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

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24
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4answers
17k views

Best way to reduce chance of hash collisions: Multiple hashes, or larger hash?

I would like to maintain a list of unique data blocks (up to 1MiB in size), using the SHA-256 hash of the block as the key in the index. Obviously there is a chance of hash collisions, so what is the ...
23
votes
1answer
5k views

No SHA-1 collision? Yet SHA-1 is broken?

Is there a known pair of distinct bit strings (A,B) such that SHA-1(A) == SHA-1(B)? If the answer is no than how can SHA-1 be considered broken?
20
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6answers
13k 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")?
15
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7answers
2k 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, ...
14
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1answer
2k 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 ...
11
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1answer
655 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? ...
10
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3answers
4k 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?
10
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2answers
2k 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? ...
9
votes
3answers
577 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. ...
9
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2answers
264 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.
9
votes
1answer
4k 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 ...
8
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1answer
1k views

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 ...
8
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1answer
1k 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 ...
8
votes
1answer
2k views

From hash to Cryptographic hash

After reading some excellent papers on SipHash, I understood that good non-cryptographic hashes such as MurmurHash and CityHash are not secure for MAC usage, due to a certain type of DDos attack ...
8
votes
1answer
823 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. ...
7
votes
2answers
441 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 ...
7
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1answer
1k 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: ...
7
votes
1answer
483 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 ...
7
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2answers
314 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 ...
7
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1answer
670 views

Implementation of Tao Xie and Denguo Feng's MD5 attack

It is well known that MD5 is completely broken today - however, to understand the theory behind the attacks I am looking for an implementation of the collision attacks described in the 2009 paper A ...
6
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2answers
4k 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 $a ≠ b$?
6
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2answers
397 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 ...
6
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3answers
380 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 owners of the server and ask them what SHA1 fingerprint they have. They start reading ...
6
votes
4answers
384 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 ...
6
votes
1answer
873 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
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1answer
1k 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 ...
6
votes
1answer
863 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
302 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. ...
5
votes
2answers
260 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 ...
5
votes
3answers
839 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 ...
5
votes
1answer
463 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 ...
5
votes
2answers
441 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 ...
5
votes
1answer
130 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 ...
5
votes
1answer
248 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 ...
5
votes
1answer
520 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, ...
5
votes
1answer
250 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 ...
5
votes
2answers
2k 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 ...
5
votes
2answers
330 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 ...
4
votes
5answers
562 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 ...
4
votes
2answers
613 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, ...
4
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3answers
2k views

Is it fair to assume that SHA1 collisions won't occur on a set of <100k strings

I'm building a system that has to take file paths, and generate a unique name for each one. I'm planning on using SHA1 as the hash function. My question is: do I have to deal with possible collisions ...
4
votes
1answer
181 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
102 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
votes
2answers
841 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 ...
4
votes
1answer
110 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 $O(\min(2^{-n/2} , 2^{-c/2}))$ ...
4
votes
3answers
260 views

The difference between being not strongly collision resistant, and not weakly collision resistant?

So I understand the two concepts: (Strong) collision resistance: it is infeasible to find a pair $x \neq x'$ such that $h(x) = h(x')$ Weak collision resistance / 2nd-preimage resistance: for all $x ...
4
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1answer
150 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
votes
1answer
175 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
votes
4answers
532 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. ...
4
votes
1answer
200 views

How hard is it to generate a partial RSA fingerprint collision?

When I use SSH to log into a new system, I get asked to verify that the fingerprint (a 32 hex digits string) of the hosts's RSA key is correct. How much if it must I actually compare (by hand/eye) to ...