## hash function [English]

### InterPARES Definition

No definition in earlier IP projects. ITrust definition not yet developed.

### Other Definitions

- Bitcoin Wiki Vocabulary [2017] (†796 s.v. "Hash function"): A computer algorithm which takes an arbitrary amount of input data and deterministically produces fixed length output, known as the data's "hash", that can be used to easily verify that data has not been altered. If you change any single bit of the original data and run the hash algorithm, the hash will completely change. Because the hash is seemingly random, it is prohibitively difficult to try to produce a specific hash by changing the data which is being hashed.
- ISO TC307 (Japan). 2017 (†857 s.v. "3.4 hash" ; p.8): mathematical function that maps values from a larger set of values into a smaller set of values, reducing a long value into a shorter hashed value. [SOURCE: ISO/IEC 14776-261:2012, 3.1.80]
- ISO TC307 N38 (United States). 2017. (†834 s.v. "Cryptographic Services"): Hash functions are often used to protect the ledger from modifications. Any change to information in the ledger will result in a computed hash that is different from the hash that was previously computed and stored for the ledger. A new hash is computed each time a transaction is added to the ledger.
- ISO/IEC 10118-1:2016 (†843 3.4): function which maps strings of bits of variable (but usually upper bounded) length to fixed-length strings of bits, satisfying the following two properties: — for a given output, it is computationally infeasible to find an input which maps to this output; — for a given input, it is computationally infeasible to find a second input which maps to the same output. · Note 1 to entry: Computational feasibility depends on the specific security requirements and environment.
- Scaling Bitcoin [2017] (†845 s.v. "Hash function"): A cryptographic hash function is a mathematical algorithm that maps data of arbitrary size to a bit string of a fixed size (a hash function) which is designed to also be one-way function, that is, a function which is infeasible to invert. The only way to recreate the input data from an ideal cryptographic hash function's output is to try a large number of possible inputs to see if they produce a match. Bruce Schneier has called one-way hash functions "the workhorses of modern cryptography". The input data is often called the message, and the output (the hash value or hash) is often called the message digest or simply the digest.
- Schneier 2004 (†825 para 2-3): One-way hash functions are a cryptographic construct used in many applications. They are used with public-key algorithms for both encryption and digital signatures. They are used in integrity checking. They are used in authentication. They have all sorts of applications in a great many different protocols. Much more than encryption algorithms, one-way hash functions are the workhorses of modern cryptography. · In 1990, Ron Rivest invented the hash function MD4. In 1992, he improved on MD4 and developed another hash function: MD5. In 1993, the National Security Agency published a hash function very similar to MD5, called the Secure Hash Algorithm (SHA). Then in 1995, citing a newly discovered weakness that it refused to elaborate on, the NSA made a change to SHA. The new algorithm was called SHA-1. Today, the most popular hash function is SHA-1, with MD5 still being used in older applications.
- Seibold et al. 2016 (†821 p.18): An application programming interface creates, through a process called hashing, a unique key or digital fingerprint for each file.

### Citations

- BlockchainHub Glossary (†807 s.v. "Cryptographic Hash Function"): is a one-way hash function which is considered practically impossible to invert, that is, to recreate the input data from its hash value alone. The input data is often called the message, and the hash value is often called the message digest or simply the digest. Main properties (1) easy to compute the hash value for any given message (2) infeasible to generate a message from its hash (3) infeasible to modify a message without changing the hash (4) infeasible to find two different messages with the same hash. Cryptographic hash functions have many information security applications, notably in digital signatures, message authentication codes (MACs), and other forms of authentication. They can also be used as ordinary hash functions, to index data in hash tables, for fingerprinting, to detect duplicate data or uniquely identify files, and as checksums to detect accidental data corruption. (†2364)
- Dupont & Maurer 2015 (†856 ): Hash functions accept arbitrary data as input and output a fixed-size unique “fingerprint” of the input. The guarantee of uniqueness is critical here and is made possible by a form of cryptography derived from “public-key” cryptography first invented in the 1970s. (†2387)