Questions tagged [diffie-hellman]

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

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Steiner GDH3 (n-party Diffie Hellman)

I am trying to implement a n-party Diffie Hellman protocol. In particular I'm implementing Steiners et. al. natural extension "GDH3" (starting on page 34). The problem is that my python ...
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Diffie-Hellman Decryption with A, B, p, g, cipher Parameters

I have a Crypto assignment, trying different ideas for couple hours landed me nowhere. Supposedly during a Diffie-Hellman key exchange Alice sends bob an encrypted message, the message (cipher) is a ...
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Diffie-Hellman parameter for use in TLS derived from given finite-field group

I'm interested in cryptography, but unfortunately haven't found the time to dive into the topic much. I have done some quantum encryption stuff back in the day, but that isn't helping me with my ...
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Why Kerberos isn't using Diffie-Hellman key exchange?

Kerberos transforms the user password to obtain a secret key, which is also stored in the kerberos database. That way what travels through the network is the derived key, not the password itself, ...
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Change in Diffie-Hellman

In the Diffie-Hellman protocol, each participant selects a secret number $x$ and sends the other participant $α^x\bmod q$ for some public number $α$. What would happen if the participants sent each ...
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Secret keys in Diffie-Hellman

I assume 1 atleast is excluded from the set of possible secret keys, since you can compromise the key just by looking at the equation if it is 1 $g^1 \mod p \equiv g$. In case g is less than $p$ ...
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Diffie-Hellman should size of group be a prime?

I have found some contradicting answers regarding finding a safe prime. When p is a "safe prime", this means that $(p−1)/2$ is also prime. We then define $q=(p−1)2$. In that situation, the order ...
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Encrypting a string of arbitrary size with a big integer

I am currently writing Python script that initiates a secure handshake. I am kind of reinventing the wheel here, so that I can properly learn it. Now, I have Diffie-Hellman mostly working from ...
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Diffie-Hellman what is the subgroup

I am trying to understand the safety precautions regarding the variables used in Diffie-Hellman and I was refered to this post answer Does the generator size matter in Diffie-Hellman?. In more ...
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Diffie-Hellmann why bigger primes prefered over bigger pseudorandom secrets?

So I have build an application which purpose is to demonstrate Diffie-Hellman. I don't fully understand some of the arguments for the sizes of the numbers used in the algorithm. It seems like there is ...
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Diffie-Hellman server/client generated secret size

So I have implemented a client and a server performing Diffie-Hellmann, however sometimes the symmetric key is not valid? Example: Prime: 23 Primitive root: 5 client secret: 14 server secret: ...
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Cracking diffie-hellman

I want am making an application, demonstrating an attack on a weakly performed diffie-hellman. I need to implement an algorithm that can use the shared variables in order to compute the key. Searching ...
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Can Eve break this public key cryptosystem if she can solve DLP or DHP?

The PKC is in this way: Alice and Bob fix a publicly known prime $p$, and all of the other numbers used are private (unless sent). Alice takes her message $m$, chooses a random exponent $a$, and ...
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KDF options for a ECDH shared secret

the answer to this question suggests that the x-coordinate of an ECDH shared secret is itself usable as an encryption key without apply a KDF.... if so, then what additional security is provided by ...
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AES ECB deterministic encryption to perform Private set intersection

There are two parties, and each has a list of plain-texts. My end goal is to assign indices to unique plain-texts across both parties. My present approach is the following. Both parties perform a ...
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What is the key length of shared secret by Curve 25519 ECDH

Both parties uses Curve 25519 key pairs for ECDH key exchange. What is the key length of shared secret after ECDH?
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Using ECDSA to sign a ECDiffieHellman-derived key

Assuming that Alice and Bob know the hash of each other's public key, take this method: Use ECDiffieHellman with temporary private keys each time to generate a per-session shared symmetric key Create ...
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Why MQV scheme need a half coordinate?

Recently I get to know the MQV key agreement in a lecture. the protocol is quite simple MQV My question is why there is a $\hat X and$ $\hat Y$ in the protocol, it does not seem natural at all, does ...
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Possibility of Man-in-the-Middle on X3DH

I'm reading about the security of the Signal protocol (used in WhatsApp) and in particular, the X3DH part used to establish authentication, all papers seem to gloss over Man in the Middle attacks. Is ...
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Small exponents and the RSA problem

I need some help with the following statement from the book A Graduate Course in Applied Cryptography* - Dan Boneh and Victor Shoup, in 8.10.1 The key derivation problem, page 320 of v0.5: ...
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Diffie-Hellman Key Exchange - Key Derivation before Execution

I am dealing with protocol design regarding the Diffie-Hellman Key Exchange (DHKE). The DHKE is performed by the Key Master and some Slave Component. Afterwards, a session key is transmitted using the ...
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Diffie-Hellman man-in-the-middle attack

Diffie-Hellman Key Exchange Basic Protocol: Let G be a cyclic group of order $n$ and generator $g$. Alice chooses $a \in \{0,\ldots,n-1\}$ and Bob chooses $b \in \{0,\ldots,n-1\}$. ...
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How to calculate the exponential inverse in elliptic curve Diffie-Hellman? [duplicate]

In Diffie-Hellman over $Z_p$, I know that if I choose a random $a$ in $Z_p$ and compute the modular inverse mod p-1, not mod p, then, with generator $g$ of my group, I can compute, for example, $(g^{...
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Diffie-Hellman + Asymetric encryption kept simple, or alternatives to TLS

I am working on a system that includes our dear friends Alice, Bob, and a server that is only partially trusted. I have spent the majority of the day reading up on various cryptography methods, but I ...
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Using Diffie Hellman with a shared secret

For a crypto course I need to realize a small software project where a client and a server want to communicate over a unsecured network. They both have the same secret. So I bascially have to secure ...
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Random Oracle to prove an Authenticated DH protocol

I am trying to understand how they use the random oracle to solve the CDH. For example, in the security proof on page 7 of the following paper; A Lightweight Message Authentication Scheme for Smart ...
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How to compute certificates using Diffie-Hellman key exchange with certificates and Elgamal signature scheme

We consider the Diffie–Hellman key exchange scheme with certificates. We have a system with the three users Alice, Bob and Charley. The Diffie–Hellman algorithm uses p = 61 and α = 18. The three ...
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The way to break Diffie-Hellman(small nonprime number)

i am wondering how to break DH(small nonprime number). Let's say we don't know: $a$: Alice's private key $b$: Bob's private key And we know: $N$: nonprime number $p$,$q$: $N = p * q$ ($p$ and $q$...
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How can breaking the ElGamal encryption scheme break the Diffie-helman protocol?

I have been studying how ElGamal is a public-key algorithm built on top of the Diffie-Hellman key exchange but I got confused. How exactly could an attacker break the Diffie-Hellman protocol (i.e. ...
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What is Post-Compromise security exactly?

After reading these papers on Post-Compromise Security: Post Compromise Security Asynchronous Ratcheting Trees My understanding is the following: it is possible for a key-agreement protocol to ...
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Diffie-Hellman with Galois field

I Google around and can't find any page mentioning Diffie-Hellman with Galois field $GF(p^n)$ with $n>1$. Is there a reason for this? For example, wouldn't Diffie-Hellman with $GF(2^n)$ be ...
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Apply HKDF After DIffie-hellman Key Exchange

Im trying to use HKDF after DIffie-hellman key exchange. However, i got stuck on HKDF part. ...
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How do Quadratic Residues work and how do they relate to Diffie Hellman?

Can someone explain Quadratic residues to me in english? I keep reading forums with all the math symbols and it's hard to follow. And how are they incorporated in Diffie-Hellman? Is there a difference ...
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Question on Simplest Oblivious Transfer Protocol

In the paper The Simplest Protocol for Oblivious Transfer, in the scenario where the receiver chooses $c=1$, then $B$ is calculated as $Ag^b$. Then $k_1$ is calculated as $H( (B/A)^a)$. That means ...
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Safe way to derive IV from the DH shared keys for AES-GCM?

I am preparing to use AES-GCM to make a secure communication program. Here is the procedure DH key exchange to get shared keys SHA256(shared keys) Use AES-GCM to encrypt a message while the key only ...
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Using ECDH for authentication

I've found this method for using ECDH for asymmetric encryption. Is there a similar method for using ECDH (rather than the more usual ECDSA, let's say my hardware can do ECDH but not ECDSA) to ...
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Why can you use shorter keys with elliptic curve Diffie Hellman key exchange?

I am a layperson interested in how cryptography works. I would like to know why you can use shorter keys with elliptic curve Diffie-Hellman (ECDH) than with the discrete log DH key exchange. Both have ...
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Diffie–Hellman key exchange, why it's hard to break it?

I'm a bit confused why it's hard to break Diffie–Hellman Let's take the example ALICE has G, a, and n BOB had G, b and n Eve ...
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ECDH Public Key Recovery - Determine if valid key exists from just signature and message

If I have just the (r,s,v) values of the signature and the message hash, what is the most efficient way to determine if a valid public key exists? I do not need to know the public key, just that the ...
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List of big strong primes

I want to make a Diffie Hellman key exchange code using Python, but I'm afraid of just randomly picking $g$. I read on the answer of Thomas Pornin to this question How does one calculate a primitive ...
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How to reduce Computational Diffie–Hellman problem and Decisional Diffie–Hellman problem to Discrete logarithm problem

I'm supposed make some reductions but don't even know where to start. Any help would or explanation on how to do this would be much appreciated.
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Standards for finite-field discrete log cryptography

The NIST and TLS standards for Diffie-Hellman key exchange over a finite field all work in a subgroup of ${\mathbb Z}_p^*$ having prime order $q$, where $p = 2q+1$. On the other hand, DSA has a larger ...
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DHE parameters: which parameters need to be random each time?

In DH key exchange, first, both parties need to agree on a generator $g$ and prime $p$. After that each party computes its key (party A: $g^a \bmod p$) and (party B: $g^b \bmod p$), then they exchange ...
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ECDH, x25519 and limitations for securing client-certificates on the server

g'day Crypto.SE I'm working on a practical application of sharing a static key to better-identify a user, between the recipient (server) and the initiator (client ie. user's machine). My core ...
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Prime computation for Diffie Hellman

While reading some cryptographic RFCs, came across the Diffie Hellman prime computation formula: 2^1024 - 2^960 - 1 + 2^64 * { [2^894 pi] + 129093 } I am curious what the expression [2^894 pi] means ...
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Using Diffie–Hellman without a key derivation function

In an answer to another question, it was suggested that the shared secret derived from Diffie–Hellman key exchange could then be used directly as a one-time pad to encrypt a message (by XORing the ...
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A doubt about Diffie-Hellman Key Exchange

I have a doubt about the Diffie-Hellman problem: Is it possible to send a message to someone using only the Diffie–Hellman key exchange? Since it's called a key exchange can you only make a key?
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Interactive ECDHE Authentication With Numeric Code

Trying to simplify my question, keeping only core concepts. Proposed solution: Both user devices generates ECDHE key pairs. Send pub keys to each other. Generate shared secret. Device that requests ...
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find prime number of diffie-hellman algorithm

at IPsec, Diffie-hellman is used in framework ISAKMP resulting shared secret. some of Diffie-Hellman group may define the length of the prime number of q and the size of the secret key of K: group 1 (...
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(How) Is DDH generally broken in groups of composite order?

In a somewhat recent lecture a claim was made that I couldn't back up myself but it got me curious whether it actually holds: If the order of the group is not prime, then the DDH assumption does ...

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