Uni Directional Payment

Dear All Smart Contract Developers,

I am an amateur in this space and would greatly appreciate your help in helping me understand certain concepts from this documentation (Uni-Directional Payment Channel).

In this documentation, it states that "Alice authorizes a payment by signing a message (off chain) and sends the signature to Bob". I am unsure what it means by signing a message (off chain) in this case, is there a function I am supposed to create or what am I supposed to do?

I sincerely apologise for my inept in this field!

Regards,
Porsche Tan

It most likely means that Alice uses her private key to sign a message to Bob that says that she commits to performing this transaction on chain at some point in the future if Bob wants to settle. I am not sure which project you’re reading the documentation from, but at least in the case of Raiden this is pretty much how it works. The parties involved in a channel exchange messages off-chain that can later be understood by a smart contract on-chain in order to settle the balances in the channel.