Internet DRAFT - draft-hoeglund-core-oscore-key-limits
draft-hoeglund-core-oscore-key-limits
CoRE Working Group R. Höglund
Internet-Draft M. Tiloca
Updates: 8613 (if approved) RISE AB
Intended status: Standards Track 25 October 2021
Expires: 28 April 2022
Key Update for OSCORE (KUDOS)
draft-hoeglund-core-oscore-key-limits-02
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. This document
defines how two OSCORE peers must follow these limits and what steps
they must take to preserve the security of their communications.
Therefore, this document updates RFC8613. Furthermore, this document
specifies Key Update for OSCORE (KUDOS), a lightweight procedure that
two peers can use to update their keying material and establish a new
OSCORE Security Context.
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
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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 28 April 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. AEAD Key Usage Limits in OSCORE . . . . . . . . . . . . . . . 3
2.1. Problem Overview . . . . . . . . . . . . . . . . . . . . 3
2.1.1. Limits for 'q' and 'v' . . . . . . . . . . . . . . . 4
2.2. Additional Information in the Security Context . . . . . 7
2.2.1. Common Context . . . . . . . . . . . . . . . . . . . 7
2.2.2. Sender Context . . . . . . . . . . . . . . . . . . . 7
2.2.3. Recipient Context . . . . . . . . . . . . . . . . . . 8
2.3. OSCORE Messages Processing . . . . . . . . . . . . . . . 8
2.3.1. Protecting a Request or a Response . . . . . . . . . 8
2.3.2. Verifying a Request or a Response . . . . . . . . . . 9
3. Current methods for Rekeying OSCORE . . . . . . . . . . . . . 9
4. Key Update for OSCORE (KUDOS) . . . . . . . . . . . . . . . . 11
4.1. Extensions to the OSCORE Option . . . . . . . . . . . . . 12
4.2. Function for Security Context Update . . . . . . . . . . 13
4.3. Establishment of the New OSCORE Security Context . . . . 15
4.3.1. Client-Initiated Key Update . . . . . . . . . . . . . 16
4.3.2. Server-Initiated Key Update . . . . . . . . . . . . . 18
4.4. Retention Policies . . . . . . . . . . . . . . . . . . . 21
4.5. Discussion . . . . . . . . . . . . . . . . . . . . . . . 21
5. Security Considerations . . . . . . . . . . . . . . . . . . . 21
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
6.1. OSCORE Flag Bits Registry . . . . . . . . . . . . . . . . 22
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.1. Normative References . . . . . . . . . . . . . . . . . . 22
7.2. Informative References . . . . . . . . . . . . . . . . . 23
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
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.
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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].
Should these limits 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. The original
OSCORE specification [RFC8613] does not consider such limits.
This document updates [RFC8613] as follows.
* It defines when a peer must stop using an OSCORE Security Context
shared with another peer, due to the reached key usage limits.
When this happens, the two peers have to establish a new Security
Context with new keying material, in order to continue their
secure communication with OSCORE.
* It specifies KUDOS, a lightweight key update procedure that the
two peers can use in order to update their current keying material
and establish a new OSCORE Security Context. This deprecates and
replaces the procedure specified in Appendix B.2 of [RFC8613].
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 the CoAP [RFC7252] and OSCORE [RFC8613] protocols.
2. AEAD Key Usage Limits in OSCORE
The following sections details how key usage limits for AEAD
algorithms must be considered when using OSCORE. It covers specific
limits for common AEAD algorithms used with OSCORE; necessary
additions to the OSCORE Security Context, updates to the OSCORE
message processing, and existing methods for rekeying OSCORE.
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.
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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 has been done with the AEAD
algorithm for that key.
When a peer uses OSCORE:
* The key used to protect outgoing messages is its Sender Key, in
its Sender Context.
* The key used to decrypt and verify incoming messages is its
Recipient Key, in 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.
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).
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For the algorithms that can be used as AEAD Algorithm for OSCORE
shows in Figure 1, 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 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.
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 2 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 2, it is especially possible to achieve the lowest
IA = 2^-54 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'.
When AEAD_AES_128_CCM_8 is used as AEAD Algorithm for OSCORE, this
document recommends to use the triplet (q, v, l) = (2^20, 2^10, 2^8)
and to never use a triplet (q, v, l) such that the resulting IA and
CA probabilities are higher than 2^-54.
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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 2: Probabilities for AEAD_AES_128_CCM_8 based on chosen q,
v and l values.
The algorithms using AES presented in this draft all use a block size
of 16 bytes (128 bits), while AEAD_CHACHA20_POLY1305 uses a block
size of 64 bytes (512 bits). As 'l' is defined as the maximum size
of each message in blocks, different block sizes will result in
different maximum messages sizes for the same value of 'l'. Figure 3
presents the resulting maximum message size in bytes for the
different algorithms and values of 'l' presented in this document.
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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 3: 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 OSCORE
Security Context MUST also include the following information.
2.2.1. Common Context
The Common Context is extended to include the following 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 is extended to include the following 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.
* '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.
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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.
2.2.3. Recipient Context
The Recipient Context is extended to include the following
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'.
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 MUST be incremented.
If 'count_q' exceeds the 'limit_q' limit, the message processing MUST
be aborted. From then on, the Sender Key MUST NOT be used to encrypt
further messages.
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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 MUST NOT be incremented.
If the decryption and verification of the COSE object using the
Recipient Key fails, the 'count_v' counter MUST be 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. Current methods for Rekeying OSCORE
Before the limit of 'q' or 'v' defined in Section 2.1.1 has been
reached for an OSCORE Security Context, the two peers have to
establish a new OSCORE Security Context, in order to continue using
OSCORE for secure communication.
In practice, the two peers have to establish new Sender and Recipient
Keys, as the keys actually used by the AEAD algorithm. When this
happens, both peers reset their 'count_q' and 'count_v' values to 0
(see Section 2.2).
Other specifications define a number of ways to accomplish this, as
summarized below.
* The two peers can run the procedure defined in Appendix B.2 of
[RFC8613]. That is, the two peers exchange three or four
messages, protected with temporary Security Contexts adding
randomness to the ID Context.
As a result, the two peers establish a new OSCORE Security Context
with new ID Context, Sender Key and Recipient Key, while keeping
the same OSCORE Master Secret and OSCORE Master Salt from the old
OSCORE Security Context.
This procedure does not require any additional components to what
OSCORE already provides, and it does not provide perfect forward
secrecy.
The procedure defined in Appendix B.2 of [RFC8613] is used in
6TiSCH networks [RFC7554][RFC8180] when handling failure events.
That is, a node acting as Join Registrar/Coordinator (JRC) assists
new devices, namely "pledges", to securely join the network as per
the Constrained Join Protocol [RFC9031]. In particular, a pledge
exchanges OSCORE-protected messages with the JRC, from which it
obtains a short identifier, link-layer keying material and other
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configuration parameters. As per Section 8.3.3 of [RFC9031], a
JRC that experiences a failure event may likely lose information
about joined nodes, including their assigned identifiers. Then,
the reinitialized JRC can establish a new OSCORE Security Context
with each pledge, through the procedure defined in Appendix B.2 of
[RFC8613].
* The two peers can run the OSCORE profile
[I-D.ietf-ace-oscore-profile] of the Authentication and
Authorization for Constrained Environments (ACE) Framework
[I-D.ietf-ace-oauth-authz].
When a CoAP client uploads an Access Token to a CoAP server as an
access credential, the two peers also exchange two nonces. Then,
the two peers use the two nonces together with information
provided by the ACE Authorization Server that issued the Access
Token, in order to derive an OSCORE Security Context.
This procedure does not provide perfect forward secrecy.
* The two peers can run the EDHOC key exchange protocol based on
Diffie-Hellman and defined in [I-D.ietf-lake-edhoc], in order to
establish a pseudo-random key in a mutually authenticated way.
Then, the two peers can use the established pseudo-random key to
derive external application keys. This allows the two peers to
securely derive especially an OSCORE Master Secret and an OSCORE
Master Salt, from which an OSCORE Security Context can be
established.
This procedure additionally provides perfect forward secrecy.
* If one peer is acting as LwM2M Client and the other peer as LwM2M
Server, according to the OMA Lightweight Machine to Machine Core
specification [LwM2M], then the LwM2M Client peer may take the
initiative to bootstrap again with the LwM2M Bootstrap Server, and
receive again an OSCORE Security Context. Alternatively, the
LwM2M Server can instruct the LwM2M Client to initiate this
procedure.
If the OSCORE Security Context information on the LwM2M Bootstrap
Server has been updated, the LwM2M Client will thus receive a
fresh OSCORE Security Context to use with the LwM2M Server.
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In addition to that, the LwM2M Client, the LwM2M Server as well as
the LwM2M Bootstrap server are required to use the procedure
defined in Appendix B.2 of [RFC8613] and overviewed above, when
they use a certain OSCORE Security Context for the first time
[LwM2M-Transport].
Manually updating the OSCORE Security Context at the two peers should
be a last resort option, and it might often be not practical or
feasible.
Even when any of the alternatives mentioned above is available, it is
RECOMMENDED that two OSCORE peers update their Security Context by
using the KUDOS procedure as defined in Section 4 of this document.
It is RECOMMENDED that the peer initiating the key update procedure
starts it before reaching the 'q' or 'v' limits. Otherwise, the AEAD
keys possibly to be used during the key update procedure itself may
already be or become invalid before the rekeying is completed, which
may prevent a successful establishment of the new OSCORE Security
Context altogether.
4. Key Update for OSCORE (KUDOS)
This section defines KUDOS, a lightweight procedure that two OSCORE
peers can use to update their keying material and establish a new
OSCORE Security Context.
KUDOS relies on the support function updateCtx() defined in
Section 4.2 and the message exchange defined in Section 4.3. The
following properties are fulfilled.
* KUDOS can be initiated by either peer. In particular, the client
or the server may start KUDOS by sending the first rekeying
message.
* The new OSCORE Security Context enjoys Perfect Forward Secrecy.
* The same ID Context value used in the old OSCORE Security Context
is preserved in the new Security Context. Furthermore, the ID
Context value never changes throughout the KUDOS execution.
* KUDOS is robust against a peer rebooting, and it especially avoids
the reuse of AEAD (nonce, key) pairs.
* KUDOS completes in one round trip. The two peers achieve mutual
proof-of-possession in the following exchange, which is protected
with the newly established OSCORE Security Context.
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4.1. Extensions to the OSCORE Option
In order to support the message exchange for establishing a new
OSCORE Security Context as defined in Section 4.3, this document
extends the use of the OSCORE option originally defined in [RFC8613]
as follows.
* This document defines the usage of the seventh least significant
bit, called "Extension-1 Flag", in the first byte of the OSCORE
option containing the OSCORE flag bits. This flag bit is
specified in Section 6.1.
When the Extension-1 Flag is set to 1, the second byte of the
OSCORE option MUST include the set of OSCORE flag bits 8-15.
* This document defines the usage of the first least significant bit
"ID Detail Flag", 'd', in the second byte of the OSCORE option
containing the OSCORE flag bits. This flag bit is specified in
Section 6.1.
When it is set to 1, the compressed COSE object contains an 'id
detail', to be used for the steps defined in Section 4.3. In
particular, the 1 byte following 'kid context' (if any) encodes
the length x of 'id detail', and the following x bytes encode 'id
detail'.
* The second-to-eighth least significant bits in the second byte of
the OSCORE option containing the OSCORE flag bits are reserved for
future use. These bits SHALL be set to zero when not in use.
According to this specification, if any of these bits are set to
1, the message is considered to be malformed and decompression
fails as specified in item 2 of Section 8.2 of [RFC8613].
Figure 4 shows the OSCORE option value including also 'id detail'.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 <----- n bytes ----->
+-+-+-+-+-+-+-+-+---+---+---+---+---+---+---+---+---------------------+
|0|1|0|h|k| n | 0 | 0 | 0 | 0 | 0 | 0 | 0 | d | Partial IV (if any) |
+-+-+-+-+-+-+-+-+---+---+---+---+---+---+---+---+---------------------+
<- 1 byte -> <----- s bytes ------> <- 1 byte -> <----- x bytes ---->
+------------+----------------------+---------------------------------+
| s (if any) | kid context (if any) | x (if any) | id detail (if any) |
+------------+----------------------+------------+--------------------+
+------------------+
| kid (if any) ... |
+------------------+
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Figure 4: The OSCORE option value, including 'id detail'
4.2. Function for Security Context Update
The updateCtx() function shown in Figure 5 takes as input a nonce N
as well as an OSCORE Security Context CTX_IN, and returns as output a
new OSCORE Security Context CTX_OUT.
As a first step, the updateCtx() function derives the new values of
the Master Secret and Master Salt for CTX_OUT, according to one of
the two following methods. The used method depends on how the two
peers established their original Security Context, i.e., the Security
Context that they shared before performing KUDOS with one another for
the first time.
* If the original Security Context was established by running the
EDHOC protocol [I-D.ietf-lake-edhoc], the following applies.
First, the EDHOC key PRK_4x3m shared by the two peers is updated
using the EDHOC-KeyUpdate() function defined in Section 4.4 of
[I-D.ietf-lake-edhoc], which takes the nonce N as input.
After that, the EDHOC-Exporter() function defined in Section 4.3
of [I-D.ietf-lake-edhoc] is used to derive the new values for the
Master Secret and Master Salt, consistently with what is defined
in Appendix A.2 of [I-D.ietf-lake-edhoc]. In particular, the
context parameter provided as second argument to the EDHOC-
Exporter() function is the empty CBOR byte string (0x40)
[RFC8949], which is denoted as h''.
Note that, compared to the compliance requirements in Section 7 of
[I-D.ietf-lake-edhoc], a peer MUST support the EDHOC-KeyUpdate()
function, in case it establishes an original Security Context
through the EDHOC protocol and intends to perform KUDOS.
* If the original Security Context was established through other
means than the EDHOC protocol, the new Master Secret is derived
through an HKDF-Expand() step, which takes as input N as well as
the Master Secret value from the Security Context CTX_IN.
Instead, the new Master Salt takes N as value.
In either case, the derivation of new values follows the same
approach used in TLS 1.3, which is also based on HKDF-Expand (see
Section 7.1 of [RFC8446]) and used for computing new keying material
in case of key update (see Section 4.6.3 of [RFC8446]).
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After that, the new Master Secret and Master Salt parameters are used
to derive a new Security Context CTX_OUT as per Section 3.2 of
[RFC8613]. Any other parameter required for the derivation takes the
same value as in the Security Context CTX_IN. Finally, the function
returns the newly derived Security Context CTX_OUT.
updateCtx(N, CTX_IN) {
CTX_OUT // The new Security Context
MSECRET_NEW // The new Master Secret
MSALT_NEW // The new Master Salt
if <the original Security Context was established through EDHOC> {
EDHOC-KeyUpdate(N)
// This results in updating the key PRK_4x3m of the
// EDHOC session, i.e., PRK_4x3m = Extract(N, PRK_4x3m)
MSECRET_NEW = EDHOC-Exporter("OSCORE_Master_Secret",
h'', key_length)
= EDHOC-KDF(PRK_4x3m, TH_4,
"OSCORE_Master_Secret", h'', key_length)
MSALT_NEW = EDHOC-Exporter("OSCORE_Master_Salt",
h'', salt_length)
= EDHOC-KDF(PRK_4x3m, TH_4,
"OSCORE_Master_Salt", h'', salt_length)
}
else {
Master Secret Length = < Size of CTX_IN.MasterSecret in bytes >
MSECRET_NEW = HKDF-Expand-Label(CTX_IN.MasterSecret, Label,
N, Master Secret Length)
= HKDF-Expand(CTX_IN.MasterSecret, HkdfLabel,
Master Secret Length)
MSALT_NEW = N;
}
< Derive CTX_OUT using MSECRET_NEW and MSALT_NEW,
together with other parameters from CTX_IN >
Return CTX_OUT;
}
Where HkdfLabel is defined as
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struct {
uint16 length = Length;
opaque label<7..255> = "oscore " + Label;
opaque context<0..255> = Context;
} HkdfLabel;
Figure 5: Function for deriving a new OSCORE Security Context
4.3. Establishment of the New OSCORE Security Context
This section defines the actual KUDOS procedure performed by two
peers to update their OSCORE keying material. Before starting KUDOS,
the two peers share the OSCORE Security Context CTX_OLD. Once
completed the KUDOS execution, the two peers agree on a newly
established OSCORE Security Context CTX_NEW.
In particular, each peer contributes by generating a fresh value R1
or R2, and providing it to the other peer. The byte string
concatenation of the two values, hereafter denoted as R1 | R2, is
used as input N by the updateCtx() function, in order to derive the
new OSCORE Security Context CTX_NEW. As for any new OSCORE Security
Context, the Sender Sequence Number and the replay window are re-
initialized accordingly (see Section 3.2.2 of [RFC8613]).
Once a peer has successfully derived the new OSCORE Security Context
CTX_NEW, that peer MUST terminate all the ongoing observations it has
with the other peer as protected with the old Security Context
CTX_OLD.
Once a peer has successfully decrypted and verified an incoming
message protected with CTX_NEW, that peer MUST discard the old
Security Context CTX_OLD.
KUDOS can be started by the client or the server, as defined in
Section 4.3.1 and Section 4.3.2, respectively. The following
properties hold for both the client- and server-initiated version of
KUDOS.
* The initiator always offers the fresh value R1.
* The responder always offers the fresh value R2.
* The responder is always the first one deriving the new OSCORE
Security Context CTX_NEW.
* The initiator is always the first one achieving key confirmation,
hence able to safely discard the old OSCORE Security Context
CTX_OLD.
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* Both the initiator and the responder use the same respective
OSCORE Sender ID and Recipient ID. Also, they both preserve and
use the same OSCORE ID Context from CTX_OLD.
The length of the nonces R1, and R2 is application specific. The
application needs to set the length of each nonce such that the
probability of its value being repeated is negligible; typically, at
least 8 bytes long.
4.3.1. Client-Initiated Key Update
Figure 6 shows the KUDOS workflow with the client acting as
initiator.
Client Server
(initiator) (responder)
| |
Generate R1 | |
| |
CTX_1 = | |
updateCtx(R1, | |
CTX_OLD) | |
| |
| Request #1 |
Protect with CTX_1 |------------------->|
| OSCORE Option: | CTX_1 =
| ... | update(R1,
| d flag: 1 | CTX_OLD)
| ... |
| ID Detail: R1 | Verify with CTX_1
| ... |
| | Generate R2
| |
| | CTX_NEW =
| | update(R1|R2,
| | CTX_OLD)
| |
| Response #1 |
|<-------------------| Protect with CTX_NEW
CTX_NEW = | OSCORE Option: |
updateCtx(R1|R2, | ... |
CTX_OLD) | d flag: 1 |
| ... |
Verify with CTX_NEW | ID Detail: R2 |
| ... |
Discard CTX_OLD | |
| |
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// The actual key update process ends here.
// The two peers can use the new Security Context CTX_NEW.
| |
| Request #2 |
Protect with CTX_NEW |------------------->|
| | Verify with CTX_NEW
| |
| | Discard CTX_OLD
| |
| Response #2 |
|<-------------------| Protect with CTX_NEW
Verify with CTX_NEW | |
| |
Figure 6: Client-Initiated KUDOS Workflow
First, the client generates a random value R1, and uses the nonce N =
R1 together with the old Security Context CTX_OLD, in order to derive
a temporary Security Context CTX_1. Then, the client sends an OSCORE
request to the server, protected with the Security Context CTX_1. In
particular, the request has the 'd' flag bit set to 1 and specifies
R1 as 'id detail' (see Section 4.1).
Upon receiving the OSCORE request, the server retrieves the value R1
from the 'id detail' of the request, and uses the nonce N = R1
together with the old Security Context CTX_OLD, in order to derive
the temporary Security Context CTX_1. Then, the server verifies the
request by using the Security Context CTX_1.
After that, the server generates a random value R2, and uses the
nonce N = R1 | R2 together with the old Security Context CTX_OLD, in
order to derive the new Security Context CTX_NEW. Then, the server
sends an OSCORE response to the client, protected with the new
Security Context CTX_NEW. In particular, the response has the 'd'
flag bit set to 1 and specifies R2 as 'id detail'.
Upon receiving the OSCORE response, the client retrieves the value R2
from the 'id detail' of the response. Since the client has received
a response to an OSCORE request it made with the 'd' flag bit set to
1, the client uses the nonce N = R1 | R2 together with the old
Security Context CTX_OLD, in order to derive the new Security Context
CTX_NEW. Finally, the client verifies the response by using the
Security Context CTX_NEW and deletes the old Security Context
CTX_OLD.
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After that, the client can send a new OSCORE request protected with
the new Security Context CTX_NEW. When successfully verifying the
request using the Security Context CTX_NEW, the server deletes the
old Security Context CTX_OLD and can reply with an OSCORE response
protected with the new Security Context CTX_NEW.
From then on, the two peers can protect their message exchanges by
using the new Security Context CTX_NEW.
4.3.2. Server-Initiated Key Update
Figure 7 shows the KUDOS workflow with the server acting as
initiator.
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Client Server
(responder) (initiator)
| |
| Request #1 |
Protect with CTX_OLD |------------------->|
| | Verify with CTX_OLD
| |
| | Generate R1
| |
| | CTX_1 =
| | updateCtx(R1,
| | CTX_OLD)
| |
| Response #1 |
|<-------------------| Protect with CTX_1
CTX_1 = | OSCORE Option: |
updateCtx(R1, | ... |
CTX_OLD) | d flag: 1 |
| ... |
Verify with CTX_1 | ID Detail: R1 |
| ... |
Generate R2 | |
| |
CTX_NEW = | |
updateCtx(R1|R2, | |
CTX_OLD) | |
| |
| Request #2 |
Protect with CTX_NEW |------------------->|
| OSCORE Option: | CTX_NEW =
| ... | updateCtx(R1|R2,
| d flag: 1 | CTX_OLD)
| ... |
| ID Detail: R1|R2 | Verify with CTX_NEW
| ... |
| | Discard CTX_OLD
| |
// The actual key update process ends here.
// The two peers can use the new Security Context CTX_NEW.
| Response #2 |
|<-------------------| Protect with CTX_NEW
Verify with CTX_NEW | |
| |
Discard CTX_OLD | |
| |
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Figure 7: Server-Initiated KUDOS Workflow
First, the client sends a normal OSCORE request to the server,
protected with the old Security Context CTX_OLD and with the 'd' flag
bit set to 0.
Upon receiving the OSCORE request and after having verified it with
the old Security Context CTX_OLD as usual, the server generates a
random value R1 and uses the nonce N = R1 together with the old
Security Context CTX_OLD, in order to derive a temporary Security
Context CTX_1. Then, the server sends an OSCORE response to the
client, protected with the Security Context CTX_1. In particular,
the response has the 'd' flag bit set to 1 and specifies R1 as 'id
detail' (see Section 4.1).
Upon receiving the OSCORE response, the client retrieves the value R1
from the 'id detail' of the response, and uses the nonce N = R1
together with the old Security Context CTX_OLD, in order to derive
the temporary Security Context CTX_1. Then, the client verifies the
response by using the Security Context CTX_1.
After that, the client generates a random value R2, and uses the
nonce N = R1 | R2 together with the old Security Context CTX_OLD, in
order to derive the new Security Context CTX_NEW. Then, the client
sends an OSCORE request to the server, protected with the new
Security Context CTX_NEW. In particular, the request has the 'd'
flag bit set to 1 and specifies R1 | R2 as 'id detail'.
Upon receiving the OSCORE request, the server retrieves the value
R1 | R2 from the request. Then, the server verifies that: i) the
value R1 is identical to the value R1 specified in a previous OSCORE
response with the 'd' flag bit set to 1; and ii) the value R1 | R2
has not been received before in an OSCORE request with the 'd' flag
bit set to 1. If the verification succeeds, the server uses the
nonce N = R1 | R2 together with the old Security Context CTX_OLD, in
order to derive the new Security Context CTX_NEW. Finally, the
server verifies the request by using the Security Context CTX_NEW and
deletes the old Security Context CTX_OLD.
After that, the server can send an OSCORE response protected with the
new Security Context CTX_NEW. When successfully verifying the
response using the Security Context CTX_NEW, the client deletes the
old Security Context CTX_OLD.
From then on, the two peers can protect their message exchanges by
using the new Security Context CTX_NEW.
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4.4. Retention Policies
Applications MAY define policies that allows a peer to also
temporarily keep the old Security Context CTX_OLD, rather than simply
overwriting it to become CTX_NEW. This allows the peer to decrypt
late, still on-the-fly incoming messages protected with CTX_OLD.
When enforcing such policies, the following applies.
* Outgoing messages MUST be protected by using only CTX_NEW.
* Incoming messages MUST first be attempted to decrypt by using
CTX_NEW. If decryption fails, a second attempt can use CTX_OLD.
* When an amount of time defined by the policy has elapsed since the
establishment of CTX_NEW, the peer deletes CTX_OLD.
4.5. Discussion
KUDOS is intended to deprecate and replace the procedure defined in
Appendix B.2 of [RFC8613], as fundamentally achieving the same goal,
while displaying a number of improvements and advantages.
In particular, it is especially convenient for the handling of
failure events concerning the JRC node in 6TiSCH networks (see
Section 3). That is, among its intrinsic advantages compared to the
procedure defined in Appendix B.2 of [RFC8613], KUDOS preserves the
same ID Context value, when establishing a new OSCORE Security
Context.
Since the JRC uses ID Context values as identifiers of network nodes,
namely "pledge identifiers", the above implies that the JRC does not
have anymore to perform a mapping between a new, different ID Context
value and a certain pledge identifier (see Section 8.3.3 of
[RFC9031]). It follows that pledge identifiers can remain constant
once assigned, and thus ID Context values used as pledge identifiers
can be employed in the long-term as originally intended.
5. 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].
Depending on the specific key update procedure used to establish a
new OSCORE Security Context, the related security considerations also
apply.
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TODO: Add more considerations.
6. IANA Considerations
This document has the following actions for IANA.
6.1. OSCORE Flag Bits Registry
IANA is asked to add the following entries to the "OSCORE Flag Bits"
registry within the "Constrained RESTful Environments (CoRE)
Parameters" registry group.
+----------+------------------+------------------------+-----------+
| Bit | Name | Description | Reference |
| Position | | | |
+----------+------------------+------------------------+-----------+
| 1 | Extension-1 Flag | Set to 1 if the OSCORE | [This |
| | | Option specifies a | Document] |
| | | second byte of OSCORE | |
| | | flag bits | |
+----------+------------------+------------------------+-----------+
| 15 | ID Detail Flag | Set to 1 if the | [This |
| | | compressed COSE object | Document] |
| | | contains 'id detail' | |
+----------+------------------+------------------------+-----------+
7. References
7.1. Normative References
[I-D.ietf-lake-edhoc]
Selander, G., Mattsson, J. P., and F. Palombini,
"Ephemeral Diffie-Hellman Over COSE (EDHOC)", Work in
Progress, Internet-Draft, draft-ietf-lake-edhoc-12, 20
October 2021, <https://www.ietf.org/archive/id/draft-ietf-
lake-edhoc-12.txt>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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>.
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[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>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.
7.2. Informative References
[I-D.ietf-ace-oauth-authz]
Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments (ACE) using the OAuth 2.0
Framework (ACE-OAuth)", Work in Progress, Internet-Draft,
draft-ietf-ace-oauth-authz-45, 29 August 2021,
<https://www.ietf.org/archive/id/draft-ietf-ace-oauth-
authz-45.txt>.
[I-D.ietf-ace-oscore-profile]
Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson,
"OSCORE Profile of the Authentication and Authorization
for Constrained Environments Framework", Work in Progress,
Internet-Draft, draft-ietf-ace-oscore-profile-19, 6 May
2021, <https://www.ietf.org/archive/id/draft-ietf-ace-
oscore-profile-19.txt>.
[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-03, 12 July 2021,
<https://www.ietf.org/archive/id/draft-irtf-cfrg-aead-
limits-03.txt>.
[LwM2M] Open Mobile Alliance, "Lightweight Machine to Machine
Technical Specification - Core, Approved Version 1.2, OMA-
TS-LightweightM2M_Core-V1_2-20201110-A", November 2020,
<http://www.openmobilealliance.org/release/LightweightM2M/
V1_2-20201110-A/OMA-TS-LightweightM2M_Core-
V1_2-20201110-A.pdf>.
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[LwM2M-Transport]
Open Mobile Alliance, "Lightweight Machine to Machine
Technical Specification - Transport Bindings, Approved
Version 1.2, OMA-TS-LightweightM2M_Transport-
V1_2-20201110-A", November 2020,
<http://www.openmobilealliance.org/release/LightweightM2M/
V1_2-20201110-A/OMA-TS-LightweightM2M_Transport-
V1_2-20201110-A.pdf>.
[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>.
[RFC7554] Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using
IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the
Internet of Things (IoT): Problem Statement", RFC 7554,
DOI 10.17487/RFC7554, May 2015,
<https://www.rfc-editor.org/info/rfc7554>.
[RFC8180] Vilajosana, X., Ed., Pister, K., and T. Watteyne, "Minimal
IPv6 over the TSCH Mode of IEEE 802.15.4e (6TiSCH)
Configuration", BCP 210, RFC 8180, DOI 10.17487/RFC8180,
May 2017, <https://www.rfc-editor.org/info/rfc8180>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC9031] Vučinić, M., Ed., Simon, J., Pister, K., and M.
Richardson, "Constrained Join Protocol (CoJP) for 6TiSCH",
RFC 9031, DOI 10.17487/RFC9031, May 2021,
<https://www.rfc-editor.org/info/rfc9031>.
Acknowledgments
The authors sincerely thank Christian Amsuess, John Mattsson and
Goeran Selander 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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