# Tagged Questions

Entropy is a measure of information content, which can be viewed as the uncertainty in a random selection.

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### Can Shannon's entropy be used to determine if a file is encrypted?

If we have to determine a file is encrypted or not, can we use Shannon's entropy algorithm on the file? As discussed here, the entropy (in bits per byte) being closer to 0 is considered as more ...
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### Entropy of Hash function [duplicate]

How is the entropy of the output of hash functions are defined? More specifically I saw in the paper https://eprint.iacr.org/2013/383.pdf Table 3 page 24. The entropy of challenge is mentioned which ...
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### Password entropy much lower than entropy of encryption keys. Why is this acceptable?

When talking symmetric encryption, a 56 bit key is known to be so weak. If you use it for your encryption, you are considered a goner. When talking passwords however, the standard these days is ...
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### How can a poor RNG impact security?

I have read this article and I do somehow "understand" that the reduction of entropy makes it easier to guess. But how exactly can the security be reduced by that? Can you give a concrete scenario, ...
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### Updating the seed key of a PRNG while maintaining initial entropy?

Consider a seed key ($n$-bit long) is completely random and entropy of random variable is $n$ and PRNG is using deterministic algorithm and that that deterministic algorithm does not add any entropy ...
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### Turing's (still?) classified inference engine algorithm?

Does anyone know the algorithm used by Turing's Colossus inference engine, so highly classified that the Brits kept it secret for decades after WW II? Indeed, it may still be classified. Several ...
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### How to prove a symmetric encryption scheme provides perfect secrecy?

I learned in class that in order to achieve perfect secrecy, the source of the plaintext $\mathcal{P}$ needs to be independent from the source of the encryption key $\mathcal{K}$. We also learned that ...
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### autocorrelation and entropy of historical ciphers

I'm learning cryptography and come across historical ciphers such as vigenere cipher,hill cipher,random substitution and etc.I'm using cryptool for learning purposes and I have a question in mind.I've ...
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### Why does AES ciphertext entropy always close to 8?

I have three binary files: 31744 byte length (1), 5712980 byte length (2) and 10806008 byte length (3). The alphabet is all 256 symbols from 00 to ff. I compute the shannon entropy, replacing ...
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### Can we bound adversarial power in terms of total entropy in universe? [duplicate]

The strengths of various cryptographic primitives are often explained (though not defined) in terms of how long it would take to break under the current best known attacks. For example, apparently the ...
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### How to calculate min entropy of the output of PRNG?

As min entropy of a random variable provides the maximum probability of getting the desired realization of the random variable, which is the most conservative way of measuring the uncertainty of ...
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### How to calculate entropy of output of a PRNG

Suppose the entropy of seed key is $H(n)$ which is $n$-bit long and random and the output bit length of the PRNG is $2^n$, which will be used as a long running key. Though it is obvious in that case, ...
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### Entropy calculation

I calculated the entropy for "variable", "property" and their concatenation "variableproperty" (with e.g. the Shannon Entropy) and got $2.75, 2.5$ and $3.33$ respectively. These are all entropy per ...
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### Secure entropy extractor for thermal noise collected from camera input?

I have read this paper (pdf) which talks about measuring the entropy of thermal noise collected from camera input. They estimate the minimum entropy at about 4 bits per pixel. Probably estimating 1 ...
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### Is the concept of provably secure hash the same as entropy smoothing hash functions?

Is the concept of provably secure hash the same as entropy smoothing hash functions? In the tutorial Sequences of Games: A Tool for Taming Complexity in Security Proofs V. Shoup shows us a proof of ...
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### Is part of an output of secure PRNG also cryptographically secure?

Lets say I generate 192 bits using some secure PRNG. If I now take first 128 bits is it equivalent of generating 128 bits in the first place? P.S. More practical application for me is that I need a ...
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### Are there any practical ways to transfer random data securely?

The owner of the random.org service recommends that you do not use randomly generated numbers from their website in cryptographic keys. This makes sense, but it got me wondering. Is it possible to ...
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### 1 bit shannon entropy CPU instructions?

It has been proved, that the xor operator is suitable for entropy accumulation, due to its theoretical shannon entropy of 1 bit. There are six truth tables with ...
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### Is CPU timing jitter a usable entropy source?

In some virtualized environments, the only source of entropy available is CPU timing jitter. Can one get enough entropy from this source for practical uses? Also, is this secure against local side-...
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### How is the One Time Pad (OTP) perfectly secure?

The Wikipedia entry on One Time Pads (OTPs) states that if this cipher is used properly; ie, the keys are truly random and each "part" of the key is independent of every other "part", it's uncrackable,...
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### How much entropy is generated by 'random' human keystrokes?

I'm wondering about using human input as an entropy generator. We all know humans are terrible at making up random numbers or keys or passwords. But what if we let a user 'randomly' type in some ...
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### Reusing same source for single-source randomness extractor

Let $ext$ be a single-source randomness extractor which takes a $d$-bit seed and a $n$-bit source as input and produces a $m$-bit output. Suppose we have a source $X$ with min-entropy $k$. Is it ...
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### Will entropy be lost by using a DRBG?

Lets assume we have a DRGB (deterministic random bit generator) which is seeded by a good true RBG (random bit generator). Before any bit has been read from the DRBG, the entropy is clearly the number ...
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### How does generating random numbers “remove entropy from your system”?

In a previous question, I quoted the --gen-random entry in the GPG Man Pages. At the end it says: PLEASE, don’t use this command unless you know what you are ...
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### Collision likelihood from /dev/urandom for VMs using the same underlying hardware

We have multiple virtual machines sharing the same physical hardware. We use /dev/urandom for session key generation. But a question comes up: if ...
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### The benefit of PBKDF2 for big keyfile?

The man page of cryptsetup says: Whenever a passphrase is added to a LUKS header (luksAddKey, luksFormat), the user may specify how much the time the passphrase processing should consume. The ...
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### Raw /dev/random output for keymat [duplicate]

Noob question: It is my understanding that the raw output from non-blocking /dev/random (where the kernel's entropy_avail estimate is equal to or greater than the number of random bits required) ...
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### Generating unbiased numbers with a biased six sided die?

Eve takes a fair six-sided die and adds some heavy paint to the side of the die with the 6 on it. This results in a biased die that rolls a 6 with probability 2/7, and each other number (1-5) with ...
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### Has there been any research on entropy efficient information-theoretically secure PRNGs?

I have an idea for an research project, but I am first wondering if it has been researched or implemented before. I think I have a way to come up with a set of PRNG algorithms which have the ...
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### Is it possible to reverse the birthday attack calculation?

I find that for every 100 password salts in our database, we only average 94.73 distinct salt values (averaged over a total of around 18 million). Is there a way to take that observation and calculate ...
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### Is it secure to concatenate two 16-bit random numbers into a 32-bit one?

I'm working on a security application for RF modules aimed to protect a communication between an autarkic sensor and a line-powered receiver against MITM (man-in-the-middle attacks). I decided to use ...
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### What is the meaning of the overlapping region on OTP-perfect secrecy diagram?

I'm new to cryptography. I want to ask about OTP-perfect secrecy diagram like figure below: On the overlapping region (middle), notated by R(X;Y;Z). R can be calculated by I(X;Y) - I(X;Y|Z). ...
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### Detecting steganography by analysing non-significant data entropy?

I am not thinking only steganography in images, I think it is also possible to encode data for example into the length of spaces of a clear html text. I suspect, the steganography changes (elevates) ...
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### Hashing entropy with only 128 bits of internal state

I am looking to implement the Fortuna algorithm (https://www.schneier.com/fortuna.pdf) on a system with very restrictive memory constraints, using AES-128 as the underlying cipher. As the key size is ...
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### Is it okay to include relative time for true random number generation?

Similar questions have been asked regarding use of time in true random number generation, but I'd like to consider using time as just one source of entropy, complementing others. True random number ...
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### How feasible would it be to generate 300 million public key pairs in 8 hours?

Assume there is a hypothetical situation where one would need to generate hundreds of millions of public key pairs of reasonable strength in a limited time. What would be required in terms of ...
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### Testing PRNG quality from ECC public keys?

Having a large set of ECC public keys $P_i = n_iB$ on a fixed curve $E$ over a prime field, is there a way to determine if coefficients $n_i$ were generated using a bad PRNG? In other words, can a ...
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### What is the difference between Shannon entropy and saying that tossing a 6-sided die 100 times has more than 256-bit entropy?

I'm confused. I thought that tossing a 6-sided die 100 times had a greater than 256-bit entropy because $6^{99} < 2^{256} < 6^{100}$. (A similar concept appeared in this XKCD comic, where ...
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### Chopping off SHA256 entropy?

I have a NoSQL key-value database. I want to insert couple million of records to it. For the key generation, I'm, using a prefix (something like data-one-) and ...
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### Is there a secure source of entropy on a typical microcontroller?

On a device that does not have a hardware RNG, what is the best source for entropy? Some options and pitfalls floating around: Use the ADC to read something analog: can become deterministic if ...
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### Can hash functions improve a lousy PRNG?

If I have a poor PRNG, could a cryptographically secure hash function make a better output? Say the PRNG can produce as much numbers as you wish, reasonably random, but not good enough to be used in ...
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### Better lower bound on min-entropy

In “Randomness Condensers for Efficiently Samplable, Seed-Dependent Sources” by Dodis, Ristenpart, and Vadhan (PDF), I have seen the statement that: any tuple of distributions $(X,Z)$ is $ε$-close ...
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### TRNG vs PRNG - Entropy?

How much more entropy gives a Hardware Random Number Generator or True Random Number Generator compared to a Pseudo Random Number Generator? I know it depends on which TRNG is used, but I'm thinking ...
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### How to crack SHA256 only letters

I recently created a program that hashes a password using SHA256 and potentially using salt. How can I crack that if I know that the password only contains lowercase letters?
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### How many bits of entropy does an elliptic curve key of length n provide?

A FAQ for an open source project makes the claim: Indeed, an elliptic curve key of length n provides $n/2$ bits of security. I have two questions: What is the practical difference between "...
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### Meaning of entropy of a bitstring in NIST SP 800 - 90A

From NIST SP 800 - 90A from January 2012 (see http://csrc.nist.gov/publications/nistpubs/800-90A/SP800-90A.pdf), page 6: For the purposes of this Recommendation, an n-bit string is said to have ...
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### Physical entropy source: How many bits entropy per byte is sufficent?

8 bit / byte would be perfect obviously. head -c 1M /dev/urandom | ent gives ~ 7.9998 bits per byte. Now, if I have a physical entropy source, how much entropy ...
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### Is randomsound for increasing entropy pool not doing what it is supposed to do?

I am using randomsound because I thought it would be a good idea to inject some additional entropy into the entropy pools of my linux machines (clearly those machines have a corresponding sound card). ...