Of course you are not going to give them the wallet password.
Oh.. they tell you that you withdrawn the coins from an exchange, so that's the proof you have the coins?
Just say that it was an hacker that stole your coins from the exchange, not you.
You got robbed, and you don't have to pay taxes on the money that was stolen from you, right?
P.S. for the geeks: use Veracrypt
You really do not need to provide your password to give the declaration, once you declare that you have how much coins in your wallet then can verify it in the blockchain without giving out any sensitive data, when it comes to government you never lie unless you want trouble, how are you planning to cash out the coins other than through banks accounts, either you have to find a user who provides cash for your coins or only through bank account and how are you planning to over come that situation. If a government ask me to sign a declaration form about the amount of coins i hold i have no other choice than to provide the details rather than facing punishment.Oh.. they tell you that you withdrawn the coins from an exchange, so that's the proof you have the coins?
Just say that it was an hacker that stole your coins from the exchange, not you.
You got robbed, and you don't have to pay taxes on the money that was stolen from you, right?
P.S. for the geeks: use Veracrypt