From d3b78fea959dfb0e1c1f5079ae516303bdb9a0f8 Mon Sep 17 00:00:00 2001
From: Jakob Unterwurzacher
Date: Sun, 9 Oct 2016 17:05:12 +0200
Subject: reverse: add panics against API abuse

These should help prevent later programming errors.
---
 internal/cryptocore/cryptocore.go    | 4 ++++
 internal/fusefrontend_reverse/rfs.go | 8 ++++++--
 2 files changed, 10 insertions(+), 2 deletions(-)

(limited to 'internal')

diff --git a/internal/cryptocore/cryptocore.go b/internal/cryptocore/cryptocore.go
index 7cb5c95..db82f56 100644
--- a/internal/cryptocore/cryptocore.go
+++ b/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.
diff --git a/internal/fusefrontend_reverse/rfs.go b/internal/fusefrontend_reverse/rfs.go
index bc8a535..35e9e50 100644
--- a/internal/fusefrontend_reverse/rfs.go
+++ b/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)
-- 
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