main: add "-info" option

Pretty-prints the config while stripping out sensitive
(and uninteresting) data

https://github.com/rfjakob/gocryptfs/issues/111

Documentation/MANPAGE.md

@@ -86,6 +86,16 @@ same name. By default, CIPHERDIR is used.
 #### -fusedebug
 Enable fuse library debug output
 
+#### -h, -help
+Print a short help text that shows the more-often used options.
+
+#### -hh
+Long help text, shows all available options.
+
+#### -info
+Pretty-print the contents of the config file for human consumption,
+stripping out sensitive data.
+
 #### -init
 Initialize encrypted directory
 

cli_args.go

@@ -20,7 +20,7 @@ type argContainer struct {
 	debug, init, zerokey, fusedebug, openssl, passwd, fg, version,
 	plaintextnames, quiet, nosyslog, wpanic,
 	longnames, allow_other, ro, reverse, aessiv, nonempty, raw64,
-	noprealloc, speed, hkdf, serialize_reads, forcedecode, hh bool
+	noprealloc, speed, hkdf, serialize_reads, forcedecode, hh, info bool
 	masterkey, mountpoint, cipherdir, cpuprofile, extpass,
 	memprofile, ko, passfile, ctlsock, fsname string
 	// Configuration file name override
@@ -126,6 +126,7 @@ func parseCliOpts() (args argContainer) {
 	flagSet.BoolVar(&args.forcedecode, "forcedecode", false, "Force decode of files even if integrity check fails."+
 		" Requires gocryptfs to be compiled with openssl support and implies -openssl true")
 	flagSet.BoolVar(&args.hh, "hh", false, "Show this long help text")
+	flagSet.BoolVar(&args.info, "info", false, "Display information about CIPHERDIR")
 	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")

help.go

@@ -7,7 +7,7 @@ import (
 )
 
 const tUsage = "" +
-	"Usage: " + tlog.ProgramName + " -init|-passwd [OPTIONS] CIPHERDIR\n" +
+	"Usage: " + tlog.ProgramName + " -init|-passwd|-info [OPTIONS] CIPHERDIR\n" +
 	"  or   " + tlog.ProgramName + " [OPTIONS] CIPHERDIR MOUNTPOINT\n"
 
 // helpShort is what gets displayed when passed "-h" or on syntax error.
@@ -27,6 +27,7 @@ Common Options (use -hh to show all):
   -h, -help          This short help text
   -hh                Long help text with all options
   -init              Initialize encrypted directory
+  -info              Display information about encrypted directory
   -masterkey         Mount with explicit master key instead of password
   -nonempty          Allow mounting over non-empty directory
   -nosyslog          Do not redirect log messages to syslog

info.go

@@ -0,0 +1,45 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"strings"
+
+	"github.com/rfjakob/gocryptfs/internal/configfile"
+	"github.com/rfjakob/gocryptfs/internal/contentenc"
+	"github.com/rfjakob/gocryptfs/internal/exitcodes"
+	"github.com/rfjakob/gocryptfs/internal/tlog"
+)
+
+// info pretty-prints the contents of the config file at "filename" for human
+// consumption, stripping out sensitive data.
+// This is called when you pass the "-info" option.
+func info(filename string) {
+	// Read from disk
+	js, err := ioutil.ReadFile(filename)
+	if err != nil {
+		tlog.Fatal.Printf("info: ReadFile: %#v\n", err)
+		os.Exit(exitcodes.LoadConf)
+	}
+	// Unmarshal
+	var cf configfile.ConfFile
+	err = json.Unmarshal(js, &cf)
+	if err != nil {
+		tlog.Fatal.Printf("Failed to unmarshal config file")
+		os.Exit(exitcodes.LoadConf)
+	}
+	if cf.Version != contentenc.CurrentVersion {
+		tlog.Fatal.Printf("Unsupported on-disk format %d", cf.Version)
+		os.Exit(exitcodes.LoadConf)
+	}
+	// Pretty-print
+	fmt.Printf("Creator:      %s\n", cf.Creator)
+	fmt.Printf("FeatureFlags: %s\n", strings.Join(cf.FeatureFlags, " "))
+	fmt.Printf("EncryptedKey: %dB\n", len(cf.EncryptedKey))
+	s := cf.ScryptObject
+	fmt.Printf("ScryptObject: Salt=%dB N=%d R=%d P=%d KeyLen=%d\n",
+		len(s.Salt), s.N, s.R, s.P, s.KeyLen)
+	os.Exit(0)
+}

main.go

@@ -214,7 +214,19 @@ func main() {
 	} else {
 		tlog.Debug.Printf("OpenSSL enabled")
 	}
-	// Operation flags: -init or -passwd; otherwise: mount
+	// Operation flags
+	if args.info && args.init || args.info && args.passwd || args.passwd && args.init {
+		tlog.Fatal.Printf("At most one of -info, -init, -passwd is allowed")
+		os.Exit(exitcodes.Usage)
+	}
+	// "-info"
+	if args.info {
+		if flagSet.NArg() > 1 {
+			tlog.Fatal.Printf("Usage: %s -info CIPHERDIR", tlog.ProgramName)
+			os.Exit(exitcodes.Usage)
+		}
+		info(args.config) // does not return
+	}
 	// "-init"
 	if args.init {
 		if flagSet.NArg() > 1 {