Preimage

A preimage is the input that produces a given output when run through a hash function. If SHA-256 of some value equals a known digest, that value is the digest's preimage. Secure hash functions make recovering a preimage from an output computationally infeasible, since an attacker faces a search space of 2^256 possibilities for a 256-bit hash.

Why it matters

Preimage resistance is one of the load-bearing assumptions of bitcoin. Addresses are hashes of public keys, contract commitments hide values until they are revealed, and proof-of-work assumes no one can reverse a hash. In the Lightning Network the word has a specific operational meaning: the preimage is the payment secret whose revelation proves a payment was completed.

How it works

In a Lightning payment, the recipient generates a random secret, hashes it with SHA-256, and embeds the hash in an invoice. Each node along the route locks funds that can only be claimed by presenting the matching preimage before a timeout. When the recipient reveals the secret to claim the final hop, it propagates backward hop by hop, settling every link in the route. This makes the multi-hop payment atomic: either the preimage is revealed and everyone is paid, or the timeouts expire and all funds return to their senders.