# Tag Info

3

If you can read the intermediate states of the encryption algorithm you could recover, one by one all the round keys. Given a AES round, all the operation between the two AddRoundKey (at the beginning and the ond of the round) are invertible. Take for example round 1: you get the internal state before AddRoundKey (of round 2), you get back at the beginning ...

3

We have a Padding Oracle if there is a different response from the server gives us an indication of the correctness of the pad (say if this needs proving). We can establish this by playing a game where we send badly padded cipher-text and random strings to the server, finally submitting some at random and seeing if we can get a non-negligible Advantage is ...

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In the standard definition of security for public key encryption schemes there exists only one public key. Therefore the decryption oracle will always decrypt with the secret key that corresponds to the public key given to the attacker. It does not matter how the $c_1$ in your question is computed, it can be computed using the real public key, a different ...

2

It is called circular security. It is problematic because it is not captured by the usual security definitions. I.e., even if an encryption scheme is proven secure by some regular definition, it is usually not a given to be circular secure. To see why consider, for example, the usual definition of semantic security for a public-key encryption scheme $\Pi = ... 2 Here's the next step in the iteration, which should be easy to understand: Let's call the oracle on 2P and 4P: Answer (even,even) means, that$P<N/4$(this is still easy: Otherwise either 2P or 4P would be greater than N). Answer (even,odd) means$N/4<P<N/2$. (odd,even) means$N/2<P<3N/4$and (odd,odd) means$3/4N<P<N$. Actually, ... 2 This represents the probability over all$K$that$A$given an oracle access to$F_K(\cdot)$outputs 1. You usually compare that to the probability of A ouptputting 1 while having oracle access to a random function and the difference tells you how well$A$does at telling$F_K$from random 1 The approach with which I solved the problem is indeed as @tylo suggested. Initially we know that the target plaintext$P$is within the bounds$[0,N]$where the lower bound$LB=0$and the upper bound$UB=N$. Now we iterate the following algorithm$log_{2}N$times to find P from the original intercepted ciphertext$CC' = (2^{e}\mod N) * C\$ if ...

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Prove? Why does the attacker need to "prove" it? For example, the attacker can check whether there is an oracle by looking at the code and seeing whether such an attack is possible. Or, the attacker can guess that such an attack might be possible and then try the attack. If the attack succeeds, the attacker knows the system is vulnerable. There might ...

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