# Tagged Questions

A hash collision is any pair of values (preimages) that when hashed produce the same digest (image). Finding even one example of a collision should be infeasible in a cryptographically secure hash function.

69 views

### Increased security weaknesses when truncating SHA1

Given the following preconditions: A plaintext A (~512bit length), that is hashed by using SHA-1 and truncated to (the first) 64 bits. I know both, the plaintext and the truncated hash, and want ...
184 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 ...
576 views

### How many possible input files are there who's length can be stored in 63-bits?

I'm trying to calculate how many possible combinations of files there can be using a signed 64-bit file length, but can't seem to find a formula (or I'm using the wrong keywords). For example, the ...
60 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 ...
67 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
19 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. ...
45 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 ...
39 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 ...
83 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: $E(brute)$ = ...
70 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 ...
71 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 ...
66 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. ...
42 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 ...
67 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 ...
35 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 $K$ ...
154 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 ...
261 views

### Tunnels used in md5

I'm reading the paper "Message Freedom in MD4 and MD5 Collisions: Application to APOP" about finding collisions for the MD5 hash algorithm involving the concept of tunnels as described in "Tunnels in ...
5k views

### 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")?
40 views

### SHA-512 hash collisions [duplicate]

Possible Duplicate: Is SHA-512 bijective when hashing a single 512-bit block? Is there any guarantee that there are no collisions for all 512-bit input values for SHA-512 (every 512-bit ...
1k views

### Are there any known collisions for the SHA-2 family of hash functions?

Are there any known collisions for the hash functions SHA-1, SHA-224, SHA-256, SHA-384, and SHA-512? By that, I mean are there known values of $a$ and $b$ where $F(a) = F(b)$ and $a ≠ b$?
2k views

### What is pre-image 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?
514 views

### Change in probability of collision when removing digits from MD5 hexadecimal hash values

I am aware that MD5 has a known collision vulnerability and should not be relied upon when uniqueness is required, but in the environment I am working on I only have access to MD5 hash function. ...
115 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 ...
129 views

### Three-way hash collision

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

### Second pre-image imply pre-image resistance

I am study hash functions. I get understand colision resistance imply second pre-image resistance, but I can't get, yet, Why Second pre-image imply pre-image resistance?. Anybody Could be help me with ...
500 views

### Counter mode secure hash algorithm

Ever since the SHA-3 competition, I've been wondering if it is possible to create a hash algorithm that is easier to parallelize. The current algorithms all seem to require building a tree of hashes. ...
43 views

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

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 ...
80 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: ...
320 views

### Is there a hash function which has no collisions?

Is there a hash function which has no collisions? To clarify: it would be some function which would produce variable-length output, and never produce the same output for differing input. It would ...
193 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 ...
137 views

### Hash function based on block cipher (and proof of security in the PRP model)

I wonder if there exists proofs of security of primitives like hash function (based on block cipher) in the PRP model. I often see proofs in the random oracle model (for hash function based on ...
300 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, ...
298 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 ...
655 views

### what kind of hash function can provide a short hash and be collision resistant?

When you try to connect via SSH, you see a signature which is short but I heard it is even stronger than sha256. It is perhaps stronger because it uses more rounds. Is there a hash function or a ...
630 views

### Is truncating a SHA512 hash to the first 40 characters 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 ...
109 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 ...
148 views

### Finding a collision for a hash function

I'm trying to find a collision for the following (modified) Merkle-Damgard hash function. Suppose we already have a hash function $h : \mathbb{Z}_2^{2·n} \to \mathbb{Z}_2^n$ for fixed length bit ...
307 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. ...
280 views

### 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 ...
54 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 ...
97 views

### Digital Signature Scheme Count

My question is How I will be able to count digital signature Scheme that can sign many documents with one private key. There are signature scheme as long as exists private keys in the keys spaces? or ...
77 views

395 views

### Using an MD5 hash as a password

Suppose Alice is using a password prompt that only accepts up to 32 characters for any particular password. Memorization of long strings of random characters is not one of Alice's strengths, so she ...
474 views

### Increased CRC collision probability when adding bits to input message

The Scenario I have a message string I need to transport over a wireless network that may be unreliable. This message string is about 100 bits long, and is packaged with an 8-bit CRC. When the ...
238 views

### Finding hash almost-collisions

A few months ago, XKCD posted a challenge to find a plaintext which hashed (using Skein 1024 1024) to a specified value. Inputs were scored based on the hamming distance between the hash of the ...