Questions tagged [elgamal-encryption]
ElGamal is a public key encryption scheme with security based on the discrete logarithm problem.
2
votes
1answer
44 views
ECIES/ ECDHE/ EC-ElGamal encryption comparison
I need to choose an encryption system, so I am trying to understand the differences between the existing options. I always find that people compare ECIES (Elliptic Curve Integrated Encryption Scheme) ...
0
votes
1answer
41 views
How much more secure is $c = mg_1^r + g_2(g_1(g_1^r-1)/(g_1-1)) \mod p$ compared to just $c = mg_1^r \mod p$ (dis. log), all known but $r$?
To encode a message $m$ to a cipher $c$ you can use the only hard solvable problem of computing the discrete logarithm with a generator $g$ in base over a prime $p$.
$c = mg_1^r$ mod p
If an ...
1
vote
1answer
65 views
Proof of correctness of an ElGamal encryption given a specific public key
Suppose Alice sends an ElGamal encryption of a value $v$ to Bob (using either the normal or exponential version of ElGamal). E.g. assuming publicly-known $pk_{BOB} = h$ and a generator $g$, Alice ...
0
votes
0answers
43 views
Beginners question regarding ElGamal
How to motivate, that in ElGamal Alice the recipient keeps her keys fixed, while Bob the sender changes the key each time - why not both of them change their keys each time?
2
votes
1answer
123 views
find $a$ and $k$ for a given el gamal cryptosystem
I am given this question:
Suppose Alice is using the ElGamal Signature scheme with parameters
$p = 31847$, $\alpha = 5$, and $\beta = 25703$
Assuming that we have received signed messages
$(x_1,(\...
0
votes
0answers
46 views
ElGamal re-encrypts data
I try to learn ElGamal re-encrypts scheme and I have a question about the example: How is a re-encryption done with elGamal?.
The $g$ value of Alice must be the same as $g$ value of Bob or not?
And ...
4
votes
3answers
588 views
How to verify if g is a generator for p?
For learning purpose, supposed I have a 16-digit prime which is 2685735182215187, how do I verify if g is a generator? (p is supposedly a special kind of prime)
0
votes
0answers
14 views
I2P Tunnel Creation Build on Request Record Question en(de)cryption method
Not sure if anyone here can help, but if so, sure would appreciate the help.
If we read the https://geti2p.net/spec/tunnel-creation documentation page on how I2P creates a tunnel, we can read on the ...
0
votes
1answer
74 views
El Gamal Cryptosystem Proof, Step-by-Step Explanation Needed. I feel like I don't understand what I have on this proof
Can someone provide a good and thorough explanation of the El Gamal proof? Basically, I need a step-by-step breakdown of what is happening at each important part in the algorithm.
I haven't been able ...
3
votes
1answer
222 views
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
votes
1answer
119 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}...
1
vote
1answer
70 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 ...
0
votes
0answers
25 views
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 ...
0
votes
1answer
56 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
votes
1answer
64 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
votes
1answer
105 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
votes
0answers
67 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
votes
1answer
111 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 ...
4
votes
0answers
85 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
votes
1answer
75 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 ?
2
votes
1answer
84 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
votes
1answer
57 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 ...
0
votes
1answer
42 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 ...
5
votes
2answers
246 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 ...
2
votes
0answers
172 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 ...
-1
votes
1answer
97 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
vote
1answer
72 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 ...
4
votes
0answers
134 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 $a,b,c$ ...
1
vote
1answer
148 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
votes
3answers
310 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
votes
0answers
136 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 $priv_a$ ...
1
vote
1answer
149 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 ...
5
votes
1answer
197 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:
...
3
votes
1answer
280 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 ...
0
votes
1answer
111 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 ...
1
vote
2answers
74 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 ...
1
vote
0answers
91 views
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 ...
1
vote
1answer
339 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,...
0
votes
1answer
141 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 ...
0
votes
1answer
213 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 ...
4
votes
0answers
96 views
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 ...
0
votes
1answer
185 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 = ...
0
votes
1answer
121 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
votes
1answer
627 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 $G * \ldots * G$ ...
1
vote
2answers
85 views
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 ...
0
votes
1answer
186 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 ...
3
votes
2answers
275 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 ...
0
votes
1answer
422 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 ...
3
votes
2answers
144 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 $n=p \cdot q$ ...
0
votes
1answer
90 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 ...
