Email + PGP vs TextSecure

The original version of PGP is no longer freeware since it was aquired by Symantec, but the source code can still be downloaded for peer review.

PGP is a protocol and not an application per se. It offers a standard on which applications can implement to offer full encryption.

PGP uses asymmetric encryption. One user generates 2 keys - private and public. Another user uses the public key to encrypt the message that can then be decrypted only using first users's private key.

Uses Curve25519, AES-256, and HMAC-SHA256. The security of these algorithms has been tested over many years of use in hundreds of different applications. Messages sent via TextSecure are end-to-end encrypted, which means that they can only be read by your intended recipients. We make it easy for you to verify that you are communicating with the right people and that no MITM attack has occurred. The keys that are used to encrypt your messages are stored on your device alone, and they are protected by an additional layer of encryption if you have a passphrase enabled.

TextSecure is free and open source, enabling anyone to verify its security by auditing the code. TextSecure is the only private messenger that uses open source peer-reviewed cryptographic protocols to keep your messages safe.

To pass EFF's security review TextSecure had to encrypt data in transit, the encryption had to be end-to-end, the app needed to provide forward secrecy, app's source code has to be open to independent review, users have to be able to verify contact's identities even with the service provider compromised, app's cryptographic design has to be well documented and app's source code has to have been audited in the last 12 months.