Difficulty of finding an input string that hashes to a given value

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3answers
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Is it possible to brute-force a hash algorithm of 32 bits

I have a 30000 user passwords and usernames stored in a table on a database in which the passwords are stored as hash where ,I need to guess the passwords for all users and I have the hash function ...
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2answers
59 views

If we know a sha256 hashed value and its message length can we find the message?

Is it possible to find the original message if we know a sha256 hashed value and its message length?
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0answers
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Does a partial preimage attack imply a preimage attack?

Let's assume we have an $n$-bit hash function and a $b$-bit partial preimage attack that is faster than brute force. Does this imply a faster than brute force preimage attack on the whole hash? It ...
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1answer
29 views

First preimage resistance check

So I have $H: \{0,1\}^* \rightarrow\{0,1\}^n$ a hash function resistant to the second preimage and to collisions. Let there be a function $H' : \{0,1\}^* \rightarrow \{0,1\}^{n+1}$ with the following ...
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2answers
111 views

Same 64-bit preimage resistance security for SipHash and SHA-512/64?

If I have to chose a 64 bit preimage resistant hash function; will there be any difference in security between SipHash and SHA-512/64 (SHA-512 truncated to 64 bits)? How long will it take an ...
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1answer
43 views

Is it safe to use fast/small hash for key identity?

I have to roll database access keys often, but it's important to know which key each system is using in order to avoid unavailability. I want the systems to report that without exposing the key itself,...
4
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1answer
153 views

Is finding collisions in a part-hash not often enough a bad problem?

My situation: I've been working now for a couple of months on my own unique hash function, I've changed it many times and had two main versions but I won't bore anyone with the details of my work; at ...
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3answers
9k views

What are preimage resistance and collision resistance, and how can the lack thereof be exploited?

What is "preimage resistance", and how can the lack thereof be exploited? How is this different from collision resistance, and are there any known preimage attacks that would be considered feasible?
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3answers
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How to reverse this hash function?

I have a function that takes an $m$ byte inputs $x_i$ and maps it a 32 byte outputs $y_j$. The hash function is defined as: $$y_j = \sum_{i=1}^{m} (x_i)^{i-1} \pmod {127}$$ The input is restricted ...
5
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1answer
57 views

What is the security strength of an n-bit HMAC?

HMACs depend on preimage resistance, or so I have read. The security strength of hash function with an $n$-bit output against collision attacks is $2^{\frac{n}{2}}$. Against preimage attacks, it is $2^...
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2answers
73 views

Decrypting SHA-256 with SHA-256 as input?

Is there a way to somewhat decrypt a SHA-256 hash, provided that the initial string was also a SHA-256 hash? If not, could anyone link me to a rainbow table containing some of the $16^{64}$ possible ...
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1answer
38 views

Create a potential input for sha-256 hash given a substring of input?

Let's say I'm given a specific SHA-256 hash. Further assume that the SHA-256 input, that yielded this hash contained a known sub-string. Is there a way to find the input (containing the specific ...
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1answer
49 views

Does absence of second pre-image resistance infer absence of pre-image resistance?

Talking about hash functions, I know the hierarchy is as follows: Pre-image resistance Second pre-image resistance Collision resistance Where each property implies the one before it so 2nd pre-...
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1answer
34 views

Determining collision resistance of a function?

I am having trouble trying to understand if the function is collision resistant. As I understand it, preimage resistance means that it is hard for an adversary to find two messages that produce the ...
1
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1answer
99 views

Why does a fixed key not make MACs like CBC-MAC or GMAC into secure hash functions? [duplicate]

Why does setting the secret key to a fixed, public value not make MACs like CBC-MAC or EMAC into secure unkeyed cryptographic hash functions? In other words, why is the resulting hash function not ...
2
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1answer
120 views

Are there any successful preimage attacks?

Are there any successful preimage attacks on any serious cryptographic hash algorithm or at least on serious-looking non-cryptographic hashes (like MurmurHash)?
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7answers
4k views

Why can't I reverse a hash to a possible input?

I'm going to provide “proof” why a hash function can be reversed, and I hope you can tell my why I'm wrong So, a hash function can be implemented as a series of logic gates. All logic gates can be ...
2
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1answer
74 views

Does including the size of the hashed data improve security over just the hash

Trying to settle a discussion at work. We are mostly concerned with pre-image resistance. Suppose we have a common secure hash function $H$ (e.g. SHA-1/SHA-2) and some message $M$ we want to hash. ...
2
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3answers
690 views

Computational feasible to reverse MD5SUM?

This might be out of ignorance, I apologize, but how complex of a problem might it be to generate a file of size $N$ whose MD5SUM is $X$? For example, ...
3
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3answers
851 views

For any hash value, is there an infinite number of inputs that hash to it?

Bruce Schneier writes (back in 2005) in a post on cryptanalysis of SHA-1: SHA-1 produces a 160-bit hash. That is, every message hashes down to a 160-bit number. Given that there are an ...
0
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2answers
74 views

hash functions preimage and second collision resistance [duplicate]

A hash function H is secure in terms of pre-image resis-tance. Does this imply that H has second pre-image (aka weak collision) resistance?
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2answers
134 views

Would it matter if my miner was hashing random vs incremental values?

I'm working on my miner for my "game" site that's basically a pre-image attack from a hash posted online. You submit a hash input, it's hashed, and your score is the hamming distance (the number of ...
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1answer
125 views

Has there ever been more then a theoretical difference between preimage resistance and second preimage resistance?

In other words, has there ever been a point in time in which having the content of a message has actually helped break a hash function?
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2answers
807 views

SHA-256 Reversing A String of Equal Length

I've done a lot of reading on how SHA-256, I've found that SHA-256 is irreversible because more data is fed into the hashed string than the hash string contains. But, what if the data that was ...
5
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1answer
258 views

Can iterated hashing be used to mitigate collision and preimage weaknesses?

How much security does double hashing add regarding collisions and preimages? Is it helpful to iterate a hash function even more times than two? For example, can MD5 be fixed (in practice) by ...
3
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1answer
97 views

Does SipHash have 64 bit (second) preimage security?

I was not able to find an explicit statement about the first and second preimage security of SipHash-2-4 or SipHash-4-8 anywhere. I think it should be 64 bits, but I'd feel safer if somebody who has ...
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0answers
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Example: pre-image resistance to second pre-image resistance

It is possible to convert a pre-image resistant function $f:\{0,1\}^{n}\rightarrow \{0,1\}^{n}$ to a second-preimage resistant function? I am thinking to use a pseudo-random generator and construct ...
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2answers
229 views

Signature and hash function properties

I understand the need for the hash function to be collision resistant and second pre-image resistant. For what reason, exactly, does a hash function need to be pre-image resistant? If this property ...
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2answers
147 views

SHA-512 partial preimage

I was looking at how SHA-512 works, and I have a question. Imagine I want an input $N$ of 512 bits whose hash in hexadecimal starts with the digits 12345678. How ...
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2answers
2k views

Why does second pre-image resistance imply pre-image resistance

I am studying hash functions. I can understand why collision resistance implies second preimage resistance, but I don't get why second preimage resistance should imply first preimage resistance. ...
2
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1answer
293 views

Concatenation of two strong hashes may have striking weakness

For any hash functions $H_0$ and $H_1$, it is easily proved that their concatenation $H_0\|H_1$, defined by $(H_0\|H_1)(X)=H_0(X)\|H_1(X)$, is at least as resistant as the strongest of $H_0$ and $H_1$ ...
0
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1answer
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Complexity of preimage resistance

Why is the complexity of pre-image resistance $2$ raised to the power of $n$? I have been looking in everything lecture about pre-image resistance, but I still didn't know the answer.
2
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1answer
113 views

Is it possible to deduce the value of a string when given its hash and multiples hashes of the string + other known strings?

To be clear, I have access to $H(S)$ and as many $H(S||K)$ as I want. Every string $K$ is known. Can I use the extra information to my advantage and try to deduce $S$, without requiring bruteforce? Or ...
3
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1answer
287 views

Is $H(x) = x^2 \bmod p$ pre-image resistant, second pre-image resistant and/or collision resistant

I have the function $H(x) = x^2\bmod p$ , where $p$ is a prime of length n bits and this function maps to the message $x$ to a n-bit hash value $H(x)$. I need to find out if it is pre-image resistant,...
10
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1answer
2k views

SHA-256: (Probabilistic?) partial preimage possible?

Currently busying myself with the Bitcoin "mining" algorithm, I am wondering if the process really cannot be simplified. For reference, the algorithm is basically SHA-256d: $success := SHA256( ...
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1answer
132 views

Hash Function to get same value for different messages [duplicate]

I am studying a function where I found the some hash properties. Hash functions: Given a code c is computationally infeasible to find m such that ...
2
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1answer
237 views

Does a break in a collision resistance property of a hash function by definition implies an attack at the first pre-image attack?

Is there a formal security proof in the shape of reduction that states that if an attacker manages to break the collision resistance property of a cryptographic hash function (a random oracle) he will ...
5
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1answer
222 views

Pseudo preimage for a hash made from a cipher

Consider the Miyaguchi–Preneel construction: $H_0 = E(0,m_0) \oplus m_0$ (0 here means a vector filled with zeros) $H_1 = E(H_0,m_1) \oplus H_0 \oplus m_1$ where $E(K,M)$ is a block cipher (for ...
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3answers
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Why can't hashes be reversed with toffoli gates?

This is a follow up question to this question: Crack cryptographic hash functions using Toffoli gates?. Suppose there is a hash function $$H(x)=y,$$ where $x$ is the input of the function and $y$ is ...
3
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0answers
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Prove preimage resistance property

Let $ f : \{0,1\}^* \rightarrow \{0,1\}^n$ and $ g : \{0,1\}^* \rightarrow \{0,1\}^m$. $h$ is created by concatenate f and g: $ h : \{0,1\}^* \to \{0,1\}^{n+m}$ with $h(x) = f(x) || g(x)$. How do I ...
3
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1answer
584 views

Is SHA-256 safe when used in this way?

I am wondering whether an attacker can gain any useful information from having several hashes of an unknown plaintext that differs in relatively few bits. For example, if provided with: ...
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1answer
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Did NIST verify “post-quantum” claims in the SHA3 proposal papers?

I have been reading Bernstein’s “Quantum attacks against Blue Midnight Wish, ECHO, Fugue, Grøstl, Hamsi, JH, Keccak, Shabal, SHAvite-3, SIMD, and Skein” paper from 2010… This document disproves ...
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3answers
473 views

Can one build a one-way function from AES?

We change the AES block cipher encryption: we delete the key schedule algorithm the user now provides a string of 1408 bits we divide the string to 11 sub keys, and use them directly in the ...
2
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0answers
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How hard is a known prefix hash preimage attack?

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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1answer
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Is it possible to find a preimage for a reduced size hash?

Given a message $M$ and a 256 bit key $K$, perform HMAC($K$, $M$) with a 256 bit hash function resulting in: $a$ = ...
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2answers
3k views

Pre-image resistant but not 2nd pre-image resistant?

Are there any cryptographic hash functions for which there is a known pre-image attack, or a known second pre-image attack, but not both? The attack doesn't have to be practical - just anything that ...
3
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1answer
130 views

How difficult is it to find the “pre-image” of a block cipher?

Say that I have $$ C_1 = AES_{k_1}(M_1) $$ How difficult would it be to find a key, K2 and plaintext M2 $$ C_2 = AES_{k_2}(M_2) $$ such that $$ C_1 == C_2 $$ How would using a block cipher ...
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1answer
413 views

Which MACs can be converted into a secure unkeyed hash function?

It is known that setting the secret key to a fixed, public value does not make MACs like CBC-MAC or GMAC into secure unkeyed cryptographic hash functions that could be used - for instance - for ...
2
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2answers
385 views

Speeding up partially known plaintext preimage recovery attack on MD5

Suppose there are three messages A, B and C of different length, that are 16 DWORDs in ...
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2answers
544 views

Finding partial pre-image of MD5 hash

I have the following requirement for hashing using MD5. H(A,B,C,X); Where values A,B & C are given. However X is not given. I would like to find out what value of X would give a hash beginning ...