# Tagged Questions

Difficulty of finding two different inputs that hash to the same value

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### Will there always be 256 collisions for the output of the MD5 hashsum function for 17-byte inputs?

By my math, if you are inputting all possible 17 byte values, and the output is 16 bytes long, then there must always be 256 possible inputs that will result in each output hashsum. However, looking ...
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### Probability of SHA256 Collisions for Certain Amount of Hashed Values

I wonder if you can help me figure out that question: Is there a known probability function f: N -> [0,1], that computes the probability of a sha256 collision for a certain amount of values to be ...
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### Cycles in SHA256

In the recommendations, it seems cycles is used usually as in "CPU Cycles", but, that's not what I mean here. What I mean, is this: Let's say I start with a particular 256 bit value. Call this v. I ...
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### 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 $H_1$ ...
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### What practical uses can random hash collisions be put to?

Using magic, I've built a machine that instantly generates hash collisions. To try it out, I set the dial to SHA256 and typed in "e7cf3ef4f17c3999a94f2c6f612e8a888e5b1026878e4e19398b23bd38ec221a". I ...
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### Problem with testing MD5 collisions

I am trying to test MD5 collisions presented in “Collisions for Hash Functions MD4, MD5, HAVA” (PDF) by Xiaoyun Wang, Dengguo Feng, Xuejia Lai, Hongbo Yu; 2004. I wrote a PHP script to test it, but ...
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### Reducing size of hash function

If I have some data I hash with SHA256 as hash=SHA256(data) and then copy only the first 8 bytes of the hash instead of the whole 32 bytes. How easy is it to find a ...
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### Hash “Preimage by product” resistance

Let H() be a hash function that achieves collision resistance as well as first and second preimage resistance. Let's equip the output set of H of a multiplicative group structure, more precisely a ...
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### Hashing - Digital Signing and Trivial Stretching

Two very basic questions: 1. Digital Signing Why is it not possible to just simply replace the hash value from the request? Examples: ...
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### How hard is it to find plaintexts whose hashes satisfy $h(a)\oplus h(b)=h(c)$?

Given a cryptographic hash function $h$, for example SHA256, how hard is it to find plaintexts $a,b,c$ such that $$h(a)\oplus h(b)=h(c) \text?$$
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### How much work is required to detect multiple collisions for a hash function?

Assume an ideal hash function of output size n bits, detecting one collision requires approximately 2^(n/2) evaluations of the ...
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### Why is EdDSA collision-resilient with SHA-512?

In the Bernstein et al. paper about EdDSA, the authors claim EdDSA is resilient against collisions (i.e. it can still be secure even if the hash function used isn't collision-resistant), drawing on a ...
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### Are there cryptographic hash functions which do not have any collisions?

I've been studying digital signatures and hash functions recently, and I was wondering: Do hash functions exist which will never produce collisions?
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### Security concern about reducing hash value using modulo operation

As stated in the title, what I am looking for is information about a "technique" that I would like to use in some of my algorithms. Sometimes I need to map a hash function's result into a range of ...
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### SHA-1 collisions - what about practical attacks?

I understand the theoretical problem with hash collision but when it comes to practice, I get very confused. Suppose a attacker would like to forge a certificate (or any kind of structured piece of ...
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### Is double hashing collision resistant?

Double hashing can surely provide more security than only one layer of hashing but does that necessarily mean it is more collision resistant? This question in a more mathematical form: If $H$ is a ...
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### Collision in Merkle–Damgård without a collision in compression function

Is it possible to find a collision in Merkle–Damgård just by omitting the extra one bit that is appended to each input without having a collision in compression function?
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### Hash collision using the leading 40-bits of SHA-1

I am trying to implement Floyd's cycle finding algorithm for finding a leading 40-bit hash collision in the SHA-1 algorithm. However, my code will not find a repetition even with 800,000,000 hashes. ...
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### Does this guarantee a unique 32 bit Hash?

I came across some source code that loosely does the below in order to achieve a 32 bit hash. The input string is passed through MD5 to get 16 bytes Hash (as usual). Then the 16 bytes are split into ...