# Relayed Meta Data Authentication
There are two methods by which the API provider can manage access to their API by on-chain smart contracts known as requesters.
- Authorizers (on-chain)
- Relayed Meta Data Security Schemes (off-chain)
Relayed meta data is known information about the requester (smart contract) passed along to the API endpoint by Airnode.
Using the relayed meta data security schemes does not require
blockchain-specific knowledge or blockchain interaction by an API provider and
there are no blockchain transaction (gas) fees. Just apply the desired security
scheme fields in the Airnode config.json file during deployment of an Airnode,
see Building an Airnode. Then have
your API operations check for any security scheme values you configured
(typically the sponsorAddress) against your customer database.
# Simple Example
The illustration below is one way to make use of relayed meta data for security
purposes. Here a sponsor (a.k.a. developer or customer) submits their
sponsorAddress to be stored by an API provider. The sponsorAddress can be
used to authenticate requests by a sponsor's smart contracts.
The following sections relate to the numbers shown in the diagram above.
# 1: Deploy Airnode
An Airnode is deployed with the sponsorAddress security scheme instructing
Airnode to pass along the sponsorAddress with all requests.
# 2: Register Sponsors
A sponsor is the developer of an on-chain smart contract that wishes to request
off-chain data from an API provider. A sponsor has a know sponsorAddress. The
API provider can request the sponsorAddress from the sponsor using a webform
as part of a paid subscription setup.
# 3: Making Requests
When a smart contract (requester) makes a request of any API operation, the
sponsorAddress will be part of the request. A sponsor can have more than one
requester.
- 3a: The requester makes a request to Airnode.
- 3b: Airnode adds the
sponsorAddressto the request before passing the request to the API operation. The API operation performs a lookup on thesponsorAddressto verify it has a subscription. If verified the API operation returns the response to Airnode. - 3c: Airnode returns the response to the requester.
# Security Schemes
Including the sponsorAddress used in the
Simple Example above, there are five
relayed meta data security schemes that can be incorporated into any Airnode.
- relayChainId - The chain ID the request came from.
- relayChainType - The chain type the request came from.
- relaySponsorAddress - The sponsor's wallet address. The sponsor owns and operates one or more requesters.
- relaySponsorWalletAddress - Other wallet addresses associated with each requester. These are owned and funded by a sponsor to pay on-chain gas costs.
- relayRequesterAddress - The on-chain address of the requester (smart contract).