TestSupportedCurvesNegotiation fails with standard Go on Linux

[potatosips]

Summary

crypto/TestSupportedCurvesNegotiation assumes that every Linux Go toolchain advertises Cloudflare's private P256Kyber768Draft00 curve. That curve is implemented by Cloudflare's Go fork, not by the standard Go TLS stack. Standard crypto/tls filters the unknown private curve ID from the ClientHello, so the test fails and makes make test fail for the standard Go development setup documented in the README.

Reproduction

At commit 2206516c3b693357d101b9a4b7a1e39c11cc265c:

go version
# go version go1.26.4 linux/arm64

go test -mod=vendor -race -count=1 \
  -run TestSupportedCurvesNegotiation -v ./crypto

Actual result:

=== RUN   TestSupportedCurvesNegotiation
    curves_test.go:121:
        Error: Should be true
--- FAIL: TestSupportedCurvesNegotiation (0.02s)
FAIL github.com/cloudflare/cloudflared/crypto

The same failure reproduces with standard Go 1.26.5. Running the full make test suite fails for the same reason.

Expected behavior

The test should pass with both supported toolchain variants:

• Cloudflare's Go fork advertises X25519MLKEM768, P256Kyber768Draft00, and P-256.
• Standard Go filters the unsupported private ID and advertises X25519MLKEM768 and P-256.

In both cases the test should continue to verify the exact advertised sequence, including preference order, and reject any other missing or added curve.

Environment

• cloudflared commit: 2206516c3b693357d101b9a4b7a1e39c11cc265c
• OS: Ubuntu 24.04.4 LTS
• Architecture: arm64
• Reproduced with: standard Go 1.26.4 and 1.26.5

Root cause

The test uses runtime.GOOS == "linux" as a proxy for TLS capability. P256Kyber768Draft00 support is a property of Cloudflare's Go fork, not of the operating system. An earlier repository change (d0a6318334b5cb0421b17c4478defac6356f4533, TUN-9161) likewise distinguished the non-FIPS/standard curve behavior.

Suggested fix

Build the expected standard-Go sequence as [X25519MLKEM768, P-256]. If the captured ClientHello contains P256Kyber768Draft00, compare it with the full configured sequence instead. This preserves exact-order validation on both TLS implementations without changing production curve configuration.

Verification of the attached fix

• Focused test passes on standard Go 1.26.4 and 1.26.5.
• Full make test passes on Go 1.26.4 (go vet plus race-enabled tests).
• make fmt-check passes.
• golangci-lint 2.12.2 reports 0 issues.

Fix commit prepared locally: 824ac288a0faca138a81968dddda2c6471eb85e2 on branch fix/standard-go-curve-negotiation-test.