As anyone who has seen “Law & Order” knows, Americans have a constitutional right against self-incrimination. Prosecutors can’t force a suspect to confess to a crime; only voluntary confessions are admissible in court.
 

But suppose a suspect writes a secret confession in a language known only to him. Can prosecutors force him to translate it into English and, thus, unwillingly incriminate himself?

In a case argued in November at the Supreme Judicial Court, state prosecutors claim that they can do exactly that.

The suit, Commonwealth v. Gelfgatt, concerns a defendant who allegedly encrypted information on computers and other electronic devices seized from his home and car. Prosecutors believe that the information, if decrypted, will show that the defendant committed mortgage fraud. But they say that they can’t crack the encryption.

In that sense, the information is like evidence written in a language known only to the defendant: He allegedly can decrypt it, but others allegedly can’t.

So the legal question is whether, despite the right against self-incrimination, prosecutors can force the defendant to decrypt that information and then use it to put him in prison.

A Superior Court judge ruled that such forced decryption would violate the Constitution because, in essence, it would require the defendant to explain the meaning of evidence seized by the government.

Prosecutors are challenging that ruling on appeal. They point to cases holding that certain physical acts, like providing a blood or handwriting sample, are not protected by the right against self-incrimination. They say that decrypting a computer is yet another physical act — like unlocking a safe — that is equally unprotected.

The problem with that argument, as the ACLU of Massachusetts and other privacy groups have explained in an amicus brief, is that it overlooks how encryption actually works.

Encryption does not simply lock up data, and decryption does not simply unlock it. Instead, encryption changes electronic data altogether. Typically, it scrambles the data into a jumble of 1s and 0s that can be unscrambled, and thus rendered understandable, by someone who knows the passphrase. So decryption actually creates a new version of the evidence.

For that reason, a defendant who is forced to decrypt his computer must use his own mind — where the passphrase is kept — to translate unreadable evidence into readable evidence. That act of translation is what separates compelled decryption from compelled acts of physical production.

In America, the Constitution prevents the government from forcing people to play such an active role in their own prosecutions. Prosecutors can’t force a foreign defendant to translate a document he wrote in Chinese. They can’t force a young defendant to translate something he said in slang. The encryption context is no different.

Of course, encryption software arguably makes it easier than ever before for people to create and store information that others cannot read. But that is no reason to discard the traditional right against self-incrimination.

If anything, upholding constitutional protection for data encrypted using the latest technology is a necessary counterweight to recent governmental assaults on privacy.

As disclosures about the National Security Agency confirm, the government has used technological advances to place powerful limits on the information that innocent people can keep from the government. Without obtaining individual warrants supported by probable cause, governments are collecting all Americans’ cellphone records, mapping social networks and monitoring the Internet.

Those tactics leave encryption as one of the few ways that innocent Americans can exert some measure of control over information they want to keep private. And there is a rich tradition of using encryption that way. Most famously, Thomas Jefferson and James Madison exchanged encoded drafts of the Bill of Rights.

Given that history, and given how governments are using the latest technology to thwart liberty, now is the time to ensure core constitutional protections remain intact so that all of us can use the latest technology without sacrificing our liberty.
 

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Matthew R. Segal and Jessie J. Rossman are legal director and staff attorney, respectively, at the ACLU of Massachusetts.