What is the difference between passkey authentication and passkey-based digital signatures?

Why This Matters for Security Teams

Passkey authentication and passkey-based digital signatures both rely on modern public key cryptography, but they solve different security problems. Authentication is about establishing that a user or agent can unlock a credential at sign-in. Digital signatures bind a specific action, document, or transaction to that credential, creating stronger non-repudiation and auditability. For security teams, the difference matters because approval workflows, delegated authority, and transaction integrity cannot be validated by login alone.

This distinction becomes especially important in environments where NHIs and agents are acting on behalf of humans. The Ultimate Guide to NHIs — What are Non-Human Identities notes that NHIs outnumber human identities by 25x to 50x in modern enterprises, which means the control problem is not just access, but proving intent and action. NIST control expectations in NIST SP 800-53 Rev 5 Security and Privacy Controls also reinforce that strong authentication and transaction integrity are separate governance objectives. In practice, many security teams discover the gap only after a login is proven legitimate but the resulting approval or transfer cannot be cryptographically tied to a specific action.

How It Works in Practice

A passkey used for authentication typically proves possession of the private key and successful local user verification, such as biometrics or a device PIN. The relying application receives an assertion that the user or agent authenticated, but that assertion is usually scoped to the login event. A passkey-based digital signature uses the same underlying cryptographic identity to sign a defined payload, such as an invoice, release approval, or API request body, so the signature can be verified later against the exact bytes that were approved.

For practitioners, the operational differences are practical:

• Authentication answers whether the credential holder may enter the system.
• Digital signatures answer what data was approved and whether it was altered after approval.
• Authentication is often sufficient for session establishment; signatures are better for high-risk authorisation, non-repudiation, and workflow integrity.
• For NHIs, signatures can help separate machine identity proof from task-specific approval evidence, especially when paired with policy and logging.

That separation aligns with current identity guidance and with emerging digital identity frameworks such as ISO/IEC 27001:2022 Information Security Management and eIDAS 2.0 — EU Digital Identity Framework, which both support stronger assurance around identity proofing and trust services. The control lesson is straightforward: do not treat a successful passkey login as evidence that a transaction was explicitly approved. These controls tend to break down when teams reuse authentication assertions as proxy approvals in fast-moving CI/CD, admin automation, or delegated payment flows because the approval context is never separately signed.

Common Variations and Edge Cases

Tighter transaction signing often increases implementation complexity, so organisations have to balance stronger assurance against workflow friction and developer overhead.

One common edge case is a system that says it “supports passkeys” but only uses them for login. That is not the same as supporting signed approvals. Another is when a platform signs a server-side challenge instead of the actual business payload, which improves session security but does not prove which record, document, or request was authorised. For NHI use cases, current guidance suggests treating workload authentication and workload approval as separate events unless the platform explicitly binds the signed object to the cryptographic operation.

This matters in environments with shared devices, service desks, delegated admin, or agentic automation. In those cases, the best practice is evolving toward explicit signing of the transaction context, with clear review, retention, and verification steps. NHIMG research on the CI/CD pipeline exploitation case study and Emerald Whale breach shows how quickly trust erodes when credentials are accepted without binding them to the exact action. The practical rule is simple: use passkey authentication to prove who or what entered, and use digital signatures when the business question is what exactly was approved.

Related resources from NHI Mgmt Group

• What is the difference between app-specific passwords and modern federated authentication for cloud applications?
• What is the difference between authentication and action-specific authorization in agent workflows?
• What is the difference between push-based MFA and phishing-resistant authentication?
• What is the difference between passwordless authentication and password-based access?