punktfunk/crates/punktfunk-core/README.md

punktfunk-core

The shared protocol core — the one place where punktfunk's transport, forward error correction, and crypto live. It's linked into the host and every native client, so there's exactly one implementation of the wire format everywhere.

Written in Rust with no async on the per-frame path (native threads only). It exposes both a normal Rust API and a stable, versioned C ABI, so the Swift and Kotlin clients — and any C embedder — link the same code as the Rust ones.

What's in here

• Transport & session (session.rs, transport/, packet.rs) — the punktfunk/1 data plane over raw UDP: packetization, reassembly (with attacker-bounded limits), pacing, and socket tuning.
• FEC (fec/) — the wall-breaker. Two codes:
  • GF(2⁸) classic Reed–Solomon with the Cauchy generator matrix — byte-identical to the nanors library Moonlight uses, so our parity is decodable by a stock Moonlight client.
  • GF(2¹⁶) Leopard-RS (SIMD, O(n log n)) — up to 65535 shards/block, which removes the ~1 Gbps FEC ceiling. punktfunk/1 negotiates this one.
• Crypto (crypto.rs) — AES-128-GCM session encryption with per-direction nonce salts and sequence-as-AAD; SPAKE2 PIN pairing lives behind the quic feature.
• QUIC control plane (quic.rs, client.rs, feature quic) — the Hello/Welcome/Start handshake, cert pinning/TOFU, reverse audio, and the embeddable NativeClient connector. This is the only place tokio/quinn are allowed; the feature is off by default so the core stays runtime-free.
• C ABI (abi.rs) — the versioned surface (punktfunk_abi_version(), PunktfunkConfig carrying its own struct_size) that generates include/punktfunk_core.h via cbindgen at build time.

Build outputs

The crate builds three ways at once (crate-type = ["lib", "cdylib", "staticlib"]):

Output	Used by
lib (rlib)	the host, probe, and tools link it as a normal Rust crate
cdylib (.so/.dylib)	the Swift / Kotlin clients via the C ABI
staticlib (.a)	the C test harness and static embedding

Test

cargo test -p punktfunk-core                 # unit + proptest + loopback
cargo run  -p loss-harness                   # FEC loss-resilience sweep (no network needed)
bash crates/punktfunk-core/tests/c/run.sh    # standalone C-ABI link + round-trip proof

Design invariants (do not regress)

• One core, linked everywhere — protocol/FEC/crypto live only here, behind the stable C ABI.
• No async on the hot path — the per-frame pipeline is native threads only; quic (tokio/quinn) is control-plane only, feature-gated, off by default.
• Security hardening stays intact — the reassembler bounds attacker-controlled fields before allocating; AES-GCM keeps per-direction nonce salts + seq-as-AAD; the ABI checks struct_size. Regression tests exist — keep them green.

Related

• punktfunk-host — the streaming host built on this core
• Clients — the apps that link this core over the C ABI (or directly, in Rust)
• punktfunk-planning: implementation-plan.md (internal planning repo) — why GF(2¹⁶) FEC, the latency budget, and the architecture thesis