main: allow password change with -masterkey

Requested at https://github.com/rfjakob/gocryptfs/issues/28

Documentation/MANPAGE.md

@@ -80,10 +80,14 @@ This flag is useful when recovering old gocryptfs filesystems using
 "-masterkey". It is ignored (stays at the default) otherwise.
 
 **-masterkey string**
-:	Mount with explicit master key specified on the command line. This
+:	Use a explicit master key specified on the command line. This
 option can be used to mount a gocryptfs filesystem without a config file.
 Note that the command line, and with it the master key, is visible to
 anybody on the machine who can execute "ps -auxwww".
+This is meant as a recovery option for emergencies, such as if you have
+forgotten your password.
+	
+	Example master key: 6f717d8b-6b5f8e8a-fd0aa206-778ec093-62c5669b-abd229cd-241e00cd-b4d6713d
 
 **-memprofile string**
 :	Write memory profile to specified file. This is useful when debugging
@@ -91,7 +95,7 @@ memory usage of gocryptfs.
 
 **-nonempty**
 :	Allow mounting over non-empty directories. FUSE by default disallows
-this because to prevent accidential shadowing of files.
+this to prevent accidential shadowing of files.
 
 **-nosyslog**
 :	Diagnostic messages are normally redirected to syslog once gocryptfs
@@ -113,7 +117,16 @@ option.
 specifying "-extpass /bin/cat FILE".
 
 **-passwd**
-:	Change password
+:	Change the password. Will ask for the old password, check if it is
+correct, and ask for a new one.
+	
+	This can be used together with `-masterkey` if
+you forgot the password but know the master key. Note that without the
+old password, gocryptfs cannot tell if the master key is correct and will
+overwrite the old one without mercy. It will, however, create a backup copy
+of the old config file as `gocryptfs.conf.bak`. Delete it after
+you have verified that you can access your files with the
+new password.
 
 **-plaintextnames**
 :	Do not encrypt file names and symlink targets

internal/configfile/config_file.go

@@ -139,7 +139,11 @@ func LoadConfFile(filename string, password string) ([]byte, *ConfFile, error) {
 
 		return nil, nil, fmt.Errorf("Deprecated filesystem")
 	}
-
+	if password == "" {
+		// We have validated the config file, but without a password we cannot
+		// decrypt the master key. Return only the parsed config.
+		return nil, &cf, nil
+	}
 	// Generate derived key from password
 	scryptHash := cf.ScryptObject.DeriveKey(password)
 

main.go

@@ -59,9 +59,14 @@ func loadConfig(args *argContainer) (masterkey []byte, confFile *configfile.Conf
 		os.Exit(ErrExitLoadConf)
 	}
 	fd.Close()
+	if args.masterkey != "" {
+		masterkey = parseMasterKey(args.masterkey)
+		_, confFile, err = configfile.LoadConfFile(args.config, "")
+	} else {
 		pw := readpassword.Once(args.extpass)
 		tlog.Info.Println("Decrypting master key")
 		masterkey, confFile, err = configfile.LoadConfFile(args.config, pw)
+	}
 	if err != nil {
 		tlog.Fatal.Println(err)
 		os.Exit(ErrExitLoadConf)
@@ -75,12 +80,24 @@ func changePassword(args *argContainer) {
 	tlog.Info.Println("Please enter your new password.")
 	newPw := readpassword.Twice(args.extpass)
 	confFile.EncryptKey(masterkey, newPw, confFile.ScryptObject.LogN())
+	if args.masterkey != "" {
+		bak := args.config + ".bak"
+		err := os.Link(args.config, bak)
+		if err != nil {
+			tlog.Fatal.Printf("Could not create backup file: %v", err)
+			os.Exit(ErrExitInit)
+		}
+		tlog.Info.Printf(tlog.ColorGrey+
+			"A copy of the old config file has been created at %q.\n"+
+			"Delete it after you have verified that you can access your files with the new password."+
+			tlog.ColorReset, bak)
+	}
 	err := confFile.WriteFile()
 	if err != nil {
 		tlog.Fatal.Println(err)
 		os.Exit(ErrExitInit)
 	}
-	tlog.Info.Printf("Password changed.")
+	tlog.Info.Printf(tlog.ColorGreen + "Password changed." + tlog.ColorReset)
 	os.Exit(0)
 }