# All Questions

47 views

### How to encode a message to MD5?

I have faced a problem to understand MD5 algorithm.My question is weather the original input message is converted to binary value in MD5 before using the input message as parameter in four Feistel ...
289 views

### Homomorphism and Zero Knowledge in FIPS 140-2 compliant systems

I am analyzing a system that needs to be at least level 3 FIPS 140-2 compliant. The system may leverage from homomorphic and Zero Knowledge constructs, but, as far as I can see, the algorithms ...
85 views

### How do they implement MAC in TLS?

I recently read questions about "MAC then Encrypt/Encrypt then MAC" which made me curious about how they actually use the algorithm. MAC only calculated once either case (according to what I read). ...
61 views

### Are there any test vectors for the CMS content type AuthEnvelopedData (for AES-GCM)?

I am looking for CMS AuthEnvelopedData test vectors for AES-GCM mode. I haven’t seen any ready-to-use test vectors for it. There are test vectors for AES-GCM mode but not for its CMS support. I even ...
93 views

### Question on 3DES keys

The official NIST publication I saw says for federal applications 3DES with three keys should be used. I have such questions: what does it mean federal? Does it mean for industrial applications that ...
86 views

### Finding collisions in SHA1

I am trying to find two collisions in SHA1 for the 50 least significant bits. I was wondering if there was a way to efficiently do this without having to brute force all of the possible hash outputs? ...
21 views

### Would using a symmetric list of subkeys in a Feistel cipher make our messages vulnerable to oracle attacks?

Suppose I have a Feistel cipher structure that has $16$ rounds. I come up with $8$ subkeys $k_1,k_2,...,k_8$ to be used for the first $8$ rounds and then assign the rest of the keys so that ...
176 views

### Mathematical formula for switching the key for OTP?

Instead of generating the random key for the one time pad cipher over and over again, is there a mathematical formula that allows you to switch the key to a new key? The new key must be as random and ...
131 views

### How vulnerable is the one-time-pad (OTP) encryption, if the OTP is used twice, with a random substitution scheme

So I was wondering, I read up about OTP encrption and can see how using an OTP encryption is in itself unbreakable. However, I looked at how OTP is broken if the OTP-key is used on more than one ...
34 views

### How to go about encrypting a digital exam?

I'm building a Digital Examination system and I'm having problems understanding some concepts in cryptography. I want use to Asymetric Cryptography and my questions are Do I encrypt the entire ...
35k views

One runtime platform provides an API that supplies PKCS#5 padding for block cipher modes such as ECB and CBC. These modes have been defined for the triple DES, AES and Blowfish block ciphers. The ...
36 views

### Secure way to hash a lookup value

I want to securely store a database table that consists of an account ID and a few columns of sensitive data. The data only has value if it can be associated with the account ID. My thinking is to ...
42 views

### Can an attacker recover the key if a Poly1305 collision is known?

Suppose that messages $m_1$ and $m_2 \neq m_1$, both known to the attacker, are both MAC'd through Poly1305 with the same key. By chance, these two messages hash to the same value. The attacker ...
27 views

### Inverse in ElGamal signature

I am learning about ElGamal signature verification. During the signature generation one has to choose a k such that ...
31 views

### Access structure in Attributes Based Encryption [duplicate]

I thought that access structure and access tree are equivalent but I've read in a paper (page 3) that there are at least three types of access structures : AND gates Access tree LSSS generating ...
65 views

### Scalar multiplication with projective coordinates

I'm implementing point addition, doubling and scalar multiplication using projective coordinates. I took reference from this link https://www.nsa.gov/ia/_files/nist-routines.pdf I have implemented ...
442 views

### Is a 4DES or 5DES system possible?

We know that 3DES is created with $E_{K_3}(D_{K_2}(E_{K_1}(m)))$ to extend DES's key length. Is it possible to extend it further by repeating this pattern? Perhaps using ...
29 views

IPSec like strongSwan can be configured to do rekeying after time or bytes transferred or packets transmitted. Is there any recommendations to set those settings? Maybe according to algorithms used?
46 views

### digital signature on image and text

i am trying to implement an android app that should "sign" an image, M and a string, S using a private key, K-1 and send the result to a server. for instance the app should be doing {M,S}K-1. the ...
2k views

### Vigenere ciphertext encrypted with another vigenere cipher

I have been learning about Vigenere ciphers and then thought of this scenario: if a cryptographer encrypts a plaintext English message with a Vigenere cipher and then another cryptographer, who wants ...
53 views

### What happens to NORX if the same (key, nonce, header data) triple is reused

As I understand it, reusing a (key, nonce, header data) triple in NORX leaks the XOR of the initial common prefix (but this is zero, so only its length is leaked) the XOR of the first distinct ...
47 views

### Does absence of second pre-image resistance infer absence of pre-image resistance?

Talking about hash functions, I know the hierarchy is as follows: Pre-image resistance Second pre-image resistance Collision resistance Where each property implies the one before it so 2nd ...
109 views

### Why gcd(r,(p-1)/r) needs to be 1 in benaloh cryptosystem

I recently discovered the benaloh cryptosystem. I am working with the system as it is discribed in the following link: https://en.wikipedia.org/wiki/Benaloh_cryptosystem However I need some help in ...
126 views

### How can prime factor of $q-1$ divide prime $q$?

In this paper - "A Subexponential Algorithm for the Discrete Logarithm Problem", author mentions (page 56), For each $p_l^{e_l} | q$ proceed.... $p_l^{e_l}$ is one of the prime factors of ...
279 views

### Ciphertext notation where the ciphertext consists of two parts

I've see in several places (here or there) the notation $c = (U, V)$ or $c =\langle U, V \rangle$, where $c$ is a ciphertext (to be transmitted). But as both $U$ and $V$ are "key-like". I can't find ...
59 views

### Unkeyed, fast, one-way PRP

Are there any fast, secure, one-way, unkeyed almost-pseudorandom permutations? I am looking for something that can hide a MAC, without requiring a secret key and while being much faster than public ...
104 views

### Can “Approximate Hashing” work in a Proof-of-Work context?

Here is a draft of an article claimed to soon appear in a peer-reviewed conference: Matthew Vilim, Uenry Duwe, Rakesh Kumar, Approximate Bitcoin Mining. It is proposed to simplifying a SHA-256 engine ...
95 views

### Fast MAC for 64-bit inputs

I am looking for a fast MAC (~few machine instructions) to be used for hash tables with 64-bit public inputs. I can't use SipHash as the hash function will be inlined in too many places and so the ...
3k views

### Why do we use XTS over CTR for disk encryption?

I'm taking Prof. Boneh's crypto class from Coursera, and am unsure on the requirement for XTS mode for disk encryption. It seems that CTR mode would do exactly what XTS can do, but is simpler to ...
33 views

### Can we share an n-bit string by using shamir's secret sharing n times?

For Shamir's Secret Sharing, with a secret of size $n$, instead of having a finite field of order $n$, can we just use Shamir's Secret Sharing $n$ times, once for each bit, using a field of order 2? ...
48 views

### Simulation of a custom build network security algorithm with ElGamal Cryptosystem using Elliptic Curve

I am trying to build an algorithm to encrypt and decrypt text using ElGamal Cryptosystem using Elliptic Curve. My algorithm generation is done. But at simulation part I stuck. My algorithm steps are ...
78 views

### Encryption with two meanings without computer or programming

Can one create a cipher with two meanings? Not two layers, two phrases in the same cipher in the way that it would have a meaning X, for example, if deciphered using Transposition cipher and meaning Y ...
91 views

### What is the difference between mutual authentication and unilateral authentication both ways?

I am currently reading Engineering Security by Peter Gutmann, on page 7 he states: proper mutual authentication is quite different from unilateral authentication in both directions He ...
41 views

### mapping function

I have read a scientific paper which covers some security schemes in mobile ad hoc networks. I don't understand this notation: $$H_0\;\;\;\text{ Mapping }\{0,1\}^*\to \mathbb Z_p^*$$ where $p$ is a ...
375 views

### ECDSA signature verifiable 1-way transformations

Alice signs a message $m$ with her private key, yielding a signature ($r$,$s$). I want to prove to someone else that I have this signature, but I don't want them to have the knowledge of what ...
128 views

### $(k, n)$ threshold secret sharing to $(k+k', n + n')$ without reissuing all shares

Given a $(k,n)$ threshold secret sharing used to back up a secret (meaning that the dealer has access to the secret at all times), is it possible to update it to a $(k+k', n+n')$ threshold secret ...
37 views

### Weighted secret sharing scheme?

Is there a secret sharing scheme where secrets can be weighted, and that you need a total weight of $1$ to unlock the secret? I know that you could, of course, just figure out every subset that has ...
252 views

### Given a linear block-cipher,how can an attacker decrypt any plaintext value encrypted,using 128 chosen ciphertexts

Given the linear block cipher $\operatorname{LinearCipher}(k, p) = c$ \operatorname{LinearCipher}(k, p_1 \oplus p_2) = \operatorname{LinearCipher}(k, p_1) \oplus \operatorname{LinearCipher}(k, ...
420 views

### Relation between attack and attack model for signatures

I would like to know: What is the relationship between an attack and an attack model. For example, let $\Pi$ be the Lamport signature scheme. This signature has its security based on any one-way ...
61 views

### RSA : Double-Encryption and order of Encryption

I'm studying cryptography and we're looking at RSA. We're required to do a double encryption from Alice to Bob and then Bob back to Alice. We've been told the order of encryption is important and it ...
232 views

### Choosing finite field size in Shamir's Secret Sharing Scheme

The Wikipedia article on Shamir's Secret Sharing says to that to have information theoretical security the splitting algorithm should be evaluated using finite field arithmetic on the field ...
132 views

### Questions about a SHA3 cipher stream encryption scheme

I have written an encryption implementation using a hash as a cipherstream for demonstration purposes. I know it is slower than more sensible options such as AES but I am interested in several things, ...
139 views

### Is there any zero-knowledge technique to verify proof of knowledge of shared secret key?

Generally, to verify knowledge/key generated between two interacting parties in an authentication protocol uses Hash, MAC, Digital Signatures or encryption (based on random challenge). But I would ...
2k views

### Desirable S-box properties

What desirable properties should an S-box have? My current standard selection process is to just pick them at random and verify that they fit the following criteria: The probability that any random ...
58 views

### Create big random number from 2 smaller numbers?

If I have 2 true random numbers from atmospheric noise or a different source. Suppose $x= 299$ and $y= 499$. Is it possible to create a bigger number with $x$ to the power of $y$? Can I use ...
117 views

### What is the typical block size in RSA?

If the input text is divided into a fixed block size for encryption and decryption, what could be the typical block size for RSA algorithm for different size of input like 8 MB, 16 MB, 126 MB and 256 ...
30 views

### Metrics of PRNGs

I am a bit confused on the metrics that can be applied to the output of (deterministic) pseudo-random number generators. I've run output from many RNGs through ENT. The only metrics that really differ ...
25 views

### Is Shamir's Secret Sharing Scheme insecure for larger field? [duplicate]

According to wikipedia, if you are using shamir's secret sharing scheme with a field of order $p$, "High values of $p$ are risky because Eve knows that the chance for $f(x)\pmod{p}=f(x)$ increases ...
Bob uses RSA with $p=83$ and $q=103$. His public key is $e=445$. Alice claims that she can calculate Bob's private key without using the definition $d=e^{−1}\bmod φ(n)$? But how?
${\text{Let }}g:{\{ 0,1\} ^n} \to {\{ 0,1\} ^n}$ be a round function of an $n$-bit iterated block cipher and let $K \in {\{ 0,1\} ^n}$ be a fixed round-key, and denote ${g_K}(x) = g(x \oplus K).$ Show ...