9
votes
1answer
2k views

Zero knowledge proof protocol example?

Alice is color blind. She never knows if her gloves are matched. Her brother Bob always teases her saying her gloves are mismatched and she should go change them. Alice wants to know if ...
0
votes
1answer
40 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
170 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 ...
6
votes
5answers
3k 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 ...
1
vote
3answers
115 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
148 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
1k 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
127 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
65 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 ...
3
votes
1answer
576 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
49 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
123 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
118 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
138 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
89 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 ...
1
vote
0answers
66 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
112 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
56 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
82 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
122 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
44 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
14k 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
177 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
42 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
121 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
51 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 ...
1
vote
1answer
150 views

Where I can find a list of certified software / hardware RNGs compliant to NIST SP 800-22?

I need to use NIST SP 800-22 approved software / hardware to generate RSA key pair. Where I can find a list of certified software / hardware RNGs compliant to NIST SP 800-22? And how is the SP ...
13
votes
1answer
15k views

Calculating RSA private exponent when given public exponent and the modulus factors using extended euclid

When given $p = 5, q = 11, N = 55$ and $e = 17$, I'm trying to compute the RSA private key $d$. I can calculate $\varphi(N) = 40$, but my lecturer then says to use the extended Euclidean algorithm to ...
0
votes
2answers
59 views

Get AES key from client request

I get a request from a client who encrypts the message with the wrong AES key. Is there a way for me to know what AES key the client actually used? I understand I may get the decrypted msg which is ...
4
votes
2answers
8k views

The modulus of RSA public key

I am studying the RSA cryptosystem. The public key consists of $(n, e)$, the modulus (product of two large primes), and the encryption exponent. I want to separate the modulus $n$ and exponent $e$. A ...
2
votes
1answer
115 views

Ring-LWE elliptic gaussian distribution

I am currently trying to understand this Ring-LWE article: http://www.cims.nyu.edu/~regev/papers/ideal-lwe.pdf and I have a question. Firstly, it is mentioned in the paper that we can view the ...
2
votes
1answer
566 views

Why should the primes used in RSA be distinct?

The two primes $p$ and $q$ part of the public key need to be distinct. What's the reason for them to be distinct? Is it because factorization of $p^2$ where $p$ is a prime is relatively easier, or is ...
-1
votes
1answer
79 views

What is the meaning for a vector mod a matrix in a lattice?

I'm reading about the lattice recently.In the paper, it gives a method of a vector mod a matrix: ⃗c mod B as ⃗c−⌊⃗c×B^(−1)⌉×B = [⃗c×B^(−1)]×B. I know that a integer A mod the other integer B is ...
1
vote
1answer
87 views

AES-CBC pre-determined IV vs. null IV

Is there any security advantage to using a pre-determined IV in CBC mode over a null IV? I'm implementing a license key system similar to this article on CodeProject, but I'm confused by the authors ...
0
votes
0answers
54 views

how to implement order preserving encryption except cryptdb

Which cryptographic algorithm is suitable for implementing order preserving encryption with out using cryptdb
9
votes
1answer
330 views

Why isn't outputting only a portion of the hash state a simple defense against length extension attacks?

As I understand length extension attacks, they depend on the coincidental property of most cryptographic hash functions that the hash value is exactly the hash function state after hashing the last ...
3
votes
2answers
395 views

Is there a multiple asymmetric encryption algorithm, which requires all private keys to reveal the secret?

I am looking for an asymmetric encryption algorithm, which allows to encrypt a secret with multiple public keys, but to reveal the secret all private keys must be used. You shouldn't be able to tell ...
3
votes
2answers
117 views

What is the difference between the standard representants of $\mathbb Z/q\mathbb Z$?

The symbol $\mathbb Z/q\mathbb Z$ (given that $q$ is prime) represents the prime field $\mathbb Z_q$. Basically, the elements of this field are represented by $\{0, 1, \dots, q-1\}$, let's call this ...
2
votes
1answer
142 views

What's the differences among Blind Computation, Secure Multi-Party Computation, Secure Circuit Evaluation and Homomorphic Encryption

We know that Blind Computation, Secure Multi-Party Computation, Secure Circuit Evaluation and Homomorphic Encryption all can process the encrypted data, but I am puzzled by them. What are their ...
5
votes
2answers
504 views

SHA-256 exhaustive search

How do I calculate the number of computations needed to break SHA256 in case we are using it for safely storing passwords (together with a salt)? Are there any formulas that can be employed? For ...
1
vote
4answers
110 views

How is an epsilon of 1/1000 non-negligible?

Lately I've been studying cryptography and in the current course I'm taking we're reviewing statistical tests and how they can be used to determine if a pseudo-random generator is secure or not. ...
0
votes
1answer
51 views

Brute forcing the secret key in Elgamal encryption

Crypto noob here, I am attempting to do this programming challenge. I do not have the secret key that is used to decrypt the message. However, the key is small enough for a brute force approach. I am ...
1
vote
2answers
64 views

How to compute two EC point multiplication?

I would like to know how to compute multiplication of two valid EC points over a curve E with generator G. i.e. Given only P and Q points then how to compute R = P * Q where $P = p G$, $Q = q G$ and ...
2
votes
0answers
51 views

Differential addition on Montgomery curve

Point multiplication using Montgomery ladder technique over Montgomery curves only require x coordinate, which in many situation leads to faster implementation as compared to point multiplication over ...
3
votes
2answers
241 views

Which concrete applications benefit from Oblivious RAM constructions?

The main motivation for Oblivious RAM (ORAM) is that, for instance, in the "cloud" setting a client outsources his data to a server in the form of encrypted blocks. Later on, he wants to perform read ...
4
votes
1answer
117 views

Does exponentiation by squaring work on Montgomery curves?

Consider the point multiplication $Q=[d]P$, where $P$ a point on elliptic curve multiplied with an integer $d$ to get another point $Q$ on the same curve. This operation can be computed by a ...
3
votes
1answer
255 views

Which is better ECDHE with TLS 1.0

I have a webserver which only supports TLS 1.0 and I am not sure about something: Which is the better cipher in this group when aiming for the best security? ...
4
votes
1answer
268 views

Fractal Merkle Traversal

I am studying Fractal Merkle Tree Traversal algorithm in the book “Post Quantum Cryptography” (PDF). On the [pag. 54] I don't understand this paragraph: We may determine the number of pebbles ...

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