Questions tagged [elgamal-encryption]
ElGamal is a public key encryption scheme with security based on the discrete logarithm problem.
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Diffie-Hellman private key recover with non-prime modulus
Say we have a classic Diffie-Hellman key exchange. We have the following parameters of a public key:
p,g,y
Where $p$ is the modulus, $g$ is the base, $y$ is the ...
3
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1answer
86 views
Elgamal problem on $\mathbb{QR}_p$ with $p$ a safe prime
I need some orientation to solve the following problem:
Let $p = 2q+1$ be a safe prime and $s(x)$ the smallest of the two square roots of $x$ modulo $p$. Then:
Determine the distribution of $s(g^{ab}...
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1answer
22 views
Give a CPA-attack on Elgamal when used with $\mathbb{Z}_p^*$
We know that if the decisional Diffie-Hellmann problem is hard then the Elgamal encryption is CPA-secure. One show that if the chosen group is $\mathbb{Z}_p^*$ then the decisional Diffie-Hellman ...
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0answers
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How to decrypt plain text x using man in the middle attack
Suppose Alice want to share a plain text $x = 5$ to Bob; using RSA, DHKE, and El-Gamal.
My question is if Man-in-the-middle attack is applied then weather oscar can compute/decrypt plain text in all ...
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1answer
42 views
Simplified ECC non-point public key example, how it works
I know that ECC public key is in fact point on curve calculated by $(x,y) = k \times G$ , while $k$ is random and $G$ is the base point, it performs "Point addition" which involves some math behind.
...
2
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1answer
34 views
ElGamal variant
I came across this problem, while currently reading "Introduction to Modern Cryptography" by Katz and Lindell.
I am new to crypto and just trying to go through the book and solve the exercises, ...
2
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1answer
76 views
Can I perform a division of two integers homomorphically using ElGamal?
How can I perform a division of two integers homomorphically?
(Simplifying assumptions can be made if needed, that is, I am fine with dividing numbers that are whole and the result will be whole as ...
2
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0answers
57 views
El-Gamal like encryption, how can i guess the key?
There is $p$ (large prime number), $g$ (generator of $p$),
and $x_a$, $x_b$, $r$ (which are between $1$ and $p-1$).
I know $g^{x_a}$, $g^{x_b}$, $g^r$, $g^{x_a\cdot x_b\cdot r} \bmod p$.
How can I ...
2
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1answer
101 views
Verification of encryption for ElGamal cryptosystem
Suppose Alice wants to convince Bob that ciphertext c = (a, b) = (gr, m*hr) is some properly encrypted plaintext (not just random numbers). Obviously she can use zk-proof of discrete logarithm ...
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73 views
Use of randomness in an Elgamal like encryption
Suppose I have the following encryption scheme: for a message $m\in\mathbb{F}_p^*$, I generate the ciphertext = $(g^r,f^mh^r)$ where $g$ is the generator of a cyclic group $G$ of unknown order $n$ and ...
2
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1answer
48 views
Why $\alpha$ in ElGamal cryptosystem has to be a primitive root? [duplicate]
When choosing the public key for ElGamal, $\alpha$ must be chosen as a primitive mod p. What if $\alpha$ is not a primitive root ? How will it influence the encryption and decryption ?
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1answer
81 views
Does this scheme break under a DHP adversary?
Consider the following cryptography scheme:
We are given a prime $p$.
Alice wants to send the plaintext $m \pmod p$ to Bob.
Alice chooses $a$ s.t. $\text{gcd}(a,p-1)=1$, and sends Bob, $u = m^a \...
2
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1answer
54 views
Usefulness of OAEP with ECC
Does OAEP make sense for use in an ECC ElGamal cryptosystem? The way I see it, OAEP makes questionable sense even for RSA because even though it's a "all or nothing" transformation, many RSA ...
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1answer
31 views
Given paramaters of an Edward's curve and x, determine a y value if it exists
I'm making a demonstration cryptosystem using ECC ElGamal. I've currently got a working implementation of Edward's Curve operations and a basic ElGamal implementation (Encrypts only points on the ...
3
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1answer
125 views
How to securely map messages to points on an elliptic curve
I'm implementing a demonstration hybrid cryptosystem in Python (FinCrypt, I know the name is bad) and I'm migrating over from my Weierstrass curve implementation, which was based off of this, to one ...
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0answers
106 views
Chinese Remainder Theorem and Elgamal
I am studying an encryption scheme which is Elgamal-like where I think CRT can help optimise the encryption and decryption but I am not sure if I am applying CRT the correct way.
I have a cyclic ...
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1answer
81 views
Is this problem still as hard as discrete logarithm (modified ElGamal)?
I am trying to find a vulnerability or proof for the following problem:
ElGamal part.
Given $g\in\mathbb Z_p$ where $g$ generates $\mathbb Z_p^\star$, select randomly $k\in\mathbb Z_p$ and calculate ...
1
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1answer
63 views
For ElGamal-based key encapsulation, is it necessary to hash before using as AES key?
I'm working with SJCL, specifically using ElGamal to encrypt messages. Behind the scenes, this is doing something similar to what's described in this SO post (emphasis mine):
Regardless how big ...
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0answers
132 views
Indistinguishability vs fixed bit-length
Suppose there is a cyclic group $\mathbb{G}$ of prime order $q$ of elements in $Z^{∗}_p$ with a generator $g$ and values $a,b,c,d \in Z_q$.
There is also an equation $g^{ab} = g^{cd}$, where $a,b,c$ ...
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1answer
136 views
What is an example of elgamal homomorphic double encryption?
I am looking for such an cipher algorithm that is based on assymetric cryptography where the order of encrypting is irrelevant.
For example:
There is a message "x", Bob encrypt that message with his ...
3
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3answers
293 views
Zero-knowledge transfer of value protocol inspired by EC El Gamal
This is a follow up on the question I asked here. I designed a scheme that allows the following:
Alice has a value $a$ which she wants to keep secret
Bob has a value $b$ which he wants to keep secret
...
2
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0answers
106 views
Is EC El Gamal the only option?
This question is related to the question I asked here. I'm looking for encryption scheme with the following properties:
Given $m$ is a 256-bit value, $pub_a$ and $pub_b$ are public keys, and $priv_a$ ...
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1answer
124 views
Commutative homomorphic encryption for zero-knowledge transfers
I am trying to design a scheme that would allow the following:
Alice has a number $a$ which she wants to keep secret
Bob has a number $b$ which he wants to keep secret
Alice can "transfer" a number ...
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0answers
89 views
How trapdoor key exposure affects security of trapdoor-based simulatable NIZKs?
I wonder if revealing a trapdoor key would help the verifier to distinguish real and fake proofs in NIZKs that are based on substituting the challenge with the result of a chameleon hash.
I don't ...
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1answer
174 views
How to encode messages in $\Bbb Z_p^*$ to be encrypted with ElGamal scheme?
In ElGamal encryption scheme, in order to achieve IND-CPA security, one must use a group where the DDH problem is assumed to be hard. As this answer suggests, one way to achieve that is the following:
...
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1answer
196 views
ElGamal ciphertext lenght
I'm studying for an exam and answering practice questions and I would love clarification on something. Apologies if it seems really simple.
My lecture notes indicate that for ElGamal:
the ...
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1answer
94 views
Non interactive ZKP that encrypted additive elgamal message is in set of valid messages
This is taken from Anonymous voting by two-round
public discussion by F. Hao P.Y.A. Ryan P. Zielinski
Votes are encrypted using the additive variant of ElGamal. I am also using ECElgamal. The idea is ...
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2answers
59 views
Implementing commitments and challenges for a distributed ElGamal encryption scheme
I am trying to implement a distributed encryption system by having as a main source of information this book (Introduction to Cryptography by Delfs and Knebl) and this Internet article (More Mix than ...
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Is it possible to re-cipher with Paillier?
I know that with ElGamal we can re-cipher and get a second ciphertext equal to the first.
Is it possible with Paillier too?
When saying "re-cipher", I mean "A sends me a message, that is encrypted ...
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1answer
219 views
How to recover secret $x$ from Elgamal signatures with repeated $k$?
Set generator $g \equiv 5 \pmod p$ where $p=647$ and $p$ is prime.
With the same $g$, $p$ and secret signing key $x$, Alice sends two messages, $428$ and $129$, with signatures $(433, 239)$ and $(433,...
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27 views
Validity of ElGamal signature
I'm trying out this practice question but dont know if I'm on the right path. Suppose I have generator $g≡5$ mod $p$ where $p$ = $647$ is a prime.
and Alice publishes her public key $y$ ≡ $57$ mod $p$...
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1answer
105 views
How to recover private key in ElGamal when random value k is known
Given following ElGamal encryption scheme: $\delta = M(\alpha^a)^k \mod{p}$. Assume that an attacker knows the random value $k$. How can he recover the private key $a$?
I know that it's possible to ...
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1answer
162 views
ElGamal with weak public key
Assuming this setup: A prime $p$, a generator $\alpha$ in $\mathbb{Z}_p^\ast$. Alice chooses her private exponent $d$ from $[2, p-2]$ and calculates her public key $\beta$ = $\alpha^d$.
Bob then ...
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Proof of knowledge of exponentiations
I am reading a paper of Furukawa and Sako, "An efficient scheme for proving a shuffle" from 2001. This paper writes a protocol for verifiable shuffling in mixnets. Their protocol make use of ...
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1answer
120 views
EL gamal cryptosystem
I have this question which gives me q=71(my prime number) , a=7(generator), Yb=3(public key), m=30(message), c1=59(first cipher text) and he wants me to find c2 (second cipher text)
I know that c2 = ...
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1answer
100 views
Proof of correctness of Exponential ElGamal
I have an assignment for proving the correctness of both ElGamal and exponential ElGamal homomorphic features (multiplicative and additive respectively). I have managed to prove the former but am ...
3
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1answer
574 views
Exponentiation in ECC
I have a naive question, what is the nomenclature of exponentiation mean in ECC?
I was reading about exponential ElGamal, what does it mean if a generator point $G^x$ ? What does $G * \ldots * G$ ...
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2answers
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Subgroup generation: should we check that order is not 2? or g in not too small?
For $p=2q+1$,where p and q are primes, we have subgroups of order $p−1$, $q$, $2$ and $1$. To find $G_q$, usually we just check that $g^q\bmod p=1$ and that's it.
However,here it's mentioned that ...
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1answer
156 views
Lenght of the ElGamal cipher
Ok, it's kind of stupid question, but I want to make sure.
What is the length of the ElGamal cipher? It's equal to the size of 2 elements of the cyclic group, right? But lengths of elements are NOT ...
2
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2answers
234 views
Information leak in ElGamal encryption with message in base group
Assume a finite commutative base group $\mathbb B$, some $g$ in $\mathbb B$, and $\mathbb{G} = \langle g\rangle$ the subgroup that $g$ generates, with choice of $\mathbb B$ and $g$ such that ElGamal ...
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1answer
351 views
ElGamal in $\mathbb Z^*_p$ with $g$ not a generator of $\mathbb Z^*_p$
Let $G$ be a subgroup of $\mathbb{Z}^*_p$, $p=23$. The order of $G$ is $|G|=11$. Let $g=4$ be a generator of $G$. Consider the key generation algorithm of the ElGamal encryption scheme. Assume that ...
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2answers
130 views
Can I use modulo $n^2$ arithmetic where $n=p \cdot q$ for ElGamal encryption?
In the paper which I was reading, the authors were using ElGamal encryption with modulo $p$, where $p$ was a prime.
Do the commitment properties still hold if we use modulo $n^2$ where $n=p \cdot q$ ...
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1answer
83 views
CDH between two groups implies weakness of El-Gamal
Let $G$ a cyclic group of prime order $p$ with a generator $g$.
Let $H$ a cyclic group of prime order $p$ with a generator $h$.
Suppose that you have an algorithm $\mathcal{A}$ that given two ...
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1answer
107 views
Does ElGamal authentication exist?
Are there any natural ways to transform ElGamal encryption system or ElGamal signature scheme into an authentication protocol?
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1answer
650 views
How to find a generator g for a large prime p?
Trying to implement ElGamal. I have a 1024-bit prime p and now need to find a primitive root g for it.
How do you find such a g in an acceptable time? It takes so long, my sage server actually ...
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0answers
116 views
Understanding ElGamal encryption for a Mental Poker Case
I am looking to understand ElGamal encryption as it is used in a specic context described below. I am interested in creating a practical form of Mental Poker for games other than poker developed by ...
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1answer
282 views
why doesn't quadratic residue attack work with Elgamal encryption with decisional diffie Hellman assumption?
I was reading this notes http://www.cs.umd.edu/~jkatz/gradcrypto2/NOTES/lecture4.pdf
It's given that discrete log assumption is not enough for semantic security, I'm assuming there maybe chance of ...
3
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1answer
665 views
ElGamal encryption and signature using same keys
In ElGamal scheme we have message $M$; $p$, $g$ and $y=g^x \bmod p$ as public key where $x$ is unknown private key.
Encrypted message $(c,d)$, where $c=g^k \bmod p$ and $d=M \cdot y^k \bmod p$.
...
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1answer
123 views
Semi static ElGamal vulnerability when the secret is not truely random
Assume Alice wants to communicate with Bob. Bob provides his public parameters $(g,n,y)$ with $y=g^x$ where $x$ is his secret key
Now Alice wants to send $m$ to Bob. She generates a random $r$ and ...
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1answer
226 views
Security proof of mix net
I am writing an exercise in cryptography about anonymous communication. One of the systems I have looked into is the mix net protocol. I want to prove that this is secure. I have seen many different ...
