Gantry: 12 - libp2p PrivateNetwork PSK

[vpatelsj]

Scope

Deferred but falsifiable libp2p PrivateNetwork PSK work.

Problem

DHT/coord exposure currently relies on NetworkPolicy rather than cryptographic peer gating. DHT-advertised provider addresses can also drive constrained SSRF if arbitrary pods can reach Gantry ports or join the DHT.

Evidence:

• internal/gantry/discovery/discovery.go:590
• internal/gantry/transfer/client.go:140

Changes

• Add libp2p PrivateNetwork(psk) in discovery.go, with the PSK sourced from a cluster Secret.
• Define key distribution and rotation workflow.
• Move this work ahead of coord authz enforcement if the deployment cannot apply an equivalent NetworkPolicy boundary, cannot deny cross-namespace coord/transfer dials, or includes arbitrary pods on the cluster network reaching Gantry ports in the threat model.

Tests

• Peers without the PSK cannot join the Gantry libp2p network.
• Gantry agents with the same PSK can bootstrap, discover providers, and exchange coord traffic.
• Rotation or missing-secret failure mode is explicit and observable.
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