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A general description is here.
Support for 'https://' URLs is available on Windows, and on other platforms if support for 'libcurl' was compiled in and if that supports the 'https' protocol (system installations can be expected to do). So 'https://' support can be expected except on rather old OSes (an example being OS X 'Snow Leopard', where a non-system version of 'libcurl' can be used).
(Windows only) The default method for accessing URLs _via_ 'download.file()' and 'url()' has been changed to be '"wininet"' using Windows API calls. This changes the way proxies need to be set and security settings made: there have been some reports of sites being inaccessible under the new default method (but the previous methods remain available).