Overview

Cryptographic hash function – Wikipedia

Overview

Hash Function Properties – Κεντρική Σελίδα

Hash Function Properties. Hash functions compress a n (abritrarily) large number of bits into a small number of bits (e.g. 512).

Properties of Secure Hash Functions | Denim Group

This is the most usual property developers think of when they think of a cryptographic hash function. Unlike an encryption, there should be no “dehashing” function. A good preimage resistant function should be “hard” to invert. An example of a hash function that …

Cryptography Hash functions – tutorialspoint.com

Features of Hash Functions

Properties of a Cryptographic Hash Function – Hash

Continuing on our exploration of the fundamental concept of cryptography, this module will explain the Hash Function, its purpose and application, potential attack vectors, and the importance of hash functions on cryptographic design.

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Hash Function Properties – Nc State University

Computer Science CSC/ECE 574 Dr. Peng Ning 5. Desirable Properties of Hash Functions • Consider a hash function H. – Performance: Easy to compute H(m) – One-way property: Given H(m) but not m, it’s computationally infeasible to find m – Weak collision resistance (free): Given H(m), it’s computationally infeasible to find m’ such that H(m’) = H(m).

blockchain – properties of a cryptographic hash function

In the week 1 lecture of the bitcoin coursera course, there is a discussion of the 3 properties of a cryptographic hash functions: Collision-resistance: A hash function H is said to be collision resistant if it is infeasible to find two values, x and y , such that x != y , yet H(x)= H(y).

I had the same thought, and the difference is indeed subtle. I had to think about this awhile.

Suppose you had a hash, BadHash. You pick x’ and a random nonce r’, compute y’ = BadHash(r’|x’), and give me y’.Best answer · 0Restructuring the definitions made it a bit clearer to me.

Collision-resistance:

Given: x and h(x).

Hard to find: y that is distinct from x and such that h(y)=h(x).

Hiding:

Given: h(r|x).

Secret: x and a highly-unlikely-and-randomly-chosen r.

Hard to find: y such that h(y)=h(r|x).0This course is extremely confusing and poorly written.

In the hiding problem, you are trying to find x, knowing the value H(r|x) (and knowing H of course). But you don’t know r! For example the set for x could be {0,1} but you can’t decide between 0 or 1 because adding a secret r to x mixes all the possible hashes.0Let’s have: y = H(x|r). Here the output is y, and inputs are r and x.

Hiding property:

Given an output of a hash function (y), it is infeasible to find an input (x). Note, r is not given.

Puzzle-friendly property:

Given an output of a hash function (y) and part of the input (r), it is difficult to find an input (x).0Consider this algo: Take any text file and assume a=1, b=2, c=3 and so on and calculate the sum for all the letters and if the net sum is less than 5 you win an award. Lets say you didn’t win the first time, so you add some arbitrary text to the end of this file(nonce) and do this calculation again, until you win.0Assume that x is the outcome of a coin toss, ie. x is 0 or 1. Given H(x) noone should be able to find x, but An attacker can easily find x, given y=H(x), since there there are only two possible hash values. He computes H(0) and H(1) and checks which one equals y.

Now, assume that you prepend a large random key to x and you compute H(k x).0

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Cryptographic properties of hash functions – Aktuálne

Informally speaking, a hash function is a function that maps messages of an arbitrary length to strings of ﬁxed length. An outputof a hash functionwith some message on its input is called a hash of the message. One of the many applications of hash functions are digital signatures.

algorithm – What is a good Hash Function? – Stack Overflow

A good hash function has the following properties: Given a hash of a message it is computationally infeasible for an attacker to find another message such that their hashes are identical. Given a pair of message, m’ and m, it is computationally infeasible to find two such that that h(m) = h(m’)

For doing “normal” hash table lookups on basically any kind of data – this one by Paul Hsieh is the best I’ve ever used. http://www.azillionmonkeys.com/qed/hash.html If you care about cryptographically secure or anything else more advanced, then YMMV. If you just want a kick ass general purpose hash function for a hash table lookup, then this is what you’re looking for.Best answer · 30There’s no such thing as a “good hash function” for universal hashes (ed. yes, I know there’s such a thing as “universal hashing” but that’s not what I meant). Depending on the context different criteria determine the quality of a hash. Two people already mentioned SHA. This is a cryptographic hash and it isn’t at all good for hash tables which you probably mean. Hash tables have very different requirements. But still, finding a good hash function universally is hard because different data types expose different information that can be hashed. As a rule of thumb it is good to consider all information a type holds equally. This is not always easy or even possible. For reasons of statistics (and hence collision), it is also important to generate a good spread over the problem space, i.e. all possible objects. This means that when hashing numbers between 100 and 1050 it’s no good to let the most significant digit play a big part in the hash because for ~ 90% of the objects, this digit will be 0. It’s far more important to let the last three digits determine the hash. Similarly, when hashing strings it’s important to consider all characters – except when it’s known in advance that the first three characters of all strings will be the same; considering these then is a waste. This is actually one of the cases where I advise to read what Knuth has to say in The Art of Computer Programming , vol. 3. Another good read is Julienne Walker’s The Art of Hashing .49There are two major purposes of hashing functions: to disperse data points uniformly into n bits. to securely identify the input data. It’s impossible to recommend a hash without knowing what you’re using it for. If you’re just making a hash table in a program, then you don’t need to worry about how reversible or hackable the algorithm is SHA-1 or AES is completely unnecessary for this, you’d be better off using a variation of FNV . FNV achieves better dispersion (and thus fewer collisions) than a simple prime mod like you mentioned, and it’s more adaptable to varying input sizes. If you’re using the hashes to hide and authenticate public information (such as hashing a password, or a document), then you should use one of the major hashing algorithms vetted by public scrutiny. The Hash Function Lounge is a good place to start.9This is an example of a good one and also an example of why you would never want to write one.

It is a Fowler / Noll / Vo (FNV) Hash which is equal parts computer science genius and pure voodoo: unsigned fnv_hash_1a_32 ( void *key, int len ) {

unsigned char *p = key;

unsigned h = 0x811c9dc5;

int i;

for ( i = 0; i < len; i++ )

h = ( h ^ p[i] ) * 0x01000193;

return h;

}

unsigned long long fnv_hash_1a_64 ( void *key, int len ) {

unsigned char *p = key;

unsigned long long h = 0xcbf29ce484222325ULL;

int i;

for ( i = 0; i < len; i++ )

h = ( h ^ p[i] ) * 0x100000001b3ULL;

return h;

}

Edit: Landon Curt Noll recommends on his site the FVN-1A algorithm over the original FVN-1 algorithm: The improved algorithm better disperses the last byte in the hash. I adjusted the algorithm accordingly.5I’d say that the main rule of thumb is not to roll your own. Try to use something that has been thoroughly tested, e.g., SHA-1 or something along those lines.3A good hash function has the following properties: Given a hash of a message it is computationally infeasible for an attacker to find another message such that their hashes are identical. Given a pair of message, m’ and m, it is computationally infeasible to find two such that that h(m) = h(m’) The two cases are not the same. In the first case, there is a pre-existing hash that you’re trying to find a collision for. In the second case, you’re trying to find any two messages that collide. The second task is significantly easier due to the birthday “paradox.” Where performance is not that great an issue, you should always use a secure hash function. There are very clever attacks that can be performed by forcing collisions in a hash. If you use something strong from the outset, you’ll secure yourself against these. Don’t use MD5 or SHA-1 in new designs. Most cryptographers, me included, would consider them broken. The principle source of weakness in both of these designs is that the second property, which I outlined above, does not hold for these constructions. If an attacker can generate two messages, m and m’, that both hash to the same value they can use these messages against you. SHA-1 and MD5 also suffer from message extension attacks, which can fatally weaken your application if you’re not careful. A more modern hash such as Whirpool is a better choice. It does not suffer from these message extension attacks and uses the same mathematics as AES uses to prove security against a variety of attacks. Hope that helps!1What you’re saying here is you want to have one that uses has collision resistance. Try using SHA-2. Or try using a (good) block cipher in a one way compression function (never tried that before), like AES in Miyaguchi-Preenel mode. The problem with that is that you need to:

Hash function for strings in C |

c – What integer hash function are good that accepts an |

Cryptographic Hash Functions Explained: A Beginner’s Guide

Hash functions are often called one-way functions because, according to the properties listed above, they must not be reversible. If an attacker could easily reverse a hash function, it …

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Chapter 6 Hash Functions – University of California, San Diego

The most basic security property of a hash function is collision-resistance, which measures the ability of an adversary to ﬁnd a collision for an instance of a family H. There are diﬀerent notions of collision-resistance, varying in restrictions put on the adversary in its quest for a collision.

Hash function property – Cryptography Stack Exchange

Let $H$ be a hash function that is both hiding and puzzle friendly. Consider $G(z) = H(z) \Vert z_l$ where $z_l$ is the last bit of $z$. Show that $G$ is puzzle

Hash Functions – Sans Technology Institute

The primary application of hash functions in cryptography is message integrity. The hash value provides a digital fingerprint of a message’s contents, which ensures that the message has not been altered by an intruder, virus, or by other means. Security Laboratory: Cryptography in Business Series. Other Related Articles in Security