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|
package cryptfs
import (
"encoding/json"
"fmt"
"io/ioutil"
"log"
)
import "os"
const (
// 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 to. Not exported to JSON.
filename string
// Encrypted AES key, unlocked using a password hashed with scrypt
EncryptedKey []byte
// Stores parameters for scrypt hashing (key derivation)
ScryptObject scryptKdf
// The On-Disk-Format version this filesystem uses
Version uint16
// List of feature flags this filesystem has enabled.
// If gocryptfs encounters a feature flag it does not support, it will refuse
// mounting. This mechanism is analogous to the ext4 feature flags that are
// stored in the superblock.
FeatureFlags []string
}
// 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) error {
var cf ConfFile
cf.filename = filename
cf.Version = HEADER_CURRENT_VERSION
// Generate new random master key
key := RandBytes(KEY_LEN)
// Encrypt it using the password
// This sets ScryptObject and EncryptedKey
cf.EncryptKey(key, password, logN)
// Set feature flags
cf.FeatureFlags = append(cf.FeatureFlags, FlagGCMIV128)
if plaintextNames {
cf.FeatureFlags = append(cf.FeatureFlags, FlagPlaintextNames)
} else {
cf.FeatureFlags = append(cf.FeatureFlags, FlagDirIV)
cf.FeatureFlags = append(cf.FeatureFlags, FlagEMENames)
}
// Write file to disk
return cf.WriteFile()
}
// LoadConfFile - read config file from disk and decrypt the
// contained key using password.
//
// Returns the decrypted key and the ConfFile object
func LoadConfFile(filename string, password string) ([]byte, *ConfFile, error) {
var cf ConfFile
cf.filename = filename
// Read from disk
js, err := ioutil.ReadFile(filename)
if err != nil {
return nil, nil, err
}
// Unmarshal
err = json.Unmarshal(js, &cf)
if err != nil {
Warn.Printf("Failed to unmarshal config file")
return nil, nil, err
}
if cf.Version != HEADER_CURRENT_VERSION {
return nil, nil, fmt.Errorf("Unsupported on-disk format %d", cf.Version)
}
for _, flag := range cf.FeatureFlags {
if cf.isFeatureFlagKnown(flag) == false {
return nil, nil, fmt.Errorf("Unsupported feature flag %s", flag)
}
}
// Generate derived key from password
scryptHash := cf.ScryptObject.DeriveKey(password)
// Unlock master key using password-based key
// We use stock go GCM instead of OpenSSL here as speed is not important
// and we get better error messages
cfs := NewCryptFS(scryptHash, false, false, false)
key, err := cfs.DecryptBlock(cf.EncryptedKey, 0, nil)
if err != nil {
Warn.Printf("failed to unlock master key: %s", err.Error())
Warn.Printf("Password incorrect.")
return nil, nil, err
}
return key, &cf, nil
}
// EncryptKey - encrypt "key" using an scrypt hash generated from "password"
// and store it in cf.EncryptedKey.
// Uses scrypt with cost parameter logN and stores the scrypt parameters in
// cf.ScryptObject.
func (cf *ConfFile) EncryptKey(key []byte, password string, logN int) {
// Generate derived key from password
cf.ScryptObject = NewScryptKdf(logN)
scryptHash := cf.ScryptObject.DeriveKey(password)
// Lock master key using password-based key
cfs := NewCryptFS(scryptHash, false, false, false)
cf.EncryptedKey = cfs.EncryptBlock(key, 0, nil)
}
// WriteFile - write out config in JSON format to file "filename.tmp"
// then rename over "filename".
// This way a password change atomically replaces the file.
func (cf *ConfFile) WriteFile() error {
tmp := cf.filename + ".tmp"
// 0400 permissions: gocryptfs.conf should be kept secret and never be written to.
fd, err := os.OpenFile(tmp, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0400)
if err != nil {
return err
}
js, err := json.MarshalIndent(cf, "", "\t")
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
}
const (
// Understood Feature Flags.
// Also teach isFeatureFlagKnown() about any additions and
// add it to CreateConfFile() if you want to have it enabled by default.
FlagPlaintextNames = "PlaintextNames"
FlagDirIV = "DirIV"
FlagEMENames = "EMENames"
FlagGCMIV128 = "GCMIV128"
)
// Verify that we understand a feature flag
func (cf *ConfFile) isFeatureFlagKnown(flag string) bool {
switch flag {
case FlagPlaintextNames, FlagDirIV, FlagEMENames, FlagGCMIV128:
return true
default:
return false
}
}
// isFeatureFlagSet - is the feature flag "flagWant" enabled?
func (cf *ConfFile) IsFeatureFlagSet(flagWant string) bool {
if !cf.isFeatureFlagKnown(flagWant) {
log.Panicf("BUG: Tried to use unsupported feature flag %s", flagWant)
}
for _, flag := range cf.FeatureFlags {
if flag == flagWant {
return true
}
}
return false
}
|
