# Tagged Questions

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

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### If you hashed a hash an infinite number of times would you end up with a unique hash?

If you took a hashing algorithm for example MD5 and repeatedly passed the output hash back into the algorithm an arbitrarily large number of times would you eventually end up with one unique hash? My ...
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### Has human-generated entropy ever been a real problem?

I know the topic of human brains generating random numbers has been discussed here and in Cognitive Sciences before, but I am interested in a particular part of it: In an introductory book to ...
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### Why perfect secrecy can be ensured when a plain message and a cipher-text based on one-time pad are correlated?

First, some well-known results are as follows. For random variables $X$ and $Y$, we have $$H(X,Y) \leq H(X) + H(Y).$$ The equality is achieved when $X$ and $Y$ are independent. Second, in one-time ...
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### Errata for NIST SP 800-63

I'm looking through the latest version of NIST SP 800-63, and there is a table (Table A.1) in the Appendix that supposed give the "entropy" of a password for a 94 character alphabet for various ...
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### Randomness in generation of Virtual Machines from pre-built Image [closed]

I am studying Multi-Tenancy vs. Virtualization, and I got thinking about randomness, true randomness, virtualization etc, and I couldn't find anything on it. From what I gather, ...
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### The effect of truncated hash on entropy

Suppose I have a 128-bit random binary string (128 bits of entropy), then I hash it using SHA-256, then I take the first 128 bits of the output hash. Does the taken bit string still have (almost) 128 ...
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### Should I remove these use cases of MD5/SHA1 from my program?

I have a web app (a register and login system) written already but i am now improving it. One of its design goals was high security. I have some use cases of md5 and sha1 in it and now i am in doubt ...
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### Is this JS library using openssl genrsa -rand improperly?

pem, a JS wrapper around openssl, has the following code in it: var params = ['genrsa', '-rand', '/var/log/mail:/var/log/messages', keyBitsize ]; Am ...
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### How to accurately calculate Unicity Distance for English?

The Unicity Distance for the DES cipher is around $8.6$ characters, and can be calculated using the $U=H(k)/D$ formula, where $D = R - r$, and where $R = 8$ is the number of bits in a byte (ASCII is 7 ...
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### Noisiest RF band for random number generation

I've been looking into the difference between PRNGs and proper RNG techniques. One that I particularly like is the idea of tuning a radio to a certain frequency and bandwidth and just listening to the ...
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### Is there a kind of OS entropy pool on Windows systems? [closed]

I am wondering if Windows provides an equivalent of /dev/urandom from linux ? Or any kind of system entropy pool ?
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### Entropy in relation to computer security [closed]

I am going over some lecture notes and my lecturer is off sick, so not sure where to turn. There is this equation that I cannot understand as it's given with a couple of examples. I was hoping ...
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### How many bits of entropy would I get from 300 coin tosses? [closed]

Bonus question, what is the formula to calculate this?
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### How secure is a pronounceable password in terms of entropy?

There are some strong studies which support the use of pronounceable passwords and multiple tools which provide generation of such passwords. According to this question the entropy of a password ...
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### How do you test randomness?

Suppose I receive a list of 1 million coinflips, and I want to know how likely it is that the list was randomly generated. My first thought would be to count the number of heads and tails, which ...
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### CSPRNG's (ELI5) What's the purpose of multiple seeds/ regenerating sources of entropy (i.e. /dev/random)?

Why is there a need to generate more than one high entropy seed? It would seem like if the original source of entropy is high quality, than some 512 bits should be more than enough to seed a CSPRNG ...
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### How to calculate the entropy of passwords? [duplicate]

Here is a cartoon about password entropy. http://xkcd.com/936/ I dont quite understand how the entropy is calculated in the cartoon assuming they are calculate correctly. But in general, I dont have ...
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### Entropy Loss of KDFs

Am I losing any entropy when using KDFs, such as the ones in NIST Special Publication 800-108? For example, can I derive 128-bit session keys from a (uniformly random) 128-bit master key?
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### Entropy of two concatenated random values

In this Intel blog posting, the author claims: The amount of work required to brute-force predict a random value that has n bits of entropy is $O(2^n)$. If you concatenate two values together, the ...
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### run length testing [closed]

Looking for guidance/references with/about calculations: I am designing a widget that creates N-bit binary sequences. My customers target spec for the sequence's minimum entropy is X bits/bit. I plan ...
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### Whitening and randomness extraction

The Von Neumann randomness extractor is explicitly mentioned in the randomness extractor page of Wikipedia, but strangely enough that page does not contain any reference to whitening the results of an ...
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### CSPRNG that cannot be used as random extractor

Are there any specific CSPRNG's that cannot be used as randomness extractor within a TRNG? In other words, can you simply provide enough entropy gathered from a good entropy source as seed in any ...
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### Collecting entropy: can I use timing of reading as a separate source from the reading itself?

I'm implementing an entropy gathering program. For that I collect 2 bits from two gpio-pins connected to a random source. I whiten this stream of bits using the von Neumann decorrelation algorithm. ...
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### Protocol: Coin Flipping over phone

Protocol: Coin Flipping over telephone Hypothesis: Alice and Bob have agreed: a one to one function $f$ which is easy to compute $f(x)$ from $x$ and while given any value $f(x)$ it is nearly ...
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### What is the entropy per Diceware word if a random symbol is inserted into a random position in the word?

On the Diceware page is this little gem: For extra security without adding another word, insert one special character or digit chosen at random into your passphrase... Inserting a letter at ...
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### Is it possible to get better randomness by using multiple PRNGs?

I don't know if this question even makes sense, but: Is there be any benefit or impact to some sort of hierarchical (e.g. a PRNG drives the decision for which PRN to pull from a set of PRNGs) or ...
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### Could scrypt salts contribute to entropy if passwords & salts are only stored in human memory?

I'm unsure of how exactly the salt is manipulated by scrypt and its potential contribution if any to overall password entropy if passwords & salts are stored only in human memory for a ...
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### Mixing Entropy Sources by XOR?

Assume that I have two sources of entropy (say, a hardware RNG and a CSPRNG) and that the two sources are independent (do not know anything about the internal state of the other RNG). Can I mix them ...
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### Padding to increase entropy and size of (otherwise) small JSON objects

Maybe this better fits, the security stackexchange... I'm building a web API. We want to enable some relatively simple encryption, instead of going full-blown OAuth2 (which frankly, IMO, sucks). So I ...
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### Generating Encryption Keys from Biometric Fingerprint Templates

How would one go about generating Encryption Keys from Biometric Fingerprint Templates extracted from a fingerprint using either Griaule SDK or DigitalPersona SDK or any other SDK that will return the ...
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### Why do “nothing up my sleeve numbers” have low entropy?

As a preface, forgive me for some of the links being from Wikipedia. I realize that academia frowns upon this. I came across this article about "nothing up my sleeve numbers". In it, it says: ...
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### Is AES key length a size of the key or entropy?

While educating my self I'm having hard time to understand what an 128 bit AES key actually means? Is it a key length or entropy? Please explain to me trough following example (assuming AES): ...
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### How can the key size be calculated [closed]

If the decrypting key is 448, then what is the key size? Suppose I am encrypting a plain text with a key by choosing some random numbers between ...
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### What's the entropy of the output for Tiger Encryption?

I'm tasked to apply tiger encryption to my string(key). Does anyone have any idea how to find the entropy of the hex output by tiger encryption?
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### Will hashing over and over eventually give the same hash?

There are many cases in maths where doing the same thing over and over again will bring you closer to a value. Derivation of polynomials is the striking example: no matter what you start with, you end ...
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### Entropy when iterating cryptographic hash functions

Consider a cryptographic hash function that maps $n$-bit strings to $n$-bit strings:  \DeclareMathOperator{\H}{H} \DeclareMathOperator{\SHA}{SHA-256} \H(x) : \left\{0,1\right\}^{n} \mapsto \left\{0,...
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### Trying to calculate the strength of a Bitcoin brain wallet created with a custom diceware password

I want to create a brain wallet using a custom diceware since I don't like the original one. Instead of 7776 words it has 46656 because I use 6 dice for each word instead of 5, and also it only has ...
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### Entropy test for AES Key Schedule

According to the article "Lest We Remember: Cold Boot Attacks on Encryption Keys" there is a quick and dirty entropy test that can help to find possible AES Key Schedules in memory dumps. Although ...
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### Entropy of Androids motion-sensor data

My app on Android uses the built-in motion sensors to collect seed-data for the PRNG (java.security.SecureRandom) that is later to be used for key-generation. The user is told to shake the device so ...
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### Standard or guidance for entropy collection

I need to write some high-level security requirements which include a random number generator suitable for use in cryptography. I don't need something very formal, this isn't a security certification ...
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### Seed / reseed DRBG too often?

There are limits to how much you can use a the DRBG before you have to reseed it, but are there any limits to how much you must use it before reseeding? Take the following scenarios as an example… ...
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### How does converting character set affect hash entropy?

Does mapping a large character-set hash onto a small character-set hash lower entropy for a fixed size substring? I'm writing a python app that involves generating passwords for websites. During the ...
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### Collecting entropy from a stream to a file

I have a potentially infinite stream of data originating in some physical phenomena that may expose random behavior (due to quantum effects, thermal noise, the butterfly effect in chaotic dynamical ...
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### Hash 22 byte secret into 512 bit hash: Full entropy?

Does it make sense to hash a 22 byte secret (which may not have full entropy since it is chosen by the user) into a 512 bit hash (using Keccak-512)? More precisely, will the 512 bit hash have full ...
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### /dev/random only needed for key generation / CPU pattern encryption

Consider the following scenarios: using OpenSSL to encrypt a file with Rijndael-256 using LUKS to encrypt a hard-drive that is used every day Exactly when is entropy from ...
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### Is there a way to generate sound one-time pads out of public information?

Is there a way to 'mangle' a public data-source (for example, the current date in YYYYMMDD or the top New York Times headline) to form a one-time pad that will sufficiently hide the pad's source? ...
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### Deterministically combine more than one source of entropy

What is the proper/canonical way to do this? For example, $0 < r1 < 1$ and $0 < r2 < 1$. Presuming uniformly distributed probabilities for the two, combining/averaging them is going to ...
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### Random Number Generation with a Entropy pool versus Seed

To give a little background I'm currently implementing a crypto library in JavaScript. I have started porting across Linux's random number generator because its both publicly scrutinized, it is open-...
What happens to entropy after hashing? Suppose you have a key with entropy $k$. Can entropy $k$ be increased by hashing the key?