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To attack a MAC in general, the attacker needs to find a valid MAC of a message that they do not have the MAC for (or find a message collision that allows a different message to have the same MAC digest). In this case, the attacker would be appending data to the original message, not the MAC itself, and trying to obtain a valid MAC for the new message. In ...


2

No, scrypt in not vulnerable to password extension attacks. Internally, scrypt passes the password to PBKDF2, which uses it as a key for the HMAC function -- hence they've effectively already done the workaround you thought of.


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It seems that all this discussion about the IV is just to confuse the reader. Ignore IV. You just have to do a simple length-extension attack on the CBC-MAC. Yes, one message is enough.



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