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Can anyone explain why CBC-MAC is not secure for variable length message? For the previous question I'll quote Matthew Green's post from 2013: A quick reminder. CBC-MAC is very similar to the classic CBC mode for encryption, with a few major differences. First, the Initialization Vector (IV) is a fixed value, usually zero. Second, CBC-MAC only ...


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Public key encryption uses a public key of the receiver; anybody can encrypt. So origin authentication would only work if you'd also have a shared secret key (in which case the whole public key encryption becomes kind of useless) or a private key (in which case you'd probably use a signature or an authenticated key agreement protocol).



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