# Questions tagged [randomness]

Usage of randomness (i.e. non-predictable data, usually in the form of bits or numbers) for cryptographic purposes.

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### Does a practical collision attack on a cryptographic hash function also mean it fails “indistinguishable from random data”?

It is knowns that SHA-1 has been broken in practice using collision attacks. Related to SHA-1 this mainly had a negative impact on the security of Digital Certificate signatures Email PGP/GPG ...
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### Why does a Hashing Function produce good Random Numbers if Input is random?

I've heard about using a hash approach to create random numbers based on two independent entropy sources A, B: $h_{i} = H(a_i+b_i)$ $H$ could be for instance SHA-1, or SHA-256. My questions are: 1)...
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### Lsb as hard core predicate in a sign-hash scheme

Given a random oracle and a deterministic signature scheme. Is the following $hc(x)$ an hardcore predicate of $H(SIG_k(x))$ ? $$hc(x)=lsb(H(SIG_k(x)))$$ How would I get to think about this? I saw a ...
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### Sub exponentially hard OWF , PRF and iO

I'm currently reading the work "Obfuscation of probabilistic circuits and Applications' by Canetti Lin Tessaro and Vaikuntanathan 2015. It says sub exponentially hard OWF implies sub exponentially ...
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### Does Benford's law hold against encrypted data?

If I can assume that the output of a block cipher like AES is a PRandom distribution, can I assume that Benford's law will hold if I partition that output into integer data?
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### Can a random permutation be restored / decrypted?

Say you have a 1,000,000 random bytes file and you split it into 1000 x (1000 bytes) pieces. You then pick two random pieces and swap them writing down the position of the pieces (eg. swapped piece ...
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### Universally verifiable random beacon

I am developing a network where potentially many many nodes (billions, in principle?) should be able to agree on a random number. I would really prefer to avoid $O(N^2)$ procedures where everyone ...
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### Relationship between "Entropy" and "Security" in crypto functions

There is a lot of confusion between "entropy" and "security" of a cryptographic function. I like to visualize it as the entropy being the water, and the crypto function being a glass. So for ...
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### Does RSA_PRBG security depend on keeping seed secret?

The HAC provides a PRBG using RSA (Chapter 5 p.185): It appears the seed must be kept secret if $e$ is a well known value. If $e$ is uniformly distributed between 1 and $\phi$, then the seed does not ...
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### ISO 9564-1 format 4: do I need an IV?

ISO 9564-1 format 4 describes an extended PIN block format. Simply said, this standard can be used to encrypt 4-digit PIN codes in a secure way. The standard mentions (simplified) to add random ...
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### Is the number still a CSPRN if we only allow an interval from a CSPRNG?

tr -dc 0-9 < /dev/urandom | fold -w 3 | awk '$0>=100 &&$0<=200' What this line does: /dev/urandom (Linux) is proven to be a CSPRNG, that is ...
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### Can physical object hiding be quantified with entropy?

If it works in the virtual world, why can't the same principle work in the real world? For example if you want to hide a private key, you need a virtual haystack to hide it inside, which is the ...
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### Can a repeated random sequence be considered random?

Imagine I have a 1000000 bit sequence that is considered random ( ie the output of a TRNG). Now imagine I 'double' that sequence : I concatenate the same sequence 2 times : " my 1000000 bits "concat"...
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### Is it safe to encode an entropy this way?

I have been analyzing the source code of a software, and I came across this encoding snippet, I would like to know if it's safe to encode an entropy source this way. So the entropy source is a 132bit ...
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### How much original entropy is lost if a part of the hash is known?

You have an entropy source of n bits You hash it with SHA-512 for example k bits of the SHA-512 output are publicly revealed, ...
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### How much entropy is lost if 1 character is fixed in HMAC SHA-512?

We have an entropy source called “Entropy”. We encode this entropy with HMAC-SHA512 using a “Key”. The Key is public, but the Entropy is secret. We are then testing for the HMAC-SHA512 output in order ...
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### Hashing entropy to generate 256bit key?

I have a 256bit fully random entropy source from dices, which is probably not cryptographically secure. I have read that it's customary to take the hash of the entropy source in order to generate a ...
Say I create a 256-bit key by taking a random number between $0$ and $2^{256}-1$. Would it be cryptographically equivalent to generate $8$ numbers $r_1\dots r_8$ between $0$ and $31$, and combining ...