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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Mersenne Twister's vulnerabilty with small-size distributions
C++ rookie here, trying to wrap his head around (CS)PRNGs.
I (assume that I) already understand the concept of the Mersenne Twister being "broken" (i.e. not cryptographically secure) due to its ...
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How well is it understood mathematically why encryption schemes are hard to crack?
I have read some intro material into cryptography. It mainly goes into the current encryption schemes like AES, but not very deeply into the mathematics of why they are secure.
I know that encryption ...
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Proofs of security for anonymity in a multi-party setting
I'm looking for material that proves the anonymity of parties in a non-interactive setting. I would like to know if there are simulation based proofs (UC) and property based proofs (game based) for ...
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Fairness and guaranteed output delivery
Why for achieving the fairness and guaranteed output delivery, less than a half of the parties should be corrupted? ($t < m/2$, where $t$ is the number of corrupted parties and $m$ is the total ...
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Any Arbitrary Function
According to the attached reference, does it mean that the adversary computes $PPT(initial~input)$, $PPT(z)$ and $PPT(f_i(x',y'))$?
For example, suppose the corrupted party, in the malicious model, ...
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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 ...
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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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Strong One-Way Function
In the book "Foundations of cryptography-Oded Goldreich-Page 33", if we use the deterministic polynomial-time algorithm instead of the probabilistic polynomial-time algorithm for case 2 (Hard to ...
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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 ...
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The interaction between the malicious Adversary and the Simulator and Real-Ideal' views
This example is from “Efficient Secure Two-Party Protocols-Carmit Hazay-Yehuda Lindell”.
And the security proof of the above example is this:
I have drawn a picture that is deduced from the above ...
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Security proof in the Malicious Model
According to the above protocol(Efficient Secure Two-Pary Protocols-Hazay-Lindell-page 27), the authors proved the security of this protocol according to the two paragraphs below:
We separately ...
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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 ...
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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 ...
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difference between the uniform and non-uniform probabilistic polynomial algorithms (ppt)
I am reading the book "Efficient Secure Two-Party Protocol". A question came to my mind. why has the author used a probabilistic polynomial time algorithm for security definition of ideal/real model ...
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Difference between semi-honest and malicious adversary using Ideal Real Model Paradigm
Would you please explain to me why in the security definition of ideal/real model paradigm which is described in the efficient secure two-party protocol book(Hazay, Lindell), the simulator tries to ...
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How small is negligible?
When proving theorems in crypto we often make use of the concept of negligible functions or, more simply, negligible parameters.
As a rule of thumb, given today (2018) computational power, what is ...
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"Power of one" as input to functions of a cryptosystem
What does $1^\lambda$ mean when you pass it as a parameter to the functions of a cryptosystem. The cryptosystem in question is this and a picture reference is this.
I have been told it signifies the ...
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A qusetion about the security game of Certificateless signatures
Recently I read Certificateless Public Auditing for Data Integrity in the Cloud , I have a question about the security of proposed HOMOMORPHIC AUTHENTICABLE CLS. （Sec. IV）. For the Type-I Adversary, ...
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In "Lecture Notes on Cryptography" what does "PTM" mean?
I came across a handbook named "Lecture Notes on Cryptography" from Shafi Goldwasser and Mihir Bellare and I read their definition 3.1 about poly-time indistinguishability:
Let $X_n,Y_n$ be ...
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Maximum value of negligible probability?
We can see on many papers for cryptography that there are many uses of negligible probability to ensure something (security, correctness, etc).
How can we say that some probability is negligible?
I ...
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Is there a computational security of public-key encryption which is similar to the one of symmetric key encryption
We say a symmetric key encryption scheme $\Pi_{1} = (\mathrm{Gen}, \mathrm{Enc}, \mathrm{Dec})$ is computational security if for every PPT algorithm $A$ and every $x_{0}, x_{1}$ ($|x_{0}| = |x_{1}|$),
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Can the key-generation algorithm always be uniform?
Assume that the encryption uses $n$-bit keys to encrypt $l(n)$-length messages.
If a symmetric key encryption scheme is defined as $\Pi_{1} = ( \mathrm{Enc}, \mathrm{Dec} )$, then for every $x_{0}, ...
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security against CPA attack?
Let $\mathcal{E} = (E,D)$ be a CPA secure cipher.
Let's define $\mathcal{E'} = (E', D')$, where $E'(k,m) = E(k, E(k,m))$.
How can I prove that $\mathcal{E'}$ will be CPA secure if $\mathcal{E}$ is?
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Design flaw in IND-CKA definition of Searchable Symmetric Encryption?
I am a graduate student interested in searchable encryption research. I carefully read the Secure Indexes paper published in 2004, and I was confused about the IND-CKA, the game-based security ...
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What is the relationship between entropy conditioning and final output bias in a TRNG?
This question concerns the conditioning and output of true random number generators. It refers to NIST Special Publication 800-90C, Recommendation for Random Bit Generator (RBG) Constructions. It ...
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Deterministic symmetric indistinguishable encryption
Are there any block-cipher modes (or symmetric encryption schemes in general) that are fully-deterministic, meaning same inputs lead to the same outputs (like for all modes with constant IV) and ...
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Secure pseudo-random generator
Suppose that $G : \{0,1\}^k \rightarrow \{0,1\}^{2k}$ is a secure pseudo-random generator. Describe what is wrong with each of the following attempts to build a pseudo-random generator with $k$-bit ...
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Hash function composition - security level
When using two hash functions, g(x)=SHA-512 and f(x)=MD5
g(x) has 512 bit output (using salt)
f(x) has 128 bit output.
Let's say that z(x)=f(g(x))
meaning the output is 128 bit long.
The Question:
...
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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 ...
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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 ...
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What does St denote?
I am reading a paper and notice that the term St is used without being defined. I suspect it is a common notation that I am not aware of.
It is used in the following paper:
https://eprint.iacr.org/...
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What exactly is a “security parameter”?
I often encounter the term “security parameter” when I read crypto related stuff. My basic understanding is that it just denotes some bit-length however, I'm not so sure.
For example, when it says
...
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Encryption Scheme & Perfect Secrecy [Katz & Lindell]
An encryption scheme consists of the following data
a plaintext space $\mathcal M$
a ciphertext space $\mathcal C$
a key space $\mathcal K$
a key generating algorithm Gen
encryption and decryption ...
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Does information theoretical security definition imply DDH, RSA, QR does not hold?
Assuming we are in the information theoretical setting, whereby there is no bound on the computational power of an adversary. Does this mean that the standard definitions for DDH, RSA or QR do no hold ...
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How do adversary models and security types relate [from InfoSec SE]
I have been told to move the question to this forum in order to get a better insight, so here we go:
Reading some questions in this forum I encountered this answer mentioning IND-CPA as a requirement ...
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I am not convinced that SHA2/3 or AES256 is secure
I have been researching cryptography for the past few weeks to try and make sense of a few things, but is simply not making any sense for me.
If I encrypt a message, send it to another computer, how ...
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RIsk mitigation: block cypher vs xor
It sort of looks like to me that there is a kind of risk mitigation in the architecture of block cyphers vs the xor cypher.
In XOR, the risk is entirely in the random number generator. The XOR ...
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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 ...
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How is the zero-knowledge proof definition violated, here?
I'm confused about the definition of an interactive zero-knowledge proof (the simulator-transcript-thingy).
Assume the following setup:
$P$ and $V$ do a Diffie-Hellman key exchange to generate the ...
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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 ...
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Formal Information-Theoretic Key Expansion Definition
Is there a formal definition for a protocol that expands keys that are pre-shared among 2 or more parties?
How about an informal definition?
Example:
Alice and Bob pre-share a $n$-bit string $k$. ...
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PPT vs NP problems
The common definition of security (for some cryptographic primitive) is to be secure against any PPT adversary (any probabilistic algorithm which runs in polynomial time). At the same time, we assume ...
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Difference between a secure PRG and semantically secure encryption
I am taking cryptography course in Coursera from Stanford University. I have following question
I have a question why append 0 in G(k) i.e., G'(k) = G(k) || 0 is
considered as not secure PRG as 0 in ...
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Interpretation of message space distribution in the definition of perfect secrecy of encryption schemes
I am a beginner to cryptography, and have started reading Katz and Lindell's book titled "Introduction to Modern Cryptography". I'm unable to understand what the probability distribution over the ...
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What does it mean the security parameter for a FHE scheme?
FHE schemes claim to use some security parameter $\lambda$ for deriving the other parameters of the scheme, the size of the key and the possible circuits that can be evaluated. Like in Fully ...
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Hash Function Properties
What I am trying to understand is shown in all three properties of a secure hash function, I will focus on Preimage attack resistance.
Preimage resistance: given a hash h, it's difficult to find m s.t....
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Homomorphic encryption and approximated GCD 2
This is a further question to the one reached here Homomorphic encryption and approximated GCD.
I am now thinking about a symmetric cipher in which the plaintext set is integers, illustrated as ...
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The notion of sub-exponential security
I have been reading a paper where they construct probabilistic IO (indistinguishability obfuscation) from sub exponential IO. I want to know if the following two notions of sub-exponential security ...
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Are there any security definitions for DNS?
Given a recent post on the Tor blog and the corresponding discussion on HN I was wondering if there are any security definitions for DNS or similar name systems.
So something similar to ACCE for TLS ...
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