cryptocore: add NonceSize to AEADTypeEnum

Have the information in one centralized place,
and access it from main as needed.

internal/configfile/config_file.go

@@ -325,3 +325,18 @@ func getKeyEncrypter(scryptHash []byte, useHKDF bool) *contentenc.ContentEnc {
 	ce := contentenc.New(cc, 4096, false)
 	return ce
 }
+
+// ContentEncryption tells us which content encryption algorithm is selected
+func (cf *ConfFile) ContentEncryption() (algo cryptocore.AEADTypeEnum, err error) {
+	if err := cf.Validate(); err != nil {
+		return cryptocore.AEADTypeEnum{}, err
+	}
+	if cf.IsFeatureFlagSet(FlagXChaCha20Poly1305) {
+		return cryptocore.BackendXChaCha20Poly1305, nil
+	}
+	if cf.IsFeatureFlagSet(FlagAESSIV) {
+		return cryptocore.BackendAESSIV, nil
+	}
+	// If neither AES-SIV nor XChaCha are selected, we must be using AES-GCM
+	return cryptocore.BackendGoGCM, nil
+}

internal/cryptocore/cryptocore.go

@@ -6,7 +6,6 @@ import (
 	"crypto/aes"
 	"crypto/cipher"
 	"crypto/sha512"
-	"fmt"
 	"log"
 	"runtime"
 
@@ -20,43 +19,35 @@ import (
 )
 
 const (
-	// KeyLen is the cipher key length in bytes.  32 for AES-256.
+	// KeyLen is the cipher key length in bytes. All backends use 32 bytes.
 	KeyLen = 32
-	// AuthTagLen is the length of a GCM auth tag in bytes.
+	// AuthTagLen is the length of a authentication tag in bytes.
+	// All backends use 16 bytes.
 	AuthTagLen = 16
 )
 
 // AEADTypeEnum indicates the type of AEAD backend in use.
-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 {
-	switch a {
-	case BackendOpenSSL:
-		return "BackendOpenSSL"
-	case BackendGoGCM:
-		return "BackendGoGCM"
-	case BackendAESSIV:
-		return "BackendAESSIV"
-	default:
-		return fmt.Sprintf("%d", a)
-	}
+type AEADTypeEnum struct {
+	Name      string
+	NonceSize int
 }
 
+// BackendOpenSSL specifies the OpenSSL backend.
+// "AES-GCM-256-OpenSSL" in gocryptfs -speed.
+var BackendOpenSSL AEADTypeEnum = AEADTypeEnum{"AES-GCM-256-OpenSSL", 16}
+
+// BackendGoGCM specifies the Go based GCM backend.
+// "AES-GCM-256-Go" in gocryptfs -speed.
+var BackendGoGCM AEADTypeEnum = AEADTypeEnum{"AES-GCM-256-Go", 16}
+
+// BackendAESSIV specifies an AESSIV backend.
+// "AES-SIV-512-Go" in gocryptfs -speed.
+var BackendAESSIV AEADTypeEnum = AEADTypeEnum{"AES-SIV-512-Go", siv_aead.NonceSize}
+
+// BackendXChaCha20Poly1305 specifies XChaCha20-Poly1305-Go.
+// "XChaCha20-Poly1305-Go" in gocryptfs -speed.
+var BackendXChaCha20Poly1305 AEADTypeEnum = AEADTypeEnum{"XChaCha20-Poly1305-Go", chacha20poly1305.NonceSizeX}
+
 // CryptoCore is the low level crypto implementation.
 type CryptoCore struct {
 	// EME is used for filename encryption.
@@ -174,7 +165,7 @@ func New(key []byte, aeadType AEADTypeEnum, IVBitLen int, useHKDF bool, forceDec
 			log.Panic(err)
 		}
 	} else {
-		log.Panicf("unknown cipher backend %q", aeadType.String())
+		log.Panicf("unknown cipher backend %q", aeadType.Name)
 	}
 
 	if aeadCipher.NonceSize()*8 != IVBitLen {
@@ -203,7 +194,7 @@ type wiper interface {
 func (c *CryptoCore) Wipe() {
 	be := c.AEADBackend
 	if be == BackendOpenSSL || be == BackendAESSIV {
-		tlog.Debug.Printf("CryptoCore.Wipe: Wiping AEADBackend %d key", be)
+		tlog.Debug.Printf("CryptoCore.Wipe: Wiping AEADBackend %s key", be.Name)
 		// We don't use "x, ok :=" because we *want* to crash loudly if the
 		// type assertion fails.
 		w := c.AEADCipher.(wiper)

internal/siv_aead/siv_aead.go

@@ -19,6 +19,11 @@ const (
 	// KeyLen is the required key length. The SIV algorithm supports other lengths,
 	// but we only support 64.
 	KeyLen = 64
+	// NonceSize is the required nonce/IV length.
+	// SIV supports any nonce size, but in gocryptfs we exclusively use 16.
+	NonceSize = 16
+	// Overhead is the number of bytes added for integrity checking
+	Overhead = 16
 )
 
 // New returns a new cipher.AEAD implementation.
@@ -42,11 +47,11 @@ func new2(keyIn []byte) cipher.AEAD {
 
 func (s *sivAead) NonceSize() int {
 	// SIV supports any nonce size, but in gocryptfs we exclusively use 16.
-	return 16
+	return NonceSize
 }
 
 func (s *sivAead) Overhead() int {
-	return 16
+	return Overhead
 }
 
 // Seal encrypts "in" using "nonce" and "authData" and appends the result to "dst"

mount.go

@@ -292,24 +292,18 @@ func initFuseFrontend(args *argContainer) (rootNode fs.InodeEmbedder, wipeKeys f
 		frontendArgs.DeterministicNames = !confFile.IsFeatureFlagSet(configfile.FlagDirIV)
 		args.raw64 = confFile.IsFeatureFlagSet(configfile.FlagRaw64)
 		args.hkdf = confFile.IsFeatureFlagSet(configfile.FlagHKDF)
-		if confFile.IsFeatureFlagSet(configfile.FlagAESSIV) {
-			cryptoBackend = cryptocore.BackendAESSIV
-			IVBits = contentenc.DefaultIVBits
-		} else if args.reverse {
+		cryptoBackend, err = confFile.ContentEncryption()
+		if err != nil {
+			tlog.Fatal.Printf("%v", err)
+			os.Exit(exitcodes.DeprecatedFS)
+		}
+		IVBits = cryptoBackend.NonceSize * 8
+		if cryptoBackend != cryptocore.BackendAESSIV && 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 cryptoBackend == cryptocore.BackendGoGCM && args.openssl {
+			cryptoBackend = cryptocore.BackendOpenSSL
 		}
 	}
 	// If allow_other is set and we run as root, try to give newly created files to