A hash is a digital fingerprint or identifier generated by passing any data through a hash function. The resulting collision-resistant hash is a fixed-size alphanumeric string, regardless of the size of the input data. Here are a few examples of keccak-256 hashing:
This illustrates how, even with a larger input, the hash output remains a fixed length.
How hashing works
The process of hashing starts when input data, like a transaction or a message, is divided into smaller chunks. The hash function then processes these chunks in stages:
Dividing the data: The input is split into blocks of a fixed size, depending on the hash function. For example, in SHA-256, data is divided into 512-bit blocks. If data isn’t divisible by 512 bits (64 bytes), the hash function uses padding to ensure that the data fits the block size. Padding involves adding extra bits to the input data until it reaches the required block length.
Compression function: Each block is passed through a compression function, which mixes the data. The compression function operates on the blocks iteratively, processing one block at a time and continually transforming the intermediate results.
Mixing and permutation: As the function processes the data, it performs operations to scramble it, such as bitwise shifts, logical operations, and modular additions. This ensures that even the smallest change in input drastically alters the output.
Final hash value: Once all blocks are processed, the final result is a fixed-length hash value. This value is a collision-resistant representation of the input data.
Characteristics of a blockchain hash
Fixed-length: Regardless of the input size, a hash always produces a fixed-length output. For example, the SHA-256 hash function used in Bitcoin always generates a 256-bit hash.
Efficient computation: Hash functions are designed to be computationally efficient, allowing quick processing of large amounts of data.
Avalanche effect: Even a tiny change in the input data will result in a completely different hash. This property helps in detecting any alterations to the data.
Cryptographic hashes are designed to be secure and resistant to attacks. They offer properties like:
Determinism: A hash function will always return the same output for any specific input. No matter how many times or where the computation occurs, as long as the input remains unchanged, the resulting hash value will stay identical.
Preimage Resistance: It is computationally infeasible to reverse-engineer a cryptographic hash. This means that, given an output (hash), finding the original input would take an impractical amount of time and computational power.
Second Preimage Resistance: Given an input and its corresponding hash, it is improbable to find another input that produces the same hash. This ensures that every input returns a unique hash, minimizing the chances of two different pieces of data generating identical hashes.
Collision Resistance: The likelihood of two distinct inputs generating the same hash is so low that finding such a collision would be computationally infeasible. Collision is the event where two distinct inputs produce the same hash value. Collision resistance is crucial for the security of digital signatures, certificates, and blockchain transactions.
These properties make cryptographic hash functions suitable for security-critical applications like password storage, digital signatures, and blockchain technology.
Non-cryptographic hashes are simpler and often faster than cryptographic ones, but they lack the security properties mentioned above. They are primarily used for data integrity checks and other non-security-related purposes. While they can be deterministic, they are generally not preimage resistant.
Importance of hashing in blockchain
Hashing is crucial for trustless, decentralized systems because it ensures transparency and security of blockchain systems by linking each block to the previous one, creating an immutable historical record of transactions.
Common uses for hashes in blockchains
A transaction hash uniquely identifies each transaction, based on the information contained within the transaction. Every transaction is processed through a cryptographic hash function and the resulting hash can be used to reference the transaction.
A block hash serves as a unique identifier for each block and acts as a "fingerprint" to ensure the data remains unchanged. It is generated by hashing key data such as the previous block’s hash, the Merkle root, and the timestamp. It then links the current block to the previous one, maintaining the integrity of the chain.
The Merkle root hash represents all transactions within a block, functioning as the top hash in a Merkle tree for efficient transaction verification.
A state root hash captures the entire state of a blockchain at a point in time, including account balances, smart contract states, and other on-chain data. The blockchain organizes account and contract data into a Merkle tree or Patricia trie and the root hash of this structure summarizes the entire dataset. The state root is stored in the block header and updated as the blockchain state evolves with each new block.
A transaction receipt hash represents the outcome of a transaction, including whether it succeeded, failed, or triggered specific events or logs. After processing a transaction, the blockchain generates a receipt that records the result and any triggered logs or events. These receipts are hashed and combined into a Merkle tree, with the root hash stored in the block header.
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