mastodon/app/services/setup_local_account_service.rb

class SetupLocalAccountService
  def call(user, username)
    user.build_account

    user.account.username = username
    user.account.domain   = nil

    keypair = OpenSSL::PKey::RSA.new(2048)
    user.account.private_key = keypair.to_pem
    user.account.public_key  = keypair.public_key.to_pem

    user.save!
  end
end