How should banks and payment providers implement Strong Customer Authentication without treating it like a simple policy control?

They should treat SCA as a cryptographic programme, not a checklist. That means binding authentication to the transaction, keeping the factors independent, and managing the certificates and keys that support every flow. In open banking, the same discipline must cover API identification, secure communication, renewal, and revocation so a valid control remains valid in practice.

Why SCA Has to Be Engineered, Not Merely Adopted

strong customer authentication only works when it is built into the security mechanics of the payment journey, not bolted on as a policy statement. For banks and payment providers, the real question is whether the control still holds when channels, devices, APIs, and certificate lifecycles change. A usable SCA design should prove the user, the session, and the transaction together.

The practical implication is that SCA should be tied to cryptographic assurance and transaction context. If the factor can be replayed, detached from the payment, or reused across flows, the control may satisfy a procedure but fail to reduce fraud or unauthorised execution. That is why implementation details such as token binding, client authentication, and secure transport matter as much as the authentication ceremony itself.

In open banking, this also means the control surface extends beyond the login step. The provider must treat API authentication, certificate management, and message integrity as part of the same trust chain, because a valid customer challenge is only meaningful if the surrounding transport and endpoint trust are sound.

What Changes When SCA Is Treated as a Cryptographic Control

The strongest SCA implementations preserve independence between factors while also binding the approved action to the exact transaction. That combination reduces the value of stolen credentials, intercepted one-time codes, and session replay because the attacker has to defeat both the authentication step and the transaction-specific proof.

For banks, this changes design decisions around certificate issuance, key rotation, revocation, and client authentication. It also changes how security and product teams think about exceptions, because a weak fallback channel can silently become the real control. A control that depends on long-lived shared secrets, reusable approvals, or unclear device trust is not delivering the same assurance as one built on strong cryptographic identity and clear transaction context.

Open banking endpoints make this even more visible. A payment provider can have strong customer verification and still be exposed if API identification is weak, if certificates are stale, or if secure communication is not consistently enforced across every participant in the flow. The point is not just to authenticate once, but to keep that assurance valid across the whole lifecycle of the payment interaction.

For implementation reference, the right technical model is the one that maps authentication to the API and certificate layer as well as the customer step, which is why practitioners often anchor this work in NIST SP 800-63 Digital Identity Guidelines, RFC 8705: OAuth 2.0 Mutual-TLS Client Authentication and Certificate-Bound Access Tokens, and RFC 7523: JWT Profile for OAuth 2.0 Client Authentication and Authorization Grants.

Why Operations, Renewal, and Revocation Decide Whether SCA Stays Valid

SCA is often weakened by lifecycle failure rather than by a bad initial design. Certificates expire, keys are rotated late, exception paths accumulate, and service integrations drift from the original security assumptions. Once that happens, the control can remain “enabled” in a policy sense while becoming unreliable in practice.

That is especially important for payment ecosystems where many parties depend on the same trust chain. A bank, an account servicing payment provider, and a third-party API consumer may each have their own operational process, but the assurance only holds if the whole chain can detect revocation, reject stale credentials, and recover cleanly when a trust anchor changes. This is why renewal and revocation are not administrative afterthoughts, they are part of the control.

Practitioners should also be wary of channel-specific exceptions. If a fallback path skips certificate checks, weakens transaction binding, or allows a lower-assurance authenticator to complete high-risk activity, the control no longer matches the risk it was meant to cover. In practice, the least visible path is often the one that breaks the assurance model first.

Risk and Threat Considerations

When SCA is treated as a policy checkbox, attackers look for the implementation seam rather than the policy statement. Replayable approvals, weak API authentication, expired certificates, and downgrade paths can let a valid-looking transaction proceed even when the user never approved the real payment.

Failure mechanism: The control fails when authentication is not bound to the specific transaction, when certificates or keys are not managed as first-class security assets, or when fallback flows weaken the original assurance model.

Impact: Fraudsters can reuse captured approvals, impersonate trusted clients, or push unauthorised payments through interfaces that still appear compliant on paper.

Standards & Framework Alignment

This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.

OWASP API Security Top 10 addresses the attack surface, NIST SP 800-63 and NIST SP 800-53 Rev 5 set the technical controls, and ISO/IEC 27001:2022 defines the regulatory obligations.

Framework	Control / Reference	Relevance
NIST SP 800-63	Digital Identity Guidelines	SCA depends on assurance levels, authenticators, and transaction binding in payment journeys
Recommendation — Map payment authentication to assurance requirements and verify the factor strength matches the transaction risk.
NIST SP 800-53 Rev 5	IA-5 — Authenticator Management	SCA relies on secure issuance, rotation, and revocation of authenticators, keys, and certificates
	IA-9 — Service Identification and Authentication	Open banking SCA depends on authenticating APIs and payment services, not only end users
	SC-12 — Cryptographic Key Establishment and Management	SCA integrity depends on key generation, distribution, rotation, and revocation
Recommendation — Manage credentials and cryptographic authenticators through their full lifecycle and revoke them promptly. Authenticate service-to-service payment interfaces with strong, mutually verified credentials. Treat payment trust keys as managed security assets and rotate or revoke them on schedule.
ISO/IEC 27001:2022	A.8.5 — Secure authentication	SCA requires authentication mechanisms that remain effective across channels and flows
	A.8.24 — Use of cryptography	Binding SCA to the transaction and protecting API traffic are cryptographic problems
	A.5.15 — Access control	SCA is part of controlling who can approve or execute payment actions
Recommendation — Implement authentication controls that preserve assurance across all payment channels. Use cryptography to protect transaction integrity, transport security, and certificate-based trust. Enforce access decisions so only properly authenticated actors can initiate payment actions.
OWASP API Security Top 10	API2 — Broken Authentication	Open banking SCA fails if API or client authentication can be bypassed or replayed
	API8 — Security Misconfiguration	Expired certificates, weak TLS, or fallback paths can undermine SCA in production
Recommendation — Harden payment APIs against authentication bypass, replay, and weak client validation. Eliminate configuration drift that weakens transport security or certificate validation.

Practitioner Guidance

What to prioritise: Start with the trust chain, not the policy text. Confirm how the customer challenge, client authentication, transaction binding, certificate status, and revocation checks fit together for every payment path, including APIs and fallback journeys.

What to verify: Prove that the control still works after certificate renewal, key rotation, device change, and provider-to-provider integration. If a test transaction can succeed with a stale trust artefact or a generic approval, the control is too loose.

Common mistake: Treating SCA as an identity prompt instead of a transaction security mechanism. The practitioner takeaway is that SCA is only strong when the cryptography, the operational lifecycle, and the payment context all fail closed together.