Senior C# Backend Engineer

Senior C# Backend Engineer

£550 - £650 per day

London 2 days per week onsite

This is an opportunity to join a greenfield platform programme focused on building a modern identity and authorisation capability from the ground up. You will help shape core platform services that will be consumed across a wider ecosystem of AI-native applications, data services, and microservices.

The Company

They are a technology-led organisation investing in a next-generation platform designed to support complex enterprise-scale products and services. The programme is in its early stages, giving contractors the opportunity to influence architecture, engineering standards, and platform capabilities. The project is expected to be a long-term initiative, with initial contractors playing a key role in its evolution and future growth.

The Role and Deliverables

• Develop and maintain authorisation SDKs, Policy APIs, identity services, and authorisation microservices.
• Design and implement secure identity and access management capabilities using OAuth2, JWT, and OIDC standards.
• Build authorisation models including RBAC, ReBAC, ACL, policy evaluation, delegated administration, and permission inheritance.
• Develop integrations with API Gateway, AI Gateway, Kubernetes, and Service Mesh technologies.
• Contribute to distributed systems design, graph-based relationship modelling, and scalable microservices architecture.
• Create automated tests, authorisation regression suites, and performance testing frameworks to ensure platform reliability.

Your Skills & Experience

• Strong experience developing backend applications using C#/.NET, with Java also considered.
• Expertise building and supporting REST APIs within microservices-based environments.
• Strong understanding of identity, authentication, and authorisation concepts.
• Experience working with OAuth2, JWT, and OpenID Connect.
• Knowledge of distributed systems and graph modelling principles.
• Ability to design and develop scalable platform services and developer-facing SDKs.
• Experience integrating systems with Kubernetes, API Gateway technologies, or Service Mesh environments.
• Understanding of policy-based access control, RBAC, ReBAC, ACL, or policy-as-code frameworks.
• Exposure to technologies such as Neo4j, OpenFGA, SpiceDB, OPA, Redis, or Kafka would be beneficial.

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