You may be able to encrypt beyond the government's ability to decrypt But how can you handle a court forcing you to reveal the contents?
IANAL, but at least in the USA, the fifth amendment protects you against self-incrimination. I do not think you can be compelled to divulge an encryption key if doing so would provide any evidence you committed a crime. Any decent lawyer and/or the ACLU's could probably prevail with this argument in court.
The trick, of course, is that the prosecution will typically give another party who has access to the encrypted data immunity from prosecution, so the 5th amendment does not apply. Then that party can be compelled to decrypt the messages. This makes getting at encrypted emails easy if the prosecution is only after one of the sender or recipient.
From what I can gather, in civil cases, so long as decryption would not provide evidence of a crime on your behalf, you could be compelled by the court to decrypt the data, even if it harms your arguments in the civil case. But if you're willing to risk perjury, you can always testify you forgot the key and likely get away with it.