Check a Captilo proof without Captilo

The certificate is as Captilo issued it

The fingerprints prove the bytes in the file, not what a PDF viewer shows: someone could leave the genuine photo in the file and draw another picture over it, shrink it to a speck, add a page, or save the file again with changes. So look at the file first. It must have exactly one page per proof (a multi-proof certificate prints “Proof N of M”) and exactly one %%EOF, because Captilo writes it in one go. On every page pdfimages must list exactly one image with enc “jpeg”, the photo, drawn at least 200 pt on its longer side (width × 72 ÷ x-ppi), and at most two other images, the logo and the QR code, each at most 100 pt (the logo also has an smask line). The script checks the rest: nothing drawn over the photo, no other fonts, patterns or layers, and every object where the file’s cross-reference says it is.

pdfinfo certificate.pdf | grep Pages
grep -c -a "%%EOF" certificate.pdf           # must print 1
pdfimages -list certificate.pdf

One page per proof, one revision, one JPEG photo per page and nothing else but the logo and the QR code.

The photo is unchanged

The photo in a certificate is the original image file, embedded byte for byte. Extract it page by page with Poppler’s pdfimages: -j writes the page’s one JPEG as a .jpg file (the logo and QR code come out in other formats). Hash that file, never “the largest image in the file”: the photo that counts is the one the page draws.

pdfimages -j -f 1 -l 1 certificate.pdf page1  # page 1: exactly one page1-*.jpg
sha256sum page1-*.jpg                        # Linux
shasum -a 256 page1-*.jpg                    # macOS
Get-FileHash page1-*.jpg -Algorithm SHA256   # Windows PowerShell

The fingerprint equals the Picture Hash.

The record is unchanged

Below the hashes, the certificate prints the signed record of the capture: time, place, device and check results, as canonical JSON (RFC 8785). The certificate states the rule itself: the SHA-256 of this exact text is the Metadata Hash. Extract the text, then join the record’s lines from its opening { to its closing } without adding or removing a single character, not even a final line break. The same record is embedded in the file; the script also checks that the printed one is identical to it.

pdftotext -raw certificate.pdf certificate.txt
# join the record’s lines into record.json, then:
sha256sum record.json

The fingerprint equals the Metadata Hash.

Both fingerprints are on the blockchain

Without writing code: open the certificate’s Sonic TX link. It shows the transaction on sonicscan.org, a public block explorer that Captilo doesn’t run. Check that the transaction succeeded and was sent to the Captilo registry, then look at its raw input data: the Picture Hash, the Metadata Hash and the Event ID without its dashes all appear there in hexadecimal. Or ask any Sonic node directly:

curl -s https://rpc.soniclabs.com -H 'Content-Type: application/json' -d '{
  "jsonrpc": "2.0", "id": 1, "method": "eth_call",
  "params": [{ "to": "0x3A6873D7A36E3FcD89AF557a24de2DaAFf09cB9b",
              "data": "0x1b80bb3a00000000000000000000000000000000<Event ID without dashes>" }, "latest"]
}'

The reply is 0x followed by eight 64-character blocks: the picture hash, the metadata hash, two internal IDs, the capture and anchoring times as Unix seconds in hex, the proof version, and a last block ending in 1 when the proof exists. The version is written by Captilo’s server, not the phone: 3 means the server consumed a live capture nonce (an online capture). For a proof anchored since 2026-09-04 08:21:52 UTC, a version of 2 means no nonce was consumed: treat it as an offline capture (device-clock time only), whatever its record says. If the node cannot be reached, nothing was checked: try again.

The registry returns both hashes under the event ID, with the time the proof was anchored.