Is security by obfuscation sufficient?
soult: There are two things to consider: First, you need to make sure that your filename is a long, unique, hard-to-guess string, that is easy to generate. This rules out all 5 UUID specifications:Version 1: MAC + timestamp => easy to predictVersion 2: MAC + some other static data + partial timestamp: => Also easy to predictVersion 3: MD5 of some file or random string => If an attacker has a file, he can generate the MD5 hash himself and see if you also have this file.Version 4: Random data => slowVersion 5: Same as version 3, but with SHA1.Your best bet IMHO is to use the HMAC of the file. This will defend you against all the flaws that using an unique ID would have.The second step is to ensure that your secret links don't leak. You can employ robots.txt to disallow robots, use dereferer.org or anonym.to to hide away referers, but you still won't be secure, as someone can still copy and paste the link. If that is ok with you, then you can stop reading now.You could of course add an EC2 machine between the user and your S3 storage that makes sure each link only works once, but this would be expensive and counter-effective. However, Amazon S3 allows you to create a request that can be made via HTTP GET and that is only valid until a specific time. (See the API documentation, chapter "Authentication and Access Control"). This will allow you to generate a new URL every time you want to serve a file to your client. The URL will only be valid for a specific time period. The downside is, that this again is time-demanding and that all caching on the user side will be useless.Good luck!