Hash

A hash is the output of a hash function that converts any input data (text, file, password, string, binary object) into a fixed sequence of characters. Hashing is used for verifying data integrity, securely storing passwords, enabling digital signatures, supporting distributed systems, and powering many network and server mechanisms.

A key property of a hash is the impossibility of reconstructing the original data from the resulting value, as well as high sensitivity to any changes in the input: even a one-character modification completely changes the output.

How a hash function works

A hash function accepts a message of variable length and processes it block by block using mathematical and logical operations. The result is a hash of fixed size. For example, SHA-256 always produces a 256-bit (64-character) hash regardless of the size of the original data.

Hashing is irreversible: the function is not designed for backward transformation. It is also computationally infeasible to find another input that produces the same hash (the collision problem) if the function is considered cryptographically secure.

Key properties of hashes

• Determinism. The same input always produces the same output.
• Sensitivity to changes. A minimal modification of the input radically changes the hash.
• Irreversibility. It is impossible to restore the original data from a hash.
• Collision resistance. Two different messages should not result in the same hash.
• High performance. Algorithms are optimized for fast computation.

Types of hash functions

• Cryptographic. Used for security: SHA-2, SHA-3, BLAKE2, Whirlpool. They are resistant to brute force and collisions.
• Non-cryptographic. Used for data indexing, hash tables, distributed computing: CRC32, MurmurHash, CityHash. They are fast but unsuitable for protecting sensitive information.
• Salted hashes. A password is combined with a random string (salt) before hashing. This protects against dictionary attacks and rainbow tables.

Where hashing is used

Hashes are used in operating systems, network protocols, databases, blockchains, cloud services, logging systems, and DevOps tools.

For example:

• when registering a user password, the system stores only the hash and not the password itself;
• in digital signatures, a hash is used to verify the integrity of a message;
• Git stores file versions using their hashes;
• a blockchain links blocks together via the hash of the previous block;
• CDNs and browsers check whether a file has changed by comparing its hash;
• hash tables use hashes for fast data lookup.

Benefits

Hashing provides data security without storing originals, speeds up lookup and integrity verification, reduces infrastructure load, and increases system reliability. It prevents unauthorized recovery of original information, and collision resistance protects against tampering.

FAQ

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