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There are a few pitfalls: File name integrity: signing files one at a time signs the contents of the files. It (typically) does not protect the file names from tampering. This could be disastrous in some situations (e.g. an attacker could change blacklist.txt to whitelist.txt). Set membership integrity: signing individual files does not prevent adding or ...


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No pitfalls I can see except that you may run into missing files more than you would if you chose to sign them individually. One negligible drawback is speed: you'd be forced to calculate one hash more for the verification step than if you did it separately. The I biggest is probably compatibility. There is no standard way of hashing multiple files in a ...


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Not, that should be not possible, at least if you have a good key. All this is only valid as long as SHA-256 is still a secure hash algorithm and not broken. To be precise, as long as HMAC-SHA-256 is not broken. Does the attacker have any informations about the messages (other than the length and that they are "random-noise")? If he/she doesn't: No, ...


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If the keys and messages are known, yes, you can distinguish which were used - because you can test them all. If not, then this is "sligtly harder" (= not really possible with big enough values). Anything of the further answer will assume that the attacker doesn't know the keys or the messages. The definition of HMAC looks like this: $HMAC(K, m) = H(K ...


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Actually, it's there on the list, just with a different name -- the approved algorithm you want is listed as "SHS" (Secure Hashing Standard). Now, the term "SHS" doesn't distinguish between the various flavors of SHA-2 (and SHA-1, which is still approved for some uses); however if you look at this more detailed list, that gives details on what vendors have ...



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