crypto/tls: fix ECH compatibility

Previously, the code only checked supportedVersions[0] for TLS 1.3
However, Chromium-based
browsers may list TLS 1.3 at different positions, causing ECH failures.
This fix:
    Iterates through supportedVersions to accept connections as long as TLS 1.3 is present.
    Improves ECH compatibility, ensuring Chrome, Edge, and other browsers work properly.

Fixes #71642

Change-Id: I32f4219fb6654d5cc22c7f33497c6142c0acb4f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/648015
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>

diff --git a/src/crypto/tls/ech.go b/src/crypto/tls/ech.go
index 8bfd47ced3895bd328f3ad8c1bd842398474d0e4..3ca9fd00b1b56242e09823c8a797b85aadb50a86 100644 (file)
--- a/src/crypto/tls/ech.go
+++ b/src/crypto/tls/ech.go
@@ -392,8 +392,28 @@ func decodeInnerClientHello(outer *clientHelloMsg, encoded []byte) (*clientHello
                return nil, errInvalidECHExt
        }
 
-       if len(inner.supportedVersions) != 1 || (len(inner.supportedVersions) >= 1 && inner.supportedVersions[0] != VersionTLS13) {
-               return nil, errors.New("tls: client sent encrypted_client_hello extension and offered incompatible versions")
+       hasTLS13 := false
+       for _, v := range inner.supportedVersions {
+               // Skip GREASE values (values of the form 0x?A0A).
+               // GREASE (Generate Random Extensions And Sustain Extensibility) is a mechanism used by
+               // browsers like Chrome to ensure TLS implementations correctly ignore unknown values.
+               // GREASE values follow a specific pattern: 0x?A0A, where ? can be any hex digit.
+               // These values should be ignored when processing supported TLS versions.
+               if v&0x0F0F == 0x0A0A && v&0xff == v>>8 {
+                       continue
+               }
+
+               // Ensure at least TLS 1.3 is offered.
+               if v == VersionTLS13 {
+                       hasTLS13 = true
+               } else if v < VersionTLS13 {
+                       // Reject if any non-GREASE value is below TLS 1.3, as ECH requires TLS 1.3+.
+                       return nil, errors.New("tls: client sent encrypted_client_hello extension with unsupported versions")
+               }
+       }
+
+       if !hasTLS13 {
+               return nil, errors.New("tls: client sent encrypted_client_hello extension but did not offer TLS 1.3")
        }
 
        return inner, nil