Biometric Passcodes and the Ease of Forced Decryption

-- By KyungjinKim - 11 Mar 2022

Introduction

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 uses their own face or their own fingerprints, hoping that the device might unlock. However, a dedicated attacker will be working under two scenarios: one in which the attacker knows the identity of the user, and one in which 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 the users' hands in which fingerprints are visible. Facial recognition technology have also been known to be vulnerable under the first scenario in the past, for instance by attackers utilizing 3D models constructed to look like the user’s face. The second scenario, while without the aforementioned vulnerabilities, 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 to be used. 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 biometrically encrypted devices owned by a certain 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 of devices protected by biometric passcodes. However, while most courts seem to agree that (3) is necessary, they have reached vastly 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 supicion 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 compelled to unlock their devices simply for being near an area that is being searched.

Fifth Amendment Protections

Users may believe that the Fifth Amendment offers 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 the 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 that hold the provision is a communicative act agree that the provision is incriminating and compelled. However, they are heavily split on whether it is testimonial. Some of the courts have held that, because the biometric key is provision of a physical characteristic and not a communicative testimony, the act is not testimonial. State v. Diamond, 905 N.W.2d 870 (Minn. 2018); Barrera; In re search of A White Google Pixel 3 Xl Cellphone in a Black Incipio Case, 398 F. Supp. 3d 785 (D. Idaho 2019). 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).

Conclusion

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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