lib/protocol: Optimize FileKey (#7440)

lib/protocol/encryption.go

@@ -8,7 +8,6 @@ package protocol
 
 import (
 	"context"
-	"crypto/sha256"
 	"encoding/base32"
 	"encoding/binary"
 	"errors"
@@ -20,6 +19,7 @@ import (
 	"github.com/gogo/protobuf/proto"
 	"github.com/miscreant/miscreant.go"
 	"github.com/syncthing/syncthing/lib/rand"
+	"github.com/syncthing/syncthing/lib/sha256"
 	"golang.org/x/crypto/chacha20poly1305"
 	"golang.org/x/crypto/hkdf"
 	"golang.org/x/crypto/scrypt"
@@ -487,8 +487,10 @@ func KeyFromPassword(folderID, password string) *[keySize]byte {
 	return &key
 }
 
+var hkdfSalt = []byte("syncthing")
+
 func FileKey(filename string, folderKey *[keySize]byte) *[keySize]byte {
-	kdf := hkdf.New(sha256.New, append(folderKey[:], filename...), []byte("syncthing"), nil)
+	kdf := hkdf.New(sha256.New, append(folderKey[:], filename...), hkdfSalt, nil)
 	var fileKey [keySize]byte
 	n, err := io.ReadFull(kdf, fileKey[:])
 	if err != nil || n != keySize {

lib/protocol/encryption_test.go

@@ -12,9 +12,11 @@ import (
 	"reflect"
 	"regexp"
 	"strings"
+	"sync"
 	"testing"
 
 	"github.com/syncthing/syncthing/lib/rand"
+	"github.com/syncthing/syncthing/lib/sha256"
 )
 
 func TestEnDecryptName(t *testing.T) {
@@ -180,3 +182,22 @@ func TestIsEncryptedParent(t *testing.T) {
 		}
 	}
 }
+
+var benchmarkFileKey struct {
+	key [keySize]byte
+	sync.Once
+}
+
+func BenchmarkFileKey(b *testing.B) {
+	benchmarkFileKey.Do(func() {
+		sha256.SelectAlgo()
+		rand.Read(benchmarkFileKey.key[:])
+	})
+
+	b.ResetTimer()
+	b.ReportAllocs()
+
+	for i := 0; i < b.N; i++ {
+		FileKey("a_kind_of_long_filename.ext", &benchmarkFileKey.key)
+	}
+}