3 files changed, 110 insertions, 4 deletions
diff --git a/cryptfs/config_file.go b/cryptfs/config_file.go
new file mode 100644
index 0000000..752ed09
--- /dev/null
+++ b/cryptfs/config_file.go
@@ -0,0 +1,104 @@
+package cryptfs
+
+import (
+	"errors"
+	"io/ioutil"
+	"encoding/json"
+)
+import "os"
+
+const (
+	// Changing this string breaks backward compatability
+	testBlockData = "gocryptfs test block"
+
+	// The dot "." is not used in base64url (RFC4648), hence
+	// we can never clash with an encrypted file.
+	ConfDefaultName = "gocryptfs.conf"
+)
+
+type confFile struct {
+	// File the config is saved in. Lowercase => not exported to JSON.
+	filename string
+	// Unencrypted AES key
+	Key [16]byte
+	// GCM ciphertext with auth tag to verify the key is correct
+	TestBlock []byte
+}
+
+// CreateConfFile - create a new config file with "key" and write to "filename"
+func CreateConfFile(filename string, key [16]byte) error {
+	var cf confFile
+	cf.filename = filename
+	cf.Key = key
+
+	// Generate test block
+	cfs := NewCryptFS(cf.Key, false)
+	cf.TestBlock = cfs.EncryptBlock([]byte(testBlockData))
+
+	// Write file to disk
+	err := cf.WriteFile()
+
+	return err
+}
+
+// LoadConfFile - read config file from disk and verify the key using the
+// embedded TestBlock
+func LoadConfFile(filename string) (*confFile, error) {
+	// Read from disk
+	js, err := ioutil.ReadFile(filename)
+	if err != nil {
+		return nil, err
+	}
+	var cf confFile
+	err = json.Unmarshal(js, &cf)
+	if err != nil {
+		return nil, err
+	}
+	cf.filename = filename
+
+	// Try to decrypt the test block to see if the key is correct
+	//
+	// Speed does not matter here. Use built-in crypto.
+	cfs := NewCryptFS(cf.Key, false)
+	d, err := cfs.DecryptBlock(cf.TestBlock)
+	if err != nil {
+		return nil, err
+	}
+	ds := string(d)
+	if ds != testBlockData {
+		return nil, errors.New("Invalid test block content: " + ds)
+	}
+	return &cf, nil
+}
+
+// WriteFile - write out config in JSON format to file "filename.tmp"
+// then rename over "filename"
+func (cf *confFile) WriteFile() error {
+	tmp := cf.filename + ".tmp"
+	fd, err := os.Create(tmp)
+	if err != nil {
+		return err
+	}
+	js, err := json.Marshal(cf)
+	if err != nil {
+		return err
+	}
+	_, err = fd.Write(js)
+	if err != nil {
+		return err
+	}
+	err = fd.Sync()
+	if err != nil {
+		return err
+	}
+	err = fd.Close()
+	if err != nil {
+		return err
+	}
+	err = os.Rename(tmp, cf.filename)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
diff --git a/cryptfs/cryptfs.go b/cryptfs/cryptfs.go
index 40a9024..8927d74 100644
--- a/cryptfs/cryptfs.go
+++ b/cryptfs/cryptfs.go
@@ -8,6 +8,7 @@ import (
 )
 
 const (
+	KEY_LEN = 16
 	NONCE_LEN = 12
 	AUTH_TAG_LEN = 16
 	DEFAULT_PLAINBS = 4096
diff --git a/cryptfs/nonce.go b/cryptfs/nonce.go
index f93f59c..3e464a3 100644
--- a/cryptfs/nonce.go
+++ b/cryptfs/nonce.go
@@ -16,8 +16,9 @@ type nonce96 struct {
 
 var gcmNonce nonce96
 
-func (n *nonce96) randBytes(len int) []byte {
-	b := make([]byte, len)
+// Get "n" random bytes from /dev/urandom or panic
+func RandBytes(n int) []byte {
+	b := make([]byte, n)
 	_, err := rand.Read(b)
 	if err != nil {
 		panic("Could not get random bytes for nonce")
@@ -26,9 +27,9 @@ func (n *nonce96) randBytes(len int) []byte {
 }
 
 func (n *nonce96) init() {
-	b := n.randBytes(8)
+	b := RandBytes(8)
 	n.low64 = binary.BigEndian.Uint64(b)
-	b = n.randBytes(4)
+	b = RandBytes(4)
 	n.high32 = binary.BigEndian.Uint32(b)
 	n.ready = 1
 	return