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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 ...


4

Although your scheme is secure - especially with a random key of 32 bytes or higher - it won't offer any benefit over HMAC. It's therefore not recommended to use such a scheme. Also note that `bcrypt has been designed for key stretching which is deliberately not efficient. You have ample entropy in your key so there is no need for key stretching.


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The question is how much is this schema secure? Not significantly more secure than sha256(m + k) is and may be less secure. An attacker who could arrange a collision for that would trivially also get a collision for the bcrypt hash of that, regardless of the salt value. While SHA-256 is collision resistant, there are MACs that have better bounds, like ...



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