Questions tagged [provable-security]

A primitive or protocol with provable security is accompanied by a mathematical proof that shows how to reduce the security claims about the protocol to a set of assumptions.

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How to evaluate number of security bits of a signature scheme?

I am trying to evaluate the effective security levels that a digital signature scheme (namely post-quantum digital schemes like WOT-S (Winternitz one time signature scheme) and Dilithium Crystals). ...
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What's the difference between H(m) $\oplus$ H(key) and H(m || key) in MAC construction?

We all know that HMAC has the form of $H(ikey || H(okey || M))$. I'm wondering what if we replace the H(okey || M) into H(okey) $\oplus$ H(M)? In other words, is $$H\Big( ikey \mathbin\| \big( H(okey) ...
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What is meant by the probability distribution of a verifier?

I'm having trouble understanding the role of probability distributions in the definitions of computational/statistical indistinguishability, especially in the definition of zero-knowledge. From A ...
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About CPA or CCA security proof

Recently reading about CPA, CCA safety security proof. After reading a few papers, the author is in the process of proof. The challenger maintains some lists. I really don’t understand why the ...
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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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Data Security and Encryption

I have a research paper on data security using proxy re-encryption, and I need to provide an answer to a reviewer's comment. However, I have no idea on what to do or how to answer it. The security ...
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1answer
425 views

Is it safe to use many nonces for XChaCha20?

I have a very simple question. Since XChaCha20 nonces are 192-bit, there's technically no limitation to the max number of message encrypted, since the chances of two random nonces being the same are ...
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55 views

How do we know a single differential/linear trail dominates the others?

I have read the proof of AES's resistance against differential cryptanalysis. In the proof the authors show that there is no single differential trail with prop ratio higher than $2^{-300}$ over 8 ...
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Constructing a mac scheme secure to regular definition but insecure to special case

The mac-forge security game for mac schemes looks like this: $k \leftarrow \operatorname{Gen}(1^n)$; $(m,t) \leftarrow A^{\operatorname{MAC}_k(⋅)}(1^n)$; Let $Q$ denote the set of all queries that $A$...
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Question on the CPA-security proof of the CTR-Mode

While reading Katz & Lindell's textbook (2nd edition) I found a proof for the CPA-Security of CTR-Mode. It can be found at the end of chapter 3.6.2 (page 93 and 94). I don't quite understand, why ...
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IND-CCA game proof of Encrypt-then-Mac

In [BN07, Subsection 4.3], the authors show that if a symmetric scheme $\mathcal{SE}$ is IND-CPA and a MAC $\mathcal{MA}$ is EUF-CMA (though they use the SUF-CMA notation, but I think it refers to the ...
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Hashes as mutually independent keys

For cascading encryption (ie. AES-Twofish-Serpent), the most secure implementation is with mutually independent keys. My question is if it is secure to hash the previous key for each layer of ...
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Authenticated sharing in MASCOT protocol

I am studying MASCOT protocol for multiparty computation and I am stuck in the comprehension of the authentication protocol $\Pi_{[[ \cdot ]]}$ and its security proof. Supposing the initialize phase ...
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Combined scheme security proof

Let CS be a combined scheme of $n$ public key subschemes. CS is composed of two algorithms Setup and KeyGen, that all the subschemes share, plus all the other algorithms of each subscheme. Suppose ...
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Can a broken cipher reveal the key?

Suppose I cascade cipher A and cipher B to encrypt a message. Both ciphers use the same key. I know that theoretically, the security of the setup when used correctly is at least as secure as the most ...
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Are key-prefixed schnorr signatures still relevant?

My question refers to the papers "On the Multi-User Security of Short Schnorr Signatures by Blocki and Lee" and "Multi-user Schnorr security, revisited by Bernstein". The main ...
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Does the implication “security classic schnorr $=>$ multi-key security key prefixed schnorr” apply to short Schnorr signatures?

My question refers to the paper "Multi-user Schnorr security, revisited" by Bernstein. Does the implication shown in the paper security of classic Schnorr $\implies $ multi-key security of ...
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Clarification of Advantage vs Probability/Success of an Adversary

In cryptography, for a polynomial time-bounded adversary $\mathcal{A}$, given a scheme $\Pi$, the success or probability of succeeding $\mathcal{A}$ is the likelihood for $\mathcal{A}$ to break $\Pi$, ...
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How to generate a random point on an elliptic curve without knowing it's corresponding scalar private key

Given an elliptic curve with generator $G$, is it possible to generate a random point on the curve $Q = a \cdot G$ without knowing the secret value $a$ that generated it? Note that just using an $a$ ...
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An exercise to show that CBC Chain is insecure

The CBC-Chain mode of operation is a CBC variant in which the IV that is used for the very first message to be encrypted is randomly selected, whereas the IV used for each subsequent encrypted message ...
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Question about IND-CCA security

We are given the following securitygame $G^{\text{LOSTNAME}}$: Generate $k \leftarrow \{0,1\}^{\kappa}$ uniformly at random Choose $h \leftarrow \{0,1\}$ uniformly random Create decryption oracle $\...
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Where to find a comprehensive content on security proof systems in cryptography

Proof systems (provable security) are seen in many research papers related to cryptography and security. Where would I be able to find a comprehensive resource covering the purpose and the ...
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Why do some people believe that humans are “bad at” generating random numbers/characters like this?

I'm not even sure if they are serious, but I've heard many times that some people refuse to not only trust their computer to generate a random string (which is understandable) but also don't trust ...
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How secure is Encrypt-And-MAC?

From this popular Crypto.SE thread, the general feeling is that Encrypt-Then-MAC is the most secure. From the first answer, the poster says that Encrypt-And-MAC provides integrity of the plaintext. If ...
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SHA3_512 as dynamic SBOX generator

I'm creating a substitution-permutation cipher (for learning purposes only of course) and I'd like to implement dynamic SBOXs. The cipher will have a block size of 128, and 32 rounds. These SBOXs will ...
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293 views

Is E_k(M)||H(m) form of MAC secure?

Would this encryption and signing scenario be considered tamper-proof? ...
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Security of ChaCha compared to AES and Serpent

I'm trying to understand how secure XChaCha20 is in comparison with other ciphers. From what I know, ChaCha is more secure than AES, but less secure than Serpent. My question is how much more "...
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Is it possible to convert a selective secure broadcast encryption scheme to an adaptive secure broadcast encryption scheme?

I was reading the paper, entitled "Multiparty Key Exchange, Efficient Traitor Tracing, and More from Indistinguishability Obfuscation", by Boneh et al. On page 23, the authors have claimed ...
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325 views

Is it theoretically possible to create an unbreakable cipher?

I know this question might sound strange, but is it theoretically possible to create an unbreakable cipher if we don't consider bruteforce? Some of us believe that it is possible to create ciphers and ...
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May it be possible that the trusted third party knows the parties' private keys in an “ideal model” of a Secure multiparty computation protocol?

May it be possible that the trusted third party knows the parties' private keys in an “ideal model” of a Secure multiparty computation protocol? I wonder if it may be possible that a trusted third ...
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Why don't we use Blowfish if it hasn't been cracked?

Since Blowfish is old, well-audited, and has no published attacks, why are we using AES instead? I know that Bruce Schneier said that Blowfish is insecure and told people to transition to Twofish, but ...
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Cascading encryption with blocks or output

For educational purposes, I'm trying to build a cascading encryption implementation with AES, Twofish, Blowfish, Salsa20, and ChaCha20. First of all, please don't judge my paranoia or choices, this is ...
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Not understanding deterministic authenticated encryption (DAE) security definition

I have a question regarding the security definition of deterministic authenticated encryption (DAE) as defined by Bellare and Shrimpton. Their definition is reproduced below, and my question pertains ...
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Unconditionally secure block cipher for a finite number of blocks

I am wondering if someone can point me toward a block cipher that provides unconditional security for only a bounded number of encryptions. For example, you can encrypt $h > 1$ length $n>1$ ...
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How to formally prove that random guess is the best attack

How to formally argue that there exists no efficient $A$ who can come up with a $e$ such that $$\begin{align}x&=PRF(e,0) \land \\ y &= PRF(e,1) \land \\ x &= H(y)\end{align}$$ where $H$ is ...
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Solving RLWE modulo a prime ideal

Suppose you have the following set up for RLWE: $K$ is a cyclotomic field of degree $n$ over $\mathbb{Q}$, and $p\in\mathbb{Z}$ is a prime integer that splits as follows in $R = \mathcal{O}_K$: $p\...
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Is it meaningful to allow parties to use a non-uniform random tape in 2-party computation?

Consider a trivial example where two semi-honest parties want to flip a biased coin. Since the sender is semi-honest, the sender can locally flip a biased coin and send it to the receiver. A natural ...
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Is this a provably secure scheme to encrypt a private key?

I got this idea to use the features of the one-time pad, an encryption technique that provably cannot be cracked, but this time applied to encrypt a private key. A possible application could be to ...
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Technological based solution needed for a real world problem of transferring a physical object between two untrusted parties [closed]

Suppose I have a physical object A. Another person also has some rights over the object A. After some litigations, a schedule has been made under which object A will be passed for some time to the ...
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Can a PRF distiguisher invoke the function's algorithm?

The definition of a function $F:\ \{0,1\}^n\times\{0,1\}^n\to\{0,1\}^n$ being a Pseudo Random Function Family (PRF) is that it's implementable by a P.P.T. algorithm $\mathcal F$, and there is no P.P.T....
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Is ORAM security definition adaptive?

Based on the "obliviousness" definition of the access pattern of the ORAM: Given the RAM, after a sequence of read/write operations, the physical addresses accessed will be computationally ...
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Which of those functions are PRFs?

Assume that $F: \{0,1 \}^n \rightarrow \{0,1 \}^n$ is PRF. Examine if the following functions are PRFs: $$ \begin{align} 1. \, F_1(k,x) &= F(k,x) \oplus x \\ 2. \, F_2(k,x) &= F\left(F(k,0^n), ...
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Using Optimal Prime Field in ECC

"Optimal Prime Field is a family of 'low-weight' prime fields that allow for efficient software implementation of all operations requiring a modular reduction, in particular the field-...
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116 views

Security proof, Sequence of games

I hope someone can help me with the sequence of games security proof for protocols. Here are my questions: What is the aim of each individual game? i.e. why can't we do this in one indistinguishable ...
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Extractability of random oracle in simulation-based proof

I am working on the simulation-based proof, where Random Oracle (RO) is used to extract malicious parties' input. In particular, the RO considered in the proof has a small range (whose size is ...
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Security Proof, Sequence of games and birthday paradox

The sequence of games security proof approach is very confusing. For example, I am trying to understand it in the following paper https://www.dropbox.com/s/dhsgsognxviwacv/Provably%20Secure%...
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Boneh DDH Paper - Sampling Integers in Random Reduction

I've been reading Dan Boneh's DDH paper, in particular section 3.1 which covers DDH randomized reduction. The first two sentences of theorem 3.1 state: Let $\Bbb G = \{G_p\}$ be a family of finite ...
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What is a quantifier in game-based definitions?

I'd like to know what are quantifiers in game-based security definitions (in cryptography) Some relevant examples and sources are also much appreciated. Edit: for example, in a formal definition, we ...
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A combination of game-based and simulation definitions to analyse a protocol security

Consider the case we want to design a crypto protocol: $P_1$, say for verifiable computation. We define the protocol by designing a game and prove its security. Consider that we want to design another ...
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Can we determine a security parameter in the hybrid argument where the number of hybrids is polynomially bounded but not known?

Let $\lambda$ be a statistical security parameter. Consider a security proof that is based on hybrid argument, where there are polynomially many (say, $n = p(\lambda)$) hybrids, $H_1, ..., H_n$. Any ...

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