Content Credentials

Content Credentials (C2PA) attach a signed record of a file's origin to the file itself. That record lives in the file's metadata, so it is lost when a platform or an editor strips the metadata. Etchv's watermark is in the pixels instead, so a copy without its metadata still carries it. Etchv's public resolver uses that watermark to give back the copy's original manifest.

How it works

• You watermark an image with provenance=c2pa. Etchv embeds the watermark, then signs a C2PA manifest into the file.
• The manifest has a c2pa.watermarked.bound action and a c2pa.soft-binding assertion. The assertion names the algorithm com.etchv.watermark.1 and carries the file's watermark ID.
• The file is opted in to public provenance: its manifest store is kept so that anyone can recover it from the watermark.
• Someone finds a copy whose metadata is gone. A C2PA tool, or anyone, asks the resolver with the copy itself or with its watermark ID. The resolver returns the manifest ID, and the manifest store is one request away.

Only opted-in files can be resolved. A watermark ID from any other file matches nothing, exactly like one that doesn't exist, and an answer names a manifest and nothing else: no workspace, asset or filename.

The watermark algorithm

The watermark ID is the 64-character hex digest that X-Watermark-ID returns when you watermark a file; see embedding. Detection reads it from the pixels, so removing the metadata doesn't affect it. Editing or recompressing the file can affect recovery.

To turn a watermark ID into a soft-binding value:

VALUE=$(python3 -c 'import base64, sys; print(base64.b64encode(bytes.fromhex(sys.argv[1])).decode())' "$WATERMARK_ID")

Sign a file with Content Credentials

Add provenance=c2pa to an image watermark request:

curl --fail-with-body "https://api.etchv.com/watermarks/images?provenance=c2pa" \
  -H "X-API-Key: $ETCHV_API_KEY" \
  -F 'file=@photo.png' \
  --form-string 'data={"generation_id":"gen_0042"}' \
  --output photo-signed.png

It applies to JPEG, PNG, WebP, single-page TIFF and GIF images, and costs the same as any image embed. Any other format returns 422.

Resolve a copy

The resolver implements the C2PA Soft Binding Resolution API at https://api.etchv.com/c2pa/v1. It takes no credentials.

Send the image itself as the request body. The resolver detects its watermark and returns the matching manifests:

curl --fail-with-body "https://api.etchv.com/c2pa/v1/matches/byContent" \
  -H "Content-Type: image/png" \
  --data-binary @copy.png

{
  "matches": [
    {
      "manifestId": "urn:c2pa:6f1e2a7c-4d3b-4c55-9a8e-2b1f0c9d7e61",
      "endpoint": "https://api.etchv.com/c2pa/v1",
      "similarityScore": 100
    }
  ]
}

It takes one JPEG, PNG, WebP, TIFF or GIF image of up to 5 MB and 20 megapixels. A detected watermark that differs from an opted-in one in up to 48 of its 256 bits still matches, with a lower similarityScore (out of 100).

If you already have the watermark ID, query by its soft-binding value, URL-encoded:

curl --fail-with-body -G "https://api.etchv.com/c2pa/v1/matches/byBinding" \
  --data-urlencode "alg=com.etchv.watermark.1" \
  --data-urlencode "value=$VALUE"

POST /c2pa/v1/matches/byBinding takes the same alg and value as a JSON body.

Returns the manifest store as application/c2pa:

curl --fail-with-body "https://api.etchv.com/c2pa/v1/manifests/$MANIFEST_ID" \
  --output photo.c2pa

GET /c2pa/v1/services/supportedAlgorithms lists com.etchv.watermark.1 under watermarks.

Both matches routes take maxResults, from 1 to 10 (default 10). The newest manifest comes first.

Validate the manifest

The Etchv Content Credentials client, open source under the MIT license, resolves a copy, fetches its manifest store and validates it in one step:

pip install "git+https://github.com/etchv-labs/integrations.git#subdirectory=c2pa"
etchv-c2pa copy.png --save copy.c2pa

Validate the copy against the manifest store with any C2PA validator, such as the C2PA SDK or c2patool, giving it the store as external manifest data.

The signature and the claim validate. The hard binding reports assertion.dataHash.mismatch, and that's expected: the data hash covers the signed file's bytes, and a copy without its metadata has different bytes. The soft binding is what ties the copy to the manifest. A validator also reports signingCredential.untrusted unless Etchv's signing certificate is on its trust list.

Limits

A request over a limit returns 429. Over the matches/byContent limits, the 429 carries Retry-After, in seconds; the shared daily limit resets at midnight UTC.