diff --git a/src/main.zig b/src/main.zig
index 97aa33b..6875ac0 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -201,3 +201,31 @@ fn digest(ally: std.mem.Allocator, data: []const u8, case: std.fmt.Case) ![]cons
     }
     return res;
 }
+
+// it even works in ReleaseFast so I think it's ok.
+test "in-place-encrypt" {
+    const al = std.testing.allocator;
+
+    var key: [AES.key_length]u8 = undefined;
+    std.crypto.random.bytes(&key);
+    const iv: [AES.nonce_length]u8 = undefined;
+    std.crypto.random.bytes(&key);
+
+    var tag: [AES.tag_length]u8 = undefined;
+
+    // 1mb message. very big. should find an error if one exists.
+    const original_message: []u8 = try al.alloc(u8, 1 << 20);
+    defer al.free(original_message);
+    std.crypto.random.bytes(original_message);
+
+    const message = try al.dupe(u8, original_message);
+    defer al.free(message);
+
+    const ad: []u8 = try al.dupe(u8, "u");
+    defer al.free(ad);
+
+    AES.encrypt(message, &tag, message, ad, iv, key);
+    try std.testing.expect(!std.mem.eql(u8, message, original_message));
+    try AES.decrypt(message, message, tag, ad, iv, key);
+    try std.testing.expect(std.mem.eql(u8, message, original_message));
+}