# All Questions

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### Why is AES not a Feistel cipher?

I am studying for an exam right now. And I wanted to make sure I got this point correct. AES is not a Feistel cipher because the operations in AES are not invertible. Is the above statement ...
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### How to verify a number encrypted with an unknown key

Alice and Bob are going to follow the protocol below. Are there any crypto-constructions to help Bob verify the correctness of the answer he gets?: Alice encrypts a set of numbers using some ...
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### Double Encrypting with two different keys

In terms of security, would it be MORE or LESS secure to take, say, an RC4 output (or Serpent) or other, that is encrypted with one key, and to encrypt that output with AES (using a different key)? ...
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### Shamir Secret sharing - Can share generator keep x values secret?

I'm wondering, in Shamir secret sharing, can generator of the shares, keep the x values which are used in evaluating the polynomial to obtain y values (i.e., the shares) secret, and whenever the ...
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### Related-key attacks on AES

According to Wikipedia: Related-key attacks can break AES-192 and AES-256 with complexities $2^{176}$ and $2^{99.5}$, respectively. What are the requirements for these attacks (i.e how many ...
890 views

### Elliptic Curves of different forms

Looking at http://safecurves.cr.yp.to/ to find a safe curve, I find that most curves described here are of a different form from that generally used. In Bouncy Castle, for example, ...
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### Is it reasonable to assure that p-1 and q-1 aren't smooth?

I came across the requirement that, in RSA, $p-1$ and $q-1$ shouldn't be smooth, shouldn't consist of lots of small factors. Therefore my question: How complicated is it to check whether $p-1$ is ...
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### Relative merits of AES ECB and CBC modes for securing data at rest

I need to store several million Payment Card Numbers (PCNs) securely in a mainframe database (that is, 'at rest'). I assume that any attacker will have access to all of the stored data. I assume the ...
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### Why are elliptic curve variants of RSA “chiefly of academic interest”?

Yesterday I was thinking about elliptic curve variants of popular protocols/algorithms (ECDH, ECES[1], etc) and the thought occured that I had never seen an elliptic curve variant of RSA. My ...
869 views

### A fair peer-based coin-flipping protocol?

I found this question on the game programming site and was intrigued. I came up with an answer off the top of my head but I'm no cryptanalyst so it is probably not water-tight. This is how my idea ...
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### In RSA, rationale for prime $p$ with $p-1$ having prime factor $u$ with $u-1$ having large prime factor?

In the 1978 RSA paper, it is recommended, among other things, to choose primes $p$ such that $(p-1)$ has a large prime factor $u$. This was motivated by Pollard's p-1 algorithm. Further, the authors ...
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### Is multiplicative secret sharing secure?

I suggested mulitiplicative secret sharing in an answer to another question, but noted that I wasn't sure if it was even secure and was hoping someone would comment on the security. Since no one did, ...
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### Why can't the IV be predictable when its said it doesn't need to be a secret?

I heard multiple times not to reuse the same IV and IV should be random but doesn't need to be secret. I also heard if the IV is something like sequential numbers or something predictable I should ...
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### Luby-Rackoff theorem confusion

The Luby-Rackoff theorem states that if a round function is a secure pseudorandom function (PRF) then 3 rounds are sufficient to make the block cipher a pseudorandom permutation (PRP). PRPs are ...
1k views

### AES timing attacks

I'm just interested in cryptography, so please don't expect me to be an expert. ;) I recently read about AES cache timing attacks and found it very interesting. I read the article Cache-timing attacks ...
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### Hill Cipher known plaintext attack

I know a plaintext - ciphertext couple of length 6 for a hill cipher where its key is a [3x3] matrix. Based on what I've read and learned, to attack and crack keys of [n x n], if we know a plaintext ...
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### Homomorphic cryptosystems in RSA

Hopefully Crypto can help me understand homomorphic cryptosystems. I'm designing a high score server for a game I made, and because of facets in the language i'm using, the player would be able to ...
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### Can we use elliptic curve cryptography in wireless sensors?

Can we use elliptic curve cryptography in wireless sensors? If so, how do you map points to message characters?
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### Why can ECC key sizes be smaller than RSA keys for similar security?

I understand how ECC is based on the discrete log problem and RSA on integer factorization. I've read several references that show how a solution to either of these problems can typically be adapted ...
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### How much data can I encrypt with AES before I need to change the key in CBC mode?

In my cryptography class, the instructor suggested that in order to give the attacker a minimal advantage of $1/2^{32}$, we have to change the key after $2^{48}$ blocks are encrypted. It seems that ...
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### Why is RSA usually limited to messages up to 1 block

I'm wondering why RSA encryption usually is only used for messages that fit into one block. For larger messages hybrid encryption in combination with symmetric ciphers like AES seem to be the solution ...
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### AES Key Length vs Block Length

This answer points out that certain key and block lengths were a requirement for the AES submissions: The candidate algorithm shall be capable of supporting key-block combinations with sizes of ...
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I've found a way to complete a task which I'd solve with passwords or by sending keys over the wire (otherwise) by using RSA's homomorphic property. I'm restricted to RSA (any padding; for hardware ...
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### Building a combined encryption scheme from two encryption schemes that's secure if at least on of them is secure

Any thoughts on how this can be done? Let $\Pi_1 = (\mathrm{Gen}_1, \mathrm{Enc}_1, \mathrm{Dec}_1)$ and $\Pi_2 = (\mathrm{Gen}_2, \mathrm{Enc}_2, \mathrm{Dec}_2)$ be two encryption schemes for ...
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### How do I encrypt with the private key? [duplicate]

Possible Duplicate: RSA encryption with private key and decryption with a public key This wording is creeping everywhere (e.g. there): "I encrypt with the private key" and even sometimes, ...
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### Hash Based Encryption (fast & simple), how well would this compare to AES? [duplicate]

First of all, I know it's a very bad idea to invent your own encryption algorithm. It's better to use existing known, trusted, extensively tested and studied algorithms with a proven track record. The ...
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### Would this method deliver a perfectly non-malleable encryption for at least two blocks? [closed]

Disclaimer: the algorithms I present here (and in other messages) are used as a hands-on way to learn about developing crypto algorithms, (and might also be of interest for other curious people), not ...
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### Would this method allow fast authenticated encryption using only a single encryption and RNG operation per block? [closed]

The most promising of the 4 approaches described here (version 4) has been posted as a separate question. This post should be considered for archiving and historical purposes only. Disclaimer: the ...
16k views

### Basic explanation of Elliptic Curve Cryptography?

I have been studying Elliptic Curve Cryptography as part of a course based on the book Cryptography and Network Security. The text for provides an excellent theoretical definition of the algorithm but ...
1k views

### Is this password migration strategy secure?

I want to upgrade the security of some existing databases of users' authentication tokens strictly for the purpose of making sure that if the database is stolen, attackers will not be able to guess ...
15k views

### What's the fundamental difference between Diffie-Hellman and RSA?

What is the difference in the purpose of DH and RSA? Aren't they both public-key encryption?
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### What is the ideal cipher model?

What is the ideal cipher model? What assumptions does it make about a block cipher? How does it relate to assuming that my block cipher is a pseudo-random permutation (PRP)? When is the ideal ...
8k views

### How can one securely generate an asymmetric key pair from a short passphrase?

Background info: I am planning on making a filehost with which one can encrypt and upload files. To protect the data against any form of hacking, I'd like not to know the encryption key ($K$) used for ...
6k views

### What is entropy?

We discuss a lot of topics and use measures of entropy to determine how difficult it is for an attacker to be successful. What does entropy mean in the context of cryptography? How is entropy ...
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### What is the lowest level of mathematics required in order to understand how encryption algorithms work?

What mathematical fields of knowledge would be required in order to get a good understanding of encryption algorithms? Is it basic algebra, or is there a "higher education" mathematical field ...
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### How were the DES S-box values determined?

It seems like the S-boxes in DES have essentially random values. How were these chosen?
17k views

### How does RSA signature verification work?

I understand how the RSA algorithm works for encryption and decryption purposes but I don't get how signing is done. Here's what I (think) I know and is common practice: If I have a message that I ...
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### repeating-key xor and hamming distance

I read that to break repeating-key xor you can do the following: try a keysize $n$ and compute the hamming distance between the first $n$ bits of the encrypted string and the bits $n+1$ to $2n$ of the ...
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### Getting started [closed]

I'm looking for a good place to start in cryptography and places to go to get free books etc on the topic. I have been looking online but I always get stuck at some point or another. I need something ...
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### What exactly is a negligible (and non-negligible) function?

The mathematical definition of neglible and non-neglible functions is fairly clear-cut, but why they are important and how they are used in cryptography?
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### What is the best way to put a backdoor in an encryption system?

How can you put a backdoor into an encryption algorithm? Are there any techniques that can be used to reduce the time it takes to break a key? I am looking for practical examples encryption schemes ...
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### Difference between stream cipher and block cipher

A typical stream cipher encrypts plaintext one byte at a time, although a stream cipher may be designed to operate on one bit at a time or on units larger than a byte at a time. A block cipher ...
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### Simulation-based security?

I've been reading Introduction to Modern Cryptography by Katz and Lindell as an introduction to cryptography. The book seems to use the term 'simulator' when it talks about a game like, for instance, ...
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### How do I apply differential cryptanalysis to a block cipher?

I've read a lot of summaries of block ciphers particularly with regards to the NIST competitions stating that reduced-round block ciphers are, for example, vulnerable to differential cryptanalysis. I ...
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### Using the same secret key for encryption and authentication in a Encrypt-then-MAC scheme

Is it a weakness to use a single shared secret for protecting messages using a Encrypt-then-MAC scheme? Assuming a system is using AES-256-CBC and a SHA1-HMAC and the same secret key for both ...
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### Is it safer to generate your own Diffie-Hellman primes or to use those defined in RFC 3526?

I was wondering if the prime numbers defined for use with Diffie-Hellman in RFC 3526 are more trustworthy than generating one's own, especially considering the recent Arjen Lenstra paper (Ron was ...
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### What are the differences between proofs based on simulation and proofs based on games?

what are the main pros and cons of proving the "security" of a crypto scheme under simulation proofs instead of game based proofs?
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### Is there a simple hash function that one can compute without a computer?

I am looking for a hash function that is computable by hand (in reasonable time). The function should be at least a little bit secure: There should be no trivial way to find a collision (by hand). For ...