Add partial XChaCha20-Poly1305 support (mount flag only)

Mount flag only at the moment, not saved to gocryptfs.conf.

https://github.com/rfjakob/gocryptfs/issues/452

5 files changed, 68 insertions, 16 deletions

diff --git a/cli_args.go b/cli_args.go
index d082113..3eb92b2 100644
--- a/cli_args.go
+++ b/cli_args.go
@@ -30,7 +30,8 @@ type argContainer struct {
 	plaintextnames, quiet, nosyslog, wpanic,
 	longnames, allow_other, reverse, aessiv, nonempty, raw64,
 	noprealloc, speed, hkdf, serialize_reads, forcedecode, hh, info,
-	sharedstorage, devrandom, fsck, one_file_system, deterministic_names bool
+	sharedstorage, devrandom, fsck, one_file_system, deterministic_names,
+	xchacha bool
 	// Mount options with opposites
 	dev, nodev, suid, nosuid, exec, noexec, rw, ro, kernel_cache, acl bool
 	masterkey, mountpoint, cipherdir, cpuprofile,
@@ -180,6 +181,7 @@ func parseCliOpts(osArgs []string) (args argContainer) {
 	flagSet.BoolVar(&args.fsck, "fsck", false, "Run a filesystem check on CIPHERDIR")
 	flagSet.BoolVar(&args.one_file_system, "one-file-system", false, "Don't cross filesystem boundaries")
 	flagSet.BoolVar(&args.deterministic_names, "deterministic-names", false, "Disable diriv file name randomisation")
+	flagSet.BoolVar(&args.xchacha, "xchacha", false, "Use XChaCha20-Poly1305 file content encryption")
 
 	// Mount options with opposites
 	flagSet.BoolVar(&args.dev, "dev", false, "Allow device files")
diff --git a/internal/configfile/feature_flags.go b/internal/configfile/feature_flags.go
index 45e1853..be5616f 100644
--- a/internal/configfile/feature_flags.go
+++ b/internal/configfile/feature_flags.go
@@ -28,19 +28,22 @@ const (
 	// FlagFIDO2 means that "-fido2" was used when creating the filesystem.
 	// The masterkey is protected using a FIDO2 token instead of a password.
 	FlagFIDO2
+	// FlagXChaCha20Poly1305 means we use XChaCha20-Poly1305 file content encryption
+	FlagXChaCha20Poly1305
 )
 
 // knownFlags stores the known feature flags and their string representation
 var knownFlags = map[flagIota]string{
-	FlagPlaintextNames: "PlaintextNames",
-	FlagDirIV:          "DirIV",
-	FlagEMENames:       "EMENames",
-	FlagGCMIV128:       "GCMIV128",
-	FlagLongNames:      "LongNames",
-	FlagAESSIV:         "AESSIV",
-	FlagRaw64:          "Raw64",
-	FlagHKDF:           "HKDF",
-	FlagFIDO2:          "FIDO2",
+	FlagPlaintextNames:    "PlaintextNames",
+	FlagDirIV:             "DirIV",
+	FlagEMENames:          "EMENames",
+	FlagGCMIV128:          "GCMIV128",
+	FlagLongNames:         "LongNames",
+	FlagAESSIV:            "AESSIV",
+	FlagRaw64:             "Raw64",
+	FlagHKDF:              "HKDF",
+	FlagFIDO2:             "FIDO2",
+	FlagXChaCha20Poly1305: "XChaCha20Poly1305",
 }
 
 // Filesystems that do not have these feature flags set are deprecated.
diff --git a/internal/cryptocore/cryptocore.go b/internal/cryptocore/cryptocore.go
index 5da88db..3e6f5e8 100644
--- a/internal/cryptocore/cryptocore.go
+++ b/internal/cryptocore/cryptocore.go
@@ -10,6 +10,8 @@ import (
 	"log"
 	"runtime"
 
+	"golang.org/x/crypto/chacha20poly1305"
+
 	"github.com/rfjakob/eme"
 
 	"github.com/rfjakob/gocryptfs/v2/internal/siv_aead"
@@ -29,11 +31,17 @@ type AEADTypeEnum int
 
 const (
 	// BackendOpenSSL specifies the OpenSSL backend.
+	// "AES-GCM-256-OpenSSL" in gocryptfs -speed.
 	BackendOpenSSL AEADTypeEnum = 3
 	// BackendGoGCM specifies the Go based GCM backend.
+	// "AES-GCM-256-Go" in gocryptfs -speed.
 	BackendGoGCM AEADTypeEnum = 4
 	// BackendAESSIV specifies an AESSIV backend.
+	// "AES-SIV-512-Go" in gocryptfs -speed.
 	BackendAESSIV AEADTypeEnum = 5
+	// BackendXChaCha20Poly1305 specifies XChaCha20-Poly1305-Go.
+	// "XChaCha20-Poly1305-Go" in gocryptfs -speed.
+	BackendXChaCha20Poly1305 AEADTypeEnum = 6
 )
 
 func (a AEADTypeEnum) String() string {
@@ -78,7 +86,7 @@ func New(key []byte, aeadType AEADTypeEnum, IVBitLen int, useHKDF bool, forceDec
 	if len(key) != KeyLen {
 		log.Panicf("Unsupported key length of %d bytes", len(key))
 	}
-	if IVBitLen != 96 && IVBitLen != 128 {
+	if IVBitLen != 96 && IVBitLen != 128 && IVBitLen != chacha20poly1305.NonceSizeX*8 {
 		log.Panicf("Unsupported IV length of %d bits", IVBitLen)
 	}
 
@@ -152,8 +160,26 @@ func New(key []byte, aeadType AEADTypeEnum, IVBitLen int, useHKDF bool, forceDec
 		for i := range key64 {
 			key64[i] = 0
 		}
+	} else if aeadType == BackendXChaCha20Poly1305 {
+		// We don't support legacy modes with XChaCha20-Poly1305
+		if IVBitLen != chacha20poly1305.NonceSizeX*8 {
+			log.Panicf("XChaCha20-Poly1305 must use 192-bit IVs, you wanted %d", IVBitLen)
+		}
+		if !useHKDF {
+			log.Panic("XChaCha20-Poly1305 must use HKDF, but it is disabled")
+		}
+		derivedKey := hkdfDerive(key, hkdfInfoXChaChaPoly1305Content, chacha20poly1305.KeySize)
+		aeadCipher, err = chacha20poly1305.NewX(derivedKey)
+		if err != nil {
+			log.Panic(err)
+		}
 	} else {
-		log.Panic("unknown backend cipher")
+		log.Panicf("unknown cipher backend %q", aeadType.String())
+	}
+
+	if aeadCipher.NonceSize()*8 != IVBitLen {
+		log.Panicf("Mismatched aeadCipher.NonceSize*8=%d and IVBitLen=%d bits",
+			aeadCipher.NonceSize()*8, IVBitLen)
 	}
 
 	return &CryptoCore{
diff --git a/internal/cryptocore/hkdf.go b/internal/cryptocore/hkdf.go
index 87ca1b9..b56f507 100644
--- a/internal/cryptocore/hkdf.go
+++ b/internal/cryptocore/hkdf.go
@@ -10,9 +10,10 @@ import (
 const (
 	// "info" data that HKDF mixes into the generated key to make it unique.
 	// For convenience, we use a readable string.
-	hkdfInfoEMENames   = "EME filename encryption"
-	hkdfInfoGCMContent = "AES-GCM file content encryption"
-	hkdfInfoSIVContent = "AES-SIV file content encryption"
+	hkdfInfoEMENames               = "EME filename encryption"
+	hkdfInfoGCMContent             = "AES-GCM file content encryption"
+	hkdfInfoSIVContent             = "AES-SIV file content encryption"
+	hkdfInfoXChaChaPoly1305Content = "XChaCha20-Poly1305 file content encryption"
 )
 
 // hkdfDerive derives "outLen" bytes from "masterkey" and "info" using
diff --git a/mount.go b/mount.go
index 810f008..f8347f1 100644
--- a/mount.go
+++ b/mount.go
@@ -19,6 +19,8 @@ import (
 	"syscall"
 	"time"
 
+	"golang.org/x/crypto/chacha20poly1305"
+
 	"github.com/hanwen/go-fuse/v2/fs"
 	"github.com/hanwen/go-fuse/v2/fuse"
 
@@ -249,12 +251,17 @@ func initFuseFrontend(args *argContainer) (rootNode fs.InodeEmbedder, wipeKeys f
 	// Reconciliate CLI and config file arguments into a fusefrontend.Args struct
 	// that is passed to the filesystem implementation
 	cryptoBackend := cryptocore.BackendGoGCM
+	IVBits := contentenc.DefaultIVBits
 	if args.openssl {
 		cryptoBackend = cryptocore.BackendOpenSSL
 	}
 	if args.aessiv {
 		cryptoBackend = cryptocore.BackendAESSIV
 	}
+	if args.xchacha {
+		cryptoBackend = cryptocore.BackendXChaCha20Poly1305
+		IVBits = chacha20poly1305.NonceSizeX * 8
+	}
 	// forceOwner implies allow_other, as documented.
 	// Set this early, so args.allow_other can be relied on below this point.
 	if args._forceOwner != nil {
@@ -287,10 +294,23 @@ func initFuseFrontend(args *argContainer) (rootNode fs.InodeEmbedder, wipeKeys f
 		args.hkdf = confFile.IsFeatureFlagSet(configfile.FlagHKDF)
 		if confFile.IsFeatureFlagSet(configfile.FlagAESSIV) {
 			cryptoBackend = cryptocore.BackendAESSIV
+			IVBits = contentenc.DefaultIVBits
 		} else if args.reverse {
 			tlog.Fatal.Printf("AES-SIV is required by reverse mode, but not enabled in the config file")
 			os.Exit(exitcodes.Usage)
 		}
+		if confFile.IsFeatureFlagSet(configfile.FlagXChaCha20Poly1305) {
+			cryptoBackend = cryptocore.BackendXChaCha20Poly1305
+			IVBits = chacha20poly1305.NonceSizeX * 8
+		}
+		// If neither AES-SIV nor XChaCha are selected, we must be using AES-GCM
+		if !confFile.IsFeatureFlagSet(configfile.FlagAESSIV) && !confFile.IsFeatureFlagSet(configfile.FlagXChaCha20Poly1305) {
+			cryptoBackend = cryptocore.BackendGoGCM
+			if args.openssl {
+				cryptoBackend = cryptocore.BackendOpenSSL
+			}
+			IVBits = contentenc.DefaultIVBits
+		}
 	}
 	// If allow_other is set and we run as root, try to give newly created files to
 	// the right user.
@@ -299,7 +319,7 @@ func initFuseFrontend(args *argContainer) (rootNode fs.InodeEmbedder, wipeKeys f
 	}
 
 	// Init crypto backend
-	cCore := cryptocore.New(masterkey, cryptoBackend, contentenc.DefaultIVBits, args.hkdf, args.forcedecode)
+	cCore := cryptocore.New(masterkey, cryptoBackend, IVBits, args.hkdf, args.forcedecode)
 	cEnc := contentenc.New(cCore, contentenc.DefaultBS, args.forcedecode)
 	nameTransform := nametransform.New(cCore.EMECipher, frontendArgs.LongNames,
 		args.raw64, []string(args.badname), frontendArgs.DeterministicNames)