Signing for DHT could work for improper secret 

Currently for DHT secret prover can generate a proof for secret not really satisfying a statement to be proven. See new failing "DH tuple - signing w. proper and improper secrets" test in https://github.com/ScorexFoundation/sigmastate-interpreter/pull/636/ 

Happily, verification fails, so there is no problem with the Ergo protocol. 