Is Dijjer legal?

Dijjer is designed to fall under the "System Caching" exemption of the Digital Millennium Copyright Act, and as such, should be perfectly legal in the US and other countries with similar laws.

Is Dijjer useful for copyright infringement?

Dijjer will only download a file if it's available on a website. So for corporate copyright holders, policing Dijjer is exactly the same as policing websites.

Then what's the goal of Dijjer?

In practice, most artists have to give up their copyrights to big corporations to reach a large audience. We hope Dijjer will change that, by letting musicians and filmmakers reach millions directly, without handing over their copyrights to anybody.