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.)