Notabit

You no longer have to worry about the actual cryptographic reliability of the messengers you use for encrypted messaging. Regardless of a messenger's promises, the Notabit encrypted messaging app adds another layer of end-to-end encryption to your messenger secret conversation with a cipher you can see and test.
Type your message into Notabit, get an encrypted cipher, and send it to your recipient via any messenger, SMS, or email.

Since Notabit is an independent, third-party encrypted messaging app, there is no way to compromise the encryption through a predefined loophole on the server side of the messenger or any other party in the middle.
Just consider any popular messenger as a useful communication channel with a cryptographic quality you can never really verify to send strongly encrypted messages with Notabit.

To send encrypted messages, click the "New Chat" button and copy the default generated encryption invitation into any messenger and send it to your recipient. Your recipient will receive the encryption, add it to his Notabit and enter his reply. And so on.
The app uses encryption and signing with public-key cryptography.
The sender encrypts the message with the recipient's public encryption key and signs it with the sender's private signing key. The recipient decrypts the message with the private signing key and verifies the signature with the sender's public signing key.

This process of key exchange means that communication takes place in ping-pong mode, i.e. each request must be answered in order for a chat to remain consistent.