# All Questions

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### ElGamal in $Z^*_{p^n}$

If $p$ is an odd prime and $n$ natural,it is known that the group $Z^*_{p^n}$ is cyclic.Explain why the selection-choice of the group $Z^*_{{3^{1000}}}$ for the construction of a cryptosystem ElGamal ...
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### What are the practical implications of ciphertext distinguishability?

Commonly there are four ways to "break" a secrecy-focused cryptosystem: Recover the secret key Recover the message Distinguish an encryption from random noise Distinguish the encryption of two ...
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### How AES treat string input to encrypt data and what will happen if the cipher key is less than that expected?

The official docs of AES have several example of how hexadecimal or byte input can be encrypted. I have some problem about the input text and cipher key. My question is: How can encrypt string input ...
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What is the advantage of AEAD ciphers? Why is the TLS working group pushing for them? I thought modern cipher suites require SHA256 for authentication. What advantage is there to including Poly1305? ...
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### Cryptanalysis on two time pad ciphers? [duplicate]

One time pad ciphers are unbreakable if the message is encrypted correctly and if only used once. However if used more than once, it becomes decipherable. How come? Also, would it be smart to use one ...
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### Let $F$ be a PRF, is the following $G(x)=F_{0…0}(x)∥F_{0…1}(x)|| … ||F_{1…1}(x)$ a PRG?

$F$ is a PRF with $n$ bit key, input, and output length. I know the answer is "NO it isn't!", but I can't understand why. Is that because on any input $x$, the keys for every $F$ in the concatenation ...
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### How to verify perfect secrecy of a cryptosystem? [closed]

I’m playing with a self-made encryption algorithm: “a new approach to data encryption” by Suhail Yazji (video available at youtube.com/watch?v=h1kABn5dfXc). How can I prove my algorithm to be ...
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### Present an attack for the combination of OTP and textbook RSA

Let $N=p \times q$ where $p$ and $q$ are two primes. Compute $\Phi (N) = (p-1)(q-1)$. Choose $e$ s.t. $1 < e < \Phi(N)$ and $gcd(e,\Phi(N))=1$. Compute $d = e^{-1}(mod \ \Phi(N))$. ...
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### Prove that certain amount of data was stored

I'm looking for a way to prove that a certain amount of data was stored, through some easily verifiable piece of information. Similarly to how proof-of-work can prove through a hash that a certain ...
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### Can we solve the Hidden Number Problem in $GF(2^n)$?

Is it possible to solve the Hidden Number Problem in extension fields? In particular in $GF(2^n)$? Let's suppose an attacker knows some least/most significant bits of $r_i = a_i \times k$ in a given ...
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### Knowing that $G(s)$ is a PRG, is the following construction $G'(s) = G(s||0)$ a PRG?

I know there already is a very similar question. However, I can't understand why $G'(s)$ is not a PRG. If $G(s)$ is a PRG, then why $G(s||0)$ can't also be a PRG? How can the distribution over ...
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### Implementation of digit serial rotators for SIMON cipher?

I am working on architecture of SIMON cipher where I have to use either bit serial, parallel or digit serial to implement the algorithm for each round function and also for sub-key generation. So I ...
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### Using AES for smaller blocks in a Feistel network

I was wondering how decryption would work for FPE in the case of cycled Feistel ciphers. I understand that simply reiterating through the networks with reversed key order will generate the ciphertext ...
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### What are the implications of a birthday attack on a HMAC?

After collecting approximately $2^{n/2}$ message-tag pairs a collision can be observed. So two different messages (m1 and m2) will have the same tag. This paper states: Then, for any string x, ...