2 @unnumbered Merkle Tree Hashing
4 NNCP uses @url{https://github.com/BLAKE3-team/BLAKE3, BLAKE3} hash
5 function in @url{https://en.wikipedia.org/wiki/Merkle_Tree, Merkle Tree}
6 mode of operation for checksumming @ref{Encrypted, encrypted packets}
7 and @ref{Chunked, chunked} files.
9 Previously ordinary BLAKE2b-256 was used, but it prevented partial
10 calculations of file parts, so you had to fully read the whole file
11 again after its resumed download.
13 MTH divides data on 128 KiB blocks, hashes each of them independently
14 and then calculates the Merkle tree root:
29 leaf0 leaf1 leaf2 leaf3 leaf4
31 block block block block block
34 Leaf's value is keyed BLAKE3-256 hash of underlying block (128 KiB,
35 except for probably the last one). Node's value is keyed BLAKE3-256 hash
36 of two underlying leafs. Keys are
37 @verb{|BLAKE3-256("NNCP MTH LEAF")|} and
38 @verb{|BLAKE3-256("NNCP MTH NODE")|}.
39 Keyed operation allows working with an aligned data (128KiB or 64B
40 boundaries), unlike popular way of prepending @verb{|0x00|} and
41 @verb{|0x01|} to the hashed data, being more efficient with an attention
42 to BLAKE3's internal Merkle tree.