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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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### Does any encryption/decryption algorithm supports linear decomposition?

I am not sure whether "linear decomposition" is appropriate to summary my question: We know that the traditional symmetric encryption/decryption algorithm (like AES, TDES) can be written as: ...
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### How big of a threat are *diffusion model* based AIs to cryptographic systems?

The diffusion model, which is used by products like Midjourney and Dall-E, trains AI systems to de-noise (remove added randomness) from data to infer what the original de-noised data is. That would ...
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### Encrypting random IV in CTR mode (no nonce!)

Use of plain random-IV's in CTR mode, without any special "nonces/counters" (or any "dedicated" bits!), can lead to problems with "partial overlaps", whereby attackers ...
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### Given $i$ keyed-$PRP$ labels $\ell_{i,x}$ from a $2^{256} \times 2^{256}$ Sudoku (Latin-square), how difficult is it for an adversary to solve?

There's a keyed-permutation I'm playing with, $\ell_{i,x} = \pi_i(x_i)$, which is a bijection $X \leftrightarrow X$, where $|X| = 2^{256}$, and whose evaluations on plaintext inputs $x_i$ perfectly ...
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### Attacking a one-round SPN

From Introduction to Modern Cryptography, 6.2.1 Substitution-Permutation Networks A description of an optimized attack on one round SPN A better attack is possible by noting that individual bits of ...
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### May I use the same One-time pad key two times if I encrypt it with a block cipher using two different block cipher keys?

Let's suppose I have two 2GiB files and I want to encrypt them using One-time pad (Vernam cipher), but I don't want to store two big keys. May I use only one key for the two files if I encrypt the One-...
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### How CTR mode is encrypted?

I referred to the explanation of encryption mode from Wikipedia, but I'm confused now, and I went to find some books about Cryptography, but it didn't solve my confusion. The CTR mode uses Counter to ...
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### If I encrypt a plaintext with different keys for each block, will I have the same security as a one-time pad?

Let's suppose I encrypt an 1MiB plaintext with AES-256 in ECB mode but using different keys for each 16-byte block (I know this is weird but it's just an example). Remembering that a different 256-bit ...
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### Is it undesirable for authentication to require decrypting a ciphertext?

A couple years ago, I devised some primitives for block ciphers and block cipher modes of operation; I was partly inspired by CAESAR. What these designs all had in common is that the encryption/...
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### Bit based division property

Bit based division property is a great method of Integral attack on block ciphers, which was invented by Y. Todo. According to the paper Bit-Based Division Property and Application to Simon Family (in ...
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### How many $S$-Boxes of length $3$ are there over $\mathbb{F}_{2}$?

Is there a simple way if counting the number of $S$-Boxes of length $\ell$ over $\mathbb{F}_{2}$? By $S$-box I mean an $S$-box satisfying the avalanche condition. I mean it is quite easy to see that ...
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### Does blockcipher decryption always offer equivalent security as blockcipher encryption? [duplicate]

I'm not sure if anyone explored this area before. Blockcipher encryption definitely fullfills IND-CCA, but what about its inverse? Is there a proof that shows blockcipher decryption is as secure as ...
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### Cryptography particular block length valid message

I am new at cryptography and start to learn some terms. I just saw an question in the book and didn't got a clue about this question. Could someone can explain me the answer of this question? Because ...
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### Is it fatal if a blockcipher uses a sbox with a fixed point at 0?

In a previous question, I asked how to implement arbitrary s-box in side-channel-free fashion. The code I posted in the question loops over all 256 values of a byte to avoid timing channel, and I want ...
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### Can we still use 64-bit block ciphers?

After thinking a lot about 64-bit block ciphers, I ask a question here. Most proposed block ciphers these days seem to have a block size of at least 128-bits. Also, I know that when using a 64-bit ...
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### Which encryption method support 256 bytes block size?

I am looking at some secret encryption method that apparently uses CBC and a block size of 256 bytes. I can do a chosen-ciphertext-attack. When I modify a single byte in the ciphertext a complete ...
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### Rule 30 based block cipher

So I went ahead an wrote this little program. What it does is it takes a password for a seed and generates a (pseudo-random) bit-pattern using rule 30 from cellular automata. It then XORs our input ...
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### If I encrypt two plaintexts with different keys, XOR the ciphertexts and send to an adversary, what can he/she do with what I sent?

Let's suppose I encrypt two blocks of 1MiB with AES-256 in CBC mode, each one using different keys, XOR the resulting ciphertexts and send this XORed block to an adversary. Remembering that the ...
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### How does the plausible deniability used by TrueCrypt work mathematically?

I have been unable to find any mathematical explanations on how TrueCrypt's plausible deniability encryption works, when using TC containers. Would someone be able to provide a mathematical ...
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### In CTR block cipher mode of operation, can I reuse the nonce with another key for encrypting another plaintext if using different counters?

Let's suppose I encrypt a plaintext with one key and a nonce in CTR block cipher mode of operation. Can I reuse the nonce with another key in another plaintext if I start the block counting with ...
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### Is my domain separation in stream cipher and MAC accepted?

I would like to implement domain separation in encryption/decryption along with MAC. The encryption algorithms consists of XChaCha20,XSalsa20,SM4 From China and MAC algorithms consist of HMAC-SHA512 ...
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### What mode in EVP for AES-128 should I use for cryptanalysis of AES?

I've been trying to use AES from OpenSSL and got to know that I need to use the EVP-based implementations rather than the AES_*. I need to use AES-128 ...
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### modern version of a book cypher [closed]

I'm a computer programmer and I'm working on a truly unbreakable cypher and I keep going back to a book cypher (each letter of the message is referenced by a page,row,and column number in a random ...
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### Resistance against timing attacks of AES candidates

It's difficult to implement AES securely and efficiently if the adversary can observe the timing and (approximate) location of memory accesses, unless you have dedicated hardware. The naive ...
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### Vulnerability due to same IV, same key, same plaintext at same position in OFB

I am currently studying OFB mode, and one of the vulnerability mentioned for it is that if two different messages have a block at the same position in the ciphertext, and have same plaintext, the ...
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### Block cipher decryption

I have a course work for university. The question is: My solution to the question is: P||R = D(K,C) However, in the question it doesn't say we are given R so ...
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### Can we achieve IND-CCA without a MAC?

In modern cryptography, IND-CPA is the lowest security we want. We want at least IND-CCAx security from encryption mode. Their relation can be found in 2007 Authenticated Encryption: Relations among ...
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