6
votes
2answers
433 views

Determine LFSR phase quickly?

I know it's possible with work backwards from the output bits of an LFSR to determine its feedback polynomial in a O(n) fashion. I'm also curious if, given an LFSR state and polynomial, is it ...
1
vote
0answers
382 views

Montgomery Reduction

I'm taking a hardware cryptography class and working on a problem that focuses on Montgomery Reduction. So by definition: Computing a x b (mod N) Pick R, s.t. R > N, gcd (R,N) = 1 Compute N^-1 ...
2
votes
1answer
269 views

Understanding Meet-in-the-Middle attack on block ciphers

I understand how to implement a Meet-in-the-Middle attack on 2DES/3DES, by computing for one half all possible keys and storing the outcome and then subsequently do the same for the other half but ...
5
votes
1answer
137 views

Keys in HMAC and NMAC

I'm quite new to this topic and have several questions concerning HMAC and NMAC: Why does NMAC need two keys? How can it be attacked if we just used some sort of initialization vector instead of the ...
4
votes
1answer
119 views

Hard-core predicates: should the adversary be given $1^n$?

In most (all?) classical sources such as the book of Goldreich (2001), hard-core predicated are defined thus: A polynomial-time computable predicate $b : \{0,1\}^* \to \{0,1\}$ is a hard-core of a ...
1
vote
1answer
1k views

Static DH/Static ECDH certificate using OpenSSL

I've been searching for some time and have found no way to create a working certificate and private key pair with a DH or ECDH public key using OpenSSL. Once I do this, I'd like to sign it with an RSA ...
4
votes
1answer
2k views

Who uses the RSA BSAFE library? [duplicate]

Who uses RSA's BSAFE library? Does anyone know what products use it, or have any statistics on how many end users use something that is built on BSAFE? Background: BSAFE is one of the oldest ...
17
votes
2answers
3k views

Why should I use an Initialization Vector (IV) when I have unique keys?

I took a look at “Why, or when, to use an Initialization Vector?” but my question is not the same. I have unique keys encrypting each plaintext (in CBC mode, AES-256) and I do not use a key to ...
0
votes
0answers
9 views

Does we need IV if we're generating a new Symmetric key for every message? [duplicate]

We need to encrypt individual messages, for every message we: Generate a new AES key Encrypt the message with the key (AES/CBC/PKCS5Padding) Encrypt the symmetric key with with a public key ...
10
votes
2answers
835 views

Why Elliptic Curves?

What is the benefit of using elliptic curves over the standard finite field, when the cyclic subgroup we consider of the EC's solution group is just isomorphic to some integer residue class of prime ...
3
votes
1answer
96 views

Implementing forward secrecy in OpenPGP with public key authentication

I am trying to consider the feasibility of implementing an encrypted file sharing system, which both provides forward secrecy but also allows public key verification. I am stuck on the marriage ...
3
votes
1answer
171 views

Why don't use random padding in RSA?

Consider the following scenario: Sender S sends a message to receiver R. He uses a hybrid encryption scheme with RSA as key-encapsulation algorithm and AES-256-CTR (4byte-counter) and SHA256-HMAC. ...
2
votes
2answers
196 views

RSA private key d knowing e,n

I need to find RSA private key $(d, N)$ knowing $(e, N)$. It's "own" RSA implementation. As i know $p$ is random 70 bit number, then $q$ is $p-2^{10} < q < p+2^{10}$ $d$ is max 16 bit long with ...
6
votes
1answer
539 views

Can hash functions improve a lousy PRNG?

If I have a poor PRNG, could a cryptographically secure hash function make a better output? Say the PRNG can produce as much numbers as you wish, reasonably random, but not good enough to be used in ...
5
votes
1answer
2k views

Reusing keys with AES-CBC

I heard that key/IV pairs must not be reused in AES-CTR, or when using any stream cipher for that matter. Yet the attacks described do not seem to apply to AES-CBC. Is reusing the same key several ...
5
votes
1answer
173 views

Can Alice send Bob a secure message with only DSA and no key exchange?

Suppose Bob has a DSA key pair, and Alice has Bob's public key. Is there any secure way that Alice can send Bob a message only he can read if Bob can not send any reply? I think the answer is no, ...
4
votes
0answers
83 views

Better lower bound on min-entropy

In “Randomness Condensers for Efficiently Samplable, Seed-Dependent Sources” by Dodis, Ristenpart, and Vadhan (PDF), I have seen the statement that: any tuple of distributions $(X,Z)$ is $ε$-close ...
3
votes
1answer
186 views

Hash functions to generate one time pad

Newbie question... may be... Stream cypher, will XOR message bits with one time pad bits Suppose I have a pseudo random number generator, like a Mersenne twister or sort of. I could use the ...
0
votes
1answer
78 views

Does the following linear equation hold in bilinear pairings?

Does the following hold in bilinear pairings? $$e(g^{a_1x_1}g^{a_2x_2},g^{c_1}g^{c_2})=e(g^{x_1+x_2},g^{a_1a_2(c_1+c_2)})$$
2
votes
1answer
86 views

Decryption of message in IBE without random oracle using bilinear pairing registration?

I find the following IBE scheme from the videos posted and i don't understand the decryption algorithm, will any one please elaborate the 6th step scheme Setup($\lambda$) : $(\mathbb{G}, ...
1
vote
0answers
80 views

Does the encryption algorithm DES perform random permutations? If so how is information not lost?

Before round 1 begins, all 64 bits of an incoming message and all 56 bits of the secret key are separately permuted (shuffled) My question is this : Is the shuffling of the bits performed ...
9
votes
1answer
180 views

Can Grover's algorithm be parallelized?

Using a quantum computer, Grover's algorithm can search an unordered list of length $N$ in time $\sqrt{N}$. Applied to cryptography this means that it can recover $n$ bit keys and find preimages for ...
0
votes
0answers
81 views

Multiplicative inverse ($17^{-1} \mod 31$)?

So. Sorry for bothering you with such a simple question, but I can't really get this done. It's just an exam question in which I need to use CRT in order to calculate the RSA signature of a msg ...
2
votes
1answer
81 views

Finding keys of modified DES encryption

This is a question I had in my exam today, and I'll be glad if someone can help me to find the answer. A student built an encryption algorithm (something between DES and 3DES), in which the ...
0
votes
1answer
41 views

What is the run-time on finding collisions with MACs,…?

I've recently read the claim that finding collisions on block cipher may be not a difficult task, if an attacker can control the key and the message. So, given a pseudo-random function $f:\{0,1\}^m ...
1
vote
1answer
239 views

ElGamal and Paillier key sizes for short messages

I am using ElGamal and Paillier schemes to encrypt a large number of short messages: typical 4-byte integers. I do this for the homomorphic properties of these schemes. However, the way the ...
1
vote
3answers
116 views

curious DHE implementation for key exchange

I have made some reverse engineering on proprietary software and came up with some DHE algorithm that works like this: First and foremost, the client and the server shares some public parameters (g, ...
4
votes
1answer
199 views

64bit nonce in AES-CTR, why not 128?

I'm just a bit confused about the nonce/IV in AES-CTR ... it seems the suggested way to go is have a 64bit random nonce, and use the last 64bits as the counter, "starting at 0". I'm just wondering ...
11
votes
1answer
2k views

Bleichenbacher 1998 “Million message attack” on RSA

I have been reading Bleichenbacher's 1998 paper on a forged message attack on RSA. The paper assumes access to an Oracle that takes a ciphertext $c$ and will check the decrypted text for valid PKCS #1 ...
3
votes
2answers
161 views

Determine AES key given encrypted and unencrypted files

Given an encrypted file, the original unencrypted (cleartext) file, and knowledge of which AES encryption algorithm was used, is it possible to determine the key that was used to encrypt the data in a ...
3
votes
1answer
67 views

Is authenticated encryption basically a lockable box?

I recently used a custom construction as a commitment scheme, which was taken from the standard picture you give people while explaining commitment schemes. Basically commitment schemes can be ...
4
votes
1answer
595 views

Why does “2xAES-256” provide “99.99%” security strength whereas “1xAES-128” provides “40%”? [closed]

I recently was "directed" to FlipsCloud, a company selling security solutions (as far as I can tell). In particular I've investigated their page and found a page on their "encryption features". This ...
0
votes
0answers
58 views

Identification schemes: Okamoto

I am reading Cryptography theory and practice by Stinson. The book is talking about the proof of security of Schnorr and Okamoto schemes. It claims that if an impersonater can find (in the case of the ...
0
votes
1answer
133 views

Using HMAC SHA256 to protect payment files in local trusted network

in a project that I am working on we have been tasked with eliminating PGP while providing sufficient security to protect payment files only within the Company's trusted network. We arrived at HMAC ...
0
votes
1answer
138 views

How many bits to flip in an RSA public key to do signature forgery?

Scenario: Alice wants to verify a signature from Bob. Alice knows Bob's RSA public key (e, n). However, Alice is getting the data, the signature of the data and the public key from an attacker Eve. ...
6
votes
1answer
163 views

Deterministic ECDSA signatures?

ECDSA signatures depend on parameter k that is chosen by the signer. As a result, there are many signatures for the same private key d and message m. What I want to achieve is a deterministic ...
-3
votes
2answers
95 views

What needs to be proved for a cryptosystem to be secure?

For example showing that it is easy to produce ciphertext from plaintext but is difficult/impossible to get plain text from ciphertext. I am asking in general but am more interested in asymmetric ...
2
votes
0answers
76 views

Why does encryption in STS protocol protect against identity attacks?

The basic STS protocol contains encryption of the signature as shown in Wikipedia. https://en.wikipedia.org/wiki/Station-to-Station_protocol If we would like to defend against MITM attack, this ...
2
votes
0answers
150 views

Blowfish Weak keys

I've read about the weak key issue with Blowfish, but can't seem to find anything that explains the practical effect of weak keys on Blowfish with the full 16 rounds. Is there any actual attack upon ...
1
vote
1answer
59 views

Unconditional authentication

I have a few questions regarding universal-hash functions: Is there a way universal hash functions can be used to provide unconditional authentication in the way the OTP provides unconditional ...
-1
votes
1answer
110 views

Problems implementing AES in CBC+CTS mode

I'm trying to implement an AES cipher in CBC+CTS mode, but I'm unable to get the second last plaintext block to decrypt properly. I've attached my code, it's based on the steps listed under CBC ...
6
votes
1answer
155 views

How to derive the curve Ed25519 from Curve25519?

According to the paper "Faster addition and doubling on elliptic curves" by Bernstein and Lange, the Montgomery curve (Curve25519) $$v^{2}=u^{3}+486662\cdot u^{2}+u$$ is birationally equivalent to the ...
1
vote
0answers
46 views

One-Time-Signature based codes

I'm trying to understand the sentence in bold, in the follow paragraph of A new one-time signature scheme from syndrome decoding Keygen: Given a security parameter $\lambda$, choose a suitable ...
10
votes
6answers
15k views

Difference between encrypting something and hashing something

What is the difference between encrypting something and hashing something? in what situations would I want one or the other?
0
votes
1answer
191 views

Is this hypothetical key-exchange secure?

I've thought about the following (generic) key-agreement protocol. The only weaknesses I could have found are missing freshness guaranteees. As for the notions: $E^1_K(P)$ is the symmetric encryption ...
2
votes
2answers
47 views

Correctness of ideal protocol of MPC in presence of malicious adversary

In presence of malicious adversaries in ideal protocol of Multi Party Computation, how can honest parties get the correct output? i.e., corrupted parties modify their inputs and give to the trusted ...
2
votes
1answer
42 views

N users must approve a request

I have a locked-down production system that we'd like to process specific requests, picked-up (poll) from a less-secure location. For security and "go slow, this is production" reasons, a request ...
3
votes
1answer
148 views

EAX cipher mode with nonce equal header

I am using EAX AEAD with an 8-byte message header which carries a 32-bit sequence number, a message length, and some other bits that I don't mind exposing. Assuming the sequence number never repeats ...
6
votes
4answers
1k views

What good is a hash accompanying a program?

After studying at length about weaknesses in SHA2, length extension attacks, forged signatures, etc. I have to ask why groups like Gnu provide hashes on their program texts? As long as the public ...
1
vote
1answer
54 views

“Order” in cryptographic terms for generators

Frequently I have seen people use the term order in cryptography (the group theoretic one). I have a mathematical background and order (say for prime modulus $p$) is defined as the smallest integer ...

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