Recently I encountered this problem: not being able to install a new plugin from "Eclipse marketplace.." or "Install new software...". The problem displayed is "HTTP Proxy Authentication Required". The problem was because my computer was behind a proxy server.
In my case, the Eclipse was configured to use the native proxy settings (configuredat the operating system level).
The problem appears to be from apache httpclient library and to fix this need to be disabled, by adding this line in eclipse.ini file:
eclipse.ini file looks like this now:
-startup plugins/org.eclipse.equinox.launcher_1.3.100.v20150511-1540.jar --launcher.library plugins/org.eclipse.equinox.launcher.win32.win32.x86_64_1.1.300.v20150602-1417 -product org.eclipse.epp.package.jee.product --launcher.defaultAction openFile --launcher.XXMaxPermSize 256M -showsplash org.eclipse.platform --launcher.XXMaxPermSize 256m --launcher.defaultAction openFile --launcher.appendVmargs -vmargs -Dosgi.requiredJavaVersion=1.7 -Dorg.eclipse.ecf.provider.filetransfer.excludeContributors=org.eclipse.ecf.provider.filetransfer.httpclient4 -Xms256m -Xmx1024m