From 15771ebe8d349f47f620519015b73497ba2399f1 Mon Sep 17 00:00:00 2001
From: Andreas Auernhammer <aead@mail.de>
Date: Thu, 12 Jul 2018 18:29:56 +0200
Subject: [PATCH] Fix decrypted object size and key derivation in
 CopyObjectPart  (#6141)

This commit fixes the size calculation for multipart
objects. The decrypted size of an encrypted multipart
object is the sum of the decrypted part sizes.

Also fixes the key derivation in CopyObjectPart.
Instead of using the same object-encryption-key for each
part now an unique per-part key is derived.

Updates #6139
---
 cmd/encryption-v1.go   | 20 ++++++++++++++++----
 cmd/object-handlers.go |  9 +++++++--
 2 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/cmd/encryption-v1.go b/cmd/encryption-v1.go
index ab3422244..fe51341d5 100644
--- a/cmd/encryption-v1.go
+++ b/cmd/encryption-v1.go
@@ -786,11 +786,23 @@ func (o *ObjectInfo) DecryptedSize() (int64, error) {
 	if !o.IsEncrypted() {
 		return 0, errors.New("Cannot compute decrypted size of an unencrypted object")
 	}
-	size, err := sio.DecryptedSize(uint64(o.Size))
-	if err != nil {
-		err = errObjectTampered // assign correct error type
+	if len(o.Parts) == 0 {
+		size, err := sio.DecryptedSize(uint64(o.Size))
+		if err != nil {
+			err = errObjectTampered // assign correct error type
+		}
+		return int64(size), err
+	}
+
+	var size int64
+	for _, part := range o.Parts {
+		partSize, err := sio.DecryptedSize(uint64(part.Size))
+		if err != nil {
+			return 0, errObjectTampered
+		}
+		size += int64(partSize)
 	}
-	return int64(size), err
+	return size, nil
 }
 
 // EncryptedSize returns the size of the object after encryption.
diff --git a/cmd/object-handlers.go b/cmd/object-handlers.go
index c8aff2351..464e5863a 100644
--- a/cmd/object-handlers.go
+++ b/cmd/object-handlers.go
@@ -1077,7 +1077,6 @@ func (api objectAPIHandlers) CopyObjectPartHandler(w http.ResponseWriter, r *htt
 				return
 			}
 
-			// Calculating object encryption key
 			var objectEncryptionKey []byte
 			objectEncryptionKey, err = decryptObjectInfo(key, dstBucket, dstObject, li.UserDefined)
 			if err != nil {
@@ -1086,7 +1085,13 @@ func (api objectAPIHandlers) CopyObjectPartHandler(w http.ResponseWriter, r *htt
 				return
 			}
 
-			reader, err = sio.EncryptReader(reader, sio.Config{Key: objectEncryptionKey})
+			var partIDbin [4]byte
+			binary.LittleEndian.PutUint32(partIDbin[:], uint32(partID)) // marshal part ID
+
+			mac := hmac.New(sha256.New, objectEncryptionKey) // derive part encryption key from part ID and object key
+			mac.Write(partIDbin[:])
+			partEncryptionKey := mac.Sum(nil)
+			reader, err = sio.EncryptReader(reader, sio.Config{Key: partEncryptionKey})
 			if err != nil {
 				pipeWriter.CloseWithError(err)
 				writeErrorResponse(w, toAPIErrorCode(err), r.URL)