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Sep
28
comment Encrypted database: how to deal with general queries?
Please try to stick to one question per question. I see two questions: (a) how does the industry deal with encrypted databases? (b) is there anything that simulates somewhat homomorphic encryption? Question (a) is too broad. Question (b) is not well-defined (what do you mean by "at least simulate somewhat") so probably not suitable for this site in its current form. What research have you done? Have you looked at CryptDB (which has seen some deployment in industry)?
Sep
28
comment Question about second preimage resistance of hash function combiner
I can't understand the question. What does your notation $H^{s_1}(x)$ mean? Does it mean repeatedly applying $H$, $s_1$ times? If so, what does the notation $H^{s_1,s_2}(x)$ mean? What's $s_1$? I suggest editing the question to clarify.
Sep
23
awarded  Taxonomist
Sep
22
comment Forward pseudorandom permutation
The question is unclear. What vectors? Where did those come from? What does it mean to have the same permutation at the server, in both vectors? I can't follow you. Try editing your question. Specify what is known to each party, what the inputs from each party are, and what the desired output to each party is. Also specify the trust model.
Sep
21
awarded  Popular Question
Sep
17
comment Are there cryptographic hash functions with homomorphic properties?
We already have questions here that provide lots of information on this subject. Simply searching on "homomorphic hash" in the search bar at the upper right turned up the following: crypto.stackexchange.com/q/6497/351, crypto.stackexchange.com/q/8074/351, crypto.stackexchange.com/q/12719/351. In the future, you might want to do a bit of research before asking, to help you ask a more informed question.
Sep
17
comment Is appending the hash of the plaintext to the end of an encrypted message sufficient to ensure integrity?
See crypto.stackexchange.com/q/16428/351 and crypto.stackexchange.com/q/9941/351 and security.stackexchange.com/q/13800/971 and crypto.stackexchange.com/q/11440/351, all of which answer your question.
Sep
17
comment On the privacy of perfect hash functions
Do you have a reference? Perfect hash functions in computer science, as normally studied, have nothing to do with cryptography and don't promise to provide any privacy or any other security properties.
Sep
8
revised “Practical” operations supported by functional encryption?
added 75 characters in body
Sep
4
comment Change Salt when Changing Password?
@SteveJessop, I confess I'm not sure if I understand all the details of your scenario. You are aware that you can't start building the rainbow table until you know the salt, and that salts and password hashes are usually not public, right? Are you assuming multiple separate breaches of the password database, and that the user has changed their password in between? That's a pretty rare scenario to target. See my answer for more details.
Sep
4
revised Change Salt when Changing Password?
added 2306 characters in body
Sep
4
comment Change Salt when Changing Password?
@BobBrown, are you assuming the password database is breached twice, at two different points in time? That is very rare.
Sep
4
comment Change Salt when Changing Password?
@otus, I suggest you clarify the attack model. The normal attack model is: at some point in time, the password database is breached. At that point in time, the attacker learns the salt and the password hash at that single point in time. Normally the salts and password hashes are not public and so the attacker cannot observe their values before or after that point in time. So even if the user changes their password many times, the attacker won't get to see more than one password hash -- the password hash at the time when the database was breached. So this doesn't help the attacker.
Sep
4
comment Change Salt when Changing Password?
@RichieFrame, there is no point in building a rainbow table to target a single account. At that point a rainbow table has no advantage over straightforward exhaustive search. What a rainbow table gets you is the ability to amortize effort across multiple accounts (to some extent). If you don't have multiple accounts, there is no amortization, and no reason to build a rainbow table. Lots of misconceptions about rainbow tables....
Sep
4
comment Change Salt when Changing Password?
@SteveJessop, if the account has a high-quality salt, a rainbow table is useless regardless. (I've noticed there's a lot of misconceptions out there about rainbow tables... They are cool, but they are not magic -- they do have limitations.) It's not correct to say that if you don't change the salt the rainbow table is useful. If your salt is unique there is absolutely no point in using a rainbow table. If the attacker knows the salt, the attacker might as well do exhaustive search; that's strictly better than a rainbow table. If the attacker doesn't know the salt, he has no hope.
Sep
4
answered Change Salt when Changing Password?
Sep
4
comment Change Salt when Changing Password?
If a different salt is used for each account, creating a rainbow table would be stupid: it won't work, and it'll be inferior to straightforward exhaustive search. Basically, rainbow tables already don't work (they don't provide any speed-up over exhaustive search) if you use random salts, even if you don't change them. So the purported advantages listed in this answer are not actually advantages of changing the salt...
Aug
31
answered Homomorphic Encryption: how does the equality test on ciphertexts work?
Aug
28
comment How does Random Oracle and Standard Model differ?
possible duplicate of What is the “Random Oracle Model” and why is it controversial?
Aug
28
comment How does Random Oracle and Standard Model differ?
What research have you done? There is lots written on this subject, both in textbooks and in Wikipedia, as well as on this site (try clicking on the random-oracle-model tag under your question to see other questions). I expect you to do a significant amount of research before asking, and to show us in your question what research you've done, and to use the research to frame a narrower question. There is little point in us repeating the information that is already widely available on the Internet. It looks like you haven't done that, so your question isn't a good fit for this site.