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seen Dec 4 at 9:17

Dec
21
comment Does this block cipher mode allow for decryption?
@poncho I see what you mean - I was probably a bit hasty on the close, sorry. Have edited your interpretation in, feel free to answer.
Dec
13
comment How do I solve this RSA instance for m?
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Dec
13
comment What is the importance of Modular arithmetic in cryptography?
I'd add that the Rijndael S-Box is designed to be the set of multiplicative inverses under a finite field $GF(2^8)$ - so whilst modular arithmetic is not strictly necessary for a symmetric cipher it certainly does find use in them.
Nov
30
comment How can I make my cipher show the avalanche effect?
I've removed comments along the lines of "don't design your own cipher" - here on crypto it's perfectly acceptable to try, although you should understand it is all at your own risk of course :) I've also edited the question a little to focus more on the avalanche effect in the absence of the relevant cipher constructions. If anyone feels that is unnecessary, feel free to roll back and or improve on what I've done.
Nov
30
comment Is it a good idea to use bitwise XOR on a set of MD5 sums?
If I understand this right - you're looking to use this method to ensure that two different aggregate values imply that the total source data remains unique? I.e. you're wondering if xoring md5 hashes will result in collisions over the data you're aggregating? If so, it might be worth adding that to the question perhaps? Just a thought - the more detail on these sort of things the better the answers, generally.
Nov
23
comment Security of simple xor and s-box cipher?
@Polynomial for generally talking about function notation, have a read of the wikipedia article. It provides an awful lot of detail. In this case, $E$ is a function that takes $K$ and $M$ and produces a result $M$, where $K$ and $M$ are the domains available to the function as opposed to specific function operations, and M is the range. Have a read of codomains for more examples of this type of notation in use.
Nov
23
comment Security of simple xor and s-box cipher?
@Polynomial Thanks. I think it now works - general question with specific example. I'm even more happy with that way than the way we had it before. Thanks for bearing with us and very sorry for the noise - but when we see a question with a lot of potential we'd rather make the effort to help it out than not.
Nov
23
comment Security of simple xor and s-box cipher?
@Polynomial I think it works with the two questions separately. I think it's perfectly fine to link them together in the question (like "I've asked about my sbox design here {link}").
Nov
23
comment Security of simple xor and s-box cipher?
As is, I'm itching to revert that edit (preferred option) or close whilst we work out how best to accommodate this. Don't get me wrong, I want to accommodate this question, I just don't think one huge open question is the way to go. Specifically, from the FAQ: "Your questions should be reasonably scoped. If you can imagine an entire book that answers your question, you’re asking too much."
Nov
23
comment Security of simple xor and s-box cipher?
Guys - I realise there's a need to discuss ciphers sometimes, but asking to review an entire cipher is very broad and likely to end up in discussion; that's why Paulo asked for it to be split up in the first place. SE is not about having discussions as a format. I think we'd broken the essential components down into two separate questions nicely. The danger is that allowing "please review my cipher" generally means we'll have 20 such questions in a week. If you want to debate this, meta is the place for it.
Nov
19
comment How does a chosen ciphertext attack work, with a simple example?
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Oct
28
comment Why would you expect to find a collision in a hash function after approximately $\sqrt{n}$ hashes?
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Oct
26
comment How can rainbow tables be used for a dictionary attack?
@ThomasPornin Ok, I'll make it more impressive...! That's not a bad way of looking at it, but I like to remember that it is possible to build a rainbow table incorporating all possible salts too (it's just more $\mathbb{P}$) it just becomes so costly we're not yet in a position to make it happen. I sort of alluded to the building cost of hashtables when mentioning slow hashing, but I didn't have any specific. Mind if I incorporate yours?
Oct
11
comment Encrypt-then-MAC Confidentiality, Integrity and Authenticity
@JohnBlack yes. In general $m$ should be the encrypted output (which may or may not include IVs depending on the protocol etc) and $k$ the MAC key. So then you calculate your MAC from the encrypted data, which prepends the MAC according to HMAC. Then to verify, the recipient can use the same encrypted data and the key - calculate the MAC and compare. If it isn't valid, they discard the message.
Oct
10
comment When is an asymmetric scheme considered broken?
@PaŭloEbermann ah, so it is!
Sep
26
comment How should one implement a delegated shared trust protocol?
I've removed all the comments under this question as I don't think there was anything positive in the discussion taking place. Just a polite reminder to everyone to keep it constructive and on-topic please.
Sep
26
comment Can a computationally unbounded adversary break any public-key encryption scheme?
Good morning everyone. So in the interests of not letting this get out of hand - here's what I think. I think it's hard to criticise constructively and not hurt feelings. That said, @D.W. I think the best course of action when you disagree with something is to use your vote (you're entitled to do that) - for minor issues (did you mean XYZ?) comments work, but otherwise propose an alternative solution. That way the better answer should be voted up more - comments have much less visibility in terms of being read.
Sep
1
comment How can we reason about the cryptographic capabilities of code-breaking agencies like the NSA or GCHQ?
@gokoon there's been a lot of support for this question, so I've re-opened it - during the discussion Paŭlo mentioned improvements were suggested - we've incorporated those. What do you think?
Aug
27
comment How does asymmetric encryption work?
@Jug True. For very simple examples this falls down fairly easily; however, the nth-root under a modulus becomes more difficult. Multiplication as a whole often has situations where division is a much more difficult concept (matrices, for example). Obviously it doesn't make a great cryptosystem, but it is a place to start to look for possible trapdoor functions compared to addition, which is usually quite simple to reverse.
Aug
26
comment If a cryptanalytic breakthrough is made, what process should be followed?
I've edited the question to make it more generic; let me know what you think (I can always roll it back, or the OP can). I think, personally, that it is a difficult case to decide if this question is in-scope, so I'll be guided by everyone else. Also, I've introduced an ethics tag which may or may not be a good idea - anyone with any better suggestions (soft-question?) please shout.