Use md5hashing.net to calculate and look up 66 hash digest types. It's common knowledge that the decryption of a "hash" is impossible. This service uses "reverse lookup" via the database to match a hash to its value. Our database is around ~3000M records in size and keeps growing.

A hash function is any algorithm that maps data of a variable length to data of a fixed length. The value returned by a hash function called hash digest, hash value, hash code, hash sum, checksum, or simply "hash."

Hash functions are primarily used to generate fixed-length output data that acts as a shortened reference to the original data. Hashing is useful when the original data is too cumbersome to use in its entirety.

One practical use is a data structure called a "hash table," where are data and its hash digest stored associatively. Searching for a string of a variable length in a list is slow, but the hashed value used to store a reference to the original data retrieved in uninterrupted time (barring collisions) — fixed-length hash digest is the perfect solution for building indexes in databases.

Hash functions used to accelerate table lookup or data comparison tasks such as finding items in a database, detecting duplicated or similar records in a large file, finding similar stretches in DNA sequences, and other data-driven tasks.

Another use is in cryptography, the science of encoding, and safeguarding data. It is easy to generate hash values from input data and easy to verify that the data matches the hash, but hard to 'fake' a hash value to hide malicious data. Hash sum is the principle behind the Pretty Good Privacy algorithm for data validation (data integrity check).

A hash function should be deterministic: when it is invoked twice on pieces of data that should be considered equal (e.g., two strings containing the same characters), the function should produce the same value. This policy is crucial to the correctness of virtually all algorithms based on hashing. In the case of a hash table, the lookup operation should look at the slot where the insertion algorithm stored the data that sought, so it must generate the same hash value as output.

Hash functions are typically not invertible, meaning that it is not possible to reconstruct the input datum x from its hash value h(x) alone. In many applications, it is common that several values hash to the same value, a condition called a hash collision. Since collisions cause "confusion" of objects, which can make exact hash-based algorithm slower rough ones, less precise, modern hash algorithms designed to minimize the probability of collisions. For cryptographic uses, hash functions engineered in such a way that it is impossible to reconstruct any input from the hash alone without expending vast amounts of computing time, such functions often referred to as "one-way function."

Hash functions are related to (and often confused with) checksums, check digits, fingerprints, randomization functions, error-correcting codes, and cryptographic. Although these concepts overlap to some extent, each has its own uses and requirements, designed and optimized differently. The Hash Keeper database maintained by the American National Drug Intelligence Center, for instance, is more aptly described as a catalog of file fingerprints than hash values.

Learn more about Hashing and Hash functions at Wikipedia: Hash function