GUIDELINE 04

Know what data is safe to store

Assume blockchain and asset data can become durable and widely readable. Store only what the application truly needs.

Classify before writing

Public: ownership, public profiles, hashes, public contract state.
Private but shareable: encrypt for explicit recipients before storage.
Local secret: passwords, private keys, mnemonics, session credentials. Never put these on-chain.
Regulated or deletion-sensitive: keep off-chain unless your legal and retention design explicitly supports it.

Minimize permanent data

Store a digest when verification is enough.
Do not place personal data in transaction parameters merely because the contract ignores it.
Validate type, length, and encoding in both client and contract.
Define ownership, readers, retention, and deletion before launch.

Allowlist public fields

function publicProfile(form) {
  const displayName = form.displayName.trim();
  if (displayName.length < 1 || displayName.length > 60) {
    throw new Error("Display name must contain between 1 and 60 characters");
  }
  return {
    version: 1,
    display_name: displayName,
    avatar_hash: form.avatarHash
  };
}

// Never spread an entire form or wallet object into transaction data.
const parameters = JSON.stringify({ function: "update_profile", profile: publicProfile(form) });

Bottom line

If publishing a field would surprise its owner, do not write it in plaintext to a transaction, wallet object, contract, SQL table, or asset.