This is a C library that implements the server-side of the U2F protocol. More precisely, it provides an API for generating the JSON blobs required by U2F devices to perform the U2F Registration and U2F Authentication operations, and functionality for verifying the cryptographic operations. For the host-side aspect, see our libu2f-host project.At registration time, an X.509 attestation certificate is provided. Ideally the whole certificate chain should be validated. However, the current version of this library does not do so. This is a known limitation, and we hope to address this as soon as possible. Please be sure to understand the implication of this before using the library.