[release-branch.go1.27] Honor the testing random reader in ML-KEM

[Quim Muntal (qmuntal)]

Summary

Backport the ML-KEM portion of main commit 93114c97deb03b0b112dc52d99e326661a2ba1a7 plus the dependency rule fix 29c2d8ac76.

crypto/internal/rand.Reader remains the default under testing/cryptotest.SetGlobalRandom, so check the public crypto/rand.Reader. This fixes deterministic TLS ClientHello ML-KEM replay mismatches without changing fixtures.

Changes

1.27 public-reader guards for 768/1024 and dependency updates in existing patch 0002.

Validation

Windows/arm64 CNG, existing TestSetGlobalRandom/mlkem.GenerateKey768 passed, ML-KEM tests/dependency TestDependencies passed, temporary 768/1024 deterministic/roundtrip/key-lifetime probes passed and were removed, MS_GO_NOSYSTEMCRYPTO=1 control passed, and git go-patch extract verified. One combined 1.26 run timed out; isolated checks/bounded reruns passed. No TLS replay/E2E suites were run.

The Go 1.26 toolchain was built from patched Go 1.26.8 source; the existing local devel toolchain was used against patched 1.27.1 source.

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[Copilot]

Copilot review overview

🟢 Approval recommended

The change is narrowly scoped, aligns with the stated intent (public crypto/rand.Reader default-reader checks), and updates dependency rules to match.

Review tier: Lite
Findings: None

What changed in this PR

Backports the Go 1.27 ML-KEM “use public crypto/rand.Reader for default-reader checks” fix into the existing crypto-backend patch, preventing backend ML-KEM keygen/encapsulation from running when testing/cryptotest.SetGlobalRandom has overridden crypto/rand.Reader (fixing deterministic TLS ClientHello replay mismatches).

Changes:

• Update crypto/mlkem backend gating to use rand.IsDefaultReader(crypto/rand.Reader) rather than crypto/internal/rand.Reader.
• Adjust go/build/deps_test.go dependency rules to reflect crypto/mlkem’s dependency on crypto/rand.Reader.

Patches are happy!

File	Description
patches/​0002-Add-crypto-backends.patch	Backports ML-KEM reader-guard logic and updates deps rules for crypto/mlkem accordingly.

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