main: Add '-devrandom' commandline option

Allows to use /dev/random for generating the master key instead of the
default Go implementation. When the kernel random generator has been
properly initialized both are considered equally secure, however:

* Versions of Go prior to 1.9 just fall back to /dev/urandom if the
  getrandom() syscall would be blocking (Go Bug #19274)

* Kernel versions prior to 3.17 do not support getrandom(), and there
  is no check if the random generator has been properly initialized
  before reading from /dev/urandom

This is especially useful for embedded hardware with low-entroy. Please
note that generation of the master key might block indefinitely if the
kernel cannot harvest enough entropy.

Documentation/MANPAGE.md

@@ -51,6 +51,13 @@ be suitable.
 #### -d, -debug
 Enable debug output
 
+#### -devrandom
+Use /dev/random for generating the master key instead of the default Go
+implementation. This is especially useful on embedded systems with Go versions
+prior to 1.9, which fall back to weak random data when the getrandom syscall
+is blocking. Using this option can block indefinitely when the kernel cannot
+harvest enough entropy.
+
 #### -extpass string
 Use an external program (like ssh-askpass) for the password prompt.
 The program should return the password on stdout, a trailing newline is

cli_args.go

@@ -22,7 +22,7 @@ type argContainer struct {
 	plaintextnames, quiet, nosyslog, wpanic,
 	longnames, allow_other, ro, reverse, aessiv, nonempty, raw64,
 	noprealloc, speed, hkdf, serialize_reads, forcedecode, hh, info,
-	sharedstorage bool
+	sharedstorage, devrandom bool
 	masterkey, mountpoint, cipherdir, cpuprofile, extpass,
 	memprofile, ko, passfile, ctlsock, fsname, force_owner, trace string
 	// Configuration file name override
@@ -132,6 +132,7 @@ func parseCliOpts() (args argContainer) {
 	flagSet.BoolVar(&args.hh, "hh", false, "Show this long help text")
 	flagSet.BoolVar(&args.info, "info", false, "Display information about CIPHERDIR")
 	flagSet.BoolVar(&args.sharedstorage, "sharedstorage", false, "Make concurrent access to a shared CIPHERDIR safer")
+	flagSet.BoolVar(&args.devrandom, "devrandom", false, "Use /dev/random for generating master key")
 	flagSet.StringVar(&args.masterkey, "masterkey", "", "Mount with explicit master key")
 	flagSet.StringVar(&args.cpuprofile, "cpuprofile", "", "Write cpu profile to specified file")
 	flagSet.StringVar(&args.memprofile, "memprofile", "", "Write memory profile to specified file")

init_dir.go

@@ -39,7 +39,7 @@ func initDir(args *argContainer) {
 	password := readpassword.Twice(args.extpass)
 	readpassword.CheckTrailingGarbage()
 	creator := tlog.ProgramName + " " + GitVersion
-	err = configfile.CreateConfFile(args.config, password, args.plaintextnames, args.scryptn, creator, args.aessiv)
+	err = configfile.CreateConfFile(args.config, password, args.plaintextnames, args.scryptn, creator, args.aessiv, args.devrandom)
 	if err != nil {
 		tlog.Fatal.Println(err)
 		os.Exit(exitcodes.WriteConf)

internal/configfile/config_file.go

@@ -5,7 +5,9 @@ package configfile
 import (
 	"encoding/json"
 	"fmt"
+	"io"
 	"io/ioutil"
+	"log"
 
 	"github.com/rfjakob/gocryptfs/internal/contentenc"
 	"github.com/rfjakob/gocryptfs/internal/cryptocore"
@@ -47,10 +49,25 @@ type ConfFile struct {
 	filename string
 }
 
+// randBytesDevRandom gets "n" random bytes from /dev/random or panics
+func randBytesDevRandom(n int) []byte {
+	f, err := os.Open("/dev/random")
+	if err != nil {
+		log.Panic("Failed to open /dev/random: " + err.Error())
+	}
+	defer f.Close()
+	b := make([]byte, n)
+	_, err = io.ReadFull(f, b)
+	if err != nil {
+		log.Panic("Failed to read random bytes: " + err.Error())
+	}
+	return b
+}
+
 // 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, creator string, aessiv bool) error {
+func CreateConfFile(filename string, password string, plaintextNames bool, logN int, creator string, aessiv bool, devrandom bool) error {
 	var cf ConfFile
 	cf.filename = filename
 	cf.Creator = creator
@@ -72,7 +89,12 @@ func CreateConfFile(filename string, password string, plaintextNames bool, logN
 	}
 
 	// Generate new random master key
-	key := cryptocore.RandBytes(cryptocore.KeyLen)
+	var key []byte
+	if devrandom {
+		key = randBytesDevRandom(cryptocore.KeyLen)
+	} else {
+		key = cryptocore.RandBytes(cryptocore.KeyLen)
+	}
 
 	// Encrypt it using the password
 	// This sets ScryptObject and EncryptedKey

internal/configfile/config_test.go

@@ -60,7 +60,7 @@ func TestLoadV2StrangeFeature(t *testing.T) {
 }
 
 func TestCreateConfDefault(t *testing.T) {
-	err := CreateConfFile("config_test/tmp.conf", "test", false, 10, "test", false)
+	err := CreateConfFile("config_test/tmp.conf", "test", false, 10, "test", false, false)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -80,8 +80,15 @@ func TestCreateConfDefault(t *testing.T) {
 	}
 }
 
+func TestCreateConfDevRandom(t *testing.T) {
+	err := CreateConfFile("config_test/tmp.conf", "test", false, 10, "test", false, true)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
 func TestCreateConfPlaintextnames(t *testing.T) {
-	err := CreateConfFile("config_test/tmp.conf", "test", true, 10, "test", false)
+	err := CreateConfFile("config_test/tmp.conf", "test", true, 10, "test", false, false)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -102,7 +109,7 @@ func TestCreateConfPlaintextnames(t *testing.T) {
 
 // Reverse mode uses AESSIV
 func TestCreateConfFileAESSIV(t *testing.T) {
-	err := CreateConfFile("config_test/tmp.conf", "test", false, 10, "test", true)
+	err := CreateConfFile("config_test/tmp.conf", "test", false, 10, "test", true, false)
 	if err != nil {
 		t.Fatal(err)
 	}

tests/cli/cli_test.go

@@ -34,6 +34,11 @@ func TestInit(t *testing.T) {
 	}
 }
 
+// Test -init with -devrandom flag
+func TestInitDevRandom(t *testing.T) {
+	test_helpers.InitFS(t, "-devrandom")
+}
+
 // Test -init with -aessiv
 func TestInitAessiv(t *testing.T) {
 	dir := test_helpers.InitFS(t, "-aessiv")