A proxy server performs communication operations on behalf of another participant. Whether it can inspect plaintext depends on the protocols and keys it uses: relaying opaque encrypted bytes differs from authorised TLS termination.
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A proxy server is an intermediary server that acts as a gateway between a client (such as a computer or a device) and another server (often a web server). When a client requests a resource, such as a web page, the request is first sent to the proxy server. The proxy then forwards the request to the intended server, retrieves the response, and sends it back to the client.