configfile: rename "kdf.go" -> "scrypt.go"

This really only handles scrypt and no other key-derivation functions.
Renaming the files prevents confusion once we introduce HKDF.

renamed:    internal/configfile/kdf.go -> internal/configfile/scrypt.go
renamed:    internal/configfile/kdf_test.go -> internal/configfile/scrypt_test.go

1 files changed, 62 insertions, 0 deletions

diff --git a/internal/configfile/scrypt.go b/internal/configfile/scrypt.go
new file mode 100644
index 0000000..31bcbe4
--- /dev/null
+++ b/internal/configfile/scrypt.go
@@ -0,0 +1,62 @@
+package configfile
+
+import (
+	"log"
+	"math"
+	"os"
+
+	"golang.org/x/crypto/scrypt"
+
+	"github.com/rfjakob/gocryptfs/internal/cryptocore"
+	"github.com/rfjakob/gocryptfs/internal/tlog"
+)
+
+const (
+	// ScryptDefaultLogN is the default scrypt logN configuration parameter.
+	// 1 << 16 uses 64MB of memory,
+	// takes 4 seconds on my Atom Z3735F netbook
+	ScryptDefaultLogN = 16
+)
+
+// ScryptKDF is an instance of the scrypt key deriviation function.
+type ScryptKDF struct {
+	Salt   []byte
+	N      int
+	R      int
+	P      int
+	KeyLen int
+}
+
+// NewScryptKDF returns a new instance of ScryptKDF.
+func NewScryptKDF(logN int) ScryptKDF {
+	var s ScryptKDF
+	s.Salt = cryptocore.RandBytes(cryptocore.KeyLen)
+	if logN <= 0 {
+		s.N = 1 << ScryptDefaultLogN
+	} else {
+		if logN < 10 {
+			tlog.Fatal.Println("Error: scryptn below 10 is too low to make sense. Aborting.")
+			os.Exit(1)
+		}
+		s.N = 1 << uint32(logN)
+	}
+	s.R = 8 // Always 8
+	s.P = 1 // Always 1
+	s.KeyLen = cryptocore.KeyLen
+	return s
+}
+
+// DeriveKey returns a new key from a supplied password.
+func (s *ScryptKDF) DeriveKey(pw string) []byte {
+	k, err := scrypt.Key([]byte(pw), s.Salt, s.N, s.R, s.P, s.KeyLen)
+	if err != nil {
+		log.Panicf("DeriveKey failed: %v", err)
+	}
+	return k
+}
+
+// LogN - N is saved as 2^LogN, but LogN is much easier to work with.
+// This function gives you LogN = Log2(N).
+func (s *ScryptKDF) LogN() int {
+	return int(math.Log2(float64(s.N)) + 0.5)
+}