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Jun
22
comment What does it mean for an adversary to run in PPT?
Regarding the last question. Within the field of computational complexity theory it's commonly believed that the class of problems solvable by probabilistic PT machines is strictly larger than the class of problems solvable by deterministic PT machines. That is, we strongly believe (note, however, that this is not formally proved) that P $\subsetneq$ BPP, hence, if we can prove something secure against an attacker in BPP, we've also automatically shown it secure against an attacker in P.
Apr
5
comment For a one-time pad, which MAC method is information-theoretically secure?
@lightspeeder Note that all the attacks mentioned in that paper are based on real-world schemes where the key is reused for many messages. I suggested a one-time polynomial MAC's for which none of these attacks apply. Since you have already insisted on using a one-time pad for encryption, why not for the MAC as well?
Apr
4
comment For a one-time pad, which MAC method is information-theoretically secure?
Why do you insist on using HMAC as your authentication mechanism? And more importantly, do you really need to use a one-time pad? Anyway, if unconditional security is your goal for integrity, why not go with a one-time polynomial-based MAC?
Mar
31
comment Any field in a PKI certificate where some text info can be stored?
You are probably aware of this already, bu the nsComments extension is also deprecated according to the OpenSSL docs: "The following extensions are non standard, Netscape specific and largely obsolete. Their use in new applications is discouraged."
Feb
2
comment The difference between these 4 breaking Cipher techniques?
[About CPA]: "this is the strongest type of attack possible". This is false. CCA is a stronger attack model than CPA. Furthermore, there exists attack models even stronger than CCA, for instance related-key models.
Jan
19
comment RSA cipher wrong use
Regarding applying unpadded RSA, that's already answered here: crypto.stackexchange.com/a/10155/1772
Sep
10
comment Question about Fermat's little theorem
@Alex For the left-to-right implication in the first equivalence to hold true, you should probably add that $p \nmid a$.
May
30
comment What are the requirements of a nonce?
...what I'm trying to say is that, I think that a definition of a nonce should not impose ANY other requirements other than that it should not be used more than once. This does not forbid you from putting additional constraints on it.
May
30
comment What are the requirements of a nonce?
You asked for a definition of a nonce, and I provided what I consider to be the "right" notion; namely a value which is simply not used more than once (that's it!). Unfortunately there's much ambiguity in how the terms nonce and IV's are used in practice. So you might see several sources calling the first input to CBC-mode a nonce, whereas I would have preferred calling it an IV.
Apr
19
comment PKC McEliece + $S$ + $P$
As per your own Wikipedia link: $S$ is simply any invertible binary $k \times k$ matrix, and $P$ is any $n \times n$ permutation matrix. What is it that you find unclear?
Mar
16
comment Proof of the standard pseudorandom generator + XOR encryption scheme in Goldreich
Exactly the answer I was looking for, thx! It's funny though: I had done exactly the same thing as you, including the consideration of the "triangle" inequality. But, when I got to your last equation, I couldn't figure out where that $1/2$ factor should come from. However, its good too see that I wasn't to far of :)
Feb
28
comment Is SHA-1 still practical secure under specific scenarios?
I think it should be mentioned that the security guarantees given by that HMAC-paper is disputed. See Another Look at HMAC and the youtube presentation Another look at provable security. While controversial, they do bring up relevant points to the practical (security) merit of the Bellare paper.
Feb
16
comment How can an S-Box be reversed?
I think you should make it clear that S-boxes often ARE invertible. For instance, the S-box in AES is invertible. @Liam Inverting S-boxes can be very easy: you simply create a lookup table that reverse all the possible substitutions of the S-box. E.g. if the S-box maps 0xA5 to 0x3F (this would be an 8x8 S-box), then the inverse transformation would map 0x3F to 0xA5. Thus, you simply enumerate all the possible values the S-box can have, and create an inverse table that "undoes" all those transformations (this effectively limits how large the S-boxes can be in practice).
Feb
15
comment Encrypting a key with the same key
@madhukar2k2 "I tried searching for an answer for the above, but couldnt not find one." - try looking at circular security, it might give you some answers. Additionally, I've added some things in my answer below.
Jan
23
comment Why is MixColumns omitted from the last round of AES?
Ah, apparently my vote has been locked in, so I'm unfortunately not able to change my downvote. Sorry about that :(
Jan
23
comment Why is MixColumns omitted from the last round of AES?
Ok, fair enough. I will remove my downvote when the timeout has passed. Still, since many modes of operation allows you get away with only the encryption direction, I don't understand why an equal RoundFunction wouldn't be preferable for all rounds (Ref. Fixee's comment about the hassle's of the last round omission, in his answer to PulpSpy's answer)
Jan
22
comment Why is MixColumns omitted from the last round of AES?
I feel this answer is not answering the question (even if accepted). Let me reformulate what I would like to know: If our goal is to make all rounds in the encryption/decryption equal (I stress that the round function will be different in enc/dec), what is the problem of adding MixColumns in the last round? As far as I can see, AES-ENC would then simply become: 1. AddKey 2. repeat RoundFunc 10 times. And for AES-DEC we get: 1. AddKey (last RndKey) 2. repeat InvRoundFunc' 10 times, where InvRoundFunc' = 1. InvMixColumns 2. InvShift 3. InvSubBytes 4. AddRndKey. Why is this not possible?
Jan
7
comment Why does WPA-PSK not use Diffie-Hellman key exchange?
@Michael Yes, if you know the 4-w-h then you can do that. However, I think you are confusing the situation you are in: in WPA-PSK, the pre-shared key is the ONLY (true) secret in the scheme. It is not obvious how you could allow all the participants to know this key, while at the same time not being able to read each others messages. A stronger variant is WPA-EAP, where all participants use their own individual secret. Here the 4-w-h will not reveal anything. However, this solution is more complex to set up, and usually not used for home-networks.
Jan
7
comment Why does WPA-PSK not use Diffie-Hellman key exchange?
@Steel City Hacker Yes, I'm aware of that, but I considered this to be details not very relevant for the question at hand. Thus I only provided the link, where those details are included.
Jan
7
comment Why does WPA-PSK not use Diffie-Hellman key exchange?
Yeah, I was a little sloppy in my explanation. I've edited the answer to (hopefully) clarify some of the uses.