Key exchange protocols allow two parties to produce a secret session key over a public channel.

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Can you exchange a shared key without any hardness assumptions?

Imagine that P=NP, and one way functions don't exist. Can two people end up with a random shared key of arbitrary length, if every exchange is public? They have true RNGs, and know who they are ...
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108 views

RSA Protocol behind Yaksha Security System

So I'm reading over the Yaksha Security System and see it is based on the RSA cryptosystem and a centralized server, easy enough. What I'm slightly confused on is the math behind the related keys. It ...
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122 views

DH key exchange with only one end authenticated

We know there is a man in the middle vulnerability with unauthenticated DH key establishment. And the way to negate that is to use authenticate the keys used. But what if I only verify the signature ...
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290 views

What is “serial concatenation”? [closed]

I have a third-party point-of-sale API (Speedflow Pay-N-Get) that I am trying to communicate with over HTTP, but I have a problem with it. In a nutshell, a request is encrypted using RSA, and the ...
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200 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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659 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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199 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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97 views

Send a message to an entire group of recipients

I'm attempting to send a message to a group of recipients, through an untrusted intermediary. Each of the recipients will have been invited to the group by someone who already has a trusted public ...
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313 views

is this authenticated key exchange protocol sound? too complicated? obviously flawed?

(significantly reworded this question to fit better into crypto.se). I have static RSA keypairs for both sides and an encrypted transport that needs symmetric keys, and need to exchange session keys. ...
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30 views

Key Exchange MAC encryption order of events? Problem

I want to write the sequence of events of how a MAC encryption works for documentation at work. For this purpose I have a document (given by my boss to me) in German and I am trying to translate it ...
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61 views

Diffie-Hellman, RSA and the types of public key cryptography

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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36 views

ECDH security when no KDF is used

Let's suppose our device performs ECDH with a fixed, unknown, private key $prv$. It accepts as input any point $Q$ lying in the proper subgroup of the proper ellipict curve, then computes: $P = ...
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68 views

What is the lightweight key exchange protocol of choice?

Assume I have to implement a secure channel protocol by myself. Assume further that for some obscure reasons (library restrictions?) I can't use any of the pre-made libraries for SSH, TLS and such. ...
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variant of Diffie–Hellman key exchange protocol

Motivation Consider the following variant of Diffie-Hellman key exchange protocol for two parties: Assume Alice and Bob share $\mathbb{G}$ of order $q$ generated by $g$ Alice samples $a$ ...
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232 views

What are the problems of IKEv1 aggressive mode (compared to IKEv1 main mode or IKEv2)?

I would like to seek confirmation/clarification of the following statements about IKE: IKEv1 aggressive mode is supposed to be “insecure” if used with PSK. But as far as I can see, correct (or more ...
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98 views

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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51 views

EC based password authenticated key exchange protocol

I would like to know: What is the currently best (efficiency and security wise) known EC based password authenticated key exchange protocol? (i.e. is there any EC based protocol may be similar to DH ...
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42 views

Using a Secure RNG in Probabilistic Fair Exchange

I am attempting to implement probabilistic fair exchange described in Probabilistic Non-Repudiation without Trusted Third Party. A secures a message using AES with 512 bit keys (in my ...
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39 views

WPA and WPS: connection method [closed]

Do clients use WPS every time when they connect to a WPS-enabled AP? Or maybe they do it just the first time and then they switch to a normal WPA/WPA2 key exchange? If the latter, it's possible for ...
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137 views

Beginner question about secure communication with one user using DES

I am currently learning about encryption and I was asked a question: If a customer only had a DES system how could I secure messages as DES is not secure. I was researching this and I came across a ...
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108 views

Attacker in a key exchange

I'm having trouble with the following question: $X(A) \rightarrow B:(A,N_X)$ $B \rightarrow X(S):(B,((A,N_X,T_B),K_{BS}),N_B)$ $X(A) \rightarrow B:(((A,N_X,T_B),K_{BS}),(N_B,N_X))$ ...
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79 views

Why is it hard to calculate Logarithm on Diffie Hellman?

i read something about Diffie Hellman. What i don't understand: Alice selects a randomly generated Number, calculates X = g^x mod p and sends X to Bob. If she calculates g^x, she needs to calcluate ...
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exchanging private key between service and iPhone app - digital signing

I need to do digital signing of my request data which will be sent to the server. My server sends distinct private keys to her clients after authentication and expects all responses to be digitally ...
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107 views

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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Random numbers for Master Key and Key_block

Exploration through RFCs concludes that random numbers used in the creation of Master Key and Key Block(Key material for Encryption Key and Mac Secret) are from ServerHello.random and ...
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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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Password checking algorithm

I'm trying to reverse engineer a key exchange protocol which is based on the Diffie-Hellman key exchange scheme and faced with the following problem: Let $g$ be the generator, $m$ the modulus. Alice ...
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37 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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191 views

Is this hypothetical key-exchange secure?

I've thought about the following (generic) key-agreement protocol. The only weaknesses I could have found are missing freshness guaranteees. As for the notions: $E^1_K(P)$ is the symmetric encryption ...
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209 views

DHE key exchange. Is it really secure?

Context I recently came across a video on the net showing the process of key exchange in Diffie Hellmann. Alice and Bob want to communicate. Eve is the (wo)man in the middle. This video is the ...
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105 views

Is this a good communication scheme?

I'm writing a wrapper for a chat program to allow for end-to-end encryption. I'm using Python and PyCrypto to accomplish this. I'd like to use a well known library to do this, but to my knowledge, ...
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84 views

How can one protect against a Man In The Middle attack during key exchange without using certificates?

Besides using a certificate to defend against a man in the middle attack during a key exchange, what methods can be used?
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163 views

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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55 views

Different keys for clients but 1 key on server

Is it possible to have different keys on alot of (embedded) devices and just have 1 key on a server (to witch all these devices connect)? The main point here being when 1 device is compromised this ...
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70 views

Rationale of “r” AES key use in OTR version 3 AKE protocol?

I just tried to review & understand AKE (Authenticated Key Exchange) protocol as defined in OTR secure messaging protocol version 3 here , and aiming to achieve Perfect Forward Secrecy I am a ...
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107 views

What are the potential (major) flaws in this security scheme?

It’s been a few years since I read anything involving crypto. I’ve come across this scheme and I wonder how secure it is: Alice generates an RSA keypair (we assume Alice is using proper random ...
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146 views

Why do PKI schemes need security proofs? [closed]

When it comes to designing new PKI-based key-exchange protocols, why are security proofs needed? Without them, can we show a protocol's security? Does there exist a PKI-based key-exchange protocol ...
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Key exchange resilience to DoS attack

I am designing a user registration protocol for a system where I control both clients and servers. The aim is to establish a symmetric key for subsequent authentication, while minimizing the work ...
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110 views

Sending the next one-time pad key in this one-time pad message? [duplicate]

I am doing a research project on cryptography next year, and I'm really new to cryptography. This isn't some really advanced thing or anything, and I'm just trying to come up with something basic. ...
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124 views

Rsa Key Exchange Protocol

Moderators: Maybe this question will be move to other sub-forum. My question is Anybody know some link with a tutorial of how implement the RSA key exchange protocol using java for chat comunication?
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New to cryptology, studying key exchange

I'm just getting an overview of some of the concepts in key exchanging at the moment, and I'm a bit disoriented on a few topics, namely key stretching, key derivation functions and perfect forward ...
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75 views

Attack on key exchange with authentication

Suppose the honest parties - Alice and Bob - exchange public key in person/via telephone so that attacker can only eavesdrop on the line. Alice has hashed the public key using a collision resistance ...
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792 views

Problem with Python Implementation of Key Exchange part OTR Protocol

Being new to software development and crypto in general, I thought I would write a simple p2p encrypted chat app using the otr protocol detailed here: https://otr.cypherpunks.ca/Protocol-v3-4.0.0.html ...
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116 views

Diffie Hellman Key Exchange (Finding # of bits/digits of secret key)

As we know that in DH key exchange, both Alice and Bob would agree on the parameter $p$ and $g$. Next, Alice would choose a secret key $A$ while Bob would choose a secret key $B$. Alice would compute ...
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Advise key management protocol

Assume that sender and recipient both already have each others public keys(certificates). Sender needs to generate session key to encrypt some data and sends encrypted data to recipient. All offline - ...
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Two nodes in p2p network exchange data and read data in same time [closed]

I want to finde paper about exchange data between two nodes in p2p network. They must read each other's data in same time. In example two player a,b play rock scissors paper. A must read b's data ...
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43 views

Key Hierarchy and key Encryption

If i have two 56-bit DES keys, and I use these two key to encrypt a 256-bit AES key. Like this $K_{AT}$ encrypts $K_{BT}$ and $K_{BT}$ encrypts $K$ What is the effective security of key $K$? Is it ...
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how MitM compute server secret number in logjam

I would like to know how man in the middle can compute the server secret number if the prime is 512 bit in length and if knowing ($g$) parameter as in the following figure help him or not? is the man ...
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44 views

How does Blom's scheme work?

How does Blom's scheme work? Is the polynomial expression of it a normal one? A few additional questions: How big does the prime modulus need to be? Can it be bigger than the size of the derived ...
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HMQV and MQV group representation attack

Currently reading Krawczyk's HMQV paper https://eprint.iacr.org/2005/176 and trying to follow what he says about the group respetnation attack on MQV: It says that for any group of prime order $q$, ...