There are many different algorithms that are used for very different reasons.
Diffie-Helmann (which is indeed what DH stands for) lets two parties compute a shared secret. It does not do encryption/decryption itself, that’s just not what it is for. Indeed, usually DH is used to find a shared key, which is then used as encryption/decryption key in a symmetric cypher, such as AES or Blowfish.
ArrayBuffers are things that were added to JavaScript to allow easier (more efficient and less error-prone) manipulations of binary data. I am not entirely sure if the HTML5 WebSocket API (regardless of Phoenix) supports transferring such an object directly; it could very well be that it needs to be converted to either a binary file or a Base64 string.
