Add this to your .profile, .bashrc, .zshrc...
decode_base64_url() {
local len=$((${#1} % 4))
local result="$1"
if [ $len -eq 2 ]; then result="$1"'=='
elif [ $len -eq 3 ]; then result="$1"'='
fi
echo "$result" | tr '_-' '/+' | openssl enc -d -base64