CVE-2026-71891 BLS12-381 key validation accepts a public key built on a foreign curve

In Bouncy Castle for Java before 1.86, BLS12381BasicScheme.keyValidate, and so BLSPublicKeyParameters and every BasicScheme, MessageAugmentation and ProofOfPossession verify and aggregateVerify that gate on it, accepted a public key built on a foreign ECCurve that merely shares BLS12-381's field...