Different uses of hashing

1.3.1 Compression, Encryption and Hashing

Compression, Encryption and Hashing — Note 4 of 4

Candidates should understand the need for and purpose of using hashing algorithms to store data. Candidates should be aware of different uses for hashing, such as the storing of passwords.

What you need to know

Hashing applies a hash function to data to produce a fixed-size value. Unlike encryption, hashing is one-way: the original data cannot be worked out from the hash. Hashing is used for storing passwords, checking data integrity and locating data quickly.

What to know

A good hash function is quick to calculate, produces very different hashes for similar inputs, and makes collisions (two inputs giving the same hash) rare.
Hashing is ONE-WAY: it cannot be reversed to get the original data, unlike encryption, which is designed to be decrypted.
Use — storing passwords: only the hash of a password is stored. At login, the entered password is hashed and compared with the stored hash. If the database is stolen, the passwords themselves are not revealed.
Use — hash tables: the hash of a key gives the location where a record is stored, so data can be found directly without searching.
Use — data integrity: a hash (checksum) of a file is sent with it; the receiver recalculates the hash to check the file has not been changed or corrupted.
Use — digital signatures: the hash of a message is encrypted to prove its origin and that it has not been altered.

Key terms

Hash function: An algorithm that converts input data of any size into a fixed-size output (the hash or digest).

Exam technique

Tip:Explain why hashing suits passwords: even the system administrator cannot see users' passwords, and stolen hashes cannot be reversed.
Mistake:Calling hashing a type of encryption. Encryption is two-way; hashing is one-way.

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