Contract (FlyingMoney.sol)

Solidity 0.8.24 with OpenZeppelin 5.1.0. No owner, no admin, no pause, no upgrade, no fee. One settlement token per deployment (the chain's Circle USDC), fixed in the constructor. Addresses for every chain: Deployments and /.well-known/flying-money.json.

Functions

Function	Who	What
issue(payee, spender, faceValue, expiresAt) → id	anyone (the funder)	Pulls faceValue USDC and opens a certificate. Lifetime 1 hour to 365 days. spender must differ from the funder and the payee
topUp(id, amount)	funder	Adds to the face value
extend(id, newExpiresAt)	funder	Moves expiry later (never earlier)
redeem(id, cumulative, memo, signature) → paid	anyone	Pays cumulative − redeemed to the payee
redeemMany(SignedNote[]) → totalPaid	anyone	Batch redeem; bad notes are skipped with NoteSkipped, not reverted
reclaim(id) → refunded	funder, after expiry	Returns the unredeemed remainder and closes the certificate
getCertificate(id)	view	The certificate struct
noteDigest(id, cumulative, memo)	view	The EIP-712 digest a spender signs
totalOutstanding()	view	Σ (faceValue − redeemed) over open certificates

Spenders are verified with ECDSA only (ECDSA.tryRecover, low-s). There is no ERC-1271: contract signatures can be revoked, so a note accepted offline could stop being valid.

Events

CertificateIssued(id, funder, payee, spender, faceValue, expiresAt) · CertificateToppedUp(id, amount, newFaceValue) · CertificateExtended(id, newExpiresAt) · NoteRedeemed(id, cumulative, paid, memo, redeemer) · NoteSkipped(id, cumulative, reason) · CertificateReclaimed(id, refunded)

NoteSkipped reasons: 1 unknown, 2 closed, 3 expired, 4 exceeds face value, 5 nothing to redeem, 6 bad signature.

Errors

InvalidParams · UnknownCertificate · NotFunder · Expired · NotExpired · Closed · ExceedsFaceValue · ExceedsCap · InvalidSignature · NothingToRedeem · UnsupportedToken

Launch caps

Two immutable caps, set at deployment (0 = unlimited):

• maxFaceValue: per-certificate cap. Mainnets: 100 USDC.
• maxTotalOutstanding: deployment-wide cap. Mainnets: 1,000 USDC.

There is no admin, so caps can never be raised, only superseded by a new deployment. Testnets are uncapped.

Invariants (tested with Foundry, 256 runs × depth 50)

• I1 redeemed ≤ faceValue for every certificate.
• I2 Solvency: the contract's USDC balance equals totalOutstanding, the sum of faceValue − redeemed over open certificates. I2b totalOutstanding ≤ maxTotalOutstanding when the cap is set.
• I3 A payee receives exactly the highest valid total redeemed for its certificates.
• I4 A funder never pays out more than the face value it issued.
• I5 No certificate pays anyone other than its payee (redeem) or its funder (reclaim).
• I6 After reclaim, no further transfers happen for that certificate.
• I7 redeemed never decreases.

Unit tests also cover ECDSA-only permanence (a spender address that later gains contract code changes nothing), high-s rejection, domain separation and key isolation. Details and gas numbers: SECURITY.md.