How to sha checksum mac9/6/2023 ![]() This can only ever be an attack when part of the security protocol is that $A$ is secret, because if $A$ is public, then everyone can calculate $H(A||S)$ for any suffix $S$. The adversary does not get to choose $S$ or $H(A||S)$. They're attacks against hashes when used for purposes that require stronger security properties.Ī length extension calculation on a Merkle-Damgård hash allows an someone to calculate $H(A||S)$ for a specific non-empty suffix $S$ given the knowledge of only $\mathsf(A)$ and $H(A)$ (but not $A$ itself). Length extension attacks are not attacks against hashes as hashes. Yes, the adversary could compute the hash of $Y = M | pad | X$, however because he knows what his image $Y$ is, he could compute its hash without any special properties of the hash function. It is believed to be infeasible to make the hashes match. If it were, that would count as a hash collision, hence disproving the collision resistance of SHA256. However, the hash of $M | pad | X$ will not (in general, that is, barring an extremely improbable coincidence) be the same as the hash of $M$. What the length extension attack allows someone to do is, given the hash of an unknown message $M$, compute the hash of a message $M | pad | X$ (for a specific string $pad$ which depends on the length of $M$, and an arbitrary string $X$). I highlighted the incorrect assumption: the checksums wouldn't match. Because Bob's fake ISO checksum matches the official ISO checksum, Bob doesn't notice that he has downloaded fake ISO Bob compares the SHA256 checksum that he generated from fake ISO file to the checksum found on official linux distribution's home page.
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