It is still risky to sign on line documents with high security requirements, such as tax returns or contracts. Trojans or viruses are able to manipulate the presentation of words and numbers on screen in such a way that a person signing sees something different from the actual document. In such cases, security is provided by a software for trusted mobile phones coming from the Institute of Cryptography and Security (IKS) of the Computer Science Faculty. Its operation comprises several steps: The person signing reads the text sent to him or her in ergonomic comfort on the large screen of a (non-trusted) computer. To check whether the text as presented on screen is identical with the version to be signed, the person expected to sign uses a camera to produce a snapshot of the computer display. The software developed by the IKS and implemented in the mobile phone then conducts a comparison: A normalized nominal version of the text is computed from the data also sent to the mobile phone by Bluetooth, MMS or 2D bar code. The snapshot and the nominal presentation are then intercompared. If the data agree, the signature can be applied by mobile phone. Where differences occur, they are displayed, and there should be no signature, or the signature is prevented automatically. Compared to other authentication systems, such as smart card readers with small displays, the trusted mobile phone offers advantages: Combination with the computer screen allows even extensive text documents to be read easily in good resolution. The software can be used in any mobile phone with Java capability and a sufficiently high camera resolution. In principle, adaptation to other platforms is possible. Applications are in authentication of documents, for instance, for presentation to public authorities, in on-line banking, on-line brokerage, but also in contracts, which then will no longer have to be sent by mail.