Trustworthy Software always acts like a friend.

It is secure.

It contains no back doors of any kind, for any reason. It does not allow arbitrary access to your system by unknown or known parties.

It does no harm.

Trustworthy software should never work to harm a user or the property that the user owns, or act in a way perceived by the user as evil.

It keeps secrets.

Your data is private and your software should never divulge private information to others unless you want it to.

It is dependable.

The software should be well tested and proven to be as defect free as reasonably possible, and be able to provide proof of this through unit tests and other standard software testing practices.

It is transparent about its own functionality and secrets.

Software should be trustworthy even if everything about it and how it works is fully public knowledge.

It is as easy to use for the average user as reasonably possible.

It has been said that the user base for strong cryptography or any other "complicated" form of software declines by half with every additional keystroke or mouseclick required to make it work. If nobody uses your software because it is hard to understand or use, it can not be a friend to your user.