The security features of current digital services are mostly defined and dictated by the service provider (SP). A user can always decline to use a service whose terms do not fulfill the expected criteria, but in many cases, even a simple negotiation might result in a more satisfying outcome. This article aims at building nonrepudiable security service-level agreements (SSLAs) between a user and an SP. The proposed mechanism provides a means to describe security requirements and capabilities in different dimensions, from overall targets and risks to technical specifications, and it also helps in translating between the dimensions. A negotiation protocol and a decision algorithm are then used to let the parties agree on the security features used in the service. This article demonstrates the feasibility and usability of the mechanism by describing its usage scenario and proof-of-concept implementation and analyzes its nonrepudiability and security aspects.