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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Can I use a block cipher in CTR mode to generate keys from a master key?

Suppose I have an AEAD mode based on a block cipher that requires two or more keys. In some cases, there is a complex proof that one key suffices. But finding such a proof is hard. An alternative ...
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tweakable block cipher and ideal block cipher

Can the set of block ciphers obtained from a tweakable block cipher (using distinct tweaks) be considered as the set of ideal block ciphers ?
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How does blocksize affect security?

I have recently read about how blockciphers and how stream ciphers work, and although I think I finally got everything right, I'm still wondering how increasing blocksize increases security.
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How to break a 32 bit block cipher without key?

From what I know Skip32 is the only 32 bit block cipher and I heard its not secure. What do I do or which tool do I use to break a 32-bit block cipher? I did search for this but all I find is attacks ...
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Difference between stream cipher and block cipher

A typical stream cipher encrypts plaintext one byte at a time, although a stream cipher may be designed to operate on one bit at a time or on units larger than a byte at a time. A block cipher ...
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How to decipher English letters that had been encrypted [closed]

I tried several decipher methods (here and here) but still cannot find any. How do effectively decipher those below. Below messages are English that had been encrypted. i) DBEJIXCV IWTDGN XH WDL ...
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Need for 3 rounds in Luby Rackoff theorem

I get that using Feistel network we can create a secure PRP from a secure PRF (in 3 rounds according to theorem). My question is why not just 2 rounds of Feistel network is sufficient. Is it that ...
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How to publish a cipher (concept)?

In the last months I was searching for a subject for my bachelor thesis. I came up with an idea for a new cipher concept that works by combining already known techniques in a (hopefully) new way. So ...
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TWO Fish MDS and PHT

Twofish was an AES candidate and it uses 4x4 Matrix as MDS followed by a PHT. The Branch Number of MDS and PHT is 5 and 2 Respectively. from formula Branch Number = Minimum of [HammingWeight(Input) ...
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How is the block size of a block cipher related to how susceptible the block cipher is to a birthday attack?

For example what can you say about how susceptible Triple-DES is to a birthday attack due to the fact that it has a block size of 64-bits.
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Security of randomized data dependant key schedules?

An example to demonstrate the point with two Feistel networks: Cipher A: \begin{align} roundkey = hash(key || counter)\\ roundkey = hash(key || roundkey)\\ left = left\oplus hash(roundkey || ...
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Block Cipher vs PRP in CBC?

What is the difference between using a Block cipher and a Pseudorandom Permutation in Cipher Block Chaining mode, which is better?
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60 views

Block cipher mode of operation with beyond-birthday-bound security

I am looking for block cipher modes of operation that are secure even when the number of blocks encrypted exceeds the birthday bound.
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55 views

Is 3 rounds more secure than 2 in this simplified DES?

Consider the following simplification of DES: a plaintext message block of $2n$ bits of length is divided into two blocks of length $n$ (left half $L_0$ and right half $L_1$). In other words, the ...
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How to prove the security of block ciphers

I see very often proofs of security for asymmetric crypto algorithms, for instance, using reductions to known hard problems, or game based proofs... In the field of protocols (like authentication) it ...
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73 views

How would an attacker perform an exhaustive key search on a block cipher using ECB mode?

Do you always assume from Kerchoff's principle, that the attacker has access to everything but the decryption key? That is, am I to assume that to perform an exhaustive key search, the attacker has a ...
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331 views

What is the fastest block cipher in the (Intel) world?

In AES competition,Rijndael cipher obtained first rank and become Standard and Serpent, Twofish, RC6 and MARS obtained another ranks and I know that Serpent took second rank because it was slower than ...
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Deoxys: disadvantages vs. AES?

The CAESAR candidate Deoxys is based on a tweakable block cipher that seems to be a strict improvement over AES, in that it is a tweakable cipher resistant to related-key attack. The mode proposed ...
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208 views

Simple two round feistel chiffre

I am currently learning for a exam in cryptgraphy. I now try to understand the Feistel Cipher with two rounds. I do not have any information on what the functions are actually doing. The following ...
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How secure would hand ciphers be using a Block Cipher Mode?

I've been looking into different ciphers that require very minimal computation power (calculator, deck of cards, index cards and a pencil), i.e. hand ciphers. For the most part all methods typically ...
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Doubling the block length of a given block cipher

IMHO the main essence of a Feistel cipher of block length $n$ is the working of a non-linear function $F$ that is applied to the two halves of the block of length $n/2$. Now suppose a certain block ...
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Known Plaintext Attack against 3-round SPECK48/96

I am attempting to break 3-round SPECK48/96 using a known-plaintext attack, provided with 10 PT-CT pairs. Here is a diagram: I have seemed to almost figure this out, but I cannot figure out how the ...
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HW acceleration for Camellia cipher (x86_64/AES-NI/AVX2)

Modern processors support hardware acceleration for various crypto functions such as AES directly, or general vector operations which can be used in crypto ...
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Why does DES implement so much Cross Wiring?

I've been going through DES, and I find that most of the blocks simply Reshuffle the input, which may be termed as "Cross Wiring". This include IP, IP-1, PC-1, PC-2 and even Expansion (although this ...
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2 round GOST_28147-89 cipher distinguisher

So, here is the scheme of how this will look graphically: The deal is to present distinguisher for this cipher. First of all here is my drafts: Way 1 Assume that distinguisher displays in ...
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How do substution-permutation networks work?

I understand what's going on in higher level. But i don't understand what's really going on in page 3 and 4 of this slide. Specifically, How S0 and S1 is used? What goes in what goes out? How ...
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How to crack unknown 8-bit encryption algorithm?

Can anyone tell me how to crack the encryption of 8 bit algorithm, when you have some clue that CBC mode is used, but the algorithm is secret. You also have some encrypted data (chosen plain text), ...
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Generating random non-repeating numbers from a small group

I read this Q&A which gave a clever solution: feed an incrementing index into a block key cipher as a way of producing non-repeating random numbers. The problem is the block size of all the good ...
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Encryption of 8 bit of block data

I'm looking for a cryptographic algorithm that allows encrypting only 8 bits of data in a block. Is there any such algorithm that is considered secure? The reason I'm asking is because I have to ...
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41 views

How to use AES-CBC to password-encrypt a small block of stored data

I have a small block of data (16 bytes) that I am required to encrypt, store (in an unsecure, visible location). Later I have to retrieve and decrypt it. I must use AES-CBC and a user-entered ...
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80 views

Block cipher block and key sizes

Is there any point in making block size less than key size in block cipher? For example, Russian GOST symmetric cipher uses 256-bit keys to encrypt 64-bit blocks. That means there are many keys which ...
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How secure is $\operatorname{AES256}_{\operatorname{ECB}}(\operatorname{ChaCha20}(\text{plaintext}))$?

Suppose I encrypt a bytestream using ChaCha20, and then encrypt the resulting ciphertext using AES in ECB mode. How secure is the combination?
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One cipher to rule them all?

I learned from Dan Boneh's course that many cryptographic primitives (prngs, stream ciphers, hashes, hmac, key derivation functions) can be built from just one a block cipher or PRF. For example, the ...
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A block cipher whose key changes after each block

I have been reading about various block cipher modes of operation, and all of them seem to treat the block cipher (including it's key and key schedule) as a sort of immutable black box. I would like ...
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Is there an avalanche effect using block cipher modes of operation?

I am studying cryptography and various block cipher modes of operation. Suppose I changed one bit in the last block of the message. Wouldn't that imply no change in all encrypted blocks except for ...
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Should I use a self-designed Block Cipher Mode of Operation?

We all know we should NOT roll our self-designed encryption algorithm, how about block cipher mode with a certificated cipher method like AES? Will a self-designed block cipher mode cause any flaws or ...
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Why can the last block contain a full block of padding in CBC Encryption?

I'm trying to understand the SSL Poodle Attack and I'm wondering why the last block of a CBC Record can be full of padding? Wouldn't that mean that the useful data was already a multiple of the key ...
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Breaking linear blockcipher - expanding recursive formula

I am trying to break a block cipher that can be defined as follows: We have 12 rounds and at each round we perform the following operation. We split 64 bit data into 2 parts each 32-bits, $\ x_r$ ...
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Using One Time Pad with a block cypher

The One Time Pad is often described with a xor cipher. This is nice to show the theoretical strengths of the one time pad but not so great in real world scenarios where key reuse is a danger. So Is ...
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What is the difference between substitution cipher and block cipher?

I'm studying(and clearing my head) about block cipher. I started from Caesar Cipher which can be considered as a kind of substitution cipher (if my understanding is right). And an ideal substitution ...
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Problem with braking SASAS type of encryptions

I am reading the paper Structural cryptanalysis of SASAS. I understood it till the section Recovering of layers S1 and S3 on page 6. However because of my limited understanding of mathematics, I am ...
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Is it possible to create a detection oracle for all block cipher modes? — ciphertext-only cryptanalysis

I have made an ECB/CBC detection oracle, and am now trying to make a detection oracle that can differentiate between CBC/CFB/OFB/CTR/etc. I can already tell ECB apart from the others due to this ...
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Tell where messages begin and end in a series of ciphertext blocks?

With block ciphers, I understand that the plaintext is first broken down into the right-sized blocks, padded, and then encrypted into the same number of cipher blocks. Once these blocks are sent over ...
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Rijndael variant using AES for key schedule?

The weakest part of Rijndael is considered to be the key schedule (I believe that this is true for all variants, including ones not standardized as AES). One solution to this problem is to use ...
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Does encrypting with MGF1/SHA-512/1024-bit seed equal to a 1024-bit key block cipher?

Suppose: Alice uses SHA-512 with MGF1(as in PKCS#1 V2) and a 1024-bit random seed to generate a mask, XORs the mask with a message(M), gets a cipher text (CT), and sends the CT to her old friend ...
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Reykeyed AES in CTR mode as a stream cipher

Suppose one uses AES in CTR mode to generate a keystream, but every $l$ bytes the $n$ bits that would follow are used as a new AES key. Assuming that AES is a secure PRP: Is this a secure stream ...
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Constant time multiplicative inverse within a word

I was playing with an algorithm which at one step, calculated $f(x) = x^{-1} \mod p$ for $0 < x < p = 2^{64}-59$ (note $p$ is a prime). I used Knuth's Vol 2 Algorithm X algorithm for calculating ...
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What information about the ciphertext can an attacker discern if a key is used too many times?

Suppose that I use Blowfish or XTEA (both secure block ciphers with 64-bit block size) in CTR mode. In these cases, I need to change the key after $2^{16}$ blocks to avoid leaking information with ...
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Does extra encryption improve the security of CBC and CTR?

I am learning about cryptography and during my research and reading I had a question. When looking at information leakage and CBC mode, I asked if there is a way to improve this with by using the ...
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Adding tweak to a block cipher

I know there are XEX, XTS and other ways to add tweak to block cipher without modifying cipher itself. However they are quite slow and/or complex. If we assume we have a secure block cipher round ...