Same word as valid:

accurate, attested, authentic, authoritative, binding, bona fide, cogent, compelling, conclusive, confirmed, convincing, credible, determinative, efficacious, efficient, good, in force, irrefutable, just, kosher, lawful, legal, legit, legitimate, logical, official, original, persuasive, potent, powerful, proven, pure, solid, sound, stringent, strong, substantial, telling, tested, true, trustworthy, ultimate, unadulterated, unanswerable, uncorrupted, weighty, well-founded, well-grounded