Hi scruffy,
Quite a bit to unpick here. As you are probably aware, software is treated as if it was a work of literature when it comes to copyright protection. In the modern age this is clearly nonsense, but it goes back to a time when all the instructions in a program were typed out manually so the process was a bit similiar to writing a book or a recipe. The original Copyright Designs and Patents Act has been extensively modified since 1988 to reflect the developments in software and the capabilities of hardware such as processors and, of course, the internet. The result is somewhat disjointed and interpretation of the law with respect to software is being amended by the courts all the time.
Anything which alters or adapts a piece of software without the permission of the copyright owner has the potential to be an infringement - see
section 21(3)(ab) and 21(4) CDPA. If it appears that your software is
specifically intended to make an infringing adaptation contrary to section 21(3)(ab), then you as the seller of the program may become liable for secondary infringement under
section 24(1) which says:
24 Secondary infringement: providing means for making infringing copies.
(1) Copyright in a work is infringed by a person who, without the licence of the copyright owner—
(a) makes,
(b) imports into the United Kingdom,
(c) possesses in the course of a business, or
(d) sells or lets for hire, or offers or exposes for sale or hire,
an article specifically designed or adapted for making copies of that work, knowing or having reason to believe that it is to be used to make infringing copies.
However this needs to be contrasted with
section 50C which says:
(1) It is not an infringement of copyright for a lawful user of a copy of a computer program to copy or adapt it, provided that the copying or adapting—
(a) is necessary for his lawful use; and
(b) is not prohibited under any term or condition of an agreement regulating the circumstances in which his use is lawful.
(2) It may, in particular, be necessary for the lawful use of a computer program to copy it or adapt it for the purpose of correcting errors in it.
It is worth noting in passing here that, in my experience, virtually all commercial software comes with a EULA which forbids the user from adapting it without permission, so sub-sub-section (b) above will apply in most cases.
Adding a warning to users of your software not to use it for such purposes is probably sensible, although it might also be seen as a subtle way of advertising that that is exactly what the software is designed to do. To use your gun analogy, it might be seen as similar to putting the words 'guns can kill' on the packaging of a firearm, prompting the buyer to say "yep, that's exactly what I want it to do". Only a court could decide, based on the evidence which was presented to it, whether your software was mainly intended for the purpose of infringing the copyright of other software.
You also mention copying software for private use. This is permitted under
section 50A for the purpose creating software backups, as a prudent safeguard against an original CD or DVD becoming unreadable. It does not apply to software as a service (SaaS) or other cloud based implementations of software. But more importantly this exception does
not authorise the making of adaptations of the original software other than for the limited purpose of correcting errors. The same part of the Act contains provisions permitting a program to be decompiled in certain circumstances (sections 50B and 50BA).
Then moving to the next level, where authors of their software seek to protect it with encryption or other technical measures, this brings into play two other parts of the law. The first is still within the Copyright Designs and Patents Act and can be found in
sections 296 - 296ZG. The gist of this part is that "a person who, knowing or having reason to believe that it will be used to make infringing copies, manufactures for sale or hire, imports, distributes, sells or lets for hire, offers or exposes for sale or hire, advertises for sale or hire or has in his possession for commercial purposes any means the sole intended purpose of which is to facilitate the unauthorised removal or circumvention of the technical device," is liable for infringement. In other words if your software can be used to defeat or remove any encryption etc applied by the original software maker, you could be liable provided that it could be shown that that was the 'sole intended purpose' of your software.
The second area of law is the Computer Misuse Act 1990 which forms part of the criminal law and is consequently much more serious. Although the CMA is largely aimed at the unauthorised access to a computer or network, there are a number of subsections (particularly
section 3A(2 ) where a person is guilty of an offence if he supplies an article (which includes software) likely to be used to commit an offence under section 1, namely gaining unauthorised access to a computer system. Obviously this aimed at the makers of viruses, trojans and so on, rather than mainstream software, but you need to be aware that this law exists.