# All Questions

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### 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 ...
103 views

### Functional Encryption (for Non Experts)

I recently stumbled across "How to Run Turing Machines on Encrypted Data". Given that it's now possible to run turing machines on encrypted data, what open problems remain in the realm of Functional ...
132 views

I know about PBKDF2, bcrypt, scrypt, etc. and while I'm not a crypt professional, I think I understand why are they better than just "hash(pass+salt)". But a colleague of mine surprised me with the ...
62 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 ...
125 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 $d_2=2663$. ...
58 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 ...
70 views

### Diffie-Hellman explicit key confirmation

Suppose I wanted to add explicit key confirmation to Diffie-Hellman key exchange, would the following scheme be secure? Alice selects a random $a$ and sends $g^a \mod p$ to Bob Bob selects a random ...
68 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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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 ...
75 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 ...
110 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 ...
98 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 ...
91 views

### Json AES128: Security against known plaintext attack

I have a system where I am transmitting json messages securely (using for example AES-128), where each message has the same format. For example ...
119 views

### Are the SHA family hash outputs practically random?

Say I hashed the output from a random number generator (with nonce), would the resulting SHA256 hash be as random as the inputted number? And If I used the first 5 hex characters, and then used the ...
68 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 ...
163 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 ...
55 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 ...
67 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 ...
109 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 ...
253 views

### Find the key to a Vigenère cipher, given known ciphertext and plaintext

I have the ciphertext and the plaintext. Is it possible to know the key? If not, what do I need to find it?
62 views

### How do we arrive to the equation for solving D in RSA?

We have been shown that DE is congruent to 1 mod phi(n). How do we arrive to the real equation for figuring out d that is d = e^-1 mod phi(n). I want to know the entire process. I am just curious, ...
107 views

### Weak Stream Cipher cryptanalysis

Hi I have a stream cipher which uses a keystream of: F(r) || F(r+1) || F(r+2) || ... Suppose F(r) is the RSA encryption algorithm: ...
170 views

### scrypt and bcrypt for benefits of both?

I've been reading about bcrypt and scrypt, and it seems that the general consensus is that bcrypt is more "tried and true", but scrypt is better in theory. Couldn't you just hash something with ...
171 views

### Shamir's Simple Sharing Scheme - preventing partial recovery of data

In SSSS, if you recreate the original secret with one of the decoder inputs being slightly damaged (e.g. one or two chars incorrect), you receive a slightly damaged version of the original secret. So ...
60 views

### What is the difference between a bijective random oracle and a random permutation?

Assume $S$ be a finite set $O$ be a random oracle from $S$ to $S$, such that $O$ is bijective $f$ be a random permutation of $S$ Is there any difference between $O$ and $f$? Does it makes any ...
99 views

### Homomorphic Encryption, LWE, and Practical Applications

I've been reading Brakerski and Vaikuntanathan's Efficient Fully Homomorphic Encryption from (Standard) LWE. When the authors discuss a small modulus p used for the transition from (n, log q) to (k, ...
71 views

### Applying differential cryptanalysis to ciphers with addition mod $2^{32}$

I am studying differential analysis and have a question. Consider the following simple cipher: $$c_1 = S(m_1 \oplus k_1) \oplus k_2$$ (Plaintext $m_1$ xor with key $K_1$, then result goes into an ...
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### Is the following aggregation scheme private?

Is the following scheme private? By private i mean an untrusted aggregator (UA) cannot reveal anything other then an aggregate function output on plaintext data Each party holds a secret key $k_i$ ...
55 views

### Why send a challenge with CHAP?

I have been reading about the Challenge-Handshake authentication Protocol (aka CHAP). Why send a challenge to the user and ask them to hash that challenge with the symmetric key and send that hash ...
78 views

### Efficient parameters for group Diffie-Hellman

I want to implement a basic version of Diffie-Hellman key agreement for groups. So, my key is $K=g^{abc} \mod p$. Following this, the parameters I would need to transfer would be $K_a = g^{bc}$ etc. ...
148 views

### Why can we always assume that $Gen$ algorithm choose key uniformly?

Talking about a (symmetric) encryption scheme $\Pi=(Gen,Enc,Dec)$ where $Gen$ take the security parameter $n$ as input and generates a key $k$ of length $n$. Generally speaking we say that $Gen$ ...
50 views

### order between adversaries and type of resources given

We have a game $G0$ for which an adversary $A$ has access to a certain amount of ressources. Let us suppose that the maximum advantage for the adversary to win this game is $Adv_{G0}$. If we modify ...