If you’re running Apache 2.4 you can use configuration sections to solve this quite easily.
Eg…
# Redirect to HTTPS
RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI} [R,L]
# Authenticate users only when using HTTPS
# Enable for <v2.4
# SSLRequireSSL
# ErrorDocument 403 /secure-folder/
# Enable for >v2.4
<If "%{HTTPS} == 'on'">
AuthType Basic
AuthName "Special things"
AuthUserFile /etc/blah.htpasswd
# Prevent this 'Require' directive from overriding any merged previously
<IfVersion >= 2.4>
AuthMerging And
</IfVersion>
Require valid-user
# Enable for >v2.4
</If>