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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What security risks does exposing the size of the plaintext entail?

For this question my focus is exclusively on symmetric block ciphers (with a particular interest in AES256 in CBC mode). I have read much discussion on various ways to pad the plaintext so that the ...
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How can padding be disambiguated from data, when encrypting using a block cipher?

How can padding be disambiguated from data, when encrypting using a block cipher? I'm by no means an expert in cryptography, but rather a programmer with a keen interest. Suppose, I've ...
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key recovery in the ideal cipher model

I'm reading up on the ideal cipher model and can't wrap my head around some basic calculation. Let $E$ is an ideal cipher with $n$-bit key and $n$-bit block. For each key $K$, $E_K$ is random ...
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How TAU function of SM4ni cipher is implemented?

I am trying to understand the SIMD implementation of TAU function of SM4 which is as follows: ...
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Very short block ciphers for equally-short plaintexts

I'm trying to develop an encryption technique for encrypting short phrases (such as reversibly-anonymized names) in text. The key-generation and ciphertext-representation details are fungible, but ...
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How many required known plaintexts for an attack are considered insecure?

I stumbled over this problem when looking at symmetrical block ciphers with a relatively small block size like 32 bit. When I was looking at more common block ciphers with a 64 bit, I learned that ...
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Encryption of 7 byte of data, output must be 7 byte [duplicate]

I'm looking for a cryptographic algorithm that inputs and outputs are both 7 bytes. Is there any such algorithm?
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What are the issues of look-up table based implementations?

Cryptographic libraries mostly use pre-computed lookup tables to implement block ciphers. These lookup tables require large amout of memory and a series of cache miss/hit makes them vulnerable to ...
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Simplifying Meet-In-The-Middle Attacks

Considering the fact that a block cipher is a bijective function on the set of possible plaintexts, if one views the encryption of a datum with a key $K_2$, which has been already encrypted with the ...
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Is the following s-box more linear or more non-linear?

I am trying to figure out whether the following simple s-box configuration I created is more linear than non-linear or vice versa more non-linear than linear? The parameters for this 16*4 table of ...
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Why Stream Ciphers can be slower in software than Block Ciphers

I made some tests on virtual environment (Virtual Box) using StrongSwan, a VPN based on IPSec. I had latency results using ping command saying that Chacha20 (stream cipher) had poor performance ...
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Is it possible to perform a meet-in-the-middle within a block cipher?

Standard meet-in-the-middle explanations show that you can perform a meet-in-the-middle attack on a repeated block cipher such as double-DES (performing DES twice in a row). However, block ciphers ...
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Why does a padding block exist for ECB and CBC modes?

In ECB and CBC mode, the size of cipher text is more than the plain-text size by one padding block. From what I read, ECB does a simple encryption to each 64 bit block of a Plain text, and produces ...
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Aside from Keccak, are there any keyed/unkeyed permutations where the choice of all rotation constants is not based on heuristic methods?

I am interested to see examples of cryptographic algorithms (namely, keyed or unkeyed permutations that transform an $n$-bit input to an $n$-bit output) where the choice of all rotation values is not ...
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What happens if you increment MAX counter in AES-CTR?

Suppose you use a IV/counter = ffffffffffffffffffffffffffffffffin AES-CTR. What will happen if you encrypt the next block, and thereby increment the counter by one? ...
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Block cipher information leakage using same key

This is from a previous exam question and I'm not really sure how to approach this properly We're using a block-cipher mode that encrypts $C_i$ = $E_k$ ($P_i$ $\oplus$ $C_{i-1}$ $\oplus$ $P_{...
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Meet-in-the-middle attack (different block cipher modes)

Does a meet-in-the-middle attack on depend on the used modes for block ciphers? encryption: $C_i$ = $D_{K2}$($D_{K1}$($E_{K1}$($P_i$ $\oplus$ $C''_{i-1}$))) $\oplus$ $C'_{i-1}$ and $IV_1$ = $C''_0$ = ...
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Block Cipher CTR mode when offset is not known by receiver

When an encrypted message is received, it can be easily decrypted if the offset (=counter) of the received message is known (and also the required private key of course). This allows for random access ...
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Bit based Division Property of AES MixColumn

I'm studying about division property that is proposed by todo et al. I have a question about division property for AES mixcolumn. In bit based division property, how can I calculate output division ...
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Where can I find test vectors for the present cipher with a 128-bit key?

Where can I find the test vectors for the present block cipher with a 128-bit key?
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Is there a maximum amount of data which can be encrypted using a symmetric (AES256) key?

I am working on an application where we will be encrypting billions of small (< 2,000 bytes) records. Combined, we are facing many TB of data. Each record will be encrypted independently, using a ...
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Help understanding how to decrypt cipher

I am trying to create a padding oracle attack to decrypt a cipher that is encrypted with cbc and PKCS#7 padding. My first step is just to try and decrypt the second block. I have read a lot about it,...
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Determine whether nonlinear boolean function is balanced or not

Is there any way to check that a nonlinear boolean function is balanced or not? I can't think any other way except to check all input variables.
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What are the advantages of SM3 and SM4 compared to NIST-approved algorithms (SHA3 and AES)

Armv8.4-A will add extended support for more cryptographic primitives, to include SM3, which is a cryptographic hash function used in the Chinese National Standard; and SM4, a 128-bit block cipher; ...
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What are the implications of XORing ciphertext with plaintext?

I was intrigued by this question: Does adding complexity mean a more secure cipher? And it led me to wonder: What are the implications (if any) of XORing a ciphertext with the original plaintext ...
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System equations for S-boxes

I`m on searching of any method of building system equations for S-boxes. Firstly, I just cannot undestand exact meaning of "system equations", is it anf for each input (I know how to make it using ...
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Does the block size of a symmetric cipher impact the security of the cipher itself? [duplicate]

Will a 128 bit block have some security implication in post-quantum cryptography?
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Can ECB be turned into a secure algorithm by adding obfuscation of the plaintext patterns?

Just to preface this I'm very new to cryptography. However, I've been reading that ECB should not be used under most circumstances due to its symmetry which allows attackers to gain information about ...
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silly possibly original AE construction

This idea is inspired by a recent question about sending 32 byte messages, encrypted, without a MAC but still achieving the equivalent protection of a MAC just by using a block cipher, extra ...
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Is it possible to determine the symmetric encryption method used by output size?

I'm attempting to identify the method of encryption for a black-box symmetric encryptor that produces blocks of output that are 4 bytes in length (e.g. small inputs fit in 16 bytes, then 20 bytes and ...
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How should the key schedule be designed for this general block cipher?

After looking at the Wikipedia page for the Fiestel cipher, I thought about making a variation of it that is very general. I basically followed the diagram on Wikipedia in order to structure the ...
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Is it safe to use ECB mode this way, w/o duplicate blocks?

I wanna make my own "way" of encrypting separate messages. And I want it to have the least possible size overhead while being completely encrypted (hide msg counter). Here's my current idea: we ...
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AES, CMAC and “3 pass authentication protocol” Explained

I am struggling a bit to understand how authentication systems are properly used in NFC tag environments so I'm posting this thread to clarify some concepts.(In advance, this is not my field, so ...
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Extract one unique SBox from Canonical Representative

In paper https://eprint.iacr.org/2011/218.pdf, the author has examined 16! different 4 bit SBoxes and at the end of investigation, introduced an optimal class which named "Golden SBoxes". How can I ...
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If the S-Box is given. How to calculate the affine transformation matrix?

Example using SAGE sage: sr = mq.SR(1, 1, 1, 8, gf2=True) sage: S = sr.sbox(); S (99, 124, 119, 123, 242, 107, 111, 197, 48, 1, 103, 43, 254, 215, 171, 118, 202, 130, 201, 125, 250, 89, 71, 240, ...
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Predicting value of E(k,0) for Block cipher with random and secret key

Some mode of operation of block ciphers rely on the fact that E(k,0) is an unpredictable value when k is random and secret (with 0 denoting the all-zero binary string). Why is this a reasonable ...
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Where can I find some thought-provoking questions on the fundamentals of Cryptography? [closed]

The question might sound broad, allow me to explain. Cryptography is a diverse domain. I have been in Crypto research since last four years. Recently, one of my colleague got interviewed by an IT ...
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Application of MDS matrices in serial and round-based implementations

We know that MDS matrices are used in the diffusion layers of block ciphers. My question: what types of MDS matrices can be applied to serial or round-based implementations or both of them? I ...
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Security of a block cipher if double encryption $E_{K_2} \circ E_{K_1}$ is always single encryption $E_{f(K_1,K_2)}$

If there always existed a $k_3$ such that $\operatorname{DES}(k_2, \operatorname{DES}(k_1, M)) = \operatorname{DES}(k_3, M)$, how would that affect the security of $\operatorname{DES}$?
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Meet-in-the-middle probability of success

In page 358 of Bruce Schneier's Apllied Crypto, when explaining the meet-in-the-middle attack, he states that the success probability is «1 in $2^{2m - 2n}$» with two plaintext/ciphertext pairs, and «...
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Differential and Linear trail propagation in Noekeon

In the Noekeon Cipher Specification they write the following : The propagation through Lambda is denoted by $(a \rightarrow A)$, also called a step. Because of the linearity of Lambda it is fully ...
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Additive differential probability

I am studying about additive differential probability. And I show a paper that is V.Velichkov et al., The Additive Differential Probability of ARX. I have a question in chapter 5.2. V.Velichkov et ...
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How to calculate the number of Linearly active Sbox

I cant understand the concept of linearly active sbox. In the case of differential analysis it is more easy to understand the concept of 'active sbox' Also how the increase in number of (linearly) ...
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Learning about Feistel Cipher

I'm learning about Feistel Ciphers at the moment, but having issues with the formulae, so I can't move on from it. Encryption: $𝐿𝐸_{𝑖+1} = 𝑅𝐸_𝑖$ $𝑅𝐸_{𝑖+1} = 𝐿𝐸_𝑖 \oplus 𝐹( 𝑅𝐸_𝑖 , 𝐾...
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Can block ciphers encrypt messages larger than the key length?

I am trying to learn some basic cryptography and I am reading some conflicting statements about block cipher. My current understanding is that we start with an XOR to obfuscate our data. We can then ...
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$\boxplus$- key addition vs $\oplus$- key addition

GOST cipher , Hight cipher and SEA cipher use key addition based on modular arithmetic addition (module key size $\boxplus$) compared to many ciphers which use arithmetic addition modulo 2 ( $\oplus$)....
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How can I determine the Hill cipher key in this case?

I have been struggling with a Hill cipher problem for many days, without any luck. I have the following ciphertext, which I know is an encrypted excerpt from an Edgar Allan Poe story: ...
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What is the purpose of multiple key addition layers in AES?

I understand the purpose of the SubBytes layer (confusion) and the ShiftRows and MixColumns layers (diffusion). But I don't see the purpose of having more than one key addition layer. What I've been ...
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Terminology: Is a block cipher in counter mode still considered a block cipher or is it classified as a stream cipher?

I'm confused about what terminology to use. I see people referring to such a cipher-MOO combination using both terms.
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The key size of an ideal block cipher

My understanding of an ideal block cipher is as follows: For a block cipher with block size $n$, there are $2^n$ possible plaintexts. The number of possible keys (mappings) would then be $2^n!$. ...