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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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Substitution cipher with diffusion

If i introduce a good diffusion property (lets assume the best one) to a substitution cipher, will the cipher be secure?
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Is a larger block size less secure against a brute-force known cipertext attack?

Suppose a ciphertext $c$ was encrypted from plaintext $m$, and we know something about the form of $m$; say, for example, that $m$ is English text. We can attempt to brute-force $m$ from $c$ by ...
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Why didn't people redesign the key scheduling algorithm of DES to use longer keys instead of using 3-DES?

To overcome short key length of DES, Triple DES was devised. What would go wrong or why didn't people redesign the key scheduling algorithm to use longer keys?
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State of the Twofish encryption cipher - is it broken? [closed]

Twofish was one of the finalists in the last cipher competition together with Serpent and AES, where AES won. Do you know of any cryptanalysis on Twofish and if its considered "broken" or still ...
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How keeloq keyfob exchange key with receiver in the first place?

After reading a lot of articles and the microship maunal for keeloq IC, I think I understand how the encryption and decrytion works. If I understand correctly, however, the decryption require the ...
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Design of SIMD-implemented cipher

Design of efficient of block ciphers needs consideration of many factors such as register size. i found that some ciphers can be SIMD implemented such as TWINE , SM4 , chacha20 ,SPIX ,and others. ...
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Would using perfect S-boxes in F function of an unbalanced Feistel network produce a simple and secure cipher?

When I read some articles about cryptography on wikipedia, I took note of the following statements (please, correct me, if any of these are wrong): Perfect S-box (based on bent functions) provides ...
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What are the general rules to follow if I want to design a block cipher suitable for file encryption? [duplicate]

Can you please tell me what should be the quality of an encryption function using a key size of 256 bits... I realized from my previous question that a simple XOR of plaintext (256 bits) with a ...
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Security of bit shift in Serpent

As far I know, the Serpent cipher is the only cipher that uses legit bit shifts (<<) along with circular left shift (...
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Serpent vs AES,, theoretical security

In the lets-choose-the-next-aes contest in 1998, they chose Rijndael because of speed. Now, not taking speed into account, is Serpent more secure or not? Since it has 32 rounds (of course that's not ...
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Why are cyclic keys in Feistel Networks insecure?

So everyone seems to agree that recurring round keys in a Feistel network (eg. using the same key for every round) are a massive security leak. When researching this, the only thing I found is the ...
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2DES and 3DES against a DES build with complimentary

I saw this complicated question and I am not really sure about it, would appreciate your view on it. Is the following build stronger than 2DES or 3DES with 2 different keys (or even normal DES)? $...
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Simple block cipher

I came up with this concept, which I thought would be pretty secure. So the idea is this, you have a message $M$, which you split up in blocks of each 256 bits. You generate a random (pseudo-random ...
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How to perform ByteSub transformation? [duplicate]

Can anybody please tell me how do we perform ByteSub transformation of AES to 01001001.
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What will be appropriate AES padding characters?

I am completely noob in terms of cryptography and security. So I decided AES Cipher for encrypting my data. I pad my input data String with blank white spaces to fit it in multiples of my cipher ...
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A bit-sliced implementation (or the algebraic normal form) of the two FIDES S-Boxes

I want to see a bit-sliced implementation of 5-bit and 6-bit S-Boxes of FIDES or, at least, a representation of these two functions in algebraic normal form. Where can I find it?
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How can AES-CCM behave as an AEAD stream cipher?

AESCCM- Couter with Cipher Block Chaining Message. As it uses cmac for generation for tag , which is internally aes-cbc (block cipher, need block size, may require padding) And Encryption is done ...
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Colliding input to compression function

So I recently started to look into cryptography and found the following question to cryptography scheme using $T = E_{S}(S \oplus M)$ It is said that this is not secure because you can have a ...
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Block cipher mode of operator with counter as part of its key or message [closed]

Consider a block cipher (such as AES256) as $E$, a randomly selected key $k$, a counter $i$, and an array of n-bits $M$ as the message. Also consider the following block cipher modes of operations: $...
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How can frequency analysis be applied to modern ciphers?

I am building a computer program that deciphers Caesar, Vigenere and monoalphabetic substitution ciphers. All of those are susceptible to frequency analysis. However, it does not seem to be real-world ...
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Perfect Forward Secrecy Stream Cipher based on Block Cipher

I'm thinking about ways to encrypt an endless stream of data (e.g. a video or audio feed) in a way, where a compromise of the current internal state of the encryption (e.g. a RAM dump) does not ...
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Is there a name for encryption which relies on repetitive calculation (=time) instead of a key-value? (e.g. block chain, hash value, block cipher)

Many encryption problems are fast to solve if a key, factor, exponent or something similar is known. Other encryption methods don't have such keys. E.g. a hash value of another value. It's used ...
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Why is ECB+CTR not a thing?

AES-CTR is nice for its parallelizability and simplicity but if you duplicate an IV you reveal plaintext. Chaining modes like CFB and CBC don't have that problem per se but they are not ...
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Is there a 'chain-function' which allows computing the value for the index which is next, previous, +/- some constants but has no shortcut for other?

Looking for an algorithm which has a cyclic 'chain'-function which only allows the computation of next, previous element and in addition to that also a shift by a small amount of constants. Example 1 ...
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8 bit block cipher in CTR mode for practice - counter is 32 bits

I'm given a very simple 8 bit block cipher in which to encrypt plaintext x with k k one performs $ x \oplus k $. I am given IV = 0101 and a CTR of value CCCCCCCC (36bits). My understanding is that ...
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Do any block cipher encryption functions (or similar functions) exist which are associative to each other and cyclic?

Most block ciphers encrypt a plaintext deterministically into a ciphertext which allows decryption in the opposite direction. Now, instead of encrypting just one plaintext, I'm looking for a cyclic '...
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Block cipher linearity (in relation to hill ciphers)

So while I knew hill ciphers were a form of block cipher (I'm assuming all the ones I have solved to date were ECB or something similar), so that got me thinking. Is it possible to use hill ciphers in ...
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Which block cipher parameters should be kept secret?

Which of the parameters of a block cipher (e.g., algorithm name, mode of operation, IV (if any), key length) are essential to keep secret?
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Attacking Double DES (OFB Mode) given the initialization vector and not much else? [closed]

I'm given a Plaintext/Ciphertext pair, a Ciphertext to break, and the IV. One thing of note is that the same IV is used for both DES Encryptions, so $$C = E_{K_2}(E_{K_1}(P, IV), IV)$$ I have limited ...
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Do there exist cryptographic algorithms where the “core” function is 96-to-32-bit?

There exists a FOX family of block ciphers (Wikipedia article: IDEA NXT). This cipher uses a function called "f32" which operates on three 32-bit words and outputs one 32-bit word. This function ...
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Are slide attacks effective if the primitive is sandwiched? And key schedule vs. Even-Mansour

Suppose you have a cryptographic permutation consisting of identical rounds, that makes it vulnerable to a slide attack. But if you sandwich it between two XORs of key material. Is it still vulnerable?...
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Why does error propagation in CBC mode encryption affect two blocks?

From Wikipedia: Error propagation It was common to discuss the "error propagation" properties as a selection criterion for a mode of operation. It might be observed, for example, that a one-...
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Are any block ciphers provably free of equivalent keys?

There are $2^n!$ possible permutations of an $n$-bit block cipher $E_k:\{0,1\}^n \rightarrow \{0,1\}^n$, and any given key $k$ selects one of these permutations at random. Define equivalent keys as a ...
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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 $IV_1$ = $C''_0$ = ...