Questions tagged [blocksize]

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

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How to handle padding when encrypting a value smaller than the block size

I want to encrypt numeric identifiers into ciphertexts that are deterministic and not too long. I am considering AES-128 in ECB mode to encrypt 64-bit integers. How should I pad the 64-bit integers, ...
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What is the security of XTS block cipher mode of operation if the key is much larger than the block size?

I made two Linux kernel modules of Hasty Pudding Cipher with key size of 16896-bits (16384-bits key + 512-bits tweak also called "spice), one has 512-bits of block size, the other 128-bits. I ...
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Can I use a password hash to encrypt the private key of a bigger block size?

I have private keys that are close to 2kb big. Now I want to password encrypt them in the following manner. Generate the private key of an asymmetric encryption algorithm (public key can be derived ...
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By using a block cipher multiple times in a row is a 128-bit BC as secure as a 256-bit BC which uses his 128-key only as part of the message?

For given 128-bit numbers $S$ and $E$ we want to find a series of keys $k_i$ with $$ E = BC(BC(BC(.....BC(S, k_1),k_2) ..k_n)$$ We can either use a 128-Bit blocksize block cipher similar to AES (ECB ...
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Could Blowfish be extended to larger block/key sizes if increasing the level of sum and XOR operations?

Blowfish encryption is made as follows: ...
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What would be the maximum acceptable block size for disk enryption?

AES-256 in XTS mode has 32x16 (512) bytes of blocksize but there are some other wide-block modes of operation such as WCFB which accepts any block size. My question is: What is the maximum acceptable ...
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Can we combine two AES (with block size 128) to to a block cipher with a block size 192 which is collision-resistant?

Encrypting values with AES we only need about $2^{64}$ trials until we finding an already used value (some collision). This can be done these days with easy. To make this harder can we combine two AES ...
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How to crack sha256 used as streamcipher with known or guessable block

Using SHA-256 as a stream cipher appears to bear cryptographic weaknesses, however, I am not quite sure how to implement them in decryption. Assume that I have an encryption function using 64-byte ...
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What would be the consequences of using a IV smaller than the block size in CBC encryption?

Here I use a Threefish kernel module with CBC mode. But there is no hash function that can supply a IV with Threefish block size (1024-bits), so I use a 64-bit random IV inserted in the dm-crypt with ...
phantomcraft's user avatar
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What would be the disadvantages of very large block sizes in block ciphers?

Let's suppose someone creates a block cipher with 8192-bytes of the block size (65536-bits) or maybe 16384-bytes of the block size (131072-bits). What would be its disadvantages over a block cipher ...
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If converting a SPN Block Cipher to a version with larger block size, should its S-boxes be enlarged too?

Let's suppose I want to convert a Substitution-permutation network block cipher like Kuznyechik to a version with a larger block size. Should its S-boxes be enlarged too in order to achieve similar ...
phantomcraft's user avatar
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RC6 Integer operations in modulo 32 between two 32-bit blocks

I am new to cryptography and I am trying to code the RC6 (Rivest cipher 6) algorithm. The algorithm requires addition, subtraction and multiplication in modulo 232. If I am performing these operations ...
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Are blockciphers with 64-bit blocks still relevant (in any scenario)?

Facts Blockciphers are used to build encryption algorithms and compression functions for hash algorithms. (Unkeyed) Permutations are also complementing blockciphers in encryption and hashing. Large ...
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Why are block ciphers mostly used as stream ciphers?

Using a block cipher with using either CTR or GCM mode gives us a stream cipher, the only difference being that CTR does not include a MAC tag or AAD but GCM does. I think these are the most commonly ...
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AES-128 CBC encryption : ciphertext size seems too large

I am trying to encrypt messages with the AES symmetric algorithm using C language. I am using a key, an Initialization Vector (IV) and a plaintext, all with a length of 16 bytes. I expected as output ...
costola's user avatar
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How does using a really small block size reduce the key space?

I read this online and was confused: Modern block ciphers tend to have block sizes of 128 bits or larger, because if the block size is too small there is the same problem as a small key space as ...
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Should I split a 3MByte block into smaller blocks if using XXTEA?

Given that I have a 3MByte block of data, a random key and need to use XXTea, is there any advantage in splitting the block of data into smaller blocks and encoding / decoding each separately? Is ...
Ant's user avatar
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PGP private key format

I have a PGP Private Key from Bob. ...
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Bulk encryption with Triple Blowfish

Blowfish has a large key setup penalty, except is fast once it gets rolling. I believe that before AES optimizations became commonplace, it was faster than AES after the initial key setup. It's top ...
GuyOnInternet's user avatar
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How to securely encrypt/decrypt data with a maximum chunk size?

I am aware of the different modes so that a "raw" block cipher such as AES (ECB) can be used to encrypt more than a single block of data (CBC, OFB, CTR, etc) and modes such as AES-GCM which ...
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Why blake2b is the fastest despite it being 512 bits hashing algorithm?

I am struggling to understand why blake2b is faster than blake2s, despite that blake2b is 512 bits hashing. I hope someone can clarify that for me. I read the official paper for blake2b, but I am not ...
Hinton Zsh's user avatar
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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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The smallest secure block size [duplicate]

I have designed an encryption algorithm. The algorithm will probably use CFB mode. The algorithm can encrypt any block size and this does not change the size of the key. There are always about $20$ ...
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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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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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Finding a dataset to benchmark cryptographic algorithms

I’m planning to compare cryptographic algorithms in terms of encryption/decryption time, memory used etc. and for that I’m in need of a standard dataset consisting of plaintexts of different bit sizes ...
Display111's user avatar
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What if the hash code of the genesis block (first block in the block chain) is changed?

If so, what would happen? And if not, what are some limitations to it? The reason I am asking is because I just started getting into the cryptocurrency and am very interested in the cybersecurity ...
kiyanuDevs's user avatar
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How many required known plaintexts for an attack are considered insecure?

I stumbled over this problem when looking at symmetrical block ciphers with a relatively small block size like 32 bit. When I was looking at more common block ciphers with a 64 bit, I learned that ...
heroxav's user avatar
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Does the block size of a symmetric cipher impact the security of the cipher itself? [duplicate]

Will a 128 bit block have some security implication in post-quantum cryptography?
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Is it possible to determine the symmetric encryption method used by output size?

I'm attempting to identify the method of encryption for a black-box symmetric encryptor that produces blocks of output that are 4 bytes in length (e.g. small inputs fit in 16 bytes, then 20 bytes and ...
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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 ...
forest's user avatar
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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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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 ...
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14 votes
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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 ...
Future Security's user avatar
1 vote
1 answer
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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 ...
Xace's user avatar
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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 ...
Ruben De Smet's user avatar
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3 answers
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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 ...
catalin's user avatar
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1 answer
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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 ...
s n's user avatar
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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 ...
user2800708's user avatar
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1 answer
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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 ...
Nikhil L's user avatar
-2 votes
2 answers
1k 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 ...
FoxDonut's user avatar
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1 answer
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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 ...
Daffy's user avatar
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1 vote
1 answer
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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 ...
Daffy's user avatar
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2 votes
1 answer
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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 (...
Daffy's user avatar
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6 votes
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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 ...
Elias's user avatar
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8 votes
2 answers
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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 (...
Dennis's user avatar
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Is the risk of a collision for OCB mode mitigated by upping the block size of the cipher?

A paper published by Niels Furguson Collision attacks on OCB indicates that processing large amounts of data (somewhere on the order of $2^{32}$ 128-bit blocks) with a single encryption operation (...
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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 ...
BufferOverflow's user avatar
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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 ...
user avatar
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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 = ...
Jamal Kareem's user avatar