5 Authenticator and Verifier Requirements

This section is normative.

This section provides the detailed requirements specific to each type of authenticator. With the exception of reauthentication requirements specified in Section 4 and the requirement for verifier impersonation resistance at AAL3 described in Section 5.2.5, the technical requirements for each of the authenticator types are the same regardless of the AAL at which the authenticator is used.

5.1 Requirements by Authenticator Type

5.1.1 Memorized Secrets

authenticator A Memorized Secret authenticator — commonly referred to as a password or, if numeric, a PIN — is a secret value intended to be chosen and memorized by the user. Memorized secrets need to be of sufficient complexity and secrecy that it would be impractical for an attacker to guess or otherwise discover the correct secret value. A memorized secret is something you know.

5.1.1.1 Memorized Secret Authenticators

Memorized secrets SHALL be at least 8 characters in length if chosen by the subscriber. Memorized secrets chosen randomly by the CSP or verifier SHALL be at least 6 characters in length and MAY be entirely numeric. If the CSP or verifier disallows a chosen memorized secret based on its appearance on a blacklist of compromised values, the subscriber SHALL be required to choose a different memorized secret. No other complexity requirements for memorized secrets SHOULD be imposed. A rationale for this is presented in Appendix A Strength of Memorized Secrets.

5.1.1.2 Memorized Secret Verifiers

Verifiers SHALL require subscriber-chosen memorized secrets to be at least 8 characters in length. Verifiers SHOULD permit subscriber-chosen memorized secrets at least 64 characters in length. All printing ASCII [RFC 20] characters as well as the space character SHOULD be acceptable in memorized secrets. Unicode [ISO/ISC 10646] characters SHOULD be accepted as well. To make allowances for likely mistyping, verifiers MAY replace multiple consecutive space characters with a single space character prior to verification, provided that the result is at least 8 characters in length. Truncation of the secret SHALL NOT be performed. For purposes of the above length requirements, each Unicode code point SHALL be counted as a single character.

If Unicode characters are accepted in memorized secrets, the verifier SHOULD apply the Normalization Process for Stabilized Strings using either the NFKC or NFKD normalization defined in Section 12.1 of Unicode Standard Annex 15 [UAX 15]. This process is applied before hashing the byte string representing the memorized secret. Subscribers choosing memorized secrets containing Unicode characters SHOULD be advised that some characters may be represented differently by some endpoints, which can affect their ability to authenticate successfully.

Memorized secrets that are randomly chosen by the CSP (e.g., at enrollment) or by the verifier (e.g., when a user requests a new PIN) SHALL be at least 6 characters in length and SHALL be generated using an approved random bit generator [SP 800-90Ar1].

Memorized secret verifiers SHALL NOT permit the subscriber to store a “hint” that is accessible to an unauthenticated claimant. Verifiers SHALL NOT prompt subscribers to use specific types of information (e.g., “What was the name of your first pet?”) when choosing memorized secrets.

When processing requests to establish and change memorized secrets, verifiers SHALL compare the prospective secrets against a list that contains values known to be commonly-used, expected, or compromised. For example, the list MAY include, but is not limited to:

If the chosen secret is found in the list, the CSP or verifier SHALL advise the subscriber that they need to select a different secret, SHALL provide the reason for rejection, and SHALL require the subscriber to choose a different value.

Verifiers SHOULD offer guidance to the subscriber, such as a password-strength meter [Meters], to assist the user in choosing a strong memorized secret. This is particularly important following the rejection of a memorized secret on the above list as it discourages trivial modification of listed (and likely very weak) memorized secrets [Blacklists].

Verifiers SHALL implement a rate-limiting mechanism that effectively limits the number of failed authentication attempts that can be made on the subscriber’s account as described in Section 5.2.2.

Verifiers SHOULD NOT impose other composition rules (e.g., requiring mixtures of different character types or prohibiting consecutively repeated characters) for memorized secrets. Verifiers SHOULD NOT require memorized secrets to be changed arbitrarily (e.g., periodically). However, verifiers SHALL force a change if there is evidence of compromise of the authenticator.

Verifiers SHOULD permit claimants to use “paste” functionality when entering a memorized secret. This facilitates the use of password managers, which are widely used and in many cases increase the likelihood that users will choose stronger memorized secrets.

In order to assist the claimant in successfully entering a memorized secret, the verifier SHOULD offer an option to display the secret — rather than a series of dots or asterisks — until it is entered. This allows the claimant to verify their entry if they are in a location where their screen is unlikely to be observed. The verifier MAY also permit the user’s device to display individual entered characters for a short time after each character is typed to verify correct entry. This is particularly applicable on mobile devices.

The verifier SHALL use approved encryption and an authenticated protected channel when requesting memorized secrets in order to provide resistance to eavesdropping and MitM attacks.

Verifiers SHALL store memorized secrets in a form that is resistant to offline attacks. Memorized secrets SHALL be salted and hashed using a suitable one-way key derivation function. Key derivation functions take a password, a salt, and a cost factor as inputs then generate a password hash. Their purpose is to make each password guessing trial by an attacker who has obtained a password hash file expensive and therefore the cost of a guessing attack high or prohibitive. Examples of suitable key derivation functions include Password-based Key Derivation Function 2 (PBKDF2) [SP 800-132] and Balloon [BALLOON]. A memory-hard function SHOULD be used because it increases the cost of an attack. The key derivation function SHALL use an approved one-way function such as Keyed Hash Message Authentication Code (HMAC) [FIPS 198-1], any approved hash function in SP 800-107, Secure Hash Algorithm 3 (SHA-3) [FIPS 202], CMAC [SP 800-38B] or Keccak Message Authentication Code (KMAC), Customizable SHAKE (cSHAKE), or ParallelHash [SP 800-185]. The chosen output length of the key derivation function SHOULD be the same as the length of the underlying one-way function output.

The salt SHALL be at least 32 bits in length and be chosen arbitrarily so as to minimize salt value collisions among stored hashes. Both the salt value and the resulting hash SHALL be stored for each subscriber using a memorized secret authenticator.

For PBKDF2, the cost factor is an iteration count: the more times the PBKDF2 function is iterated, the longer it takes to compute the password hash. Therefore, the iteration count SHOULD be as large as verification server performance will allow, typically at least 10,000 iterations.

In addition, verifiers SHOULD perform an additional iteration of a key derivation function using a salt value that is secret and known only to the verifier. This salt value, if used, SHALL be generated by an approved random bit generator [SP 800-90Ar1] and provide at least the minimum security strength specified in the latest revision of SP 800-131A (112 bits as of the date of this publication). The secret salt value SHALL be stored separately from the hashed memorized secrets (e.g., in a specialized device like a hardware security module). With this additional iteration, brute-force attacks on the hashed memorized secrets are impractical as long as the secret salt value remains secret.

5.1.2 Look-Up Secrets

authenticator A look-up secret authenticator is a physical or electronic record that stores a set of secrets shared between the claimant and the CSP. The claimant uses the authenticator to look up the appropriate secret(s) needed to respond to a prompt from the verifier. For example, the verifier may ask a claimant to provide a specific subset of the numeric or character strings printed on a card in table format. A common application of look-up secrets is the use of "recovery keys" stored by the subscriber for use in the event another authenticator is lost or malfunctions. A look-up secret is something you have.

5.1.2.1 Look-Up Secret Authenticators

CSPs creating look-up secret authenticators SHALL use an approved random bit generator [SP 800-90Ar1] to generate the list of secrets and SHALL deliver the authenticator securely to the subscriber. Look-up secrets SHALL have at least 20 bits of entropy.

Look-up secrets MAY be distributed by the CSP in person, by postal mail to the subscriber’s address of record, or by online distribution. If distributed online, look-up secrets SHALL be distributed over a secure channel in accordance with the post-enrollment binding requirements in Section 6.1.2.

If the authenticator uses look-up secrets sequentially from a list, the subscriber MAY dispose of used secrets, but only after a successful authentication.

5.1.2.2 Look-Up Secret Verifiers

Verifiers of look-up secrets SHALL prompt the claimant for the next secret from their authenticator or for a specific (e.g., numbered) secret. A given secret from an authenticator SHALL be used successfully only once. If the look-up secret is derived from a grid card, each cell of the grid SHALL be used only once.

Verifiers SHALL store look-up secrets in a form that is resistant to offline attacks. Look-up secrets having at least 112 bits of entropy SHALL be hashed with an approved one-way function as described in Section 5.1.1.2. Look-up secrets with fewer than 112 bits of entropy SHALL be salted and hashed using a suitable one-way key derivation function, also described in Section 5.1.1.2. The salt value SHALL be at least 32 in bits in length and arbitrarily chosen so as to minimize salt value collisions among stored hashes. Both the salt value and the resulting hash SHALL be stored for each look-up secret.

For look-up secrets that have less than 64 bits of entropy, the verifier SHALL implement a rate-limiting mechanism that effectively limits the number of failed authentication attempts that can be made on the subscriber’s account as described in Section 5.2.2.

The verifier SHALL use approved encryption and an authenticated protected channel when requesting look-up secrets in order to provide resistance to eavesdropping and MitM attacks.

5.1.3 Out-of-Band Devices

authenticator An out-of-band authenticator is a physical device that is uniquely addressable and can communicate securely with the verifier over a distinct communications channel, referred to as the secondary channel. The device is possessed and controlled by the claimant and supports private communication over this secondary channel, separate from the primary channel for e-authentication. An out-of-band authenticator is something you have.

The out-of-band authenticator can operate in one of the following ways:

- The claimant transfers a secret received by the out-of-band device via the secondary channel to the verifier using the primary channel. For example, the claimant may receive the secret on their mobile device and type it (typically a 6-digit code) into their authentication session.

- The claimant transfers a secret received via the primary channel to the out-of-band device for transmission to the verifier via the secondary channel. For example, the claimant may view the secret on their authentication session and either type it into an app on their mobile device or use a technology such as a barcode or QR code to effect the transfer.

- The claimant compares secrets received from the primary channel and the secondary channel and confirms the authentication via the secondary channel.

The secret's purpose is to securely bind the authentication operation on the primary and secondary channel. When the response is via the primary communication channel, the secret also establishes the claimant's control of the out-of-band device.

5.1.3.1 Out-of-Band Authenticators

The out-of-band authenticator SHALL establish a separate channel with the verifier in order to retrieve the out-of-band secret or authentication request. This channel is considered to be out-of-band with respect to the primary communication channel (even if it terminates on the same device) provided the device does not leak information from one channel to the other without the authorization of the claimant.

The out-of-band device SHOULD be uniquely addressable and communication over the secondary channel SHALL be encrypted unless sent via the public switched telephone network (PSTN). For additional authenticator requirements specific to the PSTN, see Section 5.1.3.3. Methods that do not prove possession of a specific device, such as voice-over-IP (VOIP) or email, SHALL NOT be used for out-of-band authentication.

The out-of-band authenticator SHALL uniquely authenticate itself in one of the following ways when communicating with the verifier:

If a secret is sent by the verifier to the out-of-band device, the device SHOULD NOT display the authentication secret while it is locked by the owner (i.e., requires an entry of a PIN, passcode, or biometric to view). However, authenticators SHOULD indicate the receipt of an authentication secret on a locked device.

If the out-of-band authenticator sends an approval message over the secondary communication channel — rather than by the claimant transferring a received secret to the primary communication channel — it SHALL do one of the following:

5.1.3.2 Out-of-Band Verifiers

For additional verification requirements specific to the PSTN, see Section 5.1.3.3.

If out-of-band verification is to be made using a secure application, such as on a smart phone, the verifier MAY send a push notification to that device. The verifier then waits for the establishment of an authenticated protected channel and verifies the authenticator’s identifying key. The verifier SHALL NOT store the identifying key itself, but SHALL use a verification method (e.g., an approved hash function or proof of possession of the identifying key) to uniquely identify the authenticator. Once authenticated, the verifier transmits the authentication secret to the authenticator.

Depending on the type of out-of-band authenticator, one of the following SHALL take place:

In all cases, the authentication SHALL be considered invalid if not completed within 10 minutes. In order to provide replay resistance as described in Section 5.2.8, verifiers SHALL accept a given authentication secret only once during the validity period.

The verifier SHALL generate random authentication secrets with at least 20 bits of entropy using an approved random bit generator [SP 800-90Ar1]. If the authentication secret has less than 64 bits of entropy, the verifier SHALL implement a rate-limiting mechanism that effectively limits the number of failed authentication attempts that can be made on the subscriber’s account as described in Section 5.2.2.

5.1.3.3 Authentication using the Public Switched Telephone Network

Use of the PSTN for out-of-band verification is RESTRICTED as described in this section and in Section 5.2.10. If out-of-band verification is to be made using the PSTN, the verifier SHALL verify that the pre-registered telephone number being used is associated with a specific physical device. Changing the pre-registered telephone number is considered to be the binding of a new authenticator and SHALL only occur as described in Section 6.1.2.

Verifiers SHOULD consider risk indicators such as device swap, SIM change, number porting, or other abnormal behavior before using the PSTN to deliver an out-of-band authentication secret.

NOTE: Consistent with the restriction of authenticators in Section 5.2.10, NIST may adjust the RESTRICTED status of the PSTN over time based on the evolution of the threat landscape and the technical operation of the PSTN.

5.1.4 Single-Factor OTP Device

authenticator A single-factor OTP device generates OTPs. This category includes hardware devices and software-based OTP generators installed on devices such as mobile phones. These devices have an embedded secret that is used as the seed for generation of OTPs and does not require activation through a second factor. The OTP is displayed on the device and manually input for transmission to the verifier, thereby proving possession and control of the device. An OTP device may, for example, display 6 characters at a time. A single-factor OTP device is something you have.

Single-factor OTP devices are similar to look-up secret authenticators with the exception that the secrets are cryptographically and independently generated by the authenticator and verifier and compared by the verifier. The secret is computed based on a nonce that may be time-based or from a counter on the authenticator and verifier.

5.1.4.1 Single-Factor OTP Authenticators

Single-factor OTP authenticators contain two persistent values. The first is a symmetric key that persists for the device’s lifetime. The second is a nonce that is either changed each time the authenticator is used or is based on a real-time clock.

The secret key and its algorithm SHALL provide at least the minimum security strength specified in the latest revision of SP 800-131A (112 bits as of the date of this publication). The nonce SHALL be of sufficient length to ensure that it is unique for each operation of the device over its lifetime. OTP authenticators — particularly software-based OTP generators — SHOULD discourage and SHALL NOT facilitate the cloning of the secret key onto multiple devices.

The authenticator output is obtained by using an approved block cipher or hash function to combine the key and nonce in a secure manner. The authenticator output MAY be truncated to as few as 6 decimal digits (approximately 20 bits of entropy).

If the nonce used to generate the authenticator output is based on a real-time clock, the nonce SHALL be changed at least once every 2 minutes. The OTP value associated with a given nonce SHALL be accepted only once.

5.1.4.2 Single-Factor OTP Verifiers

Single-factor OTP verifiers effectively duplicate the process of generating the OTP used by the authenticator. As such, the symmetric keys used by authenticators are also present in the verifier, and SHALL be strongly protected against compromise.

When a single-factor OTP authenticator is being associated with a subscriber account, the verifier or associated CSP SHALL use approved cryptography to either generate and exchange or to obtain the secrets required to duplicate the authenticator output.

The verifier SHALL use approved encryption and an authenticated protected channel when collecting the OTP in order to provide resistance to eavesdropping and MitM attacks. Time-based OTPs [RFC 6238] SHALL have a defined lifetime that is determined by the expected clock drift — in either direction — of the authenticator over its lifetime, plus allowance for network delay and user entry of the OTP. In order to provide replay resistance as described in Section 5.2.8, verifiers SHALL accept a given time-based OTP only once during the validity period.

If the authenticator output has less than 64 bits of entropy, the verifier SHALL implement a rate-limiting mechanism that effectively limits the number of failed authentication attempts that can be made on the subscriber’s account as described in Section 5.2.2.

5.1.5 Multi-Factor OTP Devices

authenticator A multi-factor OTP device generates OTPs for use in authentication after activation through an additional authentication factor. This includes hardware devices and software-based OTP generators installed on devices such as mobile phones. The second factor of authentication may be achieved through some kind of integral entry pad, an integral biometric (e.g., fingerprint) reader, or a direct computer interface (e.g., USB port). The OTP is displayed on the device and manually input for transmission to the verifier. For example, an OTP device may display 6 characters at a time, thereby proving possession and control of the device. The multi-factor OTP device is something you have, and it SHALL be activated by either something you know or something you are.

5.1.5.1 Multi-Factor OTP Authenticators

Multi-factor OTP authenticators operate in a similar manner to single-factor OTP authenticators (see Section 5.1.4.1), except that they require the entry of either a memorized secret or the use of a biometric to obtain the OTP from the authenticator. Each use of the authenticator SHALL require the input of the additional factor.

In addition to activation information, multi-factor OTP authenticators contain two persistent values. The first is a symmetric key that persists for the device’s lifetime. The second is a nonce that is either changed each time the authenticator is used or is based on a real-time clock.

The secret key and its algorithm SHALL provide at least the minimum security strength specified in the latest revision of [SP 800-131A] (112 bits as of the date of this publication). The nonce SHALL be of sufficient length to ensure that it is unique for each operation of the device over its lifetime. OTP authenticators — particularly software-based OTP generators — SHOULD discourage and SHALL NOT facilitate the cloning of the secret key onto multiple devices.

The authenticator output is obtained by using an approved block cipher or hash function to combine the key and nonce in a secure manner. The authenticator output MAY be truncated to as few as 6 decimal digits (approximately 20 bits of entropy).

If the nonce used to generate the authenticator output is based on a real-time clock, the nonce SHALL be changed at least once every 2 minutes. The OTP value associated with a given nonce SHALL be accepted only once.

Any memorized secret used by the authenticator for activation SHALL be a randomly-chosen numeric secret at least 6 decimal digits in length or other memorized secret meeting the requirements of Section 5.1.1.2 and SHALL be rate limited as specified in Section 5.2.2. A biometric activation factor SHALL meet the requirements of Section 5.2.3, including limits on the number of consecutive authentication failures.

The unencrypted key and activation secret or biometric sample — and any biometric data derived from the biometric sample such as a probe produced through signal processing — SHALL be zeroized immediately after an OTP has been generated.

5.1.5.2 Multi-Factor OTP Verifiers

Multi-factor OTP verifiers effectively duplicate the process of generating the OTP used by the authenticator, but without the requirement that a second factor be provided. As such, the symmetric keys used by authenticators SHALL be strongly protected against compromise.

When a multi-factor OTP authenticator is being associated with a subscriber account, the verifier or associated CSP SHALL use approved cryptography to either generate and exchange or to obtain the secrets required to duplicate the authenticator output. The verifier or CSP SHALL also establish, via the authenticator source, that the authenticator is a multi-factor device. In the absence of a trusted statement that it is a multi-factor device, the verifier SHALL treat the authenticator as single-factor, in accordance with Section 5.1.4.

The verifier SHALL use approved encryption and an authenticated protected channel when collecting the OTP in order to provide resistance to eavesdropping and MitM attacks. Time-based OTPs [RFC 6238] SHALL have a defined lifetime that is determined by the expected clock drift — in either direction — of the authenticator over its lifetime, plus allowance for network delay and user entry of the OTP. In order to provide replay resistance as described in Section 5.2.8, verifiers SHALL accept a given time-based OTP only once during the validity period. In the event a claimant’s authentication is denied due to duplicate use of an OTP, verifiers MAY warn the claimant in case an attacker has been able to authenticate in advance. Verifiers MAY also warn a subscriber in an existing session of the attempted duplicate use of an OTP.

If the authenticator output or activation secret has less than 64 bits of entropy, the verifier SHALL implement a rate-limiting mechanism that effectively limits the number of failed authentication attempts that can be made on the subscriber’s account as described in Section 5.2.2. A biometric activation factor SHALL meet the requirements of Section 5.2.3, including limits on the number of consecutive authentication failures.

5.1.6 Single-Factor Cryptographic Software

authenticator A single-factor software cryptographic authenticator is a cryptographic key stored on disk or some other "soft" media. Authentication is accomplished by proving possession and control of the key. The authenticator output is highly dependent on the specific cryptographic protocol, but it is generally some type of signed message. The single-factor software cryptographic authenticator is something you have.

5.1.6.1 Single-Factor Cryptographic Software Authenticators

Single-factor software cryptographic authenticators encapsulate one or more secret keys unique to the authenticator. The key SHALL be stored in suitably secure storage available to the authenticator application (e.g., keychain storage, TPM, or TEE if available). The key SHALL be strongly protected against unauthorized disclosure by the use of access controls that limit access to the key to only those software components on the device requiring access. Single-factor cryptographic software authenticators SHOULD discourage and SHALL NOT facilitate the cloning of the secret key onto multiple devices.

5.1.6.2 Single-Factor Cryptographic Software Verifiers

The requirements for a single-factor cryptographic software verifier are identical to those for a single-factor cryptographic device verifier, described in Section 5.1.7.2.

5.1.7 Single-Factor Cryptographic Devices

authenticator A single-factor cryptographic device is a hardware device that performs cryptographic operations using protected cryptographic key(s) and provides the authenticator output via direct connection to the user endpoint. The device uses embedded symmetric or asymmetric cryptographic keys, and does not require activation through a second factor of authentication. Authentication is accomplished by proving possession of the device via the authentication protocol. The authenticator output is provided by direct connection to the user endpoint and is highly dependent on the specific cryptographic device and protocol, but it is typically some type of signed message. A single-factor cryptographic device is something you have.

5.1.7.1 Single-Factor Cryptographic Device Authenticators

Single-factor cryptographic device authenticators encapsulate one or more secret keys unique to the device that SHALL NOT be exportable (i.e., cannot be removed from the device). The authenticator operates by signing a challenge nonce presented through a direct computer interface (e.g., a USB port). Alternatively, the authenticator could be a suitably secure processor integrated with the user endpoint itself (e.g., a hardware TPM). Although cryptographic devices contain software, they differ from cryptographic software authenticators in that all embedded software is under control of the CSP or issuer and that the entire authenticator is subject to all applicable FIPS 140 requirements at the AAL being authenticated.

The secret key and its algorithm SHALL provide at least the minimum security length specified in the latest revision of SP 800-131A (112 bits as of the date of this publication). The challenge nonce SHALL be at least 64 bits in length. Approved cryptography SHALL be used.

Single-factor cryptographic device authenticators SHOULD require a physical input (e.g., the pressing of a button) in order to operate. This provides defense against unintended operation of the device, which might occur if the endpoint to which it is connected is compromised.

5.1.7.2 Single-Factor Cryptographic Device Verifiers

Single-factor cryptographic device verifiers generate a challenge nonce, send it to the corresponding authenticator, and use the authenticator output to verify possession of the device. The authenticator output is highly dependent on the specific cryptographic device and protocol, but it is generally some type of signed message.

The verifier has either symmetric or asymmetric cryptographic keys corresponding to each authenticator. While both types of keys SHALL be protected against modification, symmetric keys SHALL additionally be protected against unauthorized disclosure.

The challenge nonce SHALL be at least 64 bits in length, and SHALL either be unique over the authenticator’s lifetime or statistically unique (i.e., generated using an approved random bit generator [SP 800-90Ar1]). The verification operation SHALL use approved cryptography.

5.1.8 Multi-Factor Cryptographic Software

authenticator A multi-factor software cryptographic authenticator is a cryptographic key stored on disk or some other "soft" media that requires activation through a second factor of authentication. Authentication is accomplished by proving possession and control of the key. The authenticator output is highly dependent on the specific cryptographic protocol, but it is generally some type of signed message. The multi-factor software cryptographic authenticator is something you have, and it SHALL be activated by either something you know or something you are.

5.1.8.1 Multi-Factor Cryptographic Software Authenticators

Multi-factor software cryptographic authenticators encapsulate one or more secret keys unique to the authenticator and accessible only through the input of an additional factor, either a memorized secret or a biometric. The key SHOULD be stored in suitably secure storage available to the authenticator application (e.g., keychain storage, TPM, TEE). The key SHALL be strongly protected against unauthorized disclosure by the use of access controls that limit access to the key to only those software components on the device requiring access. Multi-factor cryptographic software authenticators SHOULD discourage and SHALL NOT facilitate the cloning of the secret key onto multiple devices.

Each authentication operation using the authenticator SHALL require the input of both factors.

Any memorized secret used by the authenticator for activation SHALL be a randomly-chosen numeric value at least 6 decimal digits in length or other memorized secret meeting the requirements of Section 5.1.1.2 and SHALL be rate limited as specified in Section 5.2.2. A biometric activation factor SHALL meet the requirements of Section 5.2.3, including limits on the number of consecutive authentication failures.

The unencrypted key and activation secret or biometric sample — and any biometric data derived from the biometric sample such as a probe produced through signal processing — SHALL be zeroized immediately after an authentication transaction has taken place.

5.1.8.2 Multi-Factor Cryptographic Software Verifiers

The requirements for a multi-factor cryptographic software verifier are identical to those for a single-factor cryptographic device verifier, described in Section 5.1.7.2. Verification of the output from a multi-factor cryptographic software authenticator proves use of the activation factor.

5.1.9 Multi-Factor Cryptographic Devices

authenticator A multi-factor cryptographic device is a hardware device that performs cryptographic operations using one or more protected cryptographic keys and requires activation through a second authentication factor. Authentication is accomplished by proving possession of the device and control of the key. The authenticator output is provided by direct connection to the user endpoint and is highly dependent on the specific cryptographic device and protocol, but it is typically some type of signed message. The multi-factor cryptographic device is something you have, and it SHALL be activated by either something you know or something you are.

5.1.9.1 Multi-Factor Cryptographic Device Authenticators

Multi-factor cryptographic device authenticators use tamper-resistant hardware to encapsulate one or more secret keys unique to the authenticator and accessible only through the input of an additional factor, either a memorized secret or a biometric. The authenticator operates by signing a challenge nonce presented through a direct computer interface (e.g., a USB port). Alternatively, the authenticator could be a suitably secure processor integrated with the user endpoint itself (e.g., a hardware TPM). Although cryptographic devices contain software, they differ from cryptographic software authenticators in that all embedded software is under control of the CSP or issuer, and that the entire authenticator is subject to any applicable FIPS 140 requirements at the selected AAL.

The secret key and its algorithm SHALL provide at least the minimum security length specified in the latest revision of SP 800-131A (112 bits as of the date of this publication). The challenge nonce SHALL be at least 64 bits in length. Approved cryptography SHALL be used.

Each authentication operation using the authenticator SHOULD require the input of the additional factor. Input of the additional factor MAY be accomplished via either direct input on the device or via a hardware connection (e.g., USB, smartcard).

Any memorized secret used by the authenticator for activation SHALL be a randomly-chosen numeric value at least 6 decimal digits in length or other memorized secret meeting the requirements of Section 5.1.1.2 and SHALL be rate limited as specified in Section 5.2.2. A biometric activation factor SHALL meet the requirements of Section 5.2.3, including limits on the number of consecutive authentication failures.

The unencrypted key and activation secret or biometric sample — and any biometric data derived from the biometric sample such as a probe produced through signal processing — SHALL be zeroized immediately after an authentication transaction has taken place.

5.1.9.2 Multi-Factor Cryptographic Device Verifiers

The requirements for a multi-factor cryptographic device verifier are identical to those for a single-factor cryptographic device verifier, described in Section 5.1.7.2. Verification of the authenticator output from a multi-factor cryptographic device proves use of the activation factor.

5.2 General Authenticator Requirements

5.2.1 Physical Authenticators

CSPs SHALL provide subscriber instructions on how to appropriately protect the authenticator against theft or loss. The CSP SHALL provide a mechanism to revoke or suspend the authenticator immediately upon notification from subscriber that loss or theft of the authenticator is suspected.

5.2.2 Rate Limiting (Throttling)

When required by the authenticator type descriptions in Section 5.1, the verifier SHALL implement controls to protect against online guessing attacks. Unless otherwise specified in the description of a given authenticator, the verifier SHALL limit consecutive failed authentication attempts on a single account to no more than 100.

Additional techniques MAY be used to reduce the likelihood that an attacker will lock the legitimate claimant out as a result of rate limiting. These include:

When the subscriber successfully authenticates, the verifier SHOULD disregard any previous failed attempts for that user from the same IP address.

5.2.3 Use of Biometrics

The use of biometrics (something you are) in authentication includes both measurement of physical characteristics (e.g., fingerprint, iris, facial characteristics) and behavioral characteristics (e.g., typing cadence). Both classes are considered biometric modalities, although different modalities may differ in the extent to which they establish authentication intent as described in Section 5.2.9.

For a variety of reasons, this document supports only limited use of biometrics for authentication. These reasons include:

Therefore, the limited use of biometrics for authentication is supported with the following requirements and guidelines:

Biometrics SHALL be used only as part of multi-factor authentication with a physical authenticator (something you have).

An authenticated protected channel between sensor (or an endpoint containing a sensor that resists sensor replacement) and verifier SHALL be established and the sensor or endpoint SHALL be established and the sensor or endpoint SHALL be authenticated prior to capturing the biometric sample from the claimant.

The biometric system SHALL operate with an FMR [ISO/IEC 2382-37] of 1 in 1000 or better. This FMR SHALL be achieved under conditions of a conformant attack (i.e., zero-effort impostor attempt) as defined in [ISO/IEC 30107-1].

The biometric system SHOULD implement PAD. Testing of the biometric system to be deployed SHOULD demonstrate at least 90% resistance to presentation attacks for each relevant attack type (i.e., species), where resistance is defined as the number of thwarted presentation attacks divided by the number of trial presentation attacks. Testing of presentation attack resistance SHALL be in accordance with Clause 12 of [ISO/IEC 30107-3]. The PAD decision MAY be made either locally on the claimant’s device or by a central verifier.

Note: PAD is being considered as a mandatory requirement in future editions of this guideline.

The biometric system SHALL allow no more than 5 consecutive failed authentication attempts or 10 consecutive failed attempts if PAD meeting the above requirements is implemented. Once that limit has been reached, the biometric authenticator SHALL either:

The verifier SHALL make a determination of sensor and endpoint performance, integrity, and authenticity. Acceptable methods for making this determination include, but are not limited to:

Biometric comparison can be performed locally on claimant’s device or at a central verifier. Since the potential for attacks on a larger scale is greater at central verifiers, local comparison is preferred.

If comparison is performed centrally:

Biometric samples collected in the authentication process MAY be used to train comparison algorithms or — with user consent — for other research purposes. Biometric samples and any biometric data derived from the biometric sample such as a probe produced through signal processing SHALL be zeroized immediately after any training or research data has been derived.

Biometrics are also used in some cases to prevent repudiation of enrollment and to verify that the same individual participates in all phases of the enrollment process as described in SP 800-63A.

5.2.4 Attestation

An attestation is information conveyed to the verifier regarding a directly-connected authenticator or the endpoint involved in an authentication operation. Information conveyed by attestation MAY include, but is not limited to:

If this attestation is signed, it SHALL be signed using a digital signature that provides at least the minimum security strength specified in the latest revision of SP 800-131A (112 bits as of the date of this publication).

Attestation information MAY be used as part of a verifier’s risk-based authentication decision.

5.2.5 Verifier Impersonation Resistance

Verifier impersonation attacks, sometimes referred to as “phishing attacks,” are attempts by fraudulent verifiers and RPs to fool an unwary claimant into authenticating to an impostor website. In prior versions of SP 800-63, protocols resistant to verifier-impersonation attacks were also referred to as “strongly MitM resistant.”

A verifier impersonation-resistant authentication protocol SHALL establish an authenticated protected channel with the verifier. It SHALL then strongly and irreversibly bind a channel identifier that was negotiated in establishing the authenticated protected channel to the authenticator output (e.g., by signing the two values together using a private key controlled by the claimant for which the public key is known to the verifier). The verifier SHALL validate the signature or other information used to prove verifier impersonation resistance. This prevents an impostor verifier, even one that has obtained a certificate representing the actual verifier, from replaying that authentication on a different authenticated protected channel.

Approved cryptographic algorithms SHALL be used to establish verifier impersonation resistance where it is required. Keys used for this purpose SHALL provide at least the minimum security strength specified in the latest revision of SP 800-131A (112 bits as of the date of this publication).

One example of a verifier impersonation-resistant authentication protocol is client-authenticated TLS, because the client signs the authenticator output along with earlier messages from the protocol that are unique to the particular TLS connection being negotiated.

Authenticators that involve the manual entry of an authenticator output, such as out-of-band and OTP authenticators, SHALL NOT be considered verifier impersonation-resistant because the manual entry does not bind the authenticator output to the specific session being authenticated. In a MitM attack, an impostor verifier could replay the OTP authenticator output to the verifier and successfully authenticate.

5.2.6 Verifier-CSP Communications

In situations where the verifier and CSP are separate entities (as shown by the dotted line in SP 800-63-3 Figure 4-1), communications between the verifier and CSP SHALL occur through a mutually-authenticated secure channel (such as a client-authenticated TLS connection) using approved cryptography.

5.2.7 Verifier-Compromise Resistance

Use of some types of authenticators requires that the verifier store a copy of the authenticator secret. For example, an OTP authenticator (described in Section 5.1.4) requires that the verifier independently generate the authenticator output for comparison against the value sent by the claimant. Because of the potential for the verifier to be compromised and stored secrets stolen, authentication protocols that do not require the verifier to persistently store secrets that could be used for authentication are considered stronger, and are described herein as being verifier compromise resistant. Note that such verifiers are not resistant to all attacks. A verifier could be compromised in a different way, such as being manipulated into always accepting a particular authenticator output.

Verifier compromise resistance can be achieved in different ways, for example:

To be considered verifier compromise resistant, public keys stored by the verifier SHALL be associated with the use of approved cryptographic algorithms and SHALL provide at least the minimum security strength specified in the latest revision of SP 800-131A (112 bits as of the date of this publication).

Other verifier compromise resistant secrets SHALL use approved hash algorithms and the underlying secrets SHALL have at least the minimum security strength specified in the latest revision of SP 800-131A (112 bits as of the date of this publication). Secrets (e.g., memorized secrets) having lower complexity SHALL NOT be considered verifier compromise resistant when hashed because of the potential to defeat the hashing process through dictionary lookup or exhaustive search.

5.2.8 Replay Resistance

An authentication process resists replay attacks if it is impractical to achieve a successful authentication by recording and replaying a previous authentication message. Replay resistance is in addition to the replay-resistant nature of authenticated protected channel protocols, since the output could be stolen prior to entry into the protected channel. Protocols that use nonces or challenges to prove the “freshness” of the transaction are resistant to replay attacks since the verifier will easily detect when old protocol messages are replayed since they will not contain the appropriate nonces or timeliness data.

Examples of replay-resistant authenticators are OTP devices, cryptographic authenticators, and look-up secrets.

In contrast, memorized secrets are not considered replay resistant because the authenticator output — the secret itself — is provided for each authentication.

5.2.9 Authentication Intent

An authentication process demonstrates intent if it requires the subject to explicitly respond to each authentication or reauthentication request. The goal of authentication intent is to make it more difficult for directly-connected physical authenticators (e.g., multi-factor cryptographic devices) to be used without the subject’s knowledge, such as by malware on the endpoint. Authentication intent SHALL be established by the authenticator itself, although multi-factor cryptographic devices MAY establish intent by reentry of the other authentication factor on the endpoint with which the authenticator is used.

Authentication intent MAY be established in a number of ways. Authentication processes that require the subject’s intervention (e.g., a claimant entering an authenticator output from an OTP device) establish intent. Cryptographic devices that require user action (e.g., pushing a button or reinsertion) for each authentication or reauthentication operation are also establish intent.

Depending on the modality, presentation of a biometric may or may not establish authentication intent. Presentation of a fingerprint would normally establish intent, while observation of the claimant’s face using a camera normally would not by itself. Behavioral biometrics similarly are less likely to establish authentication intent because they do not always require a specific action on the claimant’s part.

5.2.10 Restricted Authenticators

As threats evolve, authenticators’ capability to resist attacks typically degrades. Conversely, some authenticators’ performance may improve — for example, when changes to their underlying standards increases their ability to resist particular attacks.

To account for these changes in authenticator performance, NIST places additional restrictions on authenticator types or specific classes or instantiations of an authenticator type.

The use of a RESTRICTED authenticator requires that the implementing organization assess, understand, and accept the risks associated with that RESTRICTED authenticator and acknowledge that risk will likely increase over time. It is the responsibility of the organization to determine the level of acceptable risk for their system(s) and associated data and to define any methods for mitigating excessive risks. If at any time the organization determines that the risk to any party is unacceptable, then that authenticator SHALL NOT be used.

Further, the risk of an authentication error is typically borne by multiple parties, including the implementing organization, organizations that rely on the authentication decision, and the subscriber. Because the subscriber may be exposed to additional risk when an organization accepts a RESTRICTED authenticator and that the subscriber may have a limited understanding of and ability to control that risk, the CSP SHALL:

  1. Offer subscribers at least one alternate authenticator that is not RESTRICTED and can be used to authenticate at the required AAL.

  2. Provide meaningful notice to subscribers regarding the security risks of the RESTRICTED authenticator and availability of alternative(s) that are not RESTRICTED.

  3. Address any additional risk to subscribers in its risk assessment.

  4. Develop a migration plan for the possibility that the RESTRICTED authenticator is no longer acceptable at some point in the future and include this migration plan in its digital identity acceptance statement.