We have made breaking changes to Willow Drop Format (formally Sideloading protocol), allowing for entries with no available payload and a more efficient encoding.

First, we have generalised the parameters for fingerprint computation in range-based set reconciliation: the old type of Fingerprints is now called PreFingerprint. After computing a PreFingerprint but before sending it to the other peer for comparison, it is transformed into an actual Fingerprint with an arbitrary function. This allows using large fingerprints with nice algebraic properties for the computations, but compressing them with a conventional hash function before transmission.

The second generalisation concerns payload transmission. Instead of transmitting payloads verbatim, peers may now transform the payloads into arbitrary other bytestrings, and exchange those instead. This opens up features such as streaming verification of partial payloads, or on-the-fly compression.