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

# Integrity Verification

> How Tally uses RFC 8785 hashing and chain linkage to support independent verification.

Every snapshot is hashed at write time and linked to the previous snapshot. This allows any party with a copy of the data to independently verify that state has not been tampered with.

## Envelope hash

The `envelope_hash` stored on each snapshot is computed as:

```
SHA-256( RFC_8785_canonicalize( entityStateEnvelopeV1 ) )
```

**RFC 8785** (JSON Canonicalization Scheme) deterministically orders keys and normalizes Unicode, producing a stable byte sequence regardless of serialization differences. This means the hash can be independently reproduced from the raw envelope without any platform-specific logic.

## Chain linkage

Each snapshot stores a `prev_hash` pointing to the `envelope_hash` of the preceding version. This forms a hash chain:

```
snapshot_1: envelope_hash = H1, prev_hash = null
snapshot_2: envelope_hash = H2, prev_hash = H1
snapshot_3: envelope_hash = H3, prev_hash = H2
```

A valid chain means that to tamper with any historical snapshot you would need to recompute all subsequent hashes — which is detectable.

## Server-side verification

Pass the `verify` query parameter on any snapshot read:

| Value   | Behavior                                                            |
| ------- | ------------------------------------------------------------------- |
| `none`  | No verification (default).                                          |
| `hash`  | Re-computes the RFC 8785 hash and compares against `envelope_hash`. |
| `chain` | Validates the full prev\_hash chain back to the root snapshot.      |

The response includes a `verification` block:

```json theme={null}
{
  "verification": {
    "valid": true,
    "computed_hash": "adf11c84...",
    "stored_hash": "adf11c84...",
    "matches": {
      "rfc8785": true
    }
  }
}
```

## Offline verification

Export the full ledger with `GET .../snapshots/export` and run the `tally verify-ledger` CLI against the bundle to verify hashes and chain integrity without any network dependency.

## Chain proof endpoint

`GET .../chain-proof` returns an ordered list of `snapshot_id`, `envelope_hash`, and `prev_hash` values — the minimum data needed for an independent chain audit without transferring full envelope payloads.