Fostering the widespread adoption of digital credentials requires trust and confidence within the civilian population, which needs to be facilitated through strong privacy guarantees. In this context, the concept of third-party deniability, i.e., a credential verifier's inability to prove the authenticity of received personal data to third parties, is particularly important. This work gives a structured overview of signature-based authentication mechanisms which offer third-party deniability and can be applied to the setting of a three-party credential system for digital identity documents. The resulting primitives are then compared with respect to practical challenges and requirements arising from credential systems.