Da Wikipedia:
The web site for distribution of LPS-Public is served over HTTPS, but uses a TLS certificate that is signed by a certificate authority managed directly by the Department of Defense. This CA is not trusted by most non-DoD operating systems and browsers. Because of this, visiting the site or downloading the ISO will generate a browser security warning indicating that the certificate is not trusted. Military-issued devices already include this certificate authority in their list of trusted CAs, and so will not see these errors.
Você terá que obter o CA do DOD.