Psalms 18:26

With the pure thou wilt shew thyself pure (barar); and with the froward (iqqesh) thou wilt shew thyself froward (pathal).

pure (barar) (Hithpael verb stem):  to purify oneself; to show oneself pure, just, kind

froward (iqqesqh):  twisted, distorted, crooked, perverse, perverted

froward (pathal) (Hithpael verb stem): to be twisted

Psalm 18:25-26 NASB:  

With the kind You show Yourself kind; with the blameless You show Yourself blameless; with the pure You show Yourself pure, and with the crooked You show Yourself astute.