Or, one could use a confession as ones password, e.g. "I shot the sheriff, and buried him in the woods under the old oak tree."
Suitably long, and also guarantees that telling law enforcement or the courts would be self-incrimination.
Don't know how one would prove that though, perhaps tell ones lawyer the password, who could then testify on your behalf that the password is self-incriminating, but they are legally obliged to keep the actual password privileged.
That should tie the courts up in deliberations for quite a while, although I guess they could offer you immunity for the crime in the password, but that runs the risk of letting you off for murder & then discovering that the evidence in the phone actually exonerates you of the current charges.