From 0e1bd3957b34efc3c45277ea43aaa6848acad11b Mon Sep 17 00:00:00 2001
From: Riccardo Manfrin <riccardomanfrin@gmail.com>
Date: Wed, 13 May 2026 16:19:59 +0200
Subject: [PATCH] [client] Moves deterministic key gen into rosenpass

---
 client/internal/peer/conn.go           | 23 ++------------
 client/internal/rosenpass/seed.go      | 42 ++++++++++++++++++++++++
 client/internal/rosenpass/seed_test.go | 44 ++++++++++++++++++++++++++
 3 files changed, 88 insertions(+), 21 deletions(-)
 create mode 100644 client/internal/rosenpass/seed.go
 create mode 100644 client/internal/rosenpass/seed_test.go

diff --git a/client/internal/peer/conn.go b/client/internal/peer/conn.go
index 1e416bfe7..79a513956 100644
--- a/client/internal/peer/conn.go
+++ b/client/internal/peer/conn.go
@@ -23,6 +23,7 @@ import (
 	"github.com/netbirdio/netbird/client/internal/peer/id"
 	"github.com/netbirdio/netbird/client/internal/peer/worker"
 	"github.com/netbirdio/netbird/client/internal/portforward"
+	"github.com/netbirdio/netbird/client/internal/rosenpass"
 	"github.com/netbirdio/netbird/client/internal/stdnet"
 	"github.com/netbirdio/netbird/route"
 	relayClient "github.com/netbirdio/netbird/shared/relay/client"
@@ -899,7 +900,7 @@ func (conn *Conn) presharedKey(remoteRosenpassKey []byte) *wgtypes.Key {
 	}
 
 	// Fallback to deterministic key if no NetBird PSK is configured
-	determKey, err := conn.rosenpassDetermKey()
+	determKey, err := rosenpass.DeterministicSeedKey(conn.config.LocalKey, conn.config.Key)
 	if err != nil {
 		conn.Log.Errorf("failed to generate Rosenpass initial key: %v", err)
 		return nil
@@ -908,26 +909,6 @@ func (conn *Conn) presharedKey(remoteRosenpassKey []byte) *wgtypes.Key {
 	return determKey
 }
 
-// todo: move this logic into Rosenpass package
-func (conn *Conn) rosenpassDetermKey() (*wgtypes.Key, error) {
-	lk := []byte(conn.config.LocalKey)
-	rk := []byte(conn.config.Key) // remote key
-	var keyInput []byte
-	if string(lk) > string(rk) {
-		//nolint:gocritic
-		keyInput = append(lk[:16], rk[:16]...)
-	} else {
-		//nolint:gocritic
-		keyInput = append(rk[:16], lk[:16]...)
-	}
-
-	key, err := wgtypes.NewKey(keyInput)
-	if err != nil {
-		return nil, err
-	}
-	return &key, nil
-}
-
 func isController(config ConnConfig) bool {
 	return config.LocalKey > config.Key
 }
diff --git a/client/internal/rosenpass/seed.go b/client/internal/rosenpass/seed.go
new file mode 100644
index 000000000..83aba1e0e
--- /dev/null
+++ b/client/internal/rosenpass/seed.go
@@ -0,0 +1,42 @@
+package rosenpass
+
+import (
+	"fmt"
+
+	"golang.zx2c4.com/wireguard/wgctrl/wgtypes"
+)
+
+// DeterministicSeedKey derives a 32-byte WireGuard preshared key from a pair
+// of peer public keys. Both peers, given the same key pair, produce the same
+// output regardless of which side runs the function: the inputs are ordered
+// lexicographically before concatenation.
+//
+// NetBird uses this value as the initial Rosenpass-side preshared key when no
+// explicit account-level PSK is configured, so both peers converge on the same
+// PSK before the first post-quantum handshake completes.
+//
+// The resulting key MUST NOT be treated as quantum-safe: it is deterministic
+// from public keys and exists only to seed WireGuard until Rosenpass rotates
+// in a real post-quantum PSK.
+func DeterministicSeedKey(localKey, remoteKey string) (*wgtypes.Key, error) {
+	lk := []byte(localKey)
+	rk := []byte(remoteKey)
+	if len(lk) < 16 || len(rk) < 16 {
+		return nil, fmt.Errorf("rosenpass: peer keys must be at least 16 bytes (got local=%d, remote=%d)", len(lk), len(rk))
+	}
+
+	var keyInput []byte
+	if localKey > remoteKey {
+		keyInput = append(keyInput, lk[:16]...)
+		keyInput = append(keyInput, rk[:16]...)
+	} else {
+		keyInput = append(keyInput, rk[:16]...)
+		keyInput = append(keyInput, lk[:16]...)
+	}
+
+	key, err := wgtypes.NewKey(keyInput)
+	if err != nil {
+		return nil, fmt.Errorf("rosenpass: deterministic seed key: %w", err)
+	}
+	return &key, nil
+}
diff --git a/client/internal/rosenpass/seed_test.go b/client/internal/rosenpass/seed_test.go
new file mode 100644
index 000000000..0dfa478c7
--- /dev/null
+++ b/client/internal/rosenpass/seed_test.go
@@ -0,0 +1,44 @@
+package rosenpass
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestDeterministicSeedKey_SameForBothSides(t *testing.T) {
+	// Peer A and peer B must derive the same PSK regardless of which side
+	// computes it: the function orders inputs internally.
+	a := strings.Repeat("a", 32)
+	b := strings.Repeat("b", 32)
+
+	keyAB, err := DeterministicSeedKey(a, b)
+	require.NoError(t, err)
+	keyBA, err := DeterministicSeedKey(b, a)
+	require.NoError(t, err)
+	require.Equal(t, keyAB.String(), keyBA.String(), "swapping arguments must yield identical key")
+}
+
+func TestDeterministicSeedKey_ChangesWithKeys(t *testing.T) {
+	a := strings.Repeat("a", 32)
+	b := strings.Repeat("b", 32)
+	c := strings.Repeat("c", 32)
+
+	keyAB, err := DeterministicSeedKey(a, b)
+	require.NoError(t, err)
+	keyAC, err := DeterministicSeedKey(a, c)
+	require.NoError(t, err)
+	require.NotEqual(t, keyAB.String(), keyAC.String(), "different peer pair must yield different key")
+}
+
+func TestDeterministicSeedKey_TooShortKey_ReturnsError(t *testing.T) {
+	short := "short" // < 16 bytes
+	long := strings.Repeat("x", 32)
+
+	_, err := DeterministicSeedKey(short, long)
+	require.Error(t, err)
+	_, err = DeterministicSeedKey(long, short)
+	require.Error(t, err)
+}
+