--- 1/draft-ietf-rats-architecture-00.txt 2020-02-04 09:13:41.579014707 -0800 +++ 2/draft-ietf-rats-architecture-01.txt 2020-02-04 09:13:41.655016620 -0800 @@ -1,28 +1,28 @@ RATS Working Group H. Birkholz Internet-Draft Fraunhofer SIT Intended status: Informational D. Thaler -Expires: 19 June 2020 Microsoft +Expires: 7 August 2020 Microsoft M. Richardson Sandelman Software Works N. Smith Intel - 17 December 2019 + 4 February 2020 Remote Attestation Procedures Architecture - draft-ietf-rats-architecture-00 + draft-ietf-rats-architecture-01 Abstract In network protocol exchanges, it is often the case that one entity - (a relying party) requires evidence about a remote peer to assess the + (a Relying Party) requires evidence about a remote peer to assess the peer's trustworthiness, and a way to appraise such evidence. The evidence is typically a set of claims about its software and hardware platform. This document describes an architecture for such remote attestation procedures (RATS). Note to Readers Discussion of this document takes place on the RATS Working Group mailing list (rats@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/rats/ @@ -39,73 +39,84 @@ Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." - This Internet-Draft will expire on 19 June 2020. + This Internet-Draft will expire on 7 August 2020. Copyright Notice - Copyright (c) 2019 IETF Trust and the persons identified as the + Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 - 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2 + 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Reference Use Cases . . . . . . . . . . . . . . . . . . . . . 4 4. Architectural Overview . . . . . . . . . . . . . . . . . . . 4 - 5. Topological Models . . . . . . . . . . . . . . . . . . . . . 5 - 6. Two Types of Environments . . . . . . . . . . . . . . . . . . 5 - 7. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 5 - 8. Conceptual Messages . . . . . . . . . . . . . . . . . . . . . 6 - 9. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . . 6 - 10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 7 - 11. Security Considerations . . . . . . . . . . . . . . . . . . . 7 - 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 - 13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7 - 14. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 7 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 + 4.1. Composite Attester . . . . . . . . . . . . . . . . . . . 5 + 5. Topological Models . . . . . . . . . . . . . . . . . . . . . 7 + 5.1. Passport Model . . . . . . . . . . . . . . . . . . . . . 7 + 5.2. Background-Check Model . . . . . . . . . . . . . . . . . 8 + 5.3. Combinations . . . . . . . . . . . . . . . . . . . . . . 9 + 6. Two Types of Environments of an Attester . . . . . . . . . . 10 + 7. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 11 + 8. Conceptual Messages . . . . . . . . . . . . . . . . . . . . . 11 + 8.1. Evidence . . . . . . . . . . . . . . . . . . . . . . . . 12 + 8.2. Endorsements . . . . . . . . . . . . . . . . . . . . . . 12 + 8.3. Attestation Results . . . . . . . . . . . . . . . . . . . 13 + 9. Claims Encoding Formats . . . . . . . . . . . . . . . . . . . 13 + 10. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . . 15 + 11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 16 + 12. Security Considerations . . . . . . . . . . . . . . . . . . . 16 + 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 + 14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17 + 15. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 17 + 16. Informative References . . . . . . . . . . . . . . . . . . . 17 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 1. Introduction -2. Terminology + + Remote Attestation, as used in this document, is a process by which + one entity (the "Attester") provides evidence about its identity and + state to another remote entity (the "Relying Party"), which then + assesses the Attester's trustworthiness for the Relying Party's own + purposes. - The document defines the term "Remote Attestation" as follows: A - process by which one entity (the "Attester") provides evidence about - its identity and state to another remote entity (the "Relying - Party"), which then assesses the Attester's trustworthiness for the - Relying Party's own purposes. +2. Terminology - This document then uses the following terms: + This document uses the following terms: * Appraisal Policy for Evidence: A set of rules that direct how a - verifier evaluates the validity of information about an Attester. + Verifier evaluates the validity of information about an Attester. Compare /security policy/ in [RFC4949]. * Appraisal Policy for Attestation Result: A set of rules that - direct how a Relying Party evaluates the validity of information - about an Attester. Compare /security policy/ in [RFC4949]. + direct how a Relying Party uses the evaluation results about an + Attester generated by the Verifiers. Compare /security policy/ in + [RFC4949]. * Attestation Result: The evaluation results generated by a Verifier, typically including information about an Attester, where the Verifier vouches for the validity of the results. * Attester: An entity whose attributes must be evaluated in order to determine whether the entity is considered trustworthy, such as when deciding whether the entity is authorized to perform some operation. @@ -127,27 +138,20 @@ authorized to configure Appraisal Policy for Attestation Results in a Relying Party. * Verifier: An entity that evaluates the validity of Evidence about an Attester. * Verifier Owner: An entity, such as an administrator, that is authorized to configure Appraisal Policy for Evidence in a Verifier. - [EDITORIAL NOTE] - - The term Attestation and Remote Attestation are not defined in this - document, at this time. This document will include pointers to - industry uses of the terms, in an attempt to gain consensus around - the term, and be consistent with the charter text defining this term. - 3. Reference Use Cases 4. Architectural Overview Figure 1 depicts the data that flows between different roles, independent of protocol or use case. ************ ************ ***************** @@ -187,34 +191,286 @@ obtained from an Endorser along with the Endorsements, or might be obtained via some other mechanism such as being configured in the Verifier by an administrator. The Relying Party uses Attestation Results by applying its own Appraisal Policy to make application-specific decisions such as authorization decisions. The Attestation Result Appraisal Policy might, for example, be configured in the Relying Party by an administrator. +4.1. Composite Attester + + A Composite Attester is an entity composed of multiple sub-entities + such that its trustworthiness has to be determined by evaluating all + these sub-entities. Each sub-entity has at least one Attesting + Environment collecting the claims from at least one Target + Environment, then this sub-entity generates Evidence about its + trustworthiness. Therefore each sub-entity can be called an + Attester. Among these Attesters, there may be only some, which can + be called Lead Attesters, that have the ability to communicate with + the Verifier. Other Attesters don't have this ability, but they are + connected to the Lead Attesters via internal links or network + connections, and they are evaluated via the Lead Attester's help. + + For example, a carrier-grade router is a composite device consisting + of a chassis and multiple slots. The trustworthiness of the router + depends on all its slots' trustworthiness. Each slot has an + Attesting Environment such as a TPM or TEE collecting the claims of + its boot process, after which it generates Evidence from the claims. + Among these slots, only a main slot can communicate with the Verifier + while other slots cannot. But other slots can communicate with the + main slot by the links between them inside the router. So the main + slot collects the Evidence of other slots, produces the final + Evidence of the whole router and conveys the final Evidence to the + Verifier. Therefore the router is a Composite Attester, each slot is + an Attester, and the main slot is the Lead Attester. + + Another example is a multi-chassis router composed of multiple single + carrier-grade routers. The multi-chassis router provides higher + throughput by interconnecting multiple routers and simpler management + by being logically treated as one router. Among these routers, there + is only one main router that connects to the Verifier. Other routers + are only connected to the main router by the network cables, and + therefore they are managed and verified via this main router. So, in + this case, the multi-chassis router is the Composite Attester, each + router is an Attester and the main router is the Lead Attester. + + Figure 2 depicts the conceptual data flow for a Composite Attester. + + .-----------------------------. + | Verifier | + '-----------------------------' + ^ + | + | Composite + | Evidence + | + .----------------------------------|-------------------------------. + | .--------------------------------|-----. .------------. | + | | .------------. | | | | + | | | Attesting |<--------| Attester B |-. | + | | |Environment | | '------------. | | + | | .----------------. | |<----------| Attester C |-. | + | | | Target | | | | '------------' | | + | | | Environment(s) | | |<------------| ... | | + | | | | '------------' | Evidence '------------' | + | | | | ^ | of | + | | | |------------/ | Attesters | + | | '----------------' Collecting | (via Internal Links or | + | | Claims | Network Connections) | + | | | | + | | Lead Attester A | | + | '--------------------------------------' | + | | + | Device/Composite Device/Attester/TBD #33 | + '------------------------------------------------------------------' + + Figure 2: Conceptual Data Flow for a Composite Attester + + In the Composite Attester, each Attester generates its own Evidence + by its Attesting Environment(s) collecting the claims from its Target + Environment(s). The Lead Attester collects the Evidence of all other + Attesters and then generates the Evidence of the whole Composite + Attester. + + The Lead Attester's Attesting Environment may or may not include its + own Verifier. One situation is that the Attesting Environment has no + internal Verifier. In this situation, the Lead Attesting Environment + simply combines the various Evidences into the final Evidence that is + sent off to the remote Verifier, which evaluates the Composite + Attester's, including the Lead Attester's and other Attesters', + trustworthiness. + + The other situation is that the Lead Attesting Environment has an + internal Verifier. After collecting the Evidence of other Attesters, + this Attesting Environment verifies them using Endorsements and + Appraisal Policies (obtained the same way as any other Verifier), for + evaluating these Attesters' trustworthiness. Then the Lead Attesting + Environment combines the Attestation Results into the final Evidence + of the whole Composite Attester which is sent off to the remote + Verifier, which might treat the claims obtained from the local + Attestation Results as if they were Evidence. + 5. Topological Models - + There are multiple possible models for communication between an + Attester, a Verifier, and a Relying Party. This section includes + some reference models, but this is not intended to be a restrictive + list, and other variations may exist. -6. Two Types of Environments +5.1. Passport Model - An Attester consists of at least one Attesting Environment and - Attested Environment. In some implementations, the Attesting and - Attested Environments might be combined. Other implementations might - have multiple Attesting and Attested Environments. + In this model, an Attester sends Evidence to a Verifier, which + compares the Evidence against its Appraisal Policy. The Verifier + then gives back an Attestation Result. If the Attestation Result was + a successful one, the Attester can then present the Attestation + Result to a Relying Party, which then compares the Attestation Result + against its own Appraisal Policy. - + Since the resource access protocol between the Attester and Relying + Party includes an Attestation Result, in this model the details of + that protocol constrain the serialization format of the Attestation + Result. The format of the Evidence on the other hand is only + constrained by the Attester-Verifier attestation protocol. + + +-------------+ + | | Compare Evidence + | Verifier | against Appraisal Policy + | | + +-------------+ + ^ | + Evidence| |Attestation + | | Result + | v + +-------------+ +-------------+ + | |-------------->| | Compare Attestation + | Attester | Attestation | Relying | Result against + | | Result | Party | Appraisal Policy + +-------------+ +-------------+ + + Figure 3: Passport Model + + The passport model is so named because of its resemblance to how + nations issue passports to their citizens. The nature of the + Evidence that an individual needs to provide to its local authority + is specific to the country involved. The citizen retains control of + the resulting passport document and presents it to other entities + when it needs to assert a citizenship or identity claim, such as an + airport immigration desk. The passport is considered sufficient + because it vouches for the citizenship and identity claims, and it is + issued by a trusted authority. Thus, in this immigration desk + analogy, the passport issuing agency is a Verifier, the passport is + an Attestation Result, and the immigration desk is a Relying Party. + +5.2. Background-Check Model + + In this model, an Attester sends Evidence to a Relying Party, which + simply passes it on to a Verifier. The Verifier then compares the + Evidence against its Appraisal Policy, and returns an Attestation + Result to the Relying Party. The Relying Party then compares the + Attestation Result against its own security policy. + + The resource access protocol between the Attester and Relying Party + includes Evidence rather than an Attestation Result, but that + Evidence is not processed by the Relying Party. Since the Evidence + is merely forwarded on to a trusted Verifier, any serialization + format can be used for Evidence because the Relying Party does not + need a parser for it. The only requirement is that the Evidence can + be _encapsulated in_ the format required by the resource access + protocol between the Attester and Relying Party. + + However, like in the Passport model, an Attestation Result is still + consumed by the Relying Party and so the serialization format of the + Attestation Result is still important. If the Relying Party is a + constrained node whose purpose is to serve a given type resource + using a standard resource access protocol, it already needs the + parser(s) required by that existing protocol. Hence, the ability to + let the Relying Party obtain an Attestation Result in the same + serialization format allows minimizing the code footprint and attack + surface area of the Relying Party, especially if the Relying Party is + a constrained node. + + +-------------+ + | | Compare Evidence + | Verifier | against Appraisal Policy + | | + +-------------+ + ^ | + Evidence| |Attestation + | | Result + | v + +-------------+ +-------------+ + | |-------------->| | Compare Attestation + | Attester | Evidence | Relying | Result against + | | | Party | Appraisal Policy + +-------------+ +-------------+ + + Figure 4: Background-Check Model + + The background-check model is so named because of the resemblance of + how employers and volunteer organizations perform background checks. + When a prospective employee provides claims about education or + previous experience, the employer will contact the respective + institutions or former employers to validate the claim. Volunteer + organizations often perform police background checks on volunteers in + order to determine the volunteer's trustworthiness. Thus, in this + analogy, a prospective volunteer is an Attester, the organization is + the Relying Party, and a former employer or government agency that + issues a report is a Verifier. + +5.3. Combinations + + One variation of the background-check model is where the Relying + Party and the Verifier on the same machine, and so there is no need + for a protocol between the two. + + It is also worth pointing out that the choice of model is generally + up to the Relying Party, and the same device may need to attest to + different Relying Parties for different use cases (e.g., a network + infrastructure device to gain access to the network, and then a + server holding confidential data to get access to that data). As + such, both models may simultaneously be in use by the same device. + + Figure 5 shows another example of a combination where Relying Party 1 + uses the passport model, whereas Relying Party 2 uses an extension of + the background-check model. Specifically, in addition to the basic + functionality shown in Figure 4, Relying Party 2 actually provides + the Attestation Result back to the Attester, allowing the Attester to + use it with other Relying Parties. This is the model that the + Trusted Application Manager plans to support in the TEEP architecture + [I-D.ietf-teep-architecture]. + + +-------------+ + | | Compare Evidence + | Verifier | against Appraisal Policy + | | + +-------------+ + ^ | + Evidence| |Attestation + | | Result + | v + +-------------+ + | | Compare + | Relying | Attestation Result + | Party 2 | against Appraisal Policy + +-------------+ + ^ | + Evidence| |Attestation + | | Result + | v + +----------+ +----------+ + | |-------------->| | Compare Attestation + | Attester | Attestation | Relying | Result against + | | Result | Party 1 | Appraisal Policy + +----------+ +----------+ + + Figure 5: Example Combination + +6. Two Types of Environments of an Attester + + An Attester consists of at least one Attesting Environment and at + least one Target Environment. In some implementations, the Attesting + and Target Environments might be combined. Other implementations + might have multiple Attesting and Target Environments. One example + is a set of components in a boot sequence (e.g., ROM, firmware, OS, + and application) where a Target Environment is the Attesting + Environment for the next environment in the boot sequence. + + Claims are collected from Target Environments. That is, Attesting + Environments collect the raw values and the information to be + represented in claims. Attesting Environments then format them + appropriately, and typically use key material and cryptographic + functions, such as signing or cipher algorithms, to create Evidence. + Examples of environments that can be used as Attesting Environments + include Trusted Execution Environments (TEE), embedded Secure + Elements (eSE), or Hardware Security Modules (HSM). 7. Trust Model The scope of this document is scenarios for which a Relying Party trusts a Verifier that can evaluate the trustworthiness of information about an Attester. Such trust might come by the Relying Party trusting the Verifier (or its public key) directly, or might come by trusting an entity (e.g., a Certificate Authority) that is in the Verifier's certificate chain. The Relying Party might implicitly trust a Verifier (such as in the Verifying Relying Party @@ -240,96 +495,308 @@ That is, it might evaluate the trustworthiness of an application component, or operating system component or service, under the assumption that information provided about it by the lower-layer hypervisor or firmware is true. A stronger level of security comes when information can be vouched for by hardware or by ROM code, especially if such hardware is physically resistant to hardware tampering. The component that is implicitly trusted is often referred to as a Root of Trust. 8. Conceptual Messages +8.1. Evidence - + Today, Evidence tends to be highly device-specific, since the + information in the Evidence often includes vendor-specific + information that is necessary to fully describe the manufacturer and + model of the device including its security properties, the health of + the device, and the level of confidence in the correctness of the + information. Evidence is typically signed by the device (whether by + hardware, firmware, or software on the device), and evaluating it in + isolation would require Appraisal Policy to be based on device- + specific details (e.g., a device public key). + +8.2. Endorsements + + An Endorsement is a secure statement that some entity (e.g., a + manufacturer) vouches for the integrity of the device's signing + capability. For example, if the signing capability is in hardware, + then an Endorsement might be a manufacturer certificate that signs a + public key whose corresponding private key is only known inside the + device's hardware. Thus, when Evidence and such an Endorsement are + used together, evaluating them can be done against Appraisal Policy + that may not be specific to the device instance, but merely specific + to the manufacturer providing the Endorsement. For example, an + Appraisal Policy might simply check that devices from a given + manufacturer have information matching a set of known-good reference + values, or an Appraisal Policy might have a set of more complex logic + on how to evaluate the validity of information. + + However, while an Appraisal Policy that treats all devices from a + given manufacturer the same may be appropriate for some use cases, it + would be inappropriate to use such an Appraisal Policy as the sole + means of authorization for use cases that wish to constrain _which_ + compliant devices are considered authorized for some purpose. For + example, an enterprise using attestation for Network Endpoint + Assessment may not wish to let every healthy laptop from the same + manufacturer onto the network, but instead only want to let devices + that it legally owns onto the network. Thus, an Endorsement may be + helpful information in authenticating information about a device, but + is not necessarily sufficient to authorize access to resources which + may need device-specific information such as a public key for the + device or component or user on the device. + +8.3. Attestation Results + + Attestation Results may indicate compliance or non-compliance with a + Verifier's Appraisal Policy. A result that indicates non-compliance + can be used by an Attester (in the passport model) or a Relying Party + (in the background-check model) to indicate that the Attester should + not be treated as authorized and may be in need of remediation. In + some cases, it may even indicate that the Evidence itself cannot be + authenticated as being correct. + + An Attestation Result that indicates compliance can be used by a + Relying Party to make authorization decisions based on the Relying + Party's Appraisal Policy. The simplest such policy might be to + simply authorize any party supplying a compliant Attestation Result + signed by a trusted Verifier. A more complex policy might also + entail comparing information provided in the result against known- + good reference values, or applying more complex logic such + information. + + Thus, Attestation Results often need to include detailed information + about the Attester, for use by Relying Parties, much like physical + passports and drivers licenses include personal information such as + name and date of birth. Unlike Evidence, which is often very device- + and vendor-specific, Attestation Results can be vendor-neutral if the + Verifier has a way to generate vendor-agnostic information based on + evaluating vendor-specific information in Evidence. This allows a + Relying Party's Appraisal Policy to be simpler, potentially based on + standard ways of expressing the information, while still allowing + interoperability with heterogeneous devices. + + Finally, whereas Evidence is signed by the device (or indirectly by a + manufacturer, if Endorsements are used), Attestation Results are + signed by a Verifier, allowing a Relying Party to only need a trust + relationship with one entity, rather than a larger set of entities, + for purposes of its Appraisal Policy. + +9. Claims Encoding Formats + + The following diagram illustrates a relationship to which attestation + is desired to be added: + + +-------------+ +-------------+ + | |-------------->| | + | Attester | Access some | Relying | Evaluate request + | | resource | Party | against security policy + +-------------+ +-------------+ + + Figure 6: Typical Resource Access + + In this diagram, the protocol between Attester and a Relying Party + can be any new or existing protocol (e.g., HTTP(S), COAP(S), 802.1x, + OPC UA, etc.), depending on the use case. Such protocols typically + already have mechanisms for passing security information for purposes + of authentication and authorization. Common formats include JWTs + [RFC7519], CWTs [RFC8392], and X.509 certificates. + + To enable attestation to be added to existing protocols, enabling a + higher level of assurance against malware for example, it is + important that information needed for evaluating the Attester be + usable with existing protocols that have constraints around what + formats they can transport. For example, OPC UA [OPCUA] (probably + the most common protocol in industrial IoT environments) is defined + to carry X.509 certificates and so security information must be + embedded into an X.509 certificate to be passed in the protocol. + Thus, attestation-related information could be natively encoded in + X.509 certificate extensions, or could be natively encoded in some + other format (e.g., a CWT) which in turn is then encoded in an X.509 + certificate extension. + + Especially for constrained nodes, however, there is a desire to + minimize the amount of parsing code needed in a Relying Party, in + order to both minimize footprint and to minimize the attack surface + area. So while it would be possible to embed a CWT inside a JWT, or + a JWT inside an X.509 extension, etc., there is a desire to encode + the information natively in the format that is natural for the + Relying Party. + + This motivates having a common "information model" that describes the + set of attestation related information in an encoding-agnostic way, + and allowing multiple encoding formats (CWT, JWT, X.509, etc.) that + encode the same information into the claims format needed by the + Relying Party. + + The following diagram illustrates that Evidence and Attestation + Results might each have multiple possible encoding formats, so that + they can be conveyed by various existing protocols. It also + motivates why the Verifier might also be responsible for accepting + Evidence that encodes claims in one format, while issuing Attestation + Results that encode claims in a different format. Evidence Attestation Results .--------------. CWT CWT .-------------------. | Attester-A |------------. .----------->| Relying Party V | '--------------' v | `-------------------' .--------------. JWT .------------. JWT .-------------------. | Attester-B |-------->| Verifier |-------->| Relying Party W | '--------------' | | `-------------------' .--------------. X.509 | | X.509 .-------------------. | Attester-C |-------->| |-------->| Relying Party X | '--------------' | | `-------------------' .--------------. TPM | | TPM .-------------------. | Attester-D |-------->| |-------->| Relying Party Y | '--------------' '------------' `-------------------' .--------------. other ^ | other .-------------------. | Attester-E |------------' '----------->| Relying Party Z | '--------------' `-------------------' - Figure 2: Multiple Attesters and Relying Parties with Different + Figure 7: Multiple Attesters and Relying Parties with Different Formats -9. Freshness +10. Freshness - + It is important to prevent replay attacks where an attacker replays + old Evidence or an old Attestation Result that is no longer correct. + To do so, some mechanism of ensuring that the Evidence and + Attestation Result are fresh, meaning that there is some degree of + assurance that they still reflect the latest state of the Attester, + and that any Attestation Result was generated using the latest + Appraisal Policy for Evidence. There is, however, always a race + condition possible in that the state of the Attester, and the + Appraisal Policy for Evidence, may change immediately after the + Evidence or Attestation Result was generated. The goal is merely to + narrow the time window to something the Verifier (for Evidence) or + Relying Party (for an Attestation Result) is willing to accept. -10. Privacy Considerations + There are two common approaches to providing some assurance of + freshness. The first approach is that a nonce is generated by a + remote entity (e.g., the Verifier for Evidence, or the Relying Party + for an Attestation Result), and the nonce is then signed and included + along with the claims in the Evidence or Attestation Result, so that + the remote entity knows that the claims were signed after the nonce + was generated. + + A second approach is to rely on synchronized clocks, and include a + signed timestamp (e.g., using [I-D.birkholz-rats-tuda]) along with + the claims in the Evidence or Attestation Result, so that the remote + entity knows that the claims were signed at that time, as long as it + has some assurance that the timestamp is correct. This typically + requires additional claims about the signer's time synchronization + mechanism in order to provide such assurance. + + In either approach, it is important to note that the actual values in + claims might have been generated long before the claims are signed. + If so, it is the signer's responsibility to ensure that the values + are still correct when they are signed. For example, values might + have been generated at boot, and then used in claims as long as the + signer can guarantee that they cannot have changed since boot. + +11. Privacy Considerations The conveyance of Evidence and the resulting Attestation Results reveal a great deal of information about the internal state of a device. In many cases, the whole point of the Attestation process is to provide reliable information about the type of the device and the firmware/software that the device is running. This information is particularly interesting to many attackers. For example, knowing that a device is running a weak version of firmware provides a way to aim attacks better. Protocols that convey Evidence or Attestation Results are responsible for detailing what kinds of information are disclosed, and to whom they are exposed. -11. Security Considerations +12. Security Considerations - + Any solution that conveys information used for security purposes, + whether such information is in the form of Evidence, Attestation + Results, or Endorsements, or Appraisal Policy, needs to support end- + to-end integrity protection and replay attack prevention, and often + also needs to support additional security protections. For example, + additional means of authentication, confidentiality, integrity, + replay, denial of service and privacy protection are needed in many + use cases. Section 10 discusses ways in which freshness can be used + in this architecture to protect against replay attacks. -12. IANA Considerations + To evaluate the security provided by a particular Appraisal Policy, + it is important to understand the strength of the Root of Trust, + e.g., whether it is mutable software, or firmware that is read-only + after boot, or immutable hardware/ROM. + + It is also important that the Appraisal Policy was itself obtained + securely. As such, if Appraisal Policy in a Relying Party or + Verifier can be configured via a network protocol, the ability to + attest to the health of the client providing the Appraisal Policy + needs to be considered. + +13. IANA Considerations This document does not require any actions by IANA. -13. Acknowledgments +14. Acknowledgments Special thanks go to David Wooten, Joerg Borchert, Hannes Tschofenig, Laurence Lundblade, Diego Lopez, Jessica Fitzgerald-McKay, Frank Xia, and Nancy Cam-Winget. -14. Contributors +15. Contributors Thomas Hardjono created older versions of the terminology section in collaboration with Ned Smith. Eric Voit provided the conceptual separation between Attestation Provision Flows and Attestation Evidence Flows. Monty Wisemen created the content structure of the first three architecture drafts. Carsten Bormann provided many of the motivational building blocks with respect to the Internet Threat Model. +16. Informative References + + [I-D.birkholz-rats-tuda] + Fuchs, A., Birkholz, H., McDonald, I., and C. Bormann, + "Time-Based Uni-Directional Attestation", Work in + Progress, Internet-Draft, draft-birkholz-rats-tuda-01, 11 + September 2019, . + + [I-D.ietf-teep-architecture] + Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler, + "Trusted Execution Environment Provisioning (TEEP) + Architecture", Work in Progress, Internet-Draft, draft- + ietf-teep-architecture-05, 12 December 2019, + . + + [OPCUA] OPC Foundation, "OPC Unified Architecture Specification, + Part 2: Security Model, Release 1.03", OPC 10000-2 , 25 + November 2015, . + + [RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token + (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015, + . + + [RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig, + "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, + May 2018, . + Authors' Addresses Henk Birkholz Fraunhofer SIT Rheinstrasse 75 64295 Darmstadt Germany + Email: henk.birkholz@sit.fraunhofer.de Dave Thaler Microsoft United States of America Email: dthaler@microsoft.com Michael Richardson Sandelman Software Works