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Nov
10
comment Perfect Forward Secrecy in MTI/A0
I encourage you to define the terminology you use (e.g., MTI/A0, LL-key), as I'm not familiar to them. Also, if you have a link to the paper(s) that introduce this scheme, I encourage you to post them. This might increase the chances that you get a good answer.
Nov
9
comment Why is it important to generate a pseudorandom permutation from a pseudorandom function?
What do you mean by "it is important [to do it this way]" or "it is necessary [to do it this way]"? I think you have a misconception/erroneous premise. This is one popular way to build a block cipher, but it's not the only way. It is neither necessary nor important to construct the block cipher this way. There are other ways to construct secure ciphers; see, e.g., AES/Rijndael.
Nov
7
comment partial-domain permutation and strong assumption
Did you read the cited paper on OAEP? I would expect that it's likely to have a definition of those terms.
Nov
7
comment What aspects of information theory are used in modern cryptography?
I suspect this is too broad to be a good fit for this site: you could probably write a whole book about it. (Also, the answer depends on what you mean by a "working cryptographer".)
Nov
7
comment Are there any bijective one-way functions not based on number-theoretic hardness assumptions?
The term you are looking for is "one-way permutation". There are many candidate constructions. Why would you insist that you are not willing to rely upon number-theoretic hardness assertions? That seems unnecessarily limiting and not well motivated.
Nov
5
comment Is this hand cipher any more secure than the Vigenère cipher?
user54609, I asked about 7 different questions, and you answered about 4 of them. Still wondering about the other 3.... If you're not going to answer requests for clarification, how do you expect to get a good answer? Seems odd to place a bounty but not answer such questions. You should also be editing the question to include these clarifications in the question, so people don't need to read the comment thread. P.S. You might also want to notify me when responding, by including "@D.W." in your response.
Nov
5
comment How to design a secure data container?
@MaxthonChan, you can encrypt the data with something like GPG, and that'll handle both data at rest and data in motion (you can send the encrypted data over a network). Make sure to use both confidentiality and integrity/authentication.
Oct
31
comment Is this hand cipher any more secure than the Vigenère cipher?
We need more information about the key. Is the key a sequence of numbers that are uniformly and independently distributed (each number is uniform of all others)? Is the key re-used for multiple messages? How long is the key? What do you do if the message is longer than the length of available key material? For instance, if the key is a sequence of length 15 (say) for each of the rounds, what do you do if your message is longer than 15 characters long? Do you repeat the key? Do you use a different independent key for each round, or do you use the same in each round?
Oct
28
comment ID-based key exchange protocol with PFS
Alex, (1) in that case your requirement should be that you want an efficient protocol (specify what you want to achieve, not how to achieve it), (2) I very much doubt that signatures make the protocol inefficient. Signatures are fast. I mean, you're OK with the inefficiency of bilinear pairings, but not with ordinary digital signatures? That's counter-intuitive.
Oct
28
comment ID-based key exchange protocol with PFS
What is the reason for your requirement "besides using digital signatures"? What's wrong with using digital signatures? This seems like an odd requirement to impose.
Oct
27
comment ID-based key exchange protocol with PFS
@Alex, you're right, that's an even better scheme. I've added it to my answer. (For future reference, if you already know of a partial answer to your question when asking your question, it would be helpful to include it in the question; that increases the chances that someone can give you a helpful answer.) I've edited my answer, and I believe that everything in my answer is accurate.
Oct
27
comment ID-based key exchange protocol with PFS
@Alex, thanks for your interest. I described a hybrid scheme (which derives ephemeral session keys using DH/ECDH) in the 2nd paragraph of my answer, right after the phrase "hybrid scheme".
Oct
25
comment Hypothetical unknown cipher - security in obscurity?
Your premises are implausible in practice. What if an alien bought a pig at the market, and the pig the alien picked just happened to be able to fly? Hey, it could happen....
Oct
24
comment How do I prove that this PRNG is easily distinguished from a random sequence of numbers (modulo m)?
What have you tried? Where did you get stuck? (We expect you to make some effort before asking here.) Do you know what the definition of "distinguishing from a random sequence" is? Have you tried plugging into the definition? Where did this problem arise? What's the context/motivation? Why do you need to prove it? If it's a textbook exercise, have you reviewed the material on PRGs and on elementary number theory?
Oct
23
comment Should I use the first or last bits from a SHA-256 hash?
@Pacerier - No. There is no reason to believe that using the last bits would be detrimental to security (and there are very strong reasons to expect that it is not detrimental). You're misreading the contents of that link.
Oct
21
comment What's wrong with MARS?
"As I understand only MARS and Serpent implement measures to counter future cryptoanalytic attacks" - Your understanding is faulty. All of the finalists (probably all of the submissions, or essentially all of them) included measures that their designers hoped would counter future cryptanalytic attacks.
Oct
21
comment What's wrong with MARS?
Did you read any of the analysis that was published on MARS? There were a series of workshops with a whole bunch of papers that came out of the proces. Did you read NIST's document explaining their final choice of the AES? If not, those would be good documents to go read!
Oct
20
comment Encoding information in packet lengths to actively sidestep encryption
The relevant concepts are: "side channels", "covert channels", and "steganography". As K.G. said, covert channels or stego are probably the ones most relevant to you.
Oct
10
comment The Goldreich-Goldwasser-Micali Construction with bad PRGS
I very much doubt that GGM will offer useful security if the underlying PRG has a significant bias. I think your argument is unconvincing and your hypothesis is unlikely to hold.
Oct
8
comment Predicting values from a Linear Congruential Generator
See also security.stackexchange.com/q/4268/971 for more on cracking LCGs (though with a different set of assumptions).