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

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Ephemeral Diffie-Hellman in a symmetric crypto context

I'd like forward secrecy in a case where the two sides have a shared long-term symmetric key. I intend to use AES in GCM. It looks like ephemeral Diffie-Hellman is the best way to generate the session ...
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Diffie Hellman Exponentiation Implementation Problem

im trying to work on key agreement schemas on embeded systems. for diffie hellman, ive written a 256 bit multiplication, on AVR core it takes about 2 seconds on 1Mhz frequency, lets say my algorithm ...
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DHKE choice of private keys

In some literature it is written that the private key should be chosen random from {2,3, ..., p-1} and in some it says {2,3, ..., p-2} I guess p-1 is a weak private key, but why is that so? The ...
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Diffie-Hellman Application

Alice and Bob want to perform a Diffie-Hellman key exchange using the group $G$, a primitive root $g$, Alice’s secret key $k_A$ and Bob’s secret key $k_B$. In each case below compute the element of $G$...
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Building a pad for OTP on-the-fly with Diffie-Hellman

Through repeated DH attempts, could Alice and Bob build a large random key for use as a one-time pad? My intuition is that this protocol would be as hard as breaking DH (discrete logarithm).
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Can Diffie-Hellman be scaled up or force-multiplied?

I'm not too familiar with the math involved with the exchange of keys, I've read it over several times, but math isn't my strong point. My understanding is that smart people are hard at work to ...
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346 views

weaknesses in ElGamal with public key of small order

Suppose $p=29$, $\alpha = 2 \in F_p^*$ is a generator of $F_p^*$. Bob picks $d \in \{2,...,27\}$ such that $\beta = \alpha ^d=28 \pmod{29}$. He then sends his $(p,\alpha ,\beta)$ to Alice who herself ...
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Key exchange if forward secrecy is not an issue

Let's say that we have a cryptosystem where Forward Secrecy and Man-In-The-Middle attacks are not an issue, purely hypothetically. Would it be correct for a key exchange with any given public-key ...
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Demonstrating Diffie-Hellman key exchange using only p, A, B;

I'm trying to figure out how to demonstrate DH key exchange using what is given to me. I'm unsure of how to tackle the problem. Question: Demonstrate Diffie-Hellman key exchange. Given p=104933, ...
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159 views

Recovering the random number r

For a padded message, M, using the El Gamal encryption schema, how can we determine the random number $r$, when we are given $p$, the prime number, $g$ which is the primitive root of $p$, $b$ and $x$ ...
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204 views

Are there any protocols that are truly secure from active and passive MITM attacks?

Are there are any cryptographic protocols or algorithms that can prevent active MITM attacks or interference when initiating a new connection to a server or someone you have not exchange keys with ...
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714 views

Reuse of a DH / ECDH public key

I was wondering whether it is safe to use the same DH or ECDH key pair in more than one key agreement, particularly if these public keys are in a public registry. These public keys could be used by ...
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203 views

Security Consideration on The Use of Random Number in Digital Signature

key-exchange protocol allows two parties to establish a shared key over public network. Lacking of authentication the original Diffie–Hellman key exchange is insecure under man-in- -the-middle ...
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314 views

Use curve25519 for ElGamal crypto [duplicate]

DJB described curve25519 in his paper which can be found here (PDF). It seems that the main purpose was for Diffie-Hellman key exchange. I think this means that Discrete Log is supposed to be hard on ...
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216 views

Request for 1024-bit primes $p$ , subgroup $q$ and subgroup generator $g$

I need to find a prime $p$ of $1024$ bits with a $160$ bit sub group size $q$, such that $q|p-1$ , and $g$ is the generator of the sub group size $q$. I'm looking for the numeric values of $p$ , $q$ ...
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42 views

What is the starting point of Hellman's TMTO method?

I understand what is happening and how the endpoint is computed but I'm still a bit unsure of where "SP" (Staring Point) came from. Are the starting points, randomly selected keys from the key-space n?...
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Are Diffie-Hellman, RSA and other types of public key cryptography different? [duplicate]

Are these schemes fundamentally different? Are other schemes fundamentally different in the sense that you can do different operations with them (or the same operations, but needing different ...
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ephemeral-ephemeral ECDH and MITM attack [duplicate]

ephemeral-ephemeral ECDH is it susceptible to MITM attack ? if yes, can you please give me a detailed answer ( Workflow that can be followed by the attacker) and how we can prevent this attack (static-...
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Is this a secure method of encrypting with authentication?

My goal is to allow two clients to send files securely over an untrusted network without the need for more than one block of information to be sent. Both clients have ECDSA keys of size 256 bits. I'd ...
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Deriving 2 shared secrets from one private key and 2 different public keys

I have a key pair, say $(d, Q)$. I send my public key $Q$ to 2 different people. I also get their public keys $Q_1$ and $Q_2$. Now I can derive 2 shared secrets, $d*Q_1$ and $d*Q_2$. The 2 other ...
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My attempt of combining socialist millionaire and Diffie-Hellman. Is it any good?

I've come up with an algorithm to solve the socialist millionaire problem, but I'm not sure if the algorithm is secure. I wasn't able to find any flaw in it, but it seems too simple to be secure, so I'...
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Are there any obvious techncial faults in F. Ruiz's 2013 URSA javascript cryptography? [closed]

The link to the cryptography is here: http://prgomez.com/ursa/ I have found the following simple javascript system of encryption/decryption portable and easy to use for emails and messaging sites, ...
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234 views

Use ElGamal to solve Diffie-Hellman problem

Say we are able to decrypt a Elgamal ciphertext $c$ using only the public key. Apparantly it is now possible to solve the Diffie-Hellman problem (given $g^a, g^b$ calculate $g^{ab}$). How? I know how ...
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Does “use of SHA-256 in cryptography” imply use of SHA-256 both in HMAC and DH key exchange, assuming these are the only uses of hash in system? [closed]

I am beginner to Cryptography. And recently I came across the requirement like "use of SHA-256 in cryptography". Does "use of SHA-256 in cryptography" imply use of SHA-256 both in HMAC and DH key ...
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Safe and computationally efficient way to verify a curve25519 identity?

A client identifies itself as a curve25519 public key. The server wants to verify the client owns the associated private key. Is there a safe and computationally efficient way of doing so? Which ...
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Is it possible to modify SSL handshakes to enable PFS while still using RSA during the handshake?

I know we could possibly be using DH key exchange, that would enable PFS. However, what if we make modifications on our SSL 3.0 handshake such that the only cipher we use is RSA for the achieving PFS? ...
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231 views

Best group if one wants the discrete log problem to be hard?

Suppose one is implementing a cryptographic scheme over a group where one needs the discrete logarithm to be hard - what is the recommended group to use? I'm looking for a group where calculations are ...
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159 views

Career advice for our readers [closed]

I am a first year math PhD student with work experience and an MS in math. Do you have any advice regarding internships involved with cryptography and other mathematics other than the NSA? Thanks.
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openssl logs showing keyblock length of only 88 bytes instead of 136 bytes

I was decrypting TLS encrypted data from wireshark using the openssl(TLS1.0 version) TLS logs which provides the complete key block which of size 136 bytes.Now i moved to TLS(1.2 version) in that Key ...
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176 views

What should the size of the private exponents in Diffie-Hellman key exchange for TLS be? [duplicate]

I'm using TLS with a Diffie-Hellman key exchange to negotiate the symmetric keys. How long should the exponents be?
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Secure modulus length for Diffie-Hellman [duplicate]

What is secure modulus length for modular exponentiation so that finding discrete log is computationaly impossible?
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Why is DDH not hard over $\mathbb{Z}^*_p$?

Why is Diffie-Hellman key exchange not hard over $\mathbb{Z}^{*}_p$?
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Show How to Efficiently Solve the Computational Diffie-Hellman Assumption given an Algorithm that Solves the Square-DH Problem

Let $q$ prime number, $G$ a cyclic group with order $q$ and let $g \in G$ be a generator of $G$. Suppose that you have an algorithm $A$ who takes input the element $g^a$ of $G$ and gives as output the ...
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240 views

Addition / Multiplication modulo 13

I am currently studying Diffie-Helman protocol. In my lecture notes I have a generator g=2 and I'm in group $Z_{13}^*$ . There I calculate a remainder cycle from 1 to 12 with $r =g^i \mod 13 \forall ...
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Key Refresh in Diffie-Hellman

Assume the case, that two participants have agreed on a key $K=g^{ab} \mod p$ via Diffie Hellman. I have the need to change the key every now and then. The first idea I had was to simply initiate a ...
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Diffie-Hellman: Calculating the private key from the public key with Java

I have the task to calculate the private key from the given public key and have the hint that the secret exponent lies in the range from $-2^{31} < e < 2^{31}$. To find the exponent I ...
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66 views

what is DHE_EXPORT cipher suite

I need to know what is exactly DHE_EXPORT cipher suite and why it is still supported by some servers. Is there any implementation or details about this weak crypto?
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what is recommended x length in Diffie Hellman algorithm

According to rfc documents http://tools.ietf.org/html/rfc2412#appendix-E.2 & http://tools.ietf.org/html/rfc3526#section-3, it has defined the prime and the <...
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DH with AES - is that an acceptable security scheme over UPnP?

I'm trying to devise a good security scheme to protect comms over Wi-Fi. Right now, I have 2 devices that already communicate via UPnP but I need to secure this communication. I've come up with this: ...
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What is Group in Diffie-Hellman?

I understand how Diffie-Hellman key-exchange works. Mainly, two parties agrees in a prime $p$ and a generator $g$. Then one party selects its private exponenet $x$, computes its public value $g^x \...
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One-time pad using RSA and Diffie-Hellman functions

Suppose that Alice and Bob use an encryption algorithm based on a one-way function $F$ (Eve knows it), and $r$, a secret key shared between Alice and Bob (Eve doesn't know it). Key is computed as: $K =...
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Secure Key exchange using javacard applet

I am working on a javacard applet project where i need... Secure communication between two applets let say there is applet A and applet B. Let say both applet has their own RSA key pairs. Now I ...
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157 views

Can one detect if two pairs of elements in Zp have the same exponential relation?

Suppose that $p$ is a safe prime of 2048 bits ($p = 2q + 1$, and $q$ is prime). Suppose that one is given two pairs $(x_1, y_1)$ and $(x_2, y_2)$ such that: $y_1 = x_1^{r_1} \pmod p$ $y_2 = x_2^{r_2}...
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Diffie Hellman Exchange Confusion [closed]

I am working on this question and I am wondering I have figured out the secret key, but my problem is I don't know how to use the secret key to decrypt the ciphertext. Thanks for the help!
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Security of Diffie Hellman in specific cyclic group

For some $k$, let's say $p = 1+ \prod_{j=1}^k q( j)$, where $q(1)=2$, $q(2)=3$, if $p$ is prime, the diffie-hellman key exchange is not secure in cyclic group $Z^*_p$. Why?
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Confusion regarding computing Multiplicative Inverse Modulo P?

May be a silly doubt, please rectify my confusion regarding below problem: For concreteness assume $g=2, p=11, a=6$ and $x=9$ $$A = g^a \bmod p = 2^6 \bmod 11 = 9$$ $$X = g^x \bmod p = 2^9 \bmod 11 ...
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Post-quantum hybrid encryption

Assume the following simplified protocol: seed = curve25519SharedKey(theirPublicKey, myPrivateKey) sharedKey = scrypt(seed, salt, sufficientlyHardWorkFactors...) cipherText = aes256(sharedKey, ...
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344 views

modfied man in the middle attack diffie hellman

I have been given a problem in preparation for my cryptography final that I'm not sure how to solve. It asks me to suppose a scenario where instead of where an attacker would intercept some message ...
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253 views

Zero Knowledge auth schemes with weak secret

In Zero Knowledge auth schemes the public DH factor of each peer is encrypted with a potentially weak pre-shared secret and the resulting ciphertexts are exchanged over an insecure channel. Why is no ...
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75 views

How Diffie-Hellman authentication works? [duplicate]

There are a lot of good videos on a simple Diffie-Hellman in the internet but unfortunately that is susceptible to the MITM attack. I understand how with DH the two parties are able to calculate a ...