13.21. DD 21: Exchange Key Continuity

13.21.1. Summary

This design document discusses what happens if an exchange’s master public key ever changes.

13.21.2. Motivation

The exchange master public key is an offline signing key. While this makes compromise of this key less likely, it makes it more likely that this key is lost.

The wallet (and merchants) must handle such a scenario where the exchange deploys a new master public key.

13.21.3. Proposed Solution

When wallets or merchants specify that they directly trust an exchange, the trust record must always explicitly mention a base URL and a master public key.

An exchange should have a single, canonical base URL. However, an exchange that’s reachable over different base URLs should still be handled gracefully. Wallet

We generally assume that the base URL of an exchange stays constant. Wallets do not support changing the base URL of an exchange.

A /keys response with an unknown exchange master public key is only accepted if the exchange is audited by a trusted auditor or the wallet has an exchange trust record with the advertized master public key.

Denomination records explicitly store which master public key signed them. Merchant

When coins are deposited, the wallet only specifies the base URL for the respective exchange, but not the master public key of the exchange. The merchant must always resolve the URL to the current master public key, and decide whether to accept the exchange based only on the master public key. That is, a merchant may not reject an exchange because the wallet is not specifying what the merchant believes to be the canonical base URL.

13.21.4. Alternatives

  • The wallet could treat each (base URL, master pub) pair as a separate exchange. This, however, does not work in practice, since the exchange with the new public key should still accept deposits and refreshes of coins with denominations signed by the old key.

13.21.5. Discussion / Q&A

  • Does the current merchant backend have support for changing master public keys of exchanges trusted via the auditor? Or does the code base make the assumption that one merchant base URL corresponds to only one master public key?