Oh, good grief.
Capacitor, I would humbly suggest that you read the DCMA (the Digital Millenium Copyright Act, 17 U.S.C § 1201 et. seq.) rather than relying on the information you receive from groups whose views are perhaps not fairly described as neutral towards this issue.
The DCMA criminalizes circumventing “a technological measure” that controls access to a copyrighted work, as well as distributing such a measure to others.
If there were a virus that used encoding similar to CSS, then the virus protection makers would, in fighting it, not be distributing a measure that controlled access to a copyrighted work. They would be fighting a virus that controlled access to works that are owned or licensed, legitimately, by the user. The mere fact that this work might also apply to breaking CSS does not automatically make it illegal.
In the same vein, it might be illegal in your jurisdiction to sell drug paraphenelia, and if I have a store that sells rolling paper, pipes, glass tubes, metal screens, and the like, I could probably be prosecuted for it. But if I own a hardware store, I can sell pipes and screens and glass tubes quite legally.
Don’t get your hopes up – you can’t distribute a DeCSS-like system and make some half-assed claim that you’re fighting a virus. One section of the DMCA says that there are factors to be considered in determining the good faith of someone claiming to be working on encryption, for instance (another legal exception to DCMA’s rules):
*(A) whether the information derived from the encryption research was disseminated, and if so, whether it was disseminated in a manner reasonably calculated to advance the state of knowledge or development of encryption technology, versus whether it was disseminated in a manner that facilitates infringement under this title or a violation of applicable law other than this section, including a violation of privacy or breach of security;
(B) whether the person is engaged in a legitimate course of study, is employed, or is appropriately trained or experienced, in the field of encryption technology; and
© whether the person provides the copyright owner of the work to which the technological measure is applied with notice of the findings and documentation of the research, and the time when such notice is provided.*
From this we see that an inquiry into the real purpose of the decryption is not only encouraged but mandated.
In short, the OP’s scenario is not realistic at all. Not only would no prosecutor ever proceed under the facts mentioned, the plain meaning of the statute protects a virus antidote company from any liability under the law.