diff --git a/watchtower/wtdb/session_id.go b/watchtower/wtdb/session_id.go
new file mode 100644
index 00000000..7980b1f6
--- /dev/null
+++ b/watchtower/wtdb/session_id.go
@@ -0,0 +1,26 @@
+package wtdb
+
+import (
+	"encoding/hex"
+
+	"github.com/btcsuite/btcd/btcec"
+)
+
+// SessionIDSize is 33-bytes; it is a serialized, compressed public key.
+const SessionIDSize = 33
+
+// SessionID is created from the remote public key of a client, and serves as a
+// unique identifier and authentication for sending state updates.
+type SessionID [SessionIDSize]byte
+
+// NewSessionIDFromPubKey creates a new SessionID from a public key.
+func NewSessionIDFromPubKey(pubKey *btcec.PublicKey) SessionID {
+	var sid SessionID
+	copy(sid[:], pubKey.SerializeCompressed())
+	return sid
+}
+
+// String returns a hex encoding of the session id.
+func (s SessionID) String() string {
+	return hex.EncodeToString(s[:])
+}