# 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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### 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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### 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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### 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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### 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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### 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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### 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 ...