libgocryptfs/internal/configfile/config_file.go

package configfile

import (
	"encoding/json"
	"fmt"
	"io/ioutil"

	"github.com/rfjakob/gocryptfs/internal/contentenc"
	"github.com/rfjakob/gocryptfs/internal/cryptocore"
	"github.com/rfjakob/gocryptfs/internal/tlog"
)
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 {
	// gocryptfs version string
	// This only documents the config file for humans who look at it. The actual
	// technical info is contained in FeatureFlags.
	Creator 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
	// File the config is saved to. Not exported to JSON.
	filename 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, creator string) error {
	var cf ConfFile
	cf.filename = filename
	cf.Creator = creator
	cf.Version = contentenc.CurrentVersion

	// Generate new random master key
	key := cryptocore.RandBytes(cryptocore.KeyLen)

	// Encrypt it using the password
	// This sets ScryptObject and EncryptedKey
	cf.EncryptKey(key, password, logN)

	// Set feature flags
	cf.FeatureFlags = append(cf.FeatureFlags, knownFlags[FlagGCMIV128])
	if plaintextNames {
		cf.FeatureFlags = append(cf.FeatureFlags, knownFlags[FlagPlaintextNames])
	} else {
		cf.FeatureFlags = append(cf.FeatureFlags, knownFlags[FlagDirIV])
		cf.FeatureFlags = append(cf.FeatureFlags, knownFlags[FlagEMENames])
		cf.FeatureFlags = append(cf.FeatureFlags, knownFlags[FlagLongNames])
	}

	// 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 {
		tlog.Warn.Printf("Failed to unmarshal config file")
		return nil, nil, err
	}

	if cf.Version != contentenc.CurrentVersion {
		return nil, nil, fmt.Errorf("Unsupported on-disk format %d", cf.Version)
	}

	// Check that all set feature flags are known
	for _, flag := range cf.FeatureFlags {
		if !cf.isFeatureFlagKnown(flag) {
			return nil, nil, fmt.Errorf("Unsupported feature flag %q", flag)
		}
	}

	// Check that all required feature flags are set
	var requiredFlags []flagIota
	if cf.IsFeatureFlagSet(FlagPlaintextNames) {
		requiredFlags = requiredFlagsPlaintextNames
	} else {
		requiredFlags = requiredFlagsNormal
	}
	deprecatedFs := false
	for _, i := range requiredFlags {
		if !cf.IsFeatureFlagSet(i) {
			// For now, warn but continue.
			fmt.Printf("Deprecated filesystem: feature flag %q is missing\n", knownFlags[i])
			deprecatedFs = true
			//return nil, nil, fmt.Errorf("Required feature flag %q is missing", knownFlags[i])
		}
	}
	if deprecatedFs {
		fmt.Printf("\033[33m" + `
    This filesystem was created by gocryptfs v0.6 or earlier. You are missing
    security improvements. gocryptfs v1.0 is scheduled to drop support for this
    filesystem, please upgrade!
    If you disagree with the plan or have trouble upgrading, please join the
    discussion at https://github.com/rfjakob/gocryptfs/issues/29 .

` + "\033[0m")
	}

	// 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 we only use 96-bit IVs,
	// speed is not important and we get better error messages
	cc := cryptocore.New(scryptHash, false, false)
	ce := contentenc.New(cc, 4096)

	tlog.Warn.Enabled = false // Silence DecryptBlock() error messages on incorrect password
	key, err := ce.DecryptBlock(cf.EncryptedKey, 0, nil)
	tlog.Warn.Enabled = true
	if err != nil {
		tlog.Warn.Printf("failed to unlock master key: %s", err.Error())
		return nil, nil, fmt.Errorf("Password incorrect.")
	}

	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
	cc := cryptocore.New(scryptHash, false, false)
	ce := contentenc.New(cc, 4096)
	cf.EncryptedKey = ce.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
}