> ## Documentation Index
> Fetch the complete documentation index at: https://docs.opencharge.network/llms.txt
> Use this file to discover all available pages before exploring further.

# Transaction Proofs

> Structure and verification of cryptographic transaction proofs in Opencharge

## Purpose

A transaction proof is a signed receipt attesting that a transfer was completed. It's the cryptographic evidence that enables trustless settlement.

## Proof Structure

```json theme={null}
{
  "proof": {
    "txid": "barclays_tx_456",
    "issuer": 100,
    "from": { "ocid": 200, "reference": "paypal_tx_123" },
    "to": { "ocid": 300, "reference": "momo_tx_789" },
    "amount": "10000.00",
    "currency": "UGX",
    "timestamp": 1706500500,
    "memo": "Settlement for ORD-2024-001"
  },
  "signature": "a1b2c3d4e5f6...7f1b"
}
```

### Proof Fields

| Field            | Type   | Required | Description                                |
| ---------------- | ------ | -------- | ------------------------------------------ |
| `txid`           | string | Yes      | Unique transaction ID from issuer          |
| `issuer`         | number | Yes      | OCID of service that executed the transfer |
| `from`           | object | Yes      | Sender information                         |
| `from.ocid`      | number | Yes      | Sender's OCID                              |
| `from.reference` | string | No       | Sender's transaction ID for correlation    |
| `to`             | object | Yes      | Recipient information                      |
| `to.ocid`        | number | Yes      | Recipient's OCID                           |
| `to.reference`   | string | No       | Recipient's transaction ID for correlation |
| `amount`         | string | Yes      | Amount as decimal string                   |
| `currency`       | string | Yes      | ISO 4217 currency code                     |
| `timestamp`      | number | Yes      | Unix timestamp when transfer completed     |
| `memo`           | string | No       | Human-readable description                 |

## Proof Signature

The signature is over the canonical JSON of the `proof` object:

```javascript theme={null}
canonical = canonicalize(proof)  // Sorted keys, no whitespace
hash = SHA256(canonical)
signature = secp256k1_sign(issuer_private_key, hash)
```

**Canonicalization**: Keys sorted alphabetically at all levels, no whitespace, minimal escaping.

## Verifying a Proof

```
1. Extract issuer OCID from proof
2. Fetch issuer's metadata from Router Registry
3. Extract publicKey from metadata
4. Canonicalize the proof object
5. Compute SHA256(canonical)
6. Verify signature against public key
7. Confirm issuer is in your settlement.accepts list
8. Confirm amount and currency match expected values
9. Store txid to prevent replay
```