c2pa-audio

Native C2PA (Content Credentials) signing & verification for WAV, MP3, M4A + FLAC audio — no c2pa-rs, no Rust, no OpenSSL.

A tiny (~160 KB), dependency-light implementation of the C2PA manifest format: canonical CBOR, JUMBF boxes, COSE_Sign1, and ES256 (ECDSA P-256 + SHA-256, via the vendored micro-ecc). It signs and verifies Content Credentials that are fully interoperable with the c2pa-rs reference implementation — in both directions.

It began as the C2PA layer inside CrispASR (marking AI-generated TTS output as c2pa.created / trainedAlgorithmicMedia), extracted here as a standalone library because it needs none of the ASR stack.

Why

The official c2pa-rs is excellent but heavy (~10 MB) — awkward for the browser (wasm) and for embedding in mobile/edge apps. This library is a clean-room reimplementation of the published C2PA spec + RFCs, small enough to compile anywhere (desktop, mobile, wasm), with a pure-JS variant needing no native code at all.

Interop matrix (all verified)

	verify: c2pa-rs	verify: our JS	verify: our C++
sign: c2pa-rs	✅ ref	✅	✅
sign: our JS	✅	✅	✅
sign: our C++	✅	✅	✅
sign: our wasm	✅	✅	✅

Every signer's output validates in every verifier for WAV, MP3, M4A, and FLAC, including the c2pa-rs reference reader; tampering the audio breaks the hard-binding hash, and a mangled signature fails COSE verification.

Languages

Language	How	Location	Status
C / C++	link the lib	include/c2pa_audio.h	✅ tested
JavaScript / TS	pure WebCrypto	js/c2pa.mjs, js/c2pa-verify.mjs	✅ tested
Dart / Flutter	dart:ffi	bindings/dart/	✅ tested
Python	ctypes	bindings/python/	✅ tested
Go	cgo	bindings/go/	✅ tested
C#	P/Invoke	bindings/csharp/	⚠️ code (no local dotnet)

The JS variant is special: js/c2pa.mjs + js/c2pa-verify.mjs are pure WebCrypto and need no native library — ideal for browsers and Workers.

Demo

demo/index.html is a self-contained, dependency-free page that signs, inspects, verifies, and tamper-tests audio entirely in the browser using the pure-JS library. Open the file directly, or regenerate it from source with node demo/build.mjs. See demo/README.md.

Build

cmake -B build -DCMAKE_BUILD_TYPE=Release
cmake --build build
ctest --test-dir build --output-on-failure     # C ABI round-trip + reference vector

Produces libc2pa_audio.{dylib,so,dll} (shared, for FFI) and .a (static).

C ABI

#include "c2pa_audio.h"

// sign (NULL cert/key -> bundled self-signed default cert)
unsigned char* out; size_t out_len;
c2pa_audio_sign(wav, wav_len, "audio/wav", NULL, NULL, &out, &out_len);

// verify -> bit flags (0xF == fully valid)
int flags = c2pa_audio_verify(out, out_len);

c2pa_audio_free(out);

Quick starts

JavaScript (no native lib):

import { c2paSignWav } from './js/c2pa.mjs';
import { c2paVerifyWav } from './js/c2pa-verify.mjs';
const signed = await c2paSignWav(wav, certPem, keyPem);
const result = await c2paVerifyWav(signed);   // { valid, signatureValid, ... }

Dart:

final c2pa = C2paAudio.open();               // loads libc2pa_audio
final signed = c2pa.signWav(wav);            // bundled default cert
final r = c2pa.verify(signed);               // r.valid == true

Python / Go / C# — see bindings/.

Scope

WAV (RIFF C2PA chunk), MP3 (ID3v2.4 GEOB frame), M4A/MP4 (ISO BMFF uuid box + c2pa.hash.bmff.v3), and FLAC (ID3v2 GEOB prepend, like c2pa-rs), sign + verify — all fully interoperable with c2pa-rs. Trust-anchor evaluation is out of scope (a self-signed cert verifies cryptographically but is "untrusted" to a full validator).

The BMFF path is codec-agnostic, so AAC-in-MP4 and Opus-in-MP4 work out of the box (pass audio/mp4). Only the raw streaming containers — ADTS AAC (.aac) and Ogg Opus (.opus/.ogg) — have no C2PA embedding path; neither does c2pa-rs (it refuses audio/aac / audio/ogg). Remux those into MP4 to sign them.

Signing API takes a MIME type: "audio/wav", "audio/mpeg", "audio/mp4", or "audio/flac". Verification auto-detects the container (RIFF / ID3 / ISO-BMFF).

Licensing

• This code: MIT (see LICENSE). It is an original, clean-room implementation of the published C2PA spec + RFCs (CBOR 8949, COSE 9052, JUMBF ISO 19566-5) — it does not derive from c2pa-rs source.
• Vendored micro-ecc (third_party/uecc/): BSD-2-Clause.
• SHA-256 (src/sha256.h): public domain.
• C2PA is an open standard with a royalty-free patent policy for conformant implementations. This is not legal advice — do your own IP review before shipping.