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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What is the “Random Oracle Model” and why is it controversial?

What is the "Random Oracle Model"? Is it an "assumption" akin to the hardness of factoring and discrete log? Or something else? And why do some researchers have a strong distrust of this model?
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2answers
6k views

Why does nobody use (or break) the Camellia Cipher?

If Camellia is of equivalent security and speed to AES, concerns arise. First of all, assuming the above, why is Camellia so rarely used in practice? Why aren't there any breaks in Camellia? Does ...
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3answers
2k views

What is the ideal cipher model?

What is the ideal cipher model? What assumptions does it make about a block cipher? How does it relate to assuming that my block cipher is a pseudo-random permutation (PRP)? When is the ideal ...
12
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1answer
820 views

Does unbalancing a feistel cipher always improve security? Does it improve security at all?

So according to Wikipedia unbalanced feistel ciphers provide greater provable security. Specifically, they state: The Thorp shuffle is an extreme case of an unbalanced Feistel cipher in which one ...
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Random oracle model proofs and programmability

Proving the security of a scheme with the random oracle model (ROM) involves two steps: first you prove that the scheme is secure in an idealized world where a random oracle exists, and then you ...
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5answers
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Can you make a hash out of a stream cipher?

A comment on another question made me wonder about something: Assume you're on a rather constrained platform — say, a low-end embedded device — with no built-in crypto capabilities, ...
9
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332 views

How did the Koblitz/Menezes papers affect the cryptography community?

Two highly-critical papers by Koblitz and Menezes (two well-regarded mathematicians who've contributed to the crypto community) were published years ago: Another Look at “Provable Security” in 2004, ...
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4answers
411 views

Automated security protocol verification tool for eCK model

I want a tool that (runs on Win7 and) can perform automated verification of a protocol in the eCK security model as described in Microsoft Research's paper "Stronger Security of Authenticated Key ...
8
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1answer
376 views

Is the inverse of a secure PRP, also a secure PRP?

If a block cipher is a secure PRP, is it's inverse a secure PRP as well? My intuition says yes but I'm not exactly sure. On a related note, if a block cipher is a secure sPRP, is it's inverse a ...
7
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2answers
704 views

Is SipHash cryptographically secure?

I'm evaluating different hash algorithms for use in my application. One of the kind of algorithms I am looking at are cryptographically secure ones to protect against DOS attacks. SipHash seems ...
7
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1answer
570 views

understanding forking lemma

Every time when I read a paper that has digital signature, when it comes to prove the security of a digital signature scheme, many chances that the author will use the forking lemma. The forking ...
7
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1answer
184 views

What is the “artificial abort” technique?

In the security proof of Brent Waters's paper Efficient Identity-Based Encryption Without Random Oracles, he uses a novel “artificial abort” step on page 6. At this point the simulator is still ...
7
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1answer
360 views

Why is EdDSA collision-resilient with SHA-512?

In the Bernstein et al. paper about EdDSA, the authors claim EdDSA is resilient against collisions (i.e. it can still be secure even if the hash function used isn't collision-resistant), drawing on a ...
6
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4answers
381 views

Is there a proof for showing any cryptogram is crackable?

I commonly hear statements along the lines of "all cryptograms are crackable - it's only a matter of time". Is there a proof to show that any cryptogram is "crackable"? The proof may be of a more ...
6
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1answer
154 views

How to turn adversary against encryption of zeroes into adversary against random bits?

I recently read the article Nonce-Based Symmetric Encryption by Rogaway, where he presents two different notions of indistinguishability, which he calls ind$ and ind, respectively. Here's the ...
6
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2answers
209 views

Where is the proof of security of Diffie's cipher?

There is an apparently provably secure cipher that was proposed by Diffie, but enhanced by R.A. Rueppel. The scheme, which was mentioned in Applied Cryptography, works like this: Measure the length ...
6
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1answer
530 views

Goldreich-Goldwasser-Micali Construction

The Goldreich-Goldwasser-Micali construction allows to build a (cryptographically secure) pseudo-random function from of a (cryptographically secure) pseudo-random generator. More formally, let $G: ...
6
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1answer
184 views

Is there a security proof for the Triple-DES construction in the ideal cipher model?

Suppose one has an ideal block cipher $E \: : \: \{0,\hspace{-0.04 in}1\hspace{-0.03 in}\}^k \times \{0,\hspace{-0.04 in}1\hspace{-0.03 in}\}^w \: \to \: \{0,\hspace{-0.04 in}1\hspace{-0.03 in}\}^w ...
6
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1answer
115 views

Secret key and Decryption oracle in the revised Fujisaki-Okamoto transformation

Recently, Fujisaki and Okamoto provided a revised version of the Fujisaki-Okamoto transformation [1], a generic transformation for achieving IND-CCA2 security in the Random Oracle model. This new ...
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5answers
2k views

Is a book cipher provably secure?

I've seen ciphers (usually in spy drama shows) that involve taking a book and writing down an index to individual characters. Essentially it's a keyed substitution cipher, where the key is the name ...
5
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4answers
686 views

Is this a pseudo random function (PRF)? F(k,x) = f(k,x) - f(k,x-1)

I have a question about Pseudo Random Functions. Let $f:\{0,1\}^m \times \{0,1\}^n → \{0,1\}^n$ be a secure PRF. Define $F(k,x) = f(k,x) - f(k, x-1 \bmod 2^{n}) \bmod 2^{n}$. Is $F$ is a secure ...
5
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3answers
158 views

Fault-based transition for crypto proof (a la Shoup) with big probability of fault - does it work?

background In [Shoup2004], Victor Shoup synthesizes the 'sequence of games' technique for proving security properties: Roughly it consists in a sequence from game_0 to game_n, game_0 consisting in ...
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1answer
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Proof that MACing a hash of the message is also a secure MAC

I found a theorem that says: Let $MAC = (S,V)$ be a MAC for short messages over $(K,M,T)$. Let $H: M^{big} → M$. Define $MAC^{big} = (S^{big},V^{big})$ over $(K,M^{big},T)$ as: $S^{big}(k,m) = ...
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532 views

Security proofs for CBC mode

I'm looking for different approaches to proofs for the security of CBC mode encryption. What are the best sources of information about this subject?
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281 views

CDH problem and Square-DH problem

CDH problem roughly says that choose $U=g^u, V=g^v$ uniformly at random from cyclic group $G$, it's hard to compute $CDH(U,V)=g^{uv}$. Square-DH problem roughly says choose $U=g^u$ uniformly at ...
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1answer
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What is it meant by a “hybrid argument”?

Can anyone explain (or point to a reference for) what a hybrid argument is in a security proof, and when it's convenient or preferable to use it? Among some of the places where I've seen it ...
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2answers
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Is it possible to construct a secure block cipher of size $2n$ given a secure block cipher of size $n$?

Given, say, the Blowfish block cipher, which is considered secure but only has a 64-bit block size, can we construct a secure block cipher of 128-bit block size? Say we run the key through two KDFs, ...
5
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1answer
112 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 ...
5
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2answers
789 views

Are there any secure commutative ciphers?

This answer lists two commutative cipher algorithms - Pohlig-Hellman and SRA. However, they don't appear to be too secure. My question is, here there any commutative ciphers out there that are secure ...
5
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1answer
197 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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600 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
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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 ...
4
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2answers
358 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 ...
4
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2answers
249 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: ...
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357 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 ...
4
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1answer
173 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? ...
4
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1answer
208 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 ...
4
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2answers
326 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 ...
4
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1answer
334 views

Proofs by reduction and times of adversaries

I have some difficulties to understand, when we construct a reduction, how we determine the time for the constructed adversary to break a target security property. In general these details are not ...
4
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2answers
160 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 ...
4
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1answer
148 views

Why does NTRUEncrypt lack a formal security proof?

Is there any particular reason why NTRUEncrypt lacks a formal security proof? That is, a demonstration that it achieves certain security notion (e.g. IND-CPA). I know there is a provable-secure ...
4
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1answer
241 views

Unforgeability and type of adversary

When trying to prove security of asymmetric signature, for instance for existential (or strong) unforgeability against chosen messages attack, do we need to consider the signer as a possible ...
4
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1answer
213 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 ...
4
votes
1answer
69 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 ...
4
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1answer
153 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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549 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?
4
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1answer
202 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 ...
4
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2answers
169 views

Hamiltonicity proof of knowledge

I'm learning the POK notion and definitions and as a self exercise I wante to prove the statement that the Hamiltonicity protocol is a POK system with knowledge error $1/2$. So the question will be ...
4
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1answer
162 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 ...
4
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1answer
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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 ...