# Questions tagged [compression-function]

A compression function takes two fixed-size inputs: a chaining value and a message and returns a fixed-size value.

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### CRYSTALS-Kyber Compress and Decompress function role

I was reading CRYSTALS-Kyber design. They have used compress_q(x,d) to scale an element of $\mathbb{Z}_q$ to $[ 0,1,...,2^d-1 ]$. The definitions of ...
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1 vote
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### Input/Output size of compression functions

I know that DES has 56 independent key bits, and that 3DES has 168 independent key bits by using 3 separate 56-bit DES keys. 3DES also has a block size of 64 bits. If I use 3DES as the underlying ...
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### Cryptanalyzing a hash compression function

I'm designing a hash function which uses a double-tree construction and a compression function $c(s,A,B,C,a)$ ($s$ and $a$ may be omitted when there's only one of them) where $s$ is a set of three ...
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### Recommended output filter for Rumba20 [closed]

Rumba20 is a compression function that maps a 192-byte (1536-bit) string to a 64-byte (512-bit) string. It's designed to provide collision resistance by using Salsa20 (or ChaCha20) with the ...
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### Iterated hash functions

In this image, It describes the basic idea of an iterative hash function. I am confused as to why the i value is set to be m+t+1 ...
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### How can a collision be generated in this hash function by inverting the encryption?

This is from Joan Daemen's Doctoral thesis Per page 64 (or PDF page 79) 4.7.1 Our approach Our goal is the design of simple and portable unkeyed and keyed cryptographic hash functions that are ...
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### What is the difference between data compression and compression in cryptographic algorithms?

What is the differences between data compression as used in e.g. the ZIP protocol and compression as performed in cryptographic hashes? Are there common properties as well, apart from creating a ...
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1 vote
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### "Reversing" SHA256 [duplicate]

Let's say I give you the sha256 hash of my password, which I'll call X. Now, the sha256 algorithm is a one-way function, meaning you can give it some input and get an output, but you can't get the ...
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In sha-256, when the words go through the compression function, they get added to the previous hash. In case of the first 64 rounds, they get added to the initial words ($\bmod 2^{32}$ of course). ...
1 vote
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### Idea behind internal SHA-2 functions

In the SHA-2 family, specifically SHA-256, there are functions called Sigma0 and Sigma1, which look like this: ...
191 views

### An unusual use-case for HMAC as compression function in web browsers

It's an unfortunate fact that, right now (2019), browsers don't expose standardized streaming hashing interfaces in SubtleCrypto. The only way to hash a file, is to ...
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### SNARK-friendly one-way compression functions

I'm looking for a one-way compression function that is secure and efficient within a zk-SNARK circuit. The motivation behind this comes from these considerations: SHA256-compression is highly ...
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### What characteristics define a function to be a block cipher

I'm trying to understand what characteristics or properties make the result of a function a block cipher. I understand that for a function to be a block cipher it has to be invertible and can't be a ...
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### Does there exist an “almost-ideal” compression function to compress an arbitrarily long bitstring into a single pseudo-random (unbiased) byte?

I need a function $F$ that compresses an arbitrary (unbounded from above) number of bytes into a single byte. The goal is creating a single unpredictable byte from an arbitrary bitstring. Here are ...
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1 vote
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### Davies-Meyer without XORing in the previous state

A Davies-Meyer ccompression function operates with $H_{i} = E(m_{i}, H_{i-1}) \oplus H_{i-1}$. Why not use $H_{i} = E(m_{i}, H_{i-1})$ instead?
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### Optimal compression ratio in compression function [closed]

So we'll have a transform that maps any of $A=2^a$ possible inputs to $B=2^b$ possible outputs. $a$ is generally fixed as the number of bits in the input, and $b$ is determined by the transform used. ...
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### Which compression functions are MACs?

The original HMAC paper based the security proof on the fact that the compression function of the Hash is already a MAC (appropriately keyed). How does this relate to common assumptions for ...
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