Not at all the main point, but tr can do rot13 and more; plus GNU Coreutils' implementation is fast:

    $ time openssl rand -base64 1000000000 | tr a-z n-za-m >/dev/null

    real    0m1.073s
    user    0m1.327s
    sys     0m0.644s
    $ time openssl rand -base64 1000000000 | rot13 >/dev/null

    real    0m19.225s
    user    0m20.101s
    sys     0m0.747s