According to the documentation message encryption is done in following way
Each message is encrypted with a one-time pad that consists of the channel ID and the index of the key used to sign the message; an additional nonce may be used as a revocable encryption key.
In mathematical form is above statement means
cipher = Message XOR ChannelID XOR IndexKey
I am not understanding how is onetime pad key is computed using channel ID and index of the key used to sign the message.