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Merkle Tree Hashing

NNCP uses BLAKE3 hash function in Merkle Tree mode of operation for checksumming encrypted packets and chunked files.

Previously ordinary BLAKE2b-256 was used, but it prevented partial calculations of file parts, so you had to fully read the whole file again after its resumed download.

MTH divides data on 128 KiB blocks, hashes each of them independently and then calculates the Merkle tree root:

               /   \
              /     \
           node2    leaf4
          /    \       \
         /      \       \
        /        \       \
       /          \       \
      /            \       \
    node0         node1    leaf4
   /    \        /    \      \
  /      \      /      \      \
leaf0  leaf1  leaf2  leaf3  leaf4
  |      |      |      |      |
block  block  block  block  block

Leaf’s value is keyed BLAKE3-256 hash of underlying block (128 KiB, except for probably the last one). Node’s value is keyed BLAKE3-256 hash of two underlying leafs. Keys are BLAKE3-256("NNCP MTH LEAF") and BLAKE3-256("NNCP MTH NODE"). Keyed operation allows working with an aligned data (128KiB or 64B boundaries), unlike popular way of prepending 0x00 and 0x01 to the hashed data, being more efficient with an attention to BLAKE3’s internal Merkle tree.