Internet DRAFT - draft-shaw-rats-rear
draft-shaw-rats-rear
RATS A. Shaw
Internet-Draft H. Tschofenig
Intended status: Informational S. Trofimov
Expires: 14 December 2020 S. Frost
T. Fossati
arm
12 June 2020
Restful Attested Resources
draft-shaw-rats-rear-00
Abstract
This memo describes a REST interface based on the RATS architecture
that can be used to retrieve attested system state, for example the
reading of a security critical sensor. The objective is to present a
common vocabulary of data formats and basic protocol transactions
that can be pieced together into a cohesive interface that is capable
of serving different attestation workflows.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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 14 December 2020.
Copyright Notice
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
Shaw, et al. Expires 14 December 2020 [Page 1]
Internet-Draft REAR June 2020
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 . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Document Organisation . . . . . . . . . . . . . . . . . . 4
1.3. Conventions used in this document . . . . . . . . . . . . 4
2. Abstract Mechanism . . . . . . . . . . . . . . . . . . . . . 4
2.1. Attester Interface . . . . . . . . . . . . . . . . . . . 4
2.1.1. Resource Validation . . . . . . . . . . . . . . . . . 5
2.2. Verifier Interface . . . . . . . . . . . . . . . . . . . 5
2.2.1. Attestation Result Validation . . . . . . . . . . . . 6
2.3. Example Compositions . . . . . . . . . . . . . . . . . . 6
2.3.1. Background Check with Nonce-based Freshness . . . . . 6
2.3.2. Background Check with Timestamp-based Freshness . . . 7
2.3.3. Passport with Timestamp-based Freshness . . . . . . . 7
2.3.4. Timestamp-based Uni-directional . . . . . . . . . . . 8
3. REST Instantiation . . . . . . . . . . . . . . . . . . . . . 9
3.1. Basic Data Formats . . . . . . . . . . . . . . . . . . . 9
3.1.1. Resource . . . . . . . . . . . . . . . . . . . . . . 9
3.1.2. Nonce . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1.3. Timestamp . . . . . . . . . . . . . . . . . . . . . . 10
3.1.4. Evidence . . . . . . . . . . . . . . . . . . . . . . 10
3.1.5. Attestation Result . . . . . . . . . . . . . . . . . 10
3.2. Request and Response Payloads . . . . . . . . . . . . . . 10
3.2.1. Requesting an Attested Resource . . . . . . . . . . . 10
3.2.2. Attested Resource . . . . . . . . . . . . . . . . . . 10
3.2.3. Request for Attestation Result . . . . . . . . . . . 11
3.2.4. Verifier Response . . . . . . . . . . . . . . . . . . 11
3.3. Interaction Model . . . . . . . . . . . . . . . . . . . . 12
3.3.1. Channel Security Considerations . . . . . . . . . . . 12
3.3.2. URLs . . . . . . . . . . . . . . . . . . . . . . . . 12
3.3.3. Methods . . . . . . . . . . . . . . . . . . . . . . . 12
3.3.4. Multicast Support . . . . . . . . . . . . . . . . . . 13
3.3.5. Examples . . . . . . . . . . . . . . . . . . . . . . 13
4. Discovery . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.1. Resource Directory . . . . . . . . . . . . . . . . . . . 17
4.1.1. Attested Resource Registration . . . . . . . . . . . 17
4.1.2. Verifier Resource Registration . . . . . . . . . . . 19
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 20
7. Security Considerations . . . . . . . . . . . . . . . . . . . 20
7.1. Model Architecture for the Origin . . . . . . . . . . . . 20
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 20
References . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Shaw, et al. Expires 14 December 2020 [Page 2]
Internet-Draft REAR June 2020
Normative References . . . . . . . . . . . . . . . . . . . . . 20
Informative References . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
This memo describes a REST [Fielding] interface based on the RATS
architecture [I-D.ietf-rats-architecture] that can be used to
retrieve attested system state, for example the reading of a security
critical sensor.
We present a simple vocabulary of data formats and basic protocol
transactions that can be pieced together into a cohesive interface
capable of serving different attestation workflows. At a minimum, we
want to cater for the "background check" and "passport" topological
models, and for freshness of attestation based on nonces as well as
timestamps.
The obvious advantage of sharing a uniform interface across different
actors is it creates an ecosystem in which variability is minimised
and so is the need to add complex and often fragile logics into the
deployed components, e.g., data format and protocol translation.
Besides, using the familiar REST toolbox provides additional benefits
in terms of developer friendliness as well as code base and
infrastructure reuse (e.g., web caching).
1.1. Use Cases
The primary use case is that of a device that needs to provide
application state to third parties with strong authenticity.
This is a common situation in critical infrastructure systems where
an actuator device needs some assurance that the sensing equipment is
in pristine state before acting on its signals. Here, the sensor
would expose its safety critical samples via an attested resource
whose authenticity can be verified by the actuator.
Another potential application is a fleet controller that needs to
know the current state of its dependent devices to inform its next
actions (e.g., scheduling a firmware update campaign). Here, the
dependent devices uniformly expose the same resource (e.g., the list
of currently installed software components) to the controller, which
can decide, based on the information provided, which devices need a
certain security patch.
Many more use cases exist.
Shaw, et al. Expires 14 December 2020 [Page 3]
Internet-Draft REAR June 2020
1.2. Document Organisation
The remainder of this document describes:
* An abstract protocol that allows a device to expose arbitrary
attested system state, which can be consumed by third parties
(Section 2);
* An instantiation of said abstract protocol as a set of uniform
data formats and interaction primitives based on the REST paradigm
for both HTTP [RFC7230] and CoAP [RFC7252] (Section 3);
* A way to advertise and discover said capability (Section 4).
1.3. Conventions used in this document
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.
2. Abstract Mechanism
The protocol principals are the three RATS actors: the attester (A),
the relying party (RP) and the verifier (V).
It is assumed that A either directly owns a resource, r, or has a
direct trust relationship with the resource owner.
In the following, "n" and "t" are freshness indicators: "n" is an
initiator provided nonce, "t" is a timestamp sourced by the
responder. When using timestamp based freshness, producers' and
consumers' clocks MUST be synchronised.
2.1. Attester Interface
The interface to the Attester is illustrated in Figure 1.
X is any entity interacting with the Attester, typically a Relying
Party, which wants to retrieve an attested resource.
A function "E(n_X, r, t_A)" is used by A to compute an evidence
report binding the device status to the resource ("r") together with
the freshness indicators "n_X" and "t_A". Typically, only one of
"n_X" or "t_A" will be present.
"E()" outputs an EAT token [I-D.ietf-rats-eat], "E", carrying a
"nonce" claim that is used as described in the following.
Shaw, et al. Expires 14 December 2020 [Page 4]
Internet-Draft REAR June 2020
The binding between "n_X", "t_A" and "r" is obtained by hashing their
concatenation, "H(n_X || r || t_A)", and storing the result in the
"nonce" claim which is then cryptographically signed by the Attester
as part of the produced evidence, "E". The presence of any freshness
indicator (i.e., "n_X" or "t_A") is optional. For the purpose of
computing "E", a nil freshness indicator is replaced by the zero-
length string, "". If "t_A != nil", then its value needs to be sent
back to the requester as an additional explicit protocol entity.
Optionally, an attestation result "R" computed on evidence "E" MAY be
returned by an Attester that acts as a forwarder for a Verifier.
X A
| n_X=nil |
o-------------------------------------->|
| r, t_A=nil, E(n_X, r, t_A), R(E)=nil |
|<--------------------------------------o
| |
Figure 1: Attester Interface
2.1.1. Resource Validation
Given an Appraisal Policy for Evidence "APE" and an Appraisal Policy
for Attestation Result "APR", X accepts "r" if and only if:
* "E | APE => true"
* "E.nonce == H(n_X || r || t_A)"
If "R(E)!=nil", two further conditions MUST hold:
* "R(E) | APR => true"
* "R.nonce == H("" || E || "")"
Note that not all the appraisal operations are computed directly by
X. For example, "E | APE" is typically delegated to a trusted
Verifier.
2.2. Verifier Interface
The interface to the Verifier is illustrated in Figure 2.
Y is any entity interacting with the Verifier, e.g., a Relying Party
or an Attester, which supplies an evidence and receives an
attestation result.
Shaw, et al. Expires 14 December 2020 [Page 5]
Internet-Draft REAR June 2020
The function "R(n_Y, E, t_V)" is used by V to compute the attestation
result over "E" using an implicit Appraisal Policy for Evidence
"APE". The result is cryptographically signed by V and bound to any
available freshness indicator.
"R()" outputs an EAT token [I-D.ietf-rats-eat], "R", carrying at a
minimum:
* a "result" claim carrying a boolean value that reflects the
validity of the submitted evidence given the Appraisal Policy for
Evidence used by the Verifier;
* a "nonce" claim that is used as described in the following.
The token MAY contain further information associated with the
evidence validation process.
The binding between "n_Y", "t_V" and "E" is obtained by hashing their
concatenation, "H(n_Y || E || t_V)", and storing the result in the
"nonce" claim which is then cryptographically signed by the Verifier
as part of the produced attestation result, "R". The presence of any
freshness indicator (i.e., "n_Y" or "t_V") is optional. For the
purpose of computing "R", a nil freshness indicator is replaced by
the zero-length string, "".
Y V
| n_Y=nil, E |
o----------------------------->|
| t_V=nil, R(n_Y, E, t_V) |
|<-----------------------------o
| |
Figure 2: Verifier Interface
2.2.1. Attestation Result Validation
Given an Appraisal Policy for Attestation Result "APR", Y accepts "R"
if and only if:
* "R(E) | APR => true"
* "R.nonce == H(n_Y || E || t_V)"
2.3. Example Compositions
2.3.1. Background Check with Nonce-based Freshness
Shaw, et al. Expires 14 December 2020 [Page 6]
Internet-Draft REAR June 2020
A RP V
| n_X | |
|<-----------------------------o |
| r, E(n_X, r, nil) | |
o----------------------------->| |
| | E |
| o----------------------------->|
| | R(nil, E, nil) |
| |<-----------------------------o
| | |
Figure 3: Background Check with Nonce-based Freshness
RP accepts "r" if and only if:
* "E | APE => true"
* "E.nonce == H(n_X || r || "")"
* "R | APR => true", or equivalently "R.result == true"
* "R.nonce == H("" || E || "")"
2.3.2. Background Check with Timestamp-based Freshness
A RP V
| nil | |
|<-----------------------------o |
| r, t_A, E(nil, r, t_A) | |
o----------------------------->| |
| | E |
| o----------------------------->|
| | R(nil, E, nil) |
| |<-----------------------------o
| | |
Figure 4: Background Check with Timestamp-based Freshness
RP accepts r if and only if:
* "R | APR => true", or equivalently "R.result == true"
* "R.nonce == H("" || E || "")"
* "E | APE => true"
* "E.nonce == H("" || r || t_A)"
2.3.3. Passport with Timestamp-based Freshness
The idea is that whenever the state of r changes, the Attester will
"self-issue" an evidence for the changed resource using a locally
sourced timestamp ("t_A") as the freshness indicator.
Shaw, et al. Expires 14 December 2020 [Page 7]
Internet-Draft REAR June 2020
A
.--o
| |
'->o--.
| | r, t_A, E(nil, r, t_A)
|<-'
| V
| E |
o------------------------------------------------------------>|
| R(nil, E, nil) |
|<------------------------------------------------------------o
| |
| RP
| nil |
|<-----------------------------o
| r, t_A, E(nil, r, t_A), |
| R(nil, E, nil) |
o----------------------------->|
Figure 5: Passport with Timestamp-based Freshness
RP accepts r if and only if:
* "R | APR => true"
* "R.nonce == H("" || E || "")"
* "E.nonce == H("" || r || t_A)"
2.3.4. Timestamp-based Uni-directional
If the transport allows it, timestamp-based uni-directional
attestation protocols, e.g., TUDA [I-D.birkholz-rats-tuda], can also
be constructed from the presented primitives. For example, using
CoAP Observe [RFC7641] the interaction pattern in Figure 6, with an
initial trigger and subsequent automatic updates on resource status
change, can be naturally implemented.
Shaw, et al. Expires 14 December 2020 [Page 8]
Internet-Draft REAR June 2020
A RP
| nil |
|<-----------------------------o
| r_1, t_A, E(nil, r_1, t_A) |
o----------------------------->|
| |
| [...] |
| |
| r_i, t_A, E(nil, r_i, t_A) |
o----------------------------->|
| |
| [...] |
| |
| r_n, t_A, E(nil, r_n, t_A) |
o----------------------------->|
Figure 6: Timestamp-based Uni-directional
3. REST Instantiation
Four new MIME types are defined for the requests and responses among
the three actors that have been identified in the abstract mechanism.
The MIME types are composed of the basic data types defined in
Section 3.1.
3.1. Basic Data Formats
* The resource to be attested;
* A caller provided nonce;
* A locally sourced timestamp;
* The evidence produced by the Attester, and
* The attestation result produced by the Verifier.
These basic types are described by the following CDDL rules, which
reuse the eat-token definition from [I-D.ietf-rats-eat].
3.1.1. Resource
An "ANY DEFINED BY"-like payload with type set to the original MIME
type, either Content-Type (HTTP) or Content-Format (CoAP), of the
resource representation.
resource-type = (
typ tstr / uint,
val any,
)
Shaw, et al. Expires 14 December 2020 [Page 9]
Internet-Draft REAR June 2020
3.1.2. Nonce
nonce-type = bstr
3.1.3. Timestamp
timestamp-type = tdate / time
3.1.4. Evidence
An EAT token signed by the attester bound to the relying party
request and the attested resource state.
evidence-type = eat-token
3.1.5. Attestation Result
An EAT token signed by the verifier and bound to an evidence.
attestation-result-type = eat-token
3.2. Request and Response Payloads
3.2.1. Requesting an Attested Resource
MIME type "application/rats-attested-resource-request"
CoAP Content-Format: TBD-rats-attested-resource-request-CT
nonce-key = 0 / "n_X"
attested-resource-request = {
? nonce-key => nonce-type,
}
This type is used in a POST request to an attested resource.
3.2.2. Attested Resource
MIME type "application/rats-attested-resource"
CoAP Content-Format: TBD-rats-attested-resource-CT
Shaw, et al. Expires 14 December 2020 [Page 10]
Internet-Draft REAR June 2020
resource-key = 1 / "r"
t-A-key = 2 / "t_A"
evidence-key = 3 / "E"
attestation-result-key = 4 / "R"
attested-resource = {
resource-key => resource-type,
? t-A-key => timestamp-type,
evidence-key => evidence-type,
? attestation-result-key => attestation-result-type,
}
This type is used in a successful response to a request to an
attested resource endpoint.
Note that an attestation result is only present when the Passport
model is used.
Note also that the fact that the inner resource representation is
embedded within the "application/rats-attested-resource" envelope
suppresses the ability to do content negotiation on it, i.e., the
inner representation format is unilaterally chosen by the origin.
3.2.3. Request for Attestation Result
MIME type "application/rats-attestation-result-request"
CoAP Content-Format: TBD-rats-attestation-result-request-CT
n-Y-key = 5 / "n_Y"
attestation-result-request = {
? n-Y-key => nonce-type,
evidence-key => evidence-type,
}
This type is used in a POST request to a verifier endpoint.
3.2.4. Verifier Response
MIME type "application/rats-attestation-result-response"
CoAP Content-Format: TBD-rats-attestation-result-response-CT
Shaw, et al. Expires 14 December 2020 [Page 11]
Internet-Draft REAR June 2020
t-V-key = 6 / "n_Y"
attestation-result-response = {
? t-V-key => timestamp-type,
attestation-result-key => attestation-result-type,
}
This type is used in a successful response to a POST request to a
verifier endpoint.
3.3. Interaction Model
(For now) we only describe a synchronous, RPC-like transaction model,
including the slight variant with a one-off trigger presented in
Section 2.3.4.
This might be not suited for devices that sit behind a NAT/firewall
box, or those that have to go through extended sleep cycles in order
to save energy. For this kind of devices, we assume in-network
support in the form of store-and-forward nodes (e.g., LwM2M queue
mode, specialised border routers, etc.).
3.3.1. Channel Security Considerations
Unless the channel can be considered free from passive and active
attackers at all times, all transactions are to be carried over a
secure transport (i.e., HTTPS or COAPS).
3.3.2. URLs
In the spirit of [RFC7320], no specific URL format is mandated. An
application is free to specify the URL scheme of its liking for the
exposed attested resources.
When an origin exposes the same underlying state both as nonce- and
timestamp-based resources, these are identified by two separate URIs.
The verifier function is exposed via an URI that accepts evidence in
form of "application/rats-attestation-result-request" typed requests
and returns attestation results in form of "application/rats-
attestation-result-response" typed responses.
3.3.3. Methods
As per usual REST conventions, the guiding principles are:
* POST is used for all requests involving a payload;
* GET is used for requests without a payload.
Shaw, et al. Expires 14 December 2020 [Page 12]
Internet-Draft REAR June 2020
The only example of the latter is when retrieving an "Attested
Resource" using the timestamp-based freshness model. Any other
request uses POST.
3.3.3.1. Response Codes and Caching
The possible status codes are:
* HTTP
- 200 (OK) for successful GET. This response is cacheable;
origins can use Cache-Control (max-age) and ETag headers in
order to instruct on-path caches.
- 201 (Created) for a successful POST. This response is not
cacheable.
* CoAP
- 2.05 (Content) for successful GET. This response is cacheable;
origins can use Max-Age and ETag Options to instruct on-path
caches;
- 2.01 (Created) for successful POST. This response is not
cacheable.
Otherwise, a suitable error response (i.e., HTTP 4xx/5xx, CoAP
4.nn/5.nn) is returned.
3.3.4. Multicast Support
TODO (This is a CoAP only feature.)
3.3.5. Examples
A few examples are given to illustrate the different interaction
models using both CoAP and HTTP transports.
3.3.5.1. Background Check with Nonce Based Freshness
* RP - Attester (CoAP)
Shaw, et al. Expires 14 December 2020 [Page 13]
Internet-Draft REAR June 2020
>> Request:
POST coap://device.example/my-attested-resource
Content-Format: TBD-application/rats-attested-resource-request-CT
Accept: application/rats-attested-resource
Payload:
{
"n_X": "bm9uY2Uh"
}
<< Response:
2.01 Created
ETag: "xyzzy"
Content-format: TBD-application/rats-attested-resource-CT
Payload:
{
"r" : {
"typ": "text/plain",
"val": "foobar"
},
"E": "eyJhbGciO...RfrKmTWk"
}
* RP - Verifier (HTTP)
>> Request:
POST /my-verify
Host: verifier.example
Content-Type: application/rats-attestation-result-request
Accept: application/rats-attestation-result-response
{
"E": "eyJhbGciO...RfrKmTWk"
}
<< Response:
HTTP/1.1 201 Created
ETag: "abccb"
Content-format: application/rats-attestation-result-response
Payload:
{
"R": "eyJhbGciO...8j5EDGYc"
}
3.3.5.2. Background Check with Timestamp Based Freshness
* RP - Attester (CoAP) with POST
Shaw, et al. Expires 14 December 2020 [Page 14]
Internet-Draft REAR June 2020
>> Request:
POST coap://device.example/my-attested-resource
Content-Format: TBD-application/rats-attested-resource-request-CT
Accept: TBD-application/rats-attested-resource-CT
Payload:
{ }
<< Response:
2.01 Created
ETag: "xyzzy"
Content-format: TBD-application/rats-attested-resource-CT
Payload:
{
"r" : {
"typ": "text/plain",
"val": "foobar"
},
"t_A": "2020-04-01T21:02:31Z",
"E": "eyJhbGciO...z0ikw9Aa"
}
* RP - Attester (CoAP) with GET
>> Request:
GET coap://device.example/my-attested-resource
Accept: TBD-application/rats-attested-resource-CT
<< Response:
2.05 Content
ETag: "xyzzy"
Max-Age: 3600
Content-format: TBD-application/rats-attested-resource-CT
Payload:
{
"r" : {
"typ": "text/plain",
"val": "foobar"
},
"t_A": "2020-04-01T21:02:31Z",
"E": "eyJhbGciO...z0ikw9Aa"
}
* RP - Verifier (HTTP) is the same as Section 3.3.5.1.
3.3.5.3. Passport Model
* Attester - Verifier (CoAP)
Shaw, et al. Expires 14 December 2020 [Page 15]
Internet-Draft REAR June 2020
>> Request:
POST coap://verifier.example/my-verify
Content-Format: application/rats-attestation-result-request
Accept: application/rats-attestation-result-response
Payload:
{
"E": "eyJhbGciO...RfrKmTWk"
}
<< Response:
2.01 Created
ETag: "jkllk"
Content-format: application/rats-attestation-result-response
Payload:
{
"R": "eyJhbGciO...Z0IKW9aA"
}
* Relying Party - Attester (CoAP) with POST
>> Request:
POST coap://device.example/my-attested-resource
Content-Format: TBD-application/rats-attested-resource-request-CT
Accept: TBD-application/rats-attested-resource-CT
Payload:
{ }
<< Response:
2.01 Created
ETag: "qwerty"
Content-format: TBD-application/rats-attested-resource-CT
Payload:
{
"r": {
"type": "text/plain",
"val": "foobar"
},
"t_A": "2020-04-01T21:02:31Z",
"E": "eyJhbGciO...RfrKmTWk",
"R": "eyJhbGciO...Z0IKW9aA"
}
* Relying Party - Attester (CoAP) with GET
Shaw, et al. Expires 14 December 2020 [Page 16]
Internet-Draft REAR June 2020
>> Request:
GET coap://device.example/my-attested-resource
Accept: TBD-application/rats-attested-resource-CT
<< Response:
2.05 Content
ETag: "qwerty"
Max-Age: 3600
Content-format: TBD-application/rats-attested-resource-CT
Payload:
{
"r": {
"type": "text/plain",
"val": "foobar"
},
"t_A": "2020-04-01T21:02:31Z",
"E": "eyJhbGciO...RfrKmTWk",
"R": "eyJhbGciO...Z0IKW9aA"
}
4. Discovery
4.1. Resource Directory
The following describes the new link format attribute values needed
for registering attested resources as well as verification endpoints
to a Resource Directory [I-D.ietf-core-resource-directory].
The same attribute values can be used by RD clients to discover
attestation related resources.
4.1.1. Attested Resource Registration
An attested resource is registered with:
* an interface description (if=) with value "rats.if.timestamp" or
"rats.if.nonce" depending on the supported freshness model, which
determines the access method (i.e., POST+nonce vs GET);
* a content format (ct=) with value "TBD-application/rats-attested-
resource-CT";
* an inner content format (ict=) that reflects the "type" field of
the returned "resource";
* a resource type (rt=) that reflects the nature of the inner
resource.
Shaw, et al. Expires 14 December 2020 [Page 17]
Internet-Draft REAR June 2020
If a resource has both a "plain" and an "attested" variant, then the
link value corresponding to the "attested" resource can be associated
to its "plain" twin by means of the link relationship "attested-
variant".
TBD: Should we have rats.if.timestamp variants for GET and POST?
Alternative includes: 1) let the client probe and server return
405/4.05 if the requested variant is not supported; 2) add another
attribute that explicitly states which request methods are supported.
4.1.1.1. Examples
The following example shows a registrant endpoint with the name
"node1" registering an attested heart rate sensor resource to an RD.
The location /rd is an example RD location discovered in a previous
.well-known/core query.
>> Request:
POST /rd?ep=node1 HTTP/1.1
Host: rd.example
Content-Type: application/link-format
</sensors/attested-heartrate>;
if="rats.if.timestamp";
rt="heart-rate-zoladz";
ct=TBD-application/rats-attested-resource-CT;
ict=0
<< Response:
HTTP/1.1 201 Created
Location: /rd/4520
The following example shows a registrant endpoint with the name
"node1" registering a temperature sensor resource along with its
attested twin to an RD.
The "attested-variant" link relation establishes the semantics of the
link between /sensors/temp and /sensors/attested-temp: the latter
being an attested version of the former. Note, in particular, that
the resource type (rt=) of the linked resource is inherited by the
attested twin. Missing an explicit inner content format (ict=) the
content type of the inner resource representation can be assumed to
be that of the linked resource. The interface description (if=)
"rats.if.nonce" says that the access to the attested resource happens
by supplying a nonce through a POST.
Shaw, et al. Expires 14 December 2020 [Page 18]
Internet-Draft REAR June 2020
>> Request:
POST /rd?ep=node1 HTTP/1.1
Host: rd.example
Content-Type: application/link-format
</sensors/temp>;
ct=41;
rt="temperature-c";
if="sensor",
</sensors/attested-temp>;
anchor="/sensors/temp";
rel="attested-variant";
if="rats.if.nonce";
ct=TBD-application/rats-attested-resource-CT;
ict=41
<< Response:
HTTP/1.1 201 Created
Location: /rd/4521
4.1.2. Verifier Resource Registration
A Verifier resource is registered with:
* An "rt" with value "rats.verifier";
* A "ct" with value "TBD-application/rats-attestation-result-
response-CT"
4.1.2.1. Examples
>> Request:
POST /rd?ep=node1 HTTP/1.1
Host: rd.example
Content-Type: application/link-format
</my-verifier>;
ct=application/rats-attestation-result-response;
rt="rats.verifier"
<< Response:
HTTP/1.1 201 Created
Location: /rd/4522
5. IANA Considerations
TODO
Shaw, et al. Expires 14 December 2020 [Page 19]
Internet-Draft REAR June 2020
6. Privacy Considerations
TODO
7. Security Considerations
7.1. Model Architecture for the Origin
The model architecture for the origin of the attested resource is
illustrated in Figure 7. The REST client (an user agent of a relying
party or verifier) interfaces directly with a REST front-end (a CoAP
or HTTP server stack) running in the Rich Execution Environment
(REE), for example a Linux operating system. The REST front-end is
paired with a back-end Trusted Application (TA) running in the
Trusted Execution Environment (TEE). The TA has exclusive control
over some "resource" (e.g., a sensor that feeds back into some kind
of critical infrastructure control system) and can talk to the
attestation service hosted inside the TEE to request EAT tokens.
In this model, it is critical that the attestation service can only
be used by the intended TA or, failing that, that the identity of the
calling TA can be securely proved to the relying party or verifier.
An example of the latter is the Client ID claim used in PSA
attestation [I-D.tschofenig-rats-psa-token].
.-----------------.---------------------------.
| REE | TEE |
.--------. | .-----------. | .----------. .------. |
| REST |<--->| REST |<--->| back-end +->|resource| |
| client | | | front-end | | | TA | '------' |
'--------' | '-----------' | '-----+----' |
| | | |
| | v |
| | .-------------. |
| | | attestation | |
| | | service | |
| | '-------------' |
'-----------------'---------------------------'
Figure 7: Model Security Architecture
Acknowledgments
TBD
References
Normative References
Shaw, et al. Expires 14 December 2020 [Page 20]
Internet-Draft REAR June 2020
[I-D.ietf-core-resource-directory]
Shelby, Z., Koster, M., Bormann, C., Stok, P., and C.
Amsuess, "CoRE Resource Directory", Work in Progress,
Internet-Draft, draft-ietf-core-resource-directory-24, 9
March 2020, <http://www.ietf.org/internet-drafts/draft-
ietf-core-resource-directory-24.txt>.
[I-D.ietf-rats-architecture]
Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
W. Pan, "Remote Attestation Procedures Architecture", Work
in Progress, Internet-Draft, draft-ietf-rats-architecture-
04, 21 May 2020, <http://www.ietf.org/internet-drafts/
draft-ietf-rats-architecture-04.txt>.
[I-D.ietf-rats-eat]
Mandyam, G., Lundblade, L., Ballesteros, M., and J.
O'Donoghue, "The Entity Attestation Token (EAT)", Work in
Progress, Internet-Draft, draft-ietf-rats-eat-03, 20
February 2020, <http://www.ietf.org/internet-drafts/draft-
ietf-rats-eat-03.txt>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC7320] Nottingham, M., "URI Design and Ownership", BCP 190,
RFC 7320, DOI 10.17487/RFC7320, July 2014,
<https://www.rfc-editor.org/info/rfc7320>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
Shaw, et al. Expires 14 December 2020 [Page 21]
Internet-Draft REAR June 2020
Informative References
[Fielding] Fielding, R., "Architectural Styles and the Design of
Network-based Software Architectures", Ph.D. Dissertation,
University of California, Irvine, 2000,
<http://www.ics.uci.edu/~fielding/pubs/dissertation/
fielding_dissertation.pdf>.
[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-02, 9
March 2020, <http://www.ietf.org/internet-drafts/draft-
birkholz-rats-tuda-02.txt>.
[I-D.tschofenig-rats-psa-token]
Tschofenig, H., Frost, S., Brossard, M., Shaw, A., and T.
Fossati, "Arm's Platform Security Architecture (PSA)
Attestation Token", Work in Progress, Internet-Draft,
draft-tschofenig-rats-psa-token-05, 6 March 2020,
<http://www.ietf.org/internet-drafts/draft-tschofenig-
rats-psa-token-05.txt>.
Authors' Addresses
Adrian Shaw
arm
Email: Adrian.Shaw@arm.com
Hannes Tschofenig
arm
Email: Hannes.Tschofenig@arm.com
Sergei Trofimov
arm
Email: Sergei.Trofimov@arm.com
Simon Frost
arm
Email: Simon.Frost@arm.com
Shaw, et al. Expires 14 December 2020 [Page 22]
Internet-Draft REAR June 2020
Thomas Fossati
arm
Email: Thomas.Fossati@arm.com
Shaw, et al. Expires 14 December 2020 [Page 23]
