A feistel-network or feistel cipher is a structure that has been used to create a number of different block ciphers (e.g., DES, Blowfish, Twofish, RC6).

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Does unbalancing a feistel cipher always improve security? Does it improve security at all?

So according to Wikipedia unbalanced feistel ciphers provide greater provable security. Specifically, they state: The Thorp shuffle is an extreme case of an unbalanced Feistel cipher in which one ...
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Why was the winner of the AES competition not a Feistel cipher?

The winner of the AES competition has a structure that does not qualify as a Feistel cipher, as explained in answers to this recent question. However, most many of the AES candidates, and all 3 out ...
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Are there any specific requirements for the function $F$ in a Feistel cipher?

In a Feistel cipher (as shown below), it seems that the security of the cipher would be entirely dependent upon the function $F$. What requirements on $F$ are necessary for a secure Feistel cipher? Is ...
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Avalanche effect in DES

I couldn't understand the avalanche effect in DES. Could someone explain how avalanche effect happens in DES
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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 correct?...
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What is an active S-Box?

I don't understand what an active s-box is (in reference of design criteria 8 of the DES). Because according to DES, all 8 s-boxes are used every round. But this is not what active means. So my ...
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Low complexity implementation of a small blocksize cipher (< 64 bit)

Searching for "small blocksize cipher" finds a number of discussions on the topic, mostly refering to FPE. This one in particular suggests using AES as the round function of a Feistel network. The ...
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Big block cipher as memory-hard function

I'm wondering, if something like block cipher with big block size is a good memory-hard function? All memory-hard key derivation functions I've seen look more complex than that, which made me ...
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Feistel-Network - why wire crossing?

In Feistel-Network, there would be a wire-crossing at the end (for the next round). Which insufficiency would occur, if the wire crossing would be omitted?
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Homogenous and heterogeneous Unbalanced Feistel Networks

Unbalanced Feistel networks can be homogenous (F-function identical in each round), or they can be heterogeneous (F-function not always identical in each round). The advantage of heterogeneous UFNs ...
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Is this a structural weakness of Feistel networks?

I'm doing a lot of reading about Feistel networks. Something occurred to me a bit ago that I hadn't realized previously, namely that in any Feistel construction there are bits of the plaintext that ...
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Need for 3 rounds in Luby Rackoff theorem

I get that using Feistel network we can create a secure PRP from a secure PRF (in 3 rounds according to theorem). My question is why not just 2 rounds of Feistel network is sufficient. Is it that ...
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Using a hash (like SHA-256) vs AES as the source for pseudo-random values in Feistel network?

This question is in relation to Wikipedia article on Format Preserving Encryption It says the following It is also possible to make a FPE algorithm using a Feistel network. A Feistel network ...
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Can we build authenticated encryption using Feistel Networks?

Most of the encryption modes of Feistel Networks especially the ones used to build fixed length block ciphers just provide confidentiality . Can we build authenticated encryption using Feistel ...
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Differential cryptanalysis - breaking the last round of FEAL4?

I've been trying to learn cryptanalysis. I've come across this resource which proved very helpful: http://theamazingking.com/crypto-feal.php So far I've been almost successful in breaking FEAL4 ...
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Can you decrypt with 1 round key?

If one has the last round key in an SP or Feistel network (the 16th round key in DES for example), would that someone be able to decrypt the rest of the encryption?
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Pseudorandom permutations on domains of arbitrary (i.e. not a power of 2) size

The Feistel cipher algorithm produces a pseudorandom permutation with domain size equal to $2^{2m}$ (for some $m \in \mathbb{Z}^+$). That is, a one-to-one function $\pi$ of the set $\{0, 1, 2, \dots, ...
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Understanding a Blowfish cryptanalysis

I'm reading a cryptanalysis on Blowfish, and I've come across something that I don't quite get. Let's denote $$\delta = a \oplus a'$$ where a and a' are bytes that cause a collision in some S-box ...
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What level of security is provided when a Feistel Cipher is used as a round function of another Feistel Cipher?

Recently, I was reading: Are there any specific requirements for the function F in a Feistel cipher?, and the answer posted mentions a Feistel Cipher named Turtle, which uses a four-round Feistal ...
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Could the Enigma algorithm be classified as a Feistel network?

The Enigma algorithm is a encryption method that was developed (I believe) by the Germans in WWII. It went a little something like this: When a letter was typed on the keyboard of the Enigma ...
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How does Feistel cipher create diffusion between blocks?

From what I've read about Feistel ciphers, I can understand that within each block, there is both confusion and diffusion. But it is unclear to me how a Feistel cipher permutates among blocks of data. ...
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Can the round function $F$ in a Feistel Network be practically any non-invertible function?

This question might seem silly, but how true is the fact that the round function $F$ does not have to be invertible? I am curious to know this, because non-invertible functions can be very lossy, i.e....
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Block size vs key size confusion

I understand that if a block cipher has $k$-bit keys and $n$-bit input/output blocks, then if $k>n$, we can expect one message-ciphertext pair to narrow us down (I think?) to $2^{k-n}$ possible ...
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Understanding Meet-in-the-Middle attack on block ciphers

I understand how to implement a Meet-in-the-Middle attack on 2DES/3DES, by computing for one half all possible keys and storing the outcome and then subsequently do the same for the other half but ...
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Would using a symmetric list of subkeys in a Feistel cipher make our messages vulnerable to oracle attacks?

Suppose I have a Feistel cipher structure that has $16$ rounds. I come up with $8$ subkeys $k_1,k_2,...,k_8$ to be used for the first $8$ rounds and then assign the rest of the keys so that $k_9=k_8,...
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How is this affine function a pair wise independent permutation?

Can anybody explain how the below is true ? $I_n$ denotes the set of all n-bit strings, ${\{0,1\}}^n$. (..) Let $A$ be a finite field then the permutation $f_{(a,b)}(x) = a \cdot x + b$, ...
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Increase number of rounds for SPN and Feistel ciphers

Read a post on Schneiers blog (and again 2011) about increasing the number of rounds for AES from to "AES-128 at 16 rounds, AES-192 at 20 rounds, and AES-256 at 28 rounds" to raise the security. ...
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What is the diffrence between Feistel networks and SPN?

I recently read about the concept of Feistel Networks and Substitution Permutation Networks but what is exactly the difference between the two ?
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How are subkeys, in Feistel block ciphers, derived for each round?

I'm bit confused about the subkeys that are used in Feistel ciphers. I know that all the subkeys ki are derived from the main key K, but how? Say I have a simplified Feistel block cipher of i rounds, ...
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Using a round function intended for an SP network in a Feistel network?

If I use the round function from a secure SP network (such as AES) and use it in a Feistel netwok, is this a good starting point for the second cipher? My thought is "yes" because: it already has ...
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reconstructing data on a noisy channel?

I have an interesting situation where I have a block cipher and a noisy channel. The interesting thing is that I happen to know much about the data. The cipher is a balanced Feistel network, and for ...
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Why is a Feistel network bijective?

EDIT: Fixed for clarity of intention I was reading "The FFX Mode of Operation for Format-Preserving Encryption" when I came across Figure 1 (see below). It's obvious why it's reversible (top-down ...
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Using AES for smaller blocks in a Feistel network

I was wondering how decryption would work for FPE in the case of cycled Feistel ciphers. I understand that simply reiterating through the networks with reversed key order will generate the ciphertext ...
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Can we parallelize the Feistel Networks?

Classic Feistel Networks and various types of FN's are sequential in nature. [1]has a good overview of different types. But is there a way we can do Feistel Networks in parallel and achieve good ...
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Doubling the block length of a given block cipher

IMHO the main essence of a Feistel cipher of block length $n$ is the working of a non-linear function $F$ that is applied to the two halves of the block of length $n/2$. Now suppose a certain block ...
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Why is a Feistel network not a type of substitution-permutation network?

I understand what substitution-permutation networks look like. I understand what Feistel networks and Feistel-like networks look like. A Feistel network or a Feistel-like network features substitution ...
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KPA on Feistel cipher?

I heard that DES is technically “broken” because of attacks involving large amounts of known plaintext. These attacks are obviously academic and highly complicated, so for some intuition I was hoping ...
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Exercise: Attack on a Two-Round DES Cipher

Working through the exercises in Cryptography Engineering (Schneier, Ferguson & Kohno) I have stalled on the following exercise: Consider a new block cipher, DES2, that consists only of two ...
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Is there a relation between security of rounds of DES and Luby Rackoff rounds?

DES uses 16 rounds of feistel networks but not considered secure . Where Luby Rackoff constructions which are inspired by DES its proven by Patarin that 7 rounds is secure . Is there a relation ...
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Luby-Rackoff on Feistel ciphers [closed]

On page 46 of these lecture notes, it seems to say that if we have a Feistel cipher, and plaintexts $(L_0, R_0)$ and $(L_0^*, R_0^*)$ with corresponding encryptions, then we can determine the key? But ...
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2 round GOST_28147-89 cipher distinguisher

So, here is the scheme of how this will look graphically: The deal is to present distinguisher for this cipher. First of all here is my drafts: Way 1 Assume that distinguisher displays in ...
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Validate per round rather than cycle-walking in FPE

I was thinking about FPE specifically for numbers, CC, phone, SS #, etc. and I thought of some small "optimization" that could be implemented. Can I validate the string length after each round in the ...
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FFX Encryption over alphabet of arbitrary radix

I understand how standard numeric FPE using a halved string in cycled Feistel networks is constructed and operates. I then read the paper regarding FFX encryption which should allow for any non-binary ...
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Algebraic Normal Form Piccolo [closed]

I'm trying to verify the claim of paper Improved Algebraic Fault Analysis: A Case Study on Piccolo and with Applications to other Lightweight Block Ciphers about the numbers of equations in algebraic ...
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Why does the DES crypto algorithm NOT use 2 rounds?

Now, if we were to go round by round, you could give a distinct reason for not using a single round since after just one round, the right half of the text comes directly, as-is, to form the left half ...
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Is the DES F-function injective for a given subkey?

Is it possible for two different half-block inputs to the DES F-function, with the same round subkey, to produce the same output? That is, if we feed two different half-block inputs $b_1$ and $b_2$ ...
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What is the difference between various modes in FPE?

Format preserving encryption has three modes FE1( FFX) , FE2(VAES) , FE3(BPS) . All of them are based on Feistel Networks .Can somebody explain how they differ from each other ?
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Simple two round feistel chiffre

I am currently learning for a exam in cryptgraphy. I now try to understand the Feistel Cipher with two rounds. I do not have any information on what the functions are actually doing. The following ...
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May the problem with DES using OFB mode be generalized for all feistel ciphers

There is a problem with using DES as the block cipher in OFB mode, eg: the feedback that goes back into the next round will be encrypted with the same key $k$ resulting back into the plaintext IV used ...
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Output of a Feistel network with all “0” round function

What is the output of an r-round Feistel network when the input is $(L_0, R_0)$ and each round function outputs all 0s, regardless of the input?