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---
title: Message flow
weight: 3
last_reviewed_on: 2019-10-22
review_in: 3 weeks
---
# Message flow
The Document Checking Service (DCS) acts as an interface between your service and HM Passport Office.
<%= image_tag "dcs-overview.svg" %>
1. Your service sends a [passport check][check-passport] request to the DCS.
2. The DCS validates the request and sends it to HM Passport Office.
3. HM Passport Office checks the passport data against their database and sends a response back to the DCS.
4. The DCS sends the response to your service as outlined in [check if a passport is valid][check-passport-response].
## Mutual authentication
Mutual authentication secures the messages your client exchanges with the DCS. This is implemented using client certificates over [Transport Layer Security (TLS) Protocol Version 1.2][TLS].
You must generate a private key and certificate signing request to obtain your mutual authentication certificate.
Details about how to raise a certificate signing request with our Certificate Authority will follow after the expression of interest stage of the DCS pilot.
<%= partial "partials/links" %>
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