RedPhone uses ZRTP, an open source Internet voice cryptography scheme created by Phil Zimmermann, inventor of the widely-used Pretty Good Privacy or PGP encryption. When a caller dials another RedPhone user, the app uses the two users’ keys to create a simple passphrase (“flatfoot eskimo” or “slingshot millionaire,” for example) and display it on each phone, allowing the speakers to verify that the codes match, and that there’s no man-in-the-middle intercepting the call.
TextSecure uses a similar scheme developed by cryptographers Ian Goldberg and Nikita Borisov known as “Off The Record” to exchange scrambled text messages. Both apps automatically generate a new key and delete the old one with every communication so that even if a user’s key is stolen, none of his or her past calls or texts can be deciphered. -- Sources google.com and forbes.com