C2PA manifest embedding, referencing, and hard binding for HTML documents

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  • License: MIT OR Apache-2.0
  • Tags c2pa , provenance , html , manifest , content-credentials
  • Requires: Python >=3.9
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Project description

c2pa-html

C2PA manifest embedding, referencing, and hard binding for HTML documents.

Implements the Embedding Manifests into HTML section of the C2PA Technical Specification: a C2PA Manifest Store carried inline as the Base64 content of a <script type="application/c2pa"> element, or referenced externally by a <link rel="c2pa-manifest"> element, both in the document head.

[dependencies]
c2pa-html = "0.1"

What it does
document discover, embed, reference, and remove a manifest association
hardbinding the exact c2pa.hash.data coverage, with compute and verify

Signature verification, certificate trust, assertion validation, and resolution of an external manifest URI are out of scope.

Reference an external manifest

The specification prefers this form.

use c2pa_html::document;

let page = b"<html>\n<head>\n    <title>Example</title>\n</head>\n<body></body>\n</html>";
let out = document::embed_reference(page, "https://fabrikam.example/m.c2pa")?;

let manifest = document::extract(&out)?;
assert_eq!(manifest.href(), Some("https://fabrikam.example/m.c2pa"));
# Ok::<(), c2pa_html::Error>(())

Carry the manifest inline

use c2pa_html::document;

let page = b"<html>\n<head>\n    <title>Example</title>\n</head>\n<body></body>\n</html>";
let out = document::embed(page, b"manifest-store-bytes")?;

assert_eq!(
    document::extract(&out)?.store(),
    Some(&b"manifest-store-bytes"[..])
);
// Removing the manifest restores the document byte for byte.
assert_eq!(document::remove(&out)?, page);
# Ok::<(), c2pa_html::Error>(())

Bind the document

use c2pa_html::document;
use c2pa_html::hardbinding::{compute_data_hash, verify_data_hash, Algorithm, Sha2};

let page = b"<html>\n<head>\n    <title>Example</title>\n</head>\n<body></body>\n</html>";
let out = document::embed(page, b"manifest-store-bytes")?;

let binding = compute_data_hash(&out, Algorithm::Sha256, &Sha2)?;
assert!(verify_data_hash(&out, &binding, &Sha2).is_ok());
# Ok::<(), c2pa_html::Error>(())

Binding coverage

  • Inline — one exclusion range covering the entire script element, <script through </script> inclusive. The hash is over the document with that range removed.
  • External — no exclusion range at all. The hash is over the entire document, link element included, so the manifest URI cannot be swapped without breaking the binding.

The hash is over the bytes of the document as stored, with no normalization. Anything that re-serializes the HTML — a CMS, a CDN, a formatter that rewrites quote styles or collapses whitespace — shifts byte offsets and invalidates the binding. That is by design: re-serialization is a content modification. Embed after the final serialization step, or use an external manifest.

The inline hash can be computed before the manifest exists

Because the exclusion covers the entire script element, the covered bytes are the document with the element cut out — and embed adds no bytes outside the element, so that is the original document. Signing an inline HTML manifest needs no placeholder-reserve-then-fill dance: hash, sign, embed.

use c2pa_html::hardbinding::inline_hash_before_embed;

An external manifest has no exclusion, so the link element is inside the hash and the order reverses: insert the link first, then hash.

Discovery

Scoped to the document head, byte-oriented, and tolerant of how HTML is actually written:

  • all three attribute quoting forms — rel="x", rel='x', rel=x
  • ASCII case-insensitive tag names, attribute names, type, and rel
  • rel as a space-separated token list, per HTML
  • link matched on rel alone; type="application/c2pa" is optional
  • comments, doctypes, and the contents of other script and style elements are never mistaken for markup
  • an implied head (both tags omitted) is still searched

A document shall carry at most one association. Two script elements, two link elements, or one of each are all rejected with the specified manifest.html.multipleManifests failure — the manifests were located, and the document is rejected for carrying too many.

Finding nothing carries no status code (it is simply an unsigned document), and neither does an element that matches but yields no Manifest Store — the specification names no code for that, and the fail-safe reading is that nothing was obtained. Error::is_no_manifest_located() draws the line a caller actually needs: "carries no provenance" versus "carried provenance that was rejected".

Bytes, not text

The specification directs a validator to treat the document "as a series of bytes (vs. text)", so every entry point takes &[u8] and returns Vec<u8>. A document in a legacy ASCII-compatible encoding scans correctly, and byte offsets mean what the hard binding says they mean.

Zero dependencies, no features

Discovery, embedding, Base64, SHA-2, and the binding algorithm are all in-crate. There is nothing to enable and nothing to pull in — the dependency list is empty in every configuration.

SHA-256/384/512 are implemented against FIPS 180-4 and tested against the NIST vectors. A hash is the one primitive where writing it yourself is uncontroversial: fully specified, no key, public input, no timing side channel, and published vectors that pin every path.

What that gives up is speed — no SHA-NI, no NEON. Immaterial for an HTML document; it would not be for a multi-gigabyte asset, which is why hashing goes through the Hasher trait. A caller with that problem injects an accelerated implementation and never touches the built-in one.

Related

crate method
c2pa-structured-text structured text: ASCII-armoured manifest in a comment
c2pa-unstructured-text unstructured text: Unicode variation selectors

HTML is a file format, so its binding hashes stored bytes with no normalization. The text methods differ deliberately: structured text also hashes raw bytes but has no element to exclude, and unstructured text normalizes to NFC because clipboard-portable text may arrive in any normalization form.

License

MIT OR Apache-2.0.

Project details

Verified details

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Unverified details

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Meta
  • License: MIT OR Apache-2.0
  • Tags c2pa , provenance , html , manifest , content-credentials
  • Requires: Python >=3.9
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0.2.0

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