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Questions tagged [collision-resistance]

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

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1answer
59 views

What is the minimum message length to obtain the full potential of a hash?

Suppose I want to commit to a uniformly picked nonce of fixed length $n$ bits without revealing the nonce, so I use a secure hash function $H$ mapping messages to digests of length $d$ bits. Suppose $...
16
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2answers
4k views

What makes SHA256 secure?

For example, RSA relies on a mathematically hard problem, factoring, while ECDSA or similar rely on discrete logarithm problem. What makes SHA256 and similar hash functions, of the same family, ...
1
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1answer
26 views

Number of preimages of hash function from bounded domain

Suppose we have an "ideal" cryptographic hash function $H: \{0,1\}^{n+k}\to \{0,1\}^{n}$ and a given bitstring $b\in \{0,1\}^{n}$, Can we estimate how many preimages $H^{-1}(b)$ there are? With ...
7
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1answer
90 views

SNARK-friendly one-way compression functions

I'm looking for a one-way compression function that is secure and efficient within a zk-SNARK circuit. The motivation behind this comes from these considerations: SHA256-compression is highly ...
4
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0answers
76 views

Partial collisions of hashes, why is this important?

There's a lot of work finding "partial collisions" in hashes, and "Parallel Collision Search with Cryptanalytic Applications" seems to be the go-to paper for this work. I'm trying to understand why ...
3
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1answer
96 views

Does Composition of compressing Collision Resistant Hash Functions $H^{*}=H_2(H_1(x))$ is Collision Resistant?

I'm trying to solve the following problem: Given two CRHF $H_1:2^{4n}\to2^{2n}$, $H_2:2^{2n}\to 2^n$, construct the following hash function $H^{*}=H_2(H_1(x))$ compressing from $2^{4n}\to 2^n$. We ...
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2answers
249 views

AES_256_ENCRYPT_CTR(key, sha-1(plainText)+plainText) VS AES-GCM, which is more secure? [duplicate]

What if we encrypt sha-1 hash of the plain text, along with the plain text using AES-256 in CTR mode? How much future proof the above method would be in comparison to AES-GCM or any other method ...
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1answer
47 views

Hash Collision Probabilities

Please give help! how can I calculate the probability of collision? I need a mathematical equation for my studying. Assume, I am using SHA256 to hash 100-bits. Thus: SHA256 {100} = 256-bits (hash ...
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2answers
67 views

On a lower bound for the birthday problem

I'm now familiar with a lower bound for the birthday problem as exposed in the theorem A.16 of Katz and Lindell book (alternatively see this webpage). If one denotes by $C(q,N)$ the probability of ...
3
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0answers
39 views

Is there an information theoretic equivalent of a trap door collision free function?

Warning: Possibly ill-posed question. I'm using the following definition from a recent paper available here. I believe their terminology is slightly different but reproduce my understanding of it ...
0
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1answer
62 views

What is the smallest last value of sequence of integers from 1, where there is a collision in md5? [duplicate]

Question is just out of interest. If we take md5 hash from all integers from 1 to n, what is the first n, where its md5 collides with some previous md5? Has anyone ever calculated and proved the ...
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1answer
73 views

Is the hash function $h(x\|y) = AES_x( y)$ collision resistant?

If $x$ and $y$ are both 128-bit strings would the function $h(x||y) = AES_x(y)$, where $x\|y$ means concatenation, be considered collision resistant ? Is there an easy way to find $(x_1, y_1)$ and $(...
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0answers
54 views

Extendable hash collision resistance of multiplication

I have a hash function $HashSet(A) = H(a_1) *\cdots * H(a_n)$, where $A = \{a_1,\ldots,a_n\}$ and $H$ is a hash function which generates values of $b$ bits (and $H$ has no collision attack that ...
15
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1answer
3k 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 ...
0
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1answer
42 views

Python hash code returns wrong result? [closed]

For a school project, I'm trying to demonstrate a hash collision using Python. This is the little piece of code that I'm trying to use (based on this answer): ...
0
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1answer
73 views

Show that if a fixed key MAC is not collision resistant then the MAC function is not computation resistant for every key K

I'm really having some trouble with this one. Let $H$ be a MAC function. For every key $K$ we can create an unkeyed hash function (i.e.an MDC) by using $H$ to hash messages with the fixed key $K$. ...
1
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1answer
96 views

How to find a collision with a given prefix and a known key in an AES ECB symmetric block cipher?

I should be able to find a collision for a vulnerable hash function which uses AES ECB given a prefix, but I am unable to find a reasonable solution (besides brute-force attack). The hash function is ...
5
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1answer
836 views

What is the probability to produce a collision under two different hash functions?

Given 2 (weak) hash functions, namely MD5 and SHA1 The fact that there exists 2 different inputs that will result in the same output under both hashing functions is explained in "Can there be two ...
2
votes
2answers
55 views

Difficulty of finding collisions for two different CRHF

Suppose that $\mathcal{H}$ is a family of collision resistant hash functions. Given $h$, which was chosen uniformly at random from $\mathcal{H}$, it is infeasible for a polynomial-time adversary to ...
4
votes
1answer
104 views

How to derive a hash function from another hash function? [duplicate]

Problem: Hash functions derived from another hash function (4 marks) Suppose that $H:\{0,1\}^* \rightarrow \{0,1\}^n$ is a collision resistant hash function. Define a new hash function by $G:\{0,1\}^* ...
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1answer
54 views

Why do m and m' both hash to H2?

I'm trying to understand the following text: I'm having trouble understanding why $m$ and $m'$ both hash to $H_2$. I tried doing truth tables for $m_1'$ and $m_2'$ given the equations but I don't ...
3
votes
1answer
123 views

What does Keccak being “efficiently invertible” mean?

Reading through Wikipedia, I saw this: It has been shown that the Merkle–Damgård construction, as used by SHA-2, is collapsing and, by consequence, quantum collision-resistant, but for the sponge ...
3
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1answer
233 views

MD5 - Chosen Prefix Collision Attack

Given 2 messages $m_1$ and $m_2$, in a chosen prefix attack, we want to find $s_1$ and $s_2$ such that: $\text{MD5}(m_1 || s_1) = \text{MD5}(m_2 || s_2)$ Are $s_1$ and $s_2$ found by brute force ...
0
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1answer
92 views

Is there an $n$-bit hash function such that $n$ is equal to the length of input and the collision resistance is close/equal to $2^{n/2}$?

Let $B$ denote a non-empty bitstring of arbitrary length. Then let $n$ denote the length of $B$. The question: does there exist a cryptographic hash function $H$ that outputs a hash $h$ of $B$ such ...
3
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2answers
133 views

AES-128 as compression function in Merkle-Damgard construction

Using a compression function $f : A × A → A$. A basic version given by: $W_0 = IV$ $W_1 = f(W_0, m_1)$ $W_2 = f(W_1, m_2)$ ... $W_n = f(W_{n-1}, m_n)$ $W_n$ is the output of the hash function, $...
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1answer
96 views

finding encoding based collision

For example, If I enter as a sample "hello world" in English, how hashing protocol generates a hash of it? is in the first step, change it to ASCII or Unicode or Binary? What if I find a word in other ...
19
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5answers
2k views

How many hex digits do I need to compare when manually checking hash functions?

I sometimes run sha256sum on large files after transferring them from one place to another, and will just skim the hash output to verify it's correct. But, I usually just look at the first/last 5 or 6 ...
4
votes
2answers
59 views

Feasible to find two sets with the same RSA accumulator value if a prime representative hash function is used?

Consider the RSA accumulator scheme with random oracle as proposed by Barić and Pfitzmann, with the random oracle replaced by a secure hash function $H$ as follows. For a set $S$, the accumulator ...
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1answer
103 views

repeatedly hashing result and using reverse sequence as RNG secure?

I want to have a source of randomness for which: no one but me can predict the next number in the sequence. I know the entire sequence in advance, but I have no way of manipulating it once the first ...
1
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1answer
143 views

Hashing based on the discrete logarithm problem

At a first look, one could use the elliptic curve discrete logarithm problem to grant for the onewayness of $H(x)=x*G$ (where $G$ is the generator point of the cyclic subgroup). Additionally, $H(x)$ ...
5
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2answers
75 views

Can RFC 5649 be used to wrap multiple keys with the same KEK?

I am unable to convince myself that RFC5649 is secure when storing multiple keys with the same KEK. My point is: RFC5649 uses AES-ECB, therefore the usual weakness of this method applies, namely an ...
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5answers
221 views

What is the mathematical property stating that it is hard to find a collision in the AES algorithm?

Finding a collision in the AES algorithm is (apparently) hard. Why ? What is the mathematical property stating that there is a negligible probability to find a collision by taking two random ...
4
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1answer
119 views

Cryptanalysis of MD4 (Wang et al) — how did the authors come up with the collision differential?

Edit: ok, after I re-read the paper a couple of times it is clear that Table 6 is easily derived from Table 5, which describes the collision differential. So the real question is: how did the ...
27
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3answers
3k views

Does “Shattered” actually show SHA-1-signed certificates are “unsafe”?

Note: I am not advocating anyone continues using SHA1-signed certificates: they are dead as far as security is concerned and should no longer be used. I'm just trying to clarify my understanding of ...
17
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3answers
7k views

After Google's collision attack, is RSA-SHA1 signature still safe?

Google succeeded to get a collision in SHA-1 last year in an attack called shattered. Does this fact make certificates based RSA-SHA1 Signature risky for creating fraud certificates? If RSA-SHA1 ...
2
votes
1answer
50 views

The Collision Differential for MD4 - a question on notation (Wang, et al)

In the paper "Cryptanalysis of the Hash Functions MD4 and RIPEMD" the authors introduce the following notation (paragraph 4.1): $\Delta$$H_0$ = 0 $\xrightarrow{(M_,M')}$ $\Delta$$H$ = 0 What exactly ...
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0answers
54 views

Anonymized identifier

I'm trying to find an algorithm which would allow me to create anonymized zero-metadata identifiers. I'm creating a system which creates integer identifiers using a counter. Using such identifiers ...
0
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2answers
221 views

MD5 hash functions to prove integrity of a download

It doesn't take a lot of time on the crypto StackExchange to realise the sentiment around MD5 hash functions is poor. My questions are: If it's such a bad idea to make use of MD5 hash functions why ...
3
votes
4answers
112 views

Find some hash function with deliberate collisions

I guess this should be well known (or trivially desperate), but I couldn't find any reference. I have a small number (say $k=15$) of messages $m_i$ ,$i=1\cdots k$ (they are fixed length and short, ...
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0answers
339 views

Security of custom JSON-Object hash

I am currently implementing a signed JSON-Blob, and thus i need to create a hash of any JSON-Object possible. Currently for the hash of the JSON-Object i ended up with the following Java-Code using ...
5
votes
2answers
109 views

Finding k collisions on hash function

Let $n$ be the size of the image-space of a hash function $H$. It is known that you can find a collision on $H$ in $O(\sqrt{n})$ time (by birthday paradox). How can I show that, in order to find $k$ ...
3
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2answers
125 views

Is it possible to generate an infinite number of collisions given an infinite amount of strings?

Let us assume that we have a string $w$ such that it belongs the set $C$, where every member of $C$ is not null, and contains every string of characters that can be permuted (numbers, letters etc.). ...
3
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0answers
49 views

Ambainis quantum collision-finding alg; triple hash output lengths?

I just stumbled onto a series of 2017 papers about applying the Ambainis quantum collision-finding algorithm to hash functions. (disclaimer: I haven't read all of them in full yet): Post-quantum ...
7
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2answers
2k views

How many trials does it take to break HMAC-MD5?

I know that you can find collision in MD5 with 2^64 trials using Birthday paradox. Now everyone is saying that HMAC-MD5 is significantly more secure. How can I ...
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2answers
53 views

Technical term for hash value being uncorrelated to input message?

Is there a technical term for the desirable property of hash functions. That a small change to the input message will produce a massive unpredictable and seemingly uncorrelated change to the output ...
2
votes
2answers
468 views

collisions on hash functions

Why do we say that collision-resistance is a "harder" property than second pre-image for hash functions whereas if you have an attack on the second pre-image then you find a collision ? Moreover, a ...
1
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1answer
209 views

Security of XORing hashes vs concatenating

On this question https://stackoverflow.com/questions/5889238/why-is-xor-the-default-way-to-combine-hashes a couple of the answers say that xoring is a bad/insecure choice for combining hashes. However,...
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2answers
346 views

(strong) collision resistance Example

I'm studying Cryptography & Network Security. It has the following example for collision resistance: "Suppose Bob writes an IOU message, sends it to Alice, and she signs it. Bob finds two ...
4
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1answer
88 views

For a random permutation $P$, what's the probability of the following event?

For a random permutation $P$ and $q$ distinct inputs $x_1,\ldots,x_q\in\{0,1\}^n$, what's the probability of the event that there exists at least one collision among $\{P(x_1)\oplus x_1,\ldots,P(x_q)\...
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2answers
355 views

Rolling your own crypto

I know this is usually frowned upon, but supposing you were to roll your own encryption cipher (completely on your own or with a small group of friends, with no peer review possible), what could you ...