Clear DefinitionsHash
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Pronunciation
[hash]
Analogy
Think of a hash like a fingerprint. Just as a fingerprint uniquely identifies a person regardless of their size, a hash uniquely identifies a piece of data regardless of its size. And just as you can't recreate a person from their fingerprint, you can't recreate the original data from its hash.
Definition
A fixed-length alphanumeric string that uniquely represents data of any size, generated by running that data through a cryptographic hash function. In blockchain systems, hashes are used to identify blocks, transactions, and create the links between blocks.
Key Points Intro
Hashes are fundamental building blocks of blockchain security and structure.
Key Points
Always produces a fixed-length output regardless of input size.
Even tiny changes in input produce completely different hash outputs.
Computationally infeasible to reverse-engineer the original data from its hash.
Extremely unlikely for two different inputs to generate the same hash (collision-resistant).
Example
A Bitcoin block header contains the hash of the previous block, creating the 'chain' in blockchain. When you look at a block explorer, you'll see blocks identified by hash values like '000000000000000000070a8d817883f43cb33a5f0b61d89baa222d41b92c2583'.
Technical Deep Dive
Common cryptographic hash functions in blockchain include SHA-256 (used by Bitcoin), Keccak-256 (used by Ethereum), and Blake2b. A cryptographic hash function must be deterministic (same input always produces same output), quick to compute, pre-image resistant (can't determine input from output), small changes to input cause large changes in output (avalanche effect), and collision-resistant (difficult to find two inputs with same output).
Security Warning
Not all hash functions are suitable for cryptographic use. MD5 and SHA-1, for example, are broken and should not be used for security-critical applications as they are vulnerable to collision attacks.
Caveat
While current cryptographic hash functions are secure against classical computers, quantum computing advancements may eventually threaten their security, requiring migration to quantum-resistant alternatives.
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