Avoid decrypting encrypted multipart final size (#6776)

Multipart object final size is not a contiguous
encrypted object representation, so trying to
decrypt this size will lead to an error in some
cases. The multipart object should be detected first
and then decoded with its respective parts instead.

This PR handles this situation properly, added a
test as well to detect these in the future.

cmd/encryption-v1_test.go

@@ -334,6 +334,66 @@ func TestDecryptObjectInfo(t *testing.T) {
 	}
 }
 
+// Tests for issue reproduced when getting the right encrypted
+// offset of the object.
+func TestGetDecryptedRange_Issue50(t *testing.T) {
+	rs, err := parseRequestRangeSpec("bytes=594870256-594870263")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	objInfo := ObjectInfo{
+		Bucket: "bucket",
+		Name:   "object",
+		Size:   595160760,
+		UserDefined: map[string]string{
+			crypto.SSEMultipart:                    "",
+			crypto.SSEIV:                           "HTexa=",
+			crypto.SSESealAlgorithm:                "DAREv2-HMAC-SHA256",
+			crypto.SSECSealedKey:                   "IAA8PGAA==",
+			ReservedMetadataPrefix + "actual-size": "594870264",
+			"content-type":                         "application/octet-stream",
+			"etag":                                 "166b1545b4c1535294ee0686678bea8c-2",
+		},
+		Parts: []objectPartInfo{
+			{
+				Number:     1,
+				Name:       "part.1",
+				ETag:       "etag1",
+				Size:       297580380,
+				ActualSize: 297435132,
+			},
+			{
+				Number:     2,
+				Name:       "part.2",
+				ETag:       "etag2",
+				Size:       297580380,
+				ActualSize: 297435132,
+			},
+		},
+	}
+
+	encOff, encLength, skipLen, seqNumber, partStart, err := objInfo.GetDecryptedRange(rs)
+	if err != nil {
+		t.Fatalf("Test: failed %s", err)
+	}
+	if encOff != 595127964 {
+		t.Fatalf("Test: expected %d, got %d", 595127964, encOff)
+	}
+	if encLength != 32796 {
+		t.Fatalf("Test: expected %d, got %d", 32796, encLength)
+	}
+	if skipLen != 32756 {
+		t.Fatalf("Test: expected %d, got %d", 32756, skipLen)
+	}
+	if seqNumber != 4538 {
+		t.Fatalf("Test: expected %d, got %d", 4538, seqNumber)
+	}
+	if partStart != 1 {
+		t.Fatalf("Test: expected %d, got %d", 1, partStart)
+	}
+}
+
 func TestGetDecryptedRange(t *testing.T) {
 	var (
 		pkgSz     = int64(64) * humanize.KiByte