Questions tagged [hash]

A cryptographic hash algorithm is a function which takes a variable size input and produces a fixed size output. The algorithm makes it difficult to find two inputs with the same output or reconstruct the input from the output.

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What level of security is provided when a Feistel Cipher is used as a round function of another Feistel Cipher?

Recently, I was reading: Are there any specific requirements for the function F in a Feistel cipher?, and the answer posted mentions a Feistel Cipher named Turtle, which uses a four-round Feistel ...
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LT codes with Homomorphic hashing

I have been working on a project implementing LT codes with Homomorphic hashing (inspired from http://blog.notdot.net/2012/08/Damn-Cool-Algorithms-Homomorphic-Hashing and http://blog.notdot.net/2012/...
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Second preimage attack complexity when the digest is malleable

An attacker has a hash $h$ and a message $m$. He wants to find $m^\prime$ with the same length as $m$ which hashes to $h^\prime$. However, $m^\prime$ can only differ from $m$ in the 1 bits (i.e. ...
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Efficient proof of knowledge using Carter-Wegman hash

A verifier wants to ensure, with only little exchange of data with other systems, that a large block of data $M$ that the verifier holds is also available to some other system(s). It is not an ...
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File Encryption (EFS method)

I'm implementing (for learning purposes) a file encryptor, which uses the following method (from http://en.wikipedia.org/wiki/Encrypting_File_System). I have 3 kind of keys: RSA public/private, AES ...
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P-Complete hashes, hashing to a larger set

Historically hashes have been from a large set (say 256 characters) to a smaller set (256 bits). Also, hash functions that are P-complete have no known parallel algorithm; they must be computed ...
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475 views

Hash function as secure as one-time pad?

We know that the one-time pad is provably secure as a cipher to encrypt some data. Is there an algorithm which does the same just as a hash function? Can we get a provably secure hash function? Maybe ...
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147 views

Hash multiset to point on elliptic curve where $A = 0$

I want to hash a multiset to a point on the elliptic curve $y^2 = x^3 + 3$ over a finite field of some 254-bit prime order, where $P = 3 \pmod 4$. Moreover, I want this hash to be incremental, in that ...
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Is there any way to check hash of homomorphic encrypted data?

I need some algorithm that satisfies: H - hash function Enc - encryption function (using public key) M - secret data $Enc(H(M)) = H(Enc(M))$ Let this system exist: the First person has a secret ...
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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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Calculating probability of finding collisions in an invertible hash function

Assuming we have an invertible hash function H that takes inputs of size 2n and outputs hashes of size n, I use the following algorithm to find collisions: Choose a random M as input. Compute its ...
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Implementation Attacks on Hashes

So I am familiar with attacking implementations of block ciphers via side channel attacks, cache-timing attacks, etc. What implementation attacks are there against hashes or hashed based functions (...
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360 views

How hard is a known prefix hash preimage attack on SHA-2?

Suppose the attacker knows $X, Z$ such that $H(X || Y) = Z$ If bit-length(Y) < 60 then a brute force attack is possible. What if ...
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A “concatenation-like” cryptographic operation that has a fixed output?

I am designing a cryptographic protocol, and I was wondering whether or not there exists a simple cryptographic operation $\diamond$ that is: Associative: $(x\diamond y)\diamond z = x \diamond(y\...
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Impossible to construct collision resistant hash function from one way function

[Simon 98] showed that it is impossible to construct collision resistant hash function from one way functions in a black-box way. I read the paper but barely understood it. Is there any source, where ...
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How to map the points of an elliptic curve cyclic group to $\mathbb{Z}_q$ using a hash function?

Let $E$ be an elliptic curve defined over $\mathrm{GF}(q)$, where $q=p^r$. Let $G$ be a cyclic group of points of $E$. Then how we can map points of $G$ into $\mathbb{Z}_q$ using a hash function.
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Combining multiple hash functions for HKDF

I'm trying to apply the hash function combination method described in https://eprint.iacr.org/2013/210.pdf to a HKDF PRNG construction. Thing is it needs 4 uniformly random parameters/keys chosen by ...
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Can a stream cipher with a big S-box create ASIC-resistance?

For a proof of work system a hash algorithm generates one pseudorandom index each round, in the same manner as RC4 and Spritz do. And it uses a bigger S-box, e.g. 2^20 numbers. The actual hash would ...
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361 views

Hash Function Properties

What i am trying to understand is shown in all three properties of secure hash function, i will focus on pre-image resistance. Pre-image resistance: given a hash h, it's difficult to find m s.t. H(m)=...
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377 views

Optimal way to get checksum digits from hash?

Let msg be some data we want to hash using an arbitrary digest size algorithm such as blake2b. From that hash, we would like to compute a checksum field with n decimal digits. If we suppose that the ...
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How to map a handful of numbers in a large range to a much smaller range deterministically without collissions

I want to map a set of long integers (say around 100 different values) that can span the entire 64 bit integer range to a much smaller range such as from 0 to 25000 deterministically with almost zero ...
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207 views

Signature verification with only hash (without full data)

I am looking for a way to sign a file, such that someone else can verify that I had this file, even if they only know a hash. I want to prove that I have the full file to someone who only has the hash....
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How does use of HMAC affect hash function combiners?

In the comments of this question, Ricky Demer proposes a function $HMAX$ to combine two $HMAC$s based on different hash functions: $HMAX_{H0,H1}(⟨k_0,k_1⟩,m) = HMAC_{H0}(k_0,m) \oplus HMAC_{H1}(k_1,m)...
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Infinite depth BLAKE2b tree hashing

In the BLAKE2 paper, the authors define Maximal depth (1 byte): an integer in [1, 255] (set to 255 if unlimited, and to 1 only in sequential mode) Node depth (1 byte): an integer in [0, 255] (...
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How many bits of cryptographic security do these XOR Mid-Square random numbers have?

This pseudo-random number generator uses only XOR and Mid-Square operations: Get one secret 128-bit truly random seed $R_0$ Generate a million secret $R_j$ by the recurrences (for $1\le j\le 1000000$)...
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Estimating the Security of SIS-Based Signature, by verifiying a subset of coordinates?

As I understood, the GPV signature scheme works as follows: KeyGen($1^n$) : Generate a Lattice with public $A \in Z_q^{n.m}$ and a secret trapdoor $t$. Sign $m$: compute $\vec y = H(m) \in Z_q^n$ ...
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Strategies for fast integrity attestation over multi party computation for large messages

I am looking for ways to efficiently perform integrity attestation over large messages using multi party computation. While, hashing (SHA1, 256...) is one alternative but it's painfully slow and grows ...
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How to ensure integrity of encrypted data having hashes only?

Having: plaintext $P$ a hash function $H$, encryption $E$ with key $k$ $h_p = H(P)$ and $h_{E_P} = H(E_k(P))$ Is there an encryption scheme $E$ or hash algorithm $H$ that allows proving that $h_{...
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Secure hashing when the input comes from a small space

I have a problem that I'll simplify here. Let's say I want to store a table of {person, fav color}. Only, I don't want anyone to ever be able to look up a person't favorite color; the only thing ...
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359 views

Is it possible to prevent length extension attacks

Is it possible to prevent length extension attacks without using any other hash algorithm than SHA-1?
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Merkle tree to prove integrity of subset of data

I'm thinking up a scheme where a merkle tree is used to ensure integrity of a set of data items, and different parties are allowed to verify the integrity of different subsets of that data. Let's ...
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The right way to encrypt sensitive data with an user password

I am developing something like 1Password and I want to be sure that method that I use is secure. Main idea of the app is storing user passwords, and is protected with a global master password. Main ...
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Does there exist a highly irreversible hash function or a highly irreversible pseudo-random number generator?

In my previous question, I asked about potential symmetric cryptosystems which are designed to be computed by a reversible computer. I am now wondering if there are any cryptographic hash functions ...
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Homomorphic set hash with membership proof

I'm trying to solve a problem that goes as follow. There is a set of item and that set is very large. Several actors needs to exchange information about this set, such as adding and removing element ...
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191 views

Hash function for sets with efficient membership testing

Is it possible to construct a collision-resistant function that hashes a set of values but also allows membership testing? Essentially, I'm looking for a pair of functions, $\text{H} : \{0,1\}^{**} \...
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Rabin fingerprint preimage with given length and alphabet

The Rabin fingerprint looks something like this: ...
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Analytically finding magic constant in reversible hash function

The below integer "hash" functions form a pair of reversible hash functions: ...
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Hash function onto a Schnorr group

Let $q$ be a given random prime of $2b$-bit for some security parameter $b$ (say $b=128$); let $p$ be a given much larger random prime with $p=q\cdot r+1$. Let $(G,*)$ be the (Schnorr) subgroup of $\...
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how to verify subset of hashes from hash list

I need to verify a couple hashed (SHA256) integers from a list of approximately 50 hashed integers in a message. The list can be divided into groups. The (plain) integer data is not in the message, ...
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Cyclic group collision resistant keyed cryptographic hash function

I am self-studying cryptography from a book and have come across a really difficult question that's doing my head in. I would be very grateful if somebody can help me out here with this difficult ...
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Signing integers and UUIDs and restoring them from a third party server without tampering

I'm educating myself about cryptography. I know this is a long one, but please, bear with me. :) Now I have a scenario in my mind and I'd like to ask if what I've thought is sound (enough), what more ...
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Discrete logarithms: large prime modulus vs. large semi prime modulus

I have a cryptography homework question, in the question it says a cryptographic hash function in the form of $f(x) = g^x \bmod n$ , where $n$ is a very large prime (1024bits and more), $g$ is a ...
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Question about modifying the MD5 plain text to cause a collision

I was trying to understand MD5 and got stuck with this question, its from Michael sipser's book on Information security Principles and Practice, 2nd edition The MD5 collision in Problem 25 is said ...
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How can I prove that this encryption scheme from a random oracle is secure?

I am reading this example: A random oracle is an ideal object. What makes a random oracle convenient for proofs is the part about knowing nothing on the output for a given input if you do not ...
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What's the difference between a hash chain, transaction chain and a blockchain?

Can anyone explain to me what the difference is between a hash chain, transaction chain and a blockchain?
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Is it safer to use timestamp in REST authentication?

I am working on a personal project to build a REST authentication system which authenticate clients via three parameters: ...
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Signal Key Stretching Regarding Their Blog Post

I just read this blog post (Signal - Technology Preview for secure value recovery) by Signal and something is not quite clear to me. In section "Stretching beyond a KDF" they generate a ...
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Best practice to hash multiple attributes

I have to generate a signature for concatenation of multiple independent values say A | B | C. Two options are : concatenate all attributes and generate the hash. Then Sign hash. $$ signature = Sign\...
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Does fixing partial bits of a SHA256 hash increase the probability of finding a preimage using the meet-in-the-middle attack?

Let’s say I want to find a partial rather than full preimage using the meet-in-the-middle attack. For example, my goal is to find a preimage so that the first 76-bit output of the hash are all ...
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Can a fuzzy hash be used as a less accurate merkle root?

If I had many, say a million random numbers (technically public keys.. ~600 bytes each) and I wanted to hash them all together using a fuzzy hash, how accurately could I test that a given hash was a ...