Below is a practical, security-first guide for exposing APIs to external partners. It covers architecture patterns, authentication/authorization options, transport and data protection, operational controls, partner onboarding, and a short checklist you can use to evaluate readiness.

1. Choose an appropriate exposure pattern

• API Gateway / Reverse Proxy: Place a gateway (commercial or cloud-managed) as the single entry point — centralizes auth, TLS termination, rate limiting, WAF, logging, transformations, routing to internal services.
• Partner-specific façade (BFF): Create partner-specific facades if partners need different payloads, translation or reduced surface area.
• Enterprise Integration Layer / Message Broker: For asynchronous integrations (events, large volumes), use a message broker (Kafka, RabbitMQ, managed pub/sub) and secure bridging.
• VPN / Private Connectivity: For high-trust partners, consider private peering (AWS PrivateLink, Azure Private Link, VPC/VNet peering) or an IPsec VPN to keep traffic off the public Internet.

2. Use strong authentication (mutual trust)

• OAuth 2.0 (recommended for most partner APIs): Use client credentials for machine-to-machine, with short-lived access tokens and refresh tokens if needed. Enforce scopes and audience restrictions.
• Mutual TLS (mTLS): Strong for machine-to-machine authentication; validates client certificates and server certificate, often combined with OAuth. Useful for high-assurance partners.
• API keys (only for low-risk, short-term): Simple but weak alone — pair with IP allow-listing, usage limits, and strict monitoring. Rotate regularly.
• JWTs for token format: Use signed (and preferably encrypted) JWTs with short expiry. Validate signature, issuer (iss), audience (aud), expiry (exp), and token revocation/blacklist checking where needed.
• Hardware-backed keys: For highest security, issue keys on HSMs or hardware tokens (YubiKey, cloud KMS bound keys).

3. Principle of least privilege & fine-grained authorization

• Scopes/claims: Use OAuth scopes and JWT claims to restrict what partners can do (read-only vs write, resource-level access).
• Role-based access control (RBAC) & attribute-based access control (ABAC): Implement server-side checks that map partner attributes to permitted actions.
• Resource-level policies: Limit access per customer/account/resource ID whenever possible.

4. Transport & data protection

• Enforce TLS 1.2+ (prefer TLS 1.3). HSTS and secure cipher suites.
• mTLS where needed (see above).
• End-to-end encryption for sensitive payloads (field-level encryption) if intermediate layers or logs might be accessible.
• Protect secrets in transit and at rest (secrets manager: AWS Secrets Manager, HashiCorp Vault, Azure Key Vault).

5. Rate limiting, quotas & abuse control

• Per-client rate limits and quotas (requests/minute, concurrent requests, data volume).
• Burst vs sustained limits, and clear throttling behavior (429 + Retry-After).
• Circuit breakers and graceful degradation for backend overload.

6. Network & perimeter protections

• IP allow-listing for trusted partner addresses (use cautiously — IPs can change).
• Web Application Firewall (WAF) to block OWASP top-10 attacks.
• DDoS protection and cloud provider protections (Cloudflare, AWS Shield, Azure DDoS Protection).
• Zero Trust network principles for internal services.

7. Input validation & secure coding

• Validate payloads (JSON schema, size limits), avoid deserialization vulnerabilities.
• Output encode to prevent injection issues.
• Use parameterized queries for DB operations; avoid direct filesystem/command access.
• Enforce strong content-type checks and reject unexpected content.

8. Logging, monitoring & observability

• Centralized, tamper-evident logging with structured logs (request id, partner id, timestamps, status, latency).
• Audit logs for authentication/authorization events and data access.
• Metrics & tracing (Prometheus, OpenTelemetry, Jaeger) for performance and troubleshooting.
• Real-time alerting for anomalies (spikes, error rates, auth failures).
• Retention and access controls for logs containing PII.

9. Threat detection & incident response

• Anomaly detection and behavioral analytics to detect misuse.
• Automated blocking for obvious abuse (credential stuffing, repeated 401s).
• Playbooks and runbooks: how to revoke tokens/certs, rotate credentials, communicate with partners.
• Test incident drills with partner representatives.

10. Partner onboarding, lifecycle & governance

• Contract & SLAs: Define security requirements, incident notification timelines, data handling, and retention regulations.
• Onboarding flow: identity proofing, issuing credentials/certs, providing sandbox environment and documentation (OpenAPI/Swagger, examples).
• Staged rollout: sandbox → limited production (canary) → full production.
• Credential rotation and revocation: set expiry, automate rotation, provide revocation endpoints or certificate revocation lists (CRL/OCSP).
• Periodic security reviews and pentests with partners.

11. Data protection & compliance

• Minimize data transfer: only share necessary fields (data minimization).
• Mask or redact sensitive fields (PII, card data). Use tokenization for payment data (PCI-DSS).
• Ensure compliance needs (HIPAA, GDPR, CCPA) by data residency, DPIAs, processing agreements.
• Maintain consent and data subject access procedures where required.

12. Developer experience & documentation

• Machine-readable OpenAPI spec, SDKs, sample code, and reproducible examples.
• Provide a sandbox with realistic rate limits and fixtures.
• Clear error codes and messages, versioning strategy, deprecation notices.

13. Versioning & backward compatibility

• Use URL-based or header-based versioning. Maintain old versions during a deprecation window.
• Communicate deprecations in advance and provide migration guides.

14. Testing & validation

• Security testing: SAST, DAST, dependency scanning, and penetration tests.
• Contract testing (Pact) to ensure integrations don’t break unexpectedly.
• Chaos testing for resilience.

15. Example secure flow (concise)

• Partner requests client credentials and/or a certificate during onboarding.
• Partner uses mTLS + client credentials to request a short-lived OAuth2 access token from your Authorization Server.
• Partner calls API Gateway presenting token and using mTLS TLS client cert.
• Gateway validates token signature/audience, checks scopes, rate limits, logs request, forwards to internal microservice over mutual TLS or private network.
• Internal service enforces resource-level authorization and returns response via gateway. Gateway masks sensitive fields and logs the interaction.

16. Operational checklist (quick)

• [ ] API Gateway in place with centralized auth.
• [ ] TLS 1.2+ enforced; prefer TLS 1.3.
• [ ] Strong auth (OAuth2 + mTLS where needed).
• [ ] Short-lived tokens, scope-based access, token revocation mechanism.
• [ ] Per-client rate limits and quotas.
• [ ] WAF and DDoS protections enabled.
• [ ] Centralized logging, tracing, and alerting.
• [ ] Sandbox and staged onboarding process.
• [ ] Secrets managed in a secrets manager; rotate keys/certs regularly.
• [ ] Contract & SLA with security & incident response terms.
• [ ] Regular security testing and audits.

17. Common pitfalls to avoid

• Relying only on API keys without rotation or scopes.
• Exposing internal endpoints directly without a gateway.
• Long-lived tokens or embedding secrets in client code.
• Insufficient logging/alerting or lack of partner-specific telemetry.
• Overly permissive CORS or failing to validate Origin/Referer where relevant.

18. Technologies and references (examples)

• API Gateways: Kong, Apigee, AWS API Gateway, Azure API Management, Istio/Envoy.
• Auth/Zones: OAuth2 / OpenID Connect, IdentityServer, Keycloak, Okta, Auth0.
• mTLS: Envoy, Nginx, AWS ALB with client certs.
• Secrets & Keys: AWS KMS/Secrets Manager, Azure Key Vault, HashiCorp Vault.
• Observability: OpenTelemetry, Prometheus, Grafana, Jaeger.

If you want, I can:

• Draft a concrete architecture diagram (textual or steps) for your environment (cloud/on-prem).
• Produce a sample OAuth2 scope policy and role matrix for a partner.
• Create a checklist tailored to your compliance requirements (PCI, HIPAA, etc.).

Which of those would help you next?