LSAT
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Authentication Flow
This chapter explains the high-level authentication flow from the perspective of a user and their client software.
The requirements from the user's point of view are simple: They want to be able to use a service as frictionless as possible. They are perhaps used to the concept of needing to obtain an API access key first in order to use a service, but do not necessarily want to register an account with their personal information to do so.
A service using the LSAT protocol supports exactly that requirement: The use of an API key without the need for creating an account first. And because no information needs to be input, the process of obtaining the API key can happen transparently to the user, in the background.
Whenever an LSAT-compatible client software connects to a server that uses the protocol, it receives a prompt to pay an invoice over a very small amount (a few satoshis). Once the client software pays that invoice (which can happen automatically if the amount does not exceed a user-defined threshold), a valid API key or authentication token can be constructed. That token is stored by the client's software and will be used for all future requests.

Detailed authentication flow

The following steps describe the diagram further below. It is the flow of calls that take place for a client software that wants to access a protected resource that is secured by an authentication server. As an example, we will look at the loopd client that wants to do a loop out swap with the Lightning Lab's loop server.
First time looping out:
    1.
    A loop user wishes to perform a swap with the loop server. They type the
    command loop out <amount> and hit return.
    2.
    The loopd client program contacts the loop server to initiate the swap.
    3.
    The call from the client must always go through the authentication server
    reverse proxy, which in this example is aperture. The authentication proxy
    notices that the client didn't send an LSAT and therefore cannot be granted
    access to the loop server.
    4.
    aperture instructs its own lnd instance to create an invoice over a small
    amount that is required to acquire a fresh token.
    5.
    In addition to the invoice, aperture also creates a fresh access token that is
    tied to the invoice. The token is cryptographically constructed in a way that
    it is only valid once the invoice has been paid.
    6.
    The token and the invoice are sent back to the client in the previously
    unused HTTP header 402 Payment Required.
    7.
    The loopd understands this returned error code, extracts the invoice from
    it and automatically instructs its connected lnd instance to pay the
    invoice.
    8.
    Paying the invoice results in the loopd client now possessing the
    cryptographic proof of payment (the pre-image). This proof is stored in the
    client's local storage, together with the access token.
    9.
    The combination of the access token and the pre-image yields a fully valid
    LSAT that can be cryptographically verified.
    10.
    The client now repeats the original request to the loop server, now
    attaching the LSAT to the request.
    11.
    The authentication server intercepts the request, extracts the LSAT and
    validates it. Because the LSAT is valid, the request is forwarded to the
    actual loop server that then initiates the swap.
    12.
    The answer of the swap server is returned to the client and the swap is
    now initiated.
    13.
    The whole process is fully transparent to the user. The only thing they
    might notice is a short delay of a few seconds on the first ever loop. Each
    successive loop will use the same token and will not be delayed at all.
e2e flow sequence diagram
All further loops:
    1.
    For every new request to the server, the client now automatically attaches
    the token that is stored locally.
    2.
    As long as the token has not expired, the steps 9-13 above will be followed.
    If/when the token expires, the server will start over at step 4 and instruct
    the client to obtain a fresh token.
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