Biometric Authentication Under the Fifth Amendment

I. The Password Problem

The Fifth Amendment protects any citizen against being “compelled in any criminal case to be a witness against himself.”¹ In other words, it prevents defendants from being coerced into providing incriminating, testimonial communications that could be used against them in court; however, it does not protect every piece of evidence that comes from a defendant.² A person can generally be compelled to provide physical evidence such as fingerprints or a blood sample in an investigation, but they cannot generally be compelled to reveal information from their mind like a password.³ Biometric phone unlocking sits at the intersection of these two categories, making it a significant issue in the development of modern case law.

Biometric authentication refers to the use of a fingerprint, face, or other physical attribute to unlock a device. The inconsistency lies not in the difference between collecting outer characteristics rather than inner knowledge but in what the defendant communicates in the process. For instance, using a fingerprint to unlock a particular phone can communicate information about the person’s relationship with that phone. The constitutional question should therefore hinge on what information compelled biometric authentication communicates in a given case.

II. United States v. Payne

Police arrested Jeremy Payne during a traffic stop.⁴ Payne identified his phone but refused to provide its passcode, so the officers physically commandeered his thumb and used it to unlock the phone.⁵ In the legal proceedings, Payne argued that the compelled biometric unlocking was testimonial and thus unconstitutional under the Fifth Amendment, but the Ninth Circuit rejected his claim.⁶

The central reasoning in the court’s ruling was that Payne’s thumb required no “cognitive exertion,” causing the act to be primarily physical rather than testimonial.⁷ One factor that led to this ruling is the fact that Payne had already identified the phone as belonging to him, so unlocking the phone failed to communicate ownership.⁸ Additionally, the officers chose to use his thumbprint instead of allowing Payne himself to choose which finger would unlock the phone.⁹ This means that the act didn’t provide any information about Payne’s knowledge of or relationship to the phone.

Notably, the court explained that if Payne had been allowed to choose which finger would unlock the phone, the act may have been considered testimonial.¹⁰ Payne’s knowledge of the correct finger to unlock the phone would have communicated both possession of the device and control over its contents.

III. United States v. Brown

FBI agents compelled Peter Schwartz to unlock his phone using his fingerprint.¹¹ While the district court found the act to be nontestimonial, the D.C. Circuit reversed the ruling, labelling the compelled biometric unlock a violation of the Fifth Amendment.¹² The circuit court viewed the act as communicating information rather than simply supplying a physical characteristic.¹³

Schwartz’s unlocking demonstrated knowledge of, control over, and access to the phone.¹⁴ Further, his understanding of which fingerprint would successfully unlock the phone communicated ownership.¹⁵ This ruling relied on the act-of-production doctrine, which holds that an otherwise physical act can be considered testimonial if the act itself communicates facts about existence, possession, control, or authenticity.¹⁶

IV. Communicative Significance

The key distinction between United States v. Payne and United States v. Brown is not necessarily whether biometric evidence itself is testimonial, but whether the defendant’s participation in the unlocking process communicates information beyond the physical attribute. In Payne, the officers took Payne’s thumb and used it to unlock his phone, meaning that Payne did not have to demonstrate knowledge of which finger would work.¹⁷ Because the phone had already been identified as his, Payne’s unlocking offered little additional information about his relationship to the device.¹⁸ In contrast, Brown involved Schwartz being compelled to unlock his phone himself.¹⁹ By choosing the correct fingerprint to successfully unlock the device, Schwartz demonstrated knowledge of how to access the phone; moreover, he asserted control over it.²⁰

This difference between the two cases illustrates why the physical nature of a characteristic should not automatically determine whether compelled biometric authentication is testimonial. A fingerprint may be physical evidence when collected by the state, but using it to unlock a device has the potential to communicate information about the defendant’s relationship to the device.²¹ In line with the act-of-production doctrine applied in Brown, the rule for judging the testimonial quality of biometric unlocking should be the following: compelled biometric authentication is testimonial when the defendant’s participation communicates knowledge of, control over, or access to a particular device.²² Rather than blanket-labelling all biometric evidence as constitutionally protected, this rule allows for evaluation on a case-by-case basis while upholding the Fifth Amendment.

V. In Context: ICE Arrests

The conversation surrounding biometric authentication and the Fifth Amendment is increasingly relevant as bureaucratic law enforcement relies on physical characteristics to unlock devices during arrests. Consider an individual arrested by ICE whose phone is protected by fingerprint authentication. If the person refuses to provide the password, but the officer forces the individual to unlock the phone via fingerprint, the state obtains information from the interaction that goes beyond the content of the phone. In his scenario, the act of unlocking the device demonstrates that the individual knows which fingerprint will be successful, effectively communicating that he or she has both access to and control over the device.²³

The distinction between physical evidence and testimonial communication should depend on what the compelled act reveals about the defendant. While the fingerprint itself may only qualify as a physical characteristic,²⁴ using that fingerprint to unlock a particular device communicates information that goes beyond the fingerprint itself.²⁵ As biometric authentication becomes increasingly common, courts should recognize that compelled biometric unlocking can constitute a testimonial act protected by the Fifth Amendment.