# All Questions

1answer
195 views

### Do Quantum Key Distribution and Physical Unclonable Functions combine, and how?

I see there's a project to combine Quantum Key Distribution, Physical Unclonable Functions, and classical crypto, in order to secure a high speed (100Gb/s) optical link. While there does not seem to ...
1answer
2k views

### Deciphering a key from XOR encrypted cypher using boolean logic

Assume there's an unencrypted message A, and an encrypted message B. You know that message B was encrypted using a simple XOR method of A with a private key K, resulting in message B. Thus, B = A ⊕ K ...
1answer
642 views

### Expectation Value of the Index of Coincidence

Can Anybody tell me why the expectation value of the Index of Coincidence is the following? For a cipher text string of length $L$, where $n$ is the number of alphabet characters, $k_r = 1/n$, and ...
1answer
84 views

### Incorporating known ciphertext into new message

Which ciphers $c_k$ enable one to define a joining function $f_{c,k}$ such that: $$f_{c,k}(c_k(p_1), p_2) = c_k(p_1 + p_2)$$ That is, the result of joining some known ciphertext $c_k(p_1)$ with a ...
1answer
628 views

### How to represent a 32-byte SHA2 hash in the shortest possible string?

I'm calculating a SHA2 hash of a certain sensitive key value. I need to store files on disk using this hash a directory path prefix. So lets say I hash the key value 150023, I get a 32-byte value ...
1answer
123 views

### Tips on conceiving safe software messaging platform

I have a personal project in mind in which I plan to use cryptography in order to let the users be confident in the fact that, even if all data get's stolen, it would be virtually impossible to crack ...
1answer
540 views

### Brute forcing Cardan grille

Having a "rotating" square Cardan Grill with sides long n cells, how could i determinate the cost of a brute-force attack? How many configurations should be considered to perform an exhaustive ...
1answer
315 views

### Twofish key length

I'm using twofish. Say you have a 120 bit key, is there any difference in using a 128 bit bit keylength or a 256 bit one? Since the most significant digits of the key will be allo zero in both cases, ...
1answer
625 views

### Is this fixed length MAC unforgeable?

Consider the following fixed length MAC for messages of length $\ell(n)=2n-2$ using a pseudorandom function $F$: On input of a mesage $m_0||m_1$ ($|m_0| = |m_1| = n-1$) and a key $k \in \{0,1\}^n$, ...
1answer
146 views

### How is the password sent across for verification?

I am familiar with Challenge-Response Authentication scheme. However I would like to know actual message content sent between client and server for 1 way authentication, over an insecure channel. ...
1answer
783 views

### iSeries (AS/400) Database File: password encryption

I am helping with a project in which an old software system on an iSeries is having a brand new .NET UI applied to it. It's going well... except... In order to allow users to login and maintain ...
1answer
15 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, ...
1answer
24 views

### What is the difference between LCG and Affine cipher?

What is the difference between LCG and Affine cipher? More specific: What are they used for, and what are the structural differences between them?
1answer
54 views

### RSA: must $d$ be an integer?

I am only taking baby steps in RSA. If $p=11$, $q=7$ and $e=3$, $$\phi(n) = 10*6 = 60$$ Then: $$d = (2 (\phi(n)) + 1 ) / 3 = 121/3$$ Should $d$ be kept as a non-integer or is such a $d$ invalid? ...
2answers
41 views

### Find plaintext of RSA by solving extended euclidean algorith for two encrptions with two different exponents for same plaintext

This is my homework question (but I am not asking the answer to it): Suppose two users Alice and Bob have the same RSA modulus n and suppose that their encryption exponents eA and eB are ...
1answer
21 views

### how to calculate maximum data size of an RSA signature

I am reading a content, and am not sure if I follow how they come up with the maximum data size which can be signed by an RSA key. The author states: "...if you are trying to sign a message using a ...
1answer
68 views

### Cracking simple RSA when just the ciphertext and one public key is known

Can someone explain to me (Or better yet, point me to a tool that does the brute forcing) how to crack a standard, simple RSA cipher (So don't bother with padding) if I know just one public key, the ...
1answer
66 views

### Proving collision resistance of a hash function?

Let $f$, $g$, and $h$ be hash functions that each map binary strings of length $2n$ to binary strings of length $n$. Suppose that $h(x) = f(g(x)||g(x))$. Prove that if $f$ and $g$ are collision ...
1answer
32 views

### Key material leak from initialized cipher?

Let's imagine that cipher (say AES256 in CBC mode) is initialized when application starts: ...
1answer
44 views

### Time complexity of trial division

Suppose $n=pq$, where p,q are prime numbers. let $p ( \le q)$ be the smallest prime, then we know that $p \le \sqrt{n}$. In trail division, we check $n \mod i$ for the values of $i$ from 2 to ...
1answer
35 views

### Help with pseudorandom functions

I started a new lecture this year about crypto and I've had a few problems in the first few weeks with the exercises we got. I would appreciate a "guide" on how to solve problems like the following ...
1answer
48 views

### How many Hashes to find a digest of 2^{n−k}?

Given an n-bit cryptographic hash function $H$, how many messages should we expect to hash before finding a message $x$ such that $H(x) < 2^{n−k}$ for some integer k? My take: Shouldn't it simply ...
1answer
35 views

### Are private and public key sizes of Elliptic curve related?

I'm new to elliptic curve cryptography. I just want to know in the case where I take a random number (private key) and find its associated public key, does the size of the public key depends upon ...
1answer
41 views

### PublicKey Unknown Encoding

I have a public key: ...
1answer
44 views

### Storing a secure key in an embedded device's memory

I am working on an embedded device which send/receive data and stored them in ciphertext mode (encrypted mode ). Now what's best approach for storing Keys (I used ARM CORTEX M series MCU) ? Storing ...
1answer
41 views

### PRF that can be distinguished after $k$ queries?

In adaptive attacks, if we design poorly, adversary can modify his queries and break the given pseudo random function (that is being able to distinguish it from uniform randomness). Is there a poor ...
3answers
55 views

### How to convert RSA public keys to one key to use for text hashing?

I've recently asked a question about RSA encryption. One of the things that I don't understand is the usage of two public keys. In the example on Wikipedia, a message is encrypted like: ...
1answer
34 views

### Deriving a Set of Seeds from a Seed

I need a set of seeds $S=\{s_1,..., s_n\}$, for pseudo-random function where seed $s_i$ is used to generate a set of pseudo-random values. Obviously I need to be able to regenerate the sets of ...
1answer
53 views

### Collision likelihood from /dev/urandom for VMs using the same underlying hardware

We have multiple virtual machines sharing the same physical hardware. We use /dev/urandom for session key generation. But a question comes up: if ...
1answer
99 views

### Encrypt large data with RSA through blocks in java

I'm working on a communication protocol where a client sends encrypted data to a server. What happens is that in some point, the client creates a symmetric key used to create data digests through the ...
1answer
49 views

### Design for file transfers to a new party

I'm pondering a problem of how to enable transfers of files from clients to people that does not yet have an application/account/keys yet. Here's my current thought for Alice wanting to send Bob ...
1answer
26 views

### Security of SRP-6a against impersonating server choosing small/wrong N

I'm looking at an implementation of SRP (Secure Remote Password) that essentially follows the Stanford documentation (http://srp.stanford.edu/design.html). I'm worried about one aspect though: In the ...
1answer
101 views

### SHA-256 “almost unique”?

I have seen numerous references on the internet of people describing SHA-256 as generating an "almost unique" hash. Exhibit A. there are more. Is there some mathematical basis to the almost ...
1answer
39 views

### Byte size of Diffie-Hellman public values

I have a scenario that uses RFC-3526's Diffie-Hellman Group 14 for 2048 bit key exchanges. From the document, it specifies the P (prime) value in hexadecimal value as: ...
1answer
54 views

### DES and using the S-Box 1

Can someone please check my work and correct me if I am wrong, I am just starting to learn about DES. I am trying to solve the following problem: Show that ...
2answers
56 views

### How to generate backup key for PBKD functions?

I would like to encrypt some files using AES, Ill derive key from password using some standard PBKD function and use that as AES encryption key. My question is, if user forgets the main password, ...
1answer
59 views

### Which ECB ciphertext stealing scheme is this?

I am stuck with identifying ciphertext stealing method with the following properties: mode of operation is ECB; recommended padding scheme for this application is PKCS#7, but apparently it is not ...
1answer
145 views

### How does the Efficient Fully Homomorphic Encryption from (Standard) LWE work?

I am interesting to learn the low level implementation of Efficient Fully Homomorphic Encryption from (Standard) LWE and I am wondering if anyone can answer the following questions: Does the BV ...
2answers
50 views

### Are Anonymity and Authentication possible together?

Anonymity and authentication look to be seemingly contradictory but are both possible together ? I tried reading couple of papers but could not get the big picture. So what are the major approaches ...
1answer
60 views

### Is this scheme to derive multiple keys secure?

I need a cryptographic scheme to generate a deterministic chain of private keys that look random to outside observer but can always be reconstructed from a single root key, also: Neither private ...
1answer
41 views

### Digital signatures using a partially public key

I'm thinking to sign a message using a key that is made up of a publicly shared key and a privately shared key. The operation for mixing the two keys could be concatenation, if it suffices. To ...
1answer
67 views

### Help defining a shift cipher mathematically

For an assignment question, I am trying to prove formally that double encryption with two keys by the shift-cipher encryption function results in a shift cipher as well. If the Shift-cipher encryption ...
1answer
39 views

### Sextic twist maps to q Eigenspace of Frobenius

Let $E(p)$ be a Barretto-Naehrig elliptic curve with r-torsion and embedding degree 12 and $E'$ a sextic twist with homomorphism $\psi$. How to show, that $E'$ has a unique r-torsion group $\psi$ ...
2answers
61 views

### AES S-Box Affine Mapping - how to do it?

For the byte substitution layer in AES how do we determine B(i)? I follow how to use the multiplicative inverse table and using that to get B`(i), but how do I actually calculate the affine mapping? ...
1answer
33 views

### Do random oracles are called even by honest users in the PAK protocol [BMK00]?

In the full-length proof of PAK scheme, the authors made extensive use of random oracles to extract (guessed) password from the messages generated by the real world adversary for the ideal world ...
1answer
39 views

### In bilinear pairings, is it possible to let someone be only able to decrypt ciphertexts in $G_1$ but not able to decrypt the ciphertexts in $G$?

For example, in Don Boneh et al.'s paper "Evaluating 2-DNF Formulas on Ciphertexts", they gave an encryption system that the cihpertext can be in either $G$ (when only additional homomorphic ...
1answer
58 views

### If my server utilises an RSA based certificate, why does firefox think I am using Diffie Hellman?

I am not a beginner in crypto, but not that far from it, so please be gentle. I look after a web server app that I generated the SSL certificate for, and I used an RSA public/private keypair. When ...
1answer
34 views

### Variable length variable output encryption with out any padding?

I would like to know if there is any encryption algorithm with variable input variable output with out any padding? For example if I give 16(17,18,67,117,..any size) bit plaintext, my ciphertext is ...
2answers
73 views

### Possibility of the encrypted text being the same as the supplied plain text! [duplicate]

I was learning DES when it occurred to me that there could be a possible case where a (key, plainText) pair could result in an ...
2answers
92 views

### On the fly signatures and zero-knowledge

I am reading some articles which explain on the fly signatures (also called online/offine signatures). The principle is that a few operations do not depend of the message we want to sign, so these ...

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