We already have relevant laws:
The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no warrants shall issue, but upon probable cause, supported by oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.
The Fourth Amendment prohibits legislation that forbids people from keeping their "papers and effects" encrypted when no warrant has been issued. When a warrant has been issued:
nor shall any person [...] be compelled in any criminal case to be a witness against himself
The Fifth Amendment forbids compelling anyone to provide self-incriminating testimony, why should compelling anyone to provide self-incriminating evidence be any different?
Mandating key escrow might be constitutional. Then they could convict for the crime of having an un-escrowed encrypted device even if they couldn't prove terrorism or pedo charges. Remember, Al Capone was convicted of tax evasion, not murder or racketeering.