channel: add realtime encryption

[andydunstall]

DSC-74
@owenpearson @sacOO7 we're looking to support encryption with in the database connector, though AFAICT encryption isn't supported for realtime channels

Looking at rest channels, looks like its just doing:

		cipher, _ := options.GetCipher()
		var err error
		*m, err = (*m).withEncodedData(cipher)
		if err != nil {
			return fmt.Errorf("encoding data for message #%d: %w", i, err)
		}

Though I know why withEncodedData isn't run on non-rest channels?

Just adding draft PR to start discussion - this just copies withEncodedData into RealtimeChannel

[sacOO7]

Need to check on how encryption works in rest channels. You can check and implement if it's easy, we will review.

[andydunstall]

You can check and implement if it's easy, we will review.

Thanks - thats what this PR is :)

[sacOO7]

You can check and implement if it's easy, we will review.

Thanks - thats what this PR is :)

Can you add integration tests for this? Also, check if changes are not failing existing integration tests.

Thanks!

[andydunstall]

Oh I guess Subscribe also needs to handle encrypted messages too

[owenpearson]

Looks good so far, although I'm hesitant to approve because I don't think we're doing enough to test this at the moment. The test added here would pass even if the library just ignores the cipher param entirely. At a minimum, could we add a test which creates two realtimes with a cipher mismatch and checks that the behaviour is correct (see ably-js example). It would also be good to test the behaviour when sending an encrypted message to a client without a cipher and vice versa.

[andydunstall]

test the behaviour when sending an encrypted message to a client without a cipher and vice versa

Sure, extended the test to include a channel without a cipher, the that the message doesn't equal the original

[andydunstall]

@owenpearson @sacOO7 can I get a review on this?

[owenpearson]

LGTM, thanks

[andydunstall]

@owenpearson Thanks can this be merged?

[sacOO7]

LGTM