Questions tagged [security-definition]

Questions about formal definitions of "security" for various cryptographic schemes (e.g. perfect secrecy, semantic security, ciphertext indistinguishability, etc.)

Filter by
Sorted by
Tagged with
1
vote
1answer
122 views

When knowledge soundness implies soundness

In the work of Bellare–Goldreich that defines knowledge soundness BG92, the discussion of Section 4.5 specifically decouples knowledge soundness from soundness. That is, proving knowledge soundness ...
3
votes
1answer
88 views

The number of cipher texts possible for each plaintext

This is from Dan Boneh's book Theorem 2.1. Let X = (E, D) be a Shannon cipher defined over (K, M, C). The following are equivalent: (i) X is perfectly secure. (ii) For every $c \in C$, there exists $...
0
votes
0answers
20 views

Correctness and security of a scheme

Is correctness of a scheme considered part of the security of that scheme or is it separate? I've seen both approaches in different papers.
0
votes
0answers
31 views

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 ...
2
votes
1answer
48 views

Why do we need multiple adversaries in a game-based definition?

Consider we have a protocol, where multiple algorithms of it are run by an adversary. When we want to define a game for that protocol (or its security properties), I have seen that an adversary is ...
1
vote
1answer
28 views

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 ...
1
vote
2answers
108 views

Proving security of $\Pi' = E_k(E_k(m))$ knowing $\Pi = E_k(m)$ is secure

I recently started studying cryptography but I am not sure I quite understand the concept of proof by reduction. Question I am trying to solve is as follow: Suppose $\Pi$ is a symmetric encryption ...
0
votes
0answers
34 views

Formal proof that the following definitions of perfect secrecy are equivalent

I've seen the following two definitions of perfect secrecy for an encryption scheme (Gen, Enc, Dec). ...
0
votes
0answers
14 views

Integrated Signatures and Encryption Scheme

Suppose we have a scheme ISE that combines $n$ signature schemes and one encryption scheme, where the same keys are used. It is composed of the following algorithms: Setup, KeyGen, $Sign_1,...,Sign_n, ...
5
votes
4answers
204 views

What does perfect mean within perfect forward secrecy and why do some cryptographers disagree with the term?

In this blog post, Matthew Green says Forward secrecy (usually misnamed “perfect forward secrecy”)... The Wikipedia article on forward secrecy (which the same sentence links to in the blog) says ...
3
votes
0answers
29 views

Differences in definitions of “Computational Special Soundness”

I am looking at Unruh's transform (ePrint 2014/587) and in particular the definition in §2.2 for computational special soundness in sigma protocols. Let $\cal{R} \subset \cal{X} \times \cal{Y}$ be ...
0
votes
0answers
93 views

ROLLO : security bits parameters

In ROLLO -Rank-Ouroboros, LAKE & LOCKER p.20, it says that "Notice that for 128 security bits parameters, the practical security is rather higher than 128 at about 170 bits." How Can I ...
3
votes
2answers
114 views

What is the difference between security parameter and $b$-bit security?

In a previous question, I read ... for $b$-bit security meaning $O(2^b)$ work for an attacker to break the system... While in Katz's Introduction to modern Cryptography, I read: The key-generation ...
11
votes
1answer
2k views

Meaning of “Security can be reduced to a problem”

I'm studying reductions in cryptography and confused about the way people use the word "reduction". My question is almost the same as a past question, but what I want to ask is slightly different. A ...
2
votes
1answer
213 views

RSA and the strong RSA assumptions

The RSA assumption: Given a randomly generated RSA modulus $n$, exponent $r$ and a random $z \in \mathbb{Z}_n^{*}$, find $y$ such that $y^r=z$. The strong RSA assumption: Given a randomly chosen ...
1
vote
1answer
255 views

Seed/key algorithm security

Currently I'm working on implementing a seed/key algorithm to limit access to a tool for authorized users. My understanding of the algorithm is that its strength is derived from the seed itself. ...
0
votes
0answers
18 views

Does multiple substitutions increase security? [duplicate]

Yes, because the attacker must decrypt the cypher text twice No, because it is equivalent to a single substitution Maybe, depending on the complexity of each substitution
0
votes
1answer
72 views

MAC security game

In the applied cryptography book by Boneh and Shoup, Chapter 6 on MACs, it is stated that one can modify SUF-CMA game by removing verify oracle access to the adversary. Therefore, the two versions are ...
8
votes
1answer
1k views

Existential unforgeability vs strong unforgeability

In the article https://crypto.stanford.edu/~dabo/pubs/papers/strongsigs.pdf there are two definitions for the security of a digital signature scheme: existential unforgeability and strong ...
0
votes
0answers
42 views

Understanding a security proof

I am reading the paper "RingCT 3.0 for Blockchain Confidential Transaction: Shorter Size and Stronger Security" (https://eprint.iacr.org/2019/508.pdf), an improvement to Ring Confidential Transactions....
0
votes
0answers
73 views

Importance of Backward and forward secrecy

Which security feature is more important in security protocols being used in different applications? Forward Secrecy or Backward Secrecy
0
votes
1answer
54 views

Research topics related to cryptography and Hamiltonian cycles

I am very interested in pursuing a research where I can show an application of Hamiltonian cycles in Cayley graphs of some group such as reflection groups to the field of cryptography. But currently ...
4
votes
3answers
514 views

Is it possible to exactly reverse engineer an unknown cipher inside a black box?

Consider the following arrangement:- There is an unknown cipher algorithm operating inside the black box. You can enter infinite key /plain text pairs, and observe all of the outputs. The box might ...
2
votes
0answers
61 views

Security aspects of a splitted Pohlig-Hellman system

Given a Polig-Hellman (is that really the name for that?) system with $$C = M^k \bmod p$$ where $M$ is the message, $C$ is the ciphertext, $k$ is a (secret) key (any integer relatively prime to $p-1$...
1
vote
0answers
105 views

Game-based proofs and semantic security

After reading this question, I understood that "game-based" and "simulation-based" have to do with the way that security properties are postulated. In particular, IND-CPA is a canonical example of ...
19
votes
2answers
5k views

What stops the Multiply-With-Carry RNG from being a Cryptographically Secure PRNG?

Despite the fact that Marsaglia's MWC PRNG (multiply-with-carry random number generator) is considered to be "the mother of all RNGs", it does not seem to be considered to be a CSPRNG (...
0
votes
3answers
214 views

Is a PRG concatenation using the same input still a PRG?

Suppose we have a secure PRG $G: \{0,1\}^s→ \{0,1\}^n$ and we define a $G'(k)$ as: $$ G'(k) = G(k) \| G(k) $$ So if $G(k)=\mathbf{i}$, $G'(k) = \mathbf{i} \| \mathbf{i}$ Is $G'$ still a secure PRG? ...
1
vote
1answer
47 views

Forward Security of Symmetric Encryption Schemes with LFSR based PRNG

Forward security can be achieved using symmetric encryption schemes by combining with a pseudo-random number generator (PRNG). Does LFSR based PRNG provide forward security in this case?
1
vote
2answers
480 views

What is the non-programmable random oracle model?

I would like to know the difference between the random oracle model and the non-programmable random oracle model. ​ What is the difference?
0
votes
0answers
54 views

Every encryption scheme that is perfectly secret it holds that for every distribution over the message space $M$, every $m, m' ∈ M$, and every $c ∈ C$

Prove or refute: For every encryption scheme that is perfectly secret it holds that for every distribution over the message space $M$, every $m, m'∈ M$, and every $c ∈ C$: $$Pr[M = m | C = c] = Pr[M = ...
0
votes
0answers
40 views

Is this simple proposed scheme semantically secure?

Suppose we have two parties, including $P_1$ and $P_2$ using a type of symmetric encryption algorithm, say, AES256. Let's assume the shared key is: $K$. Encryption: Each party generate a key such ...
1
vote
3answers
1k views

What are the vulnerabilities of XOR in the following scenario?

What are the security vulnerabilities of the XOR operator in the following scenario: The Key, The Cyphertext and the Plaintext are the same size in bits. The Key is only used once and it's secret The ...
2
votes
0answers
39 views

decrypting a one-time-pad that outputs 0,1 in preknown probabilities

assuming a two users want to use a one time pad ciphersystem, and they are using a program that was developed by a third party that was supposed to create random undependable bits, but for some reason ...
29
votes
1answer
963 views

Uniform vs discrete Gaussian sampling in Ring learning with errors

The Wikipedia article on RLWE mentions two methods of sampling "small" polynomials namely uniform sampling and discrete Gaussian sampling. Uniform sampling is clearly the simplest, involving simply ...
4
votes
1answer
212 views

How to determine the security in bits of a scheme?

This may be a stupid question. But how would we determine the security bit of a crypto scheme? For example, SHA2-256 offers 128 bit security due to birthday paradox, so do they measure the security ...
2
votes
1answer
123 views

Does an information-theoretically secure hash function exist?

Does an information theoretically secure hash function exist? (By exist I mean is discovered/invented and implemented, not whether it could exist.)
2
votes
2answers
101 views

Collision resistant functions - definition

Let $f$ be a collision resistant function i.e. it is computationally impossible to find $x_0, x_1$ such that $f(x_0) = f(x_1)$. If a computationally bounded adversary demonstrates that he knows some $...
19
votes
1answer
4k views

What do the signature security abbreviations like EUF-CMA mean?

From time to time, one stumbles across formal security definitions. This includes security definitions for signature schemes. The most common ones are the *UF-* ...
2
votes
3answers
534 views

What does trust mean in cryptography?

Terms like "trustless protocol", "trusted third party", "trusted platform", "trusted server", etc. require a notion of trust. I already wrote The term trust when used in the context of cryptography ...
7
votes
2answers
2k views

Difference left-or-right CPA security, IND-CPA security

I am trying to understand the notion of left-or-right-CPA (LOR-CPA) security for private-key encryption schemes introduced in my lecture. If I understood it correctly so far, the only difference to ...
0
votes
0answers
58 views

DLP instances for given number of congruences

Let $S$ be the set of $k$ congruences of the type $g_{i}^{x_i} \equiv h_i \pmod{n_i}, \quad \forall i,1\leq i \leq k$ What would be the best way to describe such set $S$ with a term in literature. ...
0
votes
0answers
85 views

1-out-of-2 Oblivious Transfer Security

Just started reading Oblivious Transfer. I am searching for a proof of security based on RSA implementation but I can't find anything simple to understand. Can someone direct/show me to an easy to ...
0
votes
1answer
177 views

Security of this PRF [closed]

Given $F$ a secure PRF with input size $\lambda$. Define $F'$ as $F'(k,x||x') = F(k, 0||x)\oplus F(k, 1||x')$ with $x$ and $x'$ of $\lambda-1$ bits. Is $F'$ a secure PRF?
0
votes
2answers
812 views

What are the exceptions to Kerckhoffs's principle?

Kerckhoffs's principle: A cryptosystem should be secure even if everything about the system, except the key, is public knowledge. Yet the following are three exceptions:- NSA Suite A cryptographic ...
1
vote
1answer
43 views

Looking for a formula for “Given $x_1$, $\ldots$, $x_{n-1}$, the output $F(x_1,\ldots,x_n)$ is dependent of $x_n$ and is of $l$-bits security”

Suppose I have a function $F(x_1,\ldots,x_n)$, where each $x_i$ is of $l$-bits security (suppose each $x_i$ is a binary string of length $l$). At the same time, suppose there are $n$ persons, each one ...
1
vote
0answers
39 views

Precise Definition of 1-out-of-2 Oblivious Transfer

What's the precise definition for 1-out-of-2 OT? and what's the best way to show that it can't be achieved under unconditionally secure setting? I am reading about it and just couldn't get my head ...
1
vote
0answers
135 views

Reduction to security of proof system for proving knowledge

I want to prove security of my system by reduction to the security of an underlying non-interactive proof system for proving knowledge. I know a game that captures the desired security property of my ...
1
vote
2answers
142 views

What is the Knowledge Of Secret Key Assumption (KOSK)

From what I've read it seems to be where an entity must prove that they own the secret key. Which is done by signing a message M using said secret key. Is this ...
5
votes
2answers
823 views

What is the difference between information-theoretic and perfect types of security?

I'm having a hard time pinning down an exact definition of the difference between information-theoretic and perfect types of security. A rigorous definition seems elusive... A. Wikipedia puts the ...
1
vote
0answers
58 views

How does the offline authentication work?

In these slides, offline and online PUF-based authentication protocols are explained in minute 16:30 and 17:30, respectively. I understand the way the online protocol works. Basically, the server ...