configfile: bake the "Creator" gocryptfs version into the file

This field is added for the convenience of users and
may help them to identify which gocryptfs version
they need to mount a filesystem.

The same information is essentially contained in FeatureFlags,
but this is more difficult to decode for humans.

It is completely ignored programmatically (also by older gocryptfs
versions).

internal/configfile/config_file.go

@@ -19,8 +19,10 @@ const (
 )
 
 type ConfFile struct {
-	// File the config is saved to. Not exported to JSON.
-	filename string
+	// gocryptfs version string
+	// This only documents the config file for humans who look at it. The actual
+	// technical info is contained in FeatureFlags.
+	Creator string
 	// Encrypted AES key, unlocked using a password hashed with scrypt
 	EncryptedKey []byte
 	// Stores parameters for scrypt hashing (key derivation)
@@ -32,14 +34,17 @@ type ConfFile struct {
 	// mounting. This mechanism is analogous to the ext4 feature flags that are
 	// stored in the superblock.
 	FeatureFlags []string
+	// File the config is saved to. Not exported to JSON.
+	filename string
 }
 
 // CreateConfFile - create a new config with a random key encrypted with
 // "password" and write it to "filename".
 // Uses scrypt with cost parameter logN.
-func CreateConfFile(filename string, password string, plaintextNames bool, logN int) error {
+func CreateConfFile(filename string, password string, plaintextNames bool, logN int, creator string) error {
 	var cf ConfFile
 	cf.filename = filename
+	cf.Creator = creator
 	cf.Version = contentenc.CurrentVersion
 
 	// Generate new random master key

internal/configfile/config_test.go

@@ -60,7 +60,7 @@ func TestLoadV2StrangeFeature(t *testing.T) {
 }
 
 func TestCreateConfFile(t *testing.T) {
-	err := CreateConfFile("config_test/tmp.conf", "test", false, 10)
+	err := CreateConfFile("config_test/tmp.conf", "test", false, 10, "test")
 	if err != nil {
 		t.Fatal(err)
 	}

main.go

@@ -72,7 +72,8 @@ func initDir(args *argContainer) {
 		toggledlog.Info.Printf("Using password provided via -extpass.")
 	}
 	password := readPasswordTwice(args.extpass)
-	err = configfile.CreateConfFile(args.config, password, args.plaintextnames, args.scryptn)
+	creator := toggledlog.ProgramName + " " + GitVersion
+	err = configfile.CreateConfFile(args.config, password, args.plaintextnames, args.scryptn, creator)
 	if err != nil {
 		fmt.Println(err)
 		os.Exit(ERREXIT_INIT)