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Aug
23
comment The specification of modern, non-communicating cipher machinery
These questions all seem pretty standard, for applied cryptography. Have you read Cryptography Engineering, by Ferguson, Schneier, and Kohno? Or other good textbooks on applied cryptography? They'll tell you about things like how long your IV should be, why you need a message authentication code, how to resynchronize after corruption, etc. You should start by studying what is already known about computer-based systems, then analyze for yourself how they apply to your situation, and come back if there's anything that you can't work out from the standard references.
Aug
22
comment Create a field in PBC
What you want to do has nothing to do with pairing-based cryptography. Why would you want to use PBC, or something related to pairings?
Aug
22
comment Is there any research about cryptography on nondeterministic Turing machines?
@PaŭloEbermann, thank you, yes, that's a good summary (with the caveat that you have to be able to verify whether your guess was correct or not).
Aug
22
comment Is a 1024-bit DSA key considered safe?
@bdesham, sorry, I don't know the answer to your second question. I don't know of any way to replace it without revoking it and creating a whole new key, but maybe someone else will know.
Aug
22
comment How can I accomplish Key Derivation in JavaScript?
@AbhiBeckert, If you want advice on how to get the best possible security for your web service, I recommend you ask a question that describes your web service, your security needs, your requirements, your user population, and similar information. We can't read your mind; and it's hard to provide a useful answer without that information. As it is, I can only answer the question you asked (whether it's possible to take a weak password and turn it into a strong crypto key); the answer is no, you can't. It doesn't matter how inconvenient that answer is for users; the answer remains correct.
Aug
21
comment How resilient to attackers with extreme resources available is this encryption method?
@Everlag, the short answer is you can't: you need to rely upon something more than just a passphrase. The simple answer is to use public-key key exchange as others have suggested (e.g., like SSL). Or, maybe you need secure storage somewhere (where the private key can be stored securely; possibly encrypted under the passphrase); maybe you need a hardware device or token or smartcard; maybe you need something else. It's hard to tell you what the best approach will be, without knowing anything of the application requirements or restrictions or goals beyond that you want very strong cryptography.
Aug
21
comment Understanding the “cube-root math” behind an RSA signature forgery
hlh, sorry, I don't understand your question. The paper explains why the answer is $2^{1019} - (N * 2^{34}/3)$. They've already given you this answer (magically) and are now explaining how you can verify that this answer is correct. Plug in $A=2^{1019}$, $B=N*2^{34}/3$ into (7), exactly as the paper tells you do, and then simplify, and then the paper tells you why the cube of $2^{1019} - (N * 2^{34}/3)$ is $2^{3057}-N*2^{2072}+G$.
Aug
21
comment Strength of CBC with Ciphertext Stealing
This doesn't really answer the OP's question, which was about the degree of risk due to leaking the length. This is the one respect in which CBC-CTS does not achieve the same level of security as plain CBC, as CBC-CTS leaks more precise information about the plaintext length.
Aug
21
comment Understanding the “cube-root math” behind an RSA signature forgery
The paper explains how it got this number immediately after the expression you are quoting. See equations (7) and (8) and the surrounding text (continuing onto the top of the next page).
Aug
20
comment Is there any research about cryptography on nondeterministic Turing machines?
"You appear to assume that if there is no such path, you don't get any usable output" - Yes, that's right. I agree: that's exactly the heart of it. See my comment on the question, where I asked for clarification about what the original poster (user8007) had in mind in this regard. I'm going based upon my best attempt at interpreting user8007's answer as well as my limited understanding of complexity theory. Do you know of any reference which suggests which is the right formulation? If so, can you share a reference where I can learn more? I'd love to learn more!
Aug
20
comment Is there any research about cryptography on nondeterministic Turing machines?
(cont.) It is easy to find a condition under which Bob accepts and outputs that $i=1$, but no amount of non-determinism is enough for Bob to accept and output that $i=0$. (There's no single guess that lets Bob verify that there is no solution to your equation. If Bob guesses one value of $M$ and find it isn't a solution to the equation, he has no idea whether some other value of $M$ might be a solution.) Encryption has the same problem. So, your example is not valid: neither encryption nor decryption can be computed using a non-deterministic algorithm.
Aug
20
comment Is there any research about cryptography on nondeterministic Turing machines?
Oops, forget everything I wrote earlier! I just figured out what I missed. Your example is no good because Bob cannot decrypt. Try writing out a non-deterministic algorithm Bob can use to decrypt. To see this, just try writing a non-det. algorithm Bob can use to compute a single biased bit $i$. Make sure you remember the rules of non-deterministic algorithms: you have to specify the criteria under which Bob accepts or rejects. What is the condition under which Bob accepts and outputs that $i=0$ (that there is no solution $M$)? Try to write it down -- you'll see you are stuck. (cont.)
Aug
19
comment Is there a security proof for the Triple-DES construction in the ideal cipher model?
@RickyDemer, I thought you were curious about whether 2-key 3DES is more secure than single DES (rather than whether it's more secure than double DES), and I believe the Aiello paper does demonstrate one sense in which it is. Anyway, it's not important -- if the Aiello paper isn't what you were looking for, I'm fine with that! The other papers should still be useful.
Aug
18
comment Textbook-RSA meet-in-the-middle attack against other RSA based schemes?
nlognfan, I see your comment that you are unable to edit your question or accept an answer. Make sure you are logged in as nlognfan. If you have lost access to that account, please try the account recovery process (or try clicking "click here to recover your account" on the sign-in page) and the StackExchange OpenID recovery page; if neither of those works for you, contact team@stackoverflow.com with as much information as you have to ask them to help you regain access.
Aug
16
comment Is there any research about cryptography on nondeterministic Turing machines?
It's not clear to me how you were expecting to use a non-deterministic Turing machine to encrypt. A non-deterministic Turing machine has many possible execution traces (and, typically, many possible outputs). How will you select which one to use as the ciphertext? For deciding/recognizing a language, we say that the input is treated as accepted if any of the possible execution traces accepts. What do you plan to do about this for encryption? The answer to the question might depend upon what you have in mind here.
Aug
12
comment Is it possible to subtract/multiply numbers using homomorphic encryption?
Have you read the Wikipedia article on homomorphic encryption? That'd be a good starting point. Also, search on this site. I think you'll find some answers....
Aug
11
comment Alice's forgetful banking
@NickODell, sorry, I mistyped -- I'll delete the erroneous comment. What I meant to ask was: You say that a barrier to having Alice send the bank a hash of her account number is that the hash will be brute-forceable. So why don't you just use a 40-digit account number? That won't be brute-forceable, and it seems to meet all your other requirements. (You said we could change the behavior on both ends, so this should be allowable.) So why did you reject the hash approach? Or is that a valid solution to your problem? I already asked about this once up in the 3rd comment from the top...
Aug
10
comment What is the fastest elliptic curve operation f(P) in affine coordinates such that f^n(P)=P only if n is large?
Richard, to enable us to help you more effectively, I think it's important for you to describe the problem in more detail publicly -- including your answers to the questions in the comment section. This site is intended to create archival-quality questions and answers that will be of utility not just to you but to others who might have the same question. (If you need help on a question where you cannot describe all the requirements publicly, you might be better off hiring a consultant.)
Aug
10
comment Efficient Incremental Updates to Large Merkle Tree
@bytemaster, if you like, the 4096-byte block can itself be structured as a Merkle tree. This is roughly equivalent to building a Merkle tree on all the records (each record = a leaf of the big Merkle tree) and only storing the top 20-21 levels of the Merkle tree in RAM, which is also a valid optimization. There are many possible optimizations -- hopefully my answer helps you identify some of the possibilities, so you can choose whichever one is most appropriate for your specific setting.
Aug
10
comment Alice's forgetful banking
If the account number is a capability that's all that's needed to withdraw all of the money from that account, then 8 digits isn't enough. If a bank has ten million customers, simple random guessing will succeed with probability 10%. So the problem as specified can't be quite right. Something smells like we don't have all the real requirements/constraints.... Can you try editing the question to tell us the real problem you have?