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Questions tagged [protocol-design]

Design of cryptographic protocols, i.e. ways of using algorithms (primitives) to achieve one or more security goals like integrity, confidentiality, authenticity (maybe together with non-security-related goals). If you ask about a specific protocol, tag with its name instead (or additionally, if ...

2
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1answer
65 views

Extending the Socialist millionaire problem to three parties

The usual solution of socialist millionaire problem allows to authenticate whether TWO parties have the same secret information. Are there any natural solution of the same problem for THREE parties?
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5answers
2k views

How to use proof of lack of knowledge?

This is a purely hypothetical example but is provable ignorance useful in cryptography? For example, let's say I have a trapdoor collision resistant function. I know the trapdoor and therefore some $...
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0answers
26 views

Does the following protocol provide Authenticy?

Alice: calculates and send $(m, \operatorname{SHA1}(key, m))$ Bob: receive the message and extract $m$, check the hash using the shared key Does this protocol integrity? Why is HMAC better than this ...
3
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1answer
80 views

In a TLS 1.3 connection, how do the client and server choose a mutual cipher suite?

According to KeyCDN, TLSv1.3 connections are established in a single round trip: While TLS 1.2 requires two round-trips to complete the TLS handshake, 1.3 gets the job done in just one trip If ...
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1answer
28 views

rolling key with two keyfobs

Hy i just saw this Question: How does a rolling code work? And I just wondered how it is possible that my Vehicle (A 2013 Mazda 6) has two key fobs? As i understand this shuld not be possible. ...
2
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1answer
63 views

Detect MITM with hashed session-key — Is my protocol secure?

I have the following setup: An app ("client") wants to communicate securely with a device I created ("server"). The client has the public key of the server (and a hashed password) preinstalled. I use ...
1
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1answer
44 views

Authentication protocol for communication with Arduino Uno

I am using an ECDH key exchange to establish a shared secret between an Arduino Uno and an Android device. For this purpose I am using this library and more specifically Curve25519. This is the ...
2
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2answers
92 views

What's wrong with this digital signature scheme?

With regards to user authentication... The server sends Alice a randomly chosen number. Alice signs the number and sends the signature back to the server. The server checks the signature using Alice'...
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2answers
67 views

What conditions does a provable secure cryptosystem satisfy?

A cryptosystem can be entirely dependent on its underlying problem or it can have a reduction to other known problems. Solvability can be affordable if there exists a reduction of the base problem to ...
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1answer
44 views

Computing/using RSA modulus without knowing its factors

There are some protocols, e.g. this, that require parties to compute/use RSA modulus: $N$ without knowing it's factors or $\phi(N)$. One way of doing it is to use a trusted party. Question: Is ...
1
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1answer
43 views

Instead of sending a set of random values, send a key for a PRF

Some Protocols require one party to send $n$ random challenges (or random values) to another party. For communication efficiency purpose, one can pick a random key for a pseudorandom function, and ...
0
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1answer
38 views

Economically Rational as a security proof

Is it possible to prove the security of a cryptocurrency payment protocol by saying that the actors will seek to maximize their profits. Would I need to set this up as a game and prove it that way or ...
0
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1answer
42 views

Time-release Protocol Implementation

“How to Build Time-Lock Encryption” by Jager “Time-release Protocol from Bitcoin and Witness Encryption for SAT” by Liu, Garcia, and Ryan I looked through the references, googled, searched GitHub ...
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1answer
38 views

How can process algebra help to design security protocols?

Would you please tell me how process algebra could help me to design security protocols? More specifically, can I use it for proving the security of protocols? Is process algebra used for any ...
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1answer
38 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 ...
0
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1answer
41 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
50 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 ...
6
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1answer
113 views

Reason for 3 rounds ChaCha in ChaCha20Poly1305@openssh

OpenSSH uses a slightly different ChaCha20Poly1305 AEAD construct then the one proposed by A. Langley and others for TLS. The TLS version The openSSH version (and here) The openSSH version uses two ...
1
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1answer
55 views

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 ...
0
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1answer
32 views

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 ...
0
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1answer
62 views

Zero-Knowledge Bingo

Bingo is the game of chance where each player matches the numbers on their card with the numbers that the caller draws at random. When the first player has collected enough called numbers on their ...
2
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1answer
104 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-...
5
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1answer
98 views

How do mix networks work with server downtime?

I am looking for an analysis of practical mix networks with server downtime. For example, if a message is supposed to go from A -> B -> C -> D, what happens if B wants to send the message but C is ...
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1answer
28 views

Should I use failed decryptings to handle multiple session keys?

I feel like this question is specific enough to stand on its own. Say I have a set of clients each with a unique ID and already established pair of session keys for each client. When a client ...
0
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1answer
63 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 ...
2
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0answers
41 views

Can “X3DH” instead of HMQV be used for OPAQUE?

So I recently re-read the OPAQUE paper and while reading it I found HMQV to be a bit "unnatural" / "unintuitive", which may just be because I didn't (yet) read the HMQV paper, so I wondered whether ...
1
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1answer
132 views

Zero-knowledge transfer of value protocol II [closed]

This is an improvement of the protocol described here. The protocol does not require trusted setup and is very efficient (much more efficient than anything else I could find). The protocol allows the ...
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0answers
24 views

Threshold signature scheme with combiner outside the group

Assume we have a (t,n) threshold signature scheme in which the secret key is shared between the n group users. If at least t users sign a certain message . Can we build the scheme in such away ...
2
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0answers
34 views

breaking anonymous key exchange protocol

An anonymous key exchange protocol $P$ is pair of probabilistic machines $(A,B)$ that sends messages to each other and at the end, both $A$ and $B$ will share the same key $k$. The attacker can see ...
3
votes
3answers
274 views

Zero-knowledge transfer of value protocol inspired by EC El Gamal

This is a follow up on the question I asked here. I designed a scheme that allows the following: Alice has a value $a$ which she wants to keep secret Bob has a value $b$ which he wants to keep secret ...
0
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0answers
19 views

Interpretation of the PBFT paper by Castro and Likov

I find the following paragraph at the end of pag.4 of the PBFT paper by Castro and Likov confusing: This is true because $prepared(m,v,n,i)$ and $|\mathcal{R}|=3f+1$ imply that at least $f+1$ non-...
6
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1answer
135 views

In coin flipping protocols, why aborting is allowed, and why the non-aborting party flips a coin at the end?

To convey how an adversary can bias the coin, most often a simple commitment-based two party coin-tossing protocol is given, as in [1]: Alice sends Bob the commitment $c = commit(x)$ Bob sends Alice ...
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0answers
145 views

Leak information (new setting)

This question is similar to Leak Information?, but with new settings. Let $a,b,c,d,e,f$ be selected at random from $\mathbb Z^*_q$. Also, let $r_1,r_2,r_3,z_1,z_2,z_3,p_1,p_2,p_3$ be selected at ...
3
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1answer
65 views

shared symmetric key

Suppose $R$ is a random challenge sent in the clear from Alice to Bob and $K$ is a symmetric key known only to Alice and Bob. Which of the following are secure session keys and which are not? Justify ...
0
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1answer
38 views

How can I prove I'm the recipient of an encrypted message without revealing my identity and the message?

Alice wants to leave a message for Bob. So she encrypt a signed message with Bob's public key. Then leaves it at a dead drop service. Then later Bob arrives and checks the messages and attempts to ...
2
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1answer
51 views

Would it be possible to negotiate a key of 128 or 256 bits strength using Merkle puzzles?

One can be fascinated by the simplicity of the schemes created by Ralf Merkle; like the Merkle tree or his key negociation protocol over an insecure channel. Wikipedia has some material on "Merkle ...
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1answer
140 views

Storing Salt in Ciphertext?

I found the cryptomator encryption documentation online, because I was looking for something to learn about x). I altered the process a bit to fit "my need" / so that I would not have a vault, but a ...
2
votes
1answer
115 views

Key establishment from secret answer to a question using Socialist Millionaire Protocol

I have read this as telling that two parties sharing a low-entropy secret (such as a secret answer to a question) can use it to establish a high-entropy shared secret key across a classical insecure ...
3
votes
3answers
121 views

How to thwart a cleartext message's size?

Here is a question I don't find addressed in my cryptography books: let's say Alice want to send Bob a message of length N. what is the proper way for Alice to hide N? I am thinking Alice should do ...
2
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2answers
200 views

Doing RSA-KEM with RSA-OAEP

I am fiddling around with the JS Web Crypto API and creating a hybrid encryption system that uses a symmetric key to encrypt form data with AES256-CBC and and a public/private key-pair to wrap/...
1
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1answer
37 views

Does semi-honest adversary eavesdrop honest parties communication?

Assume we have a multi-party computation protocol and the protocol definition, where both of them consider a semi-honest adversary, without explicitly assuming the type of channel the parties want to ...
0
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1answer
48 views

Conditional $k,n$ secret sharing

I would like to know if there is a way to place a condition on $k$-out-of-$n$ secret sharing such that out of $k$ participants who can pool their shares to reconstruct a secret, one of them should ...
3
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0answers
60 views

X.509 strong authentication protocols

Is there also a protocol definition in X.509? In "Handbook of Applied Cryptography" by A. Menezes there is a reference to X.509 strong authentication protocol, nevertheless, I could not find the ...
4
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0answers
92 views

Proof of knowledge of exponentiations

I am reading a paper of Furukawa and Sako, "An efficient scheme for proving a shuffle" from 2001. This paper writes a protocol for verifiable shuffling in mixnets. Their protocol make use of ...
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0answers
57 views

Payment system with anonymity towards seller and payment provider

Tl;dr: How would you construct a protocol for a payment system, where the seller doesn't know the Id of the buyer, and the payment processor doesn't know what's being bought (but can say if it's been ...
3
votes
1answer
62 views

Robust combiner for preimage resistance

My question is about chaining two hash functions to create a robust combiner for preimage resistance. @Zooko already gave a detailed answer here: $$H_{\circ}(x) = H_0(H_1(x))$$ is as strong as ...
2
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0answers
65 views

Why won't a BFT protocol using simple signing/voting work?

PBFT and most consensus algorithms are more complicated than a simple voting scheme. By simple voting I mean the following: we require all nodes to sign their local copy of the state and send this ...
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2answers
81 views

Bellovin ESP Attack on UDP Packets

In Bellovin's ESP Attack, it is stated that: If $L_A$, $L_B$ are using UDP, attack is easier: Wait till session ends Allocate $L_B$'s UDP port to $X_B$ Replay all packets So, how ...
5
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1answer
329 views

Secure group chat with large groups

I was reading in Whatsapp group chat the sender encrypts a key individually for each memeber. And everyone invalidates their key whenever anyone leaves the group. Whatsapp also limits group size to ...