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Subject: Discussion item local signature computation and the use of a third-party
Hello, The use of a third-party to compute the digital signature value may be hijacked in the channel to the mobile device (or at the third-party). For instance, if a different digest (and thus document) is used in the message that is received by the mobile device, the user will sign some other document, without knowing it. (The risk is of course related to how the third-party protects the message to the mobile device and how it deals with the messages of the DSS server.) The mobile device cannot identify this situation (the challenge code does not protect the digest...); the hijacker does not change the challenge code :-) Because the DSS server must be trusted anyhow (it's the service that calculates the digest and sends it to the third-party) we could propose that at least the digest+challengeCode must be signed by the DSS server and included into the message to the third-party (and mobile device). The mobile device must be able to verify this, of course (and it should explicitly trust the certificate of the DSS server). This imposes some requirements for the mobile device to verify the basic signature. It is very likely that there are other solutions as well (? mTAN WS-transaction ?, ...); I've not researched on this topic. Or do you think we can leave it as it is, for now... What do you think? Regards Ernst Jan
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