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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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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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Disabling the AES cipher suites without ECDHE key exchange algorithm [migrated]

Currently, we are having below cipher suites used in our platform. AES128-GCM-SHA256 AES128-SHA256 AES128-SHA ECDHE-RSA-AES128-GCM-SHA256 ECDHE-RSA-AES128-SHA256 ECDHE-RSA-AES128-SHA Post ...
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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 find efficient hardware implementation of sbox of lightweight block ciphers? [migrated]

Lightweight block ciphers are usually optimized for compact hardware implementations and sbox values are computed by implementing circuit using basic logic gates(AND, XOR, OR, NOR etc). What is the ...
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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 $2^n!$. ...
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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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Are all block and stream ciphers symmetric?

I'm trying to categorize my knowledge. Can I place block ciphers and stream ciphers under symmetric with RSA/DH under asymmetric?
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XORing with 1B in MixColumn transformations for AES encyption

In MixColumn transformations in AES encryption, do we have to Xor the shifted input with 1B only when the leftmost bit is high or is it like the number of times we shift it, and then when a one ...
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AES Diffusion at Block Level

AES provides diffusion at the block level by implementing the ShiftRows and MixColumn sublayers. The former permutes data on a byte level, wheres the latter combines blocks of four bytes. Can it ...
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Key, plaintext block and ciphertext block permutations question

Question: If a cipher’s key size is $k$ bits and its block size is $b$ bits, how many possible keys are there, how many possible plaintext blocks are there, and how many possible ciphertext blocks are ...
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Where would a “half round” come from?

In the hierocrypt-L3 description, the cipher takes 6, 7, or 8 rounds. Example source code also seems to follow this same specification of 8 rounds for 256-bit keys. Wikipedia shows 8.5 rounds for ...
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Lightweight block ciphers that support decryption with minimal overhead?

Lightweight block ciphers are mostly expressed for encryption routine and efforts are made to keep it as light as possible mostly in terms of gates count and power usage although some try to keep ...
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119 views

Encryption using matrix transformations

I have been thinking lately about a block cipher which takes a block of bits and arranges them in a square matrix. Then defining transforms on submatrices of the square matrix to scramble the bits. ...
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Implementations of GCM with 64-bit cipher

The paper The Galois/Counter Mode of Operation (GCM) describes mainly the 128-bit case, for use with 128-bit block ciphers such as AES. AES-GCM test vectors are available from NIST. The paper also ...
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To find nonlinear invariant of S box

I read a paper about a nonlinear invariant attack that is Y.Todo et. al's "Nonlinear Invariant Attack: Practical Attack on Full SCREAM, iSCREAM, and Midori64" In this paper's Appendix A., they said ...
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Combining stream cipher with block cipher for disk encryption

You take something like chacha20 and use the disk block # to seek it, xor it with the data block and then encrypt it with a block cipher in ECB mode. Would this be secure? Edit: To avoid getting ...
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What Feistel network block ciphers do not use data-dependent table lookups or modular additions?

I am looking for block cipher algorithms based on Feistel networks. The algorithm is required to not use data-dependent table lookups (or data-dependent rotations) or modular additions/multiplications....
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What characteristics define a function to be a block cipher

I'm trying to understand what characteristics or properties make the result of a function a block cipher. I understand that for a function to be a block cipher it has to be invertible and can't be a ...
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Encrypt twice with DES using same key ? $\operatorname{DES}(\operatorname{DES}(x,k), k)$

Does encrypting with $\operatorname{DES}$ in the following way $$\operatorname{DES}(\operatorname{DES}(x,k), k)$$ make $\operatorname{DES}$ as strong as $\operatorname{2DES}$ with 2 keys? Because we ...
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DES - encrypt 3 times - DES(DES(DES, k), k'), k) - stronger than 2DES with 2 different keys?

What if were to use DES in the following way: For a given plaintext x and key k we will perform: ...
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1answer
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Padding schemes reversal and space efficiency

My crypto course proposes the following padding schemes: $\operatorname{pad}(m) = m \mathbin\| 0^i$ $\operatorname{pad}(m) = m \mathbin\| 10^i$ $\operatorname{pad}(m) = m \mathbin\| 0^i \mathbin\| E(\...
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AES and DES - reusing the same round keys

Both DES and AES use key expansion to generate the round keys (for Feistel network or for the AddRoundKey operation). Each round uses a different key. Why do we need that? Why we cannot use the same ...
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321 views

Encrypting text using Caeser cipher in CFB mode

I am interested in learning and understanding cryptography and its application in every day life. One thing I can't seem to get my head around is encrypting plaintext using a block mode such as CFB. I ...
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Known Plaintext Attack for Modified OFB Mode of Operation

Question: Assume a variant of the OFB mode by which we only feedback the 8 most significant bits of the cipher output. We use AES and fill the remaining 120 input bits to the cipher with 0's: Why is ...
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Known Plaintext Attack on Block Cipher in ECB and CBC Modes of Operations

These 2 similar questions are from Paar's Understanding Cryptography. I could not see the answer so if someone can help I will appreciate: Question1: We consider known plaintext attacks on block ...
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Differences between Webcrypto (js) and Cryptodome.Cipher.AES (python) [closed]

I'm trying to understand the difference in output of AES decrypt (GCM mode) using Webcrypto and Cryptodome.Cipher.AES in python. ...
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AES - MixColumn transformation using the identity matrix

I'm analysing how different components of AES influence its security. However, I cannot find a good explanation to what happens if the MixColumn step would use the identity matrix. Intuition tells me, ...
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Combining classical attacks and quantum cryptanalysis

I'm regularly reading that AES-256 is secure against quantum computers because Grover's algorithm will only halve the key strength (i.e. "only" require $\sqrt{2^l} = 2^{l/2}$ operations instead of the ...
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Mixing block cipher modes (AES)

I'm trying to evaluate some changes to an existing codebase and one question that has come up is the block cipher mode. For legacy reasons we've been using AES in CBC mode, however it's unclear why ...
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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 is a PRP different from PRF? Both can be inverted to get the same input

PRP is said to be a bijective function which means that there is a one-to-one mapping with the output. And hence the output can be inverted using the decryption algorithm to get the same input. ...
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Why 3 round Feistel cipher are not common

Luby and Rackoff showed that 3 rounds are sufficient to make the Feistel networks a pseudorandom permutation. Question: why are 3 round Feistel Ciphers not common? My thoughts about the round ...
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283 views

How many rounds of encryption is ideal?

I am a newbie to the cryptography world. I am working on a custom algorithm that encrypts the data multiple times and results in a cipher text. It uses a different key in each round to encrypt it. I ...
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1answer
140 views

Why is stateful CTR slightly better than stateless CTR? Does the adversary know the value of counter in CTR mode of operation?

This might be a stupid question but is really fundamental in my understanding of CTR (I'm a newbie to cryptography theory). For CPA security, the adversary only knows the IV, and may query the oracle ...
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1answer
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What happens if the plaintext $m1$ from the first block of CBC is known to adversary?

The adversary is has unlimited access to the encryption oracle. My thinking is that upon receiving the ciphertext C = $<c_0,c_1,c_2...,c_n>$ it will just use $c_0$ to XOR it with $F_k(m_1)$ to ...
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Which block cipher mode to use?

I am working on securing a chat application, and I am not sure about which block cipher mode I should use. I am aware that I should avoid ECB as it's insecure, but what about CBC? I have watched ...
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1answer
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Does it matter how the IV and XOR are applied for a single AES-128 block?

I want to encrypt 128 bits of data. I understand the problem with ECB, so I generate a 128-bit Initialization Vector to use for encryption and send with the encrypted data. Is there a "best" way to ...
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3DES Feasibility and Security [closed]

Questions are as following: Is 3DES still feasible (in terms of security and performance) to use? Is 3DES safe to use in ECB mode? Every time on giving the same input (plain text) to encrypt via 3DES ...
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Tiny Encryption Algorithm (TEA) weakness

The Wikipedia states that different multiples of a magic constant are used to prevent simple attacks based on the symmetry of the rounds. The magic constant, ...
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1answer
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What happens at this part in the key schedule of a Speck…?

I am trying to figure out the process behind the Speck block cipher. I understand how XOR works (Exclusive-or) when you take 2 strings of bits and you want to XOR them together. However, in the key ...
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how to compute the correlation between two binary vectors?

We find different equations to compute the correlation or correlation coefficient: $\begin{align*} corr &= \frac{a+d}{a+b+c+d}\\ &= \frac{(a+d) - (b+d)}{a+b+c+d}\\ &= \frac{(a+...
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Intuition behind the asymmetry between invertibility of key and ciphertext in block ciphers

I have studied the design of block ciphers like DES and AES. I know the actual algorithms, but I don't intuitively "grok" the most important property of them: The most important property, is that ...