Perhaps a solution is to commit two crimes: crime A which has to do with the encrypted file, and crime B which has nothing to do with it. Then, use a phrase which is self-incriminating for crime B as the password for the encrypted volume. For example, "IBrokeTheSpeedLimitBy15MilesPerHourOnJuneTheTwenty-Seventh,InTheYearTwoThousandAnd4".
In which case the judge would compel you to reveal your password to your attorney. The attorney would not be able to reveal the password as that would violate privilege, but they would be compelled to uphold the terms of the valid search warrant on the decrypted data.
Well, suppose that you do give them the unencrypted file. They already have the encrypted file and presumably are aware of the encryption scheme you used, so (this could be totally wrong, as I don't know enough about encryption) they should be able to figure out the encryption key.
Any encryption scheme that is not worthless does not work that way. If you have the plaintext and the ciphertext (but did not get to choose the plaintext to be encrypted) then you are still no closer to getting the key.
