For those who are working on this problem, my squid.conf may be helpful:

--- squid.conf.default	2015-05-31 03:33:34.006361795 +0800
+++ squid.conf	2015-05-31 03:36:28.533034294 +0800
@@ -49,9 +49,15 @@
 # Example rule allowing access from your local networks.
 # Adapt localnet in the ACL section to list your (internal) IP networks
 # from where browsing should be allowed
-http_access allow localnet
-http_access allow localhost
+# http_access allow localnet
+# http_access allow localhost
 
+auth_param digest realm Proxy digest auth test
+auth_param digest program /usr/lib/squid/digest_file_auth -c /etc/squid/squid-passwd
+auth_param digest children 2
+
+acl squid-passwd proxy_auth REQUIRED
+http_access allow squid-passwd
 # And finally deny all other access to this proxy
 http_access deny all

And the content of /etc/squid/squid-passwd:

test_username:Proxy digest auth test:ab1e6b5de6ea6c8b072c5e513eea9c61

I'm testing for squid 3.5.5 on Arch Linux. I can test its correctness with curl:

$ curl -v --proxy-user test_username:test_password --proxy localhost:3128 --proxy-digest https://httpbin.org/ip