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 many blocks can securely be encrypted with XTS

I could not find in the NIST recommendations on XTS how many blocks can securely be encrypted with XTS-AES. Through the recommendations, I've found: The length of the data unit for any instance ...
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73 views

Are HMACs based on hashes with larger bit-lengths also more secure?

When doing encrypt-then-mac, I can choose to use a hmac as the MAC. For example, I could use a hash like SHA-256 or SHA-512 (by using it as a keyed hash) to create that HMAC. Does it increase ...
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117 views

Is there any existing analysis for this construction to turn a tweakable blockcipher into a PRF?

I'm basically looking at this construction to turn a tweakable blockcipher $E_c(x)$ taking a key $k$, nonce $n$, counter $c$ (forming tweak $t = c||n$) and an input $x$ into a PRF on an arbitrary-...
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1k views

Proof that IND$-CPA implies IND-CPA?

I've read a few papers recently that used a notion of security called "indistinguishability from random bits/strings" under chosen plaintext attack, also called IND\$-CPA. See e.g. http://pdf.aminer....
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202 views

Academic papers on ECDSA security

What are the most authoritative papers on the security analysis of ECDSAs? I`m mainly looking for comparisons of how secure different curves are considered to be, what potential methods are there of ...
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840 views

A block cipher with independent keys for each round

I want to modify a standard block cipher in the following way. I replace each round key by a key picked at random. Is this block cipher as secure as the original one ? Thank you. EDIT Some missing ...
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2answers
342 views

Can we say that if $P=NP$ there is no CPA secure public key encryption?

I've learned that public key encryption is based on the problem of Discrete Log (as regard to group theory) which believed to be hard. But, can we say that it doesn't matter on which problem our ...
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530 views

Is this a structural weakness of Feistel networks?

I'm doing a lot of reading about Feistel networks. Something occurred to me a bit ago that I hadn't realized previously, namely that in any Feistel construction there are bits of the plaintext that ...
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267 views

Can anyone give an example where (asymmetric) crypto can go wrong due to selection of wrong groups?

Basically the title says it all. It would be great if someone could tell give an example using provable security. More information about groups can be found at: https://en.wikipedia.org/wiki/Group_(...
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442 views

Is it possible to break a hash-based block cipher?

Let's define the following block cipher: $C_n = M_n \oplus H(k + n)$ where $C_n$ is the nth block of ciphertext, $M_n$ is the nth block of plaintext, $H$ is a cryptographic hash function, and $k$ is ...
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430 views

Relation between attack and attack model for signatures

I would like to know: What is the relationship between an attack and an attack model. For example, let $\Pi$ be the Lamport signature scheme. This signature has its security based on any one-way ...
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227 views

Given $g^a, g^b, g^c, g^{1/b}$, is it hard to distinguish $e(g, g)^{abc}$ from a random value?

where $g$ is a group element in bilinear group $\mathbb{G}$. I understand it is very similar to the conventional DBDH problem, but $g^{1/b}$ is also known, possibly making it easier? Does anyone know ...
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196 views

Difference between computational and statistical indistinguishabilities

What is the difference between the two notions of computational and statistical indistinguishability?
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199 views

Is this really the correct way to do an IND-CPA test?

In a description of IND-CPA (indistinguishability under the chosen plaintext attack), I have been reading the following, simple test: The adversary can generate as many messages as he wants. Then, ...
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214 views

How can I tell how many bits of security a secure hash function has?

How can I tell how many bits of security a secure hash function has? For example, how would I calculate or tell how many bits of security a secure hash function with 160 bits output would have? ...
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220 views

What should I be aware of when implementing algorithms myself?

I plan to build my own crypto library. The project will be primarily for me to learn (and if useful for no other purpose, that is fine). In the past I have implemented a few hashes, and AES quite a ...
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755 views

What does “Worst-case hardness” mean in lattice-based cryptography?

In the wiki page of Lattice-based Cryptography the "Worst-case hardness" is defined as below: Worst-case hardness of lattice problems means that breaking the cryptographic construction (even with ...
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2answers
387 views

Hash function based on block cipher (and proof of security in the PRP model)

Do there exist proofs of security for primitives like hash functions (based on a block cipher) in the PRP model. I often see proofs in the random oracle model (for hash function based on compression ...
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642 views

some of my confusions about DDH assumption

The wiki defines the decisional Diffie–Hellman assumption as follows: Decisional Diffie–Hellman assumption Consider a (multiplicative) cyclic group $G$ of order $q$, and with generator $g$. The DDH ...
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70 views

How to generate a random number so server cannot cheat?

Here is the protocol: Bunch of players connected to server. Server creates nonce and hashes it - send hash to clients as bit commitment. Clients make nonces and send hashes to server as their bit ...
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328 views

Is the reduction from left-or-right IND-CPA to real-or-random IND-CPA tight?

A modern trend in cryptography consists of defining security as rigorously as possible, and then designing schemes which are secure according to those definitions. Proving security comes in the form ...
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233 views

Is it safe to assume Salsa20 to be a PRP?

Often in security proofs a certain block cipher is assumed to be a pseudorandom permutation or PRP. I wonder if this goes for stream ciphers as well, and specifically for Salsa20. If limit ourselves ...
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158 views

Reason for difference in assumptions for practical private-key and public-key crypto

Theoretical cryptography tells us that everything in the world of private-key cryptography (CCA-secure symmetric encryption, message authentication codes, etc.) can be built from one-way functions and ...
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738 views

What is the definition of “security beyond the birthday paradox”?

I'm reading a paper about MACs and I would like to be sure about the meaning of a security beyond the birthday paradox. Is there a definition?
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217 views

Questions about the ideal cipher model

I've read that we can study the security of modes of operation by assuming the use of an ideal block cipher. I've also seen a paper suggesting that the ideal cipher model could be something else than ...
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175 views

Would a symmetric cipher with a keylength a big as the data length be information theoretically secure?

One-Time-Pad is information theoretically secure as long as the random number stream is evenly long or longer than the data stream it encrypts, for a "decyphered" message could have been any message ...
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72 views

Enhance security by combining bits of 4 different hashes

Assuming that performances do not matter, is there any real benefit in terms of security (in general) in combine hashes in this way: hash functions: ...
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72 views

On Proving That a Primitive Does Not Exist

In the paper of Hsiao and Reyzin, Section $1.4$: Note that to show that no general reduction from $P$ to $Q$ exists requires proving that $Q$ does not exist Since the statement is about trying ...
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110 views

OCB and GCM security

Is OCB as secure as GCM or CCM ? Since OCB design is quite different from GCM and CCM, I was wondering if the security properties of these latters are satisfied by OCB, as well. Thank you.
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Salary Negotiation Problem

Imagine Alice is applying for a new job. Alice has an idea of the minimum salary that she is willing to accept—let's call this value A. Bob, the hiring manager for ...
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74 views

What level of security is provided when a Feistel Cipher is used as a round function of another Feistel Cipher?

Recently, I was reading: Are there any specific requirements for the function F in a Feistel cipher?, and the answer posted mentions a Feistel Cipher named Turtle, which uses a four-round Feistal ...
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109 views

Efficient proof of knowledge using Wegman-Carter hash

A verifier wants to ensure, with only little exchange of data with other systems, that a large block of data $M$ that the verifier holds is also available to some other system(s). It is not an ...
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993 views

Tools for modelling and analysis of cryptographic protocols

I am designing some cryptographic protocols and I am new to it. Are there any well-known tools that can be used to model and design these protocols? And also verify or analyze their validity? If not ...
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What are the differences between proofs based on simulation and proofs based on games?

what are the main pros and cons of proving the "security" of a crypto scheme under simulation proofs instead of game based proofs?
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373 views

Can you help with that definition for a CCA?

The following is a definition taken from Introduction to Modern Cryptography by Katz and Lindell. I'm having a hard time understanding some basic concepts! Can you please help me? $\newcommand{\Gen}{\...
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349 views

Building a combined encryption scheme from two encryption schemes that's secure if at least on of them is secure

Any thoughts on how this can be done? Let $\Pi_1 = (\mathrm{Gen}_1, \mathrm{Enc}_1, \mathrm{Dec}_1)$ and $\Pi_2 = (\mathrm{Gen}_2, \mathrm{Enc}_2, \mathrm{Dec}_2)$ be two encryption schemes for ...
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239 views

Chance in cryptography

I was just thinking about my own chance and the way chance can defeat even the most advanced algorithm. My thought was : you can make a strong session id, but what if by chance, a hacker set this ...
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Question about the definition of a secure PRF

I'm taking a cryptography introduction course, and we're covering the definition of a secure PRF. I understand the test goes as follows: A challenger picks a function $f$ such that $f \leftarrow \...
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244 views

Security equivalent to Diffie–Hellman problem?

I've been doing the security proof for one of my Theorem. Basically, given $g^a$, $g^b$, $g^{cb}$, $g$ and $c$ as known values. Is the problem of computing $g^{acb^{-1}}$ equivalent to the Diffie ...
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518 views

security in the standard model → random oracle model?

Can a protocol proved secure in the standard model be considered secure in the random oracle model?
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55 views

Selective and existential unforgability of signature schemes

I understand that one can define EUF-CMA of a signature scheme is terms of a game where the adversary is allowed to query signatures on messages of his choosing, and at the end of the game he must ...
3
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50 views

About the necessity simulators set adversary random-tape

In the ideal/real proof paradigm we sometimes find simulators with the capacity of set adversary random tape. My question: when do we have to consider the necessity of simulators set adversary random-...
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1answer
258 views

Construct IND-CPA secure encryption scheme by combining two given schemes

I have two encryption schemes $\Pi_0, \Pi_1$, at least one of them is IND-CPA secure but I don't know which one. The task is to construct a scheme $\Pi$ that is guaranteed to be CPA secure and to ...
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1answer
565 views

How to prove that a function is not pseudorandom?

I am currently enrolled in a cryptography course, which uses the book by Katz and Lindell. I'm struggling with the exercies which ask for proofs, like the following one: Let G(k) be a PRG with |k|...
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180 views

Reductionist proofs of decisional problems to computational

Are they any reductionist proofs where an attacker $\mathcal{I}$ for a well established computationally "hard" problem $\mathsf{Π}$ is employing an attacker $\mathcal{A}$ who we assume is able to ...
3
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1answer
287 views

Security proof of FO(Fujisaki-Okamoto) hybrid encryption

The proof of FO hybrid encryption is hard to understand. $\:$ Especially, how does the challenger respond to the decryption queries when the challenger can only have some encryption queries? Can ...
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285 views

Is a random oracle controled by the challenger?

When proving a Crypto scheme security under random oracle model, is the random oracle always controlled by the challenger? What if the Hash is only used by the adversary?
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Does there exist a two-pass AKE protocol that is secure in eCK model and also has PFS?

As we can know that the best two-pass AKE protocols with DH message can achieve is the weak form of perfect forward security (wPFS) which guarantees security against the passive adversary. But "...
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562 views

Why are protocols often proven secure under the random oracle model instead of a hash assumption?

Is this true that whenever you design a protocol using a hash function, you must prove its security under the random oracle? I mean, is it possible to devise a protocol $P$ using a function $H$, and ...
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136 views

Simulation aborted because the adversary doesn't use the random oracle

I'm trying to construct a proof for an encryption scheme in the Random Oracle model. This encryption scheme is like a PKE scheme but with an additional function that kind of "alters" ciphertexts ...