Senior Engineer And Token Security

Senior engineer - token security in London

Rate: £550 day outside IR35

Start date: ASAP

End date: 18/12/2026

Clearance: Active SC Clearance

Location: Remote/Hybrid (UK-based)

Responsibilities

Design and implement OAuth 2.0 Token Exchange (RFC 8693) capabilities.

Develop secure delegation and propagation patterns across distributed services.

Implement token down-scoping and audience restriction mechanisms aligned to least-privilege principles.

Design secure service-to-service authentication frameworks within zero-trust environments.

Build and maintain robust JWT validation and cryptographic trust chains.

Integrate applications and services using OpenID Connect (OIDC) standards.

Essential

OAuth 2.0 standards.

Proven hands-on implementation experience with OAuth 2.0 Token Exchange (RFC 8693).

Strong understanding of OpenID Connect (OIDC) core specifications.

Expert-level understanding of JWT (RFC 7519).

Experience signing, validating, and encrypting tokens using JWS and JWE.

Strong knowledge of JWKS endpoints and automated key rotation strategies.

Understanding of secure alternatives to shared-secret authentication mechanisms.

Experience securing service-to-service communication within distributed microservice architectures.

Understanding of zero-trust security principles.

Knowledge of secure transit-layer protection and access controls.

Delegation and token management

Experience implementing:

Token down-scoping

Audience restriction (aud)

Actor and subject claims handling

Experience designing secure token lifecycle management strategies including:

Token caching

Token renewal

Revocation patterns

Familiar with

CDDO Secure by Design principles.

Experience with cloud security architectures.

Knowledge of API gateways, service meshes, and federated platforms.