It would really be useful if an attorney with actual knowledge of constitutional and criminal law would comment. The huge majority of the comments are pretty worthless and display substantial lack of knowledge of those subjects.
Comey, for discussion purposes, having convinced a judge with proper jurisdiction that a particular cell phone contains specified data pertinent to a criminal investigation, and on that basis obtained a search warrant, wold like to have the actual power to execute the warrant and obtain access to the data. (Note that Comey, here, is a stand in for all law enforcement officials, and that the overwhelming majority of search warrants are issued at the state or local level).
Too bad for him. Encryption algorithms in common use do not have known back doors and are believed to be infeasible to crack without the key, including by government cryptographic agencies. And use of such encryption is as close as anything to a natural right.
That said, we have governments, and in democratic regimes we grant them authority in certain areas of activity, one of which nearly always is protection of life and property, along with identifying, charging, trying, and punishing those who violate laws. The U. S. Constitution is quite clear about that; it is far more than the Bill of Rights, as important as it is. The Bill of Rights, and som later amendments, limit what both federal and state governments can do, but the protections in them are limited as well. The Fourth Amendment prohibits unreasonable searches and seizures, not all; and it requires that the government establish probable cause to obtain a warrant and to describe in some detail what the warrant is after. (The latter is part of the argument against general communication metadata collection). The Fifth Amendment, in part, relieves those charged with crimes of any requirement to give testimony against themselves, but that is not leave to hide or destroy evidence, even evidence they own, that supports the charges against them.
That the FBI and other law enforcement agencies sometimes exceeded their authority does not invalidate a claim, fully justified by the Constitution, that they can, subject to limitations and procedural requirements, properly obtain information from individuals' cell phones and other computers, just as they can from houses and file cabinets within them. It probably also does not mean that the government cannot punish refusal of a person served with a warrant to reveal the contents of an encrypted cell phone or other computer.
The person may have a natural right to encrypt the data and refuse to act to decrypt it, but the government has the authority and power, under some circumstances, to punish him for that refusal.