RFC9589: On the Use of the Cryptographic Message Syntax (CMS) Signing-Time Attribute in Resource Public Key Infrastructure (RPKI) Signed Objects
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(cms)
(signing-time)
Internet Engineering Task Force (IETF) J. Snijders
Request for Comments: 9589 Fastly
Updates: 6488 T. Harrison
Category: Standards Track APNIC
ISSN: 2070-1721 May 2024
On the Use of the Cryptographic Message Syntax (CMS) Signing-Time
Attribute in Resource Public Key Infrastructure (RPKI) Signed Objects
Abstract
In the Resource Public Key Infrastructure (RPKI), Signed Objects are
defined as Cryptographic Message Syntax (CMS) protected content
types. A Signed Object contains a signing-time attribute,
representing the purported time at which the object was signed by its
issuer. RPKI repositories are accessible using the rsync and RPKI
Repository Delta protocols, allowing Relying Parties (RPs) to
synchronize a local copy of the RPKI repository used for validation
with the remote repositories. This document describes how the CMS
signing-time attribute can be used to avoid needless retransfers of
data when switching between different synchronization protocols.
This document updates RFC 6488 by mandating the presence of the CMS
signing-time attribute and disallowing the use of the binary-signing-
time attribute.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9589.
Copyright Notice
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Table of Contents
1. Introduction
1.1. Requirements Language
2. Optimized Switchover from RRDP to rsync
2.1. Guidance for Repository Operators
2.2. Guidance for Relying Parties
3. Presence of the CMS Signing-Time Attribute in Public
Repositories
4. Updates to RFC 6488
5. Security Considerations
6. IANA Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
In the Resource Public Key Infrastructure (RPKI) [RFC6480], Signed
Objects are defined as Cryptographic Message Syntax (CMS) [RFC5652]
[RFC6268] protected content types by way of a standard template
[RFC6488]. That template includes an optional CMS signing-time
attribute, representing the time at which the object was signed by
its issuer. At the time when the standard template was defined,
rsync was the only distribution mechanism for RPKI repositories.
Since the publication of the standard template, a new, additional
protocol for distribution of RPKI repositories has been developed:
the RPKI Repository Delta Protocol (RRDP) [RFC8182]. While RPKI
repository operators must provide rsync service, RRDP is typically
deployed alongside it as well, and is preferred by default by most
Relying Party (RP) implementations. However, RP implementations also
support fallback to rsync in the event of problems with the RRDP
service. As deployment experience with RRDP has increased, the
usefulness of optimizing switchovers by RPs from one mechanism to the
other has become apparent.
This document describes how Repository Operators [RFC6481] and RPs
can use the CMS signing-time attribute to minimize the burden of
switching over from RRDP to rsync. Additionally, this document
updates [RFC6488] by mandating the presence of the CMS signing-time
attribute and disallowing the use of the binary-signing-time
attribute.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Optimized Switchover from RRDP to rsync
To avoid needless retransfers of unchanged files in consecutive rsync
synchronizations, [RPKI-PUB-SERV] recommends the use of so-called
'deterministic' (normalized) timestamps for files. When the content
of a file is unchanged, Repository Operators SHOULD ensure that the
last modification timestamp of the file remains unchanged as well.
This document advances the aforementioned concept by describing a
synchronization strategy through which needless transfers are also
avoided upon first use of rsync, by leveraging data previously
fetched via RRDP.
At the time of writing, all commonly used RP implementations will
first attempt synchronization via RRDP, as described in
[RPKI-REP-REQS]. If synchronization via RRDP fails for some reason
(e.g., malformed XML, expired TLS certificate, HTTP connection
timeout), the RP will attempt to synchronize via rsync instead.
In the rsync synchronization protocol, a file's last modification
timestamp ('mod-time' from here on) and file size are used to
determine whether the general-purpose rsync synchronization algorithm
needs to be executed for the file. This is the default mode for both
the original rsync implementation [rsync] and the OpenBSD
implementation [openrsync]. If the sender's copy of the file and the
receiver's copy of the file both have the same mod-time and file
size, the files are assumed to contain the same content, and they
will be omitted from the list of files to be transferred. Ensuring
consistency with respect to mod-time for both senders and receivers
helps to reduce the burden of rsync synchronization in terms of
network bandwidth, disk I/O operations, and CPU usage.
In order to reduce the burden of the rsync synchronization (following
an RRDP failure), Repository Operators and RPs SHOULD adhere to the
following guidelines.
2.1. Guidance for Repository Operators
When serializing RPKI Signed Objects to a filesystem hierarchy for
publication via rsync, the mod-time of the file containing the Signed
Object SHOULD be set to the value of the CMS signing-time attribute
contained within the Signed Object.
2.2. Guidance for Relying Parties
When serializing RPKI Signed Objects retrieved via RRDP to a
filesystem hierarchy, the mod-time of the file containing the Signed
Object SHOULD be set to the value of the CMS signing-time attribute
contained within the Signed Object.
If an RP uses RRDP to synthesize a filesystem hierarchy for the
repository, then synchronizing to the corresponding directory
directly is an option. Alternatively, the RP can synchronize to a
new (empty) directory using the --compare-dest=DIR rsync feature, in
order to avoid retrieving files that are already available by way of
the synthesized filesystem hierarchy stemming from previous RRDP
fetches. The DIR component is to be substituted with the name of the
directory containing previously fetched and validated RPKI data (in
its original DER-encoded form, to ensure the file size parameter
matches).
From the [rsync] man page for --compare-dest=DIR:
| This option instructs rsync to use DIR on the destination machine
| as an additional hierarchy to compare destination files against
| doing transfers (if the files are missing in the destination
| directory). If a file is found in DIR that is identical to the
| sender's file, the file will NOT be transferred to the destination
| directory. This is useful for creating a sparse backup of just
| files that have changed from an earlier backup.
From the [openrsync] man page for --compare-dest=directory:
| Use directory as an alternate base directory to compare files
| against on the destination machine. If file in directory is found
| and identical to the sender's file, the file will not be
| transferred.
3. Presence of the CMS Signing-Time Attribute in Public Repositories
Analyzing the [rpkiviews] archives containing millions of RPKI Signed
Objects discovered via the five Regional Internet Registry (RIR)
Trust Anchors (TAs) from 6 June 2022 to 29 January 2024, each Signed
Object contained a CMS signing-time attribute.
The above means that all of the commonly used TAs and their
subordinate Certification Authorities (CAs) produce Signed Objects
that contain a CMS signing-time attribute. This means that making
the CMS signing-time attribute mandatory would not cause any existing
commonly used TA or CA to become non-compliant.
As of 29 January 2024, for 83.8% of Signed Objects, the CMS signing-
time timestamp matches the file's mod-time observed via rsync. This
means that it is already the case that RPs would see a significant
reduction in the amount of processing required in rsync if they
adopted the strategy outlined in Section 2.2.
In the above-mentioned period of time, no Signed Objects were
discovered with a CMS binary-signing-time [RFC6019] attribute in the
specified repositories. Therefore, disallowing the use of the CMS
binary-signing-time attribute would not cause any existing commonly
used TA or CA to become non-compliant.
4. Updates to RFC 6488
This section updates [RFC6488] to make the CMS signing-time attribute
mandatory and to disallow the presence of the CMS binary-signing-time
attribute.
* In Section 2.1.6.4, this paragraph is replaced as follows.
OLD
| The signedAttrs element MUST be present and MUST include the
| content- type and message-digest attributes [RFC5652]. The
| signer MAY also include the signing-time attribute [RFC5652],
| the binary-signing-time attribute [RFC6019], or both
| attributes. Other signed attributes MUST NOT be included.
NEW
| The signedAttrs element MUST be present and MUST include the
| content-type, message-digest, and signing-time attributes
| [RFC5652]. Other signed attributes MUST NOT be included.
* In Section 2.1.6.4.3, the first sentence is replaced as follows.
OLD
| The signing-time attribute MAY be present.
NEW
| The signing-time attribute MUST be present.
* In Section 2.1.6.4.3, the sentence "Note that the presence or
absence of the signing-time attribute MUST NOT affect the validity
of the signed object (as specified in Section 3)." is removed.
* Section 2.1.6.4.4 is removed in its entirety.
* In Section 3, item 1.f is replaced as follows.
OLD
| f. The signedAttrs field in the SignerInfo object is present
| and contains both the content-type attribute (OID
| 1.2.840.113549.1.9.3) and the message-digest attribute (OID
| 1.2.840.113549.1.9.4).
NEW
| f. The signedAttrs field in the SignerInfo object is present
| and contains the content-type attribute (OID
| 1.2.840.113549.1.9.3), the message-digest attribute (OID
| 1.2.840.113549.1.9.4), and the signing-time attribute
| (1.2.840.113549.1.9.5).
* In Section 3, item 1.g is replaced as follows.
OLD
| g. The signedAttrs field in the SignerInfo object does not
| contain any attributes other than the following four: the
| content-type attribute (OID 1.2.840.113549.1.9.3), the
| message-digest attribute (OID 1.2.840.113549.1.9.4), the
| signing-time attribute (OID 1.2.840.113549.1.9.5), and the
| binary-signing-time attribute (OID
| 1.2.840.113549.1.9.16.2.46). Note that the signing-time
| and binary-signing-time attributes MAY be present, but they
| are not required.
NEW
| g. The signedAttrs field in the SignerInfo object does not
| contain any attributes other than the following three: the
| content-type attribute (OID 1.2.840.113549.1.9.3), the
| message-digest attribute (OID 1.2.840.113549.1.9.4), and
| the signing-time attribute (OID 1.2.840.113549.1.9.5).
* In Section 9 (Informative References), [RFC6019] is removed from
the list.
5. Security Considerations
No requirement is imposed concerning the correctness of the signing
time attribute. It does not provide reliable information on the time
the signature was produced and it bears no relevance for seamless
switchover between RRDP and rsync.
Although the Security Considerations in [RFC6019] mandate that the
signing-time and binary-signing-time attributes (if both present)
MUST provide the same date and time, there is still a chance that an
object will have values for these attributes that do not represent
the same date and time. Restricting the RPKI Signed Object profile
to a single field for storing the signing time removes any potential
for ambiguity.
6. IANA Considerations
This document has no IANA actions.
7. References
7.1. Normative References
[openrsync]
"openrsync", 2023, <https://www.openrsync.org/>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<https://www.rfc-editor.org/info/rfc5652>.
[RFC6268] Schaad, J. and S. Turner, "Additional New ASN.1 Modules
for the Cryptographic Message Syntax (CMS) and the Public
Key Infrastructure Using X.509 (PKIX)", RFC 6268,
DOI 10.17487/RFC6268, July 2011,
<https://www.rfc-editor.org/info/rfc6268>.
[RFC6480] Lepinski, M. and S. Kent, "An Infrastructure to Support
Secure Internet Routing", RFC 6480, DOI 10.17487/RFC6480,
February 2012, <https://www.rfc-editor.org/info/rfc6480>.
[RFC6481] Huston, G., Loomans, R., and G. Michaelson, "A Profile for
Resource Certificate Repository Structure", RFC 6481,
DOI 10.17487/RFC6481, February 2012,
<https://www.rfc-editor.org/info/rfc6481>.
[RFC6488] Lepinski, M., Chi, A., and S. Kent, "Signed Object
Template for the Resource Public Key Infrastructure
(RPKI)", RFC 6488, DOI 10.17487/RFC6488, February 2012,
<https://www.rfc-editor.org/info/rfc6488>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8182] Bruijnzeels, T., Muravskiy, O., Weber, B., and R. Austein,
"The RPKI Repository Delta Protocol (RRDP)", RFC 8182,
DOI 10.17487/RFC8182, July 2017,
<https://www.rfc-editor.org/info/rfc8182>.
[rsync] "rsync", 2024, <https://rsync.samba.org/>.
7.2. Informative References
[RFC6019] Housley, R., "BinaryTime: An Alternate Format for
Representing Date and Time in ASN.1", RFC 6019,
DOI 10.17487/RFC6019, September 2010,
<https://www.rfc-editor.org/info/rfc6019>.
[RPKI-PUB-SERV]
Bruijnzeels, T., de Kock, T., Hill, F., and T. Harrison,
"RPKI Publication Server Best Current Practices", Work in
Progress, Internet-Draft, draft-timbru-sidrops-
publication-server-bcp-02, 18 January 2024,
<https://datatracker.ietf.org/doc/html/draft-timbru-
sidrops-publication-server-bcp-02>.
[RPKI-REP-REQS]
Bruijnzeels, T., Bush, R., and G. Michaelson, "Resource
Public Key Infrastructure (RPKI) Repository Requirements",
Work in Progress, Internet-Draft, draft-ietf-sidrops-
prefer-rrdp-02, 23 December 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-sidrops-
prefer-rrdp-02>.
[rpkiviews]
"rpkiviews", <https://www.rpkiviews.org/>.
Acknowledgements
The authors would like to thank Ties de Kock, Niels Bakker, Mikael
Abrahamsson, Russ Housley, Zaheduzzaman Sarker, Éric Vyncke, Mahesh
Jethanandani, and Roman Danyliw, for their helpful review of this
document.
Authors' Addresses
Job Snijders
Fastly
Amsterdam
The Netherlands
Email: job@fastly.com
Tom Harrison
Asia Pacific Network Information Centre
6 Cordelia St
South Brisbane QLD 4101
Australia
Email: tomh@apnic.net