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 Apr21 comment Topics in Cryptography This question is too broad. You'd be better served by scanning the relevant Wikipedia articles and then asking specific, focused questions about points of confusion. Apr7 comment How to decrypt a '.enc' file that has been encrypted with RSA using a public key? You shouldn't destructively edit your posts like that. These questions are meant to serve as potentially useful references for other users. If you remove all details from your question, you destroy any existing content these future users might need. Apr5 comment How to decrypt a '.enc' file that has been encrypted with RSA using a public key? You cannot decrypt the file without the private key. Therefore, you need to obtain the private key somehow. Yet you are only given the public key. Hint: How can you find the private key given the public key? Apr2 comment Security strength of RSA in relation with the modulus size @mikemaccana: I'm glad to hear it! I'm unfortunately inactive on Twitter; the most I do is sometimes glance at other folks' pages. Apr2 comment Security strength of RSA in relation with the modulus size @mikemaccana: Indeed it does! Otherwise it would not be there. :) However, we're not sure of the value of $o(1)$ for values of $n$ that we care about. The complexity of the GNFS is heuristic. I recommend looking at this question and its answers/comments for some more discussion. Apr2 comment Security strength of RSA in relation with the modulus size @mikemaccana: The $o(1)$ is little-o notation. As $n \to \infty$, the value $o(1)$ tends toward 0. Mar29 comment Can machine learning analyze random number generator? It is not known to be impossible to formally prove that a CSPRNG is indistinguishable from random. We merely don't know how to prove it one way or another. If one way functions exist, we do have probably secure PRGs. Mar12 comment When is a cipher considered broken? I disagree that "compromised" is a good word for that scenario, because "compromised" and "broken" are really very similar concepts. I would argue that unsafe is a much better word for "broken in a practical sense". Indeed, your very answer itself explains "compromised" as "no longer considered safe to use". Jan20 comment How much (home PC) CPU time is required to generate a prime number of a given size? @K.G.: Ah, well, I did not intend for this post to really contain a serious prime-finding algorithm; rather I wanted to demonstrate that asymptotically, the task is not difficult, and real world improvements (like your suggestion) make that analysis messier. Jul4 comment Getting 88bytes cipher output from 48bytes input in AES Wow. That paper is....... poor. Like, really poor. Jun23 comment How to adjust scrypt parameters for Moore's Law? Those tests were performed in 2009, from what I remember, so Moore's law has not yet much touched those numbers. May4 comment Difference between a nonce and IV Indeed; in particular I tend to assume that, if the scheme wants a nonce, I can safely use a counter. On the other hand, IVs can sometimes break things if they're predictable (e.g., CBC mode). May1 comment How to argue to a paranoid that RSA is safe? An unreasonably paranoid point of view, perhaps. But none of those are terribly concerning or relevant: (1) we don't have a quantum computer capable of factoring anything near RSA-size semiprimes yet; (2) no, secure key sizes cannot be broken using a very large cluster by definition; otherwise they would be insecure (and we can 'simply' ramp up key size until security is met); (3) physical attacks are outside the purview of an asymmetric cryptosystem (mostly). Is RSA perfect? No - but the above are not really compelling imperfections. Mar12 comment How does the key size per data bit influence the security? It will take $2^{127}$ tries on average, not $2^{64}$. If the probability of each guess being correct is $1/2^n$, it takes $2^n / 2 = 2^{n-1}$ guesses to have a 50% chance of success. Mar9 comment Using a hash with a constant key to create easily verifiable codes Well, I have two problems with this answer: (1) it doesn't mention HMAC, which the question just screams to recommend IMO, and (2) it suggests signing a digest of the message, which should already be handled by the signature's padding scheme. Mar9 comment Finding an x such that xP = (11,44) on an elliptic curve Some folks write curve points with uppercase letters. Mar5 comment Any use for now-defunct Mt Gox Yubikey? 48 bits is not a whole lot. I don't know much about the system is setup, but could you possibly brute-force that? Feb15 comment The difference between these 4 breaking Cipher techniques? Hmm, (re)reading this answer, perhaps it would be prudent to reorder these from weakest to strongest, so that each attack builds on the previous one. (That is the typical presentation.) I think this answer might serve in the future as a useful high-level reference for the different attack types. I've noticed that the associated Wikipedia article(s) are of particularly poor quality in that department. Feb4 comment What exactly is the base for the KECCAK (SHA3) claim that a security strength of 256 bits is “post-quantum sufficient”? @figlesquidge: See this paper by Bernstein, particularly the third page. In it, he argues that the BHT quantum algorithm for collision-finding will cost more than Grover's. All thanks goes to nightcracker, who provided me this citation about a month ago. Feb3 comment Can passwords be stored securely so that a similarity comparison can be made? That's essentially the only (secure) way, as far as I can see. Suppose you had some password storage algorithm that allowed you to compute a "similarity metric" between two digests (or one digest and one input; the scenario doesn't change). Then couldn't one use the similarity metric to optimize a brute-force search? (That is: follow a path that maximizes the similarity metric.) This isn't a proof, but it does intuitively explain the problem here.