# 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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### Design flaws when designing a MAC-function

Suppose we want to build a very basic hash function like this: $$c_i = h(c_{i-1},x_i) = c_{i-1}\oplus x_i \quad\quad mit \;\; i\geq1, c_0 = \operatorname{IV}$$ Now suppose we'd use this hash-function ...
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### Check linear and differential cryptanalysis of a new block cipher [duplicate]

I am developing a new block cipher primitive. I would like to test its robustness against attacks of linear and differential cryptanalysis. Is there a tool to evaluate this using only plaintext and ...
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### Good key update algorithms for key evolving schemes

In schemes like SAKE(Symmetric-key Authenticated Key Exchange) , the master keys (used to authenticate parties and generate session keys) are evolving, ie undergo transition $K=update(K)$. What is a ...
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### “16-bit DES encryption algorithm”

I need to use encryption software from PSpice for my SPICE model. In a user guide on page 71 they claim to use a "16-bit DES encryption algorithm". According to the wiki DES is using a 64-...
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### Is there any cryptographic scheme with block size more than 128 bit

I am asking this question out of curiosity if there are any cryptographic algorithm that has a block length more that 128 bit. I am new to cryptography and I didn't know any. AES 128, 192, 256 all ...
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### Are the Serpent Test Vectors incorrect?

I have recently written an implementation of Serpent and was testing it against known vectors to no avail. Using 256-bit key, I compared my encryption to the test vectors located here: http://www.cs....
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### How many repetition in Ciphertext-Only-Attack?

Given the following algorithm, how many times do I need to repeat step 2&3 to complete the attack? Please note that block size and key are 128 bits. Step1: Prepare set S of size 2^k which contains ...
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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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### Can repeatedly encrypting a message with a secure cipher ever produce the original input like what happens in ROT13?

Lets say we have a message 'm'. We encrypt 'm' using a stream cipher or any other type of cipher with cryptographic security that produces an output equal in length to the length of 'm'. Then if we ...
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### how to Design an AES like block cipher with block length and key length equal to 64 bits

Design an AES like block cipher with block length and key length equal to 64 bits.
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### 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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### What is the difference between key size and block size (for AES)?

We are working on AES and want to develop a website which should provide an encryption facility but we are not understanding the difference between key and the block size. More appropriately what does ...
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### RCON for AES 192 and 256

This is question about how many RCON values are used in AES 192 and 256. I read the NIST Publication introducing AES and in Appendix A.2 and A.3 (Key Expansion for 192 and 256-bit keys), only $rcon_1$ ...
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### Is $f_N$ in this hash function is Matyas-Meyer-Oseas scheme?

I propose a hash function as following: H is an Merkle-Damgard hash function with a compression function $f: \{0,1\}^{3n} \to \{0, 1\}^{2n}$. Output function $f_N: \{0,1\}^{3n} \to \{0, 1\}^n$. M is ...
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### Do ciphers with a key size of 64 bits still have use under very limited attack models?

If we have a very performance critical interactive session, like a video game, running on a trusted platform (video game console). If our goal is to prevent exfiltration of up to date information (...
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### Block cipher modes used in real life

There is a number of block cipher modes, including ECB, CBC, CFB, OFB, CTR and others. After reading "Serious Cryptography. A Practical Introduction to Modern Encryption" by Jean-Philippe ...
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### Are there issues in how cryptography is used in this short code sample?

I want to encrypt some small text files so I can put them online, in non-restricted access (not the best idea, but it's cheap and simple). I know I should use something like Truecrypt instead of this, ...
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### Attack against $\operatorname{DESA}_{k,k_1} (x) \underset{\text{def}}= \operatorname{DES}_k(x)⊕k_1$

Problem 5.16 in this pdf, from the book: Christof Paar, Jan Pelzl, Understanding Cryptography 5.16. This is your chance to break a cryptosystem. As we know by now, cryptography is a tricky business. ...
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### What are the relations between cryptanalysis of block ciphers such as AES and Kendall's tau coefficient?

Studying AES on Wikipedia, I noticed a statement regarding some ongoing studies on the use of Kendall's Tau coefficient in cryptanalysis: According to the Snowden documents, the NSA is doing ...
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### Block cipher unconditional security under ciphertext only attacks

Does there exist a block cipher which provides security against ciphertext only attacks, which has an unconditional security proof guaranteeing this is true? I know the existence of a provably secure ...
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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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### One-time pad with public key changes

Let $k$ by a randomly generated $n$ bit string, and $m$ a $n$ bit message. Then putting $c = m \oplus k$ is the one-time pad and is perfectly secure. Now assume the transmitter and receiver wanted ...
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### PRESENT cipher algorithm Big “O” Notation

I am currently doing a research paper on the PRESENT Cipher algorithms and one of the components that I need to include is time and space complexity. I have tried reading academic articles and ...
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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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### One round of AES-128

Maybe it’s a silly question, I’m interested in the best known/chosen plaintext attack on only one round of AES-128. More specifically, what is the smallest number of known/chosen plaintext/ciphertext ...
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### Perfect secrecy and block ciphers

Consider a block cipher that encrypts bit strings of length $n$, where the key-space of the block cipher is of size $2^{kn}$, $k \geq 1$. My understanding of perfect secrecy is that a system is ...
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### Should I use ECB or CBC encryption mode for my block cipher?

Can someone tell me which mode out of ECB and CBC is better, and how to decide which mode to use? Are there any other modes which are better?
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### How to find the inverse of a 3x3 MDS matrix

I implemented a block cipher similar to AES. But the reason I can't decrypt is that I can't get the inverse MDS matrix. The MDS matrix I used is a 3x3 MDS matrix on $GF(2^8) \implies GF(2^8)$ like AES ...
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### Proof of probability of weak keys in block cipher

My textbook has the following theorem: Given a block cipher with a key length of K bits and blocksize of n bits as well as t plaintext-ciphertext pairs, the expected number of false keys which ...
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### Error during transmission with one plaintext block (Cipher block operation)

I was given a graph like this, and the problem was stated as: if something happens to P2 then only P2 would not be retrieved on the decryption side, and all other P's can be retrieved normally. I do ...
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### How is a block-chain stored?

Background of the problem: I am undertaking a blockchain course at University and I have taken up a case study for blockchain that relates to logistics. The requirements are simple, yet I am confused ...
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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 do I find out whether a message in CBC has padding?

When a program is decrypting a message encrypted by CBC mode, what does it find out, whether the last bytes of the last block is part of the padding or part of the plaintext? Example: Use CBC mode ...
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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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### Key Reuse in this simple cipher

Consider the following very simple cipher, where, to send length $n$ bit-strings, we randomly correspond the numbers $1, 2, 3, ..., N$ with $n$ bits, so the key is the correspondence between each ...
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### How to properly cypher 16 bytes message containing a CRC checksum?

This question arose from a new context under early developments. If cryptographic operations are not well chosen or badly sequenced, please do notice it along your answer or comments: I am a layman in ...
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### Correlation power analysis attack on Camellia

I'm investigating a correlation power analysis on a black-box ASIC I have. I want to learn about correlation analysis and it seems this ASIC implements Camellia. I searched for researches that did ...
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### Block ciphers as secure as AES but immune against cache timing attacks?

This is a question out of curiosity! I really don't know of any other block cipher that is as secure as AES that has a substitution permutation structure and can be used as an alternative to AES to ...
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### 32bit AES equals to 8bit AES?

I am looking into an article about 32bit CPU/MCU software implementation of AES and there are some weird changes over there. Like shift-rows. It seems that the shift-rows in the 32bit implementation ...
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### Weakness in a CBC-like XOR cipher

A simple symmetric encryption algorithm can be written as follows: Input message M and 64 bit key $K$ Divide M into 64 bit size blocks $B_1...B_n$ Get first block $B_1$ and perform bit-wise $\oplus$ ...
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### How does pairing letters increase the security of a substitution cipher?

Hello can someone help me to understand this : One way to reduce this problem is to increase the size of the cipher alphabet. Rather than considering our cipher alphabet to be just the 26 letters, ...