f.hs

arbitraryPass :: Int -> Gen BS.ByteString
arbitraryPass l = do
  i <- chooseInt (0, l)
  BS8.pack <$> vectorOf i arbitraryPrintableChar

qc_crypto :: TestTree
qc_crypto = testGroup "Crytographic functions" $
  [ testProperty "encodePassword/decodePassword roundtrip" $
      forAll (liftA3 (,,) arbitraryBS16 arbitraryBS16 (arbitraryPass 33)) $ \(auth,sec,pass) ->
        Just pass === decodePassword auth (Secret sec) (encodePassword auth (Secret sec) pass)
  ]

f.txt

  Crytographic functions
    encodePassword/decodePassword rountrip:                                FAIL
      *** Failed! Falsified (after 59 tests):
      ("\214\188x\SYN\158\&3d\208{w\224v\212\208\&9$","\141&[\176\159C\134\195\186\195\DC3\192K2\EOT\191","`p\NUL")
      Just "`p\NUL" /= Just "`p"
      Use --quickcheck-replay=864535 to reproduce.
      Use -p '/encodePassword\/decodePassword rountrip/' to rerun this test only.