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In response to concerns about protocol ossification and privacy, post-TCP transport protocols such as QUIC and WebRTC include end-to-end encryption and authentication at the transport layer. This makes their packets opaque to middleboxes, freeing the transport protocol to evolve but preventing some in-network innovations and performance improvements. This paper describes sidekick protocols: an approach to in-network assistance for opaque transport protocols where in-network intermediaries help endpoints by sending information adjacent to the underlying connection, which remains opaque and unmodified on the wire. A key technical challenge is how the sidekick connection can efficiently refer to ranges of packets of the underlying connection without the ability to observe cleartext sequence numbers. We present a mathematical tool called a quACK that concisely represents a selective acknowledgment of opaque packets, without access to cleartext sequence numbers. In real-world and emulation-based evaluations, the sidekick improved performance in several scenarios: early retransmission over lossy Wi-Fi paths, proxy acknowledgments to save energy, and a path-aware congestion-control mechanism we call PACUBIC that emulates a “split” connection.