Stack Exchange Network

Stack Exchange network consists of 175 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

Visit Stack Exchange

Questions tagged [blocksize]

A block is a sequence of bytes or bits, having a fixed length (the block-size).

3
votes
0answers
41 views

Effect of block size on diffusion rate

I've always been under the belief that larger block sizes require more rounds to achieve full diffusion. Rijndael, SPECK, and SIMON, for example, have a round multiplier based on the block size. The ...
2
votes
1answer
50 views

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 $2^n!$. ...
2
votes
0answers
25 views

All-or-nothing transform that doesn't increase message size?

All of the AON transforms I've read about involve generating a key, which increases the message by a few bytes. Is there an AON transform that preserves the message size while still having avalanche ...
10
votes
2answers
348 views

What are the risks of using CTR mode with 64 bit blocks?

On DJB's blog he writes: I was one of about 40 people sitting in a meeting where the speaker, NSA's Louis Wingers (one of the Simon and Speck authors), falsely claimed that counter mode is safe for ...
1
vote
0answers
44 views

Comparing Ciphertext when there is a change in source plaintext p1

So, i know that if there is a change in source plaintext p1, c1 and rest of cipher blocks will be affected as c is required for the calculation in cfb. What i need to know is that how can i show how ...
0
votes
1answer
66 views

Blockcipher: key space define permutations of blocks

An $n$-bit block cipher is a keyed function $f_k:\{0,1\}^n\rightarrow\{0,1\}^n$. There are $2^n!$ such functions. Most block ciphers seem to have only $2^n$ keys, although there is an AES mode that ...
0
votes
3answers
179 views

what is the block in crypto

Sorry for the newbie question, I'm new to cryptography and I have difficulty in understanding what the block is. For example for DES algorithm the block size is 64 bits and the key is equal to 2 to ...
3
votes
1answer
203 views

Crypto algorithm identification (reverse engineering)

I'm trying to identify a certain cryptographic algorithm used in a program. From examining the executable, it appears it Has 16 rounds (plus 2 "rounds" for pre/post processing) Pads the input ...
2
votes
3answers
364 views

Can data over a very low bandwidth channel still be encrypted?

If you have a very low bandwidth channel, can encryption still be used? Another way of putting this might be, does encrypting well increase the size of the data significantly? Are there good ...
0
votes
1answer
72 views

Arbitrary length to multiple of block size

I would like to apply CTR mode on a arbitrary length which isn't multiple of block size. Is this possible without applying padding. In case we apply padding - how does the decryption algorithm know ...
-2
votes
2answers
488 views

Java - AES with 20 byte packet

I have 20 bytes of data that I need to encrypt and send to a bluetooth node. The main issue being that the receiving node only accepts 20 byte packets. Further, I cannot send multiple packets, one ...
0
votes
1answer
262 views

How can I create a block cipher of a reduced size given a fixed length block cipher? [duplicate]

I've been reading papers on variable sized block cipher constructions based on fixed-length block ciphers, and they tend to focus on increasing the block size. They either gloss over or speak too ...
1
vote
1answer
57 views

Is there a construction for a variable length block cipher that uses a fixed length one?

I'm looking for some way to take a fixed length block cipher (for example AES) and turn it into a variable length one with comparable security. The new length may be bigger or smaller than the ...
2
votes
1answer
153 views

Is this an acceptable way to increase the block size of a block cipher?

Note: This is all for fun and learning. None of this will go into production code. I'm studying something (sponge functions, specifically) that benefit from large block sizes. AES has a (...
6
votes
1answer
496 views

What is a wide block cipher and why does it avoid birthday bound problems?

I've recently heard the claim that wide block ciphers avoid birthday bound problems. Trying to figure out what exactly "wide block encryption" is, a quick search turned up this paper which is trying ...
6
votes
2answers
2k views

SHA-3 block sizes / bitrate calculation?

Looking into different papers and online documents, I find the following block sizes for SHA-3: SHA3-224: 1152 Bit SHA3-256: 1088 Bit SHA3-384: 832 Bit SHA3-512: 576 Bit Somehow I am unable to find (...
3
votes
0answers
126 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 ...
8
votes
1answer
5k views

What is the maximum size of the plaintext message for RSA OAEP?

OAEP is an important technique used to strengthen RSA. However, using OAEP (or any technique that adds randomness) reduces the size of plaintexts that can be encrypted. Assume for instance that OAEP ...
0
votes
1answer
632 views

number of blocks, and size of padding at 64bit

A message has 2003 characters, encrypted using a block cipher of 64 bits. What is the size of the padding and the number of blocks? I tried this, but I'm sure it's not the right thing: 2003 mod 64 = ...
0
votes
2answers
800 views

BLOCK SIZE in CBC and GCM

In case I am using CBC cipher in that case is it must that my encrypted text is in multiple of the BLOCK SIZE? Is the same true for GCM?
0
votes
1answer
137 views

Weird size of data transmitted in SSLv3

Note - I know SSLv3 is dangerous but that is why I am using it to do demo of POODLE attack using server and client written in python. So the question is I am using "ECDHE-RSA-AES128-SHA" cipher for ...
0
votes
1answer
99 views

Determining the encryption method [closed]

I am trying to reverse engineer a program that encrypts a file with a password. So far i figured out that the start of the file is always the string "DecryptOK" encrypted with the password of course. ...
1
vote
1answer
563 views

How is key length of feistel cipher calculated?

In Feitsel Algorithim If our block size is n, then our key length would be n x 2^n . But this does not make sense for me. For example: Let's assume that we have 4 bit block size. If our block size ...
3
votes
2answers
635 views

What's the proper choice for the block size of a sponge-based hash?

While having a discussion about the future of Keccak in our library, we came to the discussion of how to properly assign a "block size" to Keccak (or more generally any sponge-based hash if possible), ...
4
votes
0answers
304 views

XTS and 256-bit data blocks

I am developer of an application that uses XTS with ciphers which use 128-bit blocks (AES, Serpent and Twofish). I now want to use XTS with a cipher which uses 256-bit blocks (Shacal-2). AFAIK, I ...
3
votes
1answer
575 views

How does Feistel cipher create diffusion between blocks?

From what I've read about Feistel ciphers, I can understand that within each block, there is both confusion and diffusion. But it is unclear to me how a Feistel cipher permutates among blocks of data. ...
4
votes
2answers
2k views

How does blocksize affect security?

I have recently read about how blockciphers and how stream ciphers work, and although I think I finally got everything right, I'm still wondering how increasing blocksize increases security.
1
vote
1answer
246 views

How is the block size of a block cipher related to how susceptible the block cipher is to a birthday attack?

For example what can you say about how susceptible Triple-DES is to a birthday attack due to the fact that it has a block size of 64-bits.
1
vote
2answers
1k views

Block cipher block and key sizes

Is there any point in making block size less than key size in block cipher? For example, Russian GOST symmetric cipher uses 256-bit keys to encrypt 64-bit blocks. That means there are many keys which ...
1
vote
0answers
47 views

Why was AES limited by NIST to a 128-bit block size, although Rijndael could support more? [duplicate]

Rijndael could support block sizes up to 256 bits; this seems to be useful in AEAD. Wht did NIST restrict AES to a 128-bit block size?
2
votes
4answers
10k views

What is the typical block size in RSA?

If the input text is divided into a fixed block size for encryption and decryption, what could be the typical block size for RSA algorithm for different size of input like 8 MB, 16 MB, 126 MB and 256 ...
16
votes
2answers
2k views

AES - What is the advantage of a 256-bit key with a 128-bit block cipher? [duplicate]

I'm just trying to wrap my mind around the decision to select the 128-bit Rijndael as the AES cipher, even with 192-bit and 256-bit keys. Even with a 256-bit key, you only get 2^128 possible outputs ...
6
votes
1answer
4k views

How can there be AES-256-GCM, when GCM is defined for 128-sized blocks?

In both node.js and OpenSSL, I am seeing cipher "aes-256-gcm". However, in NIST Special Publication 800-38D, that defines GCM, I am reading: The underlying block cipher shall be approved, the ...
8
votes
1answer
1k views

Why is it OK for secretbox to leak the message length?

Most block cipher implementations expect users to pad so that the message matches the cipher block size. I understand the purpose of the padding is to make the input match the cipher block size. It ...
3
votes
1answer
189 views

Why is GCM only usable for ciphers with a 128-bit block size?

All block cipher modes of operation that I understand are generic schemes. They are not restricted by key size or block size. So why is GCM restricted to 128-bit block size?
2
votes
1answer
247 views

Subkey generation in OMAC1/CMAC with XTEA?

All implementations of CMAC that I have found so far use a block cipher with the same key and block size. However, XTEA enciphers a 64 bit block by using a 128 bit key. If ...
5
votes
2answers
4k views

Encryption of 8 bit block of data

I'm looking for a cryptographic algorithm that allows encrypting only 8 bits of data in a block. Is there any such algorithm that is considered secure? The reason I'm asking is because I have to ...
6
votes
2answers
360 views

Turning a 64 bit block cipher into a 128 bit block cipher

There are quite a few block cipher modes of operation that require 128 bits. There are also modes of operation where a higher block size than 128, e.g. a block size of 256 bit would even be practical. ...
0
votes
1answer
33 views

Reordering of multiple signed blocks

I would like to refer you to page 430 of the Handbook of Applied Crypto, where the authors said "... reordering of multiple signed blocks presents a security risk..." This statement was made in ...
0
votes
2answers
144 views

Multiple encryption of part of block

Is there any security risk in encrypting parts of blocks twice? I'd like to get around block sizes and make encrypted messages just as long as the unencrypted ones: Block 1: Byte 0 - 7 Block 2: Byte ...
3
votes
2answers
807 views

Block size vs key size confusion

I understand that if a block cipher has $k$-bit keys and $n$-bit input/output blocks, then if $k>n$, we can expect one message-ciphertext pair to narrow us down (I think?) to $2^{k-n}$ possible ...
1
vote
1answer
429 views

block cipher algorithms with variable block lengths

Rijndael supports block lengths of 128, 192 and 256. AES does not but Rijndael does. What other algorithms support variable block lengths? Or is Rijndael unique in that regard?
4
votes
2answers
904 views

Encryption in which ciphertext is bigger than plaintext?

Is there an encryption which would produce a ciphertext longer than the plaintext? (please ignore block-chaining IV stuff, which would result in a data being generated from the plaintext). The point ...
9
votes
1answer
885 views

Why should CAST5 and 3DES not be used for encrypting files over 4GB?

On https://www.gnupg.org/faq/gnupg-faq.html#define_cast, it says that Like 3DES, its 64-bit block size means it should not be used to encrypt files larger than 4Gb in size… How and why does the ...
2
votes
1answer
236 views

Calculate the security of a cascade cipher

I’m reading “The Security of Multiple Encryption in the Ideal Cipher Model” and I’m having a hard time understanding something. How do I calculate the bounds of cascade cipher from the following ...
5
votes
1answer
353 views

GCM: Math behind update of AAD after ciphertext has been processed

In the Bouncy Castle libraries, the GCM cipher implementation has an interesting property that does not seem described in the GCM papers (neither the NIST or the original paper): Some AAD was sent ...
2
votes
1answer
3k views

How can AES be considered secure when encrypting large files?

Why is AES considered to be secure when encrypting large files since the algorithm is a block cipher? I mean, if the file is larger than the block size, the file will be broken down to fit the blocks....
2
votes
0answers
85 views

Cracking an appliance's network protocol

I'm trying to crack my thermostat's network protocol. I've captured several rounds of network traffic and here is what I've got to work with. Communicating via HTTP POST The POST data is JSON ...
5
votes
1answer
595 views

Is it possible to generate a secure permutation F over 32-bit integers even if F(0) … F(n) is public knowledge?

I'm currently interested in the problem of generating random-looking URLs from sequential database IDs, like how they do it in link shorteners. One way to do this is to encrypt the sequential database ...
2
votes
1answer
3k views

Message lengths with AES CTR mode?

Im working on my own AES implementation as a personal educational exercise (not for production use), and a book im using as a resource states: In the case of CTR, we are merely XORing the output ...