Key Usage Limits for OSCORE
draft-ietf-core-oscore-key-limits-00
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| Authors | Rikard Höglund , Marco Tiloca | ||
| Last updated | 2023-03-13 | ||
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draft-ietf-core-oscore-key-limits-00
CoRE Working Group R. Höglund
Internet-Draft M. Tiloca
Intended status: Standards Track RISE AB
Expires: 14 September 2023 13 March 2023
Key Usage Limits for OSCORE
draft-ietf-core-oscore-key-limits-00
Abstract
Object Security for Constrained RESTful Environments (OSCORE) uses
AEAD algorithms to ensure confidentiality and integrity of exchanged
messages. Due to known issues allowing forgery attacks against AEAD
algorithms, limits should be followed on the number of times a
specific key is used for encryption or decryption. Among other
reasons, approaching key usage limits requires updating the OSCORE
keying material before communications can securely continue. This
document defines how two OSCORE peers can follow these key usage
limits and what steps they should take to preserve the security of
their communications.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the Constrained RESTful
Environments Working Group mailing list (core@ietf.org), which is
archived at https://mailarchive.ietf.org/arch/browse/core/.
Source for this draft and an issue tracker can be found at
https://gitlab.com/crimson84/draft-tiloca-core-oscore-to-proxies.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on 14 September 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. AEAD Key Usage Limits in OSCORE . . . . . . . . . . . . . . . 3
2.1. Problem Overview . . . . . . . . . . . . . . . . . . . . 4
2.1.1. Limits for 'q' and 'v' . . . . . . . . . . . . . . . 5
2.2. Additional Information in the Security Context . . . . . 6
2.2.1. Common Context . . . . . . . . . . . . . . . . . . . 6
2.2.2. Sender Context . . . . . . . . . . . . . . . . . . . 6
2.2.3. Recipient Context . . . . . . . . . . . . . . . . . . 7
2.3. OSCORE Messages Processing . . . . . . . . . . . . . . . 7
2.3.1. Protecting a Request or a Response . . . . . . . . . 8
2.3.2. Verifying a Request or a Response . . . . . . . . . . 8
3. Security Considerations . . . . . . . . . . . . . . . . . . . 8
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Normative References . . . . . . . . . . . . . . . . . . 8
5.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Detailed considerations for AEAD_AES_128_CCM_8 . . . 9
Appendix B. Estimation of 'count_q' . . . . . . . . . . . . . . 10
Appendix C. Document Updates . . . . . . . . . . . . . . . . . . 11
C.1. Version -00 . . . . . . . . . . . . . . . . . . . . . . . 11
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
Object Security for Constrained RESTful Environments (OSCORE)
[RFC8613] provides end-to-end protection of CoAP [RFC7252] messages
at the application-layer, ensuring message confidentiality and
integrity, replay protection, as well as binding of response to
request between a sender and a recipient.
In particular, OSCORE uses AEAD algorithms to provide confidentiality
and integrity of messages exchanged between two peers. Due to known
issues allowing forgery attacks against AEAD algorithms, limits
should be followed on the number of times a specific key is used to
perform encryption or decryption [I-D.irtf-cfrg-aead-limits].
The original OSCORE specification [RFC8613] does not consider such
key usage limits. However, should they be exceeded, an adversary may
break the security properties of the AEAD algorithm, such as message
confidentiality and integrity, e.g., by performing a message forgery
attack. Among other reasons, approaching the key usage limits
requires updating the OSCORE keying material before communications
can securely continue. This document defines what steps an OSCORE
peer should take to preserve the security of its communications, by
stopping to use the OSCORE Security Context shared with another peer
when approaching the key usage limits.
1.1. Terminology
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.
Readers are expected to be familiar with the terms and concepts
related to CoAP [RFC7252] and OSCORE [RFC8613].
2. AEAD Key Usage Limits in OSCORE
This section details how key usage limits for AEAD algorithms can be
considered when using OSCORE. In particular, it discusses specific
limits for common AEAD algorithms used with OSCORE; parameters to
track associated to an OSCORE Security Context; and additions to the
OSCORE message processing.
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2.1. Problem Overview
The OSCORE security protocol [RFC8613] uses AEAD algorithms to
provide integrity and confidentiality of messages, as exchanged
between two peers sharing an OSCORE Security Context.
When processing messages with OSCORE, each peer should follow
specific limits as to the number of times it uses a specific key.
This applies separately to the Sender Key used to encrypt outgoing
messages, and to the Recipient Key used to decrypt and verify
incoming protected messages.
Exceeding these limits may allow an adversary to break the security
properties of the AEAD algorithm, such as message confidentiality and
integrity, e.g., by performing a message forgery attack.
The following refers to the two parameters 'q' and 'v' introduced in
[I-D.irtf-cfrg-aead-limits], to use when deploying an AEAD algorithm.
* 'q': this parameter has as value the number of messages protected
with a specific key, i.e., the number of times the AEAD algorithm
has been invoked to encrypt data with that key.
* 'v': this parameter has as value the number of alleged forgery
attempts that have been made against a specific key, i.e., the
amount of failed decryptions that have occurred with the AEAD
algorithm for that key.
When a peer uses OSCORE:
* The key used to protect outgoing messages is its Sender Key from
its Sender Context.
* The key used to decrypt and verify incoming messages is its
Recipient Key from its Recipient Context.
Both keys are derived as part of the establishment of the OSCORE
Security Context, as defined in Section 3.2 of [RFC8613].
As mentioned above, exceeding specific limits for the 'q' or 'v'
value can weaken the security properties of the AEAD algorithm used,
thus compromising secure communication requirements.
Therefore, in order to preserve the security of the used AEAD
algorithm, OSCORE has to observe limits for the 'q' and 'v' values,
throughout the lifetime of the used AEAD keys.
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2.1.1. Limits for 'q' and 'v'
Formulas for calculating the security levels, as Integrity Advantage
(IA) and Confidentiality Advantage (CA) probabilities, are presented
in [I-D.irtf-cfrg-aead-limits]. These formulas take as input
specific values for 'q' and 'v' (see section Section 2.1) and for
'l', i.e., the maximum length of each message (in cipher blocks).
For the algorithms shown in Figure 1 that can be used as AEAD
Algorithm for OSCORE, the key property to achieve is having IA and CA
values which are no larger than p = 2^-64, which will ensure a safe
security level for the AEAD Algorithm. This can be entailed by using
the values q = 2^20, v = 2^20, and l = 2^10, that this document
recommends to use for these algorithms.
Figure 1 also shows the resulting IA and CA probabilities enjoyed by
the considered algorithms, when taking the value of 'q', 'v' and 'l'
above as input to the formulas defined in
[I-D.irtf-cfrg-aead-limits].
+------------------------+----------------+----------------+
| Algorithm name | IA probability | CA probability |
|------------------------+----------------+----------------|
| AEAD_AES_128_CCM | 2^-64 | 2^-66 |
| AEAD_AES_128_GCM | 2^-97 | 2^-89 |
| AEAD_AES_256_GCM | 2^-97 | 2^-89 |
| AEAD_CHACHA20_POLY1305 | 2^-73 | - |
+------------------------+----------------+----------------+
Figure 1: Probabilities for algorithms based on chosen q, v and l
values.
When AEAD_AES_128_CCM_8 is used as AEAD Algorithm for OSCORE, the
triplet (q, v, l) considered above yields larger values of IA and CA.
Hence, specifically for AEAD_AES_128_CCM_8, this document recommends
using the triplet (q, v, l) = (2^20, 2^14, 2^8). This is
appropriate, since the resulting CA and IA values are not greater
than the threshold value of 2^-50 defined in
[I-D.irtf-cfrg-aead-limits], and thus yields an acceptable security
level. Achieving smaller values of CA and IA would require to
inconveniently reduce 'q', 'v' or 'l', with no corresponding increase
in terms of security, as further elaborated in Appendix A.
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+------------------------+----------+----------+-----------+
| Algorithm name | l=2^6 in | l=2^8 in | l=2^10 in |
| | bytes | bytes | bytes |
|------------------------+----------+----------|-----------|
| AEAD_AES_128_CCM | 1024 | 4096 | 16384 |
| AEAD_AES_128_GCM | 1024 | 4096 | 16384 |
| AEAD_AES_256_GCM | 1024 | 4096 | 16384 |
| AEAD_AES_128_CCM_8 | 1024 | 4096 | 16384 |
| AEAD_CHACHA20_POLY1305 | 4096 | 16384 | 65536 |
+------------------------+----------+----------+-----------+
Figure 2: Maximum length of each message (in bytes)
2.2. Additional Information in the Security Context
In addition to what defined in Section 3.1 of [RFC8613], the
following parameters, associated to an OSCORE Security Context, can
be used for keeping track of expiration of an OSCORE Security Context
and maintaining key usage below safe limits.
2.2.1. Common Context
The Common Context has the following associated parameter.
* 'exp': with value the expiration time of the OSCORE Security
Context, as a non-negative integer. The parameter contains a
numeric value representing the number of seconds from
1970-01-01T00:00:00Z UTC until the specified UTC date/time,
ignoring leap seconds, analogous to what specified for NumericDate
in Section 2 of [RFC7519].
At the time indicated in this field, a peer must stop using this
Security Context to process any incoming or outgoing message, and
is required to establish a new Security Context to continue
OSCORE-protected communications with the other peer.
2.2.2. Sender Context
The Sender Context has the following associated parameters.
* 'count_q': a non-negative integer counter, keeping track of the
current 'q' value for the Sender Key. At any time, 'count_q' has
as value the number of messages that have been encrypted using the
Sender Key. The value of 'count_q' is set to 0 when establishing
the Sender Context.
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* 'limit_q': a non-negative integer, which specifies the highest
value that 'count_q' is allowed to reach, before stopping using
the Sender Key to process outgoing messages.
The value of 'limit_q' depends on the AEAD algorithm specified in
the Common Context, considering the properties of that algorithm.
The value of 'limit_q' is determined according to Section 2.1.1.
Note for implementation: it is possible to avoid storing and
maintaining the counter 'count_q'. Rather, an estimated value to be
compared against 'limit_q' can be computed, by leveraging the Sender
Sequence Number of the peer and (an estimate of) the other peer's. A
possible method to achieve this is described in Appendix B. While
this relieves peers from storing and maintaining the precise
'count_q' value, it results in overestimating the number of
encryptions performed with a Sender Key. This in turn results in
approaching 'limit_q' sooner and thus in performing a key update
procedure more frequently.
2.2.3. Recipient Context
The Recipient Context has the following associated parameters.
* 'count_v': a non-negative integer counter, keeping track of the
current 'v' value for the Recipient Key. At any time, 'count_v'
has as value the number of failed decryptions occurred on incoming
messages using the Recipient Key. The value of 'count_v' is set to
0 when establishing the Recipient Context.
* 'limit_v': a non-negative integer, which specifies the highest
value that 'count_v' is allowed to reach, before stopping using
the Recipient Key to process incoming messages.
The value of 'limit_v' depends on the AEAD algorithm specified in
the Common Context, considering the properties of that algorithm.
The value of 'limit_v' is determined according to Section 2.1.1.
2.3. OSCORE Messages Processing
In order to keep track of the 'q' and 'v' values and ensure that AEAD
keys are not used beyond reaching their limits, the processing of
OSCORE messages is extended as defined in this section. A limitation
that is introduced is that, in order to not exceed the selected value
for 'l', the total size of the COSE plaintext, authentication Tag,
and possible cipher padding for a message may not exceed the block
size for the selected algorithm multiplied with 'l‘.
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In particular, the processing of OSCORE messages follows the steps
outlined in Section 8 of [RFC8613], with the additions defined below.
2.3.1. Protecting a Request or a Response
Before encrypting the COSE object using the Sender Key, the 'count_q'
counter is incremented.
If 'count_q' exceeds the 'limit_q' limit, the message processing is
aborted. From then on, the Sender Key must not be used to encrypt
further messages.
2.3.2. Verifying a Request or a Response
If an incoming message is detected to be a replay (see Section 7.4 of
[RFC8613]), the 'count_v' counter is not incremented.
If the decryption and verification of the COSE object using the
Recipient Key fails, the 'count_v' counter is incremented.
After 'count_v' has exceeded the 'limit_v' limit, incoming messages
must not be decrypted and verified using the Recipient Key, and their
processing must be aborted.
3. Security Considerations
This document mainly covers security considerations about using AEAD
keys in OSCORE and their usage limits, in addition to the security
considerations of [RFC8613].
[TODO: Add more considerations.]
4. IANA Considerations
This document has no IANA actions.
5. References
5.1. 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/info/rfc2119>.
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[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[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>.
[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/info/rfc8613>.
5.2. Informative References
[I-D.irtf-cfrg-aead-limits]
Günther, F., Thomson, M., and C. A. Wood, "Usage Limits on
AEAD Algorithms", Work in Progress, Internet-Draft, draft-
irtf-cfrg-aead-limits-06, 30 January 2023,
<https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
aead-limits-06>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>.
Appendix A. Detailed considerations for AEAD_AES_128_CCM_8
For the AEAD_AES_128_CCM_8 algorithm when used as AEAD Algorithm for
OSCORE, larger IA and CA values are achieved, depending on the value
of 'q', 'v' and 'l'. Figure 3 shows the resulting IA and CA
probabilities enjoyed by AEAD_AES_128_CCM_8, when taking different
values of 'q', 'v' and 'l' as input to the formulas defined in
[I-D.irtf-cfrg-aead-limits].
As shown in Figure 3, it is especially possible to achieve the lowest
IA = 2^-50 and a good CA = 2^-70 by considering the largest possible
value of the (q, v, l) triplet equal to (2^20, 2^10, 2^8), while
still keeping a good security level. Note that the value of 'l' does
not impact on IA, while CA displays good values for every considered
value of 'l'.
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+-----------------------+----------------+----------------+
| 'q', 'v' and 'l' | IA probability | CA probability |
|-----------------------+----------------+----------------|
| q=2^20, v=2^20, l=2^8 | 2^-44 | 2^-70 |
| q=2^15, v=2^20, l=2^8 | 2^-44 | 2^-80 |
| q=2^10, v=2^20, l=2^8 | 2^-44 | 2^-90 |
| q=2^20, v=2^15, l=2^8 | 2^-49 | 2^-70 |
| q=2^15, v=2^15, l=2^8 | 2^-49 | 2^-80 |
| q=2^10, v=2^15, l=2^8 | 2^-49 | 2^-90 |
| q=2^20, v=2^14, l=2^8 | 2^-50 | 2^-70 |
| q=2^15, v=2^14, l=2^8 | 2^-50 | 2^-80 |
| q=2^10, v=2^14, l=2^8 | 2^-50 | 2^-90 |
| q=2^20, v=2^10, l=2^8 | 2^-54 | 2^-70 |
| q=2^15, v=2^10, l=2^8 | 2^-54 | 2^-80 |
| q=2^10, v=2^10, l=2^8 | 2^-54 | 2^-90 |
|-----------------------+----------------+----------------|
| q=2^20, v=2^20, l=2^6 | 2^-44 | 2^-74 |
| q=2^15, v=2^20, l=2^6 | 2^-44 | 2^-84 |
| q=2^10, v=2^20, l=2^6 | 2^-44 | 2^-94 |
| q=2^20, v=2^15, l=2^6 | 2^-49 | 2^-74 |
| q=2^15, v=2^15, l=2^6 | 2^-49 | 2^-84 |
| q=2^10, v=2^15, l=2^6 | 2^-49 | 2^-94 |
| q=2^20, v=2^14, l=2^6 | 2^-50 | 2^-74 |
| q=2^15, v=2^14, l=2^6 | 2^-50 | 2^-84 |
| q=2^10, v=2^14, l=2^6 | 2^-50 | 2^-94 |
| q=2^20, v=2^10, l=2^6 | 2^-54 | 2^-74 |
| q=2^15, v=2^10, l=2^6 | 2^-54 | 2^-84 |
| q=2^10, v=2^10, l=2^6 | 2^-54 | 2^-94 |
+-----------------------+----------------+----------------+
Figure 3: Probabilities for AEAD_AES_128_CCM_8 based on chosen q,
v and l values.
Appendix B. Estimation of 'count_q'
This section defines a method to compute an estimate of the counter
'count_q' (see Section 2.2.2), hence not requiring a peer to store it
in its own Sender Context.
This method relies on the fact that, at any point in time, a peer has
performed _at most_ ENC = (SSN + SSN*) encryptions using its own
Sender Key, where:
* SSN is the current value of this peer's Sender Sequence Number.
* SSN* is the current value of other peer's Sender Sequence Number.
That is, SSN* is an overestimation of the responses without
Partial IV that this peer has sent.
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Thus, when protecting an outgoing message (see Section 2.3.1), the
peer aborts the message processing if the estimated est_q > limit_q,
where est_q = (SSN + X) and X is determined as follows.
* If the outgoing message is a response, X is the Partial IV
specified in the corresponding request that this peer is
responding to. Note that X < SSN* always holds.
* If the outgoing message is a request, X is the highest Partial IV
value marked as received in this peer's Replay Window plus 1, or 0
if it has not accepted any protected message from the other peer
yet. That is, X is the highest Partial IV specified in message
received from the other peer, i.e., the highest seen Sender
Sequence Number of the other peer. Note that, also in this case,
X < SSN* always holds.
Appendix C. Document Updates
RFC EDITOR: PLEASE REMOVE THIS SECTION.
C.1. Version -00
* Editorial improvements.
* Extended terminology.
* Recommendation on limits for CCM_8. Details in Appendix.
* Example of method to estimate and not store 'count_q'.
* Split out material from Key Update for OSCORE draft into this new
document.
Acknowledgments
The authors sincerely thank Christian Amsüss, Carsten Bormann, John
Preuß Mattsson, Göran Selander and Rafa Marin-Lopez for their
feedback and comments.
The work on this document has been partly supported by VINNOVA and
the Celtic-Next project CRITISEC; and by the H2020 project SIFIS-Home
(Grant agreement 952652).
Authors' Addresses
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Rikard Höglund
RISE AB
Isafjordsgatan 22
SE-16440 Stockholm Kista
Sweden
Email: rikard.hoglund@ri.se
Marco Tiloca
RISE AB
Isafjordsgatan 22
SE-16440 Stockholm Kista
Sweden
Email: marco.tiloca@ri.se
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