XiPKI: Compact open source PKI (CA, OCSP responder, certificate protocols ACME, CMP, EST, SCEP).

The Dogtag Certificate System is an enterprise-class Certificate Authority (CA) which supports all aspects of certificate lifecycle management, including key archival, OCSP and smartcard management.

SureThing location certification transparency ledger, including monitor and auditor -- By Pedro M Matias Carvalho and Rafael A R Figueiredo

Android compatible version of https://github.com/google/certificate-transparency-java