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May
19
comment Is One Time Pad considered Chosen-Plaintext Attack Secure?
... We should be able fix the first issue by making the keyspace $\{0,1\}^*$ and restricting the total length of messages that can be encrypted with each key to a polynomial function of $n$. This seems reasonable enough, since the adversary can only make a polynomial number of queries anyway. But the statefulness issue remains, although I guess we could just make the keys long enough compared to the maximum total message length to make the probability of overlap negligible. Lots of handwaving here, I admit, but it seems like it should work.
May
19
comment Is One Time Pad considered Chosen-Plaintext Attack Secure?
I guess we could define the "keyspace" for a one-time pad as $\{0,1\}^{\Bbb N}$, regardless of the security parameter $n$, and prefix each ciphertext with a starting offset in the key sequence that is guaranteed not to overlap any previously used key segment. That should be enough to (almost) make the one-time pad an IND-CPA secure encryption system, if I'm not mistaken. The only problems being that 1) $\rm Gen$ will not actually be an algorithm in the strict sense, and will surely not be polynomial-time, and 2) $\rm Enc$ would have to be stateful to guarantee the non-overlap property.
May
19
revised Can you make a hash out of a stream cipher?
add provable-security tag; remove the footnote about Carter-Wegman MACs, since none of the answers really address it (and it probably should be a separate question anyway)
May
19
comment Can you make a hash out of a stream cipher?
I don't really see how this adds anything to B-Con's second answer. In particular, as noted in the comments to that answer, it's not quite obvious how to obtain a block cipher that actually satisfies the (rather strict) security properties needed to prove the security of the resulting hash function.
May
19
comment Why cant Public Key Encryption be perfectly secure?
@RickyDemer: I don't really see any way to have sender-deniable public key encryption against a computationally unbounded adversary; in particular, the adversary can always recover the private key by enumerating the keyspace, and then decrypt the message. With overwhelming probability, the plaintext they get is what was originally encrypted, or at least what the intended recipient would have received.
May
19
comment How can a block cipher in counter mode be a reasonable PRNG when it's a PRP?
The answer is basically the PRF/PRP switching lemma, which essentially says that a pseudorandom permutation is indistinguishable from a pseudorandom function unless we observe a collision for one of them. See also this question on cstheory.SE.
May
19
reviewed Leave Open Building a combined encryption scheme from two encryption schemes that's secure if at least on of them is secure
May
19
comment What should I think about these unique certificate serial numbers
What is your specific question? "What should I think about X?" is not really something we can answer here. See our help center for more information about asking good questions.
May
19
reviewed Reviewed Is the hash function defined in this exercise collision-resistant?
May
19
answered Is the hash function defined in this exercise collision-resistant?
May
19
comment Is the hash function defined in this exercise collision-resistant?
This is true (and should certainly be counted as a correct answer), but it's probably not the answer the author of the exercise was looking for. (If it was, why bother with all the extra details about the internals of the hashing algorithm?)
May
19
comment Is the hash function defined in this exercise collision-resistant?
Generally, the point of homework exercises is to give you some practice in applying the methods you've learned, and to help you notice any gaps in your skillset. Thus, to get the full benefit from the exercises, you should first try to solve them as far as you can on your own, and only then turn to others for help. In particular, when asking for help with homework on Stack Exchange, it helps a lot if you mention how far you've got on your own (even if it's "I tried X and Y and Z, but none of them work because..."), and where specifically (you think) you're stuck.
May
19
revised Is the hash function defined in this exercise collision-resistant?
improve formatting, slightly less generic title, misc. copyedits
May
16
awarded  Enlightened
May
16
awarded  Nice Answer
May
16
awarded  cryptanalysis
May
15
reviewed Edit Does the plaintext have to be at least as long as the key in Polyalphabetic substituion cipher
May
15
revised Does the plaintext have to be at least as long as the key in Polyalphabetic substituion cipher
It would be shift cipher not a monoalphabetic substitution cipher
May
15
revised Does the plaintext have to be at least as long as the key in Polyalphabetic substituion cipher
added 685 characters in body
May
15
answered Does the plaintext have to be at least as long as the key in Polyalphabetic substituion cipher