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Questions tagged [block-cipher]

A block cipher is an encryption algorithm which encrypts fixed-size blocks of plaintext to same-sized blocks of ciphertext. For good ciphers every bit of the ciphertext block depends on every bit of the plaintext block and every bit of the key.

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IDEA (International Data Encryption Algorithm)

I don't know how to perform Multiplication Modulo and Addition Modulo in IDEA (International Data Encryption Algorithm).
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What Feistel network block ciphers do not use data-dependent table lookups or modular additions?

I am looking for block cipher algorithms based on Feistel networks. The algorithm is required to not use data-dependent table lookups (or data-dependent rotations) or modular additions/multiplications....
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What characteristics define a function to be a block cipher

I'm trying to understand what characteristics or properties make the result of a function a block cipher. I understand that for a function to be a block cipher it has to be invertible and can't be a ...
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DES twice - with k and then with complementary key?

What if we were to use DES in the following way: for a given text x and key k we would perform ...
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1answer
41 views

Encrypt twice with DES using same key ? $\operatorname{DES}(\operatorname{DES}(x,k), k)$

Does encrypting with $\operatorname{DES}$ in the following way $$\operatorname{DES}(\operatorname{DES}(x,k), k)$$ make $\operatorname{DES}$ as strong as $\operatorname{2DES}$ with 2 keys? Because we ...
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DES - encrypt 3 times - DES(DES(DES, k), k'), k) - stronger than 2DES with 2 different keys?

What if were to use DES in the following way: For a given plaintext x and key k we will perform: ...
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1answer
38 views

Padding schemes reversal and space efficiency

My crypto course proposes the following padding schemes: $\operatorname{pad}(m) = m \mathbin\| 0^i$ $\operatorname{pad}(m) = m \mathbin\| 10^i$ $\operatorname{pad}(m) = m \mathbin\| 0^i \mathbin\| E(\...
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AES and DES - reusing the same round keys

Both DES and AES use key expansion to generate the round keys (for Feistel network or for the AddRoundKey operation). Each round uses a different key. Why do we need that? Why we cannot use the same ...
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196 views

Encrypting text using Caeser cipher in CFB mode

I am interested in learning and understanding cryptography and its application in every day life. One thing I can't seem to get my head around is encrypting plaintext using a block mode such as CFB. I ...
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52 views

Known Plaintext Attack for Modified OFB Mode of Operation

Question: Assume a variant of the OFB mode by which we only feedback the 8 most significant bits of the cipher output. We use AES and fill the remaining 120 input bits to the cipher with 0's: Why is ...
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Known Plaintext Attack on Block Cipher in ECB and CBC Modes of Operations

These 2 similar questions are from Paar's Understanding Cryptography. I could not see the answer so if someone can help I will appreciate: Question1: We consider known plaintext attacks on block ...
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Differences between Webcrypto (js) and Cryptodome.Cipher.AES (python) [closed]

I'm trying to understand the difference in output of AES decrypt (GCM mode) using Webcrypto and Cryptodome.Cipher.AES in python. ...
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AES - MixColumn transformation using the identity matrix

I'm analysing how different components of AES influence its security. However, I cannot find a good explanation to what happens if the MixColumn step would use the identity matrix. Intuition tells me, ...
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1answer
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Combining known attacks and quantum cryptanalysis

I'm regularly reading that AES-256 is secure against quantum computers because Grover's algorithm will only halve the key strength (i.e. "only" require $\sqrt{2^l} = 2^{l/2}$ operations instead of the ...
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Mixing block cipher modes (AES)

I'm trying to evaluate some changes to an existing codebase and one question that has come up is the block cipher mode. For legacy reasons we've been using AES in CBC mode, however it's unclear why ...
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Double encryption : what method is the best?

Given the block-ciphers defined as follows : $1.$ $C=$ $E_{k1}($$E_{k2}$$(M))$ $2.$ $C=$ $E_{k1{\oplus}k2}$$(M)$ $3.$ $C=$ $(E_{k1}(r),E_{k2}(r$ ${\oplus}$ $M))$ where $C$ is the cipher ...
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How is a PRP different from PRF? Both can be inverted to get the same input

PRP is said to be a bijective function which means that there is a one-to-one mapping with the output. And hence the output can be inverted using the decryption algorithm to get the same input. ...
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1answer
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Why 3 round Feistel cipher are not common

Luby and Rackoff showed that 3 rounds are sufficient to make the Feistel networks a pseudorandom permutation. Question: why are 3 round Feistel Ciphers not common? My thoughts about the round ...
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How many rounds of encryption is ideal?

I am a newbie to the cryptography world. I am working on a custom algorithm that encrypts the data multiple times and results in a cipher text. It uses a different key in each round to encrypt it. I ...
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1answer
92 views

Why is stateful CTR slightly better than stateless CTR? Does the adversary know the value of counter in CTR mode of operation?

This might be a stupid question but is really fundamental in my understanding of CTR (I'm a newbie to cryptography theory). For CPA security, the adversary only knows the IV, and may query the oracle ...
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1answer
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What happens if the plaintext $m1$ from the first block of CBC is known to adversary?

The adversary is has unlimited access to the encryption oracle. My thinking is that upon receiving the ciphertext C = $<c_0,c_1,c_2...,c_n>$ it will just use $c_0$ to XOR it with $F_k(m_1)$ to ...
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Which block cipher mode to use?

I am working on securing a chat application, and I am not sure about which block cipher mode I should use. I am aware that I should avoid ECB as it's insecure, but what about CBC? I have watched ...
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1answer
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Does it matter how the IV and XOR are applied for a single AES-128 block?

I want to encrypt 128 bits of data. I understand the problem with ECB, so I generate a 128-bit Initialization Vector to use for encryption and send with the encrypted data. Is there a "best" way to ...
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3DES Feasibility and Security [closed]

Questions are as following: Is 3DES still feasible (in terms of security and performance) to use? Is 3DES safe to use in ECB mode? Every time on giving the same input (plain text) to encrypt via 3DES ...
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Tiny Encryption Algorithm (TEA) weakness

The Wikipedia states that different multiples of a magic constant are used to prevent simple attacks based on the symmetry of the rounds. The magic constant, ...
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1answer
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What happens at this part in the key schedule of a Speck…?

I am trying to figure out the process behind the Speck block cipher. I understand how XOR works (Exclusive-or) when you take 2 strings of bits and you want to XOR them together. However, in the key ...
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how to compute the correlation between two binary vectors?

We find different equations to compute the correlation or correlation coefficient: $\begin{align*} corr &= \frac{a+d}{a+b+c+d}\\ &= \frac{(a+d) - (b+d)}{a+b+c+d}\\ &= \frac{(a+...
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Intuition behind the asymmetry between invertibility of key and ciphertext in block ciphers

I have studied the design of block ciphers like DES and AES. I know the actual algorithms, but I don't intuitively "grok" the most important property of them: The most important property, is that ...
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1answer
132 views

Why do block ciphers use key schedules instead of round constants? (Even-Mansour)

Let's take AES as an example. What would be wrong with just having a 256 bit key that you XOR into your input and then XOR into your output? No key expansion at all. I believe it's even known as the ...
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how to compute Bit Independent Criteria on a 4-bit S-box?

The idea of Bit Independence Criterion (BIC ) was introduced by Webster and Tavares . For a given set of avalanche vectors generated by complementing a single plaintext bit, all avalanche variables ...
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2answers
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Cannot understand Block Cipher example

I learned modern block cipher and saw this example. How many padding bits must be added to a message of 100 characters if 8-bit ASCII is used for encoding and the block cipher accepts blocks of ...
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If $E_{k_1}(p_1) = c_1$, what is the probability that there exists key $k_2$ such that $E_{k_2}(p_1) = c_1$?

Let $k_1$ be a 56 bit DES key that performs $E_{k_1}(p_1) = c_1$. What is the probability that there exists another key $k_2$ such that $E_{k_2}(p_1) = c_1$?
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Selection of rotation constants in ARX design

My question is about choosing the rotation values in ARX design such as SIMON-like or SPECK-like ciphers to provide optimal differential and linear immunity. According to this, the selection of $a$ ...
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1answer
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Minimum number of active s box to be resistance to differential cryptanalysis

I am calculating the minimum number of active s_boxes for block cipher using mixed integer programming, when can we say that block cipher is resistance to differential cryptanalysis after which round? ...
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3answers
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Padding Oracle: Why should padding be 0x01?

I just finished reading about padding oracle attacks at https://blog.skullsecurity.org/2013/a-padding-oracle-example, and it blew my mind. I think I understand almost all of it, but I do not ...
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1answer
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How secure is a 128 bit ideal substitution cipher

A 128 bit ideal substitution block cipher substitutes 128 bit blocks with randomly assigned 128 bit blocks using a key table. How secure is this type of algorithm? Is this practical? Thanks!
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Latin dance family in block cipher

Is it secure to use Salsa20/ChaCha as the key schedule for a 512-bit block cipher using a public permutation in Even-Mansour construction?
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Is it possible to successfully encrypt/decrypt in DUKPT using different sized BDKs?

We are having our P2PE Solution validated against the PCI P2PE standard. We currently use a TDEA 2-key based BDK, and IPEK in both the encryption and decryption environments. ANSI X9.24-1 (2009) ...
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1answer
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What is the probability of un-authenticated messages to go through CCM because of channel errors?

Consider having the worst possible communications channel, where bits are flipped with probability 0.5. Is there any probability that channel errors make the change in a frame go un-detected through ...
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Can blocks in AES-CBC ciphertext be manipulated in such a way that plaintext becomes shuffled?

Say we have a sequence of N 16-byte plaintext blocks encrypted using AES in CBC mode, with an IV, resulting in N+1 16-byte ciphertext blocks. Can this ciphertext be manipulated in such a way that, ...
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3answers
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Calculate the Number of active s boxes

I really want to learn how to calculate the number of active s _boxes for block cipher cryptographic algorithms. I want to do that manually after running each round of an algorithm such as LED ...
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3answers
133 views

How to Calculate GE of Sbox?

How to count Gate Equivalent (GE) of Sbox used in block Cipher. For example how much area does the Sbox of Block Cipher Prince take? And How is it calculated?
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Is CLEFIA block cipher really lightweight? [closed]

CLEFIA is ISO/IEC 29192-2 standard in lightweight cryptography. According to this, the total GE area (I took the minimum) using .13 um is 2,488 (encryption mode) compared to piccolo-128 with GE area ...
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1answer
42 views

OCB-AES: Ambiguous definition of “Encipher” in RFC document

I'm implementing an OCB-AES cipher (which is described in RFC 7253), now I have to call the AES encipher function (I'll refer to that as AES-256, described in FIPS 197) Pre-summary: I'm not sure ...
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3answers
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What are the requirements from a Round function in Feistel network?

Let's assume a balanced network with many (dozens?) rounds. And a simple (selecting key bits) key schedule. What does it take from the round F function to make a secure block cipher? obviously we ...
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1answer
102 views

Using a PRP $\pi: \{0,1\}^{16} \to \{0,1\}^{16}$ to construct an ideal cipher

Say we have an ideal 16-bit PRP. Since it appears that a permutation with a small domain can be used to turn it into a PRF, which can then be used in a Feistel network. On the surface, it makes it ...
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83 views

Cipher based message authentication (CMAC) and block reordering

Is Cipher based message authentication (CMAC) susceptible to block reordering (just like CBC)?
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1answer
62 views

Unbalanced Feistel network only generates complete permutations with square lengths

I'm generating permutations with a generalized Feistel network which will output integer values $r_j$ in a specified range $[0,period]$ pseudo-randomly, according to the counter input, which will then ...
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Is it safe to calculate the counter based on the block number in the stream rather than increasing it every write?

I've come to use AES/SIC for my purpose as I want to write an encrypted file and read it back with the possibility to seek (I only need to read parts of it and don't want to decrypt the whole file ...