Questions tagged [diffie-hellman]

The Diffie–Hellman key agreement is an anonymous, non-authenticated key-agreement protocol.

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Designated verifier signature from Diffie–Hellman and a MAC

Here is an idea for a designated verifier signature scheme. Suppose Alice and Bob know each other’s public keys and Alice wants to send a message to Bob, such that only he will be convinced of its ...
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What Is The Maximum Value For N In Discrete Logarithm Problems?

I have some code, which can crack a discrete logarithm problem in ~ O(0.5n) time. However, this only works if, in the following, N is less than P: G^N (mod P). To be clear, my program can figure out ...
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running Project Wycheproof against crypto implementations in languages other than Java

So I guess https://github.com/google/wycheproof "tests crypto libraries against known attacks". It appears to mainly be intended for Java crypto providers but can it easily be adapted to be ...
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How to know if you have guessed the correct Diffie-Hellman shared secret?

Given only $p,$ $g,$ $A = g^a\pmod{p}$ and $B = g^b\pmod{p},$ the possible values for the shared secret are all the unique values of $A^b\pmod{p}$, where b is some integer. The shared secret is also ...
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Shortcut to working out Diffie-Hellman Key Exchange

I am trying to calculate Alice and Bob's shared key by hand without the use of a calculator as I feel this is an important trait when progressing into cryptography. I understand you can use the square ...
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Is Diffie-Hellman less secure when A and B select the same random number? [duplicate]

I understand that it is feasibly impossible for A and B to select the same random number, given the large input space, but what if it does happen? Does it effect the security of the key exchange? Can ...
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110 views

Is Diffie-Hellman Key Exchange an Asymmetric or Symmetric Algorithm? [duplicate]

To my understanding, the DHKE algorithm is symmetric since it only produces a shared secret, rather than public and private keys, however googling "is diffie hellman asymmetric?" results in ...
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Computationally indistinguishability using DDH assumption

This is part of the explanation of the commitment scheme from DDH in this lecture notes by Vipul Goyal: https://www.cs.cmu.edu/~goyal/s18/15503/scribe_notes/lecture22.pdf My question is not directly ...
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Are all public key a result of computing g^k mod p

I just read through the text book definition of Diffie–Hellman key exchange. And from what i understand, the public key that is shared based on the protocol is calculated from: ...
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280 views

DDH to DLIN reduction

I read multiple times that it should be feasible to show a reductions from Decisional Diffie Hellmann. Could you give examples?
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Understanding Practical Differences Between ElGamal and Diffie-Hellman

I've been tasked with building a Web Assembly site that implements E2EE. I was thinking of using ElGamal Encryption to encrypt the message and Diffie-Hellman to establish the key. After doing further ...
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How can we link AES with Elliptic Curve Diffie-Hellman Key Exchange Method

Actually, I am working on a project to combine symmetric and asymmetric cryptographic algorithms. The shared secret key for AES will be generated through the Elliptic Curve Diffie Hellman Key Exchange ...
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Can the same public key be used for ECDH and ECDSA [duplicate]

I want to generate a public key that I can use to sign messages and receive messages (using ECDH for exemple). I want to do so to have the smallest payload to share. Is it possible and proved secure ?
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X3DH and key exchange between Alice identity key and Bob pre signed key (DH1)

I'm getting familiar with Signal key exchange phase and as far as I understand all 3 exchanges between Alice ephemeral key and all of Bob keys from the bundle, I have some thoughts about key exchange ...
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174 views

Digital signature with Big Brother

I have read digital signature with Big Brother but don't understand the sequence. One approach to digital signatures is to have a central authority that knows everything and whom everyone trusts, say ...
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Read ECDHE traffic with Wireshark or the like

There is some https traffic from a specific server (which I have the certificate and private key) that I need legitimately be able to read. This traffic doesn’t come via browser so besides the ...
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Find a prime $p$ vulnerable to pohlig-Hellman

I need to find a prime number $p$ with the following constraints: $p$ is at least $1000$ bits long $p-1$ is a smooth number with the largest factor below $1000$ any factor of $p-1$ can be present ...
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Knowledge proof of private keys from DH key exchange

Given a group where the computational Diffie–Hellman (DH) assumption holds and generator G. Say there is a set of private randomly selected keys {a, b, c, d, e,...} and corresponding public keys set {...
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CDH in a group of square matrices

This paper says the CDH problem in a group of square matrices can be solved by a generalized Chinese remainder theorem. I wonder how this problem might be solved? DH protocol in the cyclic group of ...
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123 views

Is it safe to reveal an arbitrary EC point multiplied by a secret key?

We have a primary-order EC group. Need to perform a (sort-of) DH protocol, whereas the key is permanent, not a nonce (single-usage ephemeral key). So we receive an arbitrary group element (EC point) ...
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Strong Diffie Hellman in bilinear groups

The $n$-strong Diffie Hellman assumption state that given the subset $\{g, g^s,\cdots,g^{s^n}\} \subseteq \mathbb{G}$ in a cyclic group $\mathbb{G}$ of prime order $p$, a PPT algorithm cannot output $...
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Use Diffie Hellman to share a secret

I was trying to use Diffie Hellman to first agree on a key and then use that key to symmetrically share a secret. The python3 code using cryptography library look like ...
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Does breaking CDH also break DLP? [duplicate]

Does breaking the computational Diffie-Hellman problem in a group also always break discrete logarithms in that group?
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Is it safe to implement elliptic curve Diffie Hellman with secp256k1

I need to implement X3DH Key Agreement Protocol according to Signal specification, in the document they suggest using either X25519 or X448 curves. I assume those curves have been chosen for this ...
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87 views

How to choose the appropriate Smoothness Bound while using the Index Calculus method

While implementing the Quadratic Sieve, the textbooks give a rough formula for what Smoothness bound you should use in your Factor Base. To factor a number N using the Quadratic Sieve, we can use the ...
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What benefit is gained when combining AES GCM/GMAC with ECDSA?

This is a real world question (and as I'm not an expert in cryptography I have only some basic knowledge in terms of just using it, not a deep understanding how ist works under the hood.): A system ...
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2answers
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Verifying result of (EC-)Diffie-Hellman

I received a public key by JSON. For the example, I have 4 keys: 2 public keys and 2 private keys. ...
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106 views

Asymmetric encryption scheme with the shortest output for encrypting 1 byte of information

Imagine that one needs to periodically encrypt very short messages (i.e., a boolean Yes/No, a single byte, or 3-4 bytes in the worst case). We assume that there is no session, and we just need to ...
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Security proof for TLS 1.x

In JKSS12, a proof for the handshake in TLS-DHE 1.2 is given, assuming (among other things) the PRF-ODH hypothesis on the PRF used to derive keys. It is also stated that, if TLS 1.2 was to be modified ...
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Decisional Diffie-Hellman Assumption on Pairing Friendly Curves

It is known that the Decision Diffie-Hellman (DDH) problem can be easily solved over groups on pairing friendly curves (that is: one can use pairing to tell if $g^x$ and $g^y$ and $g^z$ forms a DH ...
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50 views

CDH solver algorithm construction

If $A$ is an efficient algorithm that solves the Computational Diffie-Hellman problem for $\frac{1}{2}$ of the inputs and returns a special symbol for the rest, how can I use $A$ to construct another ...
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Diffie-Hellman Assumption

I'm wondering if the following problem is as hard as computational or decision Diffie-Hellman problem? (Or is it actually an easy problem because $c$ is available?) Given a cyclic group $G$ and let ...
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Why is ECDHE safe when paired within signing?

I wanted several videos by Computerphile on Elliptic Curve Diffie-Hellman, digital signatures and TLS. For the most part I understand everything but something is bothering me. Computerphile made a ...
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Decisional Diffie-Hellman Assumption over Group of Quadratic Residue

Consider the Decision Diffie-Hellman (DDH) over $QR_n$ (the quadratic residue group over $n=pq$ where $p$ and $q$ are safe primes).According to Boneh's paper, DDH should be hard over $QR_n$ (https://...
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Linking Decisional Diffie-Hellman, Discrete Logarithm, and Knowledge of Exponent Assumptions

I'm curious about the relation between the Discrete Logarithm and Decisional Diffie-Hellman. Is it safe to have an assumption like the following to link the two? Given uniformly and independently ...
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1answer
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Where are the seeds for the initial key exchange phase taken from?

I know that the standard DH and ECDH key exchange algorithms require the client and server to agree on a large prime number and a generator (in the DH algorithm) or a curve and a point (in the ECDH ...
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How do I validate Curve448 public keys?

Performing an ECDH calculation with an invalid public key can leak information about your own private key. With Weierstrass curves, it's important to verify that the peer's public key is actually a ...
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Authentication Fold-In in Oracle Database

The Oracle database has a feature known as "Native Network Encryption". It is a protocol described as a 4-step procedure. Step 1 : Negotiate parameters for following steps Step 2 : Exchange ...
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Signal protocol: X3DH

I've been trying to get a grasp of how the Signal protocol works. According to the spec, DH is done on four keys: IK_A, SPK_B, <...
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106 views

Gap between DLog and CDH

Is there any concrete group in which one CDH is exponentially easier (even it's still hard) than DLog.
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Does having CDH oracle breaks El-Gamal signature scheme?

Having a oracle that solves Computational Diffie-Hellman problem which for given values $(g, g^a, g^b)$ outputs $g^{ab}$, is it possible to forge a signature in El-Gamal (wiki) signature scheme?
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184 views

Why is DH key exchange still useful, if a public key certificate is used?

EDIT: According to Gilles's comments and fgrieu's answer below, I would like to summarize what I have learned, and what's still needs explanation: DHKE is necessary for forward secrecy even when PKI ...
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Method to mitigate MitM attack for DH key exchange

Plain DH key exchange protocol is vulnerable to MitM attack. I checked the SRP (RFC 2945) but did not fully understand why it can help. I thought of a fix to mitigate MitM problem by using a pre-...
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60 views

What is the role of SHA-1 on Diffie-Hellman-group1-SHA1?

I undersntand that in Diffie Hellman each party generates a public / private key pair and distributes the public key. I know that a combined key is created with the sender private key, and the ...
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Advantage of Curve25519 (Elliptic Curve - Diffie Hellman)? [closed]

Is there any known vulnerability or attacks against Curve25519 ?? And pros and cons of using it?
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RSA vs diffie-hellman + polyalphabetic cipher

My basic question is: How does RSA compare to using Diffie-Hellman + <some symmetric key crypto, e.g. a polyalphabetic cipher> Let's assume that factoring large numbers and computing discrete ...
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Super basic questions to DLP and DH

This is a super basic cryptography and I don't get it. My professor only explained it to us on a very abstract level, which I just agreed to without questioning it. The DLP says it is easy to ...
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Why do you need to apply hash over DH shared key? [duplicate]

I`ve read multiple times that it is not safe to use the shared key resulted from DH key exchange directly and that you need to do a KDF or hash over it and then use the result as a key. Why is it not ...
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How to decrypt raw TLS bytes?

Is there any tool / way to decrypt raw bytes of a TLS 1.2 session, when the master key is known, even if forward secrecy is used? The closest I found is this. I know its possible using Wireshark, but ...
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Does the diffie helman rachet in double ratchet do anything if one side never replies?

I am a little confused on how this functions so apologies in advance if the question is naive. The way i understand it, the function of the standard sending and receiving ratchets in Double Ratchet is ...

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