Questions tagged [elgamal-encryption]

ElGamal is a public key encryption scheme with security based on the discrete logarithm problem.

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Message space when using the group of QR's for El Gamal Encryption

My textbook states, that the DDH-assumption is not satisfied when we use the group $\mathbb{Z}/p \mathbb{Z}$ and demonstrates an attack using Eulers Criterion. After that, it states that one should ...
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How to know if an ElGamal ciphertext is well-formed?

Is there a way to prove/verify that an exponential ElGamal ciphertext of a message representative $\widetilde{m}=mG$ is well-formed, meaning that $r_1 = r_2$ in: $$C = (R, S) = (r_1G, \widetilde m + ...
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Retrieving ElGamal message with negative exponent value

Although the ElGamal scheme states that the message $M$ must be $1 \le M \le p-1$ in this paper: A Secure and Optimally Efficient Multi-Authority Election Scheme they propose a method where the ...
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decrypting el gamal encrypted messages in regards to k being used

i am trying to understand the differences of using k in the following scenarios to better understand the protocol and most importantly the consequences of using such k. so i wonder, some user, call ...
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74 views

EL Gamal Attack without a secret key

I have the following data where all the inputs are Big Integers, group size $p$, group generator $g$ public key of the receiver $y$ $c_1$ and $c_2$ Random Number as well I am trying to execute an ...
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Construct another ciphertext that decrypts to same message without knowing key in Elgamal

I read about ElGamal from various resources and I was wondering if the following is true. Would appreciate if you could elaborate on it so I can understand better: Is it possible to construct, given ...
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How to create an EC point from a plaintext message for encryption

It seems that ElGamal encryption is also possible for Elliptic Curve cryptography. However, that requires the user to convert the message to a point on the curve. What strategies are there to derive a ...
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Security properties of ElGamal encryption variants

I'll use Taher ElGamal's A Public Key Cryptosystem and a Signature Scheme Based on Discrete Logarithms (July 1985 in IEEE Transactions on Information Theory, formerly in proceedings of Crypto 1984) as ...
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173 views

Randomness in Elgamel [closed]

Suppose that, instead of choosing r completely at random in ElGamal public key encryption, a lazy encryptor (Alice) derives it by following r′= 2r. Suppose also that Eve knows that Alice ...
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Are short ElGamal keysizes acceptable (1024-bit or lower) in cases where security is only necessary in a short timeframe

I am working on an efficient mental poker framework. I have implemented a functional and secure commutative encryption scheme based on ElGamal as described in: http://groups.csail.mit.edu/cis/theses/...
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Does the discrete log assumption hold if k > p

In discrete log cryptosystems like ElGamal it is noted that the "private key" $k$ should be chosen as any element of the group $G$ i.e. $k < p-1$. Does the integrity of the cryptosystem rely on $k$ ...
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Retrieving correct ciphertext from additive ElGamal

I have been studying additive ElGamal and I think I have the hang of it except the part where the message $M$ must be retrieved by computing the discrete log of $g^M$. From what I've read, the ...
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83 views

Understanding message reversible mapping function in group G

In ElGamal encryption, it says that the message M that is about to get encrypted must be mapped to its m counterpart in the group G using a reversible mapping function. Map the message M to an ...
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Can Paillier ,RSA or any other schemes be used for universal re-encryption like elGamal?

ElGamal, RSA, and Paillier cryptosystem have homomorphic property, and can be used fro re-encryption purposes. I want to use the encryption to re-encrypt ciphertext(as in proxy re-encryption but ...
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How to compute ElGamal decryption by hand

Lets say that $u=3, x=5, v=2$, how do we work out $u^{-x}*v$, so $3^{-5} * 2$. I know how to work out the answer if it was $3^5 * 2$ but how do we do it with negative exponents?
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Proving the CCA-insecurity of El Gamal encryption scheme

I am trying to prove that El Gamal Encryption is not CCA-Secure. If an adversary queried the encryption oracle for the encryption of $m=1$, he'll get a pair $(c_1,c_2)=(g^y,g^{xy}\cdot m)=(g^y,g^{xy}\...
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How can I prove that the plaintext of an elgamal ciphertext is the discrete log of an element?

Is there any (efficient) method to prove that the plaintext of an ElGamal ciphertext is the discrete log of an element? In the scenario I concerned, I have an El Gamal key pair $(pk, sk) = (g^y, y)$....
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Post-quantum alternative to ElGamal? (public key verifiability)

Are there any alternatives to ElGamal that would be resistant/annoying to quantum machines? I would like to preserve public key derivability/verifiability -- ability to ensure existence of ...
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ElGamal: different message with equal length random key [closed]

This situation applies to ElGamal digital signatures: consider messages m1 and m2 which we will encipher with a random key. The random key has the same Hamming weight. Is this situation secure? Why? ...
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Intuition for Cramer Shoup Encryption Scheme?

I was reading the Cramer Shoup CCA2 secure encryption scheme. The scheme is as follows. Public key = $(g_1, g_2, c, d, h, hk)$, where $c = g_1^{x_1}g_2^{y_1}$, $d = g_1^{x_2}g_2^{y_2}$, $h = g_1^z$, ...
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Whats wrong with my El Gamal signature example

I'm trying to digitally sign a message m using El Gamal. So far I've been unable to verify the digital signature ive made using El Gamal. I am using prime number, p = 8369. prime root g = 3031. ...
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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) ...
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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 ...
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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 ...
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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?
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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,(\...
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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 ...
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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)
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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 ...
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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 ...
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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 ...
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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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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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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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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. ...
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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, ...
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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 ...
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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 ...
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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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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 ...
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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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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 \...
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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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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 ...
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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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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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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 ...
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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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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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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 ...