# evm_auth · FT4 Rell API · Chromia Docs (concept)

> @mount("ft4.evm_ auth")
operation evm_ auth(account_ id: byte_ array, auth_ descriptor_ id: byte_ array, signatures: list< signature ?>) For an operation t

Source: https://docs.chromia.com/pages/ft4-rell/-f-t4-library/lib.ft4.external.auth/evm_auth/

OPERATION

# evm\_auth

```signature
@mount("ft4.evm_auth")
operation evm_auth(account_id: byte_array, auth_descriptor_id: byte_array, signatures: list<signature?>)
```

[Source](https://gitlab.com/chromaway/ft4-lib/-/blob/development/rell/src/lib/ft4/external/auth/operations.rell#lines-40)

For an operation to be authorized by an EVM signer, a specific message must be retrieved
using `get_auth_message_template`. That template will contain some of the following
placeholders:

* core.auth.ACCOUNT\_ID\_PLACEHOLDER
* core.auth.AUTH\_DESCRIPTOR\_ID\_PLACEHOLDER
* core.auth.NONCE\_PLACEHOLDER
* core.auth.BLOCKCHAIN\_RID\_PLACEHOLDER

Those must be replaced with the corresponding value:

* the ID of the account and auth descriptor that are currently being used to sign the
  operation auth message for the first two;
* a valid nonce, as would be created by `utils.derive_nonce` with the current counter value
  of the auth descriptor being used; if multiple operations are authenticated with evm\_auth
  using the same auth descriptor, the counter must increase by one on every subsequent
  operation;
* the rid of the blockchain.

The message must be signed following a standard EIP-191 implementation by all signers
defined in the auth descriptor used to authenticate, and the signatures must be passed
to the `signatures` parameter in the same order as they can be found in the auth
descriptor's `signer` field (inside `args`). If any signer did not need to sign (e.g. the
auth descriptor is a multi-sig, and only some signatures are required), that signature
must be left null.

The transaction will then contain

* evm\_auth
* the operation that is being authorized

with nothing else inbetween them. Other operations may be added before and after these two.

Throws if the next operation is blacklisted or this is the last operation in the transaction

**Parameters**

* **account\_id** — The ID of the account that is calling the next operation
* **auth\_descriptor\_id** — The ID of the auth descriptor that is authorizing the operation
* **signatures** — The signatures of the operation's auth message

**See Also**

* `core.auth._auth_handler` — for more details on operation authorization

[View this page on docs.chromia.com](https://docs.chromia.com/pages/ft4-rell/-f-t4%20-library/lib.ft4.external.auth/evm_auth.html)Generated API reference, mirrored from the live docs