A block signature of a basis binary, and its librsync wire encoding.

The basis is split into block_len-byte blocks (the last one may be shorter). Each block contributes a {weak, strong} pair: its rolling checksum and the first strong_sum_len bytes of its strong hash.

librsync defines four signature types, all supported:

RabinKarp with BLAKE2b is the default and the default of librsync 2.3 and later. The rollsum and MD4 types exist for peers using older librsync defaults; MD4 is not collision-resistant.

Wire format, all integers big-endian:

u32 magic
u32 block_len
u32 strong_sum_len
repeated: u32 weak, strong_sum_len bytes strong

The format does not record the basis length, so a decoded signature cannot tell whether its last block is short.

index maps each weak checksum to the strong hashes seen with it, and each of those to a block number. It is not part of the wire format; build_index/1 fills it and Rexd.delta/2 calls that when the index is missing. Building and querying it take constant time per block however the checksums are distributed, so a signature crafted to share one weak checksum across many blocks costs no more to process than any other.