# struct\_hash

Compute EIP-712 struct hash for an `UnsignedSeismicTx`.

## Signature

```python
def struct_hash(tx: UnsignedSeismicTx) -> bytes
```

## Parameters

| Parameter | Type                                                                                                                   | Required | Description                      |
| --------- | ---------------------------------------------------------------------------------------------------------------------- | -------- | -------------------------------- |
| `tx`      | [`UnsignedSeismicTx`](https://docs.seismic.systems/clients/python/api-reference/transaction-types/unsigned-seismic-tx) | Yes      | The unsigned Seismic transaction |

## Returns

| Type    | Description                                  |
| ------- | -------------------------------------------- |
| `bytes` | 32-byte keccak256 hash of the encoded struct |

## How it works

```
keccak256(
    TX_SEISMIC_TYPE_HASH
    ‖ encode(chain_id)
    ‖ encode(nonce)
    ‖ encode(gas_price)
    ‖ encode(gas)
    ‖ encode(to)
    ‖ encode(value)
    ‖ keccak256(data)              // dynamic type
    ‖ keccak256(encryption_pubkey) // dynamic type
    ‖ encode(encryption_nonce)
    ‖ encode(message_version)
    ‖ recent_block_hash            // already 32 bytes
    ‖ encode(expires_at_block)
    ‖ encode(signed_read)
)
```

## Encoding details

* Static fields are left-padded to 32 bytes
* Dynamic `bytes` fields (`tx.data`, `tx.seismic.encryption_pubkey`) are hashed with `keccak256` before inclusion
* `encryption_nonce` is converted from 12 bytes to a `uint96` integer via `int.from_bytes(…, "big")`
* `to` becomes 32 zero bytes when `tx.to is None`

## See Also

* [eip712\_signing\_hash](https://docs.seismic.systems/clients/python/api-reference/eip712/eip712-signing-hash) — uses struct hash
* [domain\_separator](https://docs.seismic.systems/clients/python/api-reference/eip712/domain-separator) — the other component of the signing hash
* [build\_seismic\_typed\_data](https://docs.seismic.systems/clients/python/api-reference/eip712/build-seismic-typed-data) — builds full typed data