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22h
comment Can cycle finding techniques reduce the memory usage of the MitM attack against 2DES and 3DES?
Yes, it works. It's a standard trick; kudos for re-discovering it. I see no reason the table size would amount to $2^{n/2}$. The complications in working out exact parameters arise when you work out the details of (a) parallelism, (b) distinguished points, (c) the cost of memory and routing networks, to get the exact optimal tradeoff (if you care about constant factors).
1d
comment Adding two public keys
@RickyDemer, I don't know. I guess that's a different question. If you're suggesting that might be what the OP wanted, if so, he needs to specify his requirements or goals in the question. Right now the question only asks for "another, more widely used" scheme. I can only answer the question that was asked....
1d
comment Is there are code hidden in Wired Magazine's December 2014 Issue?
@jww, are you sure you've got the right site? This question is clearly off-topic for PP&CG: codegolf.stackexchange.com/help/on-topic
Apr
5
comment Will ANY prior knowledge assist in determining contents of a SHA1 hash?
@InfernusDoleo, you'll be fine, if that 26-character secret key is truly secret and chosen uniformly using a crypto-quality randomness source. That large secret key means that the space of candidate inputs is very large, so brute-force won't work. The fact that you can see many outputs with related inputs doesn't help given that the secret key is so large.
Apr
4
comment Will ANY prior knowledge assist in determining contents of a SHA1 hash?
@InfernusDoleo, I can't answer that, because it's not clear what you intend the full input to the hash to be. If you want a more detailed answer, list all the parts of the input to the hash and what is known about each. An incrementing nonce counts as a known/predictable part of the input, since it can be predicted.
Apr
4
comment Will ANY prior knowledge assist in determining contents of a SHA1 hash?
You mean knowing some of the input to SHA1, or knowing some of the output? (you talk about knowing parts of the hash, but usually when we say "the hash" we mean the output of SHA1, not the input)
Apr
1
comment How can mega store my login details and still be secure?
@tagyro, thank you for the pointer. Unfortunately, a source code repository is not a substitute for a design document that describes how the architecture addresses these threats. (That said, from xemacs' answer, it sounds like they did exactly what I speculated in my answer.) I think the links to vulnerabilities in early versions of the system are relevant to understanding the history of the system. If there are public documents that respond to that analysis and explain why they're wrong or how they've been fixed, with technical details, I'd gladly update my answer with that information.
Mar
16
comment How to check if a sequence is generated by a linear congruential generator?
What do you mean by "a sequence of numbers starting at the nth bit"?
Mar
11
comment Encrypt-then-MAC: Do I need to authenticate the IV?
This answer contains incorrect statement. 1. It's not just "safer" to MAC the IV; it's unsafe if you don't MAC the IV. 2. "although Mallory can't predict the result of the decryption in any way" - This is simply false. Moreover, I don't know what you mean by "an invalid message", but there's no guarantee that the decryption will be "invalid"; it might be a completely valid message that just happens to be something other than what the sender wanted to send (which would amount to a break of integrity).
Mar
11
comment Is the 1st Encrypted Block Less Secure Than Subsequent Ones?
We expect you to do a significant amount of self-study and research before asking, including consulting standard resources (like Wikipedia and/or standard textbooks). This helps you craft a better question, and sometimes enables you to answer your question on your own. In this case, see e.g. en.wikipedia.org/wiki/Block_cipher_mode_of_operation#CBC. Look at the diagrams there. Why do you think the first block has fewer "permutations" or a closer relationship to the original message? The diagram looks identical for the first block....
Feb
28
comment What would a backdoor in symmetric key cipher look like?
This is already covered by crypto.stackexchange.com/q/1935/351 and crypto.stackexchange.com/q/675/351 (both of which you could have found by searching in the backdoors tag).
Feb
21
comment Anonymous Gravatar Problem
Thank you for the comments. That helps me understand this much better. I encourage you to edit the question to include this extra information -- comments are transitory and can disappear.
Feb
20
comment Anonymous Gravatar Problem
This answer might benefit from a bit more explanation. Where do the ephemeral keypairs come from? (how are they generated, and who knows them?), how do they come to be known to both Gravatar and the site?, and how does this differ from CodesInChaos's previous answer?
Feb
8
comment How many bits can be safely extracted from the BBS generator at each step?
@Nova, but I did answer the question of the OP. (Perhaps you missed it?) The answer is: zero bits, for practical parameters. There is no number of bits that can be extracted safely, with provable security, for practical parameter settings. That does answer the question that was asked.
Feb
7
comment Misunderstanding Broadcast Encryption
Check out non-committing encryption. Is it what you are looking for?
Feb
1
comment How hard is it to find plaintexts whose hashes satisfy $h(a)\oplus h(b)=h(c)$?
@JohnMeacham, I'm not familiar with SWIFFT, but if it satisfies $f(a+b)=f(a)+f(b)$, I would not call it a cryptographic hash function. The question specifically asks about cryptographic hash functions (and mentions SHA256 as an example); the term "cryptographic hash function" is often understand to require that the hash is effectively pseudorandom. So I think your criticism is debateable. If you have questions about what the OP meant by "cryptographic hash function", I suggest posting a comment underneath the question to ask the original poster to clarify.
Jan
31
comment Merkle hash tree updates
@user3150164, You can have a Merkle tree that has data values in internal nodes, if you want -- it's straightforward to extend all the ideas to that case. Alternatively, it's also possible to build a balanced binary tree data structure with values only in the leaves (and not in any internal nodes); the details of that are out of scope for this site (they're more appropriate for CS.SE than Crypto.SE). This is not a discussion forum, and not a place for extended back-and-forth or an interactive tutorial. I encourage you to formulate the original question more precisely in the future.
Jan
30
comment Merkle hash tree updates
@user3150164, inserting a new leaf at the leftmost edge can be done with $O(\lg n)$ operations. For instance, if $\ell$ is the leftmost leaf, you turn it into an internal node, with two children: $\ell$, and the new value. Then, you update the hashes of the nodes on the path to the root from these two new leaves. This takes $O(\lg n)$ time and hash updates. The general answer is: look at how any standard balanced binary tree data structure (e.g., AVL trees, red-black trees, etc.) handles this case -- they all handle this case, and achieve $O(\lg n)$ time. See paragraphs 2-4 of my answer.
Jan
30
comment Merkle hash tree updates
@user3150164, I've updated my answer with further explanation of these points.
Jan
30
comment Merkle hash tree updates
So, my advice is (1) take a closer look at the literature on balanced binary trees, (2) recognize that for most practical applications, the exact structure of the tree is not essential; all that matters is the set of values at the leaves, or possibly the order they are in. (If you have an application where you need something more, such as some special requirements on the shape of the tree, then the problem becomes different -- but that's an unusual requirement whose motivation is unclear, so don't be surprised if there haven't been many, or any, published papers on it.)