# Sellat Proof of Existence (SHA-256 Blockchain Anchoring)

> 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

## Facts

- Endpoint: POST https://sellat.app/api/v2/proofs?utm_source=zero.xyz
- Price: $0.01/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-23
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/sellat-proof-of-existence-sha-256-blockchain-anchoring-0acdda25
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_oe7xkn3AUqLgzXMagFthL

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability sellat-proof-of-existence-sha-256-blockchain-anchoring-0acdda25 -d '<json body>'
```

Example prompt: Can you create a blockchain proof of existence for this SHA-256 hash — 'a3f1c2d9e4b7082f1e6a5c4b3d2e1f0a9b8c7d6e5f4a3b2c1d0e9f8a7b6c5d4' — and give me a verifiable certificate anchored on both Polygon and Bitcoin?

## When to prefer this

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.

## Known failure modes

- 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

## 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.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "$schema": "https://json-schema.org/draft/2020-12/schema",
 "required": [
  "input"
 ],
 "properties": {
  "input": {
   "type": "object",
   "required": [
    "type",
    "method",
    "bodyType",
    "body"
   ],
   "properties": {
    "body": {
     "type": "object"
    },
    "type": {
     "type": "string",
     "const": "http"
    },
    "method": {
     "enum": [
      "POST",
      "PUT",
      "PATCH"
     ],
     "type": "string"
    },
    "headers": {
     "type": "object",
     "additionalProperties": {
      "type": "string"
     }
    },
    "bodyType": {
     "enum": [
      "json",
      "form-data",
      "text"
     ],
     "type": "string"
    },
    "pathParams": {
     "type": "object",
     "additionalProperties": {
      "type": "string"
     }
    },
    "queryParams": {
     "type": "object",
     "additionalProperties": {
      "type": "string"
     }
    }
   },
   "additionalProperties": false
  },
  "output": {
   "type": "object",
   "required": [
    "type"
   ],
   "properties": {
    "type": {
     "type": "string"
    },
    "example": {
     "type": "object"
    }
   }
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "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"
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/sellat-proof-of-existence-sha-256-blockchain-anchoring-0acdda25/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from sellat.app](https://www.zero.xyz/host/sellat.app/llms.txt)