Quote 31
IAGO
For whiles this honest fool
Plies Desdemona to repair his fortunes,
And she for him pleads strongly to the Moor,
I'll pour this pestilence into his ear:
That she repeals him for her body's lust;
And by how much she strives to do him good,
She shall undo her credit with the Moor.
So will I turn her virtue into pitch,
And out of her own goodness make the net
That shall enmesh them all. (2.3.373-382)
Earlier, we saw how Cassio sees his "reputation" as being the sum of his public behavior and his military service. Here, we can see that Desdemona's "reputation" hinges on her fidelity to her husband. When Iago says he's going to ruin Desdemona's "credit with the Moor," he means he's going to tarnish her reputation as a loyal wife ("turn her virtue into pitch").
Quote 32
IAGO
Demand me nothing. What you know, you know.
From this time forth I never will speak word. (5.2.355-356)
Iago refuses to explain himself. He conceals his motives and his true identity to the very last.