reverse: add panics against API abuse

These should help prevent later programming errors.

internal/cryptocore/cryptocore.go

@@ -72,6 +72,10 @@ func New(key []byte, backend BackendTypeEnum, IVBitLen int) *CryptoCore {
 	case BackendGoGCM:
 		aeadCipher, err = goGCMWrapper(blockCipher, IVLen)
 	case BackendAESSIV:
+		if IVLen != 16 {
+			// SIV supports any nonce size, but we only use 16.
+			panic("AES-SIV must use 16-byte nonces")
+		}
 		// AES-SIV uses 1/2 of the key for authentication, 1/2 for
 		// encryption, so we need a 64-bytes key for AES-256. Derive it from
 		// the master key by hashing it with SHA-512.

internal/fusefrontend_reverse/rfs.go

@@ -46,10 +46,14 @@ type reverseFS struct {
 
 var _ pathfs.FileSystem = &reverseFS{}
 
-// NewFS returns an encrypted FUSE overlay filesystem
+// NewFS returns an encrypted FUSE overlay filesystem.
+// In this case (reverse mode) the backing directory is plain-text and
+// reverseFS provides an encrypted view.
 func NewFS(args fusefrontend.Args) pathfs.FileSystem {
+	if args.CryptoBackend != cryptocore.BackendAESSIV {
+		panic("reverse mode must use AES-SIV, everything else is insecure")
+	}
 	initLongnameCache()
-
 	cryptoCore := cryptocore.New(args.Masterkey, args.CryptoBackend, contentenc.DefaultIVBits)
 	contentEnc := contentenc.New(cryptoCore, contentenc.DefaultBS)
 	nameTransform := nametransform.New(cryptoCore, args.LongNames)