Computers, Privacy & the Constitution

Biometric Passcodes and the Ease of Forced Decryption

-- By KyungjinKim - 11 Mar 2022


Since 2013, when Apple introduced "the first major smartphone to feature a fingerprint scanner," biometric passcodes - passcodes utilizing fingerprint or facial recognition technology to unlock devices - have exploded in popularity. Today, more than 71% mobile phones have biometric passcodes enabled. However, while devices with alphanumerical passcodes cannot be easily decrypted without the users' voluntary cooperation, biometric passcodes are vulnerable to attacks based on technical exploits, as well as “forced decryption” by law enforcement – users being legally obligated to unlock their devices under a court order.

Technology Exploits

Apple claims that biometric passcodes are secure because the probability of a random person having adequately identical fingerprints to match fingerprint passcodes is 1 in 50,000, and adequately identical faces is 1 in 1,000,000. However, this statistic is misleading because it only assumes one attack scenario, under which an attacker attempts decryption using only their own face or their own fingerprints. However, a dedicated attacker will be working under two scenarios: when the attacker knows the identity of the user, and when they do not.

Fingerprint passcodes are especially vulnerable under the first scenario. Attackers who have two-dimensional scans of fingerprints can print three-dimensional models to fool fingerprint scanners. Two-dimensional scans can easily be obtained by law enforcement following an arrest, but civilian attackers can also replicate the scans by utilizing photographs of users' hands with visible fingerprints. Facial recognition technology have also been historically vulnerable under the first scenario, for instance by attackers utilizing 3D models constructed to look like the user’s face. The second scenario also present serious risks. For instance, Apple’s fingerprint id can be fooled because it only checks for partial matches; thus, a “master key” designed to emulate common partial fingerprints can be used to unlock devices. Attackers can also utilize several existing bugs. Most importantly, because almost all biometric data remains unique for life, biometric passcodes cannot be changed, and attackers will be able to access all devices encrypted with biometric passcodes owned by the user after compromising the passcode just once.

Constitutional Protections to Forced Decryption

Biometric passcodes are also susceptible to forced encryption, and existing caselaw casts doubts on whether the Fourth or the Fifth Amendment can offer sufficient protection.

Fourth Amendment Protections

Because the Fourth Amendment allows a warrant to be issued upon "probable cause," the subsequent question is "[w]hat level of cause... the government might need to seize the suspect to enable the biometric access?"

The Fourth Amendment and caselaw agrees that, in a scenario in which 1) the particular devices are specified, 2) the user is specified, 3) there is probable cause for searching the device, and 4) there is probable cause for believing the device belongs to the defendant, Fourth Amendment requirements are satisfied for forced decryption using biometric passcodes. However, while most courts seem to agree that (3) is necessary, courts have reached markedly different conclusions when (1) and (4) were not satisfied. On one hand, some courts have held that a warrant that was not limited to a particular person or a particular device did not demonstrate probable cause. In re Search of a Residence in Oakland, 354 F. Supp. 3d 1010 (N.D. Cal. 2019); In re Application for a Search Warrant, 236 F. Supp. 3d 1066 (N.D. Ill. 2017). On the other hand, some district courts have held that Fourth Amendment requirements were met as long as the procedure was carried out with dispatch in the immediate vicinity of the premises to be searched, the government has reasonable suspicion that the suspect has committed a criminal act that is the subject matter of the warrant, and the government has reasonable suspicion that the individual's biometric features will unlock the device. In re Search of, 317 F. Supp. 3d 523, 532-33 (D.D.C. 2018); In re Search Warrant No. 5165, 470 F. Supp. 3d 715, 724 (E.D. Ky. 2020). Consequently, the worst case scenario is that a user will be legally subject to forced decryption simply by being near a searched area.

Fifth Amendment Protections

Alternatively, users may believe that the Fifth Amendment can offer protection, if forced decryption is found to be an act which compel the user to be a witness against himself. Generally, the question is whether providing the biometric passcode is 1) a communication or a communicative act that is 2) testimonial, 3) incriminating and 4) compelled. Barrera, at 835 (N.D. Ill. 2019). For there to be Fifth Amendment protection, all four criterion must be satisfied. However, courts remain divided concerning all factors. To begin with, some courts hold that providing a biometric passcode is not a communicative act at all, and merely a physical act. In re Search Warrant Application, 279 F. Supp. 3d 800 (N.D. Ill. 2017); In re Search of, 317 F. Supp. 3d 523 (D.D.C. 2018). Courts holding that the provision is a communicative act generally agree that the provision is also incriminating and compelled. However, they are heavily split on whether it is testimonial. Some courts have held that, because the biometric key is provision of a physical characteristic and not communicative testimony, the act of provision is not testimonial. State v. Diamond, 905 N.W.2d 870 (Minn. 2018); Barrera;. Finally, some courts have held that because provision of the key is implicit of the fact that the user owns or controls the content of the phone, the act is testimonial. In re Application for a Search Warrant, 236 F. Supp. 3d 1066 (N.D. Ill. 2017); In re Search of a Residence in Oakland, 354 F. Supp. 3d 1010 (N.D. Cal. 2019).


Biometric passcodes present both technological and legal risks. However, uses who choose to exclusively use alphanumerical passcodes as an alternative should know that there are unique legal issues with such passcodes. For instance, courts and academics agree that when the government can provide independent evidence that certain files exist on the encrypted portion of the devices and the defendant can access them, there is no Fifth Amendment protection against a decryption order.

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r6 - 07 Jun 2022 - 02:39:34 - KyungjinKim
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