Of course you can claim you forgot. But the judge has to believe you or you will get the same punishment as if you refuse to disclose. So the issue becomes how to plausibly forget. Some below suggest complicated two-factor authentication. But it might be enough just to cite the rarity of strong passwords - if they were easy to use/remember, everyone would use them. You do, but, predictably, you forgot yours. It's a stressful time, you know.

Just make sure you never write the password down. Not only because it could be discovered before you destroy it, but because destroying it could be construed as destruction of physical evidence, opening you up to obstruction of justice charges even if you are exonerated.