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For example, for a target bitstring of 100 bits, I cannot scan all bitstrings of 100 bits and XOR each with the target, hoping to recover the message. This approach will produce all messages that can be expressed with 100 bits. That's not the reason why one-time-pads are considered secure. The reason is that even if you try all possible keys that you get ...

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You asked for the practical impact, so the answer is that for \$120 I could probably have your entire password database done by tomorrow. Here is your program, or something similar to it: using System; using System.Text; using System.Security.Cryptography; class Program { static void Main(string[] args) { byte[] pwd = new byte; ... 39 1 - How feasible is it that the chip's manufacturer can predict the output of this PRNG when it passed tests from the people applying the use of this RdRand instruction in kernels? A strong stream cipher's output is random and unpredictable to anyone not knowing the key. See where this is heading? Just because something looks random doesn't mean it's random.... 37 "PRNG" means "Pseudorandom Number Generator" which means that a sequence of numbers (bits, bytes...) is produced from an algorithm which looks random, but is in fact deterministic (the sequence is generated from some unknown internal state), hence pseudorandom. Such pseudorandomness can be cryptographically secure, or not. It is cryptographically secure if ... 35 The title of this article is complete hype. Tip: when a journalist says “X could solve Y”, read “X probably won't solve Y”. Much of the content of the article is hype too. Cryptography has a random number problem, but the problem is not producing random numbers, and the proposal in this article wouldn't be useful to produce random numbers anyway. ... 33 The ISO/IEC 9899:1990 edition of the C standard contains: EXAMPLE The following functions define a portable implementation of rand and srand. static unsigned long int next = 1; int rand(void) // RAND_MAX assumed to be 32767 { next = next * 1103515245 + 12345; return (unsigned int)(next/65536) % 32768; } void srand(unsigned int ... 29 What are the criteria that make an RNG cryptographically secure? In short, a DRBG [deterministic random bit generator] is formally considered computationally secure if a computationally-limited attacker has no advantage in distinguishing it from a truly random source. What does this mean? Given a DRBG F and a truly random oracle G, let A be a probabilistic ... 26 A True Random Number Generator uses a physical phenomenon not known to be fully deterministic as origin of the discrete values (bits or integer numbers) that it outputs. That phenomenon can for example be a dice throw, thermal noise, disintegration of a radioactive substance… What detects this phenomenon can be followed by a conditioning stage to turn the ... 26 From what you have described, it sounds like your system works as follows: Consult the system clock to find a 32-bit seed$s$. Use System.Random to generate a passphrase$p = G(s)$. (Here$G$is shorthand for whatever computation happens inside System.Random.) Hash the passphrase with PBKDF(2?) into output$x = H(p, \sigma)$, where$\sigma$is a salt known ... 24 To begin with, your definition of perfect secrecy is non-standard. The standard definition is given in an excellent answer to the question how is the OTP perfectly secure?. Essentially, perfect secrecy means that observing the ciphertext does not affect the relative likelihoods of various plaintexts under the unknown key. So the fact that different ... 22 How many hex digits do I need to compare when manually checking hash functions? If you actually want the full security guarantees of the hash function to apply: all of them. I usually just look at the first/last 5 or 6 hex digits and call it good enough. This effectively reduces the security of the hash function to that of one that only outputs 10-... 20 I would characterize the service as similar to a trusted time-stamping service. Except they do not do the time-stamping, but just provide the "key". This allows a user to decide what do to with it, such as using it as a private key to sign something, or an HMAC key, proving the signature is "not older" than the timestamp. If the signature is published to a ... 20 What you are suggesting is not a good idea for a general purpose random number generator. It could be meaningful for very specific use cases if you need a random number generator whose output can be verified independently by a third party. Even in those cases there are other sources of entropy which are potentially more suitable. The oldest mention of this ... 19 I will answer considering Linux OS, as being one of most popular Unix-like OS (between OSes which have urandom). If you need other OS, please, inform me. Also I will answer using source code of random.c driver from Linux 3.3.3 Kernel, because it is one of best documentation of /dev/random mechanics. And the other is paper: Analysis of the Linux Random Number ... 18 The official documentation for System.Random explicitly says it should not be used for generating passwords. It’s predictable, and seeded only from the system clock. This means System.Random has at most 20 bits of entropy to anyone who has a clock accurate to within a second. Indeed, try creating two new instances in quick succession on different threads; ... 16 Evaluating a TRNG device positively requires knowing its structure, both to evaluate the actual amount of entropy it produces, and the possibility to detect a field failure. Some devices sold as TRNG are in fact a TRNG subsystem followed by a PRNG, which produces the output of the device. In that case, if the PRNG is any good, the output of the device may ... 16 Randomness is not a property of strings of bits (or characters of any sort). Rather it is a property of the process that generates those strings. However, it is convenient to conflate the string with the thing that produced the string, and thus to speak about strings being “random” or “not random”. The string 00000, for example, is random if it was the ... 16 What kind of numbers are needed for cryptography/security? Are those integers? Bits. Simply have your TRNG generate random bits. As mentioned in the other answer, the only difference between bits/hex/integers/etc is in the formatting and representation. It is almost certainly more appropriate and simpler to generate random bits than it is to rely on some ... 15 Have you heard of the strange story of Dual_EC_DRBG? A random number generator suggested and endorsed by the government that exhibits some very suspicious properties. http://www.schneier.com/blog/archives/2007/11/the_strange_sto.html From that article: This is how it works: There are a bunch of constants -- fixed numbers -- in the standard used to ... 15 The key element in the definition of a PRG is the observer (aka distinguisher, algorithm, test, etc) that the PRG is supposed to fool. A statistical PRG fools a specific set of observers, whereas a cryptographic PRG fools all efficient observers. This strong definition is essential for cryptography:: The only assumption the designer should make about the ... 15 We simply strive for crypto that's as close as possible to ideal. Indistinguishably is the strongest property we can demand from a PRNG/streamcipher. It's hard to predict which non ideal properties will lead to problems at some point in the future. For example the non ideal properties that lead to padding oracles, BEAST, CRIME or the RC4 biases were known ... 15 I tried to use mostly non-jargon beyond what the question already mentioned, to keep the answer understandable. What are the criteria that make an RNG cryptographically secure? From en.wikipedia.org/wiki/CSPRNG: there may be no algorithm that predicts the next output with anything better than guessing. If you can say "50.0001% chance the next bit is ... 15 How are you going to decide which tweet to use? Randomly? This quickly leads to a chicken / egg problem. What if the chosen tweet is one word? That would not add a lot of entropy. What if twitter is unavailable? Are you just stopping your service that relies on the entropy or are you going to continue regardless? How are you going to keep the chosen tweet ... 14 No, that would not be a true RNG, because these physics engines would just repeat the exact same calculation and thus repeat the whole sequence of random numbers - like a PRNG. The starting conditions are the seed of this PRNG. Dice are truly random in the real world. Well, are they? If we ignore quantum effects, we could measure all relevant values of the ... 14 Assuming that$b = 2^k-1$for some positive integer$k$, XORing two (or more) numbers in the range$[0,b]\$ will indeed yield a number in the same range. If the numbers are random, uniformly distributed over the range and independent, then the result will also be random and uniformly distributed. In fact, we can even prove a stronger result saying that if ...

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There are three general solutions to the non-duplicate random number problem: If you want a few numbers from a large range then pick one and reject it if it is a duplicate. If the range is large, then this won't cause too many repeated attempts. If you want a lot of numbers from a small range, then set out all the numbers in an array and shuffle the array. ...

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Say I hashed the output from a random number generator (with nonce), would the resulting SHA256 hash be as random as the inputted number? Let's suppose you flipped a perfectly fair coin. You flip it 1024 times to create a bit string of 1024-bits. Because the coin is perfectly fair, this means that each strings of 0s and 1s will appear with precisely the ...

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I'll try a practical example: I trade stocks. Instructions to my broker use a simple Caesar shift cipher, but the shift varies by values in a one-time encryption pad. Common 8-char instructions include: "buy more" "sell all" and "short it". You intercept an instruction to my broker: "AAAAAAAA" What is my instruction? Buy, sell, or short?

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If you control both ends as well as the transfer channel - for example if you are transferring a large file between two of your own computers via a USB drive - then it's OK to only verify the hash superficially. In fact as you are just checking integrity, you wouldn't even need a cryptographically secure hash function, a CRC should suffice. If you fear that ...

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