All of our officially supported Ably client libraries support cross-platform symmetric encryption. Symmetric encryption is defined as an algorithm for cryptography that uses the same cryptographic keys for both encryption of plaintext and decryption of the ciphertext.
If you want to ensure that all payloads delivered through the Ably system are encrypted automatically before they arrive at Ably and decrypted when they are received from Ably, then we provide this feature natively in our client libraries across every platform we support. Encryption is automatically enabled if you provide a secret key to the client library when instantiating a channel. The secret key is kept secret by our client libraries and is only used for encrypting and decrypting messages. As such, you can be rest assured that your payloads can never be intercepted whilst being transmitted through the Ably system.
Added to this, all communication with Ably can be over TLS (with 2048 bit asymmetric encryption) thus further ensuring that your messages can never be intercepted whilst in transit to and from the Ably servers.
Natively our client libraries support AES 256 and 128 bit encryption, see http://en.wikipedia.org/wiki/Advanced_Encryption_Standard for more information on the AES standard.