-passwd now takes a passed -scryptn flag into account

https://github.com/rfjakob/gocryptfs/issues/400

README.md

@@ -172,6 +172,8 @@ vNEXT, in progress
   Note: as a security precaution, the owner must still manually 
   `chmod gocryptfs.conf 0440` to allow mounting.
 * Allow the `nofail` option in `/etc/fstab`
+* `-passwd` can now change the `-scryptn` parameter for existing filesystems
+  ([#400](https://github.com/rfjakob/gocryptfs/issues/400))
 
 v1.7, 2019-03-17
 * **Fix possible symlink race attacks in forward mode** when using allow_other + plaintextnames

cli_args.go

@@ -50,6 +50,8 @@ type argContainer struct {
 	_ctlsockFd net.Listener
 	// _forceOwner is, if non-nil, a parsed, validated Owner (as opposed to the string above)
 	_forceOwner *fuse.Owner
+	// _explicitScryptn is true then the user passed "-scryptn=xyz"
+	_explicitScryptn bool
 }
 
 type multipleStrings []string
@@ -205,7 +207,8 @@ func parseCliOpts() (args argContainer) {
 
 	flagSet.IntVar(&args.notifypid, "notifypid", 0, "Send USR1 to the specified process after "+
 		"successful mount - used internally for daemonization")
-	flagSet.IntVar(&args.scryptn, "scryptn", configfile.ScryptDefaultLogN, "scrypt cost parameter logN. Possible values: 10-28. "+
+	const scryptn = "scryptn"
+	flagSet.IntVar(&args.scryptn, scryptn, configfile.ScryptDefaultLogN, "scrypt cost parameter logN. Possible values: 10-28. "+
 		"A lower value speeds up mounting and reduces its memory needs, but makes the password susceptible to brute-force attacks")
 
 	flagSet.DurationVar(&args.idle, "i", 0, "Alias for -idle")
@@ -227,6 +230,10 @@ func parseCliOpts() (args argContainer) {
 		tlog.Fatal.Printf("Invalid command line: %s. Try '%s -help'.", prettyArgs(), tlog.ProgramName)
 		os.Exit(exitcodes.Usage)
 	}
+	// We want to know if -scryptn was passed explicitely
+	if isFlagPassed(flagSet, scryptn) {
+		args._explicitScryptn = true
+	}
 	// "-openssl" needs some post-processing
 	if opensslAuto == "auto" {
 		args.openssl = prefer_openssl.PreferOpenSSL()
@@ -311,3 +318,15 @@ func countOpFlags(args *argContainer) int {
 	}
 	return count
 }
+
+// isFlagPassed finds out if the flag was explictely passed on the command line.
+// https://stackoverflow.com/a/54747682/1380267
+func isFlagPassed(flagSet *flag.FlagSet, name string) bool {
+	found := false
+	flagSet.Visit(func(f *flag.Flag) {
+		if f.Name == name {
+			found = true
+		}
+	})
+	return found
+}

main.go

@@ -94,7 +94,11 @@ func changePassword(args *argContainer) {
 		}
 		tlog.Info.Println("Please enter your new password.")
 		newPw := readpassword.Twice([]string(args.extpass), args.passfile)
-		confFile.EncryptKey(masterkey, newPw, confFile.ScryptObject.LogN())
+		logN := confFile.ScryptObject.LogN()
+		if args._explicitScryptn {
+			logN = args.scryptn
+		}
+		confFile.EncryptKey(masterkey, newPw, logN)
 		for i := range newPw {
 			newPw[i] = 0
 		}

tests/cli/cli_test.go

@@ -7,6 +7,7 @@ import (
 	"io/ioutil"
 	"os"
 	"os/exec"
+	"strconv"
 	"syscall"
 	"testing"
 	"time"
@@ -92,6 +93,9 @@ func TestInitReverse(t *testing.T) {
 	}
 }
 
+// testPasswd changes the password from "test" to "test" using
+// the -extpass method, then from "test" to "newpasswd" using the
+// stdin method.
 func testPasswd(t *testing.T, dir string, extraArgs ...string) {
 	// Change password using "-extpass"
 	args := []string{"-q", "-passwd", "-extpass", "echo test"}
@@ -225,6 +229,23 @@ func TestPasswdReverse(t *testing.T) {
 	testPasswd(t, dir, "-reverse")
 }
 
+// Test -passwd with -scryptn
+func TestPasswdScryptn(t *testing.T) {
+	dir := test_helpers.InitFS(t)
+	cf, err := configfile.Load(dir + "/gocryptfs.conf")
+	if err != nil {
+		t.Fatal(err)
+	}
+	testPasswd(t, dir, "-scryptn", strconv.Itoa(cf.ScryptObject.LogN()+1))
+	cf2, err := configfile.Load(dir + "/gocryptfs.conf")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cf2.ScryptObject.LogN() != cf.ScryptObject.LogN()+1 {
+		t.Errorf("wrong logN value %d", cf2.ScryptObject.LogN())
+	}
+}
+
 // Test -init & -config flag
 func TestInitConfig(t *testing.T) {
 	config := test_helpers.TmpDir + "/TestInitConfig.conf"