Heimdal libkrb5 library is a implementation of the Kerberos protocol.
Kerberos is a system for authenticating users and services on a network. It is built upon the assumption that the network is ``unsafe''. For example, data sent over the network can be eavesdropped and altered, and addresses can also be faked. Therefore they cannot be used for authentication purposes.
If you want to know more about the file formats that is used by Heimdal, please see: File formats
The project web page: http://www.h5l.org/