contentenc: add GCM-SIV support

Also add ReverseDummyNonce nonce generation.

init_dir.go

@@ -39,7 +39,7 @@ func initDir(args *argContainer) {
 	}
 	password := readpassword.Twice(args.extpass)
 	creator := tlog.ProgramName + " " + GitVersion
-	err = configfile.CreateConfFile(args.config, password, args.plaintextnames, args.scryptn, creator)
+	err = configfile.CreateConfFile(args.config, password, args.plaintextnames, args.scryptn, creator, args.reverse)
 	if err != nil {
 		tlog.Fatal.Println(err)
 		os.Exit(ERREXIT_INIT)

internal/configfile/config_file.go

@@ -45,7 +45,7 @@ type ConfFile struct {
 // 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 {
+func CreateConfFile(filename string, password string, plaintextNames bool, logN int, creator string, reverse bool) error {
 	var cf ConfFile
 	cf.filename = filename
 	cf.Creator = creator
@@ -67,6 +67,9 @@ func CreateConfFile(filename string, password string, plaintextNames bool, logN
 		cf.FeatureFlags = append(cf.FeatureFlags, knownFlags[FlagEMENames])
 		cf.FeatureFlags = append(cf.FeatureFlags, knownFlags[FlagLongNames])
 	}
+	if reverse {
+		cf.FeatureFlags = append(cf.FeatureFlags, knownFlags[FlagGCMSIV])
+	}
 
 	// Write file to disk
 	return cf.WriteFile()
@@ -165,7 +168,7 @@ func (cf *ConfFile) EncryptKey(key []byte, password string, logN int) {
 	// Lock master key using password-based key
 	cc := cryptocore.New(scryptHash, cryptocore.BackendGoGCM, 96)
 	ce := contentenc.New(cc, 4096)
-	cf.EncryptedKey = ce.EncryptBlock(key, 0, nil)
+	cf.EncryptedKey = ce.EncryptBlock(key, 0, nil, contentenc.RandomNonce)
 }
 
 // WriteFile - write out config in JSON format to file "filename.tmp"

internal/configfile/config_test.go

@@ -60,7 +60,7 @@ func TestLoadV2StrangeFeature(t *testing.T) {
 }
 
 func TestCreateConfFile(t *testing.T) {
-	err := CreateConfFile("config_test/tmp.conf", "test", false, 10, "test")
+	err := CreateConfFile("config_test/tmp.conf", "test", false, 10, "test", false)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -71,6 +71,20 @@ func TestCreateConfFile(t *testing.T) {
 
 }
 
+func TestCreateConfFileReverse(t *testing.T) {
+	err := CreateConfFile("config_test/tmp.conf", "test", false, 10, "test", true)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, c, err := LoadConfFile("config_test/tmp.conf", "test")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !c.IsFeatureFlagSet(FlagGCMSIV) {
+		t.Error("GCMSIV flag should be set but is not")
+	}
+}
+
 func TestIsFeatureFlagKnown(t *testing.T) {
 	// Test a few hardcoded values
 	testKnownFlags := []string{"DirIV", "PlaintextNames", "EMENames", "GCMIV128", "LongNames"}

internal/contentenc/content.go

@@ -11,11 +11,17 @@ import (
 	"github.com/rfjakob/gocryptfs/internal/tlog"
 )
 
+type NonceMode int
+
 const (
 	// Default plaintext block size
 	DefaultBS = 4096
 	// We always use 128-bit IVs for file content encryption
 	IVBitLen = 128
+
+	_                           = iota // skip zero
+	RandomNonce       NonceMode = iota
+	ReverseDummyNonce NonceMode = iota
 )
 
 type ContentEnc struct {
@@ -27,6 +33,8 @@ type ContentEnc struct {
 	cipherBS uint64
 	// All-zero block of size cipherBS, for fast compares
 	allZeroBlock []byte
+	// All-zero block of size IVBitLen/8, for fast compares
+	allZeroNonce []byte
 }
 
 func New(cc *cryptocore.CryptoCore, plainBS uint64) *ContentEnc {
@@ -38,6 +46,7 @@ func New(cc *cryptocore.CryptoCore, plainBS uint64) *ContentEnc {
 		plainBS:      plainBS,
 		cipherBS:     cipherBS,
 		allZeroBlock: make([]byte, cipherBS),
+		allZeroNonce: make([]byte, IVBitLen/8),
 	}
 }
 
@@ -94,6 +103,9 @@ func (be *ContentEnc) DecryptBlock(ciphertext []byte, blockNo uint64, fileId []b
 
 	// Extract nonce
 	nonce := ciphertext[:be.cryptoCore.IVLen]
+	if bytes.Equal(nonce, be.allZeroNonce) && be.cryptoCore.AEADBackend != cryptocore.BackendGCMSIV {
+		panic("Hit an all-zero nonce with GCMSIV off. This MUST NOT happen!")
+	}
 	ciphertextOrig := ciphertext
 	ciphertext = ciphertext[be.cryptoCore.IVLen:]
 
@@ -115,27 +127,38 @@ func (be *ContentEnc) DecryptBlock(ciphertext []byte, blockNo uint64, fileId []b
 
 // EncryptBlocks - Encrypt a number of blocks
 // Used for reverse mode
-func (be *ContentEnc) EncryptBlocks(plaintext []byte, firstBlockNo uint64, fileId []byte) []byte {
+func (be *ContentEnc) EncryptBlocks(plaintext []byte, firstBlockNo uint64, fileId []byte, nMode NonceMode) []byte {
 	inBuf := bytes.NewBuffer(plaintext)
 	var outBuf bytes.Buffer
 	for blockNo := firstBlockNo; inBuf.Len() > 0; blockNo++ {
 		inBlock := inBuf.Next(int(be.plainBS))
-		outBlock := be.EncryptBlock(inBlock, blockNo, fileId)
+		outBlock := be.EncryptBlock(inBlock, blockNo, fileId, nMode)
 		outBuf.Write(outBlock)
 	}
 	return outBuf.Bytes()
 }
 
 // encryptBlock - Encrypt and add IV and MAC
-func (be *ContentEnc) EncryptBlock(plaintext []byte, blockNo uint64, fileID []byte) []byte {
+func (be *ContentEnc) EncryptBlock(plaintext []byte, blockNo uint64, fileID []byte, nMode NonceMode) []byte {
-
 	// Empty block?
 	if len(plaintext) == 0 {
 		return plaintext
 	}
 
-	// Get fresh nonce
+	var nonce []byte
-	nonce := be.cryptoCore.IVGenerator.Get()
+	switch nMode {
+	case ReverseDummyNonce:
+		if be.cryptoCore.AEADBackend != cryptocore.BackendGCMSIV {
+			panic("MUST NOT use dummy nonces unless in GCMSIV mode!")
+		}
+		nonce = make([]byte, IVBitLen/8)
+		binary.BigEndian.PutUint64(nonce, blockNo)
+	case RandomNonce:
+		// Get a fresh random nonce
+		nonce = be.cryptoCore.IVGenerator.Get()
+	default:
+		panic("invalid nonce mode")
+	}
 
 	// Authenticate block with block number and file ID
 	aData := make([]byte, 8)

internal/cryptocore/cryptocore.go

@@ -29,6 +29,8 @@ type CryptoCore struct {
 	BlockCipher cipher.Block
 	// GCM or GCM-SIV. This is used for content encryption.
 	AEADCipher cipher.AEAD
+	// Which backend is behind AEADCipher?
+	AEADBackend BackendTypeEnum
 	// GCM needs unique IVs (nonces)
 	IVGenerator *nonceGenerator
 	IVLen       int
@@ -74,6 +76,7 @@ func New(key []byte, backend BackendTypeEnum, IVBitLen int) *CryptoCore {
 	return &CryptoCore{
 		BlockCipher: blockCipher,
 		AEADCipher:  gcm,
+		AEADBackend: backend,
 		IVGenerator: &nonceGenerator{nonceLen: IVLen},
 		IVLen:       IVLen,
 	}

internal/fusefrontend/file.go

@@ -256,7 +256,7 @@ func (f *file) doWrite(data []byte, off int64) (uint32, fuse.Status) {
 
 		// Encrypt
 		blockOffset := b.BlockCipherOff()
-		blockData = f.contentEnc.EncryptBlock(blockData, b.BlockNo, f.header.Id)
+		blockData = f.contentEnc.EncryptBlock(blockData, b.BlockNo, f.header.Id, contentenc.RandomNonce)
 		tlog.Debug.Printf("ino%d: Writing %d bytes to block #%d",
 			f.ino, uint64(len(blockData))-f.contentEnc.BlockOverhead(), b.BlockNo)
 

internal/fusefrontend/fs.go

@@ -326,7 +326,7 @@ func (fs *FS) Symlink(target string, linkName string, context *fuse.Context) (co
 		return fuse.ToStatus(err)
 	}
 	// Symlinks are encrypted like file contents (GCM) and base64-encoded
-	cBinTarget := fs.contentEnc.EncryptBlock([]byte(target), 0, nil)
+	cBinTarget := fs.contentEnc.EncryptBlock([]byte(target), 0, nil, contentenc.RandomNonce)
 	cTarget := base64.URLEncoding.EncodeToString(cBinTarget)
 
 	// Handle long file name

internal/fusefrontend_reverse/rfile.go

@@ -64,7 +64,7 @@ func (rf *reverseFile) readBackingFile(off uint64, length uint64) (out []byte, e
 	plaintext = plaintext[0:n]
 
 	// Encrypt blocks
-	ciphertext := rf.contentEnc.EncryptBlocks(plaintext, blocks[0].BlockNo, zeroFileHeader.Id)
+	ciphertext := rf.contentEnc.EncryptBlocks(plaintext, blocks[0].BlockNo, zeroFileHeader.Id, contentenc.ReverseDummyNonce)
 
 	// Crop down to the relevant part
 	lenHave := len(ciphertext)

main.go

@@ -305,6 +305,9 @@ func initFuseFrontend(key []byte, args argContainer, confFile *configfile.ConfFi
 		frontendArgs.PlaintextNames = confFile.IsFeatureFlagSet(configfile.FlagPlaintextNames)
 		if confFile.IsFeatureFlagSet(configfile.FlagGCMSIV) {
 			frontendArgs.CryptoBackend = cryptocore.BackendGCMSIV
+		} else if args.reverse {
+			tlog.Fatal.Printf("GCMSIV is required by reverse mode, but not enabled in the config file")
+			os.Exit(ERREXIT_USAGE)
 		}
 	}
 	// If allow_other is set and we run as root, try to give newly created files to