PKIMessage leverages your Apple devices built-in encryption to provide an additional layer of 2048-bit asymmetric encryption within iMessage, combining with the security already provided by iMessage, to ensure that no one can ever spy on your messages.
Messages are encrypted and signed prior to being sent out over the Internet, and decrypted and verified once received. No one but the acknowledged recipient on that specific device can ever view the message.
All messages are sent with the self-destruct flag built into iMessage, but you can also destroy keys after sending & receiving the message to ensure they are securely destroyed.
Also built into the extension is the ability to encrypt messages for external sharing, such as via email. These messages can then be copied back into iMessage by your intended recipient and decrypted within the extension.
*NOTE*: Messages can only be viewed on the device that shared their public key with you, and not every device registered to their account. This is intentional, and ensures that attackers that have registered a new device on another persons account cannot view your messages.
Exchange of public keys is made possible through the use of QR codes, which can be shared in-person (most secure), via "public key messages" inside PKIMessage (less secure, especially if attacks are suspected), or simply by taking an iOS screenshot and emailing them to your friend (least secure, especially if email account security is compromised).
Enjoy!