# All Questions

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### Is there an effective way for public key generation in homomorphic encryption schemes?

Is there any way to effectively generate valid distributed public key in schemes like ElGamal or Benaloh for a number of participants? There's no need in private keys since there's no intent to ...
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### What does pseudo pre-image of a hash function mean?

So finding a pre-image means given the hash value, one tries to find a message that gives this value using the function's standard IV. What about pseudo-preimage?
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### Is there a general method to crack this type of fractionating cipher?

I've seen a few puzzles based on a type of cipher in which letters in the plaintext are substituted with groups of characters in the ciphertext. The ciphertext only uses a handful of unique ...
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### What is the GnuPG process for going from a passphrase to a symmetric key?

Suppose I use GnuPG to symmetrically encrypt a file like so: gpg --no-options -c --cipher-algo AES256 --no-random-seed-file -o my.out my.file I then provide a ...
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### Block Cipher Mode Amicable to Fast Key Change/Rotation Like XOR?

I'm working with documents encrypted in long term storage. When a client requests the document, the document should be delivered encrypted under a distinct key (distinct from the long term storage ...
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### Elliptic Curve Encryption Ciphertext Size

I'd like to know how much bigger is the ciphertext when encrypting a message using ECC encrytpion? ECIES (or ElGamal)
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### Weaker Notion of Target Collision Resistance

I'm reading the paper “Collision-Resistant Hashing? Towards Making UOWHFs Practical” which states: While it might be easy to find a collision $M,M'$ in $F_K$ by making both $M,M'$ depend on $K$ ...
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### Is there a name for this 'enhanced' caeser shift and if so, is it trivial to break?

This cipher shifts the letter that it will shift to, after each shift... I used to play with this when I was a kid. I was thinking about it recently and realized that it wasn't as simple as I ...
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### What is the limit of plaintext required to break the Vigenère encryption?

A theoretical question about the Vigenère cipher: Without any knowledge about the key (not even it's length) can we tell how much known or chosen plaintext is needed for the adversary to completely ...
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### Good Encryption Exponent

I have placed a bet that I can create a public key such that my adversary will not be able to crack (decrypt) it for at least one week. For my primes $p$ and $q$, I chose very large numbers that are ...
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### How to design a cryptographically secure file hosting service

How one would go about design something like Google Drive/Dropbox providing the following features: files are encrypted and decrypted on the client files can be synced between devices the service ...
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Alice has two secret numbers, a and b. She publishes c1=E(a), c2=E(b) and c3=E(a+b). Is there an encryption system E such that anyone would be able to prove that the c3 as published by Alice is ...
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### ID-based key exchange protocol with PFS

The traditional authenticated DH protocol with PFS against active adversaries, ensures that the compromise of both party's long-term private keys after the session is completed will not help the ...
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### Prime number theorem - RSA

I am having trouble understanding the prime number theorem. As part of some revision for an exam, I am trying to answer the following questions (but seeing as I don't understand the concept of the ...
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### Since it gives negative power, how is rcon(0) calculated in AES?

I'm trying to calculate Rijndael key schedule "Rcon" values manually. I know how to calculate each value with the equation ...
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### Random session key + predictable IV

I'm using Blowfish in a toy Diffie-Hellman communications scheme. Random session keys are generated for each connection. In this case I can simply feed a null array to the IV right? The same ...
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### Is OTP useful in modern electronic communication?

I have seen from time to time questions about various OTP schemes and "OTP" (i.e. stream cipher) schemes. The most common OTP troubles appear to be: How to extend size of previously generated key or ...
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### Bandwidth and block size for Paillier cryptosystem

Can someone clarify what is meant by the terms cryptosystem bandwidth and block size for public key cryptosystems; The context is the Paillier cryptosystem and its Damgard-Jurik generalisation. My ...
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### Is it safe to use GZIP to avoid padding related attacks

I am designing a library that supports encrypting some in-memory data using AES in CBC mode. Now I need a padding but it seemed to me that Apple's CommonCrypto library and OpenSSL have different ...
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### Inverting the encryption equation of a symmetric cipher

I've just started a cryptography course. I am beginning to understand the concepts (I'm only in week 2) but I just can't get my head around the theories and principles when written as equations. I've ...
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### Why does openssl BN_generate_prime return only a fraction of the safe primes within the given bit range?

I went into prime generation issues the last week and eventually calculated 10^7 primes using erik tews safeprimegen python wrapper for openssl. (https://github.com/eriktews/gensafeprime) (System: ...
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### Should I use distinct MACs for each user, or should I use a *direction* flag when communicating?

Alice and Bob want to communicate using a stream cipher. At the beggining, they create a session key $K_s$ and exchange it via some secure channel (using some asymmetric algorithm). After every ...
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### What key exchange do OpenSSL and CryptoAPI prefer by default?

Here is what I know. When I connect to a brand-new Linux Web server with Chrome and click the "lock" icon, the key exchange algorithm is described as "ECDHE_RSA". (Yes I know what that is.) When I ...
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### What is it meant by a “hybrid argument”?

Can anyone explain (or point to a reference) what is a hybrid argument in a security proof, and when it's convenient or preferable to use it? Among some of the places where I've seen it mentioned, ...
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### Storing Credentials for a Third Party

(Originally posted to SOF, but I got no answers) I am designing a small application for use inside my company. My application needs to connect to a third party website on behalf of my application ...
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### random number generator and seed

I'm working with openssl cryptographic libraries, I'm new to all these cryptographic stuffs and slowly I'm learning all these. I have a doubt regarding random number generator, I'm using ...
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### Generating an IV for ESP 3DES-CBC

I have an application written in C, running in Linux. It uses IPsec (ESP) (manually putting the IPSec packet together) to send certain packets. The algorithm used ...
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### Do companies like CipherCloud really have the option to use homomorphic encryption?

I was reading How is CipherCloud doing homomorphic encryption? and was wondering: Is there a technically feasible way for companies like CipherCloud to use homomorphic encryption (HE) while ...
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### How does DES decryption work? Is it the same as encryption or the reverse? [duplicate]

If DES decryption is the same as encryption done in reverse order, then how can the reversed S-Box convert 4 bits into 6 bits?
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### Converting Ed25519 public key to a Curve25519 public key

I understand that: $$x_{montgomery} = \frac{1 + y_{edwards}}{1 - y_{edwards}}$$ Using the libsodium ed25519 implementation, I have tried to write the following: ...
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### a possibly stronger type of attack on identity-based encryption

What is known about the security of identity-based encryption schemes against attacks that involve seeing multiple ciphertexts and then receiving the private keys corresponding to some of those ...
180 views

### Digital Signature size requirement

What is an approximate length for a digital signature (SHA) given the following? Using RFID tags, size ranging from 100 to 2000 bytes (8000 bits), our project is setting aside a portion for a digital ...
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### Primality testing (deterministic vs. non-deterministic)

I noticed that non-deterministic primality testing algorithms are more commonly used in practice while there is a deterministic algorithm e.g., AKS which runs in polynomial time? I am right? if so, ...
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### The probability of finding a limited set of collisions for HMACSHA256

I need some input on the use of HMAC-SHA256 in an open source project, any help appreciated. The problem I'm solving is session fixation. We have a username, for that username we'd like to generate a ...
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### plaintext and its encrypted version known [duplicate]

I have a (long) text in two versions, encrypted and plaintext. I think it was encrypted using a substitution cipher method (I'm pretty sure, indeed). I'm not good in this matter, I know little about ...
193 views

### Is OpenPGP vulnerable to the “crypto doom principle”?

From reading RFC 2015 (MIME security with PGP), and from looking at the raw format of (GnuPG) encrypted and digitally signed email messages, it seems that first a signature is calculated from the ...
162 views

### Quantum resistance of Lamport signatures

The Lamport-Diffie signature scheme is said to be quantum-resistant. Why is that? What would a quantum attempt to attack this signature scheme look like, and how does it fail?
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### Is Triple-DES a group?

I know for a fact that DES is not a group, but are any of the Triple-Des versions a group? Why, or why not?
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### How does Poly1305 in the NaCl library calculate a MAC?

I've been reading about Poly1305 as implemented in the NaCl library, more specifically I've been referring to this Java implementation of NaCl. My understanding of XSalsa20Poly1305 (in this ...
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### length extension attack on an MD5 hash of a text file of about 10K Bytes?

How much computing time on a typical desktop computer would it take to find a new length and new data to extend a text file of about 10K Bytes with a given MD5 hash?
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### Encrypt user email but be able to find user by email

Sorry for my dumb question, but it's better to ask dumb question than to do dumb things silently. I want to encrypt user email in my DB so that if someone stole the DB (and not the key) - he won't be ...
134 views

### Does SRP reduce to DH key echange when shared password is not secret?

I can find JavaScript implementations of SRP (Secure Remote Password protocol), but nothing that inspires confidence for Diffie-Hellmen key exchange. I also have a separate need for SRP later. I ...
529 views

### Winzip AES256 vs PGP

If I use the AES256 option in Winzip to encrypt a file, is it any less safe or less secure than using pgp encryption?
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### Attacking AES/CCM with random IV

A cryptosystem is using AES-128 in CCM mode with random IV. Suppose an attacker capable of: asking the cryptosystem to encrypt a single plaintext as many times as he wants; and asking the ...
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### Attacking historical ciphers methodology

It's more a theoretical question of how would you approach it. All you know about the ciphertext it's was generated with a historical cipher. The ciphertext appears to be random, BUT it's divided into ...
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### How is ElGamal not secure under chosen ciphertext attack, but semantically secure in some cases?

I know that you can create a ciphertext c' using c and then find the corresponding m' for c' which you can use to find m for c. So, doesn't this mean that it is not semantically secure? But I also ...
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### How can mega store my login details and still be secure?

I understand how Mega's encryption works. For a quick summary of all those in the future looking for an answer on this... here is how it works: Upon first signing up for an account you make a ...
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### Is this method for exchanging private key using RSA sound?

Bob know's Alice's public key, and he wants to make sure he's connecting to the one which has that key. Furthermore, Alice wants to verify when she gets a connection from Bob who'll give his public ...