From 91bc452b9aea9d08076bf1c4abd7ee98ed7e90a1 Mon Sep 17 00:00:00 2001
From: Filippo Valsorda <hi@filippo.io>
Date: Sat, 29 Aug 2026 13:51:20 +0200
Subject: [PATCH] internal/stream: reject negative and overflowing encrypted
 sizes

---
 internal/stream/stream.go      |  4 ++++
 internal/stream/stream_test.go | 12 ++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/internal/stream/stream.go b/internal/stream/stream.go
index a2fc532..58fd9fa 100644
--- a/internal/stream/stream.go
+++ b/internal/stream/stream.go
@@ -12,6 +12,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"math"
 	"sync/atomic"
 
 	"golang.org/x/crypto/chacha20poly1305"
@@ -20,6 +21,9 @@ import (
 const ChunkSize = 64 * 1024
 
 func EncryptedChunkCount(encryptedSize int64) (int64, error) {
+	if encryptedSize < 0 || encryptedSize > math.MaxInt64-encChunkSize+1 {
+		return 0, fmt.Errorf("invalid encrypted payload size: %d", encryptedSize)
+	}
 	chunks := (encryptedSize + encChunkSize - 1) / encChunkSize
 
 	plaintextSize := encryptedSize - chunks*chacha20poly1305.Overhead
diff --git a/internal/stream/stream_test.go b/internal/stream/stream_test.go
index c5ef40f..bf955c0 100644
--- a/internal/stream/stream_test.go
+++ b/internal/stream/stream_test.go
@@ -10,6 +10,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"math"
 	"testing"
 	"testing/iotest"
 
@@ -631,6 +632,17 @@ func TestDecryptReaderAtInvalidSize(t *testing.T) {
 	if err == nil {
 		t.Error("invalid size (empty final chunk): expected error, got nil")
 	}
+
+	// Negative sizes and sizes that would overflow the chunk count computation
+	// must be rejected with an error rather than a panic.
+	for _, size := range []int64{-1, -70000, math.MaxInt64, math.MaxInt64 - 200, math.MaxInt64 - 65550} {
+		if _, err := stream.EncryptedChunkCount(size); err == nil {
+			t.Errorf("EncryptedChunkCount(%d): expected error, got nil", size)
+		}
+		if _, err := stream.NewDecryptReaderAt(key, bytes.NewReader(ciphertext), size); err == nil {
+			t.Errorf("NewDecryptReaderAt(%d): expected error, got nil", size)
+		}
+	}
 }
 
 func TestDecryptReaderAtTruncated(t *testing.T) {