posted Nov 15, 2000

Why Digital Signatures Are Not Signatures
11/15/2000; 11:32:10 PM

'The problem is that while a digital signature authenticates the document up to the point of the signing computer, it doesn't authenticate the link between that computer and Alice. This is a subtle point. For years, I would explain the mathematics of digital signatures with sentences like: "The signer computes a digital signature of message m by computing m^e mod n." This is complete nonsense. I have digitally signed thousands of electronic documents, and I have never computed m^e mod n in my entire life. My computer makes that calculation. I am not signing anything; my computer is....

'The mathematics of cryptography, no matter how strong, cannot bridge the gap between me and my computer. Because the computer is not trusted, I cannot rely on it to show me what it is doing or do what I tell it to. Checking the calculation afterwards doesn't help; the untrusted computer can't be relied upon to check the calculations properly. It wouldn't help to verify the code, because the untrusted computer is running the code (and probably doing the verification). It wouldn't even help to store the digital signature key in a secure module: the module still has to rely on the untrusted computer for input and output.'


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