crypto/tls: mlkem needs GODEBUG=tlsmlkem=1 to get enabled

[msmeissn]

Go version

go1.25.3

Output of go env in your module/workspace:

go env
AR='ar'
CC='gcc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='g++'
GCCGO='gccgo'
GO111MODULE=''
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/home/meissner/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/home/meissner/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build2704894481=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/home/meissner/projects/goserver/golang-pqc-examples/go.mod'
GOMODCACHE='/home/meissner/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/meissner/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/lib64/go/1.25'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/meissner/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='local'
GOTOOLDIR='/usr/lib64/go/1.25/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.25.3'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

The current TLS mlkem draft support needs to have set

GODEBUG=tlsmlkem=1

to be enabled.

This is contrary to the documented behaviour:

Go 1.24 enabled the post-quantum key exchange mechanism
X25519MLKEM768 by default. The default can be reverted using the
tlsmlkem setting.
This can be useful when dealing with buggy TLS servers that do not handle large records correctly,
causing a timeout during the handshake (see TLS post-quantum TL;DR fail).
Go 1.24 also removed X25519Kyber768Draft00 and the Go 1.23 tlskyber setting.

What did you see happen?

tls mmlkem draft support did not activate by default.

What did you expect to see?

tls mlkem draft support activated by default.

[seankhliao]

How are you determining that it's not enabled by default?

[gabyhelp]

Related Issues

• crypto/tls: X25519MLKEM768 not enabled by default #75453 (closed)
• crypto/tls: X25519MLKEM768 curve causes panics under GODEBUG=fips140=only #75528 (closed)
• crypto/tls: add X25519MLKEM768 and use by default; remove x25519Kyber768Draft00 #69985 (closed)
• crypto/fips140: TLS handshake fails with go1.25 and FIPS-140-3 strict mode #75166 (closed)
• crypto/tls: implement and enable X25519Kyber768Draft00 #67061 (closed)
• crypto/tls: add support for NIST curve based ML-KEM hybrids #71206 (closed)
• crypto/ecdh: use of X25519 is not allowed in FIPS 140-only mode #75148 (closed)
• crypto/ecdh: Basic HTTPS request broken when FIPS-140 is enforced #76112 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[msmeissn]

I ran a super simple testserver (any standard HTTPS server written in GO wil do)

e.g.
https://github.com/hub-adda/golang-pqc-examples.git

then tried to connect with e.g. openssl 3.5:
openssl s_client -connect 10.144.110.114:4443

in the debug output it shows:

Negotiated TLS1.3 group: X25519MLKEM768

ONLY when I run the go server with GODEBUG=tlsmlkem=1

(The problem is I guess that:
go/src/crypto/tls/defaults.go

var tlsmlkem = godebug.New("tlsmlkem")

// defaultCurvePreferences is the default set of supported key exchanges, as
// well as the preference order.
func defaultCurvePreferences() []CurveID {
        if tlsmlkem.Value() == "0" {
                return []CurveID{X25519, CurveP256, CurveP384, CurveP521}
        }
        return []CurveID{X25519MLKEM768, X25519, CurveP256, CurveP384, CurveP521}
}

if no GODEBUG is set , the default value of tlsmlkem seems to be 0.

[seankhliao]

Your go.mod go directive determines the default Go version / GODEBUG settings you have. You've set Go 1.23, which is before tlsmlkem was enabled by default.

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