Implementation Considerations

Customer Privacy

The merchant must use the same zkAbacus public parameters across all channels. Tne size of a customer's anonymity set is determined by how many channels are open with a given merchant using a particular set of public parameters, namedly those used in zkAbacus, which consist of a zkAbacus public key and range proof parameters. The customer must therefore have a mechanism to retrieve these parameters that provides the assurance not only that the parameters are indeed from the merchant, but also that the merchant has only set of parameters for all their customers. Potential options for this include the provision and use of an on-chain registry service for merchant parameters.
Traffic analysis. The zkChannels protocol does not provide any protection against a network-level adversary. Implementations should use a privacy-enhancing networking tool such as Tor.
Timing attacks on closes after adversarial merchant aborts. The zkChannels protocol provides cryptographic unlinkability if a merchant aborts during a customer payment. That is, the customer is always able to close using information that the merchant cannot, from the content of the close, use to link a channel to a particular aborted payment. However, this does not prevent timing attacks. Implementations should therefore consider mechanisms to avoid trivial trivial timing attacks in this case, such as by including a random delay interval before closing a channel after an aborted payment.

Optimizing merchant storage. A merchant may periodically drop closed channels from memory, but must not naively drop any information from their revocations database. In order to re-initialize this database, the merchant would have to stop processing payments and close all their channels at the same time.
Self delay values. The parties must agree on the values used for on-chain transaction timeouts. If global defaults are used across all of a merchant's channels, these should be set to relatively large values, thereby allowing all parties sufficient time to respond even in the presence of periodic network failures. If parties wish to specify channel-specific delay values, these must be set in accordance with each party's risk tolerances: the customer should choose the self-delay period of expiry and the merchant should choose the self-delay period of custClose.