2
votes
1answer
1k views

Inverting the encryption equation of a symmetric cipher

I've just started a cryptography course. I am beginning to understand the concepts (I'm only in week 2) but I just can't get my head around the theories and principles when written as equations. I've ...
3
votes
2answers
263 views

bcrypt and pbkdf2 double hashing

If I want to protect myself from implementation of algorithm issues and inherent design flaws of algorithms in password hashing, what would be most secure way to combine two hashes? ...
9
votes
1answer
1k views

What stops the Multiply-With-Carry Random Number Generator from being a Cryptographically Secure Pseudo-Random Number Generator?

Despite the fact that Marsaglia's MWC PRNG (multiply-with-carry random number generator) is considered to be "the mother of all RNGs", it does not seem to be considered to be a CSPRNG ...
2
votes
1answer
425 views

AES with different modes of operation

I'm taking up a cryptography class and this is for an assignment. We're asked to find the differences between AES/RSA in the different modes of operation vs the one timepad method. From wiki, I can ...
2
votes
1answer
116 views

Is it dangerous to use plaintext as IV in CBC?

Is it dangerous to use plaintext as IV in CBC? For example: I want to encrypt $p1$, $p2$ with CBC. However I use $p2$ as the IV for CBC purposes. What would be the risk of not using a IV based on ...
2
votes
1answer
317 views

Why does openssl BN_generate_prime return only a fraction of the safe primes within the given bit range?

I went into prime generation issues the last week and eventually calculated 10^7 primes using erik tews safeprimegen python wrapper for openssl. (https://github.com/eriktews/gensafeprime) (System: ...
5
votes
1answer
229 views

HMAC construction based on the combination of two hash functions

I would like an $\operatorname{HMAC}$ that's based on two different hash functions ($H_1$ and $H_2$), so that a break of the combined $\operatorname{HMAC}$ would imply a break of ...
12
votes
3answers
1k 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 ...
2
votes
1answer
4k views

Difference between linear cryptanalysis and differential cryptanalysis

What is the main difference between linear cryptanalysis and differential cryptanalysis? For example in relation to DES?
5
votes
1answer
317 views

Zero knowledge proof of possession of key

Suppose Alice has at some point in time produced ciphertext $C$ from message $M$ with key $k$. Suppose Alice has then passed $C$ to Bob, and also made a commitment to her key $k$ (thanks to ...
3
votes
1answer
186 views

Salsa20 in Davies-Meyer mode with a fixed key (message): is it one-way?

Say I have a function that calculates a pseudo random permutation and this function is easy to invert. For example $P(i) = AES_k(i)$ where $k$ is a publicly known key. So anyone can compute $P(i)$ ...
1
vote
2answers
537 views

Combining multiple symmetric encryption algorithms - implications?

I was just wondering if I add more security by combining two or more symmetric encryptions on a plain text. For example: Plaintext → AES → Twofish → Serpent Of ...
3
votes
1answer
344 views

Proof of correct construction of a private key in distributed cryptography

In an exponential ElGamal encryption scheme where the key generation is done in a distributed way among $n$ trustees we have that each trustee $i$ (where $1 \leq i \leq n$): Selects a private key ...
2
votes
3answers
334 views

Can I dynamically calculate an appropriate number of iterations for PBKDF2 based on the system time, rather than using a fixed value?

Could you choose the number of PBKDF2 iterations based upon the system time? I've heard a few people recommend that the number of iterations for PBKDF2 should be doubled every two years (starting ...
6
votes
1answer
248 views

What exactly could be accomplished with a backdoor in Dual_EC_DRBG?

Assume that some entity really holds the private key corresponding to the recommended/dubious constants of Dual_EC_DRBG. According to this presentation, they would be able to reconstruct the internal ...
3
votes
1answer
786 views

Possible ECC backdoor and its impact on Internet traffic

In a recent article, Bruce Schneier suggested that he prefers classic discrete log crypto over ECC because "I no longer trust the constants. I believe the NSA has manipulated them through their ...
4
votes
1answer
233 views

Can the AES encryption and decryption algorithms be described as a polynomial for a specific key?

AES is a block cipher that consists of non-linear and linear parts to provide the Shannon requirements of confusion and diffusion respectively. The linear parts can for sure be described as a ...
2
votes
2answers
163 views

Privately Sum Values without Dealers, MPC

Suppose $n$ actors each hold a plaintext $p_i$. We wish to find $\sum p_i$, without leaking any information about individual $p_i$. Any actor (or any link in the network) could be controlled by an ...
2
votes
1answer
137 views

Can you help me understand PFS and wPFS?

Every time I encounter the concepts of PFS (perfect forward secrecy) and wPFS (weak perfect forward secrecy), I feel uncertain about them. My understanding is that: PFS ensures that, if the ...
2
votes
1answer
103 views

Recasting randomly generated numbers to other widths

Suppose I have a list of randomly generated uint_8's, uniform over the whole range of uint_8. I write them into a file as raw ...
6
votes
2answers
245 views

Meaning of “family” in “family of hash functions”

Many definitions related to universal hashing mention the term "a family of hash functions, say $H$". What does this 'family' mean exactly? A numerical example would be appreciated. Also, what does it ...
3
votes
1answer
150 views

Stream vs Block cipher: Synchronising and the receiver

I am learning cryptography at school and I was wondering: How are the blocks/streams after decryption synchronized at the receiver to form one long meaningful message? Am I right by saying that ...
3
votes
1answer
386 views

Hide a weakness in ECC by choosing the prime or one of the curve coefficients

Suppose you are given a value $c$. Can you find a prime $p$ and an integer $b$ such that the elliptic curve $$E: y^2 \equiv x^3 -3x + b \pmod p$$ is cryptographically weak? You need to choose ...
4
votes
1answer
153 views

Can a PRNG with a fixed amount of entropy always be detected?

Given a fixed seed (i.e. no ongoing source on entropy), is there (practically or theoretically) any PRNG algorithm that can generate an infinite sequence of random numbers such that an observer cannot ...
1
vote
2answers
218 views

Is there a theoretical maximum useful keysize given the block-size?

Consider a block-cipher $F$ with a $N$-bit block-size and a $M$-bit key-size. That is, if $k$ is a $M$-bit key, $p$ is a $N$-bit block of plain-text and $c$ is a $N$-bit block of cipher-text, then: ...
4
votes
2answers
272 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 ...
17
votes
5answers
2k views

Could RDRAND (Intel) compromise entropy?

I was recently discussing the issue of RDRAND in Intel chips and the whole issue about how NSA could potentially be influencing Intel to weaken or create backdoors in their design. This petition was ...
3
votes
1answer
336 views

Should new applications still use RSA? Is it worth going down the ECDH route for protocols?

I am currently designing an application for communication between two and more users. At the moment I am trying to figure out what methods to use for the asymmetric cryptology. With the recent ...
1
vote
1answer
103 views

Question about Fermat's little theorem

Why is $g^e \mod p = g^{e \mod (p-1)} \mod p$ if p is prime. I don't get it. It follows from Fermat's little theorem.
5
votes
2answers
267 views

Relation between key size and PRNG state size

Some (supposedly) cryptographically secure PRNGs have an internal state of only 160 bits or less. When the algorithm is otherwise properly designed, that seems like enough to generate a 128 bit key ...
1
vote
1answer
148 views

Security of (cryptographic) padding compared to other methods for blockcompletion for block ciphers

As said in the heading, I want to know how the security of different padding methods, e.g. ANSI X.923, ISO 10126 and PKCS7, is compared to other methods to reach the needed block size, like ciphertext ...
1
vote
1answer
167 views

General-purpose non-interactive multi-party computation protocol

Most of general purpose multi-party computation protocols are highly interactive. Especially the ones with Shamir's secret scheme, as there is a degree reduction step in multiplication that needs the ...
71
votes
3answers
22k views

Should we trust the NIST-recommended ECC parameters?

Recent articles in the media, based upon Snowden documents, have suggested that the NSA has actively tried to enable surveillance by embedding weaknesses in commercially-deployed technology -- ...
2
votes
1answer
91 views

Is it possible to generate a message using the mac and the key?

Say that you have the Key and the MAC value that was generated from the Key using CBC. Is it possible to generate a message with just the mac and the key? If so, how would you do it?
15
votes
1answer
865 views

What NIST protocol was allegedly backdoored by NSA in 2006?

From a recent NY Times article: Cryptographers have long suspected that the agency planted vulnerabilities in a standard adopted in 2006 by the National Institute of Standards and Technology ...
3
votes
1answer
67 views

Should I use distinct MACs for each user, or should I use a *direction* flag when communicating?

Alice and Bob want to communicate using a stream cipher. At the beggining, they create a session key $K_s$ and exchange it via some secure channel (using some asymmetric algorithm). After every ...
1
vote
1answer
308 views

Why are the first few bytes of a gpg encryption always the same?

If I run the command 'gpg -a -e < input.txt > output.txt' with completely different inputs (e.g. from /dev/urandom), the first 19 characters are always the same. What is this header? Is it ...
2
votes
1answer
445 views

Question regarding multiple SHA-256 rounds on a Bitcoin Brain Wallet passphrase…

I am attempting to come up with a way of memorizing a seed that could lead to any number of brain wallets for bitcoin. I need multiple wallets because a) I don't want all my eggs in one basket, and ...
2
votes
2answers
156 views

Security of S/MIME in case of CA compromise

Suppose Alice and Bob are exchanging messages using S/MIME, protected by certificates that have been issued by either the same CA or by two mutually independent CAs. There exists an adversary Mallory ...
3
votes
1answer
281 views

Which PRNG security requirements do I need for key generation

Do I need to worry about "predictability" an an allegedly secure CSPRNG if I am just using it to generate key material from time to time? Suppose that for some odd reason, I am less trustful today ...
4
votes
2answers
175 views

Can S/MIME be still considered secure?

Previoulsy I had asked this question at http://stackoverflow.com/questions/18235983/can-s-mime-be-still-considered-secure but I feel this forum is topic-wise the right place. Recently there has been ...
8
votes
4answers
339 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 ...
-1
votes
2answers
162 views

What are the weaknesses of this message exchange protocol?

Here is a protocol for a key exchange: Bob encrypts a message $m$ with his key $K_B$: $m_1 = E_{K_B}(m)$. Bob send the result $m_1$ to you. Alice encrypts my encrypted above message with her own ...
0
votes
2answers
121 views

Why do PKI schemes need security proofs? [closed]

When it comes to designing new PKI-based key-exchange protocols, why are security proofs needed? Without them, can we show a protocol's security? Does there exist a PKI-based key-exchange protocol ...
3
votes
1answer
626 views

RSA key pair generation using PRNG with same seed

I see a lot of Q/A where persons are trying to generate a specific key pair using static data such as a password. Now say we use a known PRNG (dangerous assumption), seeded with a the data as a static ...
5
votes
2answers
471 views

Camellia or AES - which should I use?

Imagine I am a software developer. I want a block cipher, and I have the choice of using AES or Camellia. Which should I use? Which makes more sense from a technical perspective, and why? I need ...
2
votes
1answer
130 views

Can stream ciphers (usually) be “run backwards”?

Is it possible to reconstruct the previous output bits of a stream cipher, e.g. RC4, when only the current state is known, or is that computationally hard, or even impossible (due to ambiguous ...
1
vote
1answer
140 views

Is the “secure-as-worst-case” version of NTRU patented?

Does the improved version of NTRU — which is described in "Making NTRU as Secure as Worst-Case Problems over Ideal Lattices" by Stehlé and Steinfeld — fall under the same patents as the original?
1
vote
1answer
568 views

Break a simple compression function of a cryptographic hash function

I have a very simple compression function, which looks like this in C++: ...
2
votes
1answer
480 views

What key exchange do OpenSSL and CryptoAPI prefer by default?

Here is what I know. When I connect to a brand-new Linux Web server with Chrome and click the "lock" icon, the key exchange algorithm is described as "ECDHE_RSA". (Yes I know what that is.) When I ...

15 30 50 per page