I was following the vignette for the package, updated and loaded all necessary packages, seemed like it would be a pretty straight-forward process to authenticate. Instead, I get an error
> setup_twitter_oauth(consumer_key, consumer_secret, access_token, access_secret)
 "Using direct authentication"
Error in check_twitter_oauth() : OAuth authentication error:
This most likely means that you have incorrectly called setup_twitter_oauth()'
download.file(url="http://curl.haxx.se/ca/cacert.pem", destfile="cacert.pem") #read this was necessary for Windows machines
consumer_key <- 'abc'
consumer_secret <- 'abc'
access_token <- '123-abc'
access_secret <- 'abc'
setup_twitter_oauth(consumer_key, consumer_secret, access_token, access_secret)
Just install/load base64enc. That solves the problem.