# Security

City Protocol's card domain is designed for backend-first deployment. This constraint is intentional and mirrors compliance standards found in traditional card programs.

<table><thead><tr><th width="172.421875">Concern</th><th>Guidance</th></tr></thead><tbody><tr><td><strong>API Key Security</strong></td><td>All provider API keys must remain server-side. Card operations should be invoked from backend services, never from browser or mobile clients directly.</td></tr><tr><td><strong>PAN Reveal</strong></td><td>Revealing full card numbers is a sensitive operation governed by the provider's security model. Access should be gated behind your own authorization and audit layers.</td></tr><tr><td><strong>Card Controls</strong></td><td>Freeze, unfreeze, and replace actions should be correlated with your internal user records and compliance state before execution.</td></tr><tr><td><strong>State Correlation</strong></td><td>Use the <code>meta</code> fields at creation time to tie card records back to your internal user IDs, email addresses, and contact information for downstream reconciliation.</td></tr></tbody></table>


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