The police cannot force you to tell them the passcode for your phone. Forcing you to turn over or type in your passcode violates the Fifth Amendment privilege against self-incrimination—the privilege that allows people to “plead the Fifth” to avoid handing the government evidence it could use against them. And if you have a phone that’s encrypted by default (which we hope you do), forcing you to type in your passcode to unlock the device means forcing you to decrypt your phone, too. That forced translation—of unintelligible information to intelligible—also violates the Fifth Amendment.

But there’s a problem: not all law enforcement officers have received the memo. In one particularly egregious case, military investigators forced the defendant, Sergeant Edward J. Mitchell, to unlock and decrypt his iPhone 6 after he asked for a lawyer. Not only was the investigators’ continued interrogation of Sgt. Mitchell without a lawyer a clear violation of U.S. Supreme Court precedent, but compelling him to unlock and decrypt his phone also violated the Fifth Amendment. The case is currently on appeal to a federal military appeals court, and we filed an amicus brief with the court explaining why.

The Fifth Amendment privilege against compelled self-incrimination protects “testimonial” communications. Testimonial communications are those that require a person to use “the contents of his own mind” to communicate some fact. Testimonial communications don’t have to be verbal; the key is that the information conveyed must come from the suspect’s own mind. As we explain in our brief, compelled passcode-based decryption is inherently testimonial—and thus always prohibited by the Fifth Amendment—for two reasons.

First, the compelled entry of a memorized passcode forces a person to reveal the contents of their mind to investigators—contents that are absolutely privileged by the Fifth Amendment. As far as the Fifth Amendment is concerned, there’s no difference between forcing a person to type their passcode directly into their phone and forcing them to say it out loud to an investigator. The trial judge in this case understood that and found that typing in a passcode was a “testimonial act.” So just by forcing the defendant to unlock his phone, the investigators violated his Fifth Amendment right.

Second, the process of decryption itself is testimonial because it involves translating unintelligible, encrypted evidence into a form that can be used and understood by investigators—again relying on the contents of the suspect’s mind.

Encryption transforms plain, understandable information into unreadable letters, numbers, or symbols using a fixed formula or process. When information is encrypted on a phone, computer, or other electronic device, it exists only in its scrambled format. If Sgt. Mitchell’s phone had merely been locked but not also encrypted, had the officers broken into the phone, they would have been able to access and understand the information stored on the phone. But since the phone was encrypted, if they had tried to break into the phone, they would have found only scrambled, encrypted data; they wouldn’t have been able to understand it. The officers needed Sgt. Mitchell, and his unique knowledge, to translate the information on the phone into its unscrambled, intelligible state for them to be able to use it against him. In other words, they were seeking transformation and explanation of data by an accused of the very data they sought to incriminate him with. This thus violated the Fifth Amendment for a second and independent reason—because of the nature of compelled decryption.

Oral argument in this case is scheduled for 12:30 p.m. on April 4, 2017 at the University of Notre Dame Law School in Indiana, as part of the Court of Appeals for the Armed Forces’ student outreach program. We hope the court holds that, because of the very nature of decryption, compelled passcode-based decryption hits at the heart of the Fifth Amendment’s privilege against self-incrimination.

Thanks to the American Civil Liberties Union and ACLU of the District of Columbia for joining our brief.