From f378e631472f22df653e0880e1ded251b7e856bf Mon Sep 17 00:00:00 2001
From: Jakob Borg <jakob@kastelo.net>
Date: Sat, 18 Mar 2023 10:22:18 +0100
Subject: [PATCH] lib/protocol: Handle encrypted requests without encrypted
 hash (fixes #8277) (#8827)

The layout of the request differs based on whether it comes from an
untrusted device or a trusted device with encrypted enabled. Handle
both.

Closes #8819.
---
 lib/protocol/encryption.go | 28 ++++++++++++++++++----------
 1 file changed, 18 insertions(+), 10 deletions(-)

diff --git a/lib/protocol/encryption.go b/lib/protocol/encryption.go
index ea2a4ad01..322dfedcf 100644
--- a/lib/protocol/encryption.go
+++ b/lib/protocol/encryption.go
@@ -96,18 +96,26 @@ func (e encryptedModel) Request(deviceID DeviceID, folder, name string, blockNo,
 		return nil, errors.New("short request")
 	}
 
-	// Decrypt the block hash.
+	// Attempt to decrypt the block hash; it may be nil depending on what
+	// type of device the request comes from. Trusted devices with
+	// encryption enabled know the hash but don't bother to encrypt & send
+	// it to us. Untrusted devices have the hash from the encrypted index
+	// data and do send it. The model knows to only verify the hash if it
+	// actually gets one.
 
+	var realHash []byte
 	fileKey := e.keyGen.FileKey(realName, folderKey)
-	var additional [8]byte
-	binary.BigEndian.PutUint64(additional[:], uint64(realOffset))
-	realHash, err := decryptDeterministic(hash, fileKey, additional[:])
-	if err != nil {
-		// "Legacy", no offset additional data?
-		realHash, err = decryptDeterministic(hash, fileKey, nil)
-	}
-	if err != nil {
-		return nil, fmt.Errorf("decrypting block hash: %w", err)
+	if len(hash) > 0 {
+		var additional [8]byte
+		binary.BigEndian.PutUint64(additional[:], uint64(realOffset))
+		realHash, err = decryptDeterministic(hash, fileKey, additional[:])
+		if err != nil {
+			// "Legacy", no offset additional data?
+			realHash, err = decryptDeterministic(hash, fileKey, nil)
+		}
+		if err != nil {
+			return nil, fmt.Errorf("decrypting block hash: %w", err)
+		}
 	}
 
 	// Perform that request and grab the data.