Cryptography

Questions tagged [key-exchange]

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

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Fast key exchange algorithm in hardware with small key size

Which asymmetric algorithm will be best only for key exchange to set up communication using symmetric cryptographic algorithm. Comparison should be in term of speed, key length and their hardware ...
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Combining Post-Quantum and Classical KEM

I came across this paper "Hybrid Key Encapsulation Mechanisms", were three methods are defined that allow a secure combination of a classical key encapsulation with a post-quantum key ...
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Secret sharing such that all shareholders obtain access to the secret (one shareholder can't just run off with the shares)

Say, using something like Shamir's polynomial scheme, you split a secret $x$ among $n$ people (each given a "share" of the secret) such that you need all $n$ shares to recover the secret. ...
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A source of randomness that anyone can independently, conveniently and robustly access?

Does there exist a source of randomness that anyone in the world can independently, conveniently and robustly access? For example, the 10th decimal place of the temperature in Mexico City is ...
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Verifying result of (EC-)Diffie-Hellman

I received a public key by JSON. For the example, I have 4 keys: 2 public keys and 2 private keys. ...
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XDH vs DH and ECDSA vs EDDSA

I am new to security area. I came across mutiple words I can't understand and there is little infomration I can get from google. What is XDH/XEC, is the X means 'enhanced'? Are they just have a ...
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Why is an ephemeral key required to prove possession of a static private key in Key-Establishment Schemes

In the NIST 800-56A rev3 "Recommendation for Pair-Wise Key-Establishment Schemes Using Discrete Logarithm Cryptography" in section 5.6.2.2.3.2 "Recipient Obtains Assurance [of the ...
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25 views

How to calculate a common encryption key between sender and receiver

In the picture below, for the text underlined in red color: 7X(MOD 11) = 72(MOD 11) =49(MOD 11) My questions are: (1) obviously there is no equal relationship between 72(MOD 11) and 49 (MOD 11), and ...
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45 views

What can I use for key exchange?

I am trying to send AES keys from one computer to another, but I need to provide some form of attestation of the key's provenance. How should I go about doing this?
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CK vs BR Key Exchange Security Models

I'm writing a paper on Authenticated Key Exchange Protocols. I've read Bellare and Rogway's seminal paper on the subject and I think I understand BR Model and I'm now reading Cenetti and Krawczyk's ...
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37 views

What are the explicit usage of different keys derived from SKEYSEED in IKE?

We have seven different keys derived from DH key and nonces via PRF in IKEv2 as skd, skai, skar, skei, sker, skpi, skpr. Why different keys are generated for initator and responder for encryption? ...
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37 views

Why pre-shared key is not involved to key derivation in IKEv2?

In IKEv1 (RFC 2409), preshared secret is involved to key derivation where IKEv2 (RFC 7296) use it for only authentication. When we consider post-quantum security, this property makes IKEv1 suitable if ...
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SIDH: What if the two kernel generators are chosen in the same torsion group?

In SIDH, either party chooses its secret point $R_A = [m_A]P_A+[n_A]Q_A \in E[\ell_A^{e_A}]$, $R_B = [m_B]P_B+[n_B]Q_B \in E[\ell_B^{e_B}]$ from two different sets $E[\ell_A^{e_A}]$ and $E[\ell_B^{e_B}...
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When is a PQ key-exchange algorithm suitable for use with long-term static keys?

I took a look at Cloudflare Circl because I'm curious which Post-Quantum algorithms are implemented in Go, which could be used to exchange a key. I read this comment that SIDH is only good for ...
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Can Merkle signatures be leveraged for key exchange?

A Merkle signature scheme is post-quantum-suitable as it relies only on the security of a one-way function. However, this construction seems to only be capable of authentication, and not ...
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Under which conditions certificate is required for IKEv2?

In IKEv2 document, there are expressions as [CERT,] or [CERTREQ,] in the parts of IKE_SA_INIT or IKE_AUTH exchanges. In this notation brackets indicates that it is optional. I didn't see expressions ...
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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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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 ...
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How to decrypt raw TLS bytes?

Is there any tool / way to decrypt raw bytes of a TLS 1.2 session, when the master key is known, even if forward secrecy is used? The closest I found is this. I know its possible using Wireshark, but ...
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105 views

What is the simplest post-quantum asymmetric cryptographic algorithm for key exchange?

This question has been burning in the back of my head for the past few days, and a quick Google search doesn't yield any usable results. As the title suggests: What is the easiest-to-understand post-...
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Comparing Diffie Hellman and EC Diffie Hellman sizes

So, I am currently trying to understand the differences between DH and ECDH. I understand the basics between how the two algorithms work, however, I do not clearly understand how to compare the ...
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47 views

Intuitive explanation for key exchange with public/private keys

I want to teach some people about crypto basics, and one of the topic will be key exchange with a public and private keys. The audience is made of people working in InfoSec, but mostly junior fresh ...
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1answer
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What does DH(IPKa, IPKb) notation mean? Diffie-Hellman confusion

I'm trying to implement X3DH (extended triple Diffie-Hellman) in a network of mine. The way I understand it, a bundle (IPKb,SPKb,OTKb) is retrieved by Alice from Bob, through the server, and using her ...
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Is the following PAKE protocol UC secure?

Consider the following (simplification of a) PAKE protocol: Alice and Bob start with a pre-agreed password pw. To establish a new session key $k$, first Alice samples a random nonce $r$ and sends it ...
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56 views

what motivated the design decisions of RFC 8291 ("Message Encryption for Web Push")?

Related question here. I'm reading RFC 8291, which describes a protocol to protect web push messages sent between an application server and a user agent (typically a mobile browser or other mobile ...
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Does IND-CPA to INC-CCA effects on the error probability?

If we have a key encapsulation mechanism (KEM) that is IND-CPA secure, with some error probability $P_e$ (probability that the shared keys are not equal). The goal is to use the Fujisaki-Okamoto ...
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AKE using Noise_NNpsk2 vs Noise_NKpsk2

I am working on an implementation based on the Disco library, which itself is based on Noise and strobe framework. The goal is to do bilaterally entity-authenticated key agreement with perfect forward ...
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56 views

How efficiently can an attacker forge parts of a fingerprint?

If two devices do not trust each other yet, you can't simply send the correct fingerprint across: you have to manually verify it. I am looking into the security of comparing only random parts of a ...
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Simple description of a multi-party fair exchange protocol with trusted third-party

Imagine Alice wants to sell a file with important information to Bob for money. The file must be encrypted, so nobody can eavesdrop. It must also be signed upfront, so Bob knows it has not been ...
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1answer
81 views

Is there any standard present to measure strength of a generated key using true random number generator?

I was wondering how to measure the security strength of a generated secret key for the below protocol: "A 128 bit random number is generated by TRNG and the random number will be used as a ...
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Finding Diffie-Hellman secret key with only some know values [closed]

I currently have the following information and I assume the first line separated by the: represents the prime number and generator. Is there any way I can find the secret key shared for the Diffie-...
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In group key agreement/exchange protocols, does key independence implies forward and backward secrecy?

Kim et al. [here] define Foward and Backward secrecy as follows: Forward Secrecy (not perfect foward secrecy): the inability for a passive attacker to obtain past group keys from a subset of new ...
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What is the extra defense that McEliece 6960119 and 6688128 have that 8192128 does not?

From https://csrc.nist.gov/CSRC/media/Projects/post-quantum-cryptography/documents/round-3/official-comments/Classic-McEliece-round3-official-comment.pdf The 8192128 parameter set is bigger, but the ...
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Dragonfly masked DH

I was looking at Dragonfly key exchange protocol RFC 7664, and it seems to make use of masked Diffie-Hellman in the final part. Generating two random numbers less than group order $q$, $m$, and $r$ ...
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63 views

Can we do Multi-Prime Diffie-Hellman?

What is the correct way, if any, to do Multi-Prime DH? From security point of view, is there any benefit to do it? Multi-Prime is not about multi participants. Multi prime is when we use two or more ...
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Diffie Hellman with Smartwatch

I am trying to reverse engineer the Handshake wit a Smartwatch by sniffing the BLE traffic. There is what I figured out and were able to reproduce: The manufacturer-app requests a random number from ...
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1answer
324 views

Curve25519 Key Validation

According to the original paper of Bernstein, there is no key validation needed when using Curve25519 for Diffie-Hellman Key Exchange. However, where does this property come from? Is there any proof ...
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How to exchange keys despite a MITM, using some short known password?

The simple scenario is two devices on one WiFi network sitting side by side. The user can copy a short code from one device to the other to be the "password" however this code can't be too ...
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1answer
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How to sign messages using the NOISE Protocol Framework?

I wanted to learn more about secure messaging and started a personal project using the libhydrogen library. From my understanding, instead of using public-key ...
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1answer
154 views

ECDH/ECDSA key exchange on embedded devices

Two devices are communicating only with each other and shall do this with AES encryption. Both devices are offline, have very limited storage and use a small cryptographic coprocessor which is able to ...
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195 views

MAC-then-encrypt in SIGMA protocol for authenticated key exchange

The SIGMA protocol proposed in 2003 and used in TLS 1.3 and IKE stands for "SIGn-and-MAc" and can optionally protect identity using encryption. The SIGMA-I variant illustrated below ...
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Post-quantum asymmetric encryption algorithms

One could use Falcon-512 for establishing a private and public key for an asymmetric context. Whereas in a symmetric context, we could use firesaber/saber/variants in order to obtain a shared secret ...
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Key storage management of session keys and long term keys

In key management, one of the topics is key storage, I want to know where and how these keys(long-term and short-term) are stored? For example I read that android password apps use KeyStore api to ...
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1answer
67 views

Symmetric Exchange Key Sharing

Question: Suppose that there are 10000 = 10^4 banks and 10 payment card organisations (PCOs). How many secret keys will be needed if each PCO shares a unique secret key with each bank? How many extra ...
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Is this variant of Diffie-Hellman viable and quantum resilient?

In this paper the author suggests using a variant of Diffie-Hellman which involves floating-point numbers of arbitrary size in the generation of a shared secret. There are no primes, calculations are ...
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Securely transmit signed token (e.g., JWT) over insecure channel

Apologies if this is already answered somewhere, but I could not find it. I have a setup where a client has a JWT (RSA- or ECDSA-signed), and needs to prove that fact to the peer which issued the ...
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Why is this not a viable key exchange algorithm? [duplicate]

I was just wondering why this kind of algorithm can't be used instead of, say, Diffie-Hellman to exchange keys: Alice decides on a key she wishes to share with Bob. Alice generates a stream of bytes ...
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Rounding function used in Saber Key Exchange

In Saber: Module-LWR based key exchange, the authors use a rounding function called $\textit{bits}$, defined (in page 3) as follows: $bits(x, i, j)$, with $j \leq i$, gives $j$ consecutive bits of a ...
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104 views

TLS 1.2 Server Key Exchange problem

Below the Wireshark snapshot of the handshake compliant to the TLS 1.2. The position 1 displays the enum ServerECDHParams where I can use any curve having the own ID RFC8422. I'm free for example to ...
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Can TLS be used simply to exchange keys using the handshake protocol?

From what I understand, in general, TLS consists of two protocols: 1 - Handshake protocol: uses public-key cryptography (ECDH, DH) to establish a shared secret between the client and server. 2 - ...

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