on a radios not programmed will hear garbled voice so they won't be able to understand the conversation.
There are different encryption levels such low, medium and high and the implementation of these levels are dependent on the commercial use of the devices. One of the more secure encryption techniques is called P25, which is a suite of wireless communications protocols used in the US and elsewhere for two-way voice radio systems. The protocols include security options in which voice and data traffic can be cryptographically protected from eavesdropping. Lower level radios may simply have voice scrambling that could easily be picked up with someone who has the same radio from the same manufacturer.