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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Where exactly do you add the zeroes to a URL in a padding oracle attack? Also, how would you decrypt a file this way?

I have been studying various crypto attacks, and one of the attacks that I have recently studied was the padding oracle attack. Now, I mostly understand it, but there is one aspect of the padding ...
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Construction of key recovery attack in O(2^(n/2))

I have to construct a key recovery attack on symmetric key encryption using a publicly known permutation $\Pi$ in $O(2^\frac{n}{2})$ time using $2^\frac{n}{2}$ queries to an encryption oracle. The ...
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CTR mode calculate messages before new key is required

I am studying a cryptography video on Coursera here titled: Modes of Operation: Many Time Key (CTR). I have just two simple questions: At around 4:30 in the video they show 2^48 without saying where ...
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How to properly guarantee authentication, confidentiality, and replay-resistance for multiple messages using a pre-shared key?

I have a device that needs to communicate with another host and exchange fixed-length messages. All traffic should be encrypted and authenticated, and it should be resistant to replay attacks. ...
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AESGCM with arbitrary tag length in Java [migrated]

For algorithm test vector evaluation, I am trying to perform an AESGCM encryption and decryption with arbitrary tag length values such as 32 bits. When I try to initialize my cipher with such an ...
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Why are block ciphers mostly used as stream ciphers?

Using a block cipher with using either CTR or GCM mode gives us a stream cipher, the only difference being that CTR does not include a MAC tag or AAD but GCM does. I think these are the most commonly ...
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Is there a round block cipher that is secure for n round but not secure when the rounds increased

The common knowledge is that increasing the round of a block cipher can make it more secure. This is quite true if we consider the linear and differential attacks. The Tiny encryption algorithm is ...
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Do groups generated by round functions generate the alternating group?

Let $K,X$ be sets and let $F:K\times X\rightarrow X$ be a function. For each $k\in K$, let $f_{k}:X\rightarrow X$ be the function where $f_{k}(x)=F(k,x)$ whenever $k\in K,x\in X$. Assume that each $f_{...
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So is AES-256 more secure or less secure than AES-128 after all?

It seems there are attacks that work more effectively on AES-256 than AES-128, which makes it less secure in some cases. But the bigger key size should add some safety margin on the other hand, for ...
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Proper implementation AES-256 for single block E_k then being used on AES-GCM

My first objective is to implement AES-GCM in PowerShell 5.1 since there is not an implementation that not use external dll libraries. According to the GCM schema of encryption: the E_k block should ...
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Online AES Calculator

I am currently looking into how AES works and found a calculator online: http://testprotect.com/appendix/AEScalc I understand that the AES process has 10 rounds of calculations. Does this mean that ...
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How to minimize AES output size?

I work on a project about CAN bus and I want to encrypt the data among nodes but I have have a problem because CAN bus nodes receive and send 8 bytes among each others and I know the size of AES ...
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Why is this DES key considered weak?

I understand the premise of weak keys in DES and cryptography. From searching online, I understand that keys that are comprised of all zeroes / all ones / alternating ones and zeroes / alternating ...
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Is it possible to crack AES-256 ECB when provided with the ciphertext and a portion of the corresponding plaintext? [duplicate]

AES-ECB encrypts the message by breaking it into blocks and encrypting them one by one. Provided the knowledge of the ciphertext, a portion of the plaintext that corresponds to a block in the message, ...
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Does anybody have test vectors for the LBlock block cipher?

LBlock is a lightweight block cipher with 64-80 bits block-key size, designed by a group of Chinese scholars. Does anybody have LBlock test vectors? I am looking for test vectors, which contain the ...
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Complexity of Boomerang Attack on COCONUT98

I am trying to understand the paper The Boomerang Attack from David Wagner. On page 162 about complexity of boomerang attack the paper says that: The attack requires $8 \cdot 2 \cdot 32 \cdot 2^{32} ...
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Want to implement a cipher efficiently on Hardware with verilog, is there any standard resources to start?

I want to implement a new block cipher on hardware. The cipher has a look-up table and the rounds are not self-similar. So, the code size, as well as the chip area, is expected to be larger. I just ...
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How to use sagemath to find nonlinear invariants of S-box?

I read a paper about a nonlinear invariant attack that is "Nonlinear Invariant Attack: Practical Attack on Full SCREAM, iSCREAM, and Midori64" And I've found a website of sagemath : https://...
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Vulnerability in DES3 when key can be chosen

In the question there are two functions encrypt basically does IV ⨁ DES3(P⨁IV) and encrypt_flag just encrypts the flag after being padded into a multiple of 8. IV is a random 8 byte number which is ...
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Does XORing a random block into a layer of CBC encrypted message add some bit strenght?

Let's suppose I encrypt a message with a block cipher in CBC mode, keep the key and IV secret and XOR a random block of data in each encrypted CBC block, being the XORed block the same for each CBC ...
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How do I preform the inverse key expansion of aes decryption? [duplicate]

I’m working on an aes encryption and decryption project in 8086 assembly! I managed to code an encryption algorithm and it worked perfectly. No I’m working on the decryption algorithm. I’ve managed to ...
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Quadratic Equations to Describe SBox

I am using a 8 bit Sbox having 16 entries with a lookup table of 4x4 in cryptographic algorithm. Sbox is constructed on subgroup of galois field 2^8. I want to calculate number of quadratic equations ...
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Security Analysis

I have designed my own lightweight cryptographic algorithm and now measuring its security. I was finding bounds on probabilities against linear and differential attacks. For differential ...
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How to prove the pseudorandomness of CryptoPAn

in my cryptography course, we are learning about pseudo randomness and the prefix-preserving function known as CryptoPAn invented in 2002. It is said that the scheme is highly insecure. So in that ...
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Random Oracle Model for an ideal block cipher vs Ideal Cipher Model

Some security proof involving an ideal function use the Random Oracle Model (ROM). They posit a thing that given integer $b$ and bistring $M$, returns what it previously returned for that input $(b,M)$...
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how can identify which cryptanalysis is used ( Indistinguishability cryptanalysis against classical ciphers) [duplicate]

I have 10 ciphertexts and 10 plain texts I want to know which cipher is from which plain text how can I know this I know the name of substitution cipher, transposition cipher, Vigen`ere cipher and ...
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Number of false keys in exhaustive key search for block ciphers

I'm doing a self study through Understanding Cryptography by Christof Paar and I have a question about Theorem 5.2.1 in the book: Given a block cipher with a key length of $k$ bits and block size of $...
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AES plaintext is smaller than 128 bits [duplicate]

I'm currently developing a AES implementation in a graduation project at university. As far as I know, AES uses a 128 bit block length as input . if the entered string to encrypt is not 128 bits such ...
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How does using a really small block size reduce the key space?

I read this online and was confused: Modern block ciphers tend to have block sizes of 128 bits or larger, because if the block size is too small there is the same problem as a small key space as ...
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Number of plaintext–ciphertext pairs needed to rule out false keys with a reasonable likelihood in AES

I am new to cryptography and I've got the following question from Understanding Cryptography: Imagine that aliens — rather than abducting earthlings and performing strange experiments on them — drop a ...
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1:1 algorithmic shuffle of massive numerical space

For a personal project I'm currently tinkering on, I'm looking for a function which has the following properties: For any number X in the range 0..MAX, there is a 1:1 random mapping with another ...
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Proving Security of a Tweakable Block Cipher in CBC Mode (Threefish)

I want to prove the security of a Tweakable Block Cipher in CBC Mode How does one prove the security of a Tweakable block cipher (ie Threefish) in CBC mode with a random IV and a constant (public) ...
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LibTomCrypt Documentation on LRW Mode/Tweakable Block Cipher - Apparent Contradiction?

In the LRW mode discussion on page 34/161 of the LibTomCrypt manual, available here for example https://opensource.apple.com/source/CommonCrypto/CommonCrypto-55010/Source/libtomcrypt/doc/...
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Listing first 8 bits generated by LFSR

Consider the primitive polynomial P(x) = x^4 + x^3 + 1, initialized with the bit string (shifting occurs from left to right, were the right-most bit represents the LFSR output): 1101. List the first 8-...
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IND-CCA insecurity of modified CBC encrypt + MAC scheme

I've been puzzling over the following proof for the past couple of days. Since a block cipher in CBC mode can be used to build both an IND-CPA secure encryption scheme and a EUF-CMA secure block, a ...
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Finding Multiplicative Inverse In Field

Consider the finite set Z_257 of non-negative integers less than 257. The number 257 is a prime, so Z_257 forms a field with addition and multiplication mod 257. How can I use the Extended Euclidean ...
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OpenSSL DES encryption for KCV calculation

I'm trying to implement a simple command-line tool to calculate the (Key Check Value) KCV of some encryption keys. The algorithm for the KCV is simply to encrypt a block of 8 zeros with DES and get ...
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Question on the CPA-security proof of the CTR-Mode

While reading Katz & Lindell's textbook (2nd edition) I found a proof for the CPA-Security of CTR-Mode. It can be found at the end of chapter 3.6.2 (page 93 and 94). I don't quite understand, why ...
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Backdoor Designer Key Recovery in LowMC-M

The paper https://eprint.iacr.org/2020/986.pdf proposed a framework for embedding a malicious backdoor in LowMC cipher, that will later help the designer to recover the secret key in the known-...
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Is there any way to encrypt the path in between a small number of 3D positions?

Or more general: given two valid random cipher $c_0, c_1$ a function $D$ with $$D(c_0,c_1) = (a,b,c)$$ should exist but hard to compute (for most cases). The result $(a,b,c)$ represents the path from $...
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Hashes as mutually independent keys

For cascading encryption (ie. AES-Twofish-Serpent), the most secure implementation is with mutually independent keys. My question is if it is secure to hash the previous key for each layer of ...
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Is AES strictly speaking a SP network?

Forgive me if this seems like a trivial question. I've read that AES is a SP network. However, when reading about P-boxes in SPNs they say that they simply shuffle bits around. In AES the ShiftRows ...
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nonlinearity and autocorrelation of common s-boxes

I cannot find publications comparing the properties of popular cipher s-boxes. First of all, I am interested in nonlinearity and autocorrelation of s-boxes, which are used in seed, cast-128, camellia ...
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Can we design a block cipher based on SP-network without loss of E/D similarity theoretically?

I understand what SP-network (Substitution-Permutation Network) and Feistel Network look like. And those ciphers based on Feistel (e.g. DES) often achieve so called E/D similarity, which indicates ...
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Is there any block-cipher, format-preserving or similar encryption method with known cycle size? (without short-cut, with known source/runtime vars)

E.g. given a $n$-bit message $m_0$ and an encryption function $f$ After applying the function $c$ times it will result in $m_0$ again. $f^1(m_0) = f(m_0) = m_1$ $f^2(m_0) = f(f(m_0)) = f(m_1) = m_2$ .....
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Can a broken cipher reveal the key?

Suppose I cascade cipher A and cipher B to encrypt a message. Both ciphers use the same key. I know that theoretically, the security of the setup when used correctly is at least as secure as the most ...
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AES-CBC decrypts to wrong output?

Someone I know showed this to me, and I can't put a finger to why it's happening. Am I missing something? They encrypt the text, and the output is really guessable. $u\to t$, $i\to h$ (it's one letter ...
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Division property table of SBox

In Table 12 (page 23) of the GIFT paper, the "Division property" table is given. I could not understand how it has been computed. My questions: May I have an algorithm on how to compute the ...
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Where is the SIV in AES-GCM-SIV?

This is my understanding of Synthentic IVs You have 2 keys $K_1$ & $K_2$. $F$ is a PRF Instead of choosing a separate IV, you instead generate the IV from the PlainText. $IV = F(K_1, m)$ $c = E(...
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Can anyone explain to me the difference between the OTP (one-time pad) and Vernam Cipher?

I heard they are very similar, although there are subtle differences between the two.

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