Tagged Questions

5
votes
1answer
129 views

Is Wikipedia's table about SHA-2 collisions correct?

I was looking a Wikipedia article on SHA-2, and the "Comparison of SHA functions" table seems to indicate that SHA-2 is less secure than SHA-1. Is this true, or is the table wrong / misleading? ...
3
votes
1answer
117 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 ...
6
votes
2answers
161 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 ...
2
votes
4answers
163 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 ...
4
votes
2answers
183 views

Are cryptographic hash functions perfect hash functions?

For a cryptographic hash function and input values of shorter length than the hash function output, it's pretty obvious that there should be as few collisions as possible. But are there guaranteed to ...
0
votes
0answers
54 views

Strength of Combining Hash functions [duplicate]

If I combine two hash functions, what will the impact on the strength of the resulting function. If I combine in following way: H1*H2 (multiply) H1 + H2 (concat) H1 Xor H2 H1 (H2) EDIT: Lets say H1 ...
1
vote
0answers
69 views

Is SHA-1 collision free on data up to 20 bytes long? [duplicate]

Is SHA-1 collision free on data up to 20 bytes long (lenght of hash / internal state)? That means that every input produce unique output, but you surely know that, i just write it in order my question ...
4
votes
2answers
162 views

Is SHA-1 still practical secure under specific scenarios?

It is conjectured that SHA-1 has been broken from the "research" perspective but no in real world. That is that there is an algebraic attack that explores weaknesses on its algrebraic construction. ...
6
votes
1answer
168 views

Can I find two specific words with the same md5 hash?

I want to find two strings containing special words like "yes" or "no", mixed with random characters, for which the MD5 hash is equal. An example of what I'm looking for: ...
2
votes
2answers
205 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 ...
1
vote
1answer
101 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 ...
4
votes
3answers
191 views

How many bits of hash are realistically needed for key verification?

Say I'm connecting to a web server secured with TLS but with a self-signed certificate. Accordingly, I call the owner of the server and ask him what SHA1 fingerprint he has. He starts reading out the ...
0
votes
1answer
75 views

Does a break in a collision resistance property of a hash function by definition implies an attack at the first pre-image attack?

Is there a formal security proof in the shape of reduction that states that if an attacker manages to break the collision resistance property of a cryptographic hash function (a random oracle) he will ...
3
votes
4answers
148 views

Can there be two hash functions without common collisions?

Is there a way to prove/create (or are there known hash functions) two hash functions that never have the same collision? I mean, like provable in way that someone who took one cryptography class in ...
11
votes
1answer
832 views

No SHA-1 Collision? Yet SHA1 is broken?

Is there a known pair of distinct bit strings (A,B) such that SHA1(A) == SHA1(B)? If the answer is no than how can SHA1 be considered broken?
4
votes
2answers
187 views

Do MD5's weaknesses affect Oplop?

Oplop is an algorithm that generates account-specific passwords from a master password and user-chosen nickname (typically username@domain). From the website: Concatenate the master password with ...
4
votes
0answers
87 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 ...
6
votes
1answer
972 views

How does a birthday attack on a hashing algorithm work?

A "normal", brute-force attack on a cryptographic hashing algorithm $H$ should have a complexity of about $2^{n}$ for a hash algorithm with an output length of $n$ bits. That means it takes about ...
1
vote
1answer
186 views

How to represent a 32-byte SHA2 hash in the shortest possible string?

I'm calculating a SHA2 hash of a certain sensitive key value. I need to store files on disk using this hash a directory path prefix. So lets say I hash the key value 150023, I get a 32-byte value ...
2
votes
1answer
125 views

Question about hash collisions

If we have a hash function $h(x)$ and then a hash function $H(X) = h(h(X_0) || h(X_1))$ where $X_0$ is the first half of $X$, $X_1$ is the second half of $X$ and $||$ is concatenation. Then assuming ...
4
votes
2answers
550 views

128 bit hash with least chance of collision

I'm building a storage system for JSON documents where they are looked up on a 128 bit key. These JSON documents have a timestamp within them, but apart from that are user-entered data. These JSON ...
6
votes
3answers
405 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. ...
0
votes
1answer
304 views

Creating a hash of XOR'd blocks

Suppose a message $m$ is divided into blocks of length $160$ bits: $m > = M_1 || M_2 || ... || M_l$ And define $h(m) = M_1 \oplus M_2 \oplus ... \oplus M_l$ Which of the three desirable ...
8
votes
5answers
2k 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 hash (MD5) value, i.e. an MD5 collision?
-2
votes
1answer
301 views

How many possible combinations of input files are possible using a 63-bit length?

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 ...
7
votes
3answers
1k 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? Are there any known preimage attacks that would be considered feasible?
8
votes
2answers
194 views

Why would you expect to find a collision in a hash function after approximately $\sqrt{n}$ hashes?

I can't get an intuitive understanding of why it's $2^{(\frac{n}{2})}$ and not $2^n$, where $n$ is the number of bits of which the key consists.
2
votes
1answer
480 views

Why doesn't preimage resistance imply the second preimage resistance?

Let the preimage resistance be defined as »given a hash value $h$, it is hard to find any message $m$ such that $\operatorname{hash}(m)=h$«, and let the second preimage resistance be defined as »given ...
5
votes
2answers
683 views

Strength of multiple hash iterations?

Is it correct that increasing the iteration possibly decreases the cipher strength but increases the amount of time it would take to find the original hash values if using brute-force on a given hash? ...
4
votes
3answers
415 views

Is the number of creatable torrents limited?

Currently, a magnet link containing a 40-digits long SHA-hash value, is assigned to every torrent which is created. Therefore, this hash should be unique to identify a torrent and send the right bytes ...
9
votes
7answers
1k views

Is calculating a hash code for a large file in parallel less secure than doing it sequentially?

I would like to improve the performance of hashing large files, say for example in the tens of gigabytes in size. Normally, you sequentially hash the bytes of the files using a hash function (say, ...
7
votes
1answer
368 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. ...