# Questions tagged [security-definition]

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

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### Why the definition in $\epsilon$-differential privacy is multiplicative rather than additive？

According to its mathematical definition, a random algorithm $M: D\rightarrow R$ satisfies $\epsilon$-differential privacy if the adjacent datasets $x, y \in D$ where $D$ is a whole dataset and ...
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### Comparison of security definitions for signatures

The two main security definitions for signatures are EUF-CMA and the strong version of it sEUF-CMA. What I see that their difference is that in EUF-CMA experiment, the adversary needs to produce a ...
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### Why is the security level defined by $\log _{2} \inf \{ t_{i}/\varepsilon_{i} \mid i \in I \}$

In public-key cryptography, the security level indicates the strength of an adversary in breaking a scheme or solving a problem, which can be seen as the time cost of breaking a scheme or solving a ...
411 views

### Formal definition of collision resistance for hash function

In my cryptography class, the professor said that collision resistance for a fixed hash function is not a "precise" definition. The reason is since a fixed hash function is a single instance of a ...
515 views

### Differential privacy definition

Differential privacy defines "privacy" of a mechanism $A$ as the "closeness" of the two distribution $Pr[A(D) \in S]$ and $Pr[A(D') \in S]$ where $D,D'$ differ in one element. And the distance between ...
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### Does GCM (or GHASH) only provide 64-bit security against forgeries?

In a recent comment a doubt was voiced about my answer, which claims GCM to requires $2^{128}$ for a successful forgery. The doubt was that the square root needs to be taken meaning the security would ...
167 views

### What is the security model of the FHE system introduced in Fully Homomorphic Encryption Using Ideal Lattices?

How would one construct a security model to play against the adversary, and define the security of the overall scheme? This is in reference to the scheme introduced in "Fully Homomorphic Encryption ...
164 views

### Definition of symmetric cipher and definition of asymmetric cipher

So I'm currently taking a cryptography class right now and my professor has told the class that she can't even define symmetric cipher for us without getting into asymmetric ciphers and the textbook ...
386 views

### Is there a cryptographic approach to availability

Background Cryptography can be said to provide the tools used to fulfill the goals of information security. The three pillars of information security are confidentiality, integrity, and ...
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### What's the difference between perfect security and unconditional security?

This paper refers to two distinct security models: perfect security and unconditional security. Until now, I knew of them as the same thing and equivalent to information-theoretic security. From the ...
145 views

### Non-Repudiation vs Identity-Binding

There are 3 properties offered by a cryptographic digital signature scheme: Integrity guarantee of the message signed. Authenticity guarantee of the message signed. Non-repudiation of the message ...
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### Identifying negligible functions

I am having a hard time understanding and applying the formulas that are used to identify a function is negligible or not? One text defines it as; a function $f$ from the natural numbers to the non-...
284 views

### On the security definition of password-authenticated key exchange

I found in all PAKE papers, the security is defined as something like this: Let $Succ(A)$ be the probability that an attacker $A$ successfully distinguished the session key from a random string. Then ...
211 views

### Indistinguishable encryptions and CPA-secure example

Let $F$ be a PRF and $G$ be a PRG with expansion factor $n \to n +1$ For the following encryption scheme, decide whether it has indistinguishable encryptions and whether it is CPA-secure. ...
215 views

### How is n-bit security calculated with nested encryption?

What will the total $n$-bit security be if I encrypt a message with a 128-bit XOR cipher and then take that ciphertext and encrypt it with a 128-bit key. Is the total security level now 256? One ...
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### How exactly is "true randomness" defined in the realms of cryptography?

Especially in relation to stream ciphers, I frequently read about (sometimes theoretical, sometimes practical) attacks that are able to "distinguish a ciphertext from a truly random stream". What's ...