The most famous key exchange algorithm with perfect forward-secrecy.

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1answer
178 views

ID-based key exchange protocol with PFS

The traditional authenticated DH protocol with PFS against active adversaries, ensures that the compromise of both party's long-term private keys after the session is completed will not help the ...
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2answers
170 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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1answer
144 views

Derive a public EC key from two public EC keys

Alice has two EC key pairs: $a_1$, $a_2$ are private keys (integers), $A_1$, $A_2$ are the corresponding public keys (points). Alice and Bob want to create a new public key $C$. Alice must prove that ...
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92 views

Self authenticating tamper resistant first meetings

I'm not sure what the precise words I am looking for are so I apologize if there is a better way to phrase this. What I am thinking of is ways to transfer an identity in a way that it is tamper ...
3
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2answers
147 views

Is prime regeneration necessary for every new session using a Diffie-Hellman key exchange?

As I understand it, bruteforcing a Diffie-Hellman generated secret key takes $P-1$ attempts to crack the shared secret, where $P$ is a very large prime used for modulus. If your $P$ is quite large, ...
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1answer
331 views

Why must curve25519 shared secret be hashed?

When using the curve25519 DH function, D.J. Bernstein recommends hashing the shared secret before using it as a session key for a symmetric block cipher. Why is that? Hashing won't increase the ...
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0answers
87 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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1answer
143 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 ...
2
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1answer
180 views

some of my confusions about DDH assumption

The wiki defines the decisional Diffie–Hellman assumption as follows: Decisional Diffie–Hellman assumption Consider a (multiplicative) cyclic group $G$ of order $q$, and with generator $g$. The DDH ...
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2answers
110 views

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 ...
8
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1answer
759 views

Diffie-Hellman: choosing wrong generator “g” parameter and its implications of practical attacks

In traditional DH one chooses two shared parameters: a large prime "p" and base "g", which is primitive root mod "p". Suppose generation algorithm is broken and "g" generates only a subgroup (group ...
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2answers
424 views

Vulnerabilities if encrypting the same data with 2 different keys

Let's say that Alice and Bob complete a key exchange and then Alice uses that key k1 to encrypt a 256-bit key kaes1 and sends it ...
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1answer
89 views

Does there exist a two-pass AKE protocol that is secure in eCK model and also has PFS?

As we can know that the best two-pass AKE protocols with DH message can achieve is the weak form of perfect forward security (wPFS) which guarantees security against the passive adversary. But ...
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2answers
2k views

AES encryption using a Diffie-Hellman key exchange

I have two systems which need to encrypt their communication by AES-128. The key for the AES encryption should be exchanged with the Diffie-Hellman (DH) algorithm. Now, I've read about DH and got a ...
4
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1answer
534 views

Why elgamal is still discussed

As I understand, if two users have a shared key, it is possible to encrypt a message using symmetric key encryption. So when a secret key is shared through Diffie-Hellman asymmetric key exchange, it ...
4
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1answer
96 views

Hardness of CDH in different groups

What is the difference of the CDH problem in different groups? In particular, given a group $\mathbb{G}_1$ of order $q$ that is a subgroup of $\mathbb{Z}_q^*$, $q$ prime, and another group ...
5
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1answer
226 views

How random is the shared secret in the Diffie Hellman key agreement

Being triggered by this horrible implementation in I2P shown by Sergei at Stackoverflow, I would like to know how random the value ZZ in the DH protocol is. ...
2
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1answer
110 views

Is generating public key inevitably slow in classic Diffie-Hellman if the private key is large?

Let's say I pick a 3000+ bit prime $p$ for which I also know that $q = (p - 1) / 2$ is a prime. If I've understood correct, then I can pick pretty much any $g$. Is generating $g^x$ now inevitably ...
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2answers
148 views

Stream cipher with Diffie-Hellman and Cryptographic Hash Function?

(Already asked the same question on stackoverflow but I was told to better ask here. http://stackoverflow.com/questions/17527192/one-time-pad-with-dh-and-cryptographic-hash-function) I know: Never do ...
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0answers
88 views

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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2answers
803 views

How to find generator $g$ in a cyclic group?

As generator $g$ is use in DH how to find combination of prime $p$ and $g$? eg: if we choose $p=23$ its generator is $7$ (given in book) how to find it?
2
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1answer
132 views

Using plain Diffe-Hellman for server authentication with known public key

Other considerations aside, is it possible to use DH with an established public key (together with fixed g, p, q) to safely authenticate a server instead of using some signing algorithm? In other ...
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4answers
340 views

Authenticated DH, what protocols are secure?

I read about STS + variants being insecure in the SIGMA paper, which then proposes SIGMA as a replacement. Are the SIGMA variants still considered secure or are there some other protocol that's ...
5
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1answer
175 views

Why use variable p, q, g for Diffie-Hellman?

In the book Cryptographic Engineering, it is said that fixing p, q, g for a key negotiation protocol based on DH is a bad idea (page 228 1st ed). But allowing for flexible p, q and g requires a lot ...
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3answers
13k views

How can I use SSL/TLS with Perfect Forward Secrecy?

I'm new to the field of cryptography, but I want to make the web a better web by setting up the sites that I host with Perfect Forward Secrecy. I have a list of questions regarding the setup of ...
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157 views

PAK Diffie-Hellman vs. sharing high-entropy key

In order to setup an authenticated shared secret key between two clients, I am faced with the choice between two possibilities: A: Let users set a (low-entropy) shared password and then perform some ...
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1answer
113 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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1answer
158 views

Does SRP reduce to DH key echange when shared password is not secret?

I can find JavaScript implementations of SRP (Secure Remote Password protocol), but nothing that inspires confidence for Diffie-Hellmen key exchange. I also have a separate need for SRP later. I ...