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Sellat Proof of Existence (SHA-256 Blockchain Anchoring)
x402
Sellat Proof of Existence (SHA-256 Blockchain Anchoring) is a paid API for AI agents from sellat.app, paid per call via x402, $0.01/call, status unknown (last checked 2026-10-23).
Anchors a SHA-256 hash on Polygon blockchain and attests it on Bitcoin via OpenTimestamps, returning a verifiable certificate, proof.json, and evidence ZIP
Price / call$0.01
Success—
StatusUnknown
How this service works
Timestamp and notarize any file or data by its SHA-256 hash: blockchain proof of existence and document integrity, anchored on Polygon in minutes and attested on Bitcoin (OpenTimestamps); blockchain notarization, not a notary act. Returns proof_id, certificate, proof.json and evidence ZIP, verifiable without Sellat. Use when you must prove that exact data existed by now and is intact.
Output
Returns a verifiable proof certificate, a proof.json file, and an evidence ZIP containing the blockchain anchoring on Polygon (within minutes via fast batch) and a Bitcoin attestation via OpenTimestamps. All outputs are independently verifiable without relying on Sellat's infrastructure.
Example use cases
Timestamping intellectual property before filing
“I've got the SHA-256 hash of my new software design document and I need to prove it existed today before I file the patent — can you anchor this hash 'b4e2f1a0c3d5e6f7a8b9c0d1e2f3a4b5c6d7e8f9a0b1c2d3e4f5a6b7c8d9e0f1' on the blockchain and get me a certificate?”
Legal evidence preservation for a contract
“We need to preserve proof that this contract draft existed at today's date — please timestamp the SHA-256 hash 'c5f3e2b1a0d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9b0c1d2e3f4a5b6c7d8e9f0' on Polygon and Bitcoin and return the proof.json and certificate.”
Audit trail for compliance records
“Our compliance team needs a blockchain-anchored timestamp for this audit log file's SHA-256 hash '1a2b3c4d5e6f7a8b9c0d1e2f3a4b5c6d7e8f9a0b1c2d3e4f5a6b7c8d9e0f1a2b' — please submit it to Sellat and get back the verifiable certificate and evidence ZIP.”
Technical details
Endpoint
POSThttps://sellat.app/api/v2/proofs?utm_source=zero.xyzWhen to prefer
Choose this endpoint when you need a low-cost ($0.01 USDC), fast, multi-chain proof of existence that is independently verifiable without trusting Sellat — combining Polygon's speed with Bitcoin's security via OpenTimestamps. Prefer over eIDAS/RFC 3161 timestamps when you want decentralized blockchain anchoring rather than EU qualified trust service provider signatures.
Side effects
- Invalid or malformed SHA-256 hash input returns a 400 error
- Payment failure via x402 protocol prevents proof creation
- Polygon network congestion may delay batch anchoring beyond stated minutes
- Duplicate hash submission may return existing proof or create new timestamp
- OpenTimestamps Bitcoin attestation may be delayed due to Bitcoin block times
Input
Schema · Request body
| Field | Type | Description |
|---|---|---|
| inputrequired | object | |
| output | object |
Output
Schema · Response
{
"type": "json",
"example": {
"id": "<proof_id>",
"hash": "<sha256>",
"next": "POST /api/v2/proofs/{id}/qualified adds the eIDAS qualified timestamp (RFC 3161) for 0.90 USD",
"urls": {
"self": "https://sellat.app/api/v2/proofs/<id>",
"proof_json": "https://sellat.app/api/v2/proof/<id>.json",
"certificate": "https://sellat.app/certificate/<id>",
"evidence_zip": "https://sellat.app/api/v2/proofs/<id>/evidence.zip"
},
"state": "queued → anchored (minutes)",
"api_key": "<shown once: read-only key for this wallet’s proofs>",
"created": true,
"payment": {
"charged": true,
"amount_usd": "0.01",
"transaction": "<tx hash on Base>"
},
"proof_id": "<proof_id: use it in POST /api/v2/proofs/{id}/qualified>",
"anchoring": {
"policy": "fast"
}
}
}Tags
x402
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