Hash functions are primarily used in hash tables, to quickly locate a data record ( for example, a dictionary definition ) given its search key ( the headword ).

It is possible to achieve a time-space tradeoff by pre-computing a list of hashes of dictionary words, and storing these in a database using the hash as the key.

Pre-computed dictionary attacks can be thwarted by the use of salt, a technique that forces the hash dictionary to be recomputed for each password sought, making precomputation infeasible provided the number of possible salt values is large enough.

A standard solution to the dictionary problem is a hash table ; in some cases it is also possible to solve the problem using directly addressed arrays, binary search trees, or other more specialized structures.

The LM hash also does not use cryptographic salt, a standard technique to prevent pre-computed dictionary attacks.

It offers minimal security ; the MD5 hash function is vulnerable to dictionary attacks, and does not support key generation, which makes it unsuitable for use with dynamic WEP, or WPA / WPA2 enterprise.

Brute-force attacks and dictionary attacks are the simplest methods available, however these are not adequate for systems that use large passwords, because of the difficulty of both storing all the options available, and searching through such a large database to perform a reverse-lookup of a hash.

Some hash functions may map two or more keys to the same hash value, causing a collision.

Such hash functions try to map the keys to the hash values as evenly as possible because collisions become more frequent as hash tables fill up.

Specifically, the hash function is used to map the search key to the hash.

When storing records in a large unsorted file, one may use a hash function to map each record to an index into a table T, and collect in each bucket T a list of the numbers of all records with the same hash value i. Once the table is complete, any two duplicate records will end up in the same bucket.

A good hash function should map the expected inputs as evenly as possible over its output range.

), and each input may independently occur with uniform probability, then a hash function need only map roughly the same number of inputs to each hash value.

Some of those algorithms will map arbitrary long string data z, with any typical real-world distribution — no matter how non-uniform and dependent — to a 32-bit or 64-bit string, from which one can extract a hash value in 0 through n − 1.

The basic ingredients are hash tables that map rationals to strings.

* Albireo Index – software to rapidly map hash values of chunks to their storage locations, delivering high performance writes in deduplication environments.

A checksum or hash sum is a fixed-size datum computed from an arbitrary block of digital data for the purpose of detecting accidental errors that may have been introduced during its transmission or storage.

Differential cryptanalysis is a general form of cryptanalysis applicable primarily to block ciphers, but also to stream ciphers and cryptographic hash functions.

The discovery of differential cryptanalysis is generally attributed to Eli Biham and Adi Shamir in the late 1980s, who published a number of attacks against various block ciphers and hash functions, including a theoretical weakness in the Data Encryption Standard ( DES ).

If it's not found, the key's hash is turned into another number in the same range, and the request is routed to the node whose location is closest to the key.

A CHK is a SHA-256 hash of a document ( after encryption, which itself depends on the hash of the plaintext ) and thus a node can check that the document returned is correct by hashing it and checking the digest against the key.

A hash function is any algorithm or subroutine that maps large data sets of variable length, called keys, to smaller data sets of a fixed length.

A hash function should be referentially transparent ( stable ), i. e., if called twice on input that is " equal " ( for example, strings that consist of the same sequence of characters ), it should give the same result.

This is a contract in many programming languages that allow the user to override equality and hash functions for an object: if two objects are equal, their hash codes must be the same.

This is crucial to finding an element in a hash table quickly, because two of the same element would both hash to the same slot.

Although the idea was conceived in the 1950s, the design of good hash functions is still a topic of active research.

The HashKeeper database maintained by the American National Drug Intelligence Center, for instance, is more aptly described as a catalog of file fingerprints than of hash values.

Therefore, each slot of a hash table is associated with ( implicitly or explicitly ) a set of records, rather than a single record.

For this reason, each slot of a hash table is often called a bucket, and hash values are also called bucket indices.

This observation identified the gnutella network as an unscalable distributed system, and inspired the development of distributed hash tables, which are much more scalable but support only exact-match, rather than keyword, search.

The table is often an array with two or more indices ( called a grid file, grid index, bucket grid, and similar names ), and the hash function returns an index tuple.

Basically, if some hash values are more likely to occur than others, a larger fraction of the lookup operations will have to search through a larger set of colliding table entries.

( In an ideal " perfect hash function ", no bucket should have more than one record ; but a small number of collisions is virtually inevitable, even if n is much larger than m – see the birthday paradox ).

It will however have more collisions than perfect hashing, and may require more operations than a special-purpose hash function.

This approach has proven to speed up hash code generation by a factor of five or more on modern microprocessors of

In more technologically advanced areas, modern hashish is now made with more advanced technologies by compressing and / or refining or purifying a great quantity of trichomes harvested from Cannabis sativa plants using such processes as ice-water separation, also known as " bubble melt ", producing " bubble hash " or " water hash ".

The design of the HMAC specification was motivated by the existence of attacks on more trivial mechanisms for combining a key with a hash function.

The worst-case lookup speed in an imperfect hash table is O ( N ) time, but far more typically is O ( 1 ), with O ( m ) time spent evaluating the hash.

* Buckets in a trie which are analogous to hash table buckets that store key collisions are necessary only if a single key is associated with more than one value.

* There is no need to provide a hash function or to change hash functions as more keys are added to a trie.

If the options are identified by a hash, then writing the function so that it takes a callback makes it more flexible: its user can choose whatever hashing algorithm is desired and the function will continue to work, since it uses the callback to turn option names into hashes ; thus, callbacks allow the user of a function to fine-tune it at runtime.