# All Questions

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### Solve a Modular Exponentiation

It might be common, but if we had to solve an equation like this $m=s^{e}$ mod $n$ where $m,e,n$ are known. How can we find $s$. What optimisations could be applied? And what would the complexity of ...
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### ECB weakness and its exploitation

Lets say that I have a server which encrypts information at the server side using ECB mode with the blocksize of 16. If the server appends data that I control with a secret and sends back the ...
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### What happens to entropy after hashing?

What happens to entropy after hashing? Suppose you have a key with entropy $k$. Can entropy $k$ be increased by hashing the key?
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### Can we ensure the security of a crypto-algorithm and -implementaton against acoustic cryptanalysis?

Like people always say: “Attacks only get worse…” — which is why I'm asking early. I have been reading the paper “RSA Key Extraction via Low-Bandwidth Acoustic Cryptanalysis” published December 18, ...
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### How can uniformity of hash functions (e.g. SHA-256) be proved?

In reading about the Bitcoin protocol I noticed how much its proofs-of-work apparently depend on uniformity of the SHA-256 hash function. And so presumably do many other applications. How do ...
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### Brute Force on 3DES with Reduced Keyspace and Unknown IV

I'm trying to brute force a 3DES problem given a reduced keyspace (ie I know the first half of the key) but with an unknown IV. The code decrypts to plaintext. My first thought was that I could set ...
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### Is this algorithm demonstrating “proof of work”

Is this algorithm demonstrating proof of work? This is a algorithm where a "token" is combined with a time stamp and then a cryptographic hash is generated from the combination, next the objective is ...
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### Can $r$ and $s$ from an ECDSA signature be negative numbers?

I have an ECDSA implementation in Mathematica over secp256k1 and $r$ and $s$ are always positive numbers. But if you encode them as bytes, the most significant byte can be over ...
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### Calculate entropy of key derived from PBKF2 function

I am using PBKF2 to derive a encryption key from a password. suppose entropy of my password is y and my PBKF2 function has x number of iterations. So how to calculate the entropy of finale key derived ...
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### Is SSL broken? If so, what can I do about it? [closed]

But recently, with the scandals about the (in)filtrations, I read that the NSA (and of course others, but I'm not particularly worried about NSA) is able to break SSL and track your data. It is true? ...
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### Guessing encryption algorithm based on key and ciphertext

Is it possible to tell which encryption algorithm was used, assuming I have a key and a encrypted text? To not complicate the situation let assume we are speaking about symmetric algorithms only. ...
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### How to perform authentication without central server in P2P?

How can one be sure that the man who you're talking with is the one who you think he is? i.e. How can one perform authentication in P2P network without a central trust server or Certificate Authority? ...
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### How to use HKDF to combine two keys

I have heard that to combine two keys HKDF is better than using XOR function. But if you look at the HKDF RFC I could not figure out how to use it for concatenating two keys. What I can see is HKDF ...
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### Pollard’s Rho Method

I can't get my head around Pollard’s Rho Method for solving discrate log problem I have read in a book: The basic idea is to pseudorandomly generate group elements of the form α^i · β^j ...
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### Outsourcing arbitrary computations securely

Consider the following scheme. Alice wants Bob to make some computations for her, but she doesn't want to reveal the data on which he's going to do it. So, she encrypts the data, sends them to Bob, he ...
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The Decision composite residuosity problem problem states that is impossible to distinguish between those two ensembles: $\{x^N \mod {N^2} | x \in \mathbb{Z^*_{N{^2}}}\}$ and $\{r \in \mathbb{Z^*_{N{^... 3answers 305 views ### Is the following authentication protocol insecure? I am a student taking cryptography this semester. I was just wondering why can't we use this simple protocol? Alice → Open Connection → Bob Alice ← Cert$_b$← Bob Alice → {K$_{AB}$}$_{Bob}$→ Bob ... 1answer 589 views ### RSA Encryption and Signature - Weak Padding Assume that I have an plaintext$m$and it is padded with$randompad||00||m$and then it is encrypted with RSA and a public encryption key so we get the encrypted$Sm$. Then to assure its integrity ... 1answer 313 views ### Unforgeability and type of adversary When trying to prove security of asymmetric signature, for instance for existential (or strong) unforgeability against chosen messages attack, do we need to consider the signer as a possible adversary?... 1answer 102 views ### Can I use the cipher text ouput of only one DES rotation as plaintext to check validity? I have made a python code for one DES rotation. Can I use the resulting cipher text as a plain text for the same rotation with the same key? I think the answer is no, I can't do so. But I need to be ... 2answers 701 views ### RSA Signature Weak Padding Attack Assume that we have a message$m$of size$n$, and it is padded with two 01 bytes in front. Then the signature$s$is computed using a private key$ks$. Can we ... 5answers 8k views ### What is the difference between CSPRNG and PRNG? What is the difference between CSPRNG and PRNG? Is there performance differential between them? For example: We use PRNG for key generation which is very expensive and CSPRNG for IV/nonce in block ... 1answer 58 views ### What vulnerabilities would open up if PGP authentication didn't have integrity checking? It is my understanding that PGP has both an integrity check and an authentication check. What vulnerabilities would open up if it didn't have that integrity check? It seems like the authentication ... 2answers 3k views ### Zero knowledge proof protocol example? Alice is color blind. She never knows if her gloves are matched. Her brother Bob always teases her saying her gloves are mismatched and she should go change them. Alice wants to know if Bob ... 1answer 115 views ### What is the best (thoroughly covering) textbook for application of LFSRs in cryptography? [closed] What is the best (thoroughly covering) textbook for application of LFSRs in cryptography? (Beside Cipher Systems by Beker & Piper) 4answers 3k views ### Is a Mersenne-twister cryptographically secure if I truncate the output? I want to create an online roulette game. Is it OK if the PRNG is seeded only once, when the user starts the game, or is it recommended to re-seed sometimes during game? Maybe just seeded once a day ... 1answer 1k views ### Why are the lower 3 bits of curve25519/ed25519 secret keys cleared during creation? I am currently experimenting with ed25519 and I noticed that on secret key creation, bit 254 is always set and the lower 3 bits are always cleared. I found that bit 254 is always set to protect ... 1answer 1k views ### Is this the mathematical equation for the Vigenere Cipher Is this the mathematical equation for the Vigenere Cipher or something else and if something else what is it? Where t is the letter of plain text and n is the position of t within the text and c is ... 1answer 63 views ### Type A Curves of Supersinglular In the Type A curve used in Pairing based Cryptography, what is meant by sign0 & sign1 in the expression below? ... 1answer 143 views ### Private Messaging Suppose Alice and Bob wants to send messages through Sue without revealing to Eve that the exchange took place. Slightly more formally, I don't want Eve to have any advantage telling if Alice is ... 1answer 4k views ### Multi-party encryption algorithm To give some foreground information: I acknowledge that I am a cryptography newb and not by any means an expert (and probably never will be). In a recent CS class we had several assignments writing ... 1answer 205 views ### Logical OR operation in a homomorphic additive cryptosystem Suppose we have a cryptosystem homomorphic for addition (say Paillier's). Is there a way to perform a logical OR operation between two binary values (with a binary result). We can, of course, obtain ... 2answers 225 views ### Diffie-Hellman insecure on addition modulo$n$Assume that the group$G$is the set$\mathbb{Z}_{n} = \{0,\ldots, n-1\}$for a 1024 bit integer and$+$is addition modulo$n$. Then why would Diffie-Hellman key exchange in this group be insecure? 1answer 115 views ### Which data to share in CTR mode? Suppose I've got two clients$A$and$B$. Client$A$encrypts data in CTR (Counter) mode to get a ciphertext. What information will client$B$have to receive from$A$to decrypt the ciphertext? Just ... 1answer 247 views ### Elliptic Curve Verifiable Secret Sharing I'm reading this paper, which on page 3(Section IV.C) presents a Jointly Random Verifiable Secret Sharing Scheme for Elliptic curves. The algorithm makes sense to me save for this part: "Each$P_{i\...
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 $a$ ...