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Questions tagged [protocol-analysis]

Protocol analysis is the detailed analysis of the security of an abstract or concrete cryptographic protocol.

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1answer
46 views

Information Theoretic Oblivious Transfer?

Do we have information theoretic oblivious transfer protocols that are that can not be broken by a computationally unbounded attacker?
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42 views

Digital signature forging on a server, after entity authentication [duplicate]

Lets assume there is a guy called Ben, he contacts some server for a ticket number and the server returns a random number to him. Ben then signs that number and sends it back to the server, which ...
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39 views

No freshness protection for single messages, but for the overall protocol

I am given a key exchange protocol between a component $C$ and a key master $K$. During the overall execution 4 messages are transferred between $C$ and $K$. DH keypairs are always chosen randomly for ...
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1answer
32 views

Can I Implement a Well-Trusted Protocol with Well-Trusted Libraries in JavaScript securely?

Just for anyone reading this: DO NOT ROLL YOUR OWN ALGORITHM. Besides that, is implementing a well-trusted protocol with well-trusted algorithms and libraries a good idea? Is this likely to ...
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1answer
33 views

Order of OTs and commitments in Yao's Protocol for malicious adversaries

I was reading the book Efficient Secure Two-Party Protocols by Y. Lindell and C. Hazay. Page 92 of the book says This check is crucial and thus the order of first running the oblivious transfer and ...
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28 views

Constant number of rounds in Yao's Protocol for semi honest adversaries

According to me, every OT takes a constant number of rounds and since we have to do an OT for every input of one party, we'll have to do O(n) OTs which would correspond to O(n) rounds. But I have read ...
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1answer
55 views

What time is used in a TOTP counter?

I am currently working on a desktop TOTP client. From the TOTP spec at https://tools.ietf.org/html/rfc6238#section-4.1, the 6 digit OTP is generated from counter (in this case time). But what is "...
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52 views

From 2-round key-exchange protocol to a public-key encryption scheme CPA-secure

I have to solve this exercise and I really could use some help: Show that any 2-round key-exchange protocol (that is, where each party sends a single message) satisfying $Definition$ $9.1$ can be ...
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When Shamir secret sharing used in one time pad scheme

In the following, all operations are defined over a finite field of prime order. Please note that what I'm explaining below is a part of a bigger protocol, however, to keep things simple I just ...
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1answer
298 views

Contradiction to the Sequential Self-Composability of Black-Box Zero-Knowledge?

In short: it is well-known that black-box zero-knowledge protocols are sequentially self-composable. However, Goldreich and Krawczyk [GK90] present a protocol which is proven to be zero-knowledge (in ...
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How to prevent reflection attack in Needham-Schroeder in ECB mode?

We know that Needham-Schroeder is vulnerable to a reflection attack as shown here. How can I make it secure using ECB and without using timestamps? I need to modify the protocol. Otwan-Rees is a ...
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Equivalents of Yao's Xor lemma to rounds, or other hardness amplification methods?

Simple question: I just learn the existence of the great Yao's Xor lemma (see a quick sum-up at the end of this post). I'd like to use a similar argument, but instead of a simple function, I've a two ...
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Is my reasoning sound in the included scenario?

There is a package that contains a device, the device is activated/deactivated over a short range radio signal - it's function is to detect any unauthorised tampering of the package during the time it ...
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78 views

Security of the Needham-Schroeder public key protocol after a minor adjustment

I'm preparing for a computer security exam by going through some past paper questions. And I came across this particular one that I could not answer: We have the Needham Schroeder Public Key Protocol:...
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1answer
210 views

Interlock Protocol for Preventing Man-in-the-Middle (MitM) Attack

Trying to turn bookish knowledge to practical: Alice sends half of the encrypted data to eve, eve eavesdrop and tries to decrypt the data and fails as eve has only half of the encrypted block. So, ...
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51 views

Is it necessary to transmit two or three points of an elliptic curve?

Are there cryptographic protocols, where a party should transmit by communication channel simultaneously two or three $\mathbb{F}_q$-points of an elliptic curve over a finite field $\mathbb{F}_q$? ...
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3answers
2k views

Finding flaw in cryptographic protocol

If you find a flaw or bug for example in Linux kernel you can create an issue in GitHub, or if you can solve it you can contribute. How about Finding a flaw in cryptographic protocol?! How can you ...
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1answer
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Are the following asymmetric encryption schemes equivalent?

Consider the scenario where you want a machine to be able to send daily encrypted backups to a storage server. You'd prefer to not use simply a symmetric key for encryption, because if the machine ...
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73 views

Proving the lack of Perfect Forward Secrecy

In specific regarding the NSA's Key Exchange Algorithm, I read in an analysis of the protocol that perfect forward secrecy cannot be provided. Is there any way to prove or justify this? I couldn't ...
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1answer
46 views

Fairness and guaranteed output delivery

Why for achieving the fairness and guaranteed output delivery, less than a half of the parties should be corrupted? ($t < m/2$, where $t$ is the number of corrupted parties and $m$ is the total ...
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1answer
52 views

Which of these deductions are true about the simulator and the adversary?

Which of these deductions make sense? The simulator is actually being run by the adversary. It makes sense that the adversary initializes the simulator (which is just a program) with the input that ...
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1answer
62 views

Is function Deterministic or Probabilistic in the malicious security definition?

As you see, according to the attached definitions from "Efficient Secure Two-Party Protocols", the definition of the security in the semi-honest world has two cases for $f=(f_1,f_2)$.(deterministic ...
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1answer
169 views

How does future secrecy fail for Signal group messaging?

In the paper More is Less: On the End-to-End Security of Group Chats in Signal, WhatsApp, and Threema, the abstract claims that We additionally show that strong security properties, such as ...
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1answer
67 views

A problem exists with the security proof in the secure AND example in the malicious model [closed]

Imagine that the malicious adversary in the real world uses $Y = 0$ as its input, therefore he computes the output as ($X$ && $y=0$) = $0$. We can conclude that the real view in the real world ...
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1answer
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The interaction between the malicious Adversary and the Simulator and Real-Ideal' views

This example is from “Efficient Secure Two-Party Protocols-Carmit Hazay-Yehuda Lindell”. And the security proof of the above example is this: I have drawn a picture that is deduced from the above ...
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1answer
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How to use Real-Ideal model in a specific scenario (two party with cloud)

I have designed a two-party protocol where two parties outsource their input to the cloud. For example, Alice and Bob have inputs $X$ and $Y$, respectively, after which Alice and Bob encrypt their ...
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1answer
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Using the security definition in the malicious model to prove security in the semi honest model

Can I use the security definition in the malicious model in order to prove the security of a two-party protocol which is in the semi-honest model? (I am using IDEAL/REAL Model paradigm) Would you ...
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1answer
325 views

Getting a Keeloq manufacturer key

I have seen a lot of articles and research papers about breaking the Keeloq algorithm. My understanding is that the hardest part is getting the 64 bit manufacturer key. Some methods use crypto-...
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1answer
151 views

Basic encrypted custom protocol

I've been suggested to ask here instead of stackoverflow, so here I am. I'd like to encrypt the data of my application that uses a custom protocol I've created. I know about TLS and other protocols ...
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1answer
123 views

Advantages of a TLS handshake over pre-shared public keys

Lets assume the following setup: A vendor manufactures IoT devices where each is able to generate an individual private/public key-pair. Only the public key is exposed, the private key is assumed to ...
55
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2answers
10k views

Is the software that uses PGP broken, or is it PGP itself?

PGP is all over the news (even on TV) and there seems to be a lot of confusion about it. For the time being, people face articles like Attention PGP users: new vulnerabilities require you to take ...
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3answers
469 views

Using weak hash functions to construct a stream cipher

I am in the awkward position of developing a secure encryption scheme that provides data-at-rest security for a very limited embedded system. The system is only guaranteed to have 4 MiB of memory (...
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1answer
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Analysing communication complexity of a protocol

My question is more about the practical side of designing crypto. protocols and it is related to complexity. So, if you think it's irrelevant to this forum please kindly let me know and I will remove ...
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2answers
110 views

Fully homomorphic encryption with polynomials

I have read some texts concerning fully homomorphic encryption but didn't find the following simple idea. Let $p$ be a prime. We choose secret key $x_0$ in the interval $1\le x_0<p$. Let $\mathrm{...
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Has Telegram security been significantly improved with MTProto 2.0?

Telegram messenger's original encryption scheme, MTProto 1.0, has been shunned by most cryptographers for a number of reasons, like being vulnerable to IND-CCA attack; being unorthodox in general, ...
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1answer
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Why would we use a Key Agreement Protocol if we already have a key between 2 entities?

For example in my course there is this protocol : Initialisation: A and B are sharing a symmetric key S. (1) A -> B: Ra ; A generates a random number and sends it to A (2) A <- B: Rb ; B does ...
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1answer
160 views

Universal forgery based on mathematical problem

It is known that DSA admits universal forgery under assumption that the Attacker can solve the equation $x\equiv R^x\pmod p.$ Are there any other protocols admitting universal forgery based on non-...
2
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1answer
94 views

GCM and decrypting with Krack

I'm trying to understand how Krack works on GCM. I thought that looking at GCM would be easier since it's a full break and the video shows a tool that is able to decrypt everything. I see that the IV ...
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0answers
98 views

List of security protocols that have been proved to have bugs using formal methods

I'm looking for articles about finding bugs in security protocols using formal methods so I can test them using tools like proverif I've searched a lot of references but most of them seem to be about ...
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1answer
57 views

Proving knowledge of a value and computation that uses it

Alice picks random points P and Q in a group where DLP w.r.t. a base point G is hard (I am ...
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3answers
572 views

Security concern with TOTP

I am pretty new to cryptography and I have been studying up on TOTP (Time Based OTP) and while I have learnt how the computation works, I'm unable to figure out why using TOTP makes a system more ...
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0answers
285 views

what is scyther code for this protocol? [closed]

I wrote the following protocol. I wrote the code. I do not know how to write the rest of the code. please guide me. usertype SessionKey; usertype TimeStamp; hashfunction H1; macro sessionkey = H();...
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Is the crypto riot.im claims to use safe? [closed]

My friends want to communicate with me using “about.riot.im”, I tried to research them and the protocols they claim to use and I only found their own websites promoting it, so the whole thing seems ...
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1answer
180 views

Prove that this protocol based on square roots modulo $n$ is zero knowledge

Consider the following (zero knowledge) protocol based on the hardness of finding square roots modulo $n$. The Prover states that he knows a square root $x$ modulo $n$ of $y$, i.e., $y = x^2 \pmod n$. ...
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1answer
987 views

Free Start Collision In SHA-3

Given that the five sub-functions that comprise SHA-3 are reversible an individual can produce specific outputs of their choosing. The following is to my knowledge an example of a free start collision ...
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1answer
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How do I construct and use the Okamoto protocol simulator?

Proof Okamoto protocol has zero knowledge if we constructing a suitable simulator I found the following schema of Okamoto: But I don't really understand —how can I construct such a simulator and ...
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2answers
222 views

Modelling broadcast functionality in Universally Composable Framework

I'm studying the UC framework by Canetti. While I understand the basic idea, there are still a lot of details that I need to work through. One thing that's puzzling me is how to achieve concurrency ...
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1answer
134 views

Use of extra AES on top of SSL protocol [closed]

A client is communicating with a server through https, which should provide them with the security they need. I would like to add an extra layer of security. The protocol would look something like : ...
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Design a simplified version of the protocol for secure addition

To learn secure multiparty communication, I want to design a simplified protocol for secure addition, where instead of receiving two (random) numbers in $Z_p$, a player only receives a single (random) ...
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1answer
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Is a symmetric cryptographic code secure if only the sender and recipient know the method used to encrypt? [closed]

I recently designed an encoding method that I'm fairly sure is unique. Is it universally secure, provided that anyone intercepting the message doesn't know anything about how it was encrypted? Could ...