crypto/tls: add GetEncryptedClientHelloKeys callback

[mholt]

Go version

go version go1.24.0 linux/amd64

Output of go env in your module/workspace:

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/matt/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/home/matt/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build1380606169=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/home/matt/Dev/caddyserver/caddy/go.mod'
GOMODCACHE='/home/matt/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/matt/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/lib/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='on'
GOTELEMETRYDIR='/home/matt/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/lib/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.24.0'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

I'm attempting to implement the ECH spec with regards to key rotation. It states in 10.10.5:

This design is not forward secret because the server's ECH key is static. However, the window of exposure is bound by the key lifetime. It is RECOMMENDED that servers rotate keys frequently.

The application (Caddy in this case) requires use of GetConfigForClient.

What did you see happen?

The problem is, tls.Config.EncryptedClientHelloKeys is a static field, not a callback like GetCertificate.

This means we have to start a new TLS server with the updated ECH keys, which is complex and can be tricky to do without downtime.

I also realized that GetConfigForClient is not recursive... at first I tried using GetConfigForClient to return a tls.Config populated especially for ECH:

// ECH keys loaded dynamically for the outer SNI, which
// solves the key rotation problem
// ...
return &tls.Config{
	MinVersion:               tls.VersionTLS13,
	GetConfigForClient:       getConfigForClient, // loads config for inner SNI
	EncryptedClientHelloKeys: stdECHKeys,
}, nil

But this results in "no certificates configured", because the returned Config's GetConfigForClient isn't called, and sure enough it has no certs. Relevant code:

https://cs.opensource.google/go/go/+/master:src/crypto/tls/handshake_server.go;l=148-168?q=GetConfigForClient&ss=go%2Fgo&start=11

But we can't return the final tls.Config yet (the one with certificates) because we don't have the inner SNI yet.

What did you expect to see?

I was hoping that GetConfigForClient + EncryptedClientHelloKeys could play well together... in the case of ECH, maybe allow GetConfigForClient to be used to unwrap the outer Hello, and also be used for the inner. (i.e. allow calling it twice, once before once after processing ECH) Or, make a new field, GetEncryptedClientHelloKeys callback function, much like GetCertificate.

• We can't forfeit the use of GetConfigForClient due to dynamic configuration requirements.
• But EncryptedClientHelloKeys must be changed after the server has started

Rotating keys is beneficial for security and encouraged by the spec.

Do you have any other suggestions for making this work as-is? Or is this something that can be enhanced?

Thanks again for server-side ECH 💯

[gabyhelp]

Related Issues

• crypto/tls: wrong error returned in ECHConfig parsing #71706 (closed)
• crypto/tls: After the ech key verification fails, the new key sent by the ech server will not be used for handshake. #70073 (closed)
• crypto/tls: support server side Encrypted Client Hello #68500 (closed)
• crypto/tls: encryptedExtensions.echRetryConfigs not returned to client #70915 (closed)
• crypto/tls: should apply default NextProtos to the result of GetConfigForClient #70214
• crypto/tls: server should send a `illegal_parameter` alert when it received an invalid `ECHClientHello.type` #71061 (closed)

Related Code Changes

• crypto/tls: add GetClientConfig to Config
• crypto/tls: add GetClientCAs to Config
• crypto/tls: add Config.GetConfigForClient

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

[prattmic]

cc @FiloSottile @rolandshoemaker @golang/security

[rolandshoemaker]

Is the only dynamic configuration (other than the ECH keys) the certificates for the unwrapped SNI? If so can you just offload that logic to a GetCertificate callback (which can be populated in the config returned from GetConfigForClient, if that helps)?

[mholt]

I did consider that, and it's tempting to do that, but unfortunately, Caddy users can customize much about the TLS connection: https://caddyserver.com/docs/json/apps/http/servers/tls_connection_policies/

I guess in TLS 1.3 (especially with ECH enabled), many of the parameters have no effect or are overridden by hard-coded values to ensure compliance, but parameters like client authentication (ClientAuthMode) and, in some cases, even KeyLogWriter are customized by the user on a dynamic basis. (In Caddy, users can customize TLS configs based not only on ServerName, but on any property of the ClientHelloInfo.)

So, yes I think that would be a workaround, but I'd have to disclaim in our docs that enabling ECH essentially ignores all other TLS configuration properties.

I guess I'm hoping for a way to still have control over the TLS config for the unwrapped ClientHello when GetConfigForClient is used.

That'd be ideal -- but I don't want to try to force a square peg into a round hole. It could be an odd use case maybe.

[gstrauss]

The problem is, tls.Config.EncryptedClientHelloKeys is a static field, not a callback like GetCertificate.

	// If a client attempts ECH, but it is rejected by the server, the server
	// will send a list of configs to retry based on the set of
	// EncryptedClientHelloKeys which have the SendAsRetry field set.

When a new (refreshed) TLS ECH key is created, it is desirable that the prior key still be valid for a short period, but the prior key should now have SendAsRetry field unset. Also, after a short period of time, the prior key should be removed from EncryptedClientHelloKeys.

[Edit: so not only is the list of keys dynamic, but it is desirable to be able to change the properties on each key. (Alt: Could remove key and re-add same key but with altered properties)]

[rolandshoemaker]

My memory of how I actually implemented the client hello switching paged out of my brain almost immediately after 1.24 was finished, looking back at the actual code I see how this is problematic. We currently switch out the client hello before calling GetConfigForClient, so the returned config only ever applies to the inner hello.

I think this makes sense, since the outer config should never really require special config settings, except EncryptedClientHelloKeys. Unfortunately this doesn't leave any way to currently change EncryptedClientHelloKeys safely once the Config is passed to a Conn because we say the Config cannot be mutated. So unless the caller is manually creating configs with Server(..), which is a pretty niche usage of the package, they are forced to essentially restartthe server if they want to change the keys.

I think the only viable option here is to add a new callback, e.g. GetEncryptedClientHelloKeys func(*ClientHelloInfo) ([]EncryptedClientHelloKey, error), which lets the caller return a set of keys based on the outer client hello (we may actually want to reduce the scope of the client hello to the outer SNI, since the rest of the information in the outer hello should generally be ignored.)

[rolandshoemaker]

Note, because this will be a new API, it'll need to wait for 1.25.