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7

What needs to be memorized in applied science (physics, crypto) is not a set of formulas. It's, for a few of the simplest formulas studied: what the formula yields, for what inputs, the units for inputs and output, and how to derive the formula when it does not boil down to multiplying or dividing the inputs in a way that can be found from the units. When ...


4

You're wrong. Hint: a value of 8 bits has 256 possible values, not 8.


2

If $f$ is a cryptographically secure hash function and $X$ is kept secret while $f(X)=Y$ is published, then an attacker can find $X$ if and only if $X$ contains too little entropy. That is, there are too few possibilities what $X$ may be, and the attacker can try them all out and test if $Y$ results. Knowing only the length of $X$ does not help, given that $...


5

I think you are misinterpreting the security notion of pre-images. That $X$ is considered unknown for a $Y$ doesn't make $X$ a secret in the cryptographic sense. My answer basically evolves around this. From my understanding, if $H(X)=Y$, knowing $Y$, I should not be able to know $X$ is the fundamental behind the first preimage attack. First of all, ...


5

The questions Does the security of a hash function from the preimage attack also depend on the secrecy of the length of X.? The preimage attack: given a hash function $h$ and a hash value $y$ the computationally bounded adversary tries to find a pre-image $x$ such that $y = h(x)$. This is like an adversary trying to find a password given the hash of it ...


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