Integrity of digital documents is a very important issue if they are to be legally binding. Common solutions, like digital signatures or message authentication codes, are based on cryptography and require computers or similar hardware to be produced. They can be trusted only as long as the employed hardware can be trusted. We present a complementary scheme for document authentication in untrusted environment. The scheme combines visual authentication, human-recognizable watermarks, and image transformations, and is suitable also for larger text document. Because it relies on visual cryptography, it requires practically no computational power on the receiver side. To prevent potential attackers from obtaining signatures without author’s knowledge, we propose using a simple challenge-response protocol.