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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Decrypting an AES string

I'm dealing with a website that loads a javascript file which eventually sets an AES hash/value (whatever it's called) as a cookie. The file is heavily obfuscated making it hard to understand certain ...
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Security of block cipher PRP(k⊕m)⊕k

Let $\mathcal S=\{0,1\}^n$ be the set of bitstrinsg of $n$ bits (for security parameter $n$). Let $P$ be a public Pseudo-Random Permutation of $\mathcal S$, efficiently computable in both directions. ...
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E.T.P …LCM can you decipher this / how to? [closed]

How to go about deciphering the text below? E.T.P. CAM 381A2LC M Any ideas would be gladly welcomed
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blockcipher-based MACs with variable length that are UF-CMA secure?

There are may papers and notes explain why CBC-MAC is only UF-CMA secure for fix length messages. Are there any other blockcipher-based (blockcipher and hash combined included) MACs that is UF-CMA ...
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What is Associated Data in AEAD?

I am trying to understand what is AEAD exactly. I am application developer not a cryptographer. I have been searching online for a while watched a bunch of Youtube videos, read through a couple of ...
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When using openssl enc -camellia256, what block mode does it use? [migrated]

When encrypting a file with openssl using the Camellia 256-bit cipher, you can explicitly specify the block mode (e.g. ...
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Ciphers with identical round keys

Ciphers like PRINCE or LED seem to use identical round keys. Does anybody know other cipher (may include hash functions, MACs or AEADs) that has the same property?
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Detection of Substitution attack on Block Cipher ECB mode [migrated]

This is from Christoff Paar's book on Cryptography Let’s assume a protocol for wire transfers between banks (Fig. 5.2). There are five fields which specify a transfer: the sending bank’s ID and ...
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Question about IND-CPA with block cipher mode with random IV (CTR$ mutation)

I know for a fact that CTR random is IND-CPA secure due to if an adversary want to break it, it will have to run a long loop where the $$\mathit{Adv}^{ind-cpa}_{CTR~random} = C(2^{n},q) - 0$$ However, ...
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Can combining two secure block ciphers be insecure?

Let's say we have two secure block ciphers FOO and BAR. Will a new block cipher built by combining them into a single new block cipher result in any insecurity? $$\begin{align} \operatorname{BAZ}_K(x)&...
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Randomness Testing of Block Ciphers with 256-bit Block/ Key length

NIST explained the process of "Randomness Testing of the AES Candidate Algorithms" in the document. The document explains the generation of 9 datasets for AES candidates with 128-bit block ...
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OpenSSL aes-128-cfb does not produce expected size

I try to implement aes-128-cfb myself for learning. To confirm whether my implementation is correct, I want to verify it with a well-known library such as OpenSSL. But then I noticed something strange ...
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Is there any block cipher which is also a pseudo-random-permutation over the key

As block ciphers are defined as a pseudo-random-permutation over the data (keyed with the key), I was wondering, if there are also constructions for which key and data can be switched and the cipher ...
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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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S-box for byte substitution where the s-box values are multiplicative inverses of 257

The title basically says it. We have a 16 by 16 s-box, $256$ entries total. And the first value is the multiplicative inverse of $257$, so $1 \times k \bmod 257, \space k = 1$. $2 \times k \bmod ...
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What are some good softwares to help create block ciphers? [duplicate]

I'm working on my own unnamed block cipher currently. It uses permutation networks, rotations (bitwise and quitwise) and bitwise operators. Are there any good softwares and/or libraries to program in ...
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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, ...
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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 to measure brute force speed of a computer for a give cipher with given number of keys for second for a given key space

As I understood brute force attack is measured in number of keys which in turns give time required based on speed of computer It is quite interesting find out how effective an arbitrary computer is ...
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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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