Hash

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What is a hash?

A hash is the output of a hash function: a mathematical algorithm that takes input data of any size and produces a fixed-size string (typically represented in hexadecimal). Hash functions are deterministic (the same input always yields the same hash) and designed so that deriving the original input from the hash is computationally infeasible.

How hash functions work (overview)

A hash function processes input through a sequence of operations—binary conversion, chunking, mixing/compression rounds—and produces a fixed-length value called a digest. For example, the SHA-256 algorithm always outputs a 256-bit (64-hex-character) digest, no matter the input length:

Hello
: 185f8db32271fe25f561a6fc938b2e264306ec304eda518007d1764826381969

Hello world
: 64ec88ca00b268e5ba1a35678a1b5316d212f4f366b2477232534a8aeca37f3c

Hello John
: a8119595d77342cc73c93697a7f70920d3f4ded5d458e31907607e997ff76868

Small changes to input (including capitalization) produce entirely different hashes.

Key cryptographic properties

Cryptographic hash functions used in security contexts typically provide:

  • Collision resistance: it is infeasible to find two different inputs with the same hash.
  • Preimage resistance (hidden): given a hash, it is infeasible to recover an input that produces it.
  • Second-preimage resistance (puzzle-friendly): given one input, it is infeasible to find a different input that hashes to the same value, and it should be hard to construct an input that yields a specific hash.

These properties make hashes useful for verifying integrity and preventing tampering.

Common hashing algorithms

Different systems use different algorithms depending on security and performance needs. Examples include:
– SHA-256 (widely used in Bitcoin)
– Keccak-256 (used by Ethereum)
– Scrypt, Ethash, Equihash (used by various PoW cryptocurrencies)
– Blake3 (modern, high-performance hash)
– Argon2, bcrypt, PBKDF2 (designed for password hashing with resistance to brute-force)

Role of hashing in blockchain

In a blockchain, each block contains transaction data plus metadata (the block header). The header is hashed and that hash links blocks together; any change to a block’s contents changes its hash and breaks the chain unless subsequent blocks are recalculated. Comparing hashes is the primary way blockchains detect tampering and ensure data integrity, which helps prevent fraud such as double-spending.

How Bitcoin uses hashing (simplified)

Bitcoin’s proof-of-work requires miners to find a nonce (a variable input) such that the double SHA-256 hash of the block header is less than or equal to a network target. Miners repeatedly change the nonce and rehash until they find a value meeting the target. When a valid hash is found, nodes verify the hash (by hashing the header twice) and accept the block if it meets the target and the transactions are valid. This computational puzzle secures the network and establishes consensus.

Other practical uses

  • Data integrity checks (file downloads, backups)
  • Password storage (with salts and slow hash functions)
  • Digital signatures and message authentication
  • Deduplication and content-addressable storage
  • Compact identifiers for large data blobs

Simple hash example (non-cryptographic)

A pedagogical example is the mid-square method: square a number and take the middle digits as the hash. For instance, 61^2 = 3721 → hash = 72. This method is not cryptographically secure but illustrates fixed-size output mapping.

FAQs

Conclusion

Hashes are compact, deterministic representations of data that provide integrity, tamper resistance, and foundational security for systems such as blockchains. While simple to compute, properly designed cryptographic hashes are effectively irreversible and collision-resistant within practical limits, making them essential tools in modern digital security.