libcurl in use.
libcurlVersion()"none". It is intended for the version
of OpenSSL used, but not all implementations of libcurl use
OpenSSL --- for example OS X reports "SecureTranspart", its
wrapper for SSL/TLS.libssh version,
which may or may not be available (it is used for
e.g.\ifelse{latex}{\out{~}}{ } scp and sftp protocols). Where present,
something like "libssh2/1.5.0".extSoftVersion for versions of other third-party
software. curlGetHeaders, download.file and
url for functions which (optionally) use libcurl.
http://curl.haxx.se/docs/sslcerts.html and
http://curl.haxx.se/docs/ssl-compared.html for more details on
SSL versions. Normally libcurl used with R uses
SecureTransport on OS X, OpenSSL on Windows and GnuTLS, NSS or OpenSSL
on Unix-alikes. (At the time of writing Debian-based Linuxen use
GnuTLS, RedHat-based ones use NSS.)