Start with who is acting, not what is easiest to configure
API keys, OAuth tokens, and service accounts can all authenticate software, but they express different identities. An API key usually identifies an application or project, OAuth represents delegated user authorization, and a service account represents a non-human workload. Choosing an API key simply because it is easy to generate often creates oversized permissions, weak attribution, and difficult offboarding later.
Before selecting a mechanism, identify the caller: a browser, backend service, scheduled job, CI pipeline, or field device. Then ask whether it acts under its own authority or on behalf of a user, and whether the resource owner must give consent. A system reading a user's cloud files, calendar, or CRM records generally needs OAuth. A nightly backend job writing to a warehouse is better represented by a service account with a narrow role. An API key can be appropriate when only the calling application matters and the provider supports strict scope, source, and quota restrictions.
Give each mechanism a clear stopping point
API keys answer which application is calling, but not which person approved an action. They can fit mapping, translation, model inference, or low-risk internal read endpoints when they can be restricted by API, origin, IP address, environment, and quota. They do not belong in browser bundles, mobile application packages, public spreadsheets, or source control. Software distributed to end users cannot reliably keep an embedded secret.
OAuth is valuable because it models delegation, not merely login. Requested scopes should match visible product functions instead of granting an entire mailbox, drive, or CRM account for convenience. Service accounts fit machine-to-machine communication, CI/CD, scheduled processing, and data pipelines, but they should never become shared administrator identities. Useful decision rules are:
- User attribution: Use OAuth when an operation must belong to an individual or requires that person's consent. Preserve the mapping among user, tenant, scopes, and authorization grant.
- Workload attribution: Use a service account or workload identity for unattended backend work. Separate identities by application, responsibility, and environment.
- Application identification: Use a restricted API key only when identifying the caller is sufficient and exposure has limited consequences.
- Public clients: Browser, desktop, and mobile applications should use Authorization Code with PKCE. A client secret shipped with them is not confidential.
- Cross-cloud access: Prefer short-lived tokens and identity federation over downloading and storing long-lived private-key files.
Lifecycle controls create the real security boundary
Credential creation is only the beginning. Every key, OAuth client, and service account needs an owner, purpose, environment, permission scope, creation date, and rotation procedure. Development, testing, and production must use separate identities. The same credential should not be copied across developer laptops, CI systems, and production servers. Store secrets in a managed secret service, vault, or encrypted deployment environment, and retrieve them at runtime rather than committing configuration files.
API-key rotation should allow a short overlap: issue a new key, deploy it, verify that traffic has moved, and then revoke the old one. OAuth access tokens should be short-lived. Refresh tokens need encrypted storage and explicit handling for revocation, changed scopes, suspended users, and renewed consent. Treating a refresh token as a permanent password or printing it into logs, error trackers, and support screens defeats the benefits of OAuth.
For service accounts, assign the smallest practical role and prefer cloud workload identity mechanisms such as IAM roles or workload federation. If a platform requires a private-key file, limit who can download it, record access, and rehearse rotation. One service account per defined workload makes audit records meaningful: the team can identify which system, in which environment, accessed which resource without trying to reconstruct activity behind a shared identity.
Migrate shared credentials and design for compromise
A common legacy pattern is one API key—or even one administrator login—shared by every integration. Migrate by observing current use, creating separate identities, assigning minimum permissions, and switching callers one at a time. Record credential identifiers, source systems, environments, and API routes in searchable audit data, but log only hashes or partial identifiers rather than complete secrets. Revoke the old credential after traffic has stopped, then add expiry reminders and automated detection of unused credentials.
Compromise handling should be part of the design, not an improvised response. The team should know how to revoke access immediately, issue replacements, identify services requiring redeployment, determine the affected data scope, and preserve evidence for investigation. No mechanism is automatically secure because of its name. Security comes from a clear identity, constrained authority, safe secret storage, observable use, and a revocation path that actually works. Integrations spanning LINE, ERP, CRM, cloud platforms, and IoT systems benefit from one consistent identity and credential-governance model, which an experienced integration team can help translate into architecture and operating controls.