Reference Integrity Measurement Extension for Concise Software Identities
draft-birkholz-rats-coswid-rim-00

Abstract

Concise Software Identification (CoSWID) tags identify and describe individual software components, patches, and installation bundles. CoSWID is based on ISO/IEC 19770-2:2015 2:2015 that provides a complementary XML schema definition (XSD) for Software Identification (SWID) tags. CoSWID supports the same features as the corresponding XML SWID tags. The CoSWID specification also includes more structured extensibility features and reduces a few of ambiguities that are not explicitly resolved in the ISO XSD. In this document, these extensibility features (extension points) are used to add attributes to the CoSWID specification. The new attributes allow for the use of CoSWID as Reference Integrity Measurements (RIM). There are three set of RIM features defined in this specification. 1.) attributes that support RIM manifests for Measured Boot, 2.) attributes that support package manager managed structures, and 3.) attributes that allow for OID to be used in the description of Reference Integrity Measurements.

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 September 10, 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 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
• 1.1. Requirements Notation
• 2. CoSWID Attribute Extensions
• 2.1. RIM requirements on existing CoSWID attributes
• 2.2. RIM extensions for Measured Boot
• 2.3. RIM extension for Software Package Management
• 2.4. Additional Measurement Attributes for CoSWID
• 2.5. CoSWID RIM CDDL
• 3. Privacy Considerations
• 4. Security Considerations
• 5. References
• 5.1. Normative References
• 5.2. Informative References
• Authors' Addresses

1. Introduction

Reference Integrity Measurements describe the intended state of (composite) software components installed on a (composite) device. A measurement of all installed software components of a devices allows for assertions about the trustworthiness of the given device. In combination with a root of trust (RoT) for reporting (RTM), these measurements can be refined into evidence and enable IETF Remote Attestation ProcedureS (RATS). RATS support the decision process of whether to put trust in the trustworthiness of a device - or not.

The RATS architecture [I-D.ietf-rats-architecture] defines the roles Verifiers, Attesters, Endorsers, and Relying Parties. The RATS architecture also specifies that Attestation Evidence is created by Attesters and consumed by Verifiers. Ultimately, the goal is to enable a Relying Party to put trust in the trustworthiness of a remote peer (the Attester). Attestation Evidence is composed of believable assertions about an Attester’s trustworthiness characteristics. In RATS, these assertions are called Claims. The Verifier conducts a set of appraisal procedures in order to assess the compliance of an Attester’s trustworthiness characteristics.

The most prominent appraisal procedure in RATS is the comparison of Claim values included in Attestation Evidence with Reference Claim values provided by Endorsers (e.g. supply chain entities). The comparison of Claim values via Reference Claim values is vital for the assessment of compliance metrics with respect to software components installed on an Attester. A typical objective here is the remediation of already vulnerabilities discovered in certain versions of software components.

The Integrity Measurement Architecture (IMA) of the Linux Security Modules (LSM) provides a detailed Event Log (sometimes also referred to as a Measurement Log) that retains a sequence of hash measurements of every software sub-component (e.g. a firmware, an ELF executable, or a configuration file) that is created and appended to the sequence of measurements that composes the event log before the software component in question is started or read.

In essence, to enable this appraisal procedure conducted by Verfiers an Attester’s IMA provides Event Logs that include the hash values of every started software component and therefore are part of the Attestation Evidence an Attester creates. The complementary well-known-values that Verifiers require are included in the Reference Integrity Measurements (RIM). RIMs can be provided via Software Identification tags created by Endorsers, such as the software creators, manufacturers, vendors, or other trusted third parties (e.g. supply chain or certification entities).

This document provides an extension to the CoSWID specification defined in [I-D.ietf-sacm-coswid]. The extension adds attributes to CoSWID tags that enable them to express RIMs. A vital subset of these attributes are illustrated in the TCG Reference Integrity Manifest Information Model [ref] specification. These attributes are added to the existing CoSWID specification via the most general extension point the CoSWID specification provides: $$coswid-extensions. An additional map definition named “reference-measurements” is added and is defined in section [ref] of this document.

Furthermore, a usage profile for signed CoSWID tags is defined in this specification in support of the software-component structure of systems managed by package managers. Signed CoSWID tags that are aligned with that software model can be used to describe the contents of one or multiple of the packages that make up the contents of a system. In this case, it may be that multiple CoSWID tags are provided as Reference Claim values for a attestation by a single Attester. In order to minimize the impact on the sizes of packages, it is likely that any CoSWID tags delivered as part of packages as part of a package manager managed system will not contain actual reference values, but instead a link-entry to a CoSWID tag published by the vendor in a repository.

Reference Integrity Measurements provide a vital resource to be consumed by … TBD

1.1. Requirements Notation

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. CoSWID Attribute Extensions

This specification defines three types of attribute sets that can be added to the CoSWID specification via its defined extension points.

1. Attributes that support RIM manifests for Measured Boot (often still referred to as Secure Boot),
2. Attributes that support the RPM package manager structure, and
3. Attributes that allow for OID to be used in the description of Reference Integrity Measurements.

2.1. RIM requirements on existing CoSWID attributes

As defined by NIST IR 8060 [ref], there are required “Meta Attributes” for XML SWID tags that have to be included in a SWID tag in order to compose a valid SWID RIM. In this section, these attributes are mapped to CoSWID attributes and corresponding requirements on attributes defined in the CoSWID specification to compose valid NIST IR 8060 signed Payload content in the Concise Software Identity Reference Integrity Measurement (CoSWID RIM) representation.

The ‘software-meta-entry’ type defined in the CoSWID specification includes the optional members ‘product’, ‘colloquial-version’, ‘revision’, and ‘edition’. These four members MUST be included in a CoSWID RIM in order to compose a valid Reference Integrity Measurement in alignment with NIST IR 8060. Furthermore, the semantics of the text (tstr) typed values MUST convey content that allows for semantic interoperability in a given scope (e.g. an administrative domain). The software-meta-entries provide vital support for steering decisions made by the RATS verifier role in order to enable discovery and matching of related or additional CoSWID RIM available to or discoverable by the verifier.

2.2. RIM extensions for Measured Boot

The following attributes are derived from the TCG Reference Integrity Manifest Information Model [ref] specification. These attributes support the creation of very small CoSWID RIM tags that enable the Remote Integrity Verification (RIV [I-D.fedorkow-rats-network-device-attestation]) of small things, i.e. constrained devices in constrained network environments. In consequence, the majority of the attributes listed in this section represent metadata about firmware and supply chain entities that provide firmware for a device (platform). Analogously to the mandated software-meta-entries illustrated above, the attributes defined in the table below provide more context and enable steering decisions for the appraisal procedures of a Verifier. Consecutively, RIM have to be managed and curated in a consistent manner so that there is no significant threshold for a Verifier to make use of them during an appraisal procedure.

The design of the additional RIM attributes in this section is motivated by the vast variety of identifier types used in production today, e.g. endorsement documents [I-D.ietf-rats-architecture] that are enrolled or on-boarded on the Attester itself. It is vital to highlight that this variety can render semantic self-descriptiveness more difficult. Most importantly though: interoperability beats self-descriptiveness. A convergence towards a common identification scheme with respect to software components and its subset that is firmware is highly encouraged - alas not achieved at the time of creating this proposed standard. The following table defines the semantics of the set of new members that are added via the reference-measurement-entry map. The reference-measurement-entry map is added using the $$coswid-extension CDDL extension point.

2.3. RIM extension for Software Package Management

To enable very small CoSWID tags that basically are signed references to full Base RIMs for each software package that ultimately include all the hash values required by the appraisal procedure of a Verifier, the member rim-reference is added using the $$payload-extension CDDL extension point.

2.4. Additional Measurement Attributes for CoSWID

[I-D.ietf-sacm-coswid] specifies a well-defined file hash structure for integrity measurements of individual files in a file-system. This documents extends this feature to all additional members of the resource-collection group: ‘directory’, ‘resource’, and ‘process’. These new well-defined measurement options extend the scope of RIM to (sub-)trees of files, software running in memory (e.g. available from Trusted Execution Environments, such as SGX enclaves), and even external resources, such as remote services of collections of RIM bundles.

2.5. CoSWID RIM CDDL

The following CDDL specification uses the existing CDDL extension points as defined in [I-D.ietf-sacm-coswid]:

• $$coswid-extension
• $$payload-extension
• $$directory-extension
• $$resource-extension
• $$process-extension

<CODE BEGINS>
$$coswid-extension //= (reference-measurement => reference-measurement-entry)

reference-measurement-entry = {
  ? payload-type => direct / indirect / hybrid,
  ? platform-configuration-uri-global => any-uri,
  ? platform-configuration-uri-local => any-uri,
  binding-spec-name => text,
  binding-spec-version => text,
  platform-manufacturer-id => uint,
  platform-manufacturer-name => text,
  platform-model-name => text,
  ? platform-version => uint,
  ? firmware-manufacturer-id => uint,
  ? firmware-manufacturer-name => text,
  ? firmware-model-name => text,
  ? firmware-version => uint,
  rim-link-hash => bytes,
}

$$payload-extension //= ( ? support-rim-type-kramdown => direct / indirect, )
$$payload-extension //= ( ? support-rim-format => text, )
$$payload-extension //= ( ? support-rim-uri-global => any-uri, )
$$payload-extension //= ( ? rim-reference => any-uri, )

reference-measurement = 58
payload-type = 59
payload-rim = 60
platform-configuration-uri-global = 61
platform-configuration-uri-local = 62
binding-spec-name = 63
binding-spec-version = 64
platform-manufacturer-id = 65
platform-manufacturer-name = 66
platform-model-name = 67
platform-version = 68
firmware-manufacturer-id = 69
firmware-manufacturer-name = 70
firmware-model-name = 71
firmware-version = 72
rim-link-hash = 73
support-rim-type-kramdown = 74
support-rim-format = 75
support-rim-uri-global = 76
rim-reference = 77

direct = 0
indirect = 1
hybrid = 2

$$directory-extension //= ( ? hash => hash-entry )

$$resource-extension //= ( ? hash => hash-entry )

$$process-extension //= ( ? hash => hash-entry )
<CODE ENDS>

3. Privacy Considerations

TBD

4. Security Considerations

To be elaborated on

5. References

5.1. Normative References

5.2. Informative References

Authors' Addresses