CryptoGraf is a freeware software for Nokia S60 2nd Edition mobile devices that allows you to encrypt and digitally sign SMS and MMS messages that you send just like PKI certificates.

The following encryption methods are supported:

-RSA 1024/2048 bits Encryption
-AES 256 bits Encryption
-SHA256 with RSA Digital Signature
-X.509 Public Key Digital Certificates

Here’s how the software works:

Both the sender and reciever need to have ths software installed. Before sending messages, you will need to create a CryptoContact for the recipient whom you want to send an encrypted message. After exchanging the CryptoContact with that reciepient, you are ready to exchange SMS/MMS messages securely.

Read the QuickStart guide here

Download the application here