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Mar
20
comment Does the SHA hash function always generate a fixed length hash?
@CodeInChaos perhaps I should add extra explanation, but, hash functions essentially eat blocks of data and give you a bit string output. Each bit string maps to a particular combination of input blocks (mapped so by the hash function). That's all there is to it. When you look at the size of the number of unique such combinations you can have ($2^256$ in this case) and the fact these unique combinations map to whole amounts of blocks, you realise there's a huge amount of potential inputs.
Mar
20
comment john the ripper, usages
I'm closing this as off topic for the moment - we don't really handle "what can I do with this tool" - for topics that are on topic, have a nosey at the faq. I've talked to some potential sites about migration; if you have a specific problem you you're interested in understanding if John the Ripper can help with, do feel free to edit the question and flag it and then they'd be happy to have it - there's an edit link under your question and a flag one too :)
Mar
10
comment MD5 encryption in Windows batch script
The best place for this sort of question is super user - I'd move it over there but they already have a question that might help. If it doesn't, I can always migrate this one; however, they may close it there. Feel free to respond in comments if you need migration :)
Mar
8
comment Shannon entropy calculation: is $H(A|R·A) = H(A)$?
Moderator note: please don't cross post near identical questions; if your question isn't being answered you can always flag it for migration if you intend to ask the same one. It is acceptable to cross post questions where you're looking for differing points of view from different communities, but this way we just end up moving the Q to one site or the other :) In this case, don't worry about it too much, I've contacted the maths mods and we'll move/merge one of the Qs.
Mar
8
comment Does a public key absolutely need to be used to initiate an encrypted session?
Hello, welcome to crypto :) Can I make a suggestion - would some of your queries be better off as separate questions? E.g. 2/3 might suit on security.se - they can and do regularly handle what-should-I-do implementation questions. There is absolutely no problem with you asking multiple questions if your problem needs it; and there's no problem you linking to one question from another, either. I just think it might help to focus the answers, creating separate questions :) You can always edit your questions at any time, by the way.
Mar
7
comment Does security under ROM imply exactly what?
I'm passing through very quickly, so haven't read your Q fully, but if you've got two separate questions you are more than welcome to ask them both as separate questions. I know most forums dislike that; we however prefer it :) Anyway, up to you, don't take this comment as meaning you have to (it might be best to keep them as one) but just so you know you can, if you want to - and welcome to crypto :)
Feb
23
comment How do public key rings work in cryptographic applications?
Hello, welcome to crypto.se :) Just a friendly note from a mod - your original title was "what are some advantages...". Normally, we see titles like that and expect a poor question (they're usually asking things that are off topic) but you had a good one underneath! My advice: don't be afraid to make your question titles specific to what you're asking - that way people are much more likely to read it and answer :) Whilst I'm here, there's an edit link under the question if you want to improve on my title - feel free to do so.
Feb
1
comment Why RSA encryption key is based on modulo $\varphi(n)$ rather than modulo $n$
@poncho ah of course. I'll edit that in. I was focusing too hard on my explanation and missed the obvious. Thanks :)
Feb
1
comment How do I correctly derive a Key and IV?
Rfc2898DeriveBytes generates your key byte array and itself takes a salt - for generating the salt you'd need a CSPRNG - like this: stackoverflow.com/questions/1668353/…. You can use the same CSPRNG to generate your IV bytes - they simply need to be random - we might have a question already about deriving IV and keys from the output of PBKDF2 but that's stretching my personal expertise a bit.
Feb
1
comment How do I correctly derive a Key and IV?
@JohnBubriski Ah, I'm thinking about a system where you have a message to send somewhere. In this case yes, since the "message" is in fact an entry in a database, you'd need to store them both. Now suppose you were going to store a list of holidays someone has made, beginning with a date. These are different messages and need different IVs (unless you encrypt the whole list in one go) so that an attacker cannot, by persuading you to insert dates they choose, derive any information about the plaintext or key.
Feb
1
comment How do I correctly derive a Key and IV?
@JohnBubriski Not quite. The IV needs to not predictable between messages. You can (and indeed have to, to recover the first block) send the IV with the message, so there isn't a requirement to store it, just that it needs to be unpredictable. The salt you'll need to store somewhere for key generation.
Jan
31
comment How do I correctly derive a Key and IV?
@fgrieu true - the "birthday problem". Not sure what the OP can do to mitigate that, unless (s)he can enforce uniqueness by storing IVs centrally. Which might well come with its own set of risks. Any ideas for improvement?
Jan
30
comment Can you help me understand what a cryptographic “salt” is?
I've edited your question to encourage explanations that should help, and the inclusion of references to help you out. I would however be inclined to take Jalaj's advice - definitely have a read around. If you can include in your questions what you're not understanding, you'll get a lot more out of the site. It is completely OK to quote the relevant parts of a reference you're not understanding and ask for clarification - in fact, that's better. The more precise you are about what you don't understand, the more we can help you :) Do feel free to edit in anything you specifically want covered.
Jan
26
comment How can I avoid calculating with huge numbers when implementing the RSA algorithm
@CodeInChaos (S)He does - and I'd expect most bignum libraries would provide such exponentiation methods - for example mpz_powm(). It wouldn't be a massively efficient implementation if it didn't :) More generally, bignum libraries usually provide built-in methods for doing many number-theoretic operations because building such algorithms on top of their primitive operations is inefficient and there are usually shortcuts you can take.
Jan
13
comment What is the most secure hand cipher?
I'd tend to agree with @mikeazo that this question seems almost identical to the previous question in terms of likely answers. Would it be worth clarifying (via an edit) in what way that question doesn't meet your needs? There should be a link under your question for editing, just beneath the classical cipher tag.
Jan
12
comment Two mutually untrusted parties want to exchange data: how to ensure each one gets the data it needs?
Hi @David, I merged the duplicate cross-posted question into this one as they're identical and there were good answers there - however, you've answered both! Two answers isn't a problem, just letting you know so if you want to make any edits/amendments in light of the merge, you can.
Jan
12
comment Two mutually untrusted parties want to exchange data: how to ensure each one gets the data it needs?
@IlmariKaronen now done.
Jan
10
comment RSA cracking: The same message is sent to two different people problem
I know it's just an example, but to extend Thomas' point, also note the modulus itself is prime, allowing you to decrypt a single message by computing the inverse of $e$ - in other words, you don't actually need the same message to be sent to two different people in this scenario - the inverse of $e=13$ is $d=137$ and you can compute $127^{137} \equiv 10 \mod(179)$.
Jan
7
comment How can I break REDSHIRT / REDSHIRT2 encryption?
For what it's worth, whilst not originally asked here I'm reasonably happy this is an OK question. It's about a used in-the-wild algorithm, albeit not a very good algorithm by the answers given. The key part that line in the FAQ was to address questions that say: "here is some data 0x123456 0x123456 0x123456 can anyone decode it?". What this question is really asking is "can data encrypted by REDSHIRT be decrypted trivially?". That said, if anyone disagrees feel free to start a question on meta.
Jan
5
comment Cycle attack on RSA
@EmilioFerrucci well the order of $e$ will be $k$ since $e^k = 1$ is the solution you're trying to find. $e$ can't be a subgroup on its own as it doesn't have an identity element wrt multiplication. I think you might be thinking of the notes here - finding $k$ is at least as hard as factoring unless $ord(\mathbb{Z}_{\phi(n)}^{\times})$ is known. Of course, finding that order is going to be hard in and of itself - you don't know how the group is generated in the first place (remember $\phi(n)$ is (part of the) the private key).