| RATS Working Group | G. Mandyam |
| Internet-Draft | Qualcomm Technologies Inc. |
| Intended status: Standards Track | L. Lundblade |
| Expires: September 25, 2019 | Security Theory LLC |
| M. Ballesteros | |
| J. O'Donoghue | |
| Qualcomm Technologies Inc. | |
| March 24, 2019 |
The Entity Attestation Token (EAT)
draft-mandyam-rats-eat-00
An attestation format based on concise binary object representation (CBOR) is proposed that is suitable for inclusion in a CBOR Web Token (CWT), know as the Entity Attestation Token (EAT). The associated data can be used by a relying party to assess the security state of a remote device or module.
TBD
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Remote device attestation is fundamental service that allows a remote device such as a mobile phone, an Internet-of-Things (IoT) device, or other endpoint to prove itself to a relying party, a server or a service. This allows the relying party to know some characteristics about the device and decide whether it trusts the device.
Remote attestation is a fundamental service that can underlie other protocols and services that need to know about the trustworthiness of the device before proceeding. One good example is biometric authentication where the biometric matching is done on the device. The relying party needs to know that the device is one that is known to do biometric matching correctly. Another example is content protection where the relying party wants to know the device will protect the data. This generalizes on to corporate enterprises that might want to know that a device is trustworthy before allowing corporate data to be accessed by it.
The notion of attestation here is large and may include, but is not limited to the following:
The required data format should be general purpose and extensible so that it can work across many use cases. This is why CBOR (see [RFC7049]) was chosen as the format — it already supports a rich set of data types, and is both expressive and extensible. It translates well to JSON for good interoperation with web technology. It is compact and can work on very small IoT device. The format proposed here is small enough that a limited version can be implemented in pure hardware gates with no software at all. Moreover, the attestation data is defined in the form of claims that is the same as CBOR Web Token (CWT, see [RFC8392]). This is the motivation for defining the Entity Attestation Token, i.e. EAT.
An “entity” can be any device or device subassembly (“submodule”) that can generate its own attestation in the form of an EAT. The attestation should be cryptographically verifiable by the EAT consumer. An EAT at the device-level can be composed of several submodule EAT’s. It is assumed that any entity that can create an EAT does so by means of a dedicated root-of-trust (RoT).
Modern devices such as a mobile phone have many different execution environments operating with different security levels. For example it is common for a mobile phone to have an “apps” environment that runs an operating system (OS) that hosts a plethora of downloadable apps. It may also have a TEE (Trusted Execution Environment) that is distinct, isolated, and hosts security-oriented functionality like biometric authentication. Additionally it may have an eSE (embedded Secure Element) - a high security chip with defenses against HW attacks that can serve as a RoT. This device attestation format allows the attested data to be tagged at a security level from which it originates. In general, any discrete execution environment that has an identifiable security level can be considered an entity.
Fundamentally this attestation format is a verifiable data format. It is a collection of data items that can be signed by an attestation key, hashed, and/or encrypted. As per Section 7 of [RFC8392], the verification method is in the CWT using the CBOR Object Signing and Encryption (COSE) methodology (see [RFC8152]).
In addition, the reported attestation data could be determined within the secure operating environment or written to it from an external and presumably less trusted entity on the device. In either case, the source of the reported data must be identifiable by the relying party.
This attestation format is a single relatively simple signed message. It is designed to be incorporated into many other protocols and many other transports. It is also designed such that other SW and apps can add their own data to the message such that it is also attested.
At least the following three participants exist in all EAT operating models. Some operating models have additional participants.
In all operating models, the manufacturer provisions some secret attestation key material (AKM) into the entity during manufacturing. This might be during the manufacturer of a chip at a fabrication facility (fab) or during final assembly of a consumer product or any time in between. This attestation key material is used for signing EATs.
In all operating models, hardware and/or software on the entity create an EAT of the format described in this document. The EAT is always signed by the attestation key material provisioned by the manufacturer.
In all operating models, the relying party must end up knowing that the signature on the EAT is valid and consistent with data from claims in the EAT. This can happen in many different ways. Here are some examples.
This standard supports all these operating models and does not prefer one over the other. It is important to support this variety of operating models to generally facilitate deployment and to allow for some special scenarios. One special scenario has a validation service that is monetized, most likely by the manufacturer. In another, a privacy proxy service processes the EAT before it is transmitted to the relying party. In yet another, symmetric key material is used for signing. In this case the manufacturer should perform the verification, because any release of the key material would enable a participant other than the entity to create valid signed EATs.
EATs may be transmitted by any protocol. For example, they might be added in extension fields of other protocols, bundled into an HTTP header, or just transmitted as files. This flexibility is intentional to allow broader adoption. This flexibility is possible because EAT’s are self-secured with signing (and possibly additionally with encryption and anti-replay). The transmission protocol is not required to fulfill any additional security requirements.
For certain devices, a direct connection may not exist between the EAT-producing device and the Relying Party. In such cases, the EAT should be protected against malicious access. The use of COSE allows for signing and encryption of the EAT. Therefore even if the EAT is conveyed through intermediaries between the device and Relying Party, such intermediaries cannot easily modify the EAT payload or alter the signature.
The term “signing scheme” is used to refer to the system that includes end-end process of establishing signing attestation key material in the entity, signing the EAT, and verifying it. This might involve key IDs and X.509 certificate chains or something similar but different. The term “signing algorithm” refers just to the algorithm ID in the COSE signing structure. No particular signing algorithm or signing scheme is required by this standard.
There are three main implementation issues driving this. First, secure non-volatile storage space in the entity for the attestation key material may be highly limited, perhaps to only a few hundred bits, on some small IoT chips. Second, the factory cost of provisioning key material in each chip or device may be high, with even millisecond delays adding to the cost of a chip. Third, privacy-preserving signing schemes like ECDAA (Elliptic Curve Direct Anonymous Attestation) are complex and not suitable for all use cases.
Eventually some form of standardization of the signing scheme may be required. This might come in the form of another standard that adds to this document, or when there is clear convergence on a small number of signing schemes this standard can be updated.
The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
This document reuses terminology from JWT [RFC7519], COSE [RFC8152], and CWT [RFC8392].
UEID’s identify individual manufactured entities / devices such as a mobile phone, a water meter, a Bluetooth speaker or a networked security camera. It may identify the entire device or a submodule or subsystem. It does not identify types, models or classes of devices. It is akin to a serial number, though it does not have to be sequential.
It is identified by Claim Key X (X is TBD).
UEID’s must be universally and globally unique across manufacturers and countries. UEIDs must also be unique across protocols and systems, as tokens are intended to be embedded in many different protocols and systems. No two products anywhere, even in completely different industries made by two different manufacturers in two different countries. should have the same UEID (if they are not global and universal in this way then relying parties receiving them will have to track other characteristics of the device to keep devices distinct between manufacturers).
The UEID should be permanent. It should never change for a given device / entity. In addition, it should not be reprogrammable.
UEID’s are binary byte-strings (resulting in a smaller size than text strings). When handled in text-based protocols, they should be base-64 encoded.
UEID’s are variable length with a maximum size of 33 bytes (1 type byte and 256 bits). A receivers of a token with UEIDs may reject the token if a UEID is larger than 33 bytes.
UEID’s are not designed for direct use by humans (e.g., printing on the case of a device), so no textual representation is defined.
A UEID is a byte string. From the consumer’s view (the rely party) it is opaque with no bytes having any special meaning.
When the entity constructs the UEID, the first byte is a type and the following bytes the ID for that type. Several types are allowed to accommodate different industries and different manufacturing processes and to give options to avoid paying fees for certain types of manufacturer registrations.
| Type Byte | Type Name | Specification |
|---|---|---|
| 0x01 | GUID | This is a 128 to 256 bit random number generated once and stored in the device. The GUID may be constructed from various identifiers on the device using a hash function or it may be just the raw random number. |
| 0x02 | IEEE EUI | This makes use of the IEEE company identification registry. An EUI is made up of an OUI and OUI-36 or a CID, different registered company identifiers, and some unique per-device identifier. EUIs are often the same as or similar to MAC addresses. (Note that while devices with multiple network interfaces may have multiple MAC addresses, there is only one UEID for a device) TODO: normative references to IEEE. |
| 0x03 | IMEI | This is a 14-digit identifier consisting of an 8 digit Type Allocation Code and a six digit serial number allocated by the manufacturer, which SHALL be encoded as a binary integer over 48 bits. The IMEI value encoded SHALL NOT include Luhn checksum or SVN information. |
| 0x04 | EUI-48 | This is a 48-bit identifier formed by concatenating the 24-bit OUI with a 24-bit identifier assigned by the organisation that purchased the OUI. |
| 0x05 | EUI-60 | This is a 60-bit identifier formed by concatenating the 24-bit OUI with a 36-bit identifier assigned by the organisation that purchased the OUI. |
| 0x06 | EUI-64 | This is a 64-bit identifier formed by concatenating the 24-bit OUI with a 40-bit identifier assigned by the organisation that purchased the OUI. |
The consumer (the Relying Party) of a UEID should treat a UEID as a completely opaque string of bytes and not make any use of its internal structure. For example they should not use the OUI part of a type 0x02 UEID to identify the manufacturer of the device. Instead they should use the OUI claim that is defined elsewhere. The reasons for this are:
This claim describes the parts of the device or entity that are creating the EAT. Often it will be tied back to the device or chip manufacturer. The following table gives some examples:
| Name | Description |
|---|---|
| Acme-TEE | The EATs are generated in the TEE authored and configured by “Acme” |
| Acme-TPM | The EATs are generated in a TPM manufactured by “Acme” |
| Acme-Linux-Kernel | The EATs are generated in a Linux kernel configured and shipped by “Acme” |
| Acme-TA | The EATs are generated in a Trusted Application (TA) authored by “Acme” |
The claim is represented by Claim Key X+1. It is type StringOrURI.
TODO: consider a more structure approach where the name and the URI and other are in separate fields.
TODO: This needs refinement. It is somewhat parallel to issuer claim in CWT in that it describes the authority that created the token.
This claim identifies a device OEM by the IEEE OUI. Reference TBD. It is a byte string representing the OUI in binary form in network byte order (TODO: confirm details).
Companies that have more than one IEEE OUI registered with IEEE should pick one and prefer that for all their devices.
Note that the OUI is in common use as a part of MAC Address. This claim is only the first bits of the MAC address that identify the manufacturer. The IEEE maintains a registry for these in which many companies participate. This claim is represented by Claim Key TBD.
EATs have a claim that roughly characterizes the device / entities ability to defend against attacks aimed at capturing the signing key, forging claims and at forging EATs. This is done by roughly defining four security levels as described below. This is similar to the security levels defined in the Metadata Service definied by the Fast Identity Online (FIDO) Alliance (TODO: reference).
These claims describe security environment and countermeasures available on the end-entity / client device where the attestation key reside and the claims originate.
This claim is identified by Claim Key X+2. The value is an integer between 1 and 4 as defined below.
This claim is not intended as a replacement for a proper end-device security certification schemes such as those based on FIPS (TODO: reference) or those based on Common Criteria (TODO: reference). The claim made here is solely a self-claim made by the Entity Originator.
The “nonce” (Nonce) claim represents a random value that can be used to avoid replay attacks. This would be ideally generated by the CWT consumer. This value is intended to be a CWT companion claim to the existing JWT claim IANAJWT (TODO: fix this reference). The nonce claim is identified by Claim Key X+3.
The “secbootenabled” (Secure Boot Enabled) claim represents a boolean value that indicates whether secure boot is enabled either for an entire device or an individual submodule. If it appears at the device level, then this means that secure boot is enabled for all submodules. Secure boot enablement allows a secure boot loader to authenticate software running either in a device or a submodule prior allowing execution. This claim is identified by Claim Key X+4.
The “debugdisabled” (Debug Disabled) claim represents a boolean value that indicates whether debug capabilities are disabled for an entity (i.e. value of ‘true’). Debug disablement is considered a prerequisite before an entity is considered operational. This claim is identified by Claim Key X+5.
The “debugdisabledsinceboot” (Debug Disabled Since Boot) claim represents a boolean value that indicates whether debug capabilities for the entity were not disabled in any way since boot (i.e. value of ‘true’). This claim is identified by Claim Key X+6.
The “debugpermanentdisable” (Debug Permanent Disable) claim represents a boolean value that indicates whether debug capabilities for the entity are permanently disabled (i.e. value of ‘true’). This value can be set to ‘true’ also if only the manufacturer is allowed to enabled debug, but the end user is not. This claim is identified by Claim Key X+7.
The “debugfullpermanentdisable” (Debug Full Permanent Disable) claim represents a boolean value that indicates whether debug capabilities for the entity are permanently disabled (i.e. value of ‘true’). This value can only be set to ‘true’ if no party can enable debug capabilities for the entity. Often this is implemented by blowing a fuse on a chip as fuses cannot be restored once blown. This claim is identified by Claim Key X+8.
The “loc” (location) claim is a CBOR-formatted object that describes the location of the device entity from which the attestation originates. It is identified by Claim Key X+10. It is comprised of an array of additional subclaims that represent the actual location coordinates (latitude, longitude and altitude). The location coordinate claims are consistent with the WGS84 coordinate system [WGS84]. In addition, a subclaim providing the estimated accuracy of the location measurement is defined.
The “lat” (latitude) claim contains the value of the device location corresponding to its latitude coordinate. It is of data type FloatOrNumber and identified by Claim Key X+11.
The “long” (longitude) claim contains the value of the device location corresponding to its longitude coordinate. It is of data type FloatOrNumber and identified by Claim Key X+12.
The “alt” (altitude) claim contains the value of the device location corresponding to its altitude coordinate (if available). It is of data type FloatOrNumber and identified by Claim Key X+13.
The “acc” (accuracy) claim contains a value that describes the location accuracy. It is non-negative and expressed in meters. It is of data type FloatOrNumber and identified by Claim Key X+14.
The “altacc” (altitude accuracy) claim contains a value that describes the altitude accuracy. It is non-negative and expressed in meters. It is of data type FloatOrNumber and identified by Claim Key X+15.
The “heading” claim contains a value that describes direction of motion for the entity. Its value is specified in degrees, between 0 and 360. It is of data type FloatOrNumber and identified by Claim Key X+16.
The “speed” claim contains a value that describes the velocity of the entity in the horizontal direction. Its value is specified in meters/second and must be non-negative. It is of data type FloatOrNumber and identified by Claim Key X+17.
The “ts” (timestamp) claim contains a timestamp derived using the same time reference as is used to generate an “iat” claim (see Section 3.1.6 of [RFC8392]). It is of the same type as “iat” (integer or floating-point), and is identified by Claim Key X+18. It is meant to designate the time at which a measurement was taken, when a location was obtained, or when a token was actually transmitted. The timestamp would be included as a subclaim under the “submod” or “loc” claims (in addition to the existing respective subclaims), or at the device level.
The “age” claim contains a value that represents the number of seconds that have elapsed since the token was created, measurement was made, or location was obtained. Typical attestable values are sent as soon as they are obtained. However in the case that such a value is buffered and sent at a later time and a sufficiently accurate time reference is unavailable for creation of a timestamp, then the age claim is provided. It is identified by Claim Key X+19.
The “uptime” claim contains a value that represents the number of seconds that have elapsed since the entity or submod was last booted. It is identified by Claim Key X+20.
Some devices are complex, having many subsystems or submodules. A mobile phone is a good example. It may have several connectivity submodules for communications (e.g., WiFi and cellular). It may have sub systems for low-power audio and video playback. It may have one or more security-oriented subsystems like a TEE or a Secure Element.
The claims for each these can be grouped together in a submodule.
Specifically, the “submods” claim is an array. Each item in the array is a CBOR map containing all the claims for a particular submodule. It is identified by Claim Key X+22.
The security level of the submod is assumed to be at the same level as the main entity unless there is a security level claim in that submodule indicating otherwise. The security level of a submodule can never be higher (more secure) than the security level of the EAT it is a part of.
Each submodule should have a submod_name claim that is descriptive name. This name should be the CBOR txt type.
It is allowed for one EAT to be embedded in another. This is for complex devices that have more than one subsystem capable of generating an EAT. Typically one will be the device-wide EAT that is low to medium security and another from a Secure Element or similar that is high security.
The contents of the “eat” claim must be a fully signed, optionally encrypted, EAT token. It is identified by Claim Key X+23.
EAT is a one-way protocol. It only defines a single message that goes from the entity to the server. The entity implementation will often be in a contained environment with little RAM and the server will usually not be. The following requirements for interoperability take that into account. The entity can generally use whatever encoding it wants. The server is required to support just about every encoding.
Canonical CBOR encoding is explicitly NOT required as it would place an unnecessary burden on the entity implementation.
The entity may use any integer encoding allowed by CBOR. The server MUST accept all integer encodings allowed by CBOR.
The entity can use any string encoding allowed by CBOR including indefinite lengths. It may also encode the lengths of strings in any way allowed by CBOR. The server must accept all string encodings.
Major type 2, bstr, SHOULD be have tag 21, 22 or 23 to indicate conversion to base64 or such when converting to JSON.
The entity can use any array or map encoding allowed by CBOR including indefinite lengths. Sorting of map keys is not required. Duplicate map keys are not allowed. The server must accept all array and map encodings. The server may reject maps with duplicate map keys.
The entity should send dates as tag 1 encoded as 64-bit or 32-bit integers. The entity may not send floating point dates. The server must support tag 1 epoch based dates encoded as 64-bit or 32-bit integers.
The entity may send tag 0 dates, however tag 1 is preferred. The server must support tag 0 UTC dates.
URIs should be encoded as text strings and marked with tag 32.
Encoding data in floating point is to be used only if necessary. Location coordinates are always in floating point. The server must support decoding of all types of floating point.
Use of Other types like bignums, regular expressions and so SHOULD NOT be used. The server MAY support them, but is not required to. Use of these tags is
Claims defined for EAT are compatible with those of CWT so the CWT Claims Registry is re used. New new IANA registry is created. All EAT claims should be registered in the CWT Claims Registry.
TODO: add the rest of the claims in here
How an EAT consumer determines whether received CBOR-formatted data actually represents a valid EAT is application-dependent, much like a CWT. For instance, a specific MIME type associated with the EAT such as “application/eat” could be sufficient for identification of the EAT. Note however that EAT’s can include other EAT’s (e.g. a device EAT comprised of several submodule EAT’s). In this case, a CBOR tag dedicated to the EAT will be required at least for the submodule EAT’s and the tag must be a valid CBOR tag. In other words - the EAT CBOR tag can optionally prefix a device-level EAT, but a EAT CBOR tag must always prefix a submodule EAT. The proposed EAT CBOR tag is 71.
Certain EAT claims can be used to track the owner of an entity and therefore implementations should consider providing privacy-preserving options dependent on the intended usage of the EAT. Examples would include suppression of location claims for EAT’s provided to unauthenticated consumers.
A UEID is usually not privacy preserving. Any set of relying parties that receives tokens that happen to be from a single device will be able to know the tokens are all from the same device and be able to track the device. Thus, in many usage situations ueid violates governmental privacy regulation. In other usage situations UEID will not be allowed for certain products like browsers that give privacy for the end user. it will often be the case that tokens will not have a UEID for these reasons.
There are several strategies that can be used to still be able to put UEID’s in tokens:
Note that some of these privacy preservation strategies result in multiple UEIDs per device. Each UEID is used in a different context, use case or system on the device. However, from the view of the relying party, there is just one UEID and it is still globally universal across manufacturers.
TODO: Perhaps this can be the same as CWT / COSE, but not sure yet because it involves so much entity / device security that those do not.
| [ASN.1] | International Telecommunication Union, "Information Technology -- ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)", ITU-T Recommendation X.690, 1994. |
| [IDevID] | "IEEE Standard, "IEEE 802.1AR Secure Device Identifier"", December 2009. |
| [Webauthn] | Worldwide Web Consortium, "Web Authentication: A Web API for accessing scoped credentials", 2016. |
This is shown in CBOR diagnostic form. Only the payload signed by COSE is shown.
{
/ nonce / 11:h'948f8860d13a463e8e',
/ UEID / 8:h'0198f50a4ff6c05861c8860d13a638ea4fe2f',
/ secbootenabled / 13:true,
/ debugpermanentdisable / 15:true,
/ ts / 21:1526542894,
}
{
/ nonce / 11:h'948f8860d13a463e8e',
/ UEID / 8:h'0198f50a4ff6c05861c8860d13a638ea4fe2f',
/ secbootenabled / 13:true,
/ debugpermanentdisable / 15:true,
/ ts / 21:1526542894,
/ seclevel / 10:3, / secure restriced OS /
/ submods / 30:
[
/ 1st submod, an Android Application / {
/ submod_name / 30:'Android App "Foo"',
/ seclevel / 10:1, / unrestricted /
/ app data / -70000:'text string'
},
/ 2nd submod, A nested EAT from a secure element / {
/ submod_name / 30:'Secure Element EAT',
/ eat / 31:71( 18(
/ an embedded EAT / [ /...COSE_Sign1 bytes with payload.../ ]
))
}
/ 3rd submod, information about Linux Android / {
/ submod_name/ 30:'Linux Android',
/ seclevel / 10:1, / unrestricted /
/ custom - release / -80000:'8.0.0',
/ custom - version / -80001:'4.9.51+'
}
]
}