Biometrics and Derived Credentials

This is Part 4 of a series discussing the public comments on Draft NIST SP 800-157, Guidelines for Derived Personal Identity Verification (PIV) Credentials and the final version of the publication. Links to all the posts in the series can be found here.

As reviewed in Part 3, a PIV card carries two fingerprint templates for off-card comparison, and may also carry one or two additional fingerprint templates for on-card comparison, one or two iris images, and an electronic facial image. These biometrics may be used in a variety of ways, by themselves or in combination with cryptographic credentials, for authentication to a Physical Access Control System (PACS) or a local workstation. The fingerprint templates for on-card comparison can also be used to activate private keys used for authentication, email signing, and email decryption.

By contrast, neither the draft version nor the final version of SP 800-157 consider the use of any biometrics analogous to those carried in a PIV card for activation or authentication. Actually, they “implicitly forbid” the storage of such biometrics by the Derived PIV Application that manages the Derived PIV Credential, according to NIST’s response to comment 30 by Precise Biometrics.

But several comments requested or suggested the use of biometrics by the Derived PIV Application. In this post I review those comments, and other comments expressing concern for biometric privacy. Then I draw attention to privacy-preserving biometric techniques that should be considered for possible use in activating derived credentials.

Comments requesting the use of biometrics

Comments 13 by Oberthur Technologies, 233 by Apple and 397 by CertiPath requested the option of using biometrics for private key activation. NIST responded that additional activation methods will be considered in the next version of the document. Comments 27 and 29 by Precise Biometrics suggested the use of biometrics for remote issuance of derived credentials, and for remote reset of the activation PIN or password.

Comment 26 by Precise Biometrics and NIST’s response are hard to understand, but deserve to be explained.

The comment refers to FIPS 201-2 which “specifies different authentication mechanisms that can be used to fulfill LOA 4” and “can be used together as multiple authentication factors to achieve even higher authentication confidence at LOA 4“. This no doubt refers to Table 6-1, Table 6-2, Footnote 35, and Table 6-3 of FIPS 201-2. Table 6-1 renames the four levels of assurance (LOA) of Section 2.1 of OMB memorandum M-04-04. Table 6-2 allows the use of the authentication mechanisms called BIO-A, OCC-AUTH and PKI-AUTH, which I explained in Part 3, for physical access control at LOA 4. Footnote 35 refers to SP 800-116, which, as I also explained in Part 3, discusses combinations of biometric and cryptographic authentication mechanisms for authentication to a PACS. Table 6-3 is concerned with “authentication for logical access“, i.e. with authentication to obtain access to an information system rather than to a physical facility; it allows the use of BIO-A, OCC-AUTH and PKI-AUTH for authentication to a local workstation, i.e. a computer used by the cardholder, but not to a remote server. In the “Suggested Change” column, Precise Biometrics seems to be suggesting that, although SP 800-157 is not concerned with the biometric authentication mechanisms, nor with combinations of authentication mechanisms, it should leave the door open for other documents to do so.

NIST’s response declined the suggested change arguing that SP 800-157 is aligned with FIPS 201-2 and SP 800-63. SP 800-63, the Electronic Authentication Guideline, is concerned with “remote authentication of users (such as employees, contractors, or private individuals) interacting with government IT systems over open networks“.

Regarding FIPS 201-2, NIST’s response says that Table 6-3 allows only PKI-AUTH (authentication with the PIV Authentication private key and certificate) for “remote access control“, i.e. for logical access to a remote information system. It is left unsaid that SP 800-157 assigns derived credentials an extremely narrow scope, ruling out their use for authentication to a local workstation or physical access control. As reviewed in Part 1, this narrow scope was protested in many comments.

SP 800-63 is quoted as saying that biometric authentication uses “information that is private rather than secret“, which makes it unsuitable for remote access control according to SP 800-63. But the reason why it is unsuitable for remote access control but not for local authentication is that

In the local authentication case, where the Claimant is observed by an attendant and uses a capture device controlled by the Verifier, authentication does not require that biometrics be kept secret.

(SP 800-63, page 2, lines 2-5). So the real issue is not whether authentication is local or remote, but whether it is attended or unattended. As I explained in Part 3, in agreement with the above quote, when there is no assurance of liveness, security depends on the relative secrecy of the biometric, and more precisely on whether an adversary has access to a biometric sample. If biometric authentication is deemed unsuitable for remote authentication because it is unattended, it should also be deemed unsuitable for the unattended biometric authentication methods BIO and OCC-AUTH of FIPS 201-2 that I described in Part 3, and maybe even for credential activation, which is also unattended.

I actually agree that biometric authentication, if used by itself rather than as a means of activating a cryptographic authentication credential, is unsuitable for remote access control, but for a different reason. Such authentication would require the person who wishes to authenticate to surrender his or her biometric privacy by submitting a biometric sample to that system, exposing it to abuse or compromise. Possible consequences of biometric abuse or compromise were briefly discussed in Part 3.

Comments expressing concern about biometric privacy

Biometric authentication is too often used in the private sector with complete disregard for the privacy implications that I discussed in Part 3. So I was happy to see that some comments expressed concern for biometric privacy.

SP 800-157 does not call for storing any biometrics in a mobile device that carries derived credentials, but requires the use of a biometric sample taken from the user for in-person credential issuance at LOA 4, and for in-person reset of the activation PIN or password. The draft version required the sample used at issuance to be retained for future reference and to be used for in-person reset. Comment 171 by the Department of State, and duplicate comment 337 by the Federal Public Key Infrasctructure Certificate Policy Working Group (FPKI CPWG), asked for a reference to the Privacy Act on the need to protect the retained sample. Similarly, comment 284 by Sam Wilke asked for a reference to authority on retaining biometric information. Comment 243 questioned the need to collect the sample, since “In cases where the same agency issues the PIV card and the derived credential, we would already be in possession of the biometric template.” (Actually, SP800-157 allows the issuer of the PIV card and the issuer of derived credential to be different, but several comments objected to this. I reviewed those comments in Part 1, and argued that NIST is allowing the issuers to be different for reasons that are not applicable.)

In response to the comments, NIST added a footnote stating that “The retained biometric shall be protected in a manner that protects the individual’s privacy“, and allowed the option of comparing the biometric sample used for in-person reset against a stored biometric on the PIV Card or biometric data stored in the “chain-of-trust” (i.e. collected for use in a background check before issuance of the PIV card) instead of comparing it against the retained sample. But it declined to remove the request to retain a sample, arguing that “The requirement is derived from the common policy and it provides an audit trail for dispute resolution“.

Privacy-preserving biometrics

The use of a biometric sample to activate a cryptographic credential that can in turn be used for authentication to a remote system avoids the privacy risk inherent in sending the sample to the remote system. But other risks remain. In particular, if the sample is compared to a biometric template that is stored in a mobile device without tamper protection, the template can be obtained by an adversary who captures the device. Biometric information in the template can then be used to construct a biometric sample that matches the template and can be used to impersonate the user.

Perhaps the best way to avoid biometric privacy risks is to not store biometrics in mobile devices. So I’m actually sympathetic to the stance taken by SP 800-157 of not allowing any biometrics to be carried in mobile devices along with derived credentials. But this is not a realistic stance. PIV cards use biometrics to perform a variety of functions, as we saw in Part 3, and there is a desire to be able to perform the same functions with a mobile device, strongly expressed in the comments. Also, the inclusion of software tokens among the options for implementing derived credentials in SP 800-157, in spite of the security challenges that I discussed in Part 2, shows that there is a desire to not be constrained by the unavailability of physical tamper resistance.

So NIST should look for ways of using biometrics without compromising privacy. There is a class of privacy-preserving biometric authentication techniques can make that possible. These techniques use the following general approach. An enrollment biometric sample is used to compute helper data, which is later combined with an authentication sample to produce a biometric key. The same biometric key is produced consistently by varying but genuine samples with a high probability, whose complement is akin to the false rejection rate (FRR) of ordinary biometric authentication. A different key is produced by a non-genuine sample with a high probability, whose complement is similarly akin to the false acceptance rate (FAR) of ordinary biometric authentication.

For a technique that follows this approach to be privacy-preserving, it must be deemed unfeasible to derive significant biometric information that might be useful to an adversary from the helper data. This is the case, for example, in a technique originally proposed by Juels and Wattenberg in their paper entitled A Fuzzy Commitment Scheme, and later successfully applied to authentication with an iris image by Hao, Anderson and Daugman in their paper Combining cryptography with biometrics effectively (available as a technical report). In that technique, the helper data is the exclusive-or of a random codeword of an error-correction system and a binary feature vector derived from a biometric sample. The random codeword is not a random string, since it contains redundancy. But it is not known how to derive significant biometric information from the fact that the helper data is the exclusive-or of a feature vector with a string that contains such redundancy.

In the technique of Juels et al. and Hao et al., the biometric key is generated at random at enrollment time, and expanded into the random codeword by adding redundancy. At authentication time, the random codeword is recovered by x-oring the helper data with a feature vector derived from the authentication sample, which produces a string that differs from the codeword where the enrollment and authentication feature vectors differ, and applying error correction to the string. The biometric key is then obtained by removing the redundancy from the codeword.

While this technique is well suited to biometric modalities where a sample can be readily translated into a binary feature vector, other privacy-preserving biometric techniques may be better suited to other modalities. For example, the Fuzzy Vault technique of Juels and Sudan may be better suited for modalities where a sample is more readily translated into a set of feature points rather than a binary feature vector.

Surveys of privacy-preserving biometric techniques are available here and here.

Using privacy-preserving biometric techniques for credential activation

The simplest way to use privacy-preserving biometric techniques to activate one or more credentials stored in a mobile device is to store the helper data and a cryptographic hash of the biometric key in the device. An activation sample entered by the user is then combined with the helper data to derive a biometric key. A hash of the derived key is computed and compared to the stored hash. If they are equal, the user is allowed to use the credentials.

This simple method protects the privacy of the biometric against an adversary who captures the mobile device and extracts the helper data and the hash of the biometric key. But it does not protect the credentials themselves, which can be extracted by the adversary if there is no physical tamper protection.

A more elaborate method is to store the helper data and encrypt the credentials under the biometric key. This would protect the credentials if the biometric key had sufficient entropy. But unfortunately it does not. In their paper referenced above, Hao et al. estimate that their biometric key has 44 bits of entropy. That’s much more than the 20 bits of entropy of a 6-digit PIN that the final version of SP 800-157 allows for activation of credentials in a software token unprotected by physical tamper resistance (cf. first paragraph of Section 3.4 and Footnote 12). But it is not nearly enough against an adversary who extracts the encrypted keys and mounts a brute force attack against the biometric key using a botnet.

An effective method is to encrypt the credentials under a key that is entrusted to a key storage service in the cloud, and retrieved from the cloud using a device-authentication credential to authenticate to the service. Because it is only used for that particular purpose, the device-authentication credential can be derived on-demand from a protocredential stored in the device and a secret not stored in the device, in a way that deprives an adversary who captures the device and obtains the protocredential of any information usable for mounting an offline guessing attack against the non-stored secret (or any parameter derived from the non-stored secret).

In our comments on the draft version of SP 800-157 we sketched a protocredential-based activation technique that uses the activation passcode (PIN or password) as the non-stored secret. The protocredential consists of the domain parameters of a DSA key pair, a salt, and a device record handle referring to a record in the key storage service. The device-authentication credential derived from the passcode and the protocredential consists of the DSA key pair and the device record handle.

That technique can be easily modified to use a biometric key in lieu of the passcode, as described in our technical report entitled A Comprehensive Approach to Cryptographic and Biometric Authentication from a Mobile Perspective. In the modified technique, the non-stored (relative) secret is the biometric sample supplied by the user, the helper data is part of the protocredential, and the DSA key pair is regenerated using the biometric key derived from the helper data and the biometric sample instead of the passcode.

We have recently found an alternative way of deriving a device-authentication credential from a protocredential and a non-stored secret, where the non-stored secret may also be either a passcode or a biometric sample.

If the non-stored secret is a passcode, the protocredential consists of a device record handle and an uncertified key pair, and the credential is derived simply by adding the passcode as an additional component. To authenticate to the key storage service, the device sends the passcode, the public key component of the key pair, and a signature on a challenge computed with the private key. The service verifies the signature, computes a joint hash of the passcode and the public key, and compares the computed joint hash against a stored joint hash. An adversary who captures the device may obtain the key pair, but can only test guesses of the passcode by attempting to authenticate online to the service, which limits the number of attempts. An adversary who breaches the security of the key storage service cannot mount an offline guessing attack against the PIN using the joint hash without knowing the public key.

If the non-stored secret is a biometric sample, usable in conjunction with helper data to derive a biometric key, the protocredential consists of a device record handle, an uncertified key pair, and the biometric helper data. The device-authentication credential consists of the device record handle, the uncertified key pair, and the biometric key derived from the helper data and the biometric sample. To authenticate to the key storage service, the device sends the biometric key, the public key component of the key pair, and a signature on a challenge computed with the private key. The service verifies the signature, computes a joint hash of the biometric key and the public key, and compares the computed joint hash against a stored joint hash. The biometric key is protected against an offline guessing attack by an adversary who captures the device or breaches the security of the key storage service.

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