# 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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### Difference between an authentication scheme and a identification scheme in ZK proofs?

I know that a signature scheme is the "strongest" level of security, and correct me if I'm wrong, but that should be basically just a identification scheme where you can also sign a message ...
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### KPA-Security definition

In cryptography there are 4 basic attack classifications: Ciphertext-Only Attack Known-Plaintext Attack Chosen-Plaintext Attack Chosen-Ciphertext Attack In Katz & Lindell's textbook (2nd edition)...
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### Exact security requirements for extendable output functions (XOF)?

In the FIPS202 document "SHA-3 Standard: Permutation-Based Hash and Extendable-Output Functions" an extendable-output functions is defined as: An extendable-output function (XOF) is a ...
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### Reference for a formal definition of universal forgery and EUF-CMA

In many papers, I see EUF-CMA and SUF-CMA referenced as a canonical term used, but I did not find a reference paper/book that ...
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### Besides block-cipher which other methods can only be computed step-by-step even with known secret (but fast per step) and can be inverted?

Depending at the cryptographic function used applying it $i$-times to a given input can be computed in different complexity classes (based at their input size). $$f^i(m_0) = c_i$$ For example for most ...
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### How can a concatenation of $N$ block-cipher with known keys be more secure?

General problem / Intro: encrypting the (computable) relation in between two random numbers which are members of a as small as possible set while anything except the order of execution is known to the ...
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### Superscript vs subscript notation in cryptographic formulation

I'm currently reading this paper [PDF]. On page 4, I bumped into these notations : \begin{equation} \text { Experiment } \operatorname{Exp}_{\mathcal{F} \mathcal{E}, A}^{\text {ind-mode }}(k) \text { :...
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### Can a block-cipher considered secure if a bit-change of the input leads to a 50% chance change for every single output bit? -> round number?

Block-cipher use self-inverse ($f(f(x)) = x$) operations which then will be applied to the plaintext and most likely contain some constants which can be based at a key. To get security such ...
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### Question about game based security

When a game between an adversary $A$ and a challenger $CH$ is used in the security model of a scheme, and oracles are defined, does the adversary only have access to those oracles in the attack or ...
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### How to compare two post quantum digital signature schemes in terms of latency?

I want to compare two digital signature schemes of different nature. One is Hash-based (Winternitz one time signature scheme) and the other is lattice-based (Dilithium Crystals). What about be an ...
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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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### show that key recovery is not possible in a computationally secure system

(G, E, D) is a computationally secure encryption scheme over the message space $\{0,1\}^n$. Show that the probability that a PPT adversary can recover the key after seeing the encryption of a random (...
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### How does the security of Elliptic curve compare to normal discrete logarithm?

Intro: EC are often compared with RSA but how about a more safe version of the discrete logarithm? All 3 can be reduced to the problem: $$b = g^a \mod{P}$$ In RSA $P$ is a product of two primes. To ...
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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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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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### 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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### How long would it take all of the supercomputers or cloud computing on Earth to bruteforce a significantly long password?

I was arguing with a colleague who thinks that SHA256 (password + 64 character static salt) is "insecure." My argument is that nothing in cryptography is "secure," it's all a ...
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### How to formally define the security of Random Oblivious Transfer

Assume that there is a protocol $(A,B)$ such that receives no input and satisfies: $A$ - outputs two random bits $x_0, x_1 \in \{0,1\}$ $B$ - outputs a random bit $b \in \{0,1\}$ and also outputs $x_b$...
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### Alternatives to simple bit-measured security for ciphers

For a cryptographic primitive, we usually see the security level measured in bits, where n-bit security means that the attacker would have to perform $2^n$ operations to break it. For key-derivation ...
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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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### Bits of security vs collision resistance?

I have been doing some research on security, and I am confused on the bits of security vs collision resistance, and I was wondering if someone could clarify my understanding. For instance, if you have ...
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### What are the “costs” to find a pre-image, weak collision, or strong collision?

For a secure, n-bit hash function, what are the “costs” to find a pre-image, weak collision, or strong collision?