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pqc-admission-webhook: set Envoy ecdh_curves on a Cilium Gateway API listener
Cilium's Gateway API translation has no field for Envoy TLS parameters, so there is no supported way to set ecdh_curves on a Gateway-managed listener and no way to negotiate the hybrid post-quantum group X25519MLKEM768. The upstream request for one used post-quantum key exchange as its motivating example, drew no maintainer response, and was auto-closed as stale. Editing the generated CiliumEnvoyConfig does not work either: the operator reverts drift on the resource it owns within about a second. That is also the opening. A controller that reliably rewrites a resource is a controller whose writes pass through admission control, so this injects the curve preference at admission time and lets the operator's own regeneration carry it in. Fails open by construction: failurePolicy Ignore, and every path through the handler returns an allowing response including on input it cannot parse. A webhook in front of the resource that configures your ingress can take a site down; this one can only degrade key exchange, never block a write. Finds the TLS chain by inspection rather than by index, because a hardcoded index silently stops matching when a listener is added or reordered, and skips chains that already carry tlsParams so reinvocation cannot clobber the result. No third-party dependencies: the AdmissionReview wire shape is small and stable and hand-modelling it keeps apimachinery out of the API server's admission path. Extracted from a private monorepo where it has run in production since 2026-07-25. TARGET_CEC has no default here and the curve list is configurable, neither of which was true internally.
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.github/workflows/ci.yaml

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name: CI
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on:
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push:
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branches: [main]
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pull_request:
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permissions:
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contents: read
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jobs:
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test:
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runs-on: ubuntu-latest
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steps:
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- uses: actions/checkout@v4
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- uses: actions/setup-go@v5
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with:
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go-version: "1.24"
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- run: gofmt -l . | tee /tmp/fmt && test ! -s /tmp/fmt
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- run: go vet ./...
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- run: go test -race ./...
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- run: docker build -t pqc-admission-webhook:ci .

.gitignore

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/pqc-admission-webhook

Dockerfile

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# syntax=docker/dockerfile:1
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# --- build stage -----------------------------------------------------------
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FROM golang:1.25-alpine AS build
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WORKDIR /src
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# No third-party dependencies (the AdmissionReview contract is hand-modelled),
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# so there is no go.sum and nothing to download. go.mod is still copied first
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# so the layer caches independently of the sources.
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COPY go.mod ./
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COPY . .
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# Tests run in the image build as well as in CI, so a red test can never be
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# published as a green tag.
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RUN go vet ./... && go test ./...
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# Static, stripped binary for distroless.
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RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 \
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go build -trimpath -ldflags="-s -w" -o /out/pqc-admission-webhook .
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# --- runtime stage ---------------------------------------------------------
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# distroless static nonroot: no shell, runs as uid 65532, PSS-restricted friendly.
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FROM gcr.io/distroless/static:nonroot
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USER 65532:65532
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EXPOSE 8443
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ENTRYPOINT ["/pqc-admission-webhook"]
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COPY --from=build /out/pqc-admission-webhook /pqc-admission-webhook

LICENSE

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