Hash Functions Compared
SHA-256, Keccak-256, Blake3, and Poseidon — which one each chain uses, why ZK systems needed a new family of 'arithmetic-friendly' hashes, and what tradeoffs each makes.
What a Hash Function Is
A hash function takes any input and produces a fixed-size output that looks random. Same input always gives the same output. Find two inputs giving the same output? You've found a collision, and most security guarantees collapse. Good cryptographic hashes are designed so finding collisions is computationally infeasible — measured in the 2^128 to 2^256 range, well beyond anything physically realisable. Hashes are everywhere in crypto: transaction IDs, block hashes, Merkle trees, signature precomputations.
The Big Four You'll Meet
**SHA-256**: Bitcoin's choice. Designed by the NSA in 2001, ubiquitous in TLS and general computing. **Keccak-256**: Ethereum's choice. Designed by the team that won the SHA-3 competition. Note: Ethereum's `keccak256` is the original Keccak submission, not the slightly-different finalised SHA-3 standard. **Blake3**: A modern fast hash optimised for software speed. Used by IPFS, some L2 systems, and emerging chains. **Poseidon**: A 'ZK-friendly' hash designed for efficient verification inside zero-knowledge proofs.
- SHA-256: Bitcoin, classical security, slowish
- Keccak-256: Ethereum, also slowish but with cleaner properties
- Blake3: Newest of the classics, much faster on commodity hardware
- Poseidon: ZK-friendly, used inside zk-SNARKs and zk-STARKs
Key Takeaways
- All four are cryptographic hashes; pick based on chain and context
- SHA-256 (Bitcoin) and Keccak-256 (Ethereum) are the legacy heavyweights
- Blake3 is the modern speed king for non-ZK contexts
- Poseidon exists because regular hashes are very expensive to compute inside ZK proofs
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References & further reading
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