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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How many additions modulo $2^k$ and multiplications in $\mathbb F_{2^k}$ are needed to resist cryptanalysis?

Consider a $k$-bit block cipher with $r$ rounds, and key composed of $r$ subkeys $K_i\in\{0,1\}^k-\{0^k\}$ (that is, non-zero $k$-bit bitstrings), for $i\in[0,r)$. Plaintext is $P=S_0\in\{0,1\}^k$, ...
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Statistical saturation attack on block ciphers

I was wondering if anyone around here could give me some explanation on this type of attacks. Pretty much the only thing that I could find is A Statistical Saturation Attack against the Block Cipher ...
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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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Safely using ciphers that take small key sizes

Is it feasible to use triple encryption for block ciphers with the maximum keysize being the only significant security issue? I mean $C = E_{k_1}(E_{k_2}(E_{k_3}(P)))$ and $P = D_{k_1}(D_{k_2}(D_{k_3}(...
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Differential Path Search of a Block Cipher

Camellia Design and Analysis document refers to a paper: [24] M. Matsui, "Differential Path Search of the Block Cipher E2," (in Japanese), Technical report of IE- ICE, ISEC99-19, pp.57-64, The ...
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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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Markov ciphers - generalized group operation

I’m reading this paper of Lai and Massey where they give a very nice perspective on differential attacks in general http://www.isiweb.ee.ethz.ch/papers/arch/xlai-mass-inspec-1991-3.pdf They do their ...
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How to decide which diffusion scheme is better for a block cipher design

There are several schemes that can be used to achieve the diffusion property in a block cipher. Schemes include MDS code, Bit permutation, Byte/ Nibble permutation, Diffusion Matrix, Diffusion ...
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246 views

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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mCrypton - does anyone know where I can find test vectors or a reference implementation?

I am currently writing an implementation of the mCrypton lightweight block cipher (as presented in this paper). Unfortunately, the authors do not provide any test vectors. I tried to contact the ...
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161 views

Implement Twofish cipher with 16-bit blocksize?

I've seen implementation of Rijndael cipher with 16-bit blocksize. And I have also tried to implement 16-bit Rijndael cipher by myself with only a little sneak peeking at the example of the ...
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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, $\ x_r$ and ...
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Are there key collisions in AES-192, AES-256?

Suppose we have a single, fixed plaintext block $p$ of 128 bits. Call the set of all possible 128 bit blocks $B$. We could define fixed plaintext AES-128 as a function $f(k) \to B$ that takes a 128 ...
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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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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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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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Why is confusion & diffusion not required PKI encryption schemes like Elgamal encryption?

The Elgamal encryption scheme is a Multiplicative Mask with the shared key. It doesn't seem to have any confusion or diffusion. So why is it not necessary here as compared to Block Ciphers?
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Whats the probability a random cipher has a high probability differential?

This is a bit vauge and long winded. Any partial answers or comments/ways to make this more precise would be greatly appreciated. Fix some general structure for a cipher, for example Sbox then ...
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Why does the random oracle receive much more criticism than the ideal cipher?

The random oracle model seems to be criticized a lot as lacking practical relevance, making wrong predictions, being programmable, etc. However, I have never heard such criticism about the ideal ...
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40 views

Attackability of one-time MAC when using block mode

Consider an encrypted message m which is encrypted with AES-CBC, so in addition to the message there is an IV supplied initially....
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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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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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429 views

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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Why commutativity gives an invariant subspace?

In A Generic Approach to Invariant Subspace Attacks the authors defined an invariant subspace as: $$F(u + A) = v + A$$ Where $F:\mathbb{F}_2^n \to \mathbb{F}_2^n$,$u, v \in \mathbb{F}_2^n$, and $A$ ...
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447 views

Round function in Feistel

Following situation. We got a Feistel network. And in each round we get following outcome on the left side: $$L^\prime = L_1 \oplus F(K_1, R_1)$$ Is there a possible key to get an unchanged outcome ...
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334 views

Problems key AES 128 key from last round key

I have written an algorithm in Python to find the main AES 128 key from the last round key. However, it doesn't work; I can not find back my main key. I know that the general principle is to do $w[i] ...
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How can I identify the linear equations for a block cipher with 4 different s-boxes?

For a block cipher with 4 different s-boxes (each used in an individual round) how can I identify the linear equations? Normally, for ciphers like AES with a single s-box, linear cryptanalysis can be ...
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1answer
533 views

Cracking a key using a combination of partial keys

As part of a cryptanalytic attack I've carried out, I used a divide-and-conquer strategy to divide the secret key $k$ into several equal-sized partial keys $k = {k_1, k_2, ...}$. The attack works ...
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328 views

Bit slice format of 4x4 s-boxes?

I have studied bit-sliced implementation in SERPENT block cipher and I need to use this technique to implement some 4x4 s-boxes. I couldn't find any procedure to obtain bit-sliced format in any paper ...
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Quantitatively measure the private information revealed by a matrix?

Suppose I have a private matrix $A$ that I want to conceal. I do the matrix multiplication $A^\prime =AK$ and $A^\prime$ is public. How can I quantitatively measure "how well" the matrix $A$ is ...
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Why the same characteristics cannot be used to recover all FEAL4 keys

I am trying to understand how differential Cryptanalysis works. Here I read how to use differential cryptoanalysis for cracking FEAL4 cipher. The big gotcha with this process is that you cannot ...
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How well analyzed are giant block length ciphers?

I have heard about ciphers that have a block length of 256 megabytes... That is huge. How well are they understood by the encryption community? I'm referring to PMC Ciphers. Has anyone used such a ...
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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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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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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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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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Usage of block and stream ciphers

I see that in many applications stream ciphers are common than block cipher - whether is Salsa20, AES-GCM, etc. In which cases would a stream cipher be preferred over a block cipher and vice versa? ...
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Can XEX or XTS modes be used with only one tweak? (part 2)

This question is similar to Can XEX or XTS modes be used with only one tweak? except I am interested to know if one can instead use $X = E_{k}(0) \otimes \alpha^j \\ C = E_{k}(P \oplus X) \oplus X$ in ...
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Is there a universal construction for Davies-Meyer hash functions?

My understanding is that there exists no strong pseudorandom permutation for which the Davies-Meyer construction is known to yield a provably collision-resistant hash function. Were I mean provably ...
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Memory required for known plaintext attack on general block cipher

Consider a block cipher on binary strings of length $k$ and keys which can be any binary string of length $n$. With $k2^n+k$ memory we can store all key/ciphertext pairs for a fixed plaintext and this ...
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Rivest - Time Memory Tradeoff Using Distinguished Points

A while ago, Hellman introduced a time-memory tradeoff for chosen plaintext attack on block ciphers. There is an innovation due Rivest using a concept of 'distinguished points'. I am looking for the ...
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What is the exact algorithm for encryption and decryption with hummingbird-1 cryptography?

I have really googled the entire internet but could not find a clear algorithm for hummingbird-1 cryptography.The problem emerges in the decryption module: What is the initial value of state register ...
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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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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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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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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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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, 173, ...
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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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Double encryption : what method is the best?

Given the block-ciphers defined as follows : $1.$ $C=$ $E_{k1}($$E_{k2}$$(M))$ $2.$ $C=$ $E_{k1{\oplus}k2}$$(M)$ $3.$ $C=$ $(E_{k1}(r),E_{k2}(r$ ${\oplus}$ $M))$ where $C$ is the cipher ...
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how to compute Bit Independent Criteria on a 4-bit S-box?

The idea of Bit Independence Criterion (BIC ) was introduced by Webster and Tavares . For a given set of avalanche vectors generated by complementing a single plaintext bit, all avalanche variables ...