The SCO Group Inc. has found what it says is proof that it owns all copyrights related to the Unix operating system, a claim rival Novell Inc. had contested last month.

SCO said Friday the proof is in an amendment to the asset purchase agreement through which SCO acquired Unix from Novell in 1995. The amendment dates from 1996.

Last month, Novell said it hadn’t transferred Unix’s copyrights nor patents to SCO as part of their Unix purchase agreement.

But in light of SCO’s finding, Novell on Friday reluctantly acknowledged that the amendment “appears to support SCO’s claim that ownership of certain copyrights for Unix did transfer to SCO in 1996.” However, Novell reiterated its claim that it holds the Unix patents.

At any rate, the companies are involved in nothing more than a war of words so far. Neither has raised the issue with a court.

Novell launched its challenge last month in order to poke holes into SCO’s legal challenges against the open-source Linux operating system.

SCO has made allegations that Unix code it owns has been illegally copied into Linux software, including the Linux operating system kernel.

So far, SCO has only formally sued one company over Unix and Linux: IBM Corp. In March, SCO sued IBM, seeking at least US$1 billion, alleging misappropriation of trade secrets, unfair competition, contract interference and breach of contract, in attempts to damage Unix to benefit its Linux business.

SCO has purposefully decided not to proceed with a copyright or patent violation charge against IBM, or anybody else for that matter so far, because it believes the avenue it has taken against IBM puts it on much stronger legal ground, its executives have said.

SCO has also sent letters to about 1,500 large companies warning them they could be held liable for intellectual property violations related to their use of Linux software. It has also warned commercial Linux vendors of possible liabilities.