c2pa-text-binding 0.3.0

Overview

A concrete family of text soft-binding algorithms compatible with the C2PA Soft Binding framework, each registered in the C2PA soft binding algorithm list. A soft binding derives a content-keyed value that survives reformatting, re-encoding, excerpting, and light editing, so a manifest is recoverable when the hard binding — a byte-exact hash — has been broken.

The alg string is what a c2pa.soft-binding assertion carries and what a recovery service matches on; it is the identifier that matters.

This crate is the perceptual/watermark recovery layer. It is distinct from the Variation-Selector transport used elsewhere in WritersProof, which is a hard binding (a c2pa.hash.data over normalized text) and is not soft binding. Registration in the algorithm list is not C2PA conformance certification.

Quick Start

[dependencies]
c2pa-text-binding = "0.2"

Emit and sign a c2pa.soft-binding assertion

soft_binding builds the normative CBOR assertion (it round-trips through the c2pa-rs reader — see tests/c2pa_roundtrip.rs); manifest signs it as a COSE_Sign1 / EdDSA envelope.

use c2pa_text_binding::{simhash::Fingerprint, soft_binding, sign_cose, SOFT_BINDING_LABEL};

let text = "…the document being bound…";
let secret_key = [7u8; 32];                   // caller-supplied Ed25519 secret
let assertion = soft_binding::from_fingerprint(&Fingerprint::compute(text));
let cbor = assertion.to_cbor()?;              // store under SOFT_BINDING_LABEL
let signed = sign_cose(&cbor, &secret_key)?;  // detached-key COSE_Sign1
# Ok::<(), c2pa_text_binding::Error>(())

Recover and classify a candidate

The verify path recomputes the fingerprint from the current text, compares it to the stored value at the algorithm's registered threshold, and returns a confidence tier. BOUND requires a durable fingerprint match (41/44, measured zero false matches) plus the anti-transfer cross-check; a structural (43) match or a watermark hit alone caps at LIKELY. Tier thresholds are grounded in ROBUSTNESS.md.

use c2pa_text_binding::{simhash::Fingerprint, soft_binding::{self, SoftBinding}, verify, Confidence};

let text = "…the document being bound…";
let cbor = soft_binding::from_fingerprint(&Fingerprint::compute(text)).to_cbor()?;

let candidate = SoftBinding::from_cbor(&cbor)?;
let tier = verify(text, &candidate, /*watermark_verified=*/ false, /*crosscheck_ok=*/ true);
assert_eq!(tier, Confidence::Bound);
# Ok::<(), c2pa_text_binding::Error>(())

Register with the Soft Binding Resolution API

These algorithms are registered in the C2PA soft binding algorithm list, so a c2pa.soft-binding assertion referencing them can drive decentralized manifest recovery.

Transport Survivability Research

The soft-binding layer above exists because hard bindings are fragile in text pipelines. TRANSPORT.md is a reproducible benchmark quantifying that fragility, comparing every invisible-text carrier — A.8 variation selectors (v1 and a proposed v2), the zero-width Reed-Solomon watermark, Unicode Tag smuggling, naive zero-width binary — against the fingerprint recovery layer, across deterministic probes and real transports (HTML sanitizers, Office, email, Markdown, pandoc, macOS RTF).

Key results, all reproducible with cargo run --release --example transport_survivability and harness/tier1.py:

• Carrier survival is not provenance survival. Email, Markdown, and pandoc round trips leave the invisible carrier intact but reflow the visible text, breaking the A.8 hard binding. Only the reflow-tolerant fingerprint survives.
• A.8 needs a checksum, not just a length. Property testing shows the v1 length field is self-corruptible — a single dropped length byte yields a wrong payload. Only an integrity field (this crate's HMAC watermark, or a v2 checksum) fails safe.
• Error correction is decisive. The Reed-Solomon watermark tolerates ~30–40% code-point loss where every un-coded carrier fails by 20%.
• Sanitizers preserve invisible payloads. Ten of thirteen real pipelines — including bleach and nh3 — pass the payload through untouched; a security-relevant result.

Interoperability. This crate's A.8 codec is byte-identical to Encypher's c2pa-text reference library in both directions (tests/interop.rs, harness/interop.py), so the comparison is against the real deployed format, not a stand-in.

Runs are reproducible via a pinned venv (harness/requirements.txt) or the Dockerfile, which stamps tool and library versions with every result.

Related Crates

Part of a family of single-purpose crates, one per C2PA embedding method. Each is standalone and independently versioned.

Security

Found a vulnerability? Please report it privately — see SECURITY.md.

License

Licensed under either of Apache License, Version 2.0 or MIT License at your option.

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