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

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2answers
83 views

Are variable-length crypto hash functions still susceptible to collisions?

I just recently learned that the SHA-3 finalist Keccak allows for variable length output. As the only answer to this question states, "[it] need to have an output length at least equal to the input ...
4
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1answer
130 views

Is hashing a list of hashes safe?

I have an arbitrary long list of hashes (e.g. git tree hashes), and I'm wondering by how much I am increasing the risk of hash collisions by condensing them into a single hash, like so: ...
4
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2answers
550 views

A patched SHA1 attempt for password verification

This is a classical scenario: For our web based authentication platform we have a username and a password field. User enters these values and presses Log In. The browser gathers the user data, ...
2
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1answer
34 views

How can finding a collision help an attacker with tampering messages with HMAC

As stated in the HMAC RFC (RFC 2104): The strongest attack known against HMAC is based on the frequency of collisions for the hash function H. How can a collision benefit an attacker? I would ...
5
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1answer
167 views

Does collision resistance imply (or not) second-preimage resistance?

I've seen contradictory results. Sometimes hash functions are collision-resistant but not necessarily second-preimage resistant. I've seen this kind of things in papers from Bart Preneel: “Security ...
1
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1answer
129 views

SHA1 collision event probability after n iterations

There are String1 and String2 - some variables-strings. The probability of collision for different ...
2
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2answers
105 views

Is a collision attack possible on MD5 if message is known partially

Is it possible to find $m'$ such that $\operatorname{MD5}(m'||m_1)=\operatorname{MD5}(m_1||m_2)$ if I know $m_2$ and $\operatorname{MD5}(m_1||m_2)$? Is this a collision or a preimage attack? Also in ...
3
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2answers
151 views

Generate a unique 64 bit numeric database key from a string without collisions at $2^{32}$ birthday threshold?

What is the best way to generate a unique 64 bit numeric database key from a string? For example: if the string is a URL, you could generate a 64 bit key using SipHash or by truncating a SHA256 hash ...
5
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1answer
128 views

How can one detect if a hash function doesn't have a full image?

Suppose we have a hash function $H: \{0,1\}^* \mapsto \{0,1\}^n$, that doesn't get all the $n$ length bitstrings as a value for any input, but is otherwise secure. Can that fact be detected? For ...
2
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0answers
42 views

space complexity of quantum collision search

Is there a known way to reduce the space complexity of quantum collision search beyond what is offered by the built-in time-space tradeoff, while keeping the time complexity significantly below what ...
7
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2answers
238 views

What is the MD5 collision with the smallest input values?

I am interested in MD5 collisions for small input messages. The collision examples given at http://www.mscs.dal.ca/~selinger/md5collision/ show two different strings, where only a tiny amount of data ...
4
votes
1answer
147 views

Why are collision attacks important when talking about MAC schemes?

I'm reading Ferguson, Schneier, and Kouno's Cryptography Engineering, and it has a section on MACs. It discusses different types of MACs, but it seems to claim that to acheive 128-bit level security, ...
4
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2answers
453 views

Is PBKDF2-HMAC-SHA1 really broken?

I just read through this article which demonstrates practical (and seemingly trivial) collisions in PBKDF2-HMAC-SHA1, and provides a few examples of collisions. Am I missing something here? Is ...
5
votes
1answer
187 views

I need a 64-bit cryptographic hash for 96 bits of data

I have a situation in which I need to combine a 32-bit datum, G, and a 64-bit datum, I, to produce a 64-bit datum. No two ...
2
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1answer
136 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 ...
1
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1answer
85 views

Prove that two MACs with incremendal PRF application are not secure

How do I prove the following MACs are insecure?                                                        1                                                                                   2
3
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2answers
227 views

Hashing SHA-512 1000 times

SageMathCloud security (http://goo.gl/dCujW1) is as follows: Only the hash of your password is stored by the server, which uses 1000 iterations of the sha-512 hash function, with a salt length of ...
1
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1answer
118 views

Hash functions with or without secret key

The classical definition of hash functions (found in books) are OK, but when studying a little more there is a lot of notions that seem ambiguous (contradictory) to me : Sometimes we have hash ...
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2answers
140 views

Current standard for hash function security parameter?

When designing a hash function, what should be the secure input and output lengths? In particular, what lengths do I need for collision resistance?
5
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1answer
154 views

Is MD5 second-preimage resistant when used only on FIXED length messages?

I fully realize that MD5 should not be used in any new project, but in my particular situation I have severe CPU performance issues, so MD5 is convenient. I have read a lot about MD5 security for this ...
4
votes
1answer
87 views

Why has the sponge construction's generic collision finding attack a complexity of O(min(2^(-n/2) , 2^(-c/2)))?

I'm trying to learn about sponge function for creating hash functions and generic attacks on it. I'm looking for the collision finding attack scenario which leads to $O(\min(2^{-n/2} , 2^{-c/2}))$ ...
1
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1answer
88 views

Would this prevent FNV1a 32 bit hash collision?

I am considering the FNV1a 32 bit hash as part of a simple software security and authentication system. I am aware that FNV1a is NOT ideal but it may suit my needs. I am aware that FNV1a has rather ...
0
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2answers
89 views

Is the composition of collision resistant hash with non collision resistant hash a collision resistant hash?

Suppose $h_1$ is collision resistant, and $h_2$ isn't. Will $h(x)=h_2(h_1(x))$ always be collision resistant? [Edited:] the output of h2 must be uniformly distributed
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0answers
203 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). ...
6
votes
2answers
333 views

Can the XOR of two non-collision-resistant hashes be collision resistant?

Suppose I have two hash functions, of which neither (or only one) is collision resistant, and I want to create a new hash function by taking the bitwise exclusive or (XOR) of the results of those two ...
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0answers
41 views

TCR hash functions from MD5

I'm reading Collision-Resistant Hashing: Towards Making UOWHFs Practical (pag. 10 and 11) and here say The most direct way to construct a TCR hash function is to key a function like MD5 or SHA-1. ...
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1answer
75 views

Adversary to Any Collision Resistance Hash Function

I'm reading a notion Any Collision Resistance for Hash Functions. In the lecture define a collision finder CF. CF is said to $(t,\epsilon)$-break the family of hash functions $F_K$ if the ...
0
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1answer
48 views

Asymptotic Hash Function and Concrete Hash Function

I'm reading Collision-Resistant Hashing: Towards Making UOWHFs Practical, and they say that its formalization is non-asymptotic. Why is the formalization non-asymptotic? What is an example of hash ...
0
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0answers
41 views

Merkle-Damgard transform: necessity of the Length block [duplicate]

The Merkel-Damgard transform append the message length (in blocks) to the message itself and then computing the 'chained hash' algorithm. Does the last block (which contains the length of the ...
2
votes
1answer
166 views

Brute force attack expected running time

I am a bit confused about the expected running times of brute force attacks on different cryptosystems. So let's assume a key size of $2^n$ bits. Symmetric key cryptography: $E(brute)$ = ...
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0answers
36 views

MD Construction Doesn't Propagate TCR

I'm reading a proof of the proposition of CollisionResistant Hashing Towards Making UOWHFs Practical Suppose there exists a compression function $F: \Sigma^k\times\Sigma^{c+m'} \rightarrow ...
2
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2answers
86 views

MD5 theoretical question

Ok, so this is a very possible stupid question, but one of those things that has sat at the back of my mind for a while. We all know that MD5 has issues, and these have been known for about a decade. ...
1
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1answer
90 views

SHA-1 Keyed Hash Function

I know that SHA-1 is an unkeyed cryptographic hash function when used in practice. But, in the theory, all hash function are defined with keys. My question is: How I will be able to formalize the ...
0
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1answer
95 views

Explicit Key SHA-1

I'm reading “Collision-Resistant Hashing? Towards Making UOWHFs Practical” and in the Section 2 it says: Hash functions like MD5 or SHA-1 have no explicit key. But no notion of ...
0
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1answer
106 views

No Birthday Attack to TCR

I'm reading the paper “Collision-Resistant Hashing? Towards Making UOWHFs Practical” , which compared TCR (Target Collision Resistant) and ACR (Any collision Resistant). It says we wish to stress ...
2
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1answer
55 views

Weaker Notion of Target Collision Resistance

I'm reading the paper “Collision-Resistant Hashing? Towards Making UOWHFs Practical” which states: While it might be easy to find a collision $M,M'$ in $F_K$ by making both $M,M'$ depend on $K$ ...
2
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1answer
69 views

Dependence on Keyed Hash Function

I'm reading the paper “Collision-Resistant Hashing? Towards Making UOWHFs Practical” which states: With an ACR hash function $F$ the key $K$ is announced and the adversary wins if she manages to ...
2
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2answers
251 views

Difference between collision resistance and target collision resistance

For a hash function, what's the difference between Collision Resistance and Target Collision Resistance?. I understand the definition of hash function collision resistance, but I don't know about ...
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2answers
159 views

Are there attacks that break collision resistance but not preimage resistance?

Are there any examples of attacks on hash functions which: break collision resistance and second preimage resistant, or break collision resistance and preimage resistant? I have looked at ...
2
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0answers
187 views

Question about modifying the MD5 plain text to cause a collision

I was trying to understand MD5 and got stuck with this question, its from Michael sipser's book on Information security Principles and Practice, 2nd edition The MD5 collision in Problem 25 is said ...
7
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1answer
793 views

Three-way hash collision

According to the birthday paradox we need approximately $O(|T|^{1/2})$ samples from the tag-space to find a collision for a hash function $h:K\times M \to T$. But how many samples are needed to find a ...
2
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0answers
78 views

Can you help me understand this “Message Modification by Meet-in-the-Middle” related to Tiger Hash? [closed]

I found this paper that explains how to attack a reduced round tiger hash, but I'm having a hard time understanding some of the mathematical notation. Could someone explain the section "Message ...
3
votes
1answer
105 views

md5sum collision question

I've been reading the wikipedia page, trying to get a better understanding of how to produce a collision. http://en.wikipedia.org/wiki/MD5#Collision_vulnerabilities The wikipedia page above states: ...
0
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1answer
392 views

probability of N hash digits colliding

What is the probability of the first N hash digits colliding? for example i made a script that appends the first 5 digits of the file's sha1 hash to the name of the file. So will the probability of ...
4
votes
1answer
471 views

Will rehashing an SHA256 hash continually, eventually produce every possible value?

So let's say you had infinite time and energy. You have a hashed string of some sort. Because you have infinite time and energy, you can produce a collision(or the original value) easily enough. But, ...
4
votes
5answers
477 views

Is it theoretically possible to construct a string that contain its own hash value?

After saw the xkcd comic self-description, I wonder is it theoretically possible to construct a self-descriptive string that contains its own hash value? Let's say the string's md5 value is ...
12
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1answer
1k views

Is truncating a SHA512 hash to the first 160 bits as secure as using SHA1?

I am from a web development background (I don't know an awful lot about cryptography or how the algorithms themselves work), so I am asking this question in simple terms. Consider a hash of the word ...
3
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1answer
59 views

Applications of 3-collisions

I recently read Improved Generic Algorithms for 3-Collisions by Joux and Lucks (Asiacrypt 2009), available as http://eprint.iacr.org/2009/305.pdf. I was wondering about applications of this technique ...
4
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4answers
427 views

Compare two hashes with different salt

I'm storing the salted hash of a credit card number in a database. What I'd like to be able to do is determine if two different entries in the same database correspond to the same credit card. ...
2
votes
1answer
135 views

Probability for collisions of a one-way compression function

Given a one-way compression function $h:\{0,1\}^n \rightarrow \{0,1\}^m$ and an attacker that picks $x_1 \ldots x_t \in \{0,1\}^n$ (uniformly distributed), I have to show that the probability to find ...