The API can be found at: http://wpcentral.io/api.
Warning
All API’s aren’t stable yet and can change till REST API is in core of WordPress. This include the footprint of the APIs.
Getting checksums of plugins & themes
Currently you can get the checksums of plugins by passing the name and the version number. See for endpoints below.
/checksums/plugin/(?P<plugin>[a-z-]+)/(?P<version>\\w.+)
/checksums/theme/(?P<theme>[a-z-]+)/(?P<version>\\w.+)