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Yes, this scheme preserves confidentiality and integrity of messages. Pre-image resistance of the secure hash function ensures confidentiality of the message (in conjunction with an appropriately strong key). The use of a nonce ensures that an eavesdropper can't tell whether a genuine message is the same as any previous messages. The properties of a MAC ...


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What you think of is called an extension attack and it turns out that this is the way to go if you would like to break the general CBC-MAC when the message length is not fixed. All that an adversary needs to do is to mount a chosen message attack. Suppose he asks for the tag on the message $m=m_1||m_2||...||m_l$. The resulting CBC MAC would be ...


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In general (without talking about MD5): Suppose our hashfunction $H$ is a Merkle-Damgard construction using a Davies-Meyer compression function $h=(H_i,m)=E_{m_i}(H_{i-1})\oplus H_{i-1}$. Since the compression function is public, everybody is able to compute the input to the final round of the MD-Hash. In addition, if you know the input to the final round ...


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We can attack the MAC defined by: MAC(k,m)=MD5(m||k), in a chosen-messages setup, basically because MD5's collision-resistance is broken. The adversary chooses m and m' of the same length $b\ge64$ bytes, differing only in their first $\lfloor b/64\rfloor$ 64-byte blocks, such that there is a collision after hashing these blocks of m and m'. If follows that ...


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First of all there does exist information theoretically secure message authentication codes suitable for use with a one time pad. An HMAC is not one of those information theoretically secure. As far as I recall the first article presenting such a construction is the 1981 article by Wegman and Carter: New hash functions and their use in authentication and ...


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If key 2 and key 3 has a nonnegligible chance to be the same, then the attacker has a nonnegligible chance of being able to generate a valid (Message, MAC) pair. Here's how it works, if the message is not a multiple of 16, then XCBC pads the message out to the next multiple of 16; if it already is, the message remains the same. Then, XCBC logically does a ...



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