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

3 files changed, 44 insertions, 14 deletions

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