simplify generation of stateless reset tokens (#4858)

stateless_reset.go

@@ -0,0 +1,42 @@
+package quic
+
+import (
+	"crypto/hmac"
+	"crypto/rand"
+	"crypto/sha256"
+	"hash"
+	"sync"
+
+	"github.com/quic-go/quic-go/internal/protocol"
+)
+
+type statelessResetter struct {
+	mx sync.Mutex
+	h  hash.Hash
+}
+
+// newStatelessRetter creates a new stateless reset generator.
+// It is valid to use a nil key. In that case, a random key will be used.
+// This makes is impossible for on-path attackers to shut down established connections.
+func newStatelessResetter(key *StatelessResetKey) *statelessResetter {
+	var h hash.Hash
+	if key != nil {
+		h = hmac.New(sha256.New, key[:])
+	} else {
+		b := make([]byte, 32)
+		_, _ = rand.Read(b)
+		h = hmac.New(sha256.New, b)
+	}
+	return &statelessResetter{h: h}
+}
+
+func (r *statelessResetter) GetStatelessResetToken(connID protocol.ConnectionID) protocol.StatelessResetToken {
+	r.mx.Lock()
+	defer r.mx.Unlock()
+
+	var token protocol.StatelessResetToken
+	r.h.Write(connID.Bytes())
+	copy(token[:], r.h.Sum(nil))
+	r.h.Reset()
+	return token
+}