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First, a fact. For some polynomial $f_x$ and some random polynomial of the same degree, say $t$, an adversary given only $f_x+t$, knows no additional information about $f_x$. Basically (due to the finite ring), this operation is the same thing as the one-time-pad. On to the problem at hand. Let $f_3=f_2\cdot(s+1)$. Since $f_2$ and $s$ have the same ...

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Fairness is, loosely speaking, the property of secure protocols that guarantees that either all honest parties will receive their output or no party will receive output. We know that this property can not be achieved for all functionalities unless when a majority of parties are honest.* As I recall this is a classic example of fairness not being ...

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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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Look up the words sound/complete from logic. Complete roughly means that a method can solve every instance. Sound roughly means that the answer it gives is correct. For example, assume that we have a program that's supposed to tell when an element belongs to a set. A sound program will only answer "yes" when the element actually belongs to the set. An ...

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