SHA-256 is the hash function that powers Bitcoin. You've already seen what a hash function does — now let's look at SHA-256 specifically: why Satoshi chose it, how it processes data internally, and why it's still considered secure while older algorithms like MD5 and SHA-1 have been broken.

Try it: SHA-256 in action

Type anything below and watch the SHA-256 digest update in real time. Notice that the output is always exactly 64 hex characters — that's 256 bits — regardless of whether you type one word or a whole paragraph.

Type anything — the digest updates live. Change one character to see the avalanche effect.
SHA-256 · 256 bits · 64 hex
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Change any character in the text
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Tap «Long input» below
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Look at the blocks — they are linked
BLOCK DATA — try changing any character:
SHA-256 HASH (64 chars = 256 bits):
Web Crypto unavailable (use HTTPS or a modern browser)
TRY SAMPLE STRINGS — click to load:
↑ each sample demonstrates a different hash property
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Why SHA-256 and not MD5 or SHA-1?

Not all hash functions are created equal. MD5 and SHA-1 were once the industry standard — until researchers found ways to create collisions (two different inputs with the same hash). Once a hash function is broken, it cannot be used to secure financial history. Satoshi chose SHA-256 because it was the strongest standardised option available in 2008, and it remains unbroken today.

Click each point to see why it matters for blockchain ▶
How SHA-256 processes data internally

You don't need to implement SHA-256 yourself — you'll always use an audited library. But understanding the processing model helps you reason about performance and protocol choices: why inputs are padded, why processing happens in 512-bit blocks, and where that 256-bit output comes from.

How SHA-256 turns any input into a 256-bit digest ▶
Input
any length (bytes)
Pad
append 1-bit + length → multiple of 512 bits
Split
into 512-bit blocks
Compress
64 rounds per block, mix 8 state words
Output
256-bit (64 hex chars) digest
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SHA-256 vs Keccak-256: choosing the right algorithm
SHA-256 and Keccak-256 are both secure — they're just used in different blockchains
AspectSHA-256Keccak-256
Internal designMerkle-Damgård constructionSponge construction (different family)
Output size256 bits (32 bytes)256 bits (32 bytes)
Standardised asSHA-2 family (FIPS 180-4)SHA-3 family — but Ethereum uses pre-standard padding
Used inBitcoin: PoW, Merkle root, double-SHA256dEthereum: address derivation, storage slots, events
StatusNo known practical attackNo known practical attack
SpeedFast; hardware acceleration (SHA-NI)Fast; slightly different performance profile
SHA-256 in code
Click any line to see the explanation ▼
python
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import hashlib
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# Standard SHA-256:
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digest = hashlib.sha256(b'Hello Bitcoin').hexdigest()
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print(digest)  # 64 hex characters, always
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# Double SHA-256 (SHA-256d) — used in Bitcoin:
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inner = hashlib.sha256(b'Hello Bitcoin').digest()  # raw bytes, not hex
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sha256d = hashlib.sha256(inner).hexdigest()
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# Keccak-256 — used in Ethereum (NOT standard SHA-3):
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from Crypto.Hash import keccak  # pip install pycryptodome
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k = keccak.new(digest_bits=256)
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k.update(b'Hello Ethereum')
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print(k.hexdigest())  # Different from SHA-256 of same input
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Protocol specificity: never use 'hash' as a generic term in blockchain code. Always name the exact function: SHA-256d for Bitcoin block headers, Keccak-256 for Ethereum addresses. Using the wrong function produces a completely different digest with no error — it silently breaks compatibility with the rest of the network.