Hashing

Hashing refers to the process of converting input data of arbitrary length into a fixed-size string of characters, typically produced by a hash function. The output is commonly called a hash value or hash code. A hash function is deterministic: the same input always yields the same hash. A good hash function also aims to distribute outputs uniformly and be efficient to compute. The output length is fixed for a given hash function, regardless of input size.

There are cryptographic hash functions and non-cryptographic hash functions. Cryptographic hashes, such as SHA-256 and SHA-3, are designed to be resistant to various attacks: preimage resistance (difficult to reverse), second preimage resistance, and collision resistance (difficult to find two inputs with the same hash). They are used for data integrity, digital signatures, and password storage when combined with salts and key stretching. Non-cryptographic hashes, such as those used in hash tables or checksums, prioritize speed and uniform distribution and do not provide strong security guarantees.

Common applications include using hashes in hash tables to map keys to buckets; in content-addressable storage and version control systems where data is addressed by its hash; in data deduplication and caching; and in distributed systems using consistent hashing to balance load.

Limitations and concerns include the possibility of hash collisions, since many inputs can share the same hash. Some older algorithms, such as MD5 and SHA-1, have vulnerabilities and are being phased out for security-sensitive use. A hash is not encryption, and reversing a hash to retrieve the original data is not generally feasible, but the original data can sometimes be inferred with sufficient effort if the hash is weak. Password storage typically uses salted hashing to enhance security.