Appendix A. BLAKE3

Reference · informative · the hash the content identifier is built on

The content identifier of chapter 3 is the first 16 bytes of a BLAKE3 hash over the preimage of section 3.2. BLAKE3 is specified by its authors in the BLAKE3 paper and reference implementation, published at github.com/BLAKE3-team. The standard uses the plain hash mode with the preimage’s own domain tag as its first bytes; it does not use BLAKE3’s keyed or key-derivation modes.

The reference implementation hashes with the Rust blake3 crate. The WebAssembly binding compiles the same crate. The Python and Node bindings and the satellite implementations use independent BLAKE3 implementations, and the vectors of chapter 8 are what show that all of them produce the same 16 bytes for the same preimage; that equality is the cross-language byte-equality contract of section 3.6.

Truncating a 32-byte BLAKE3 output to 16 bytes keeps the identifier the size of a node id and a Pulse’s envelope_ref; the 128-bit identifier is a content address, not a signature, and the standard makes no claim about it beyond collision resistance at that width for the purpose of addressing.