Phoenix has a built-in tool for this:
mix phx.gen.cert
Check instructions at config/dev.exs:
# ## SSL Support
#
# In order to use HTTPS in development, a self-signed
# certificate can be generated by running the following
# Mix task:
#
# mix phx.gen.cert
#
# Note that this task requires Erlang/OTP 20 or later.
# Run `mix help phx.gen.cert` for more information.
#
# The `http:` config above can be replaced with:
#
# https: [
# port: 4001,
# cipher_suite: :strong,
# keyfile: "priv/cert/selfsigned_key.pem",
# certfile: "priv/cert/selfsigned.pem"
# ],
#
# If desired, both `http:` and `https:` keys can be
# configured to run both http and https servers on
# different ports.
