Questions tagged [preimage-resistance]

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

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53
votes
11answers
18k views

How do hashes really ensure uniqueness?

This might seem an impractical and unnecessary conversation, but I feel it's something I need to clarify. Especially, as I just got my first developer job in a blockchain startup. So hashes are said ...
36
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3answers
34k 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?
29
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11answers
7k 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 ...
20
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2answers
7k views

What makes SHA-256 secure?

For example, RSA relies on a mathematically hard problem, factoring, while ECDSA or similar rely on discrete logarithm problem. What makes SHA-256 and similar hash functions, of the same family, ...
19
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1answer
6k views

Could we break MD5 entirely in the future?

Even of today MD5 is (sadly) still heavyly used in some applications. Even big tools like ApacheMD5. But even today there are more then enough MD5 hashes which are still not cracked. According to ...
19
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2answers
2k views

What is the general justification for the hardness of finding preimages for cryptographic hash functions?

Since most cryptographic hash functions are simple, compact constructions does this simplicity impose a limit on the complexity and the size of a function that can generate preimages? That is, given a ...
18
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2answers
6k 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: $$\mathit{success} := \...
17
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2answers
6k 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 ...
17
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1answer
4k views

What are the consequences of removing a single byte from a sha256 hash?

I'm working on a system (Ethereum) where it is significantly cheaper to store 32 bytes than 33 bytes. I'd like to create a table where data is stored based on its hash. Sha256 would meet this ...
15
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3answers
909 views

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 ...
14
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1answer
2k views

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 the ...
13
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2answers
6k 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. Could ...
12
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3answers
2k views

SHA3-255, one bit less

I need a SHA3-255 or 511. What if I simply truncate a standard SHA3-256 or 512? Apart from the doubled probability of hash collision, are there any other things I should be aware of? I could also ...
11
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1answer
3k views

Why doesn't preimage resistance imply the second preimage resistance?

Let the preimage resistance be defined as »given a hash value $h$, it is hard to find any message $m$ such that $\operatorname{hash}(m)=h$«, and let the second preimage resistance be defined as »given ...
11
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1answer
526 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 ...
10
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1answer
641 views

How does a preimage attack on MD5 break the security of HMAC-MD5?

It was mentioned in an answer to a different question that it's possible that, any day now, someone might figure out a way to turn those into a preimage attack, which would compromise the ...
8
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1answer
614 views

“official” guidelines about the minimum length of a truncated hash

I am looking for some reference documentation suggesting a minimum reasonable length for a truncated hash (specifically, a truncated SHA-256 hash) to ensure preimage resistance. Obviously, truncating ...
8
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1answer
331 views

Would finding a Merkle-Damgård preimage that doesn't change the initial state allow an attacker to prepend it to any hashed message?

Suppose, a message M was found so that MD5(M) = S, where S is the initial state of the MD5 function (0x01234567, ...). Given a hash MD5(m), would this allow computing MD5(M∥m∥padding), where padding ...
7
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2answers
223 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 ...
6
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2answers
3k views

Why might SHA-384 throughput be lower than SHA-512 throughput in hashcat and more secure?

I found a hashcat benchmark results in the internet: hashcat results: SHA-384 is 17065.4 MH/s SHA-512 is 17280.3 MH/s Why does SHA-512 take less time? SHA-512 is longer and I thought it therefore ...
6
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3answers
520 views

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 ...
6
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3answers
867 views

Minimum input size of a hash function

This is a theoretical question, to improve my understanding of hash functions. Hash functions have a one-wayness such that they are protected from the first preimage attack. From my understanding, if $...
6
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2answers
11k views

Computational requirements for breaking SHA-256?

Let's define "breaking" a hash function $H$ as being threefold (corresponding to the main properties of a cryptographic hash function): preimage attacks to get $m$ knowing $H(m)$ second-preimage ...
6
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1answer
619 views

Does preimage resistance and/or collision resistance imply the infeasiblility of finding fixed points in hash functions?

For example, if $H$ is a secure cryptographic hash function, is it infeasible to find $x$ such that $H(x)=x$ or such that $H(H(x))=x$ ? What about finding $x, y$, and $z$ such that $H(y||H(z||x))=x$ ?...
6
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3answers
502 views

How to prove that finding a cycle in a cryptographic hash function is hard?

I want to show that finding cycles in a cryptographic hash function is hard. My thought: assume there a black box, that given a cryptographic hash function $h$, finds some $x$ and $r$ in polynomial ...
6
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1answer
159 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?
6
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1answer
532 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 ...
6
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1answer
906 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^...
5
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1answer
4k views

Pre-image attack on MD5 hash

I read somewhere MD5 is broken in terms of collision, but I am wondering if it is broken in terms of pre-image resistance? Given a hash of Md5, is it possible to find the original message of it? If ...
5
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2answers
2k views

How do you find the preimage of a hash?

What is a preimage and how do you find a preimage of a hash?
5
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1answer
274 views

Is the last step of an iterated cryptographic hash still as resistant to preimage attacks as the original hash?

Considering a cryptographic hash, such as MD5 or SHA2, denoted by the function $H(m)$ where $m$ is an arbitrary binary string, there is a lot of material available that deals with potential weakness ...
5
votes
1answer
678 views

Is there a feasible preimage attack for any hash function (no matter how deprecated) today?

Has there ever been a hash function that was actually used in the field, no matter how long ago, for which there is now a feasible preimage attack? All hashes that are nowadays considered 'broken' (...
5
votes
1answer
300 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 ...
5
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0answers
298 views

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 ...
4
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2answers
1k views

Are My Answers to This Hash Question Correct?

Question When determining the security of a hash system, the cryptanalyst tries the following attacks. (a) If the attacker is NOT allowed to modify the original message, determine the number of hash ...
4
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1answer
1k 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,...
4
votes
1answer
558 views

How can I determine if a hash function is secure?

I'm doing an exam in computer security and I encountered this problem which I'm unsure of how to attack properly. Should I get down and dirty with the Hash function on paper or is there a more general ...
4
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1answer
325 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 ...
4
votes
2answers
5k 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 ...
4
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2answers
556 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 ...
4
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1answer
575 views

Do well-known hash functions have any “impossible” output values?

What I'm wondering is whether the codomain and image of common hash functions are strictly equal… or whether there are any known values which have no possible preimage. For example, SHA-1 outputs a ...
4
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1answer
1k views

What gives SHA-256 its preimage resistance?

I was reading some guy's description of the SHA-256 algorithm and I noticed that the basic operations it uses appear trivial to reverse: addition, rotate bits, etc. When I say "reverse" I don't mean ...
4
votes
1answer
1k 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 $H_1$ ...
4
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2answers
663 views

If you wrote a reversible SHA-256 algorithm, how many “metadata” bits would be required for reversability?

I was reading about reversible computing while thinking about FPGA circuits. I realized that it would be possible to write a reversible SHA-256 algorithm if you stored some additional information as "...
4
votes
1answer
466 views

What if using a block cipher as compression function in Merkle–Damgård?

The Merkle–Damgård construction builds a hash by iterating a compression function $F$, with $S_{j+1}=F(B_j,S_j)$ where $B_j$ is one of $n$ padded message blocks, $S_0$ is the IV, and $S_n$ is the hash....
4
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1answer
161 views

Why does this SHA-1 relation hold?

In De Canniere & Rechberger's 2008 paper "Preimages for Reduced SHA-0 and SHA-1", the following statement appears on page 6: Suppose that we restrict ourselves to the first $j$ + 1 bits of each ...
4
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0answers
404 views

Hash Function Properties

What I am trying to understand is shown in all three properties of a secure hash function, I will focus on Preimage attack resistance. Preimage resistance: given a hash h, it's difficult to find m s.t....
4
votes
0answers
384 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 ...
3
votes
3answers
850 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
votes
3answers
1k 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 ...