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What is entropy in crypto?
Entropy, in cryptography, is the amount of unpredictability in a secret, measured in bits. A private key or seed phrase is only as strong as the randomness used to create it: 128 bits of entropy means an attacker faces as many possibilities as 128 fair coin flips.
What it means for you
A seed phrase you make up yourself, take from a book or song, or generate with a faulty random number generator can be far easier to guess than its length suggests, and whoever guesses it holds your coins. The 12 or 24 words carry their full strength only when the underlying bits were truly random.
A common mistake: “A 24-word seed phrase is always stronger than a 12-word one.”
In fact: Only if both were generated randomly. A random 12-word BIP-39 phrase already carries 128 bits of entropy; a 24-word phrase built from words a person chose can carry far less than either.
How it works
NIST defines entropy for secrets as the uncertainty an attacker faces, where n bits of entropy equals the uncertainty of a uniformly random n-bit value. BIP-39 seed phrases start from 128 to 256 bits of entropy, in multiples of 32: 128 bits becomes 12 words and 256 bits becomes 24, after a short checksum is added. Devices gather raw randomness from entropy sources, which NIST SP 800-90B specifies and tests, and combine them with deterministic random bit generators. A phrase's word count says nothing about whether its bits were actually random.
Sources: NIST CSRC glossary: entropy, NIST SP 800-90B: Recommendation for the Entropy Sources Used for Random Bit Generation, BIP-39: Mnemonic code for generating deterministic keys · checked 4 October 2026
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