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

@@ -786,6 +786,7 @@ func (o *ObjectInfo) DecryptedSize() (int64, error) {
 	if !o.IsEncrypted() {
 		return 0, errors.New("Cannot compute decrypted size of an unencrypted object")
 	}
+	if len(o.Parts) == 0 {
 		size, err := sio.DecryptedSize(uint64(o.Size))
 		if err != nil {
 			err = errObjectTampered // assign correct error type
@@ -793,6 +794,17 @@ func (o *ObjectInfo) DecryptedSize() (int64, error) {
 		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 size, nil
+}
+
 // EncryptedSize returns the size of the object after encryption.
 // An encrypted object is always larger than a plain object
 // except for zero size objects.

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)