3
votes
2answers
509 views

AES-ECB as an authentication mechanism

ECB is considered to be insecure when used for confidentiality because identical plaintext result with identical ciphertext. But what if we use ECB for authentication? Assume A wants to transmit an ...
3
votes
1answer
69 views

Memory hardness of key derivation function through XOR-ring multiple matrix values

For theoretical purposes in order to enhance my own understanding, and NOT in order to create my own cryptography, I am asking a question about the memory-hardness of a key derivation function ...
3
votes
1answer
65 views

Is this transposition cipher information-theoretically secure for 1 message?

Given: $m = \{0,1\}^{n}$; a plaintext message of length $n$ encoded in binary $k = randomshuffle([0, 1, ..., 2n-1])$; A secret key consisting out of unique numbers between 0 and $2n-1$ in a true ...
3
votes
1answer
3k views

Knowing the key, how do I decrypt the affine cipher?

I have an encryption function $$E(p) = (ap + b) \mod{26}$$ where the parameters are $a=7$, $b = 10$, and $p$ is the plaintext letter. I was asked to decrypt the ciphertext ...
3
votes
1answer
3k views

Why is Serpent faster than AES in this benchmark?

I have a laptop without the AES-NI CPU instructions but with SSE4.1. I'm using Linux and tried the (in-memory) cryptsetup benchmark to get the rough tendency which returned: ...
3
votes
4answers
932 views

RSA by hand - did I do something wrong? (c = m on encryption)

to understand RSA better I am doing a little calculation by hand, this is what I got: Choosing: $p = 3$ $q = 5$ $n = 15$ $\phi(pq) = 2 \cdot 4 = 8$ $e > \phi(n) => e = 13$ $e \cdot d = 1 \...
3
votes
1answer
585 views

Counter Mode: static IV but different keys [duplicate]

Given we are using AES counter mode, suppose we randomly generate several keys, all of them are using same IV (say, zeros). Does this lead to any security issue? I know that in CTR mode, same key-iv ...
3
votes
2answers
318 views

Quadratic residuosity problem reduction to integer factorization

How can one show how to reduce the quadratic residuosity problem to an integer factorization?
3
votes
1answer
1k views

Non-linearity of a boolean function

What's the definition of non-linearity of a boolean function? Roughly saying it is minimum number of times it equals any affine function. But I don't get it mathematically. For example, if $f = ...
3
votes
1answer
457 views

What differences between Menezes–Vanstone ECC and ElGamal ECC?

After researching ECC encryption, I found that we can use ElGamal cryptosystem with elliptic curve and can we use Menezes-Vanstone cryptosystem with elliptic curve. What is the essential difference ...
3
votes
3answers
156 views

The meaning of “scheme”

This question is a bit different from other questions here, but I think it is suitable to correctly understand the terminology of cryptography. Consider the following two sets of terms: Encryption ...
3
votes
2answers
680 views

How can I add more rounds to AES?

I'd like to understand a bit more about the AES key schedule if anyone would mind explaining that. For instance, why is the way the schedule generated (with recursive xors and occasional substitutions)...
3
votes
2answers
2k views

Known-plaintext attack on Blowfish in ECB mode

The protection scheme I faced recently seems so weak nowadays that a simple exhaustive key search would be enough to recover the user key in an acceptable amount of time (it's OK, since almost no ...
3
votes
1answer
306 views

efficient authentication with broadcast encryption?

(I am not a crypto expert, so I apologize if the terminology I use is incorrect.) Suppose I have the following simple broadcast encryption scheme for securely sending content to $n$ authorized ...
3
votes
1answer
148 views

What does “adaptively secure” mean?

In a paper it says "In the generic group model, the PRF is adaptively secure for inputs of $\mathbb{Z}_q^n$". Maybe a stupid question, but what does "adaptively secure" mean exactly?
3
votes
1answer
74 views

Combining secure hashes with insecure hashes?

Assuming I'm a bank which seeks for secure hash functions to use in the process of signing on digital contents. I'd like to explain / prove why using each one of the following hash functions might ...
3
votes
2answers
516 views

Is there a way to do fair exchange between two parties who don't trust each other?

Let's suppose we have an Alice who knows a secret key A, and Bob who knows key B. Using their own keys, they each encrypt a message (Alice encrypts $m_A$, Bob encrypts $m_B$) with their own key, and ...
3
votes
2answers
823 views

Encrypted counter as IV in CBC mode encryption using same key

I'm currently working with a secure transport protocol that defines the IV to be a counter (incremental nonce) to be encrypted with the same key. This is a followup to a protocol that did not provide ...
3
votes
5answers
588 views

Can hashing become encrypting?

hashing some data and encrypting data share some similar attributes. Surely this is why "Difference between encrypting something and hashing something" has been asked. The point of this question is ...
3
votes
0answers
123 views

RSA security assumptions - does breaking the DLP also break RSA? [duplicate]

Possible Duplicate: Would the ability to efficiently find Discrete Logs have any impact on the security of RSA? I'm wondering if breaking the DLP, that is the basis for ElGamal and DSA, would ...
3
votes
1answer
112 views

Does $i^n=j^n$ for $i, j \in GF(2^q)$ and $i \neq j$ for some $n<2^q-1$

Let $i, j \in GF(2^q)$ and $i \neq j$ and $i,j\neq0$. Is that possible that $i^n=j^n$ for some $n$ such that $0 < n < 2^q-1$? I am looking for a proof if the answer is no, or for a method to ...
3
votes
1answer
308 views

A situation where security by obscurity might be the best solution - or am I wrong?

Imagine the following situation. There is an application for a microcontroller-driven system. The profits are made from manufacturing and selling the device itself. However, it is expected, that the ...
3
votes
1answer
497 views

Rfc2898DeriveBytes - password length

Trying to find related answer for a long time, but not convinced yet. What I am trying is to encrypt using RijndaelManaged. To create Key, I am passing password, ...
3
votes
2answers
544 views

Two-timing a one-time pad [duplicate]

Is a one-time pad still breakable on a depth of 2+ if the plaintext is completely random? I'm assuming this idea is wrong, mostly because it occurred to me, but let me walk you through my logic so ...
3
votes
2answers
136 views

Small Quantum Signatures - Reality check needed

I've been thinking a bit lately about how to get quantum resistant signatures fast and (relatively) small. One idea I've been keen on exploring is finding a crypto PRNG that allows fast-forwarding, e....
3
votes
1answer
44 views

May I use Random Oracle for Inversed Look-up?

Consider there is a protocol in real world calling a random oracle $\mathcal{H}$ for a priavte input $k$. Then in the ideal world, after the calling of $r \leftarrow \mathcal{H}(k)$ by a honest party, ...
3
votes
0answers
46 views

How to test a FHE implementation?

I want to implement a FHE scheme based on NTRU, namely the scheme described here https://eprint.iacr.org/2014/039.pdf . How to test the security of my implementation ? Do I have to implement the ...
3
votes
1answer
584 views

Is SHA-256 safe when used in this way?

I am wondering whether an attacker can gain any useful information from having several hashes of an unknown plaintext that differs in relatively few bits. For example, if provided with: ...
3
votes
1answer
355 views

Decrypt all RSA ciphertexts if you can break 1% of them

I was given a cryptography question during an interview the other day. Imagine you are given an efficient algorithm which for a given RSA public key (n,e) is able to decrypt 1% of the messages ...
3
votes
1answer
52 views

Need help understanding this exercise's solution

We were given this exercise, which asks us to identify the types of keys used on this diagram: The solution was Kxa -> A's private key Kya -> A's public key KxB -> B's private key My question ...
3
votes
1answer
273 views

Homomorphic encryption for vector addition

Building on the question and answer from Addition-only PHE in F# which ponders homomorphic cryptosystems to navigate, relatively, a single dimension without revealing absolute position (an encrypted ...
3
votes
1answer
262 views

Simple digital signature example that one could compute without a computer?

I am working on a document to explain Bitcoin to students. But I am having a hard time translating the principle described in §2 of the Bitcoin whitepaper in layman's terms. There is a great question ...
2
votes
1answer
2k views

Why does plain RSA not work with big messages ($m>n$)? [duplicate]

From what I can remember, RSA is something like this: Generate 2 distinct prime numbers $p$ and $q$ that have similar bit length. Compute $n=pq$ and $\phi(n)=(p-1)(q-1)$ Compute $e$ such that $1<...
2
votes
1answer
491 views

inverse problem about scalar multiplication on elliptic curve

Let $E$ be an elliptic curve over a finite field $F_p$. Given $n$ be a positive integer and $Q$ be a point on $E$, assume that $Q=nP$, how can we find this $P$? We can assume that $n|p-1$. If $n$ is "...
2
votes
2answers
365 views

Generate secure password hashes without access to PBKDF2 or bcrypt

Given is a system, which does provide only implementations of fast hash algorithms (MD5, SHA1, SHA-256, SHA-512). There is no implementation of PBKDF2, bcrypt or scrypt available. The system does ...
2
votes
2answers
261 views

64 bit Elliptic Curve key?

For a simple proof of concept project i'm (attempting!) to do, i've started looking into openSSL elliptic curve cryptography. However instead of the standard key lengths, 160-512. I'm interested in ...
2
votes
2answers
535 views

Why is the following RSA PRNG cryptographically secure?

One requirement states that the generator has to withstand the next-bit test. Consider the following PRNG, where we calculate next output $x_i$ via the formula $x_i = x_{i-1}^ e\mod n$. I can see ...
2
votes
2answers
673 views

How does this happen in RSA malleability?

I don't understand how the $E(m)$ turns into $E(mt)$. I mean, I don't know how does that transformation happen and how does the equation occur. $$E(m) \cdot t^e \bmod n = (mt)^e \bmod n = E(mt)$$ ...
2
votes
2answers
185 views

Speed up modular exponentation with fixed base and modulus

Can someone explain, how $a^x \mod N$ can be speeded up, when $a$ and $N$ are known constants? How big is the gain and what resources are needed? https://www.imperialviolet.org/2013/05/10/...
2
votes
1answer
412 views

Questions about proof of correct encryption in the Paillier cryptosystem

In the Paillier cryptosystem [1] the encryption of $m \in \mathbb{Z}_N$ with randomness $r \in \mathbb{Z}_n^*$ is $c = g^m r^n \bmod{n^2}$. A proof of correct encryption could look like presented in ...
2
votes
1answer
460 views

Fast modular reduction

I am looking at ways to speed up modular reduction for the polynomial $$2^{256}-2^{32}-2^9-2^8-2^7-2^6-2^4-1$$ I have read the paper "Generalized Mersenne numbers" by J.A. Solinas, but it does not ...
2
votes
1answer
492 views

distribution for a subset of md5

I need to uniquely assign a 32 bit integer to a collection of human readable strings (i.e. they will not be generated with hash collision in mind). I don't expect more than a few thousand strings. I ...
2
votes
1answer
77 views

Can the key-complement property of DES be used against 3DES?

The comments in this recent question about the security of 3DES made me wonder if the key complement property of DES (which reduces its security by ~1 bit) can be used in the meet-in-the-middle attack ...
2
votes
1answer
211 views

How does ccrypt work?

On many Linux/GNU distributions CCrypt (ccrypt) by Peter Selinger is available as a command line utility to encrypt files. How does the protocol work and is it ...
2
votes
1answer
149 views

AES Message Authentication Vulnerability

My securities instructor recently posed this question to the class and I am bending my brain in half trying to figure it out. Suppose $y_1 = AES(K, x_1)$ and $y_2 = AES(K,x_2)$, and suppose you ...
2
votes
1answer
123 views

How do I know if a given curve requires a FpCurve F2mCurve or ECCCurve?

I'm trying to read a public key into Bouncy Castle (secp256k1) and need to choose from the following objects FpPoint; FpCurve; or ...
2
votes
0answers
364 views

Elliptic Curve based blind signature implementation

I want to use Elliptic Curve based blind signature scheme for my research. There is no proper implementation of ECC-based blind signatures. Can someone describe to me which things I need to follow ...
2
votes
3answers
164 views

How much processing power should you assume an attacker has?

The answer to this question says that you should assume an attacker can do one billion operations per second: Key Size for Symmetric Homomorphic Encryption Over the Integers Is that a single attacker ...
2
votes
2answers
275 views

Point addition and doubling in Ed25519 (ref10)?

I just migrated CodesInChaos' C# port of Ed25519(ref10) to Java, and everything works fine. (I.e. I get the same results for key generation, signature and verification.) Now, I would like to do a ...
2
votes
1answer
54 views

Question about block erasure codes

I have a question about linear block erasure codes that are described in this paper. I briefly describe the idea behind the linear erasure codes and then I ask my question. Given a set $d=\langle x_i ...

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