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Train uses HKDF (HMAC-based Key Derivation Function) to generate deterministic and secure secrets for atomic swaps. HKDF is a standardized key-derivation algorithm, which lets us turn one signature into any number of unique, swap-specific secrets. If you are not familiar with HKDF you can consider it as just hashing.
1

User Sign

The users signs the message I am using Train.
2

Initial Key

initial_key = HKDF-Extract(signature) This key is cached locally by the dApp, as long as the cache persists, more signatures are not needed.
3

Derive secret

secret = HKDF-Expand(initial_key, salt = (source_chain, destination_chain, timelock)) For each swap, dapp derives a new secret using initial_key, and the salt.

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Advanatges of HKDF derived secrets.

  • Deterministic: The dApp can always recompute the secret as salt is not random, but is based on public data from the blockchain.
  • Unique: Even if two swaps involve the same user, they will have different secrets because the salts differ.
  • No user Friction: The user signs only once, and the dapp produces unlimited secrets from that one signature.
  • Recoverable: If the cached initial_key is lost, the user can simply sign the same message again and regenerate the identical initial key.