libcurl in use.
libcurlVersion()libcurl version in use or
"" if none is. If libcurl is available, has attributes"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.)