configfile: add Validate() function, support FlagXChaCha20Poly1305

We used to do validation using lists of mandatory feature flags.

With the introduction of XChaCha20Poly1305, this became too
simplistic, as it uses a different IV length, hence disabling
GCMIV128.

Add a dedicated function, Validate(), with open-coded validation
logic.

The validation and creation logic also gets XChaCha20Poly1305
support, and gocryptfs -init -xchacha now writes the flag into
gocryptfs.conf.

1 files changed, 12 insertions, 13 deletions

diff --git a/internal/configfile/scrypt.go b/internal/configfile/scrypt.go
index 7ac822e..0ce8777 100644
--- a/internal/configfile/scrypt.go
+++ b/internal/configfile/scrypt.go
@@ -1,6 +1,7 @@
 package configfile
 
 import (
+	"fmt"
 	"log"
 	"math"
 	"os"
@@ -62,8 +63,10 @@ func NewScryptKDF(logN int) ScryptKDF {
 
 // DeriveKey returns a new key from a supplied password.
 func (s *ScryptKDF) DeriveKey(pw []byte) []byte {
-	s.validateParams()
-
+	if err := s.validateParams(); err != nil {
+		tlog.Fatal.Println(err.Error())
+		os.Exit(exitcodes.ScryptParams)
+	}
 	k, err := scrypt.Key(pw, s.Salt, s.N, s.R, s.P, s.KeyLen)
 	if err != nil {
 		log.Panicf("DeriveKey failed: %v", err)
@@ -81,26 +84,22 @@ func (s *ScryptKDF) LogN() int {
 // If not, it exists with an error message.
 // This makes sure we do not get weak parameters passed through a
 // rougue gocryptfs.conf.
-func (s *ScryptKDF) validateParams() {
+func (s *ScryptKDF) validateParams() error {
 	minN := 1 << scryptMinLogN
 	if s.N < minN {
-		tlog.Fatal.Println("Fatal: scryptn below 10 is too low to make sense")
-		os.Exit(exitcodes.ScryptParams)
+		return fmt.Errorf("Fatal: scryptn below 10 is too low to make sense")
 	}
 	if s.R < scryptMinR {
-		tlog.Fatal.Printf("Fatal: scrypt parameter R below minimum: value=%d, min=%d", s.R, scryptMinR)
-		os.Exit(exitcodes.ScryptParams)
+		return fmt.Errorf("Fatal: scrypt parameter R below minimum: value=%d, min=%d", s.R, scryptMinR)
 	}
 	if s.P < scryptMinP {
-		tlog.Fatal.Printf("Fatal: scrypt parameter P below minimum: value=%d, min=%d", s.P, scryptMinP)
-		os.Exit(exitcodes.ScryptParams)
+		return fmt.Errorf("Fatal: scrypt parameter P below minimum: value=%d, min=%d", s.P, scryptMinP)
 	}
 	if len(s.Salt) < scryptMinSaltLen {
-		tlog.Fatal.Printf("Fatal: scrypt salt length below minimum: value=%d, min=%d", len(s.Salt), scryptMinSaltLen)
-		os.Exit(exitcodes.ScryptParams)
+		return fmt.Errorf("Fatal: scrypt salt length below minimum: value=%d, min=%d", len(s.Salt), scryptMinSaltLen)
 	}
 	if s.KeyLen < cryptocore.KeyLen {
-		tlog.Fatal.Printf("Fatal: scrypt parameter KeyLen below minimum: value=%d, min=%d", s.KeyLen, cryptocore.KeyLen)
-		os.Exit(exitcodes.ScryptParams)
+		return fmt.Errorf("Fatal: scrypt parameter KeyLen below minimum: value=%d, min=%d", s.KeyLen, cryptocore.KeyLen)
 	}
+	return nil
 }