SCION Control-Plane PKI
draft-dekater-scion-pki-05
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Corine de Kater , Nicola Rustignoli , Samuel Hitz | ||
| Last updated | 2024-03-05 (Latest revision 2024-03-04) | ||
| RFC stream | Independent Submission | ||
| Intended RFC status | Informational | ||
| Formats | |||
| Additional resources |
Related Implementations
Related Implementations |
||
| Stream | ISE state | Submission Received | |
| Consensus boilerplate | Unknown | ||
| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-dekater-scion-pki-05
Network Working Group C. de Kater
Internet-Draft N. Rustignoli
Intended status: Informational SCION Association
Expires: 5 September 2024 S. Hitz
Anapaya Systems
4 March 2024
SCION Control-Plane PKI
draft-dekater-scion-pki-05
Abstract
This document presents the trust concept and design of the SCION
_control-plane Public Key Infrastructure (PKI)_, SCION's public key
infrastructure model. SCION (Scalability, Control, and Isolation On
Next-generation networks) is a path-aware, inter-domain network
architecture. The control-plane PKI, or short CP-PKI, handles
cryptographic material and lays the foundation for the authentication
procedures in SCION. It is used by SCION's control plane to
authenticate and verify path information, and builds the basis for
SCION's special trust model based on so-called Isolation Domains.
This document first introduces the trust model behind the SCION's
control-plane PKI, as well as clarifications to the concepts used in
it. This is followed by specifications of the different types of
certificates and the Trust Root Configuration. The document then
specifies how to deploy the whole infrastructure.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://scionassociation.github.io/scion-cppki_I-D/draft-dekater-
scion-pki.html. Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-dekater-scion-pki/.
Source for this draft and an issue tracker can be found at
https://github.com/scionassociation/scion-cppki_I-D.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
de Kater, et al. Expires 5 September 2024 [Page 1]
Internet-Draft SCION CP-PKI March 2024
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 5 September 2024.
Copyright Notice
Copyright (c) 2024 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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Conventions and Definitions . . . . . . . . . . . . . . . 5
1.3. Trust Model . . . . . . . . . . . . . . . . . . . . . . . 6
1.4. Trust Relations within an Isolation Domain . . . . . . . 7
1.4.1. Updates and Trust Resets . . . . . . . . . . . . . . 8
1.4.2. Substitutes to Certificate Revocation . . . . . . . . 8
1.5. Overview of Certificates, Keys, and Roles . . . . . . . . 8
1.6. Trust as a Function . . . . . . . . . . . . . . . . . . . 10
1.6.1. Input . . . . . . . . . . . . . . . . . . . . . . . . 10
1.6.2. Output . . . . . . . . . . . . . . . . . . . . . . . 11
2. Certificate Specification . . . . . . . . . . . . . . . . . . 11
2.1. SCION Control-Plane PKI Keys and Certificates -
Overview . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1.1. Trust Hierarchy . . . . . . . . . . . . . . . . . . . 12
2.1.2. Control-Plane Root Certificate . . . . . . . . . . . 12
2.1.3. Control-Plane CA Certificate . . . . . . . . . . . . 13
2.1.4. Control-Plane AS Certificate . . . . . . . . . . . . 13
2.1.5. Voting Certificates . . . . . . . . . . . . . . . . . 13
2.1.6. Certificates - Formal Overview . . . . . . . . . . . 15
2.2. Certificate Specification . . . . . . . . . . . . . . . . 18
2.2.1. Basic Fields: SCION-Specific Constraints and
Conditions . . . . . . . . . . . . . . . . . . . . . 18
de Kater, et al. Expires 5 September 2024 [Page 2]
Internet-Draft SCION CP-PKI March 2024
2.2.2. Extensions . . . . . . . . . . . . . . . . . . . . . 22
3. Trust Root Configuration Specification . . . . . . . . . . . 29
3.1. TRC Specification . . . . . . . . . . . . . . . . . . . . 30
3.1.1. TRC Types and States . . . . . . . . . . . . . . . . 30
3.1.2. TRC Format . . . . . . . . . . . . . . . . . . . . . 33
3.1.3. TRC Signature Syntax . . . . . . . . . . . . . . . . 41
3.1.4. Certification Path - Trust Anchor Pool . . . . . . . 43
3.1.5. TRC Updates . . . . . . . . . . . . . . . . . . . . 45
3.2. Initial TRC Signing Ceremony . . . . . . . . . . . . . . 50
4. Deploying the CP PKI - Specifications . . . . . . . . . . . . 51
4.1. Deploying a TRC . . . . . . . . . . . . . . . . . . . . . 51
4.1.1. Base TRC . . . . . . . . . . . . . . . . . . . . . . 51
4.1.2. TRC Update . . . . . . . . . . . . . . . . . . . . . 51
4.2. Signing and Verifying Control-Plane Messages . . . . . . 52
4.2.1. Signing a Control-Plane Message . . . . . . . . . . 52
4.2.2. Verifying a Control-Plane Message . . . . . . . . . 53
4.3. Creating a New Control-Plane AS Certificate . . . . . . . 54
5. Security Considerations . . . . . . . . . . . . . . . . . . . 55
5.1. Dependency on Certificates . . . . . . . . . . . . . . . 55
5.1.1. Compromise of an ISD . . . . . . . . . . . . . . . . 56
5.1.2. Recovery from Compromise . . . . . . . . . . . . . . 56
5.2. Denial of Service Attacks . . . . . . . . . . . . . . . . 57
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 58
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 58
7.1. Normative References . . . . . . . . . . . . . . . . . . 58
7.2. Informative References . . . . . . . . . . . . . . . . . 59
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 59
Appendix A. Signing Ceremony Initial TRC . . . . . . . . . . . . 60
Ceremony Participants . . . . . . . . . . . . . . . . . . . . 60
Ceremony Preparations . . . . . . . . . . . . . . . . . . . . . 61
Location . . . . . . . . . . . . . . . . . . . . . . . . . . 61
Devices . . . . . . . . . . . . . . . . . . . . . . . . . . 61
Preparation Steps . . . . . . . . . . . . . . . . . . . . . . 62
Ceremony Process . . . . . . . . . . . . . . . . . . . . . . . 63
Phase 1: Certificate Exchange . . . . . . . . . . . . . . . 64
Phase 2: Generation of the TRC Payload . . . . . . . . . . . 65
Phase 3: TRC Signing . . . . . . . . . . . . . . . . . . . . 66
Phase 4: TRC Validation . . . . . . . . . . . . . . . . . . 66
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 66
1. Introduction
The control-plane PKI (CP-PKI) lays the foundation for the
authentication procedures in SCION. It handles all cryptographic
material used in the public key infrastructure of SCION's control
plane. This section first introduces the key concepts of the SCION
CP-PKI, including the trust model, its core elements (certificates,
keys, and roles), and their relationships. The sections after the
de Kater, et al. Expires 5 September 2024 [Page 3]
Internet-Draft SCION CP-PKI March 2024
Introduction provide detailed specifications of the building blocks
of the CP-PKI.
*Note:* For extended information on the SCION next-generation inter-
domain architecture, see [CHUAT22], especially Chapter 2, as well as
the IETF Internet Drafts [I-D.scion-overview] and
[I-D.scion-components].
1.1. Terminology
*Control-Plane PKI (CP-PKI)*: The control-plane PKI is the public-key
infrastructure upon which SCION's control plane relies for the
authentication of messages. It is a set of policies, roles, and
procedures that are used to manage trust root configurations (TRCs)
and certificates.
*Autonomous System (AS)*: An autonomous system is a network under a
common administrative control. For example, the network of an
Internet service provider, company, or university can constitute an
AS. If an organization operates multiple networks that are not
directly connected together, then the different networks are
considered different ASes.
*Isolation Domain (ISD)*: In SCION, autonomous systems (ASes) are
organized into logical groups called isolation domains or ISDs. Each
ISD consists of ASes that span an area with a uniform trust
environment (i.e., a common jurisdiction). A possible model is for
ISDs to be formed along national boundaries or federations of
nations.
*Core AS*: Each isolation domain (ISD) is administered by a set of
distinguished autonomous systems (ASes) called core ASes, which are
responsible for initiating the path-discovery and -construction
process (in SCION called "beaconing").
*Trust Root Configuration (TRC)*: A trust root configuration or TRC
is a signed collection of certificates pertaining to an isolation
domain (ISD). TRCs also contain ISD-specific policies.
*Authoritative AS*: Authoritative ASes are those ASes in an ISD that
always have the latest TRCs of the ISD. As a consequence,
authoritative ASes also start the announcement of a TRC update.
de Kater, et al. Expires 5 September 2024 [Page 4]
Internet-Draft SCION CP-PKI March 2024
*Base TRC*: A base TRC is a trust root configuration (TRC) that other
parties trust axiomatically. In other words, trust for a base TRC is
assumed, not derived from another cryptographic object. Each ISD
must create and sign a base TRC when the ISD is established. A base
TRC is either the first TRC of the ISD or the result of a trust
reset.
*TRC Signing Ceremony*: The ceremony during which the very first base
TRC of an ISD, called the initial TRC, is signed. The initial TRC is
a special case of the base TRC where the number of the ISD is
assigned.
*TRC Update*: A _regular_ TRC update is a periodic re-issuance of the
TRC where the entities and policies listed in the TRC remain
unchanged. A _sensitive_ TRC update is an update that modifies
critical aspects of the TRC, such as the set of core ASes. In both
cases, the base TRC remains unchanged.
*Voting ASes*: Those ASes within an ISD that may sign TRC updates.
The process of appending a signature to a new TRC is called "casting
a vote".
*Voting Quorum*: The voting quorum is a trust root configuration
(TRC) field that indicates the number of votes (signatures) needed on
a successor TRC for it to be verifiable. A voting quorum greater
than one will thus prevent a single entity from creating a malicious
TRC update.
*Grace Period*: The grace period is an interval during which the
previous version of a trust root configuration (TRC) is still
considered active after a new version has been published.
*Trust Reset*: A trust reset is the action of announcing a new base
TRC for an existing ISD. A trust reset should only be triggered
after a catastrophic event involving the loss or compromise of
several important private keys.
1.2. Conventions and Definitions
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.
de Kater, et al. Expires 5 September 2024 [Page 5]
Internet-Draft SCION CP-PKI March 2024
1.3. Trust Model
Given the diverse nature of the constituents in the current Internet,
an important challenge is how to scale authentication of network
elements (such as AS ownership, hop-by-hop routing information, name
servers for DNS, and domains for TLS) to the global environment. The
roots of trust of currently prevalent public key infrastructure (PKI)
models do not scale well to a global environment, because (1)
mutually distrustful parties cannot agree on a single trust root
(monopoly model), and because (2) the security of a plethora of roots
of trust is only as strong as its weakest link (oligopoly model) -
see also [BARRERA17].
The monopoly model suffers from two main drawbacks: First, all
parties must agree on a single root of trust. Secondly, the single
root of trust represents a single point of failure, the misuse of
which enables the forging of certificates. Also, its revocation can
result in a kill-switch for all the entities it certifies. The
oligopoly model relies on several roots of trust, all equally and
completely trusted. However, this is not automatically better:
Whereas the monopoly model has a single point of failure, the
oligopoly model has the drawback of exposing more than one point of
failure.
Thus, there is a need for a trust architecture that supports
meaningful trust roots in a global environment with inherently
distrustful parties. This new trust architecture should provide the
following properties:
* Trust agility (see further below);
* Resilience to single root of trust compromise;
* Multilateral governance; and
* Support for policy versioning and updates.
Ideally, the trust architecture allows parties that mutually trust
each other to form their own trust "union" or "domain", and to freely
decide whether to trust other trust unions (domains) outside their
own trust bubble.
To fulfill the above requirements, which in fact apply well to inter-
domain networking, SCION introduces the concept of *Isolation
Domains*. An Isolation Domain (ISD) is a building block for achieving
high availability, scalability, and support for heterogeneous trust.
It consists of a logical grouping of ASes that share a uniform trust
environment (i.e., a common jurisdiction). An ISD is administered by
de Kater, et al. Expires 5 September 2024 [Page 6]
Internet-Draft SCION CP-PKI March 2024
one or multiple ASes, called the *voting ASes*. Furthermore, each ISD
has a set of ASes that form the ISD core; these are the *core ASes*.
The set of core and voting ASes can, but not necessarily have to,
overlap. It is governed by a policy called the *Trust Root
Configuration* (TRC), which is negotiated by the ISD core. The TRC
defines the locally scoped roots of trust used to validate bindings
between names and public keys.
Authentication in SCION is based on digital certificates that bind
identifiers to public keys and carry digital signatures that are
verified by roots of trust. SCION allows each ISD to define its own
set of trust roots, along with the policy governing their use. Such
scoping of trust roots within an ISD improves security, as compromise
of a private key associated with a trust root cannot be used to forge
a certificate outside the ISD. An ISD's trust roots and policy are
encoded in the TRC, which has a version number, a list of public keys
that serves as root of trust for various purposes, and policies
governing the number of signatures required for performing different
types of actions. The TRC serves as a way to bootstrap all
authentication within SCION. Additionally, TRC versioning is used to
efficiently revoke compromised roots of trust.
The TRC also provides _trust agility_, that is, it enables users to
select the trust roots used to initiate certificate validation. This
implies that users are free to choose an ISD they believe maintains a
non-compromised set of trust roots. ISD members can also decide
whether to trust other ISDs and thus transparently define trust
relationships between parts of the network. The SCION trust model,
therefore, differs from the one provided by other PKI architectures.
1.4. Trust Relations within an Isolation Domain
As already mentioned previously, the control-plane PKI, SCION's
concept of trust, is organized on ISD-level. Each ISD can
independently specify its own Trust Root Configuration (TRC) and
build its own verification chain. Each TRC consists of a collection
of signed root certificates, which are used to sign CA certificates,
which are in turn used to sign AS certificates. The TRC also
includes ISD-policies that specify, for example, the TRC's usage,
validity, and future updates. A TRC is a fundamental component of an
ISD's control-plane PKI. The so-called *base TRC* constitutes the
ISD's trust anchor. It is signed during a signing ceremony by the
voting ASes and then distributed throughout the ISD.
de Kater, et al. Expires 5 September 2024 [Page 7]
Internet-Draft SCION CP-PKI March 2024
1.4.1. Updates and Trust Resets
There are two types of TRC updates: regular and sensitive. A
*regular TRC update* is a periodic re-issuance of the TRC where the
entities and policies listed in the TRC remain unchanged, whereas a
*sensitive TRC update* is an update that modifies critical aspects of
the TRC, such as the set of core ASes. In both cases, the base TRC
remains unchanged. If the ISD's TRC has been compromised, it is
necessary for an ISD to re-establish the trust root. This is
possible with a process called *trust reset* (if allowed by the ISD's
trust policy). In this case, a new base TRC is created.
1.4.2. Substitutes to Certificate Revocation
The CP-PKI does not explicitly support certificate revocation.
Instead, it relies on the two mechanisms described above and on
short-lived certificates. This approach constitutes an attractive
alternative to a revocation system for the following reasons:
* Both short-lived certificates and revocation lists must be signed
by a CA. Instead of periodically signing a new revocation list,
the CA can simply re-issue all the non-revoked certificates.
Although the overhead of signing multiple certificates is greater
than that of signing a single revocation list, the overall
complexity of the system is reduced. In the CP-PKI the number of
certificates that each CA must renew is manageable as it is
limited to at most the number of ASes within an ISD.
* Even with a revocation system, a compromised key cannot be
instantaneously revoked. Through their validity period, both
short-lived certificates and revocation lists implicitly define an
attack window (i.e., a period during which an attacker who managed
to compromise a key could use it before it becomes invalid). In
both cases, the CA must consider a tradeoff between efficiency and
security when picking this validity period.
1.5. Overview of Certificates, Keys, and Roles
The base TRC constitutes the root of trust within an ISD. Figure 1
provides a first impression of the trust chain within an ISD, based
on its TRC. For detailed descriptions, please refer to Section 2 and
Section 3.
de Kater, et al. Expires 5 September 2024 [Page 8]
Internet-Draft SCION CP-PKI March 2024
TRC 2
+--------------------------------------+
|+------------------------------------+|
||- Version - Core ASes ||
+--------+ ||- ID - Description || +--------+
| TRC 1 | ||- Validity - No Trust Reset || | TRC 3 |
| (Base |---->||- Grace Period - Voting Quorum ||--->| |
| TRC) | ||- ... || | |
+--------+ |+------------------------------------+| +--------+
|+----------------+ +----------------+|
|| Regular Voting | |Sensitive Voting||
|| Certificate | | Certificate ||
|+----------------+ +----------------+|
|+----------------+ +----------------+|
|| Votes | | Signatures ||
|+----------------+ +----------------+|
|+------------------------------------+|
|| CP Root Certificates ||
|+----------+-------------+-----------+|
| | | |
+-----------+-------------+------------+
| |
| |
v v
+-----------+ +-----------+
| CP CA | | CP CA |
|Certificate| |Certificate|
+-+-------+-+ +-----+-----+
| | |
| | |
v v v
+-----------+ +-----------+ +-----------+
| CP AS | | CP AS | | CP AS |
|Certificate| |Certificate| |Certificate|
+-----------+ +-----------+ +-----------+
Figure 1: Chain of trust within an ISD
All certificates used in SCION's control-plane PKI are in X.509 v3
format [RFC5280]. Additionally, the TRC contains self-signed
certificates instead of plain public keys. Self-signed certificates
have the following advantages over plain public keys: (1) They make
the binding between name and public key explicit; and (2) the binding
is signed to prove possession of the corresponding private key.
All ASes in SCION have the task to sign and verify control-plane
messages. However, certain ASes have additional roles:
de Kater, et al. Expires 5 September 2024 [Page 9]
Internet-Draft SCION CP-PKI March 2024
* *Core ASes*: Core ASes are a distinct set of ASes in the SCION
control plane. For each ISD, the core ASes are listed in the TRC.
Each core AS in an ISD has links to other core ASes (in the same
or in different ISDs).
* *Certification authorities (CAs)*: CAs are responsible for issuing
AS certificates to other ASes and/or themselves.
* *Voting ASes*: Only certain ASes within an ISD may sign TRC
updates. The process of appending a signature to a new TRC is
called "casting a vote"; the designated ASes that hold the private
keys to sign a TRC update are "voting ASes".
* *Authoritative ASes*: Authoritative ASes are those ASes in an ISD
that always have the latest TRCs of the ISD. They start the
announcement of a TRC update.
All further details of the SCION control-plane PKI are specified in
the following sections.
1.6. Trust as a Function
The SCION control-plane PKI can be seen as a function that transforms
potential distrust among different parties into a mutually accepted
trust contract including a trust update and reset policy as well as
certificates used for authentication procedures in SCION's control
plane.
For the function to work, it is not necessary that the ASes of the
ISD all trust each other. However, all ASes MUST trust the ISD's
core ASes, authoritative ASes, voting ASes, as well as its CA(s).
These trusted parties negotiate the ISD trust contract in a
"bootstrapping of trust" ceremony, where cryptographic material is
exchanged, and the ISD's trust anchor (the initial Trust Root
Configuration) is created and signed.
1.6.1. Input
Prior to the ceremony, the trusted parties must decide about the
validity period of the TRC as well as the number of votes required to
update a TRC. They must also bring the required keys and
certificates, the so-called root and voting keys/certificates.
During the ceremony, the trusted parties decide about the number of
the ISD. This must be an integer in the inclusive range between 64
and 4094. The next table shows the current allocation of ISD numbers
in SCION:
de Kater, et al. Expires 5 September 2024 [Page 10]
Internet-Draft SCION CP-PKI March 2024
+========+================================================+
| ISD | Description |
+========+================================================+
| 0 | The wildcard ISD. |
+--------+------------------------------------------------+
| 1 - 15 | Reserved for documentation and sample code |
| | (analogous to [RFC5398]. |
+--------+------------------------------------------------+
| 16 - | Private use (analogous to [RFC6996]). Can be |
| 63 | used for testing and private deployments. |
+--------+------------------------------------------------+
| 64 - | Public ISDs. Should be allocated in ascending |
| 4094 | order, without gaps and "vanity" numbers. |
+--------+------------------------------------------------+
| 4095 - | Reserved for future use. |
| 65535 | |
+--------+------------------------------------------------+
Table 1: ISD Number Allocations
1.6.2. Output
The output of the bootstrapping of trust ceremony, or the trust
"function", are the ISD's initial Trust Root Configuration as well as
mutually trusted and accepted CA and AS certificates--the latter are
used to verify SCION's control-plane messages. Together with the
ISD's control-plane root certificates, the CA and AS certificates
build the ISD's trust and verification chain.
2. Certificate Specification
This section provides a detailed specification of all certificates
used in SCION's control-plane PKI. It starts with an overview of the
main keys and certificates.
2.1. SCION Control-Plane PKI Keys and Certificates - Overview
There are three types of control-plane (CP) certificates: root
certificates, CA certificates, and AS certificates. Together, they
build a chain of trust that is anchored in the trust root
configuration (TRC) file of the respective Isolation Domain (ISD).
Additionally, there are regular and sensitive voting certificates,
which define the keys to cast votes in a regular and a sensitive TRC
update, respectively.
All certificates in SCION's control-plane PKI are in X.509 v3 format
[RFC5280].
de Kater, et al. Expires 5 September 2024 [Page 11]
Internet-Draft SCION CP-PKI March 2024
The next section shows SCION's trust hierarchy. This is followed by
sections that describe the main certificates and corresponding key
pairs of SCION's control-plane PKI as well as the voting certificates
and keys.
2.1.1. Trust Hierarchy
The trust is anchored in the Trust Root Configuration (TRC) for each
ISD. The trust root is axiomatic: All trust derived from this anchor
relies on all parties transitively trusting the TRC.
The trust hierarchy looks like this:
TRC
── Regular Voting Certificates
└── TRC (next version, regular update)
── Sensitive Voting Certificates
└── TRC (next version, sensitive update)
── CP Root Certificates
└── CP CA Certificates
└── CP AS Certificates
2.1.2. Control-Plane Root Certificate
The control-plane root private key is used to sign control-plane CA
certificates. Consequently, the control-plane root certificate with
the control-plane root public key is used to verify control-plane CA
certificates, i.e., root certificates determine which ASes act as CA
in an ISD.
In X.509 terms, CP root certificates are _self-signed_ CA
certificates. That is, issuer and subject of the certificate are the
same entity, and the public key in the root certificate can be used
to verify the root certificate's signature. The CP root public key
and proof of ownership of the private key are embedded in the Trust
Root Configuration (TRC) of an Isolation Domain (ISD), via the self-
signed CP root certificate. This facilitates the bootstrapping of
trust within an ISD, and marks the CP root certificates as the
starting point of an ISD's certificate verification path.
The recommended *maximum validity period* of a CP root certificate
is: 1 year.
*Note*: The TRC of each ISD contains a trusted set of control-plane
root certificates. This set builds the root of each ISD's
verification path. For more information on the selection of this
trusted set of root certificates, see Section 3.
de Kater, et al. Expires 5 September 2024 [Page 12]
Internet-Draft SCION CP-PKI March 2024
2.1.3. Control-Plane CA Certificate
The control-plane CA private key is used to sign control-plane AS
certificates. Consequently, control-plane CA certificates holding
the control-plane CA public key are used to verify control-plane AS
certificates.
The public key needed to verify the CA certificate is in a CP root
certificate. CA certificates do not bundle the root certificate
needed to verify them. In order to verify a CA certificate, a pool
of root certificates must first be extracted from one or more active
TRCs (as described in Section 4.2).
The recommended *maximum validity period* of a CP CA certificate is:
11 days.
2.1.4. Control-Plane AS Certificate
SCION ASes sign control-plane messages, such as Path Construction
Beacons, with their AS private key. Consequently, control-plane AS
certificates holding the corresponding AS public key are required to
verify control-plane messages.
In X.509 terms, control-plane AS certificates are end-entity
certificates. That is, they cannot be used to verify other
certificates.
The recommended *maximum validity period* of a CP AS certificate is:
3 days.
2.1.5. Voting Certificates
There are two types of voting certificates: the (1) regular voting
certificates and the (2) sensitive voting certificates. They contain
the public keys associated with the private keys that are allowed to
cast votes in the TRC update process. Voting certificates are
X.509-style certificates.
Regular and sensitive voting certificates are used to verify regular
and sensitive TRC updates, respectively, and are embedded in the TRC.
de Kater, et al. Expires 5 September 2024 [Page 13]
Internet-Draft SCION CP-PKI March 2024
2.1.5.1. Regular Voting Certificate
Regular voting certificates state which keys are allowed to cast
votes in a regular update. In X.509 terms, regular voting
certificates are self-signed end-entity certificates. This means
that the issuer and subject of a regular voting certificate are the
same entity, and the public key within the certificate can be used to
verify the certificate's signature. However, a regular voting
certificate cannot be used to verify other certificates.
The recommended *maximum validity period* of a regular voting
certificate is: 1 year.
2.1.5.2. Sensitive Voting Certificate
Sensitive voting certificates specify which keys are allowed to cast
votes in a sensitive update. In X.509 terms, sensitive voting
certificates are self-signed end-entity certificates. This means
that the issuer and subject of a sensitive voting certificate are the
same entity, and the public key within the certificate can be used to
verify the certificate's signature. However, a sensitive voting
certificate cannot be used to verify other certificates.
The recommended *maximum validity period* of a sensitive voting
certificate is: 5 years.
*Note*:
Both SCION's control-plane root certificates and control-plane CA
certificates are in fact CA certificates. That is, they can both
be used to verify other certificates.
One important difference between both certificate types lies in
their validity period: A SCION control-plane root certificate has
a recommended maximum validity period of one year, whereas the
recommended maximum validity period of a SCION control-plane CA
certificate is 11 days. This is because a root certificate is
part of the Trust Root Configuration of an ISD, which itself also
has a recommended maximum validity period of one year (see Table 2
below). This ensures that the TRC must not be updated all the
time and is thus relatively stable.
The SCION root private key and public key/certificate are used to
sign and verify the control-plane CA certificates, respectively.
The control-plane CA certificates are explicitly NOT part of the
TRC, for reasons of security. The control-plane CA certificates
are used to verify the control-plane AS certificates, which in
turn are used to verify control-plane messages. Routing is made
de Kater, et al. Expires 5 September 2024 [Page 14]
Internet-Draft SCION CP-PKI March 2024
more secure if both the SCION control-plane CA and AS certificates
can be renewed on a very regular basis. Would the control-plane
CA and AS certificates be part of the TRC, then the TRC would have
to be updated constantly, which is undesirable.
2.1.6. Certificates - Formal Overview
Table 2 and Table 3 below provide a formal overview of the different
types of key pairs and certificates in the control-plane PKI.
+======================+==============+============================+
| Name | Notation (1) | Used to verify/sign |
+======================+==============+============================+
| Sensitive voting key | K_sens | TRC updates (sensitive) |
+----------------------+--------------+----------------------------+
| Regular voting key | K_reg | TRC updates (regular) |
+----------------------+--------------+----------------------------+
| CP root key | K_root | CP CA certificates |
+----------------------+--------------+----------------------------+
| CP CA key | K_CA | CP AS certificates |
+----------------------+--------------+----------------------------+
| CP AS key | K_AS | CP messages, path segments |
+----------------------+--------------+----------------------------+
Table 2: Key chain
(1) K_x = PK_x + SK_x, where x = certificate type, PK_x = public key,
and SK_x = private key
de Kater, et al. Expires 5 September 2024 [Page 15]
Internet-Draft SCION CP-PKI March 2024
+==============+==========+============+=================+==========+
| Name | Notation | Signed | Contains | Validity |
| | | with | | (2) |
+==============+==========+============+=================+==========+
| TRC (trust | TRC | SK_sens, | C_root, C_sens, | 1 year |
| root conf.) | | SK_reg (1) | C_reg (1) | |
+--------------+----------+------------+-----------------+----------+
| Sensitive | C_sens | SK_sens | PK_sens | 5 years |
| voting | | | | |
| cert. | | | | |
+--------------+----------+------------+-----------------+----------+
| Regular | C_reg | SK_reg | PK_reg | 1 year |
| voting | | | | |
| cert. | | | | |
+--------------+----------+------------+-----------------+----------+
| CP root | C_root | SK_root | PK_root | 1 year |
| certificate | | | | |
+--------------+----------+------------+-----------------+----------+
| CP CA | C_CA | SK_root | PK_CA | 11 days |
| certificate | | | | (3) |
+--------------+----------+------------+-----------------+----------+
| CP AS | C_AS | SK_CA | PK_AS | 3 days |
| certificate | | | | |
+--------------+----------+------------+-----------------+----------+
Table 3: Certificates
(1) Multiple signatures and certificates of each type may be included
in a TRC.
(2) Recommended maximum validity period.
(3) A validity of 11 days with 4 days overlap between two CA
certificates is recommended to enable the best possible operational
procedures when performing a CA certificate rollover.
Figure 2 illustrates, at a high level, the relationship between a TRC
and the five types of certificates.
+--------------------+ +--------------------+ +--------------+ +---------------+
| TRC 1 +---->| TRC 2 -+------>╳ | TRC 3 +---->| TRC 4 |
| (base, initial) | | (regular update) | | (base, trust | | (sensitive |
+--+--------------------+ +--------------------+------+ | reset) | | update) |
| | +--------------+ +---------------+
| |
+--------------------------------------------+ +---+----------------------------------------+
| TRC 1 (base, initial) | | TRC 2 (regular update) |
|+------------------------------------------+| |+------------------------------------------+|
||- Version - Core ASes || ||- Version - Core ASes ||
||- ID - Description || ||- ID - Description ||
de Kater, et al. Expires 5 September 2024 [Page 16]
Internet-Draft SCION CP-PKI March 2024
||- Validity - No Trust Reset || ||- Validity - No Trust Reset ||
||- Grace Period - Voting Quorum || ||- Grace Period - Voting Quorum ||
||- ... || ||- ... ||
|+------------------------------------------+| |+------------------------------------------+|
|+--------------------++--------------------+| |+--------------------++--------------------+|
||Votes (cert.indices)|| Regular Voting || ||Votes (cert.indices)|| Regular Voting ||
|| || Certificates || || || Certificates ||
|| (empty) || || || (1),(2)... || ||
|| ||+-----+ +-----+ || || ||+-----+ +-----+ ||
|| ||| (1) | | (2) | || || ||| (1) | | (2) | ||
|| |||C | |C | ... || || |||C | |C | ... ||
|| ||| reg | | reg | || || ||| reg | | reg | ||
|+--------------------+|+--+--+ +--+--+ || |+--------------------+|+-----+ +-----+ ||
|+--------------------+| | | || |+--------------------+| ||
|| || | +--------++-----+ || || ||
|| || +----------------++-+ | || || ||
|| Signatures |+--------------------+| | | || Signatures |+--------------------+|
|| |+--------------------+| | | || |+--------------------+|
||+------------------+|| Sensitive Voting || | | ||+------------------+|| Sensitive Voting ||
|||73 A9 4E AO 0D ...||| Certificates || | +--+>|48 AE E4 80 DB ...||| Certificates ||
||+------------------+||+-----+ +-----+ || | ||+------------------+||+-----+ +-----+ ||
||+------------------+||| (3) | | (4) | || | ||+------------------+||| (3) | | (4) | ||
|||53 B7 7C 98 56 ...||||C | |C | || +------+>|7E BC 75 98 25 ...||||C | |C | ||
||+------------------+||| sens| | sens| ... || ||+------------------+||| sens| | sens| ... ||
|| ... ||+-----+ +-----+ || || ... ||+-----+ +-----+ ||
|+--------------------++--------------------+| |+--------------------++--------------------+|
|+------------------------------------------+| |+------------------------------------------+|
|| CP Root Certificates || || CP Root Certificates ||
|| || || ||
|| +-----+ +-----+ +-----+ +-----+ || || +-----+ +-----+ +-----+ +-----+ ||
|| | (5) | | (6) | | (7) | | (8) | || || | (5) | | (6) | | (7) | | (8) | ||
|| |C | |C | |C | |C | || || |C | |C | |C | |C | ||
|| | root| | root| | root| | root| ..... || || | root| | root| | root| | root| ..... ||
|| +-----+ +--+--+ +-----+ +--+--+ || || +-----+ +--+--+ +-----+ +--+--+ ||
|+------------+---------------+-------------+| |+------------+---------------+-------------+|
+-------------+---------------+--------------+ +-------------+---------------+--------------+
| | | |
+---------v-+ +-v---------+ +---------v-+ +-v---------+
| CP CA | | CP CA | | CP CA | | CP CA |
|Certificate| |Certificate| |Certificate| |Certificate|
+-----+-----+ +-----+-----+ +-+-------+-+ +-----+-----+
| | | | |
| | | | |
v v v v v
+-----------+ +-----------+ +-----------+ +-----------+ +-----------+
| CP AS | | CP AS | | CP AS | | CP AS | | CP AS |
|Certificate| |Certificate| |Certificate| |Certificate| |Certificate|
+-----------+ +-----------+ +-----------+ +-----------+ +-----------+
de Kater, et al. Expires 5 September 2024 [Page 17]
Internet-Draft SCION CP-PKI March 2024
Figure 2: TRC update chain and the different types of associated
certificates. Arrows show how signatures are verified; in other
words, they indicate that a public key contained in a certificate
or TRC can be used to verify the authenticity of another item.
2.2. Certificate Specification
All certificates used in the SCION control-plane PKI are X.509 v3
certificates. However, the SCION specification is in some places
more restrictive. This section defines these additional constraints
and conditions compared to [RFC5280] for each type of SCION control-
plane PKI certificate.
*Note*: The settings for the SCION-specific constraints and
conditions are based on the SCION open-source implementation
scionproto (https://github.com/scionproto/scion/). Adjusting these
settings to the requirements of a customer implementation may be
possible and is allowed.
2.2.1. Basic Fields: SCION-Specific Constraints and Conditions
This section briefly describes the fields of the SCION control-plane
PKI certificates based on X.509. These fields are relevant for each
SCION certificate used in the control plane, regardless of the
certificate type. For detailed descriptions of the full generic
format of X.509 v3 certificates, see [RFC5280] and X509
(https://handle.itu.int/11.1002/1000/13031), clause 7.2.
Additionally, the section lists the SCION-specific constraints and
conditions compared to [RFC5280], per certificate field.
TBSCertificate sequence: Contains information associated with the
subject of the certificate and the CA that issued it. It includes
the following fields:
* version field: Describes the version of the encoded certificate.
- *SCION constraints*: "v1" and "v2" are not allowed.
- *Additional conditions and remarks*: MUST be set to "v3" (as
extensions are used and mandatory in SCION).
* serialNumber field: A positive integer assigned by the CA to each
certificate. It MUST be unique for each certificate issued by a
given CA.
* signature field: Contains the identifier for the algorithm used by
the CA to sign the certificate.
de Kater, et al. Expires 5 September 2024 [Page 18]
Internet-Draft SCION CP-PKI March 2024
- *SCION constraints*: Currently, SCION only supports the ECDSA
signature algorithm. Find all details here: Section 2.2.1.1.
- *Additional conditions and remarks*: As a consequence, the
parameters field in the AlgorithmIdentifier sequence MUST NOT
be used.
* issuer field: Contains the distinguished name (DN) of the entity
that has issued and signed the certificate (usually a CA).
- *SCION constraints*:
o This field MUST be non-empty.
o SCION implementations MUST ONLY use the “UTF8String” value
type for all attributes (including the SCION-specific
attribute ISD-AS number).
- *Additional conditions and remarks*: All SCION implementations
MUST support the additional SCION-specific attribute ISD-AS
number. For details, see Section 2.2.1.2 and
Section 2.2.1.2.1.
* validity field: Defines the validity period of the certificate.
- *SCION constraints*: All certificates used in SCION's control-
plane PKI MUST have a well-defined expiration date.
Certificates with a generalized time value are not valid and
MUST be rejected.
- *Additional conditions and remarks*: SCION recommends a
specific maximum validity period for each type of control-plane
PKI certificate. For details, see Section 2.1.6. SCION
implementations should adopt these values.
* subject field: Defines the entity that owns the certificate.
- *SCION constraints*:
o This field MUST be non-empty.
o SCION implementations MUST ONLY use the “UTF8String” value
type for all attributes (including the SCION-specific
attribute ISD-AS number).
- *Additional conditions and remarks*: The subject field is
specified in the same way as the issuer field. For details,
see Section 2.2.1.2 and Section 2.2.1.2.1.
de Kater, et al. Expires 5 September 2024 [Page 19]
Internet-Draft SCION CP-PKI March 2024
* subjectPublicKeyInfo field: Carries the public key of the
certificate's subject (the entity that owns the certificate, as
defined in the subject field). The subjectPublicKeyInfo field
also identifies which algorithm to use with the key.
- *SCION constraints*: For constraints regarding the algorithm,
see the signature field.
* issuerUniqueID field: If set, it enables reusing the issuer name
over time.
- *SCION constraints*: This field is disallowed in SCION and MUST
NOT be used.
* subjectUniqueID field: If set, it enables reusing the subject name
over time.
- *SCION constraints*: This field is disallowed in SCION and MUST
NOT be used.
* extensions sequence: Defines the extensions of the certificate.
For a description of all extensions used in SCION, see
Section 2.2.2.
2.2.1.1. signature Field - Additional Information
For security reasons, SCION uses a custom list of acceptable
signature algorithms. This list of acceptable signature algorithms
is specified in the signature field. The list currently only
contains the ECDSA signature algorithm (defined in X962
(https://webstore.ansi.org/standards/ascx9/ansix9621998)). However,
the list might be extended in the future. The Object Identifiers
(OIDs) for ECDSA are defined as ecdsa-with-SHA256, ecdsa-with-SHA384,
and ecdsa-with-SHA512 in [RFC5758].
*Important:* The accepted cryptographic algorithms listed in this
document are the only currently accepted cryptographic algorithms.
SCION implementations MUST reject cryptographic algorithms not found
in the list.
The only accepted curves for ECDSA are:
* NIST P-256 (NISTFIPS186-4, section D.1.2.3) (named secp256r1 in
[RFC5480])
* NIST P-384 (NISTFIPS186-4, section D.1.2.4) (named secp384r1 in
[RFC5480])
de Kater, et al. Expires 5 September 2024 [Page 20]
Internet-Draft SCION CP-PKI March 2024
* NIST P-521 (NISTFIPS186-4, section D.1.2.5) (named secp521r1 in
[RFC5480])
The OIDs for the above curves are specified in section 2.1.1.1 of
[RFC5480].
The appropriate hash size to use when producing a signature with an
ECDSA key is:
* ECDSA with SHA-256, for a P-256 signing key
* ECDSA with SHA-384, for a P-384 signing key
* ECDSA with SHA-512, for a P-521 signing key
*Important:* SCION implementations MUST include support for P-256,
P-384, and P-521.
2.2.1.2. issuer Field - Additional Information
The issuer field contains the distinguished name (DN) of the CA that
created the certificate. [RFC5280], section 4.1.2.4, describes the
field's syntax and attributes. In addition to these attributes,
SCION implementations MUST also support the SCION-specific attribute
ISD-AS number. This attribute is specified below.
2.2.1.2.1. ISD-AS number Attribute
The ISD-AS number attribute identifies the SCION ISD and AS. In the
SCION open source implementation, the attribute type is id-at-ia,
defined as:
id-at-ia AttributeType ::= {id-scion id-cppki(1) id-at(2) 1}
where id-scion specifies the root SCION object identifier (OID).
*Note*: The root SCION object identifier (OID) for the SCION open-
source implementation is the IANA Private Enterprise Number '55324':
id-scion ::= OBJECT IDENTIFIER {1 3 6 1 4 1 55324}
The following points apply when setting the attribute value of the
ISD-AS number attribute:
* The string representation MUST follow the canonical formatting
defined in ISD and AS numbering
(https://github.com/scionproto/scion/wiki/ISD-and-AS-numbering).
* The canonical string representation uses a dash separator between
the ISD and AS numbers.
de Kater, et al. Expires 5 September 2024 [Page 21]
Internet-Draft SCION CP-PKI March 2024
* The ISD numbers are formatted as decimal.
* The canonical string formatting of AS numbers in the BGP AS range
(0, 2^(32-1)) is the decimal form. Larger AS numbers, i.e., from
2^32 to 2^(48-1), use a 16-bit, colon-separated, lower-case, hex
encoding with leading zeros omitted: 1:0:0 to ffff:ffff:ffff.
*Example:* AS ff00:0:110 in ISD 1 is formatted as 1-ff00:0:110.
The ISD-AS number attribute MUST be present exactly once in the
distinguished name of the certificate issuer or owner, specified in
the issuer or subject field, respectively. Implementations MUST NOT
create nor successfully verify certificates whose issuer and subject
fields do not include the ISD-AS number at all, or include it more
than once.
*Note*: Voting certificates are not required to include the ISD-AS
number attribute in their distinguished name.
2.2.2. Extensions
[RFC5280], section 4.2.1, defines the syntax of the Extensions
sequence in a X.509 certificate. Descriptions of each standard
certificate extension can be found in [RFC5280], section 4.2.1. The
corresponding clauses in X509
(https://handle.itu.int/11.1002/1000/13031) (10/2016) are clause 7.2
and clause 9, respectively.
Currently, the following extensions are relevant for SCION:
* authorityKeyIdentifier
* subjectKeyIdentifier
* keyUsage
* extKeyUsage
* basicConstraints
The following sections describe the SCION-specifics in regard to
these extensions.
2.2.2.1. authorityKeyIdentifier Extension
The authorityKeyIdentifier extension identifies the public key
corresponding to the private key used to sign a certificate.
de Kater, et al. Expires 5 September 2024 [Page 22]
Internet-Draft SCION CP-PKI March 2024
For the syntax and definition of the authorityKeyIdentifier
extension, see [RFC5280], section 4.2.1.1, and X509
(https://handle.itu.int/11.1002/1000/13031), clause 9.2.2.1.
The authorityKeyIdentifier extension provides three attributes to
specify the public key:
* keyIdentifier
* authorityCertIssuer
* authorityCertSerialNumber
In SCION, using the keyIdentifier attribute is the preferred way to
specify the authorityKeyIdentifier extension.
*Important:* SCION implementations may also support the use of the
authorityCertIssuer and authorityCertSerialNumber attributes.
However, if these attributes are set and support for them is missing,
implementations should error out.
This extension MUST always be non-critical. However, SCION
implementations MUST error out if the extension is not present AND
the certificate is not self-signed.
2.2.2.2. subjectKeyIdentifier Extension
The subjectKeyIdentifier extension identifies certificates that
contain a particular public key. It can be used, for example, by
control-plane messages to identify which certificate to use for
verification. The extension allows for overlapping control-plane CA
keys, for example during updates.
For the syntax and definition of the subjectKeyIdentifier extension,
see [RFC5280], section 4.2.1.2, and X509
(https://handle.itu.int/11.1002/1000/13031), clause 9.2.2.2.
This extension MUST always be non-critical. However, SCION
implementations MUST error out if the extension is not present.
2.2.2.3. keyUsage Extension
The keyUsage extension identifies the intended usage of the public
key in the corresponding certificate. For the syntax and definition
of the keyUsage extension, see [RFC5280], section 4.2.1.3, and X509
(https://handle.itu.int/11.1002/1000/13031), clause 9.2.2.3.
de Kater, et al. Expires 5 September 2024 [Page 23]
Internet-Draft SCION CP-PKI March 2024
The attributes of the keyUsage extension define possible ways of
using the public key. The attributes have the following meaning in
SCION:
* digitalSignature: The public key can be used to verify the digital
signature of a control-plane payload.
* contentCommitment: Not used.
* keyEncipherment: Not used.
* dataEncipherment: Not used.
* keyAgreement: Not used.
* keyCertSign: The public key can be used to verify the CA signature
on a control-plane certificate.
* cRLSign: Not used.
* encipherOnly: Not used.
* decipherOnly: Not used.
*Important:* If a certificate’s public key is used to verify the
signature of a control-plane payload (digitalSignature attribute), it
must be possible to trace back the private key used to sign the
certificate. This is done by referencing the ISD-AS and the subject
key identifier (via the subjectKeyIdentifier extension). For more
information about the subjectKeyIdentifier extension, see
Section 2.2.2.2.
If present, the keyUsage extension should be marked as "critical".
That is, the critical Boolean attribute of this extension must be set
to TRUE (the default is FALSE).
*Note*: If a certificate extension is marked "critical", the public
key in the certificate should only be used for the purpose set in the
critical extension.
Each control-plane PKI certificate type uses the public key
differently, and consequently also specifies the attributes of the
keyUsage extension differently. The next table shows the
specifications per certificate type.
de Kater, et al. Expires 5 September 2024 [Page 24]
Internet-Draft SCION CP-PKI March 2024
+==================+=======+=========+=========+==================+
| Certificate Type |Root | CA | AS | Voting (regular |
| | | | | and sensitive) |
+==================+=======+=========+=========+==================+
| _Attribute:_ | | | | |
+------------------+-------+---------+---------+------------------+
| keyUsage |MUST be| MUST be | MUST be | MAY be present |
| extension itself |present| present | present | (but is not |
| | | | | required) |
+------------------+-------+---------+---------+------------------+
| digitalSignature |MUST | MUST | MUST be | If the extension |
| |NOT be | NOT be | set | is present, the |
| |set (1)| set (2) | | digitalSignature |
| | | | | attribute MUST |
| | | | | NOT be set |
+------------------+-------+---------+---------+------------------+
| keyCertSign |MUST be| MUST be | MUST | If the extension |
| |set | set | NOT be | is present, the |
| | | | set | keyCertSign |
| | | | | attribute MUST |
| | | | | NOT be set |
+------------------+-------+---------+---------+------------------+
Table 4: keyUsage extension - Specifications per certificate type
(1) The root certificate should not be used to verify control-plane
messages.
(2) The CA certificate should not be used to verify control-plane
messages.
2.2.2.4. extKeyUsage Extension
The extKeyUsage extension specifies additional usages of the public
key in the certificate. For the syntax and definition of the
extKeyUsage extension, see X509
(https://handle.itu.int/11.1002/1000/13031), clause 9.2.2.4.
SCION uses the following attributes of the Extended Key Usage
extension, as defined in Section 4.2.1.12 of [RFC5280]:
* id-kp-serverAuth: If set, the public key can be used for SCION
control-plane server authentication.
* id-kp-clientAuth: If set, the public key can be used for SCION
control-plane client authentication.
* id-kp-timeStamping: If set, the public key can be used for the
verification of timestamps.
de Kater, et al. Expires 5 September 2024 [Page 25]
Internet-Draft SCION CP-PKI March 2024
Additionally, the Extended Key Usage extension sequence may include
the SCION-specific attributes id-kp-root, id-kp-regular, and id-kp-
sensitive. These attributes are used in the Trust Root Configuration
setup, to distinguish root certificates, regular voting certificates,
and sensitive voting certificates from each other. For more
information, see Section 3.1.2.2.11.
The specifications of the extKeyUsage extension differ per SCION
control-plane PKI certificate type. The next table provides an
overview of the specifications per certificate type.
+==============+===========+===========+=============+==============+
| Certificate | Root | CA | AS | Voting |
| Type | | | | (regular |
| | | | | and |
| | | | | sensitive) |
+==============+===========+===========+=============+==============+
| _Attribute:_ | | | | |
+--------------+-----------+-----------+-------------+--------------+
| extKeyUsage | MUST be | MAY be | MUST be | MUST be |
| extension | present | present | present | present |
| itself | | (not | | |
| | | required) | | |
+--------------+-----------+-----------+-------------+--------------+
| id-kp- | MUST NOT | MUST NOT | MUST be | MUST NOT be |
| serverAuth | be | be | included, | included |
| | included | included | if the | |
| | | | certificate | |
| | | | is used on | |
| | | | the server- | |
| | | | side of a | |
| | | | control- | |
| | | | plane TLS | |
| | | | session. | |
+--------------+-----------+-----------+-------------+--------------+
| id-kp- | MUST NOT | MUST NOT | MUST be | MUST NOT be |
| clientAuth | be | be | included, | included |
| | included | included | if the | |
| | | | certificate | |
| | | | is used on | |
| | | | the client- | |
| | | | side of a | |
| | | | control- | |
| | | | plane TLS | |
| | | | session. | |
+--------------+-----------+-----------+-------------+--------------+
| id-kp- | MUST be | | MUST be | MUST be |
| timeStamping | included | | included | included |
de Kater, et al. Expires 5 September 2024 [Page 26]
Internet-Draft SCION CP-PKI March 2024
+--------------+-----------+-----------+-------------+--------------+
| SCION- | id-kp- | | | Regular |
| specific | root MUST | | | voting |
| | be | | | cert: id- |
| | included. | | | kp-regular |
| | For | | | MUST be |
| | details, | | | included. |
| | see | | | For |
| | Section | | | details, |
| | 2.2.2.4.1 | | | see Section |
| | | | | 2.2.2.4.1 |
| | | | | Sensitive |
| | | | | voting |
| | | | | cert: id- |
| | | | | kp- |
| | | | | sensitive |
| | | | | MUST be |
| | | | | included. |
| | | | | For |
| | | | | details, |
| | | | | see Section |
| | | | | 2.2.2.4.1 |
+--------------+-----------+-----------+-------------+--------------+
Table 5: extKeyUsage extension - Specifications per certificate type
2.2.2.4.1. SCION-Specific Attributes
The id-kp-root, id-kp-regular, and id-kp-sensitive attributes must be
specified as follows:
* Root certificate:
id-kp-root AttributeType ::= {id-scion id-cppki(1) id-kp(3) 3}
* Regular voting certificate:
id-kp-regular AttributeType ::= {id-scion id-cppki(1) id-kp(3) 2}
* Sensitive voting certificate:
id-kp-sensitive AttributeType ::= {id-scion id-cppki(1) id-kp(3)
1}
where id-scion specifies the root SCION object identifier (OID).
*Note*: The root SCION object identifier (OID) for the SCION open-
source implementation is the IANA Private Enterprise Number '55324':
id-scion ::= OBJECT IDENTIFIER {1 3 6 1 4 1 55324}
de Kater, et al. Expires 5 September 2024 [Page 27]
Internet-Draft SCION CP-PKI March 2024
2.2.2.5. basicConstraints Extension
The basicConstraints extension specifies whether the certificate
subject may act as a CA. For the syntax and definition of the
basicConstraints extension, see X509
(https://handle.itu.int/11.1002/1000/13031), clause 9.4.2.1.
The basicConstraints extension includes the following attributes
relevant for SCION:
* cA attribute: Specifies whether the certificate subject may act as
a CA. If yes, this attribute MUST be set to TRUE.
* pathLenConstraint attribute: This attribute is only relevant if
the cA attribute is set to TRUE. It specifies the maximum number
of CA certificates that may follow this CA certificate in the
certification chain. Value "0" means that this CA may only issue
end-entity certificates, but no CA certificates. If the attribute
is not set, there is no limit to the allowed length of the
certification path.
The settings of the basicConstraints extension differ for each SCION
control-plane PKI certificate type. The next table shows the
specifications per certificate type.
de Kater, et al. Expires 5 September 2024 [Page 28]
Internet-Draft SCION CP-PKI March 2024
+===================+==========+============+=========+============+
| Certificate Type |Root | CA |AS | Voting |
| | | | | (regular |
| | | | | and |
| | | | | sensitive) |
+===================+==========+============+=========+============+
| _Attribute:_ | | | | |
+-------------------+----------+------------+---------+------------+
| basicConstraints |MUST be | MUST be |SHOULD | SHOULD NOT |
| extension itself |present | present |NOT be | be present |
| | | |present | |
+-------------------+----------+------------+---------+------------+
| cA |MUST be | MUST be |If the | If the |
| |set to | set to |extension| extension |
| |TRUE | TRUE |is | is |
| | | |present, | present, |
| | | |this | this |
| | | |attribute| attribute |
| | | |MUST be | MUST be |
| | | |set to | set to |
| | | |FALSE | FALSE |
+-------------------+----------+------------+---------+------------+
| pathLenConstraint |SHOULD be | SHOULD be |If the | If the |
| |set to | set to "0" |extension| extension |
| |"1", MUST | (1), MUST |is | is |
| |be marked | be marked |present, | present, |
| |as | as |this | this |
| |"critical"| "critical" |attribute| attribute |
| | | |MUST be | MUST be |
| | | |absent. | absent. |
+-------------------+----------+------------+---------+------------+
Table 6: basicConstraints extension - Specifications per
certificate type
(1) Control-plane CAs can only issue end-entity certificates.
3. Trust Root Configuration Specification
This section provides an in-depth specification of the trust root
configuration (TRC) file (see Section 3.1). The TRC contains policy
information about an ISD and acts as a distribution mechanism for the
trust anchors of that ISD. It enables securing the control-plane
interactions, and is thus an integral part of the SCION
infrastructure.
de Kater, et al. Expires 5 September 2024 [Page 29]
Internet-Draft SCION CP-PKI March 2024
The initial TRC of an ISD is signed during a signing ceremony and
then distributed throughout the ISD. This signing ceremony follows
specific rules; Section 3.2 describes these rules.
3.1. TRC Specification
The trust root configuration (TRC) is a signed collection of X.509
(https://handle.itu.int/11.1002/1000/13031) v3 certificates.
Additionally, the TRC contains ISD-specific policies encoded in a
Cryptographic Message Syntax (CMS) [RFC5652] envelope.
The TRC's certificates collection consists of a set of control-plane
root certificates, which build the root of the certification chain
for the AS certificates in an ISD. The other certificates in the TRC
are solely used for signing the next TRC, a process called "voting".
The verification of a new TRC thus depends on the policies and voting
certificates defined in the previous TRC.
*Note:* See Section 2 for the general specifications of SCION's
control-plane PKI certificates, as well as Section 2.1.2 and
Section 2.1.5, for the specifications of the control-plane root
certificates and voting certificates, respectively.
This section provides a detailed specification of the TRC. It
presents the TRC format definitions and describes the TRC payload
fields. The section uses the ITU-T X.680
(https://handle.itu.int/11.1002/1000/14468) syntax.
3.1.1. TRC Types and States
The following types of TRCs exist:
* Initial: The very first TRC of an ISD is the initial TRC of that
ISD. It is a special case of the base TRC, where the number of
the ISD is specified.
* Base: A base TRC is either the initial TRC, or the first TRC after
a trust reset (see Section 1.4.1). Trust for a base TRC cannot be
inferred by verifying a TRC update; base TRCs are trusted
axiomatically, similarly to how root CA certificates are trusted
by clients in the Web PKI.
* Update: All non-base TRCs are updated TRCs. They are the product
of either a regular or a sensitive update.
A TRC can have the following states:
de Kater, et al. Expires 5 September 2024 [Page 30]
Internet-Draft SCION CP-PKI March 2024
* Valid: The validity period of a TRC is defined in the TRC itself,
in the validity field (see Section 3.1.2.2.3). A TRC is
considered valid if the current time falls within its validity
period.
* Active: An active TRC is a valid TRC that can be used for
verifying certificate signatures. This is either the latest TRC
or the predecessor TRC, if it is still in its grace period (as
defined in the gracePeriod field of the new TRC, see
Section 3.1.2.2.4). No more than two TRCs can be active at the
same time for any ISD.
Figure 3 shows the content of both a base/initial TRC and the first
regularly-updated TRC based on the base TRC. All elements of the
shown TRCs are specified in detail in the following subsections.
de Kater, et al. Expires 5 September 2024 [Page 31]
Internet-Draft SCION CP-PKI March 2024
+--------------------------------------------+ +--------------------------------------------+
| TRC 1 (base, initial) | | TRC 2 (regular update) |
|+------------------------------------------+| |+------------------------------------------+|
||- Version - Core ASes || ||- Version - Core ASes ||
||- ID - Description || ||- ID - Description ||
||- Validity - No Trust Reset || ||- Validity - No Trust Reset ||
||- Grace Period - Voting Quorum || ||- Grace Period - Voting Quorum ||
||- ... || ||- ... ||
|+------------------------------------------+| |+------------------------------------------+|
|+--------------------++--------------------+| |+--------------------++--------------------+|
||Votes (cert.indices)|| Regular Voting || ||Votes (cert.indices)|| Regular Voting ||
|| || Certificates || || || Certificates ||
|| (empty) || || || (1),(2)... || ||
|| ||+-----+ +-----+ || || ||+-----+ +-----+ ||
|| ||| (1) | | (2) | || || ||| (1) | | (2) | ||
|| |||C | |C | ... || || |||C | |C | ... ||
|| ||| reg | | reg | || || ||| reg | | reg | ||
|+--------------------+|+--+--+ +--+--+ || |+--------------------+|+-----+ +-----+ ||
|+--------------------+| | | || |+--------------------+| ||
|| || | +--------++-----+ || || ||
|| || +----------------++-+ | || || ||
|| Signatures |+--------------------+| | | || Signatures |+--------------------+|
|| |+--------------------+| | | || |+--------------------+|
||+------------------+|| Sensitive Voting || | | ||+------------------+|| Sensitive Voting ||
|||73 A9 4E AO 0D ...||| Certificates || | +--+>|48 AE E4 80 DB ...||| Certificates ||
||+------------------+||+-----+ +-----+ || | ||+------------------+||+-----+ +-----+ ||
||+------------------+||| (3) | | (4) | || | ||+------------------+||| (3) | | (4) | ||
|||53 B7 7C 98 56 ...||||C | |C | || +------+>|7E BC 75 98 25 ...||||C | |C | ||
||+------------------+||| sens| | sens| ... || ||+------------------+||| sens| | sens| ... ||
|| ... ||+-----+ +-----+ || || ... ||+-----+ +-----+ ||
|+--------------------++--------------------+| |+--------------------++--------------------+|
|+------------------------------------------+| |+------------------------------------------+|
|| CP Root Certificates || || CP Root Certificates ||
|| || || ||
|| +-----+ +-----+ +-----+ +-----+ || || +-----+ +-----+ +-----+ +-----+ ||
|| | (5) | | (6) | | (7) | | (8) | || || | (5) | | (6) | | (7) | | (8) | ||
|| |C | |C | |C | |C | || || |C | |C | |C | |C | ||
|| | root| | root| | root| | root| ..... || || | root| | root| | root| | root| ..... ||
|| +-----+ +-----+ +-----+ +-----+ || || +-----+ +-----+ +-----+ +-----+ ||
|+------------------------------------------+| |+------------------------------------------+|
+--------------------------------------------+ +--------------------------------------------+
Figure 3: The TRC on the left-hand side is the initial base TRC.
The TRC on the right is the product of the first regular update
of the base TRC.
de Kater, et al. Expires 5 September 2024 [Page 32]
Internet-Draft SCION CP-PKI March 2024
3.1.2. TRC Format
The trust root configuration (TRC) of an ISD defines the roots of
trust of the ISD, and builds the base of the ISD's control-plane PKI.
It holds the root and voting certificates of the ISD and defines the
ISD's trust policy.
3.1.2.1. TRC Schema
The following code block shows the format of a TRC specification file
(the payload schema):
TRCPayload ::= SEQUENCE {
version TRCFormatVersion,
iD TRCID,
validity Validity,
gracePeriod INTEGER,
noTrustReset BOOLEAN DEFAULT FALSE,
votes SEQUENCE OF INTEGER (SIZE (1..255)),
votingQuorum INTEGER (1..255),
coreASes SEQUENCE OF ASN,
authoritativeASes SEQUENCE OF ASN,
description UTF8String (SIZE (0..1024)),
certificates SEQUENCE OF Certificate }
TRCFormatVersion ::= INTEGER { v1(0) }
TRCID ::= SEQUENCE {
iSD ISD,
serialNumber INTEGER (1..MAX),
baseNumber INTEGER (1..MAX) }
ISD ::= INTEGER (1..65535)
Validity ::= SEQUENCE {
notBefore Time,
notAfter Time }
ASN ::= INTEGER (1..281474976710655)
The TRCPayload sequence contains the identifying information of a TRC
as well as policy information for TRC updates. Furthermore, it
defines the list of certificates that build the trust anchor of the
ISD.
de Kater, et al. Expires 5 September 2024 [Page 33]
Internet-Draft SCION CP-PKI March 2024
For signature calculation, the data that is to be signed is encoded
using ASN.1 distinguished encoding rules (DER) X.690
(https://handle.itu.int/11.1002/1000/14472). For more details, see
Section 3.1.3.
3.1.2.2. TRC Fields
This section describes the syntax and semantics of all TRC payload
fields.
3.1.2.2.1. version Field
The version field describes the version of the TRC format
specification.
Currently, the version MUST always be "v1".
3.1.2.2.2. iD Field
The iD field specifies the unique identifier of the TRC.
The identifier is a unique sequence of
* ISD number (iSD attribute),
* base number (baseNumber attribute), and
* TRC serial number (serialNumber attribute).
All numbers MUST be positive integers.
* The *ISD number* MUST be an integer in the inclusive range from 64
to 4094 (i.e., the numbering range for public ISDs, see
Section 1.6.1).
* The *base number* indicates the starting point of the current TRC
update chain. This starting point is either the ISD's initial TRC
or the currently valid base TRC, if the valid base TRC differs
from the initial TRC. The latter MUST be the case after a trust
reset.
* The *serial number* represents the current update cycle, counting
from the initial TRC of a specific ISD.
de Kater, et al. Expires 5 September 2024 [Page 34]
Internet-Draft SCION CP-PKI March 2024
A TRC where the base number is equal to the serial number is a base
TRC. The initial TRC is a special case of a base TRC. An ISD's
initial TRC MUST have a serial number of 1 and a base number of 1.
With every TRC update, the serial number MUST be incremented by one.
This facilitates uniquely identifying the predecessor and successor
TRC in a TRC update chain.
If a trust reset is necessary, a new base TRC is announced, in order
to start a new and clean TRC update chain. The base number of this
new TRC update chain SHOULD be the number following the serial number
of the latest TRC that was produced by a non-compromised TRC update
for this ISD.
*Example*
The following simple example illustrates how to specify the ID of the
TRCs in an TRC update chain for _ISD 74_. The IDs are given in a
human-readable notation, where Bxx is the base number, and Sxx the
serial number.
de Kater, et al. Expires 5 September 2024 [Page 35]
Internet-Draft SCION CP-PKI March 2024
+===========+=================+====================================+
| Update | TRC ID | Remarks |
+===========+=================+====================================+
| Initial | ISD74-B01-S01 | |
+-----------+-----------------+------------------------------------+
| Regular | ISD74-B01-S02 | Only the serial number is |
| | | incremented. |
+-----------+-----------------+------------------------------------+
| Regular | ISD74-B01-S03 | Only the serial number is |
| | | incremented. |
+-----------+-----------------+------------------------------------+
| Sensitive | ISD74-B01-S04 | Only the serial number is |
| | | incremented. |
+-----------+-----------------+------------------------------------+
| Trust | ISD74-*B05*-S05 | A trust reset includes the |
| reset | | creation of a new base TRC. The |
| | | new base number follows the serial |
| | | number "04" of the latest TRC |
| | | resulting from a non-compromised |
| | | TRC update for this ISD. |
+-----------+-----------------+------------------------------------+
| Regular | ISD74-B05-S06 | Only the serial number is |
| | | incremented. |
+-----------+-----------------+------------------------------------+
| Regular | ISD74-B05-S07 | Only the serial number is |
| | | incremented. |
+-----------+-----------------+------------------------------------+
| And so on | | |
+-----------+-----------------+------------------------------------+
Table 7: ID of TRCs in TRC update chain
3.1.2.2.3. validity Field
The validity field defines the validity period of the TRC. This is
the period of time during which the TRC is in the "valid" state. The
notBefore and notAfter attributes of the validity field specify the
lower and upper bound of the time interval during which a TRC can be
active.
*Note:* An active TRC is a valid TRC that can be used for verifying
certificate signatures. The time period during which a TRC is active
can be shorter than the time period during which the TRC is valid.
For more information, see Section 3.1.1.
The validity field consists of a sequence of two dates, as defined in
section 7.2. of X.509 (https://handle.itu.int/11.1002/1000/13031).
de Kater, et al. Expires 5 September 2024 [Page 36]
Internet-Draft SCION CP-PKI March 2024
In addition to this standard definition, the following constraint
applies to the validity field of the TRC used in SCION:
* All TRCs MUST have a well-defined expiration date. SCION
implementations MUST NOT create TRCs that use the
"99991231235959Z" generalized time value, and verifiers MUST error
out when encountering such a TRC.
3.1.2.2.4. gracePeriod Field
The gracePeriod field of a TRC specifies the period of time during
which the predecessor TRC can still be considered active (the "grace
period"). The grace period starts at the beginning of the validity
period of the new TRC.
The validity period of the predecessor TRC ends when
* the grace period has passed,
* the predecessor’s expiration time is reached, or
* the successor TRC of the new TRC has been announced.
*Note:* The event that happens first marks the end of the
predecessor's validity period.
The gracePeriod field defines the grace period as a number of seconds
(positive integer).
The value of the gracePeriod field in a base TRC MUST be zero. The
value of the gracePeriod field in a non-base TRC SHOULD be non-zero.
It should be long enough to provide sufficient overlap between the
TRCs in order to facilitate interruption-free operations in the ISD.
If the grace period is too short, some control-plane AS certificates
might expire before the corresponding AS can fetch an updated version
from its CA.
3.1.2.2.5. noTrustReset Boolean
The noTrustReset Boolean specifies whether a trust reset is forbidden
by the ISD. Within a TRC update chain, this value CANNOT be changed
by a regular or sensitive update. However, it is possible to change
the noTrustReset value in the event of a trust reset, where a new
base TRC is created.
The noTrustReset field is optional and defaults to FALSE.
de Kater, et al. Expires 5 September 2024 [Page 37]
Internet-Draft SCION CP-PKI March 2024
*Important:* Note that once the noTrustReset Boolean is set to TRUE
and a trust reset is disallowed, this cannot be reversed. Therefore,
ISDs SHOULD always set this value to FALSE, unless they have
sufficiently assessed the risks and implications of making a trust
reset impossible.
*Note:* A trust reset represents a special use case where a new base
TRC is created. It therefore differs from a TRC update (regular or
sensitive), as the signatures in the new base TRC cannot be verified
with the certificates contained in the predecessor TRC. Instead, a
trust reset base TRC must be axiomatically trusted, similarly to how
the initial TRC is trusted.
3.1.2.2.6. votes Field
The votes field contains a sequence of indices that refer to the
voting certificates in the predecessor TRC. If index i is part of
the votes field, then the voting certificate at position i in the
certificates sequence of the predecessor TRC casted a vote on the
successor TRC. For more information on the certificates sequence,
see Section 3.1.2.2.11.
*Note:* In a base TRC, the votes sequence is empty.
Every entry in the votes sequence MUST be unique.
Further restrictions on votes are discussed in Section 3.1.5.
*Note:* The votes sequence of indices is mandatory in order to
prevent stripping voting signatures from the TRC. Absence of the
votes sequence makes it possible to transform a TRC with more voting
signatures than the voting quorum into multiple verifiable TRCs with
the same payload, but different voting signature sets. This would
violate the requirement of uniqueness of a TRC.
3.1.2.2.7. votingQuorum Field
The votingQuorum field defines the number of necessary votes on a
successor TRC to make it verifiable.
A voting quorum greater than one will prevent a single entity from
creating a malicious TRC update.
3.1.2.2.8. coreASes Field
The coreASes field contains the AS numbers of the core ASes in this
ISD.
de Kater, et al. Expires 5 September 2024 [Page 38]
Internet-Draft SCION CP-PKI March 2024
Each core AS number MUST be unique in the sequence of core AS
numbers. That is, each AS number must appear only once in the
coreASes field.
3.1.2.2.8.1. Revoking or Assigning Core Status
* To revoke the core status of a given AS, remove the respective AS
number from the sequence of AS numbers in the coreASes field.
* To assign the core status to a given AS, add the respective AS
number to the sequence of AS numbers in the coreASes field.
*Important:* Revoking or assigning the core status of/to an AS always
requires a (sensitive) TRC update.
3.1.2.2.9. authoritativeASes Field
The authoritativeASes field contains the AS numbers of the
authoritative ASes in this ISD.
Authoritative ASes are those ASes in an ISD that always have the
latest TRCs of the ISD. As a consequence, authoritative ASes also
start the announcement of a TRC update.
* Every authoritative AS MUST be a core AS and be listed in the
coreASes field.
* Each authoritative AS number MUST be unique in the sequence of
authoritative AS numbers. That is, each AS number must appear
only once in the authoritativeASes field.
3.1.2.2.9.1. Revoking or Assigning Authoritative Status
* To revoke the authoritative status of a given AS, remove the
respective AS number from the sequence of AS numbers in the
authoritativeASes field.
* To assign the authoritative status to a given AS, add the
respective AS number to the sequence of AS numbers in the
authoritativeASes field.
*Important:* Revoking or assigning the authoritative status of/to an
AS always requires a (sensitive) TRC update.
3.1.2.2.10. description Field
The description field contains a UTF-8 encoded string that describes
the ISD.
de Kater, et al. Expires 5 September 2024 [Page 39]
Internet-Draft SCION CP-PKI March 2024
* The description field SHOULD NOT be empty.
* The description of the ISD MUST be in English. Additionally, the
description field MAY contain information in other languages.
3.1.2.2.11. certificates Field
The voting ASes and the certification authorities (CAs) of an ISD are
not specified explicitly in the ISD's TRC. Instead, this information
is defined by the list of voting and root certificates in the
certificates field of the TRC payload.
The certificates field is a sequence of self-signed X.509
certificates. Each certificate in the certificate sequence must be
one of the following types:
* a sensitive voting certificate,
* a regular voting certificate, or
* a CP root certificate.
A certificate that is no control-plane root or voting certificate
MUST NOT be included in the sequence of certificates in the
certificates field.
*Note*: A certificate's type (voting or root) is specified in the
extKeyUsage extension of the certificate, by means of the SCION-
specific attributes id-kp-regular, id-kp-sensitive, and id-kp-root,
respectively. For more information, see Section 2.2.2.4.
The following constraints MUST hold for each certificate in the
certificates field of the TRC payload:
* Each certificate MUST be unique in the sequence of certificates.
That is, each certificate must appear only once in the
certificates field.
* The issuer / serialNumber pair for each certificate MUST be
unique.
* If an ISD-AS number is present in the distinguished name of the
certificate, this ISD number MUST be equal to the ISD number of
the TRC (which is defined in the iD field (see Section 3.1.2.2.2).
* Every certificate MUST have a validity period that fully contains
the validity period of this TRC. That is, the notBefore date of
this TRC's validity period MUST be equal to or later than the
de Kater, et al. Expires 5 September 2024 [Page 40]
Internet-Draft SCION CP-PKI March 2024
certificate's notBefore date, and the notAfter date of this TRC's
validity period MUST be before or equal to the certificate's
notAfter date.
* Per certificate type, every certificate distinguished name MUST be
unique.
The following must hold for the entire sequence of certificates in
the certificates field:
* votingQuorum <= count (sensitive voting certificates)
That is, the quorum defined in the TRC's votingQuorum field
(Section 3.1.2.2.7) must be smaller than or equal to the number of
_sensitive_ voting certificates specified in the TRC's
certificates field.
* votingQuorum <= count (regular voting certificates)
That is, the quorum defined in the TRC's votingQuorum field
(Section 3.1.2.2.7) must be smaller than or equal to the number of
_regular_ voting certificates specified in the TRC's certificates
field.
3.1.3. TRC Signature Syntax
A TRC contains policy information about an ISD and acts as a
distribution mechanism for the trust anchors of that ISD. Each TRC
(payload) is digitally signed. The syntax used to sign and
encapsulate the TRC payload is the Cryptographic Message Syntax
(CMS), as defined in [RFC5652]. The signed TRC payload is of the CMS
signed-data content type, as defined in Section 5 of [RFC5652], and
encapsulated in a CMS ContentInfo element, as defined in Section 3 of
[RFC5652]. For detailed information on the general syntax
definitions of the Cryptographic Message Syntax, see sections 3 and 5
of [RFC5652].
3.1.3.1. SCION-specific rules
SCION implementations have to fulfil the following additional rules,
on top of the general syntax rules from [RFC5652]:
* EncapsulatedContentInfo sequence:
- The eContentType field must be set to "id-data".
- The content of the eContent field must be the DER-encoded TRC
payload. This has the benefit that the format is backwards
compatible with PKCS #7, as described in Section 5.2.1 of
[RFC5652].
de Kater, et al. Expires 5 September 2024 [Page 41]
Internet-Draft SCION CP-PKI March 2024
* SignedData sequence:
- The certificates field MUST be left empty. The certificate
pool used to verify a TRC update is already specified in the
certificates field of the predecessor TRC's payload (see also
Section 3.1.2.2.11).
- The version field MUST be set to "1". This is because SCION
uses the "id-data" content type to encapsulate content info,
and does not specify any certificate in the SignedData sequence
(see also Section 5.1 of [RFC5652]).
* SignerIdentifier choice:
- The type of signer identifier chosen here MUST be
IssuerAndSerialNumber.
* SignerInfo sequence:
- The version field MUST be set to "1". This is because SCION
uses the IssuerAndSerialNumber type of signer identifier (see
also Section 5.3 of [RFC5652]).
- The algorithm specified in the signatureAlgorithm field MUST be
one of the algorithms supported by SCION (for details, see
signature Field - Additional Information (Section 2.2.1.1)).
- The digestAlgorithm is determined by the algorithm specified in
the signatureAlgorithm field.
3.1.3.2. TRC Equality
The signer information in the signed TRC is part of an unordered set,
per [RFC5652]. This implies that the signer information can be
reordered without affecting verification. Certain operations,
however, require TRCs to be equal, according to the following
equality definition:
*Two TRCs are equal, if and only if their payloads are byte equal.*
Two TRCs with byte equal payloads can be considered as equal, because
the TRC payload exactly defines which signatures must be attached in
the signed TRC:
de Kater, et al. Expires 5 September 2024 [Page 42]
Internet-Draft SCION CP-PKI March 2024
* The required signatures from voting certificates are explicitly
mentioned in the votes field of the payload: If index "i" is part
of the votes field, then the voting certificate at position i in
the certificates sequence of the predecessor TRC casted a vote on
the successor TRC. See also Section 3.1.2.2.6.
* The required signatures for new certificates are implied by the
currently valid TRC payload, and, in case of a TRC update, the
predecessor payload.
3.1.4. Certification Path - Trust Anchor Pool
The certification path of a control-plane AS certificate starts in a
control-plane root certificate. The control-plane root certificates
for a given ISD are distributed via the TRC. However, AS
certificates and the corresponding signing CA certificates are *not*
part of the TRC, but bundled into certificate chains and distributed
separately from the corresponding TRC. This separation makes it
possible to extend the validity period of the root certificate, and
to update the corresponding TRC, without having to modify the
certificate chain. To be able to validate a certification path, each
AS must build a collection of root certificates from the latest TRC
of the relevant ISD. The following section explains how to build a
trust anchor pool.
*Note:* Any entity sending information that is secured by the CP-PKI,
such as control-plane messages, MUST be able to provide all the
necessary trust material, such as certificates, to verify said
information. If any cryptographic material is missing in the
process, the relying party MUST query the originator of the message
for the missing material. If it cannot be resolved, the verification
process fails. For more details, see 4.2 "Signing and Verifying
Control-Plane Messages" Section 4.2.
3.1.4.1. TRC Selection For Trust Anchor Pool
The selection of the right set of TRCs to build the trust anchor pool
depends on the time of verification. The trust anchor pool is
usually used to verify control-plane messages. In this case, the
time of verification is the current time. However, if the trust
anchor pool will be used for auditing, the time of verification is
the point in time for which you want to check whether a given
signature was verifiable.
The selection algorithm for building the trust anchor pool is
described in pseudo-python code below.
de Kater, et al. Expires 5 September 2024 [Page 43]
Internet-Draft SCION CP-PKI March 2024
def select_trust_anchors(trcs: Dict[(int,int), TRC], verification_time: int) -> Set[RootCert]:
"""
Args:
trcs: The dictionary mapping (serial number, base number) to the TRC for a given ISD.
verification_time: The time of verification.
Returns:
The set of CP Root certificates that act as trust anchors.
"""
# Find highest base number that has a TRC with a validity period
# starting before verification time.
base_nr = 1
for trc in trcs.values():
if trc.id.base_nr > base_nr and trc.validity.not_before <= verification_time:
base_nr = trc.id.base_nr
# Find TRC with highest serial number with the given base number and a
# validity period starting before verification time.
serial_nr = 1
for trc in trcs[isd].values():
if trc.id.base_nr != base_nr:
continue
if trc.id.serial_nr > serial_nr and trc.validity.not_before <= verification_time:
serial_nr = trc.id.serial_nr
candidate = trcs[(serial_nr, base_nr)]
# If the verification time is not inside the validity period,
# there is no valid set of trust anchors.
if not candidate.validity.contains(verification_time):
return set()
# If the grace period has passed, only the certificates in that TRCs
# may be used as trust anchors.
if candidate.validity.not_before + candidate.grace_period < verification_time:
return collect_trust_anchors(candidate)
predecessor = trcs.get((serial_nr-1, base_nr))
if not predecessor or predecessor.validity.not_after < verification_time:
return collect_trust_anchors(candidate)
return collect_trust_anchors(candidate) | collect_trust_anchors(predecessor)
def collect_trust_anchors(trc: TRC) -> Set[RootCert]:
"""
Args:
trc: A TRC from which the CP Root Certificates shall be extracted.
de Kater, et al. Expires 5 September 2024 [Page 44]
Internet-Draft SCION CP-PKI March 2024
Returns:
The set of CP Root certificates that act as trust anchors.
"""
roots = set()
for cert in trc.certificates:
if not cert.basic_constraints.ca:
continue
roots.add(cert)
return roots
3.1.5. TRC Updates
All non-base TRCs of an ISD are updates of the ISD's base TRC(s).
The TRC update chain consists of regular and sensitive TRC updates.
The type of update determines the (payload) information that changes
in the updated TRC. This section describes the rules that apply to
updating a TRC in regard to the payload information contained in the
TRC. Some rules are valid for both update types, some only apply to
a regular or a sensitive TRC update, respectively. Based on the type
of update, a different set of voters is necessary to create a
verifiable TRC update - the section includes a description of the
corresponding voting (signing) process. To verify a TRC update, a
relying party must perform a couple of checks. These checks are
listed at the end of the section.
3.1.5.1. Changed or New Certificates
In the context of a TRC update,
* A certificate is _changing_, if the certificate is part of the
certificates sequence in the predecessor TRC, but no longer part
of the certificates sequence in the updated TRC. Instead, the
certificates sequence of the updated TRC holds another certificate
of the _same type_ and with the _same distinguished name_.
* A certificate is _new_, if there is *no* certificate of the same
type and distinguished name at all in the certificates sequence of
the predecessor TRC.
*Note:* Every new sensitive or regular voting certificate in a TRC
attaches a signature to the TRC. This is done to ensure that the
freshly included voting entity agrees with the contents of the TRC it
is now part of.
de Kater, et al. Expires 5 September 2024 [Page 45]
Internet-Draft SCION CP-PKI March 2024
3.1.5.2. Update Rules - Overview
The following table gives an overview of the types of TRC update as
well as the rules that must apply in regard to the updated TRC's
payload information.
The sections that follow provide more detailed descriptions of each
rule.
de Kater, et al. Expires 5 September 2024 [Page 46]
Internet-Draft SCION CP-PKI March 2024
+===========+====================+==============+===================+
| Type of | Payload Updated | Payload | Payload Updated |
| Update | TRC - Unchanged | Updated TRC | TRC: Other Rules |
| | Elements | - Required | to Hold |
| | | Changes | |
+===========+====================+==============+===================+
| Both | - iD field: iSD | iD field: | votes field: |
| Regular | and baseNumber | serialNumber | Number of votes |
| AND | - noTrustReset | MUST be | (indices) => |
| Sensitive | field | incremented | number set in the |
| Updates | | by 1 | votingQuorum |
| | | | field of the |
| | | | predecessor TRC |
+-----------+--------------------+--------------+-------------------+
| Regular | - Quorum in the | | votes field: |
| TRC | votingQuorum | | - All votes must |
| Update | field | | only refer to |
| | - Core ASes in | | _regular_ voting |
| | the coreASes | | certificates in |
| | field | | the predecessor |
| | - ASes in the | | TRC |
| | authoritativeASes | | - Must include |
| | field | | votes of each |
| | - Nr. and | | changed regular |
| | distinguished | | voting |
| | names of root & | | certificate from |
| | voting | | the predecessor |
| | certificates in | | TRC |
| | the certificates | | signatures field: |
| | field | | - Must include |
| | - Set of | | signatures of |
| | sensitive voting | | each changed root |
| | certificates in | | certificate from |
| | the certificates | | the predecessor |
| | field | | TRC |
+-----------+--------------------+--------------+-------------------+
| Sensitive | If the update | | votes field: |
| TRC | does not qualify | | - All votes must |
| Update | as a regular | | only refer to |
| | update, it is a | | _sensitive_ |
| | sensitive update | | voting |
| | | | certificates in |
| | | | the predecessor |
| | | | TRC |
+-----------+--------------------+--------------+-------------------+
Table 8: Overview of the update types and corresponding rules
de Kater, et al. Expires 5 September 2024 [Page 47]
Internet-Draft SCION CP-PKI March 2024
3.1.5.3. General Update Rules
The following rules MUST hold for each updated TRC, independent of
the update type:
* The iSD and baseNumber in the iD field MUST NOT change (see also
Section 3.1.2.2.2).
* The serialNumber in the iD field MUST be incremented by one.
* The noTrustReset field MUST NOT change (see also
Section 3.1.2.2.5).
* The votes sequence of the updated TRC MUST only contain indices
that refer to sensitive or regular voting certificates in the
predecessor TRC. This guarantees that the updated TRC only
contains valid votes authenticated by sensitive or regular voting
certificates in the predecessor TRC. For more information, see
Section 3.1.2.2.6 and Section 3.1.2.2.11.
* The number of votes in the updated TRC MUST be greater than or
equal to the number set in the votingQuorum field of the
predecessor TRC (see Section 3.1.2.2.7). The number of votes
corresponds to the number of indices in the votes field of the
updated TRC.
3.1.5.4. Regular TRC Update
A regular TRC update is a periodic re-issuance of the TRC where the
entities and policies listed in the TRC remain unchanged.
A TRC update qualifies as a regular update, if the following rules
apply in regard to the TRC's payload information.
* The settings of the following fields in the updated TRC MUST
remain the same compared to the predecessor TRC:
- The voting quorum set in the votingQuorum field.
- The core ASes specified in the coreASes field.
- The authoritative ASes specified in the authoritativeASes
field.
- The number of sensitive and regular voting certificates as well
as CP root certificates included in the certificates field, and
their distinguished names.
de Kater, et al. Expires 5 September 2024 [Page 48]
Internet-Draft SCION CP-PKI March 2024
- The set of sensitive voting certificates specified in the
certificates field.
* For every regular voting certificate that changes, the regular
voting certificate in the predecessor TRC is part of the voters on
the updated TRC. That is, for each changed regular voting
certificate, an index in the votes field of the updated TRC MUST
refer to the changed regular voting certificate in the predecessor
TRC.
* For every CP root certificate that changes, the updated TRC MUST
include a signature created with the private key belonging to the
changed CP root certificate (which is part of the predecessor
TRC).
* In order for a regular TRC update to be verifiable, all votes MUST
be cast by _regular_ voting certificates. That is, each index in
the votes field of the regularly updated TRC MUST refer to a
_regular_ voting certificate in the certificates field of the
predecessor TRC.
3.1.5.5. Sensitive TRC Update
If a TRC update does not qualify as a regular update, it is
considered a sensitive update.
* In order for a sensitive update to be verifiable, all votes MUST
be cast by _sensitive_ voting certificates. That is, each index
in the votes field of the sensitively updated TRC MUST refer to a
_sensitive_ voting certificate in the certificates field of the
predecessor TRC.
3.1.5.6. Signing a TRC Update
As described above, a set of voters must cast votes on the updated
TRC to make it verifiable. The votingQuorum field of the predecessor
TRC (see Section 3.1.2.2.7) defines the required number of voters,
which will represent regular or sensitive voting certificates,
respectively. Furthermore, if one or more _new_ certificates are
added to the updated TRC, the corresponding voting representatives
must also sign the updated TRC, in order to show that they have
access to the private keys listed in these fresh certificates. This
is called "showing proof-of-possession", and done by signing the TRC
with the respective private key. For the distinction between changed
and new certificates in a TRC update, see Section 3.1.5.1.
de Kater, et al. Expires 5 September 2024 [Page 49]
Internet-Draft SCION CP-PKI March 2024
It is up to the ISD members to decide how the "casting a vote"
procedure for updated TRCs will take place. Some ISDs will make a
distinction between regular and sensitive updates. These ISDs divide
the regular and sensitive signing keys in different security classes
and act accordingly. For example, they keep the regular key in an
online vault while the sensitive key would be stored offline in cold
storage. This way, the regular TRC update would lend itself to being
automated (since the keys are accessible online) whereas the
sensitive one would require manual actions to access the offline key.
Other ISDs, however, keep both regular and sensitive keys online and
perform both updates automatically.
3.1.5.7. TRC Update Verification
To verify a TRC update, the relying party must perform the following
checks:
* Check that the specified update rules as described above are
respected.
* Check whether the update is regular or sensitive.
- In case of a regular update,
o check that the signatures for the changing certificates are
present and verifiable, and
o check that all votes are cast by a regular voting
certificate.
- In case of a sensitive update, check that all votes are cast by
a sensitive voting certificate.
* In both cases, check that all signatures are verifiable, and no
superfluous signatures are attached.
If one or more of the above checks gives a negative result, the
updated TRC should be rejected.
3.2. Initial TRC Signing Ceremony
The very first base TRC of an ISD, called the initial TRC, is a
special case of the base TRC where the number of the ISD is chosen.
The initial TRC must be signed during a special signing ceremony--all
voting representatives of the initial TRC need to take part in this
signing ceremony to sign the agreed-upon TRC. As part of the
ceremony, the public keys of all voters are exchanged. The TRC is
then distributed throughout the ISD. All entities within an ISD can
de Kater, et al. Expires 5 September 2024 [Page 50]
Internet-Draft SCION CP-PKI March 2024
initially obtain an authentic TRC, by means of a secure off- or
online mechanism.
Appendix "Appendix A. Signing Ceremony Initial TRC" describes a
possible procedure for the signing ceremony of an ISD's initial TRC.
It is in principle up to the initial members of an ISD how to shape
the signing ceremony. However, it is recommended having a process in
line with the ceremony described in the Appendix.
4. Deploying the CP PKI - Specifications
This section provides several specifications regarding the deployment
of the control-plane PKI.
4.1. Deploying a TRC
4.1.1. Base TRC
Base TRCs are trust anchors and thus axiomatically trusted. All ASes
within an ISD must be pre-loaded with the currently valid base-
version TRC of their own ISD. For all specifications regarding the
creation and distribution of initial/base TRCs, see Section 3.2.
4.1.2. TRC Update
All non-base TRCs of an ISD are updates of the ISD's base TRC(s).
The TRC update chain consists of regular and sensitive TRC updates.
The specifications and rules that apply to updating a TRC are
described in Section 3.1.5.
4.1.2.1. TRC Update Discovery
Relying parties MUST have at least one valid TRC available. Relying
parties MUST discover TRC updates within the grace period defined in
the updated TRC. They SHOULD discover TRC updates in a matter of
minutes to hours. Additionally, the following requirement must be
satisfied:
*Requirement*
Any entity sending information that is secured by the CP-PKI MUST be
able to provide all the necessary trust material to verify said
information.
SCION provides the following mechanisms for discovering TRC updates
and fulfilling the above requirement:
de Kater, et al. Expires 5 September 2024 [Page 51]
Internet-Draft SCION CP-PKI March 2024
* _Beaconing Process_
The TRC version is announced in the beaconing process. Each AS
must announce what it considers to be the latest TRC.
Furthermore, each AS must include the hash value of the TRC
contents to facilitate the discovery of discrepancies. Therefore,
relying parties that are part of the beaconing process discover
TRC updates passively. That is, a core AS notices TRC updates for
remote ISDs that are on the beaconing path. A non-core AS only
notices TRC updates for the local ISD through the beaconing
process. The creation of a new TRC should trigger the generation
of new control-plane messages, as the propagation of control-plane
messages will help other ASes rapidly discover the new TRC.
* _Path Lookup_
In every path segment, all ASes must reference the latest TRC of
their ISD. Therefore, when resolving paths, every relying party
will notice TRC updates, even remote ones.
*Note:* The above mechanism only works when there is an active
communication between the relying party and the ISD in question.
4.2. Signing and Verifying Control-Plane Messages
SCION requires that control-plane messages are signed. The main
purpose of the SCION control-plane PKI is providing a mechanism to
distribute and authenticate public keys that are used to verify
control-plane messages and information. For example, each hop
information in a path segment is signed by the respective AS.
Consequently, all relying parties must be able to verify signatures
with the help of the CP-PKI.
The following sections specify the requirements that apply to the
signing and verification of control-plane messages.
4.2.1. Signing a Control-Plane Message
An AS signs control-plane messages with the private key that
corresponds to the (valid) AS' certificate.
The AS MUST attach the following information as signature metadata.
It is the minimum information a relying party requires to identify
which certificate to use to verify the signed message.
* ISD-AS number: The ISD-AS number of the signing entity. For
specification details, see Section 2.2.1.2.1.
* Subject key identifier: The identifier of the public key that must
be used to verify the message. For specification details, see
Section 2.2.2.2.
de Kater, et al. Expires 5 September 2024 [Page 52]
Internet-Draft SCION CP-PKI March 2024
Additionally, the signer SHOULD include the following information:
* Serial and base number of the latest TRC: Including this
information allows relying parties to discover TRC updates and
trust resets. For specification details, see Section 3.1.2.2.2.
* Timestamp: For many messages, the time at which it was signed is
useful information to ensure freshness.
4.2.2. Verifying a Control-Plane Message
When the relying party receives a control-plane message they want to
verify, the relying party first needs to identify the certificate
needed to validate the corresponding signature on the message.
AS certificates are bundled together with the corresponding signing
CA certificate into certificate chains. For efficiency, SCION
distributes these certificate chains separately from the signed
messages. A certificate chain is verified against the CP root
certificate. However, the root certificate is *not* bundled in the
chain, but with the TRC. This makes it possible to extend the
validity period of the root certificate, and to update the
corresponding TRC, without having to modify the certificate chain.
To verify a control-plane message, the relying party must perform the
following steps:
1. Build a collection of root certificates from the latest TRC of
the relevant ISD (that is, the ISD referenced in the signature
metadata of the message). If the grace period (see
Section 3.1.2.2.4) introduced by the latest TRC is still on-
going, the root certificates in the second-to-latest TRC must
also be included. For a description on how to build the correct
collection of certificates, see Section 3.1.4.1.
2. If the signature metadata of the message contains the serial and
base number of the latest TRC, the relying party must check that
they have this latest TRC. If not, the relying party must
request the latest TRC.
3. After constructing the pool of root certificates, the relying
party must select the certificate chain used to verify the
message. The AS certificate included in this certificate chain
MUST have the following properties:
* The ISD-AS number in the subject of the AS certificate MUST
match the ISD-AS number in the signature metadata. See also
Section 2.2.1.2.1.
de Kater, et al. Expires 5 September 2024 [Page 53]
Internet-Draft SCION CP-PKI March 2024
* The subject key identifier of the AS certificate MUST match
the subject key identifier in the signature metadata. See
also Section 2.2.2.2.
* The AS certificate MUST be valid at verification time.
Normally, this will be the current time. In special cases,
e.g., auditing, the time can be set to the past to check if
the message was verifiable at the given time.
4. After selecting a certificate chain to verify the control-plane
messages, the relying party must verify the certificate chain,
by:
* Executing the regular X.509 verification procedure. For
details, see X.509
(https://handle.itu.int/11.1002/1000/13031).
* Checking that
- all subjects of the certificates in the chain carry the
same ISD number (see also Section 2.2.1.2.1,
- each certificate is of the correct type (see also
Section 2.1), and
- the CA certificate validity period covers the AS
certificate validity period.
5. If the verification of the certificate chain was successful, the
relying party can now verify the control-plane messages, with the
root certificates from the certificate chain.
If any cryptographic material is missing in the process, the relying
party must query the originator of the message for the missing
material. If it cannot be resolved, the verification process fails.
*Important:* An implication of the above procedure is that path
segments should be verifiable at time of use. It is not enough to
rely on path segments being verified on insert, since TRC updates
that change the root key can invalidate a certificate chain.
4.3. Creating a New Control-Plane AS Certificate
The steps required to create a new AS certificate are the following:
1. The AS creates a new key pair and a certificate signing request
(CSR) using that key pair.
de Kater, et al. Expires 5 September 2024 [Page 54]
Internet-Draft SCION CP-PKI March 2024
2. The AS sends the certificate signing request to the relevant CA
within the ISD.
3. The CA uses its CA key and the CSR to create the new AS
certificate.
4. The CA sends the AS certificate back to the AS.
When an AS joins an ISD, the first CSR is sent out of band to one of
the CAs as part of the formalities to join the ISD. Subsequent
certificate renewals may be automated and can leverage the control-
plane communication infrastructure.
5. Security Considerations
The goal of SCION is to provide a secure inter-domain network
architecture, therefore this paragraph focuses on _inter_-AS security
considerations. Specifically, it covers potential attacks and
mitigations for the control-plane PKI. All _intra_-AS trust- and
security aspects are out of scope.
5.1. Dependency on Certificates
In public key infrastructures, certificate authorities have both the
responsibility and power to issue and revoke certificates. If a CA
is compromised or malicious, this power can be abused. A misbehaving
CA could refuse to issue certificates to legitimate entities, or even
issue illegitimate certificates to allow impersonation of another
entity. In the context of the SCION control-plane PKI, refusing to
issue or renew a certificate to an AS will ultimately cut that AS off
from the network, turning the CP-PKI into a potential network "kill
switch". In order to prevent this, within each ISD there are usually
multiple independent CAs.
SCION’s trust architecture fundamentally differs from a global
monopolistic trust model. In SCION, each ISD manages its own trust
roots instead of a single global entity providing those roots. This
structure gives each ISD autonomy in terms of key management and in
terms of trust, and prevents the occurrence of a global kill switch
affecting all ISDs at once. However, each ISD is still susceptible
of compromises that could affect or halt other components (control
plane and forwarding). The following sections explain these cases
and possible countermeasures.
de Kater, et al. Expires 5 September 2024 [Page 55]
Internet-Draft SCION CP-PKI March 2024
5.1.1. Compromise of an ISD
Compared to other trust architectures, in SCION there is no central
authority that could "switch off" an ISD, as each ISD relies on its
own independent cryptographic trust roots. Each AS within an ISD is
therefore dependant on its ISD's PKI for its functioning. This
section discusses potential compromises of the PKI at various levels
of the hierarchy.
* On TRC level: The private root keys of the root certificates
contained in an TRC are used to sign CA certificates. If one of
these private root keys is compromised, the adversary could issue
illegitimate CA certificates which may be used in further attacks.
To maliciously perform a TRC update, an attacker would need to
compromise multiple voting keys. This number depends on the
voting quorum set in the TRC: the higher the quorum, the more
unlikely a malicious update will be.
* On CA level: The private keys of an ISD's CA certificates are used
to sign the AS certificates. All ASes within an ISD obtain
certificates directly from the CAs. If one of the CA’s keys is
compromised, an adversary could issue illegitimate AS
certificates, which may be used in further attacks.
* On AS level: Each AS within an ISD signs control-plane messages,
such as the Path Construction Beacons (PCBs) used for path
discovery, with their AS private key. If the keys of an AS are
compromised by an adversary, this adversary can illegitimately
sign control-plane messages, including PCBs. This means that the
adversary can also manipulate the PCBs, and propagate the
manipulated PCBs to neighboring ASes or register/store them as
path segments.
5.1.2. Recovery from Compromise
This section deals with possible recovery from the compromises
discussed in the previous paragraph. As described in Section 1.4.2,
there is no revocation in the CP-PKI.
* On TRC level: If any of the root keys or voting keys contained in
the TRC are compromised, the TRC must be updated as described in
Section 3.1.5. Note that this is a sensitive TRC update, as the
certificate related to the compromised private key must be
replaced with an entirely new certificate (and not just changed).
A trust reset is only required in the case the number of
compromised keys at the same time is greater or equal than the
TRC's quorum (see Section 3.1.2.2.7).
de Kater, et al. Expires 5 September 2024 [Page 56]
Internet-Draft SCION CP-PKI March 2024
* On CA level: If the private key related to a CA certificate is
compromised, the impacted CA AS must obtain a new CA certificate
from the corresponding root AS. CA certificates are generally
short lived to limit the impact of compromise. Alternatively,
with a TRC update, a new root keys can also be forced,
invalidating the compromised CA.
* On AS level: In the event of a key compromise of a (non-core) AS,
the impacted AS needs to obtain a new certificate from its CA.
This process will vary depending on internal issuance protocols.
5.2. Denial of Service Attacks
As described previously, the SCION's control-plane PKI lays the
foundation for the authentication procedures in SCION. It provides
each AS within a specific ISD with a certified key pair. These keys
enable the authentication of all control-plane messages - every AS
and endpoint can verify all control-plane messages by following the
certificate chain.
The relying party must be able to discover and obtain new or updated
cryptographic material. For the control plane messages, this is
simplified by the observation that the sender of a message (e.g. of a
path construction beacon during path exploration or a path segment
during a path lookup) always has all the cryptographic material to
verify it. Thus, the receiver can always immediately obtain all the
cryptographic material from the message originator. As the
corresponding PKI messaging thus only occurs when the control plane
is already communicating, these requests to obtain cryptographic
material are not prone to additional denial of service attacks. We
therefore refer to [I-D.scion-cp] for a more detailed description of
DoS vulnerabilities of control-plane messages.
For certificate renewal, on the other hand, this does not apply.
Denial of Service on the CA infrastructure or on the communication
links from the individual ASes to the CA, could be used by an
attacker to prevent victim ASes from renewing their certificates,
halting the path discovery process. This risk can be mitigated in
multiple ways:
* CAs only need to be accessible from ASes within the ISD, reducing
the potential DoS attack surface
* ISDs usually rely on multiple CAs
* ISDs could create policies and processes to renew certificates
out-of-band
de Kater, et al. Expires 5 September 2024 [Page 57]
Internet-Draft SCION CP-PKI March 2024
6. IANA Considerations
This document has no IANA actions.
The SCION AS and ISD number are SCION-specific numbers. They are
currently allocated by Anapaya Systems, a provider of SCION-based
networking software and solutions (see Anapaya ISD AS assignments
(https://docs.anapaya.net/en/latest/resources/isd-as-assignments/)).
This task is currently being transitioned from Anapaya to the SCION
Association.
7. References
7.1. Normative References
[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/rfc/rfc2119>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/rfc/rfc5280>.
[RFC5398] Huston, G., "Autonomous System (AS) Number Reservation for
Documentation Use", RFC 5398, DOI 10.17487/RFC5398,
December 2008, <https://www.rfc-editor.org/rfc/rfc5398>.
[RFC5480] Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
"Elliptic Curve Cryptography Subject Public Key
Information", RFC 5480, DOI 10.17487/RFC5480, March 2009,
<https://www.rfc-editor.org/rfc/rfc5480>.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<https://www.rfc-editor.org/rfc/rfc5652>.
[RFC5758] Dang, Q., Santesson, S., Moriarty, K., Brown, D., and T.
Polk, "Internet X.509 Public Key Infrastructure:
Additional Algorithms and Identifiers for DSA and ECDSA",
RFC 5758, DOI 10.17487/RFC5758, January 2010,
<https://www.rfc-editor.org/rfc/rfc5758>.
[RFC6996] Mitchell, J., "Autonomous System (AS) Reservation for
Private Use", BCP 6, RFC 6996, DOI 10.17487/RFC6996, July
2013, <https://www.rfc-editor.org/rfc/rfc6996>.
de Kater, et al. Expires 5 September 2024 [Page 58]
Internet-Draft SCION CP-PKI March 2024
[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/rfc/rfc8174>.
7.2. Informative References
[BARRERA17]
Barrera, D., Chuat, L., Perrig, A., Reischuk, R., and P.
Szalachowski, "The SCION internet architecture",
Association for Computing Machinery (ACM), Communications
of the ACM vol. 60, no. 6, pp. 56-65, DOI 10.1145/3085591,
May 2017, <https://doi.org/10.1145/3085591>.
[CHUAT22] Chuat, L., Legner, M., Basin, D., Hausheer, D., Hitz, S.,
Mueller, P., and A. Perrig, "The Complete Guide to SCION",
ISBN 978-3-031-05287-3, 2022,
<https://doi.org/10.1007/978-3-031-05288-0>.
[I-D.scion-components]
Rustignoli, N. and C. de Kater, "SCION Components
Analysis", 2022, <https://datatracker.ietf.org/doc/draft-
rustignoli-panrg-scion-components/>.
[I-D.scion-cp]
de Kater, C., Rustignoli, N., and S. Hitz, "SCION Control
Plane", 2023, <https://datatracker.ietf.org/doc/draft-
dekater-scion-controlplane/>.
[I-D.scion-overview]
de Kater, C., Rustignoli, N., and A. Perrig, "SCION
Overview", 2022, <https://datatracker.ietf.org/doc/draft-
dekater-panrg-scion-overview/>.
Acknowledgments
Many thanks go to Fritz Steinmann (SIX Group AG), Juan A. Garcia
Prado (ETH Zurich) and Russ Housley (IETF) for reviewing this
document. We are also very grateful to Adrian Perrig (ETH Zurich),
for providing guidance and feedback about each aspect of SCION.
Finally, we are indebted to the SCION development teams of Anapaya
and ETH Zurich, for their practical knowledge and for the
documentation about the CP PKI, as well as to the authors of
[CHUAT22] - the book is an important source of input and inspiration
for this draft.
de Kater, et al. Expires 5 September 2024 [Page 59]
Internet-Draft SCION CP-PKI March 2024
Appendix A. Signing Ceremony Initial TRC
The following sections describe a possible signing ceremony for the
first (initial) base TRC of an ISD. Although each ISD is free to
decide how to shape this signing ceremony, it is recommended
establishing a procedure similar to the one below.
Ceremony Participants
A signing ceremony includes participants from member organizations of
the respective Isolation Domain. The participants of the signing
ceremony fulfill different roles:
* The *ceremony administrator* is in charge of moderating the
signing process. He/she guides all participants through the steps
they need to take. The ceremony administrator may also act as an
intermediary between participants when they share information with
each other.
* A *voting AS representative* is capable of creating voting
signatures on the TRC. This means the voting representative is in
possession of a device with the private keys of the respective
certificates in the TRC.
* A *witness* is any person that participates in the ceremony as a
passive entity. The witness has no active role in any of the
steps of the ceremony, but can stop the process and inquire for
more information if they feel the integrity of the process might
have been compromised.
*Note:* It is assumed that the member organizations of the ISD have
decided in advance, before the signing ceremony, on the roles of the
ceremony participants. That is, they have reached agreement about
the Certificate Authority (CA) ASes (that will also issue the root
certificates), the voting ASes, the representatives of the voting
ASes, the ceremony administrator and the witnesses.
*Note:* For the signing ceremony, it is assumed that all parties are
trustworthy. Issues encountered during the ceremony are assumed to
be caused by honest mistakes, and not by malicious intent. Hash
comparison checks are included to counter mistakes, such that every
participant is sure that they operate on the same data. Furthermore,
the private keys of each participant never leave their machine. The
ceremony administrator does not have to be entrusted with private
keys.
de Kater, et al. Expires 5 September 2024 [Page 60]
Internet-Draft SCION CP-PKI March 2024
Ceremony Preparations
Prior to the ceremony, participants decide on the physical location
of the ceremony, the devices that will be used during the ceremony
and the policy of the ISD. Specifically, the voting entities agree
on the following parameters:
* validity of the TRC,
* voting quorum,
* core ASes/authoritative ASes,
* description, and
* list of CP root certificates.
When these values are agreed upon, a number of voters, equal to or
larger than the specified voting quorum, needs to execute the signing
ceremony. For the base TRC, all voting entities need to be present
with both their sensitive and regular voting keys. The ceremony
process is structured in multiple rounds of data sharing. The
ceremony administrator leads the interaction and gives instructions
to each participant.
Location
The location must provide electricity and enough power sockets for
each participant. Furthermore, it should provide a monitor (or
projector) that allows the ceremony administrator to screencast.
Devices
Each party brings their own device that is provisioned with the
required material, as described below.
* Device to exchange data. This device can either be provided by
the ceremony administrator, or, if preferable, by any of the
voting representatives.
* Ceremony administrator's device: The ceremony administrator should
bring a machine that is capable of creating and verifying a TRC.
Furthermore, it needs to be able to compute the SHA-512 digest
(hash value) of files.
* Voting representative's device: The voting representative should
bring a machine that is capable of signing and verifying TRCs.
Thus, the machine needs to have access to all the voting private
de Kater, et al. Expires 5 September 2024 [Page 61]
Internet-Draft SCION CP-PKI March 2024
keys. Furthermore, it needs to be able to compute the SHA-512
digest (hash value) of the files. The exact binaries that are
required are described in a separate document.
*Important:* It is very important that all devices, especially the
data exchange device, are not compromised. Therefore, the ceremony
should ideally include a procedure to verify that the devices are
secure.
Preparation Steps
Each party involved in a TRC signing ceremony must go through a few
steps in preparation for the ceremony. This section outlines these
steps.
Preparatory Tasks of the Ceremony Administrator
In the preparation phase of the TRC Signing Ceremony, the ceremony
administrator has the following tasks:
1. Send out the high-level TRC Signing Ceremony description and the
document describing the TRC Signing Ceremony Phases to the
participants, all in digital form.
2. Remind all representatives of the voting ASes that they need to
agree on a common TRC policy before scheduling the TRC ceremony.
3. Bring all digitally distributed documents as a printout for all
parties that take part.
Preparatory Tasks of the Voting AS Representatives
The preparatory task of the representatives of the voting ASes
(short: the voters) is to generate the necessary certificates.
*Important:* Before generating the certificates, all voters need to
agree on a preliminary TRC policy, in particular on the *validity
period of the TRC*. This is necessary because all the certificates
that are generated in advance must *cover the full TRC validity
period*. The other policy values could be amended during the ceremony
itself.
Each representative of a voting AS must create the following keys and
certificates:
* A sensitive voting private key, and a certificate holding the
corresponding public key.
de Kater, et al. Expires 5 September 2024 [Page 62]
Internet-Draft SCION CP-PKI March 2024
* A regular voting private key, and a certificate holding the
corresponding public key.
Preparatory Tasks of the Certificate Authority ASes
Each AS that will be a Certificate Authority (a so-called CA AS) must
ensure that the following key and certificate is available:
* A control-plane root private key, and a certificate holding the
corresponding public key.
This implies that there will be one control-plane root certificate
per CA AS.
*Note*: Representatives of CA ASes must not be present at the signing
ceremony themselves, as they do not have to put a signature on the
TRC. However, if a CA AS does not attend the signing ceremony in
person, it must ensure that the corresponding root certificate is
available at the ceremony to be shared.
Ceremony Process
The ceremony process for the initial base TRC is structured in
multiple rounds of data sharing. The ceremony administrator leads
the interaction and instructs each participant with what to do.
The ceremony process contains the following phases:
* Phase 1: Certificate Exchange (Appendix "Phase 1: Certificate
Exchange"). In the first phase of the ceremony, all voting
parties share the certificates that must be part of the TRC with
the ceremony administrator.
* Phase 2: Generation of the TRC Payload (Appendix "Phase 2:
Generation of the TRC Payload"). In the second phase, the
ceremony administrator generates the TRC payload based on the
bundled certificates and the agreed-upon ISD policy.
* Phase 3: TRC Signing (Appendix "Phase 3: TRC Signing"). In the
third phase, each voting representative attaches the required
signatures to the TRC.
* Phase 4: TRC Validation (Appendix "Phase 4: TRC Validation"). In
the final phase of the ceremony, all voting representatives share
the signed TRC with the ceremony administrator, who aggregates it
in a single signed TRC document.
A detailed description of each phase follows below.
de Kater, et al. Expires 5 September 2024 [Page 63]
Internet-Draft SCION CP-PKI March 2024
Phase 1: Certificate Exchange
In Phase 1 of the signing ceremony, all parties share the
certificates that must be part of the TRC with the ceremony
administrator. For the representatives of the voting ASes, these are
the sensitive and the regular voting certificates. For the
representatives of the CA ASes, these are the CP root certificates.
If a CA AS does not attend the signing ceremony in person, it must
ensure that the corresponding root certificate is available at the
ceremony to be shared.
The actual sharing happens over the data exchange device, which goes
from one voting representative to the next. Each representative
copies the requested certificates from their own machine onto the
data exchange device, before forwarding the device to the next voter.
The last representative returns the device to the ceremony
administrator.
*Important:* Note that only the *certificates* must be shared during
this step, *not* the private keys. Copying a private key by mistake
invalidates the security of the ceremony.
For each provided certificate, the ceremony administrator checks that
its validity period covers the previously agreed-upon TRC validity,
that the signature algorithms are correct, and that the certificate
is of the valid type (root, sensitive voting or regular voting
certificate). If the results of these checks are as expected, the
ceremony administrator computes the SHA256 sum for each certificate.
The ceremony administrator then aggregates and bundles the provided
certificates, and calculates the hash value (SHA-512 digest) over the
entire bundle. Additionally, the ceremony administrator displays all
hash values on the monitor.
The ceremony administrator now shares the bundle with the
representatives of the voting and CA ASes. This could happen again
via the data exchange device, which goes from one representative to
the next. Each representative verifies that the certificates they
contributed have the same hash value as the displayed value on the
monitor. Furthermore, all representatives must confirm that the hash
value of the bundled certificates on their machine is equal to the
value on the monitor.
Phase 1 is concluded when every representative has confirmed that the
SHA256 sums are correct.
*Note:* If there is a mismatch in any of the SHA256 sums, Phase 1
needs to be repeated.
de Kater, et al. Expires 5 September 2024 [Page 64]
Internet-Draft SCION CP-PKI March 2024
Phase 2: Generation of the TRC Payload
In Phase 2 of the ceremony, the ceremony administrator generates the
TRC payload based on the bundled certificates and the agreed-upon ISD
policy. The result is displayed on the monitor along with a hash
value (SHA-512 digest).
To be able to generate the payload, the ceremony administrator must
ask the voting representatives for
* The ISD number of the ISD. The number (identifier, ID) of an ISD
must be chosen and agreed upon by the participants during the
signing ceremony of the ISD's initial TRC. The ceremony
administrator needs the ISD number to specify the identifier (ID)
of the initial TRC. This iD is part of the TRC payload. For more
information, see Section 3.1.2.2.2.
* The description of the TRC. For more information, see
Section 3.1.2.2.10.
* The AS numbers of the core ASes of the ISD. For more information,
see Section 3.1.2.2.8.
* The AS numbers of the authoritative ASes of the ISD. For more
information, see Section 3.1.2.2.9.
* The voting quorum for the next TRC update. For more information,
see Section 3.1.2.2.7.
* The validity period of the new TRC. For more information, see
Section 3.1.2.2.3.
*Note:* It is assumed that the voting ASes have agreed on the answers
to the above questions in advance, before the signing ceremony.
The ceremony administrator can now specify the TRC payload variables
in the payload template file, and show the filled-in template on the
monitor. When the voters have verified the data, the ceremony
administrator can compute the DER encoding of the TRC data as well as
the SHA256 sum of the TRC payload file. The ceremony administrator
then distributes the TRC payload (via the data exchange device) to
all voting representatives, who verify the payload's hash value. The
voters do this by computing the hash value of the TRC payload on
their machine and checking whether their value matches the one on the
monitor.
Phase 2 successfully concludes once every voting representative
confirms that the contents of the TRC payload are correct.
de Kater, et al. Expires 5 September 2024 [Page 65]
Internet-Draft SCION CP-PKI March 2024
Phase 3: TRC Signing
In Phase 3, each voting representative attaches a signature created
with each one of their private voting keys to the TRC (payload file).
They do this on their own machine. The purpose of signing a TRC that
contains newly introduced public keys with the corresponding private
keys is to prove the possession of the private keys.
Phase 3 concludes after all voting representatives have cast their
votes.
Phase 4: TRC Validation
In Phase 4, all voting representatives share the signed TRC with the
ceremony administrator. This happens again over the data exchange
device, which goes from one voter to the next. Each voting
representative copies the TRC payload signed with the voter's private
keys from their own machine onto the data exchange device. The last
voter returns the device to the ceremony administrator, who assembles
the final TRC by aggregating the payload data with the votes
(signatures) cast by the voting representatives.
The signed TRC is validated by inspecting its contents on the monitor
and verifying the signatures based on the exchanged certificates in
Phase 1. The ceremony administrator then shares the signed TRC with
all participants. Each of them must then inspect it once more, and
verify it based on the certificates exchanged in Phase 1. At this
point, the ceremony is completed. All participants have the signed
TRC, and can use it to distribute the trust anchors for their ISD.
Authors' Addresses
Corine de Kater
SCION Association
Email: c_de_kater@gmx.ch
Nicola Rustignoli
SCION Association
Email: nic@scion.org
Samuel Hitz
Anapaya Systems
Email: hitz@anapaya.net
de Kater, et al. Expires 5 September 2024 [Page 66]