Add dummy key agreement with codepoint 0xfe33 (#162)

DummyKex is a key agreeement similar in size but purposefully incompatible with
X25519. The goal is to have a key agreement that servers will not support, so
we can test HelloRetryRquest.

src/crypto/tls/cfkem.go

@@ -22,6 +22,7 @@ import (
  "fmt"
  "io"
 
+ "github.com/cloudflare/circl/hpke"
  "github.com/cloudflare/circl/kem"
  "github.com/cloudflare/circl/kem/hybrid"
 )
@@ -42,6 +43,11 @@ var (
  X25519Kyber768Draft00Old = CurveID(0xfe31)
  P256Kyber768Draft00 = CurveID(0xfe32)
  invalidCurveID = CurveID(0)
+
+ // A key agreeement similar in size but purposefully incompatible with
+ // X25519. The goal is to have a key agreement that servers will not
+ // support, so we can test HelloRetryRquest.
+ DummyKex = CurveID(0xfe33)
 )
 
 func singleClientKeySharePrivateFor(ks clientKeySharePrivate, group CurveID) singleClientKeySharePrivate {
@@ -58,6 +64,8 @@ func curveIdToCirclScheme(id CurveID) kem.Scheme {
  return hybrid.Kyber768X25519()
  case P256Kyber768Draft00:
  return hybrid.P256Kyber768Draft00()
+ case DummyKex:
+ return hpke.KEM_X25519_HKDF_SHA256.Scheme()
  }
  return nil
 }

src/crypto/tls/cfkem_test.go

@@ -95,6 +95,7 @@ func TestHybridKEX(t *testing.T) {
  X25519Kyber768Draft00,
  X25519Kyber768Draft00Old,
  P256Kyber768Draft00,
+ DummyKex,
  } {
  run(curveID, true, true, false, false)
  run(curveID, true, false, false, false)