Tagged Questions

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

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 ...
816 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 ...
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?
417 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 ...
2k views

223 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 ...
868 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 ...
159 views

Why do different applications of hash functions demand different properties of the hash function?

A strong cryptographic hash function in general must guarantee several properties, including: Preimage resistance, second preimage resistance and collision resistance. It turns out, however, that ...
158 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 ...
234 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 ...
234 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 ...
706 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, ...
853 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 ...
298 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,...
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 ...
291 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 ...
123 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 ...
73 views

Proving a function is one way hash function

I am working on a project which computes the hash of the given input, I have to prove that the function is pre-image resistant that is given an output we cannot get back to the input. While proving ...
100 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 ...
606 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: ...
100 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 ...
229 views

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 ...
476 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 ...
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 ...
238 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 ...
125 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)?
304 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 $H_1$ ...
32 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 ...
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. ...
115 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 ...
155 views

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 ...
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-...
167 views

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: $a$ = ...
554 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 ...
45 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,...
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 ...
35 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 ...
101 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 ...
164 views

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 ...
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 ...
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.