Use Cases

Hash functions have a variety of important uses in computer science, cryptography, and other fields.

Hash functions are often used to verify the integrity of data by comparing hash values.

Checksums: To verify that files are not corrupted during transmission, a hash of the file can be computed before transmission and compared after receipt.
Digital Signatures: Hash functions are used in combination with cryptography to verify the authenticity and integrity of messages or documents.

In cryptographic systems, hash functions play a vital role in ensuring security:

Password hashing: Instead of storing raw passwords, websites store hashed passwords. When a user enters a password, it is hashed, and the hash is compared with the stored value. Even if attackers steal the hashed passwords, they cannot easily recover the original passwords.
Message Authentication Codes (MACs): Hash functions are used to ensure message authenticity and integrity by combining a message with a secret key before hashing.
Digital Signatures: Cryptographic hash functions are used to create digital signatures, ensuring that messages or documents are not tampered with.

Hash functions are essential in data structures like hash tables or hash maps:

Efficient data retrieval: In a hash table, a hash function maps keys to positions in an array where the corresponding values are stored. This allows for efficient data insertion, deletion, and lookup (with an average time complexity of O(1)).
Collision handling: Good hash functions minimize collisions (when two inputs produce the same hash) to ensure efficient data retrieval.

Hash functions are used to organize and index data in databases:

Hash indexes: In databases, hash functions can be used to create hash indexes that allow for faster lookups of data. The hash function maps a value to a specific location in the database, enabling quick searches.

Some file systems use hash functions to optimize file storage and lookup:

Content-addressable storage (CAS): In systems like Git, files are stored based on the hash of their content. This ensures that only unique content is stored, reducing redundancy.
Deduplication: Hash functions can be used to identify identical chunks of data, allowing file systems or backup systems to store only one copy.

Hash functions can be used to generate unique "fingerprints" of large files, images, or media:

Data deduplication: Hashing can identify duplicate files or pieces of data for deduplication, reducing storage costs.
File comparisons: Two files with the same hash are likely to be identical, allowing for fast file comparison.

Hash functions can be used in distributed systems to evenly distribute data or tasks across multiple servers:

Consistent hashing: This technique allows a system to efficiently distribute and reassign resources (like keys or tasks) when new nodes are added or removed from a system.