My recent post on the legality of warrants that permit phone unlocking prompted some reader comments on how the Fifth Amendment might apply to using fingerprint readers such as Apple’s Touch ID. I think this is a hard issue, so it might be worth explaining my thinking in detail. Here are my current thoughts, with the caveat that my views aren’t completely settled and I may revisit them in the future.

Let’s start with an easy case, which I’ll label Case 1. The government orders a person to place a particular finger on a particular phone’s fingerprint reader. The Fifth Amendment is not implicated here because the person is not testifying. He is not saying anything about what is going on in his mind, so no Fifth Amendment testimony has been compelled. The police could force him to put the finger on the fingerprint reader if he were asleep or otherwise unconscious. No testimonial statement from the person is implied by the act of placing his finger on the reader.

Now consider Case 2. The police search a home with a warrant. The police find only one electronic device in the house — a single iPhone with Touch ID enabled — on the kitchen table. There are seven residents present at the house. When the police ask the seven residents who owns that phone, they all refuse to say. An officer approaches one of the residents and says: “I’m guessing that this is your phone. If you don’t go over there and unlock it with Touch ID, I’m going to arrest you for refusing my order.” It turns out the officer was right: It is that resident’s phone. The question is, does that resident have a Fifth Amendment right not to comply with the order?

Case 2 is different from Case 1, I think, because I see complying with the officer’s order as including testimony. It amounts to testimony that says, “yes, this is my phone,” or at least, “yes, this phone was set to recognize a part of my body as a means of access.” It further says: “I am familiar enough with this phone to know that the fingerprint reader was enabled and which part of me was used by me to program the fingerprint reader.”

Case 2 implies testimony because Touch ID is programmed to respond to only one body part. That choice of body part acts like a password. It’s a simple password, but it’s still a password. Most people use a finger, but they can use any of their 10 fingers. And Touch ID can be programmed to use other body parts such as the palm of a hand or even toes instead of fingers. (Very unlikely, sure — but possible.) Most people choose which body part to use based on convenience, not security. But the owner knows which body part was used and the government doesn’t, so responding to the order by unlocking the phone using the correct body part tends to show that the person is the owner.

Why might that matter? Imagine the investigation in Case 2 began when someone texted a threatening communication from a cellphone to an undercover officer. The government has reason to believe the originating phone was inside the house to be searched. The warrant authorized the government to search the house and then seize and search the phone to confirm that it was used to send the threat. In that scenario, the act of going over to the phone and unlocking it provides evidence of familiarity with and likely control of the phone. That alone isn’t enough to show that the unlocking resident sent the threat, but it’s a “link in the chain” of evidence that might support that conclusion. So I would think that unlocking the phone using Touch ID has testimonial aspects that can be incriminating and therefore trigger the Fifth Amendment.

Now consider Case 3. Imagine the police know that a particular phone belongs to a particular person. They saw the suspect use the phone to make calls and send some text messages. The suspect refers to that particular phone as “my phone,” and he gives out a number as his that rings that phone. Now imagine the police seize the phone with a warrant and want to order the suspect to use the fingerprint reader to unlock it. The officer says, “Unlock this with Touch ID, or else I’m going to arrest you for refusing my order.” Does the suspect have a Fifth Amendment right not to comply with the order?

I think the testimonial aspect of Case 3 is the same as in Case 2, but that there’s a big Fifth Amendment difference between them: Compliance would be a foregone conclusion in Case 3, while it was not in Case 2. In Case 2, the unlocking helped prove that it was the suspect’s phone. But the government already knows that in Case 3, so the foregone conclusion doctrine should apply and that should defeat the privilege. See here for a detailed explanation of the point.

In cases 2 and 3, the Fifth Amendment rule ends up being the same as it should be if the suspect had used a passcode instead of Touch ID and the officer ordered the person to enter in the passcode without revealing it to the officer. In those two cases, passcode vs. Touch ID should make no difference.

The government could argue that cases 2 and 3 are not testimonial under Doe v. United States, 487 U.S. 201 (1988). Let’s take a close look at Doe to see whether the government is right.

In Doe, the government was seeking Doe’s foreign bank records. The banks wouldn’t reveal the records without Doe’s consent, which required his signature. The government sought an order requiring Doe to sign a blanket consent form permitting the disclosure of any and all accounts he held. Doe asserted his Fifth Amendment privilege and argued that writing his signature on the blanket consent form amounted to testimony because the government was compelling the content of his speech — his signature — to gather evidence against him. The government responded that a signature does not testify about anything and didn’t trigger the Fifth Amendment.

The Supreme Court agreed with the government. “In order to be testimonial,” the court wrote, “an accused’s communication must itself, explicitly or implicitly, relate a factual assertion or disclose information.” That is, “the privilege may be asserted only to resist compelled explicit or implicit disclosures of incriminating information.” Signing the form “makes no statement, explicit or implicit, regarding the existence of a foreign bank account or his control over any such account.” True, signing the form does communicate the fact that Doe signed the form. But “this kind of simple acknowledgment — that the suspect in fact performed the compelled act — is not sufficiently testimonial for purposes of the privilege.”

My sense is that Doe is distinguishable from Case 2 and 3 because picking the right body part to unlock the phone does make an implicit statement concerning “his control over” the phone. Doe could sign the form regardless of whether he had zero bank accounts or hundreds. Signing the form merely showed that he could sign a form; that had nothing to do with Doe’s relationship to any accounts that existed. In contrast, unlocking the phone with Touch ID admits to control of the phone, in that it requires knowing the “password” of which body part unlocked the phone.

Although that’s my best sense of how Doe applies, there’s an ambiguous passage that can be read a different way in the government’s favor. In his dissent, Justice John Paul Stevens argued that signing the form was testimonial because it was akin to being “compelled to reveal the combination to his wall safe.” Stevens wrote:

[A person] may in some cases be forced to surrender a key to a strongbox containing incriminating documents, but I do not believe he can be compelled to reveal the combination to his wall safe — by word or deed.

The majority responded in Footnote 9 as follows:

We do not disagree with the dissent that “[t]he expression of the contents of an individual’s mind” is testimonial communication for purposes of the Fifth Amendment. Post at 487 U. S. 220, n. 1. We simply disagree with the dissent’s conclusion that the execution of the consent directive at issue here forced petitioner to express the contents of his mind. In our view, such compulsion is more like “be[ing] forced to surrender a key to a strongbox containing incriminating documents,” than it is like “be[ing] compelled to reveal the combination to [petitioner’s] wall safe.” Post at 487 U. S. 219.

The government could argue that being forced to unlock the phone using Touch ID is like “being forced to surrender a key to a strongbox containing incriminating documents,” rather than “being compelled to reveal the combination” to a safe. That’s what it looks like, after all. The fingerprint is the key, and the phone is the strongbox. Unlocking the phone is like surrendering the key.

This is a possible argument, but at this point I’m unconvinced. The problem is that this passage is really vague. Stevens wrote that a person may “in some cases” be forced to surrender a key, but that suggests that there are other cases in which a person couldn’t be forced to surrender a key. And the majority’s Footnote 9 is unhelpful on the point. It says that the facts of Doe are “more like” the surrendered key than a disclosed combination. But it doesn’t say why, or when a key can or can’t be surrendered. (In case you’re wondering, Justice Harry Blackmun wrote Doe. Perhaps the uncertainty is to be expected.) On the whole, I don’t think this is enough for the government to go on.

In short, my current view is:

Case 1: No Fifth Amendment privilege because there is no testimony.
Case 2: Valid Fifth Amendment privilege.
Case 3: No Fifth Amendment privilege because the foregone conclusion doctrine applies even though there is testimony.

Comments would be very welcome.