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

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

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 ...
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1answer
90 views

Block cipher not resistant to collisions

If we have the following block cipher, $E(k,m)$, which is a permutation, why is $f_2 (x,y)$ not collision resistant? $$f_2(x, y) = E(x, x \oplus y)$$
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How to generate a random number so server cannot cheat?

Here is the protocol: Bunch of players connected to server. Server creates nonce and hashes it - send hash to clients as bit commitment. Clients make nonces and send hashes to server as their bit ...
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Are there two known strings which have the same MD5 hash value?

Is there an example of two known strings which have the same MD5 hash value (representing a so-called "MD5 collision")?
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2answers
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How many RSA keys before a collision?

I was wondering how many possible private/public keys exist? If a million people – for whatever reason – would try to generate 5 keys each in the same minute (on the same date and time) is there a ...
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2answers
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Long passwords and Bcrypt

I need to store passwords for a web application, and was looking at Bcrypt for a possible solution. After some research, it appears only the first 72 bytes influence the final hash output. While it is ...
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2answers
57 views

Is there a standard for hashing a list of strings?

I'm working in Python, and my goal is to hash a list of strings with a cryptographic hash function such as sha256. It would need the property that a change in any string, or the order of the strings, ...
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31 views

Are collisions in AES-128 theoretically possible when the plaintext is the same? [duplicate]

Is it theoretically possible to find $P$, $K$, and $K^\prime$ such that $E_K(P) = E_{K^\prime}(P)$ where $E$ is AES-128 and $K \ne K^\prime$? I'd imagine this would be infeasible, but is it actually ...
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Collision free one way function

I was playing with a function that I think is collision free and uninvertible assuming the hardness of integer factorization. I am unfortunately not as skilled at math as I would like to be, and do ...
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2answers
67 views

Discrete logarithm hash function Exercises

I have a problem with this exercise: Let $G$ be a group of order a prime $q$ and let $g, h$, be two randomly selected elements of $G$, with $g,h\ne 1$. Consider the following hash function on ...
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1answer
97 views

Could we share a secret key using the Birthday problem?

I stumbled upon this code on-line, it seems obvious to me that generating all 2^40 keys in trivial time can always be achieved as can hashing each and everyone of them, using the SHARED Salt that ...
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1answer
119 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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1answer
99 views

Effect of message length on collision attacks

I know that message collisions are supposed to be hard to calculate for a good cryptographic hash. Given 1k worth of data, it's hard to find another 1k worth of data that collides with the same hash. ...
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1answer
47 views

Finding collisions for the DES F-function

It's known that for a given subkey, the F-function of the DES is not injective (see Is the DES F-function injective for a given subkey?). I am interested in efficiently finding collisions for this ...
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1answer
45 views

Question about lower bound and upper bound key collision

I'm reading this article and I have a doubt in the page 7. First I going to describe the required definitions and next I formulate my question. Let be $$F(n)=\{f_k: \{0,1\}^n\rightarrow \{0,1\}^n| ...
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Why do we need Merkle–Damgård strengthening [duplicate]

Reading through some manuals, wikis and other documentation I'm learning about Merkle–Damgård construction. I understand why padding beginning with '1', followed by '0's is needed, but I can't ...
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1answer
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What is a “freestart collision”?

In their work on SHA-1 collisions (cf. the currently unpublished paper “Freestart collision on full SHA-1” by Stevens, Karpman, and Peyrin) Stevens et al show that they are able to generate "freestart ...
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1answer
93 views

Would SHA-1 be safe for certificates with short validity?

There are articles floating around the topic of SHA-1 deprecation. The gist of it is that in (mostly) developing countries, people are still using mobile/computing devices that are 5 years old (or ...
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1answer
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Is it possible to actually verify a “sponge function” security claim?

When using a “sponge function” to create a cryptographic hash, we can look at the flat sponge claim, which flattens the claimed success probabilities of all attacks using a single parameter: the ...
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1answer
135 views

Testing hash functions for collision resistance

I was hoping to implement a software that allows testing a user-defined hash function for cryptographic properties (Meant to pique interest in cryptographic hash function for a school showcase), as ...
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1answer
31 views

bcrypt collision probabilities assuming same password and work factor?

What is the likelihood of 2 bcrypt hashes colliding if they use the same work-factor and input? Are bcrypt's salts large enough to prevent this from happening?
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1answer
133 views

Why Merkle Tree doesn't suffer Birthday attack?

I am trying to understand why Merkle Tree doesn't suffer the birthday attack? Can you help me? Thank you!
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2answers
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Probability approximation of no collisions

I am comparing the actual probability of no collisions to the probability approximation formula of no collisions from the Understanding Cryptography text. The approximation is as follows: ...
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1answer
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Rogue CA Certificate MD5 Collision Control

Papers from Marc Stevens et al show that X509 certificates that rely on MD5 signatures can break the assumed security model, that is, it allows for a forged certificate to have the same signature as a ...
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1answer
54 views

Memory hardness of key derivation function through XOR-ring multiple matrix values

For theoretical purposes in order to enhance my own understanding, and NOT in order to create my own cryptography, I am asking a question about the memory-hardness of a key derivation function ...
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1answer
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Taking half of a X bits hash key as secure as using a X/2 bit hash function?

This question comes from a (nitpicking) comment by me in this SO question. I said that using the half of the output from a 256 bit hash function is worse than using a 128 bit hash function. My ...
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1answer
90 views

Proving collision resistance of a hash function?

Let $f$, $g$, and $h$ be hash functions that each map binary strings of length $2n$ to binary strings of length $n$. Suppose that $h(x) = f(g(x)||g(x))$. Prove that if $f$ and $g$ are collision ...
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If a PRG is constructed by iterating a PRF, is it backtracking-resistant?

Say we construct a pseudo-random number generator from a pseudo-random function $f$ (using some constant key $k$ and some initial value $v_0$). We do this by feeding the output block of the PRF back ...
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2answers
59 views

Easy hash function collision

I'm designing a cryptography-breaking assignment for a college-level introductory security course, and I'm looking for a hash function which is reasonably easy (but not too easy) to generate ...
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2answers
627 views

Toy hash algorithm that can be broken

I'm looking for a toy hash function, where the idea is to have high school students break (i.e. find a collision) a hash function by hand, in order to teach them how one way functions and hashing ...
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5answers
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Is it theoretically possible to construct a string that contains its own hash value?

After saw the xkcd comic Self-Description, I wonder if it is theoretically possible to construct a self-descriptive string that contains its own hash value? Let's say the string's MD5 value is ...
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3answers
825 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 ...
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1answer
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What is the complexity of finding SHA-1 collisions?

Given the recent news about SHA-1 collisions, what is the complexity of finding a SHA-1 collision? Marc Steven's HashClash is still detailing about 260. Does the previous complexity still hold?
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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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586 views

What is the chance of a collision when using SHA-3?

I currently am writing a web app and am using two random hashes that are before and after the string and are both 50 char long (alphabet, numbers, and special chars). ...
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2answers
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How does perfect hash function works?

According to wikipedia, a perfect hash function is a hash function that uses algorithms that has a certain random aspect to their logic. It is suppose to be collision-free. However due to the pigeon ...
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MD5 SSL Collision Attack

I want to create fake certificate for test purposes. I have read about the http://marc-stevens.nl/p/hashclash/index.php project but I am unable to understand the two files that I have to put as input ...
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Are there MD5 collisions for inputs of different length?

There are many examples of MD5 collisions (some of them can be found here Are there two known strings which have the same MD5 hash value?). But as far as I know two inputs should have the same length ...
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1answer
116 views

SHA-256 “almost unique”?

I have seen numerous references on the internet of people describing SHA-256 as generating an "almost unique" hash. Exhibit A. there are more. Is there some mathematical basis to the almost ...
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72 views

Perfect hash function

Is a perfect hash function always bijective ? In Wiki "In mathematical terms, it is a total injective function" but it does not mean, that it is bijective.
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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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Combining md5 collisions to create more collisions

Given X1 and X2 such that md5(X1) = md5(X2) and Y1 and Y2 such that md5(Y1) = md5(Y2), and knowing the following property of md5: if md5(a) = md5(b), md5(a|s) = md5(b|s) is there a way to find ...
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If I compute the SHA-1 of all possible 40 digits hexadecimal strings, will I get all possible sha1 hashes? [duplicate]

Wikipedia says sha1 produces a 160-bit (20-byte) hash value. So if I compute the SHA-1 of all possible SHA-1 values, which as far as I understand is at current time impossible both to calculate and ...
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Are common cryptographic hashes bijective when hashing a single block of the same size as the output?

It's been said that CRC-64 is bijective for a 64-bit block. It the corresponding statement true for typical cryptographic hashes, like MD5, SHA-1, SHA-2 or SHA-3? For example, would SHA-512 be ...
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Bobcat hash, trying to find a 48-bit hash collision for x

I am trying to find a collision with the Bobcat hash of the 48-bit hash. I am a beginner coder so please point out any flaws in my modification of the bobcat program. It does find a truncated 12-bit ...
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3answers
266 views

What was the first MD5 collision ever constructed?

We all know that MD5's collision resistance is severly broken. But when thinking of "random" strings with great cryptographic importance I've come up with NIST's curve seeds and MD5 collisions. But ...
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Is there a way to slow down collision attacks using iteration?

The simple way to iterate a hash function $H(H(m))$ only makes brute force preimage finding slower, but leaves collision attacks as fast since a collision in the inner hash function is a collision in ...
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1answer
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1024 RSA Hash Signature attack. Forge a valid signature

I have an RSA signature of 1024 bit where i know the following: Public modulus N Public exponent (0x03) Cypher message Summary: An MD5 hash is calculated from a collection of byte and is used as ...
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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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1answer
208 views

Formula for the number of expected collisions

Say we have a hash function that produces $n$ bit outputs. From the birthday problem that after around $\sqrt{2^n}$ different inputs to the has function, we can expect a collision. Say instead that ...