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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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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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, ...
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Encrypted text length in AES

I have created an application that will be able to read any file and encrypt it using AES Encryption. For efficiency, I am reading a block of data, encrypting it and so on. So for decrypting, I just ...
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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 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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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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Is there a possibility to create a public key block cipher?

Why there is no public key-based block cipher? It is known that the block ciphers are symmetric-key encryption. However, what is the motivation to not design a public key encryption that encrypts ...
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What happens if the first input block is identical to the key in DES?

In Data Encryption Standard (DES), M is the input message and K is the key of 64 bits size. M is divided into n 64 sized blocks (B1,B2,B3...Bn).
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Is there any way to find out two numbers such that the XOR between them is a given number?

I have the following operation I took from a cryptanalysis I'm performing for a specific CBC encryption where the challenger has provided the key: ...
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How can I decrypt this kind of variation of block cipher?

I have a block cipher $E$ that is a permutation of $\{0,1\}^\ell$ for key $K\in\{0,1\}^\ell$. $c = E_{K_1}(m \oplus K_1 \oplus K_2)$ for unknown keys $K_1, K_2$ is given. I need to figure out the ...
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How random are permutations generated from Feistel networks with a small number of rounds?

I wrote a toy pseudo-random permutation out of a Feistel network using blake2b. However, looking at the distribution of permutations for small n = 6, it's clearly not uniform unless many rounds are ...
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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 to apply a block cipher to (large) packetized communication when packets can get lost/erased?

In light of current events (Zoom getting strong headwinds for using AES-128 in ECB mode), I was wondering what the general approach is to packetized data encryption, where the packet length (UDP for ...
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Why do people criticize and mistrust the e-voting based block chain?

I am planning to implement an e-voting system based on hyperledger fabric blockchain, however, I came across many criticisms from well-known security experts like Josh Benaloh and others. The problem ...
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A possible error in the book

So I have been reading this book called "Implementing SSL/TLS using Cryptography and PKI" While going through it, this snippet of code got me baffled ...
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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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Is CBC really dead?

I developed a p2p-app in C# which sends and receives encrypted text messages (50kB). For encryption, my app uses 128-bit AES in CBC cipher mode. For each message it uses a new randomly-generated IV. ...
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Block cipher linearity (in relation to hill ciphers)

So while I knew hill ciphers were a form of block cipher (I'm assuming all the ones I have solved to date were ECB or something similar), so that got me thinking. Is it possible to use hill ciphers in ...
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Why mining bitcoins is difficult?

As I understand to obtain a Golden Block is to finding a nonce that matches a hash lower than a given target, as shown in this research-gate article. And here is a "py" kernel: Bitcoin mining python (...
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Does Grover's algorithm effect block size or only key size?

We know that Grover's algorithm can speed up cracking symmetric keys. Basically the keyspace is halved. This means we have to use at least a 256-bit key (to get 128-bit security). I heard somewhere ...
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Pohlig-Hellman exponentiation block cipher

Let $G$ be a group of order $n$ and let $e,d$ be integers such that $ed\equiv 1 \pmod{n}$. Then the exponentiation maps $x \mapsto x^e$ and $y \mapsto y^d$ are inverse maps on $G$. These maps give us ...
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Security of Pohlig-Hellman exponentation cipher?

I am looking into implementing Pohlig-Hellman exponentiation cipher and I would like to know how secure that algorithm is? I am guessing it's security relates greatly to the prime number used in it. ...
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How to break CBC cipher with partly known plaintext?

I'm learning some IT security and during practice I found an exercise I couldn't solve. I have an encrypted message with block cipher in CBC mode. The blocks are one byte long and the first block is ...
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Is there any practical use of reduced rounds of AES

There are lots of attacks which are on reduced block ciphers. There are practical attack on five rounds of AES-128five rounds aes broken in six minutes. I was just wondering if there is any practical ...
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How to prove IND-CPA on a cipher [duplicate]

I came across an exercise "custom" block cipher which looks like a counter mode implementation, but it is not exactly it. I want to check if this scheme is IND-CPA secure. Here is a picture of the ...
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Longer IV generation from a single block IV for ICBC

I was reading a question about ICBC here and it mentioned that you would need a $s \cdot n$-bit $IV$ if you would use $s$ "stripes", where $n$ is the blocksize. Say that we don't like transmit that ...
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Interleaved CBC

I've been researching cryptographic modes and came across an obscure mode called interleaved CBC (ICBC). I can't find a lot about it and am wondering if anyone knows anything more. It's basically CBC ...
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18 rounds encryption in embedded?

I have a power trace of a decryption. It seems like I'm watching a 18 rounds decryption algorithm. I cannot think about any other algorithm implemented in ASIC except Cammelia who has 18 rounds. ...
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Lai-Massey scheme

One rationale for Lai-Massey design is to achieve full diffusion in a single round compared to SPN and Feistel (hence less rounds number) due to use of multiplication-􏰅addition (MA) function. However ...
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Is CTR really equally secure than CBC?

Here is a typical cryptographic situation: A secret key exists that is only known to a sender and a receiver of messages. As it is hard to replace that key, since you either need a secure channel for ...
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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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Is a larger block size less secure against a brute-force known cipertext attack?

Suppose a ciphertext $c$ was encrypted from plaintext $m$, and we know something about the form of $m$; say, for example, that $m$ is English text. We can attempt to brute-force $m$ from $c$ by ...
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Is AES-256 a post-quantum secure cipher or not?

We know Grover's algorithm speedup brute-force attacks two time faster in block ciphers (e.g brute-forcing 128 bit keys take $2^{64}$ operations not $2^{128}$). That explains why we are using 256 bit ...
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Why didn't people redesign the key scheduling algorithm of DES to use longer keys instead of using 3-DES?

To overcome short key length of DES, Triple DES was devised. What would go wrong or why didn't people redesign the key scheduling algorithm to use longer keys?
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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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Why use an Initialization Vector (IV)?

Why use an Initialization Vector (IV)? How are IV's used? What are the advantages/disadvantages of using an IV? Why use an IV instead of a longer key in which some section of the key is public? What ...
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Rijndael vs. Serpent vs. Twofish: General comparison

Can anyone explain (or give a link to document about) why Rijndaal won the AES, especially comparing it to other finalists (Serpent and Twofish)? What criteria were used to make decision? Or is there ...
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Is it possible to perform a meet-in-the-middle within a block cipher?

Standard meet-in-the-middle explanations show that you can perform a meet-in-the-middle attack on a repeated block cipher such as double-DES (performing DES twice in a row). However, block ciphers ...
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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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What is the difference between PKCS#5 padding and PKCS#7 padding

One runtime platform provides an API that supplies PKCS#5 padding for block cipher modes such as ECB and CBC. These modes have been defined for the triple DES, AES and Blowfish block ciphers. The ...
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Would using perfect S-boxes in F function of an unbalanced Feistel network produce a simple and secure cipher?

When I read some articles about cryptography on wikipedia, I took note of the following statements (please, correct me, if any of these are wrong): Perfect S-box (based on bent functions) provides ...
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Luby-Rackoff theorem for Generalized Feistel

I was reading about Luby-Rackoff theorem from various sources: [1], [2], [3], which says you need at least 3 rounds of a $2$-branch Feistel network to get a PRP if the underlying $f$ function is a PRF....
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What are the general rules to follow if I want to design a block cipher suitable for file encryption? [duplicate]

Can you please tell me what should be the quality of an encryption function using a key size of 256 bits... I realized from my previous question that a simple XOR of plaintext (256 bits) with a ...
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Does changing the order of the steps within a round affect the security of AES?

I was trying to understand the internal structure of AES (Advanced Encryption Standard) The Standard order of steps within a round: Substitute Bytes Shift Rows Mix Columns Add Round Key Substitute ...

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