lzvn

A safe, dependency-free, no_std pure-Rust LZVN decompressor that actually reads real macOS files — length-tolerant where every other Rust decoder rejects them.

// A raw Apple LZVN block (small-literal "hello" + end-of-stream).
let block = [0xe5, b'h', b'e', b'l', b'l', b'o', 0x06, 0, 0, 0, 0, 0, 0, 0];
let mut out = [0u8; 5];
let n = lzvn::decode_into(&block, &mut out)?;
assert_eq!(&out[..n], b"hello");
# Ok::<(), lzvn::Error>(())

[dependencies]
lzvn = { package = "lzvn-core", version = "0.1" }

Why this exists

LZVN is Apple's compression codec for HFS+/APFS transparent compression (decmpfs types 7 and 8) and the bvxn block type inside an LZFSE stream. Real macOS decmpfs resource-fork blocks end with the LZVN end-of-stream opcode (0x06) and are then followed by 80–300 bytes of arbitrary trailing data per block. Apple's kernel and lzvn_decode_buffer ignore those bytes; strict whole-stream Rust decoders (lzfse_rust, the lzvn crate) reject them — so they fail on genuine macOS system files (verified against macOS 26.5 "Tahoe": 0/25 real type-8 files decoded by the strict path).

lzvn stops at the end-of-stream marker and returns. That single property is the difference between decoding a real evidence disk and erroring out.

Trust, but verify

• #![forbid(unsafe_code)], zero dependencies, no_std — bounds-checked, returns a typed [Error] on malformed input rather than panicking or reading out of bounds.
• Validated against an independent oracle. Committed fixtures are real LZVN streams produced by Apple's own COMPRESSION_LZVN encoder (libcompression), padded with trailing bytes to mirror a decmpfs block — and the decoder was additionally validated against 25 genuine macOS 26.5 system-file blocks against the same Apple oracle (0/25 decoded by strict decoders, 25/25 here).
• Fuzz-hardened. cargo fuzz run decode drives arbitrary input through the decoder; the invariant is "never panic."

Scope

This crate is the codec — it decodes a raw LZVN block to bytes. The decmpfs framing (the resource-fork block table, the inline-marker conventions) belongs to the filesystem reader that calls it (e.g. hfsplus-forensic). Decode-only; no encoder.

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