1 files changed, 39 insertions, 0 deletions
diff --git a/src/cookie.go b/src/cookie.go
new file mode 100644
index 0000000..a6987a2
--- /dev/null
+++ b/src/cookie.go
@@ -0,0 +1,39 @@
+package main
+
+import (
+	"errors"
+	"golang.org/x/crypto/blake2s"
+)
+
+func CalculateCookie(peer *Peer, msg []byte) {
+	size := len(msg)
+
+	if size < blake2s.Size128*2 {
+		panic(errors.New("bug: message too short"))
+	}
+
+	startMac1 := size - (blake2s.Size128 * 2)
+	startMac2 := size - blake2s.Size128
+
+	mac1 := msg[startMac1 : startMac1+blake2s.Size128]
+	mac2 := msg[startMac2 : startMac2+blake2s.Size128]
+
+	peer.mutex.RLock()
+	defer peer.mutex.RUnlock()
+
+	// set mac1
+
+	func() {
+		mac, _ := blake2s.New128(peer.macKey[:])
+		mac.Write(msg[:startMac1])
+		mac.Sum(mac1[:0])
+	}()
+
+	// set mac2
+
+	if peer.cookie != nil {
+		mac, _ := blake2s.New128(peer.cookie)
+		mac.Write(msg[:startMac2])
+		mac.Sum(mac2[:0])
+	}
+}