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1answer
180 views

How can I find the multiplicative inverse in the first transformation of the SubBytes() transformation in AES?

In FIPS-197 §5.1.1 , it says the first transformation in the SubBytes() transformation is: Take the multiplicative inverse in the finite field $\text{GF}(2^8)$ described in Sec. 4.2; the ...
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1answer
127 views

Public and private key lengths

I am generating RSA Public and Private keys with node.js instead of reading them from pem/crt files as I can hide them inside a private object using this module https://www.npmjs.org/package/private ...
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1answer
107 views

LFSR and Markov chain question [closed]

This may seem an elementary question about LFSRs, and their link to Markov chains. LFSRs show Markov chain behaviour in that there can be a transition matrix defined over the LFSR, this follows from ...
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1answer
93 views

How can a digital signature algorithm incorporate a random data if it is to be authenticated on a separate system

I am trying to understand how RSA digital signatures can incorporate any data from a PRNG into the final signature. Does this mean that separate implementations might fail to verify each others ...
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1answer
96 views

Number of different substitution alphabet ciphers possible with given conditions?

If you consider a 26 letter alphabet, and a cipher where 24 of the letters are sent to themselves, and only 2 letters switch, how many different substitution alphabet ciphers are there, and what ...
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1answer
148 views

ElGamal digital signature. why mod p-1 in delta?

Im trying to figure out why $\delta$ in the signature part of ElGamal has mod $p-1$ when $\gamma$ has mod $p$? $\gamma = \alpha^k \mod p$ $\delta = (x-a\gamma)k^{-1} \mod p-1$
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1answer
45 views

Computational Diffie-Hellman problem over the group of quadratic residues

Suppose that $N=pq$ where $p$ and $q$ are safe primes. $\mathbb{QR}_N$ is the group of quadratic residues which is a cyclic group with order $\frac{\phi(N)}{4}$. Let $g$ be the generator of ...
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1answer
318 views

About the Rijndael/AES sbox polynomial (subBytes)

I've recently read a question about the irreducible polynomial behind the subBytes() operation in the Rijndael that has awakened and old curiosity I have: Why $\,m(x)$ was chosen as ...
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2answers
158 views

reverse an identicon to its source string

Identicons are used to create unique images based on IP address or strings. Lots of programs are available to generate identicons based on an input string. My ...
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1answer
136 views

Are the RFC3526 MODP groups Schnorr groups?

I was wondering if a group like the 1536-bit MODP Group from RFC 3526 was a Schnorr group? A Schnorr group must apparently have: $p$ and $q$ being primes $p = q\cdot r+1$ $1 < h < p$ ...
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1answer
101 views

Numeric combination of passwords

Pls I need explanation on How i can design a Cryptography that use multiple passwords or passphrase to open a safe(Lock). For example, if i need five people to unlock a secured device whereby all the ...
1
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1answer
94 views

Definition of one-way functions: randomly-chosen point vs every point

Notation: $\Sigma^k$ is the set of $k$-strings on the alphabet $\Sigma$; $\Sigma^\ast$ is the set of all finite dimensional strings on the alphabet $\Sigma$. In the context of computer-science a ...
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1answer
367 views

Usage of different size chunks during AES encryption/decryption

I want to encrypt user's files on our server. For this purpose I decided to use the PHP Mcrypt library. I chose to use the Rijandel-128 algorithm and CBC mode for encryption. Since files are quite ...
1
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1answer
141 views

XORing a header with SHA512-hash - secure?

I am writing a application that encrypts text several times and stores the result. The result looks like complete random bytes, therefore I add some random bytes at the end. To let the program know ...
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1answer
64 views

MAC security and adversaries with memory

The usual definition of MAC security is (roughly) that there is no "efficient" adversary that has non-negligible advantage in the "MAC game" where the MAC game is as follows: the challenger chooses a ...
1
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1answer
79 views

Exchanging Keyspace and Message space in PRF

Suppose there is a secure PRF $F: K \times X \to Y$. Then is $F': X \times K \to Y$, defined by $F'(x,k):=F(k,x)$, a secure PRF? In my opinion it should be because exchanging the keyspace with ...
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1answer
271 views

AES-256 Shift-Rows Offsets

I am currently trying to implement a AES-256 cipher according to this sepcificaciton: http://techheap.packetizer.com/cryptography/encryption/spec.v36.pdf As well as on Wikipedia and in the document ...
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1answer
47 views

Alert protocol in TLS

Assuming the alert protocol is encrypted AFTER a session has been established and the structure below, how does the client know whether to expect encrypted data or not? ...
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1answer
89 views

Is it possible to encrypt data in such a way that it cannot be decrypted with the key used to encrypt it but can be decrypted with another key

I'm very new to cryptography and encryption so I apologize if the answer to this is well known, but I can't find anything in my research. The crux of the problem is the need to store data on an ...
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1answer
61 views

How to invalidate decryption key in multi user model?

I want to develop an application that will allow a user to create a public/private key pair, and encrypt it symmetrically with a key such that once they enter their password, they will be able to ...
1
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1answer
233 views

Rock-paper-scissors over network, how to protect from cheating server?

I'm trying to design cryptographic protocol to play Rock-Paper-Scissors with two parties, neither trusting each other, nor trusting server they use for communication, so game is 'provably fair'. So ...
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2answers
140 views

McEliece key size

There's a lot of references about McEliece key size being the barrier for proper usage of the algorithm, exactly (or roughly) how large are the keys?
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2answers
85 views

Subexponential algorithms for DLP in $\mathbb{Z}_s \times \mathbb{Z}_t$

Consider the accepted answer to the question: Why are elliptic curves better than cyclic groups? It seems to suggest there are subexponential algorithms (i.e., algorithms with running time $$ ...
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1answer
191 views

Traitor-tracing PRF

I'm looking for a pseudorandom function (PRF) with "traitor-tracing" properties. More specifically, I'd like there to be multiple equivalent keys, so I can give different parties different keys, but ...
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1answer
324 views

Can a monoalphabetic substitution cipher attain perfect secrecy?

Can a monoalphabetic substitution cipher attain perfect secrecy? Definition of perfect secrecy: $${\rm Pr}[\,{\rm Enc}_k(m_1) = c\,] = {\rm Pr}[\,{\rm Enc}_k(m_2) = c\,]$$
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1answer
119 views

Modular exponentiation with Chinese Remainder Theorem

I'm learning modular exponentiation with Chinese remainder theorem. I found a great answer from below How can I use eulers totient and the chinese remainder theorem for modular exponentiation? But I ...
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1answer
70 views

Convincing “someone” on the (in)validity of DH tuple without revealing $u,v$

Is there a way of convincing someone on (in)validity of Diffie Hellman tuple $<g, g^u, g^v, g^{w}>$ without giving away $u,v$. or at least solve the Decisional Diffie Hellman Problem without ...
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2answers
398 views

pbkdf2 password validation

My question is related to PBKDF2. I am unsure how to validate user entered password with already existing hashes and salts. I have heard of the so called "length-constant" comparison which basically ...
1
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1answer
302 views

Simplified Fiat-Shamir example generates wrong output

I am trying to implement the Fiat-Shamir identification protocol, however the end results always fail to match. I am using algorithm's description from here. Preparation: Select 2 prime integers ...
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1answer
99 views

Modular protocol design

What does modular protocol design mean? Why does TLS not have modular protocol design? What protocols have modular design? (IPSec, SSH)
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1answer
275 views

Prime factorization of RSA modulus

Consider RSA public-key encryption with public modulus $N=3953$. Suppose we know that the public keys $e_1=337$ and $e_2=23$ correspond with the decryption information $d_1=3385$ and $d_2=2663$. ...
1
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1answer
158 views

simple multiplication in GF(8)

I am trying to do multiplication in the GF($2^3$) defined by the irreducible minimum binary polynomial $X^3+X^2+1$. I want to multiply $A(x) * B(x)$ where $A(x) = x$ and $B(x) = x^2$. The ...
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1answer
393 views

Known vs. Chosen (plaintext and ciphertext) and RSA

I want to know whether I can break RSA (textbook) with known plaintext/ciphertext attacks. Known plain-text Let $c$ be the ciphertext and $m$ the plaintext. $$c = m^a \mod n$$ Since all I have is ...
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1answer
256 views

Hill cipher is not perfectly secure

I am on cryptography course and there is a homework question to show that Hill cipher doesn't have perfect security. So assume we have an cryptosystem $(P,C,K)$, where $P = C = \mathbb Z_{26}^N$ and ...
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1answer
85 views

Addition-only PHE in F#

Using homomorphic encryption, I would like to be able to take an encrypted integer and either add 1 or -1 for a new encrypted value. I do not want the encrypted value to be recoverable - just the ...
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1answer
153 views

Usage of Fermat primes in RSA

Why are Fermat primes ($2^{2^n}+1$) useful as $e$ (the public key) in RSA encryption? I understand why $2^n+1$ primes are useful, because they would simply be ...
1
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1answer
51 views

Would requiring reciprocal signatures create a stronger Web of Trust?

In this post on Cryptome it's noted that a person called Michael Vario is willy-nilly signing people's public key's and uploading those signatures. He is effectively forging a web of trust. I'm sure ...
1
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1answer
480 views

AES mode for file encryption

I'm working on software that should eventually result in a crypto file system similar to encfs. So far, i have a user password that goes through PBKDF2 using ...
1
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2answers
319 views

AES-CBC and IV — Encrypting multiple blocks of a file with the same IV?

I have large files which are broken into blocks of 512 KB and uploaded to a server. In my DB I store the IV, and the user knows the security key themselves. When downloading, I use the same IV and ...
1
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1answer
371 views

Is it possible to decrypt double-key crypted file knowing public key and a having one or more original files?

I have some encrypted files with double-key. I know public key and don't know private key. Can I obtain the private key (and decrypt all files) using two version of one file - crypted and decrypted? ...
1
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1answer
4k views

AES Algorithm processing time in Arduino vs Raspberry Pi

The Arduino employs an 8-bit ATmega series microcontroller whereas the Raspberry Pi is based around a 32-bit ARM processor, and the Arduino is typically clocked at between 8-16MHz and with 2-8kB of ...
1
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1answer
161 views

Trustless, Multiparty Random Number Generation

I've been having trouble with coming up with a random number generator that produces a random number between two parties. My current scheme is this: User 1 gives user 2 the hash of their randomly ...
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1answer
562 views

In this example, which is a premaster secret, and which is a master secret?

In the context of this data, as shown in the image below, which one is a pre-master-secret and which one is a master-secret. It looks to me as if the final value of 2, for Alice and Bob is a ...
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1answer
593 views

Forgery Attack Against RSA Digital Signature

I have been reading about attacks against RSA Digital Signature in a book The attacker chooses the signature first and then computes the message. As a consequence, he cannot control the ...
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1answer
65 views

Are those two distributions indistinguishable?

The Decision composite residuosity problem problem states that is impossible two distinguish between those two ensembles: $\{x^N \mod {N^2} | x \in \mathbb{Z^*_{N{^2}}}\}$ and $\{r \in ...
1
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1answer
192 views

Diffie-Hellman on additive group

Given the finite cyclic, additive group (G, +), with |G| = n and generator = g, what are the computations and exchanged messages for Diffie-Hellman? What I tried myself: Alice chooses a private $a$ ...
1
vote
2answers
347 views

Game traffic encryption AES CTR + HMAC

I was asked to implement traffic encryption between the game server and clients. Currently, the traffic is not encrypted in any way, which lets players use cheats to listen to the traffic in order to ...
1
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1answer
103 views

Sample RSA-style signature confusion

I'm trying to study a simplified TLS-style protocol, including verifying RSA signatures. It's an attempt to verify using only a valid certificate. However, I can't manage to get anything to verify ...
1
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2answers
100 views

Can a zero knowledge proof (or MAC) be generated from human-typable input?

Edit: My goal is to create a voting scheme that doesn't require a lot of crypto infrastructure to be deployed on the client side. Smartcards, and tokens are impractical for these purposes. I ...
1
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1answer
302 views

How should I manage Diffie-Hellman parameters on a Web Server?

Is there a best practice for Diffie-Hellman parameters (p, g, and q) used on a web server? The server is using ephemeral keys, but should the parameters be rotated on a regular basis? Is it OK for ...

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