With the launcher, after entering my e-mail and password I get the following message:

I purchase minecrft on mojang web site and i select my operating system Windows 8 and i start to download. Then, there was trhe problem i have:
With the launcher, after entering my e-mail and password I get the following message:
(SunCertPathBuilderException: unable to find valid certification path to requested target)

what can i do to play with the mineccraft i purchase?