A hash function that maps names to integers from 0 to 15 ..

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.

The values returned by a hash function are called hash values, hash codes, hash sums, checksums or simply hashes.

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.

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

Thus, the hash function only hints at the record's location — it tells where one should start looking for it.

Still, in a half-full table, a good hash function will typically narrow the search down to only one or two entries.

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.

For that purpose, one needs a hash function that maps similar keys to hash values that differ by at most m, where m is a small integer ( say, 1 or 2 ).

If one builds a table T of all record numbers, using such a hash function, then similar records will end up in the same bucket, or in nearby buckets.

For this application, the hash function must be as insensitive as possible to data capture or transmission errors, and to " trivial " changes such as timing and volume changes, compression, etc.

In this case, the input strings are broken into many small pieces, and a hash function is used to detect potentially equal pieces, as above.

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.

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

A good randomizing function is ( barring computational efficiency concerns ) generally a good choice as a hash function, but the converse need not be true.

( 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 ).

When testing a hash function, the uniformity of the distribution of hash values can be evaluated by the chi-squared test.

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.

* A hash or dictionary or map is a more flexible variation on a record, in which name-value pairs can be added and deleted freely.

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.

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.

Arrays are used to implement other data structures, such as heaps, hash tables, deques, queues, stacks, strings, and VLists.

However, block ciphers may also be used as building blocks in other cryptographic protocols, such as universal hash functions and pseudo-random number generators.

This led to the addition of the number sign (#, sometimes called ' octothorpe ,' ' pound ' or ' diamond ' in this context-' hash ' or ' gate ' in the UK ) and asterisk or " star " (*) keys as well as a group of keys for menu selection: A, B, C and D. In the end, the lettered keys were dropped from most phones, and it was many years before these keys became widely used for vertical service codes such as * 67 in the United States of America and Canada to suppress caller ID.

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 ).

When this approach is used, the hash function must be chosen so that the result has fairly uniform distribution between 0 and n − 1, for any value of n that may occur in the application.

When the hash function is used to store values in a hash table that outlives the run of the program, and the hash table needs to be expanded or shrunk, the hash table is referred to as a dynamic hash table.

For such data, one must use a hash function that is compatible with the data equivalence criterion being used: that is, any two inputs that are considered equivalent must yield the same hash value.

In cryptographic hash functions, a Merkle – Damgård construction is usually used.

Any cryptographic hash function, such as MD5 or SHA-1, may be used in the calculation of an HMAC ; the resulting MAC algorithm is termed HMAC-MD5 or HMAC-SHA1 accordingly.

In cryptography, oracles are used to make arguments for the security of cryptographic protocols where a hash function is used.

The most common type of structured P2P networks implement a distributed hash table ( DHT ), in which a variant of consistent hashing is used to assign ownership of each file to a particular peer, in a way analogous to a traditional hash table's assignment of each key to a particular array slot.

More efficient directed search strategies, including supernodes and distributed hash tables, are now used.

MD5 and SHA1 are frequently used cryptographic hash functions but they are not recommended for password hashing unless they are used as part of a larger construction such as in PBKDF2.