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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
revised What is the fastest elliptic curve operation f(P) in affine coordinates such that f^n(P)=P only if n is large?
Include information from the comments section. Fix the incorrect statement about Koblitz curves.
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?
Aug
10
comment Alice's forgetful banking
I don't understand the constraints. What parts of the system can we modify? Can we modify both the client (the software Alice runs or the procedure Alice follows) and the server (the code that the banks run for authenticating Alice or that process they use to help Alice check about that account number)? If you can change both endpoints, why are you limited to just 8-digit account numbers: why can't you choose arbitrarily long account numbers? (Also, you later mention cryptographic keys. If we're talking about keys, those won't be 8 digits long and won't be brute-forceable, so which is it?)
Aug
9
comment why AKS is so slow in practice?
This question would be better suited to Computer Science Stack Exchange. The question is not about cryptography per se and so does not appear to be on-topic on this site.
Aug
9
comment Any problems with this secure time synchronization scheme?
@nightcracker, alas, I don't know of any literature on it. On first glance, it hardly seems possible (though maybe I shouldn't say that, maybe someone has some clever way that hasn't occurred to me). I wasn't aware that there was any issue with literature recommendations; my impression is that there's an issue with open-ended questions that solicit many answers ("what's the best book on cryptography?"), but not focused questions ("what's the state of the art on X?"), particularly if you done some searching on your own first.
Aug
8
answered Any problems with this secure time synchronization scheme?
Aug
8
comment Why is Merkle-Damgård construction insecure?
Possible dup: crypto.stackexchange.com/q/3978/351
Aug
7
comment Securing content and login with “same” password
I think the scheme in your last paragraph is better, given the requirement from the question that "the service should not be able to decode the information that it stores for the user". That scheme (from the last paragraph) seems like a good one to me!
Aug
7
comment Securing content and login with “same” password
@user12889, Regarding your first question, services should not be storing password hashes that are computed using a standard cryptographic hash function; this is standard advice. I recommend that you read standard material on password hashing. There's lots of good stuff on IT Security.Stack Exchange on password hashing, and on PAKE (see also Wikipedia): I encourage you to search there and read up on these topics before asking too many more follow-up questions.
Aug
7
comment Securing content and login with “same” password
@user12889, that's not enough. 1. If $H,H'$ is a standard hash functions, it'd be too easy for an attacker to recover $P$ from that, since standard hash functions are too fast. 2. The user still sends $H(P)$ to the server, so if the server is hacked, an attacker learns $H(P)$ and then may be able to recover $P$ (if $H$ is a fast hash). This is bad, because users often reuse the same password on other sites, and that means a successful security breach of your site may adversely affect the user's security on other sites. I suggest reading standard resources on password hashing.
Aug
7
answered Securing content and login with “same” password
Aug
7
comment Securing content and login with “same” password
@user12889, can you edit your question to indicate that fact? That wasn't clear from the question. We want questions to be self-contained (people shouldn't need to read comments to understand your intent), so I hope you can edit your question.
Aug
7
answered Securing content and login with “same” password
Aug
7
comment Efficient Incremental Updates to Large Merkle Tree
Questions about Merkle trees, about cryptographic schemes for protecting integrity, or about performance optimization of cryptographic data structures are all completely on-topic for this site. bytemaster, I'm not sure why you're getting feedback suggesting that your question is out of scope (it isn't; it is perfectly within the scope of this site), but please don't be discouraged: there's nothing wrong with your question. Actually, your question is excellent and just the sort of question I'd love to see more of!
Aug
7
answered Efficient Incremental Updates to Large Merkle Tree
Aug
5
awarded  aes