Internet DRAFT - draft-ietf-uta-tls13-iot-profile
draft-ietf-uta-tls13-iot-profile
UTA H. Tschofenig
Internet-Draft
Updates: 7925 (if approved) T. Fossati
Intended status: Standards Track Linaro
Expires: 4 September 2024 M. Richardson
Sandelman Software Works
3 March 2024
TLS/DTLS 1.3 Profiles for the Internet of Things
draft-ietf-uta-tls13-iot-profile-09
Abstract
This document is a companion to RFC 7925 and defines TLS/DTLS 1.3
profiles for Internet of Things devices. It also updates RFC 7925
with regards to the X.509 certificate profile.
Discussion Venues
This note is to be removed before publishing as an RFC.
Source for this draft and an issue tracker can be found at
https://github.com/thomas-fossati/draft-tls13-iot.
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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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 4 September 2024.
Copyright Notice
Copyright (c) 2024 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/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions and Terminology . . . . . . . . . . . . . . . 4
2. Credential Types . . . . . . . . . . . . . . . . . . . . . . 4
3. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 5
4. Session Resumption . . . . . . . . . . . . . . . . . . . . . 5
5. Compression . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Perfect Forward Secrecy . . . . . . . . . . . . . . . . . . 6
7. Keep-Alive . . . . . . . . . . . . . . . . . . . . . . . . . 6
8. Timers and ACKs . . . . . . . . . . . . . . . . . . . . . . . 6
9. Random Number Generation . . . . . . . . . . . . . . . . . . 7
10. Server Name Indication . . . . . . . . . . . . . . . . . . . 7
11. Maximum Fragment Length Negotiation . . . . . . . . . . . . 7
12. Crypto Agility . . . . . . . . . . . . . . . . . . . . . . . 7
13. Key Length Recommendations . . . . . . . . . . . . . . . . . 8
14. 0-RTT Data . . . . . . . . . . . . . . . . . . . . . . . . . 8
15. Certificate Profile . . . . . . . . . . . . . . . . . . . . . 8
15.1. All Certificates . . . . . . . . . . . . . . . . . . . . 9
15.1.1. Version . . . . . . . . . . . . . . . . . . . . . . 9
15.1.2. Serial Number . . . . . . . . . . . . . . . . . . . 9
15.1.3. Signature . . . . . . . . . . . . . . . . . . . . . 9
15.1.4. Issuer . . . . . . . . . . . . . . . . . . . . . . . 10
15.1.5. Validity . . . . . . . . . . . . . . . . . . . . . 10
15.1.6. Subject Public Key Info . . . . . . . . . . . . . . 11
15.1.7. Certificate Revocation Checks . . . . . . . . . . . 11
15.2. Root CA Certificate . . . . . . . . . . . . . . . . . . 12
15.2.1. Subject . . . . . . . . . . . . . . . . . . . . . . 12
15.2.2. Authority Key Identifier . . . . . . . . . . . . . . 12
15.2.3. Subject Key Identifier . . . . . . . . . . . . . . . 12
15.2.4. Key Usage . . . . . . . . . . . . . . . . . . . . . 13
15.2.5. Basic Constraints . . . . . . . . . . . . . . . . . 13
15.3. Subordinate CA Certificate . . . . . . . . . . . . . . . 14
15.3.1. Subject . . . . . . . . . . . . . . . . . . . . . . 14
15.3.2. Authority Key Identifier . . . . . . . . . . . . . . 14
15.3.3. Subject Key Identifier . . . . . . . . . . . . . . . 14
15.3.4. Key Usage . . . . . . . . . . . . . . . . . . . . . 14
15.3.5. Basic Constraints . . . . . . . . . . . . . . . . . 15
15.3.6. CRL Distribution Point . . . . . . . . . . . . . . . 15
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15.3.7. Authority Information Access . . . . . . . . . . . . 15
15.4. End Entity Certificate . . . . . . . . . . . . . . . . . 15
15.4.1. Subject . . . . . . . . . . . . . . . . . . . . . . 15
15.4.2. Authority Key Identifier . . . . . . . . . . . . . . 16
15.4.3. Subject Key Identifier . . . . . . . . . . . . . . . 16
15.4.4. Key Usage . . . . . . . . . . . . . . . . . . . . . 16
16. Certificate Overhead . . . . . . . . . . . . . . . . . . . . 17
17. Ciphersuites . . . . . . . . . . . . . . . . . . . . . . . . 18
18. Fault Attacks on Deterministic Signature Schemes . . . . . . 19
19. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 19
20. Security Considerations . . . . . . . . . . . . . . . . . . . 19
21. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
22. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
22.1. Normative References . . . . . . . . . . . . . . . . . . 19
22.2. Informative References . . . . . . . . . . . . . . . . . 21
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 25
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
This document defines a profile of DTLS 1.3 [DTLS13] and TLS 1.3
[RFC8446] that offers communication security services for IoT
applications and is reasonably implementable on many constrained
devices. Profile thereby means that available configuration options
and protocol extensions are utilized to best support the IoT
environment.
For IoT profiles using TLS/DTLS 1.2 please consult [RFC7925]. This
document re-uses the communication pattern defined in [RFC7925] and
makes IoT-domain specific recommendations for version 1.3 (where
necessary).
TLS 1.3 has been re-designed and several previously defined
extensions are not applicable to the new version of TLS/DTLS anymore.
The following features changed with the transition from TLS 1.2 to
1.3:
* TLS 1.3 introduced the concept of post-handshake authentication
messages, which partially replaced the need for the re-negotiation
feature [RFC5746] available in earlier TLS versions. However,
rekeying defined in Section 4.6.3 of [TLS13] does not provide
forward secrecy and post-handshake authentication defined in
Section 4.6.2 of [TLS13] only offers client-to-server
authentication. The "Exported Authenticator" specification, see
[RFC9261], recently added support for mutual, post-handshake
authentication but requires payloads to be exchanged by the
application layer protocol.
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* Rekeying of the application traffic secret does not lead to an
update of the exporter secret (see Section 7.5 of [TLS13]) since
the derived export secret is based on the exporter_master_secret
and not on the application traffic secret.
* Flight #4, which was used by EAP-TLS 1.2 [RFC5216], does not exist
in TLS 1.3. As a consequence, EAP-TLS 1.3 [RFC9190] introduced a
dummy message.
* [RFC4279] introduced PSK-based authentication to TLS, a feature
re-designed in TLS 1.3. The "PSK identity hint" defined in
[RFC4279], which is used by the server to help the client in
selecting which PSK identity to use, is, however, not available
anymore in TLS 1.3.
Finally, ciphersuites were depreciated and the RSA-based key
transport is not yet supported in TLS 1.3.
1.1. Conventions and 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.
This document reuses the terms "SHOULD+", "SHOULD-" and "MUST-" from
[RFC8221].
2. Credential Types
In accordance with the recommendations in [RFC7925], a compliant
implementation MUST implement TLS_AES_128_CCM_8_SHA256. It SHOULD
implement TLS_CHACHA20_POLY1305_SHA256.
Pre-shared key based authentication is integrated into the main TLS/
DTLS 1.3 specification and has been harmonized with session
resumption.
A compliant implementation supporting authentication based on
certificates and raw public keys MUST support digital signatures with
ecdsa_secp256r1_sha256. A compliant implementation MUST support the
key exchange with secp256r1 (NIST P-256) and SHOULD support key
exchange with X25519.
A plain PSK-based TLS/DTLS client or server MUST implement the
following extensions:
* Supported Versions,
* Cookie,
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* Server Name Indication (SNI),
* Pre-Shared Key,
* PSK Key Exchange Modes, and
* Application-Layer Protocol Negotiation (ALPN).
For use of external pre-shared keys [RFC9258] makes the following
recommendation:
Applications SHOULD provision separate PSKs for (D)TLS 1.3 and
prior versions.
Where possible, the importer interface defined in [RFC9258] MUST be
used for external PSKs. This ensures that external PSKs used in
(D)TLS 1.3 are bound to a specific key derivation function (KDF) and
hash function.
The SNI extension is discussed in this document and the justification
for implementing and using the ALPN extension can be found in
[RFC9325].
For TLS/DTLS clients and servers implementing raw public keys and/or
certificates the guidance for mandatory-to-implement extensions
described in Section 9.2 of [RFC8446] MUST be followed.
3. Error Handling
TLS 1.3 simplified the Alert protocol but the underlying challenge in
an embedded context remains unchanged, namely what should an IoT
device do when it encounters an error situation. The classical
approach used in a desktop environment where the user is prompted is
often not applicable with unattended devices. Hence, it is more
important for a developer to find out from which error cases a device
can recover from.
4. Session Resumption
TLS 1.3 has built-in support for session resumption by utilizing PSK-
based credentials established in an earlier exchange.
5. Compression
TLS 1.3 does not have support for compression of application data
traffic, as offered by previous versions of TLS. Applications are
therefore responsible for transmitting payloads that are either
compressed or use a more efficient encoding otherwise.
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With regards to the handshake itself, various strategies have been
applied to reduce the size of the exchanged payloads. TLS and DTLS
1.3 use less overhead, depending on the type of key confirmations,
when compared to previous versions of the protocol. Additionally,
the work on Compact TLS (cTLS) [I-D.ietf-tls-ctls] has taken
compression of the handshake a step further by utilizing out-of-band
knowledge between the communication parties to reduce the amount of
data to be transmitted at each individual handshake, among applying
other techniques.
6. Perfect Forward Secrecy
TLS 1.3 allows the use of PFS with all ciphersuites since the support
for it is negotiated independently.
7. Keep-Alive
The discussion in Section 10 of [RFC7925] is applicable.
8. Timers and ACKs
Compared to DTLS 1.2 timeout-based whole flight retransmission, DTLS
1.3 ACKs sensibly decrease the risk of congestion collapse which was
the basis for the very conservative recommendations given in
Section 11 of [RFC7925].
In general, the recommendations in Section 7.3 of [DTLS13] regarding
ACKs apply. In particular, "[w]hen DTLS 1.3 is used in deployments
with lossy networks, such as low-power, long-range radio networks as
well as low-power mesh networks, the use of ACKs is recommended" to
signal any sign of disruption or lack of progress. This allows for
selective or early retransmission, which leads to more efficient use
of bandwidth and memory resources.
Due to the vast range of network technologies used in IoT
deployments, from wired LAN to GSM-SMS, it's not possible to provide
a universal recommendation for an initial timeout. Therefore, it is
RECOMMENDED that DTLS 1.3 implementations allow developers to
explicitly set the initial timer value. Developers SHOULD set the
initial timeout to be twice the expected round-trip time (RTT), but
no less than 1000ms. For specific application/network combinations,
a sub-second initial timeout MAY be set. In cases where no RTT
estimates are available, a 1000ms initial timeout is suitable for the
general Internet.
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For RRC, the recommendations in Section 7.5 of
[I-D.ietf-tls-dtls-rrc] apply. Just like the handshake initial
timers, it is RECOMMENDED that DTLS 1.2 and 1.3 implementations offer
an option for their developers to explicitly set the RRC timer.
9. Random Number Generation
The discussion in Section 12 of [RFC7925] is applicable with one
exception: the ClientHello and the ServerHello messages in TLS 1.3 do
not contain gmt_unix_time component anymore.
10. Server Name Indication
This specification mandates the implementation of the Server Name
Indication (SNI) extension. Where privacy requirements require it,
the ECH (Encrypted Client Hello) extension [I-D.ietf-tls-esni]
prevents an on-path attacker to determine the domain name the client
is trying to connect to.
Since the Encrypted Client Hello extension requires use of Hybrid
Public Key Encryption (HPKE) [I-D.irtf-cfrg-hpke] and additional
protocols require further protocol exchanges and cryptographic
operations, there is a certain overhead associated with this privacy
feature.
Note that in industrial IoT deployments the use of ECH may not be an
option because network administrators inspect DNS traffic generated
by IoT devices in order to detect malicious behaviour.
Besides, to avoid leaking DNS lookups from network inspection
altogether further protocols are needed, including DoH [RFC8484] and
DPRIVE [RFC7858] [RFC8094]. For use of such techniques in managed
networks, the reader is advised to keep up to date with the protocols
defined by the Adaptive DNS Discovery (add) working group [ADD].
11. Maximum Fragment Length Negotiation
The Maximum Fragment Length Negotiation (MFL) extension has been
superseded by the Record Size Limit (RSL) extension [RFC8449].
Implementations in compliance with this specification MUST implement
the RSL extension and SHOULD use it to indicate their RAM
limitations.
12. Crypto Agility
The recommendations in Section 19 of [RFC7925] are applicable.
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13. Key Length Recommendations
The recommendations in Section 20 of [RFC7925] are applicable.
14. 0-RTT Data
Appendix E.5 of [TLS13] establishes that:
Application protocols MUST NOT use 0-RTT data without a profile
that defines its use. That profile needs to identify which
messages or interactions are safe to use with 0-RTT and how to
handle the situation when the server rejects 0-RTT and falls back
to 1-RTT.
At the time of writing, no such profile has been defined for CoAP
[CoAP]. Therefore, 0-RTT MUST NOT be used by CoAP applications.
15. Certificate Profile
This section contains updates and clarifications to the certificate
profile defined in [RFC7925]. The content of Table 1 of [RFC7925]
has been split by certificate "type" in order to clarify exactly what
requirements and recommendations apply to which entity in the PKI
hierarchy.
The content is also better aligned with the IEEE 802.1AR [_8021AR]
specification, which introduces the terms Initial Device Identifier
(IDevID) and Locally Significant Device Identifiers (LDevIDs).
IDevIDs and LDevIDs are Device Identifier (DevID) and a DevID
consists of
* a private key,
* a certificate (containing the public key and the identifier
certified by the certificate's issuer), and
* a certificate chain up to a trust anchor. The trust anchor is is
usually the root certificate).
The IDevID is typically provisioned by a manufacturer and signed by
the manufacturer CA. It is then used to obtain operational
certificates, the LDevIDs, from the operator or owner of the device.
Some protocols also introduce an additional hierarchy with
application instance certificates, which are obtained for use with
specific applications.
IDevIDs are primarily used with device onboarding or bootstrapping
protocols, such as the Bootstrapping Remote Secure Key Infrastructure
(BRSKI) protocol [RFC8995] or by LwM2M Bootstrap [LwM2M]. Hence, the
use of IDevIDs is limited in purpose even though they have a long
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lifetime, or do not expire at all. While some bootstrapping
protocols use TLS (and therefore make use of the IDevID as part of
client authentication) there are other bootstrapping protocols that
do not use TLS/DTLS for client authentication, such as FIDO Device
Onboarding (FDO) [FDO]. In many cases, a profile for the certificate
content is provided by those specifications. For these reasons, this
specification focuses on the description of LDevIDs.
While the IEEE 802.1AR terminology is utilized in this document, this
specification does not claim conformance to IEEE 802.1AR since such a
compliance statement goes beyond the use of the terminology and the
certificate content and would include the use of management
protocols, fulfillment of certain hardware security requirements, and
interfaces to access these hardware security modules. Placing these
requirements on network equipment like routers may be appropriate but
designers of constrained IoT devices have opted for different
protocols and hardware security choices.
15.1. All Certificates
To avoid repetition, this section outlines requirements on X.509
certificates applicable to all PKI entities.
15.1.1. Version
Certificates MUST be of type X.509 v3. Note that TLS 1.3 allows to
convey payloads other than X.509 certificates in the Certificate
message. The description in this section only focuses on X.509 v3
certificates and leaves the description of other formats to other
sections or even other specifications.
15.1.2. Serial Number
CAs MUST generate non-sequential serial numbers greater than or equal
to eight (8) octets from a cryptographically secure pseudo-random
number generator. [RFC5280] limits this field to a maximum of 20
octets. The serial number MUST be unique for each certificate issued
by a given CA (i.e., the issuer name and the serial number uniquely
identify a certificate).
This requirement is aligned with [RFC5280].
15.1.3. Signature
The signature MUST be ecdsa-with-SHA256 or stronger [RFC5758].
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Note: In contrast to IEEE 802.1AR this specification does not require
end entity certificates, subordinate CA certificates, and CA
certificates to use the same signature algorithm. Furthermore, this
specification does not utlize RSA for use with constrained IoT
devices and networks.
15.1.4. Issuer
The issuer field MUST contain a non-empty distinguished name (DN) of
the entity that has signed and issued the certificate in accordance
to [RFC5280].
15.1.5. Validity
In IoT deployment scenarios it is often expected that the IDevIDs
have no maximum validity period. For this purpose the use of a
special value for the notAfter date field, the GeneralizedTime value
of 99991231235959Z, is utilized. If this is done, then CA
certificates and certificates of subordinate CAs cannot have a
maximum validity period either. Hence, it requires careful
consideration whether it is appropriate to issue IDevID certificates
with no maximum validity period.
LDevID certificates are, however, issued by the operator or owner,
and may be renewed at a regular interval using protocols, such as
Enrollment over Secure Transport (EST) [RFC7030] or the Certificate
Management Protocol (CMP) [RFC9483]. It is therefore RECOMMENDED to
limit the lifetime of these LDevID certificates using the notBefore
and notAfter fields, as described in Section 4.1.2.5 of [RFC5280].
Values MUST be expressed in Greenwich Mean Time (Zulu) and MUST
include seconds even where the number of seconds is zero.
Note that the validity period is defined as the period of time from
notBefore through notAfter, inclusive. This means that a
hypothetical certificate with a notBefore date of 9 June 2021 at
03:42:01 and a notAfter date of 7 September 2021 at 03:42:01 becomes
valid at the beginning of the :01 second, and only becomes invalid at
the :02 second, a period that is 90 days plus 1 second. So for a
90-day, notAfter must actually be 03:42:00.
For devices without a reliable source of time we advise the use of a
device management solution, which typically includes a certificate
management protocol, to manage the lifetime of all the certificates
used by the device. While this approach does not utilize
certificates to its widest extent, it is a solution that extends the
capabilities offered by a raw public key approach.
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15.1.6. Subject Public Key Info
The SubjectPublicKeyInfo structure indicates the algorithm and any
associated parameters for the ECC public key. This profile uses the
id-ecPublicKey algorithm identifier for ECDSA signature keys, as
defined and specified in [RFC5480]. This specification assumes that
devices supported one of the following algorithms:
* id-ecPublicKey with secp256r1,
* id-ecPublicKey with secp384r1, and
* id-ecPublicKey with secp521r1.
There is no requirement to use CA certificates and certificates of
subordinate CAs to use the same algorithm as the end-entity
certificate. Certificates with longer lifetime may well use a
cryptographic stronger algorithm.
15.1.7. Certificate Revocation Checks
The guidance in Section 4.4.3 of [RFC7925] still holds: for
certificate revocation, neither the Online Certificate Status
Protocol (OCSP) nor Certificate Revocation Lists (CRLs) are used by
constrained IoT devices.
Since the publication of RFC 7925 the need for firmware update
mechanisms has been reinforced and the work on standardizing a secure
and interoperable firmware update mechanism has made substantial
progress, see [RFC9019]. RFC 7925 recommends to use a software /
firmware update mechanism to provision devices with new trust
anchors.
The use of device management protocols for IoT devices, which often
include an onboarding or bootstrapping mechanism, has also seen
considerable uptake in deployed devices and these protocols, some of
which are standardized, allow updates of certificates on demand.
This enables a deployment model where IoT device utilize end-entity
certificates with shorter lifetime making certificate revocation
protocols, like OCSP and CRLs, less relevant. Whenever certificates
are updated the TLS stack needs to be informed since the
communication endpoints need to be aware of the new certificates.
This is particularly important when long-lived TLS connections are
used. In such a case, the a post-handshake authentication exchange
needs to be triggered. TLS 1.3 provides client-to-server post-
handshake authentication only. Mutual authentication via post-
handshake messages is available via [RFC9261] but requires the
application layer protocol to carry the payloads.
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Hence, instead of performing certificate revocation checks on the IoT
device itself this specification recommends to delegate this task to
the IoT device operator and to take the necessary action to allow IoT
devices to remain operational.
The CRL distribution points extension has been defined in RFC 5280 to
identify how CRL information is obtained. The authority information
access extension indicates how to access information like the online
certificate status service (OCSP). Both extensions SHOULD NOT be
set. If set, they MUST NOT be marked critical.
15.2. Root CA Certificate
This section outlines the requirements for root CA certificates.
15.2.1. Subject
[RFC5280] defines the Subject field as follows: "The subject field
identifies the entity associated with the public key stored in the
subject public key field." RFC 5280 adds "If the subject is a CA
then the subject field MUST be populated with a non-empty
distinguished name matching the contents of the issuer field in all
certificates issued by the subject CA."
The Subject field MUST be present and MUST contain the commonName,
the organizationName, and the countryName attribute and MAY contain
an organizationalUnitName attribute.
15.2.2. Authority Key Identifier
Section 4.2.1.1 of [RFC5280] defines the Authority Key Identifier as
follows: "The authority key identifier extension provides a means of
identifying the public key corresponding to the private key used to
sign a certificate. This extension is used where an issuer has
multiple signing keys."
The Authority Key Identifier extension MAY be omitted. If it is set,
it MUST NOT be marked critical, and MUST contain the
subjectKeyIdentifier of this certificate.
[Editor's Note: Do we need to set the Authority Key Identifier in the
CA certificate?]
15.2.3. Subject Key Identifier
Section 4.2.1.2 of [RFC5280] defines the Subject Key Identifier as
follows: "The subject key identifier extension provides a means of
identifying certificates that contain a particular public key."
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The Subject Key Identifier extension MUST be set, MUST NOT be marked
critical, and MUST contain the key identifier of the public key
contained in the subject public key info field.
[Editor's Note: Do we need to set the Subject Key Identifier in the
CA certificate?]
15.2.4. Key Usage
[RFC5280] defines the key usage field as follows: "The key usage
extension defines the purpose (e.g., encipherment, signature,
certificate signing) of the key contained in the certificate."
The Key Usage extension SHOULD be set. If it is set, it MUST be
marked critical and the keyCertSign or cRLSign purposes MUST be set.
Additional key usages MAY be set depending on the intended usage of
the public key.
[Editor's Note: Should we harden the requirement to "The Key Usage
extension MUST be set.]
[RFC5280] defines the extended key usage as follows: "This extension
indicates one or more purposes for which the certified public key may
be used, in addition to or in place of the basic purposes indicated
in the key usage extension."
This extendedKeyUsage extension MUST NOT be set.
15.2.5. Basic Constraints
[RFC5280] states that "The Basic Constraints extension identifies
whether the subject of the certificate is a CA and the maximum depth
of valid certification paths that include this certificate. The cA
boolean indicates whether the certified public key may be used to
verify certificate signatures."
For the pathLenConstraint RFC 5280 makes further statements:
* "The pathLenConstraint field is meaningful only if the cA boolean
is asserted and the key usage extension, if present, asserts the
keyCertSign bit. In this case, it gives the maximum number of
non-self-issued intermediate certificates that may follow this
certificate in a valid certification path."
* "A pathLenConstraint of zero indicates that no non-self-issued
intermediate CA certificates may follow in a valid certification
path."
* "Where pathLenConstraint does not appear, no limit is imposed."
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* "Conforming CAs MUST include this extension in all CA certificates
that contain public keys used to validate digital signatures on
certificates and MUST mark the extension as critical in such
certificates."
The Basic Constraints extension MUST be set, MUST be marked critical,
the cA flag MUST be set to true and the pathLenConstraint MUST be
omitted.
[Editor's Note: Should we soften the requirement to: "The
pathLenConstraint field SHOULD NOT be present."]
15.3. Subordinate CA Certificate
This section outlines the requirements for subordinate CA
certificates.
15.3.1. Subject
The Subject field MUST be set and MUST contain the commonName, the
organizationName, and the countryName attribute and MAY contain an
organizationalUnitName attribute.
15.3.2. Authority Key Identifier
The Authority Key Identifier extension MUST be set, MUST NOT be
marked critical, and MUST contain the subjectKeyIdentifier of the CA
that issued this certificate.
15.3.3. Subject Key Identifier
The Subject Key Identifier extension MUST be set, MUST NOT be marked
critical, and MUST contain the key identifier of the public key
contained in the subject public key info field.
15.3.4. Key Usage
The Key Usage extension MUST be set, MUST be marked critical, the
keyCertSign or cRLSign purposes MUST be set, and the digitalSignature
purpose SHOULD be set.
The extendedKeyUsage extensed MAY be set depending on the intended
usage of the public key.
[Editor's Note: Should we harden the requirement to "The
extendedKeyUsage MUST NOT be present."]
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15.3.5. Basic Constraints
The Basic Constraints extension MUST be set, MUST be marked critical,
the cA flag MUST be set to true and the pathLenConstraint MUST be set
to 0.
[Editor's Note: Should we soften the requriement to "The
pathLenConstraint field MAY be present."]
15.3.6. CRL Distribution Point
The CRL Distribution Point extension SHOULD NOT be set. If it is
set, it MUST NOT be marked critical and MUST identify the CRL
relevant for this certificate.
15.3.7. Authority Information Access
The Authority Information Access extension SHOULD NOT be set. If it
is set, it MUST NOT be marked critical and MUST identify the location
of the certificate of the CA that issued this certificate and the
location it provides an online certificate status service (OCSP).
15.4. End Entity Certificate
This section outlines the requirements for end entity certificates.
15.4.1. Subject
The requirement in Section 4.4.2 of [RFC7925] to only use EUI-64 for
end entity certificates as a Subject name is lifted.
Two fields are typically used to encode a device identifer, namely
the Subject and the subjectAltName fields. Protocol specifications
tend to offer recommendations what identifiers to use and the
deployment situation is fragmented.
The Subject field MAY include a unique device serial number. If the
serial number is included, it MUST be encoded in the serialNumber
attribute.
[RFC5280] defines: "The subject alternative name extension allows
identities to be bound to the subject of the certificate. These
identities may be included in addition to or in place of the identity
in the subject field of the certificate."
The subject alternative name extension MAY be set. If it is set, it
MUST NOT be marked critical, except when the subject DN contains an
empty sequence.
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If the EUI-64 format is used to identify the subject of an end entity
certificate, it MUST be encoded in a subjectAltName of type DNS-ID as
a string of the form HH-HH-HH-HH-HH-HH-HH-HH where 'H' is one of the
symbols '0'-'9' or 'A'-'F'.
Domain names MUST NOT be encoded in the subject commonName. Instead
they MUST be encoded in a subjectAltName of type DNS-ID. Domain
names MUST NOT contain wildcard (*) characters. The subjectAltName
MUST NOT contain multiple names.
Note: The IEEE 802.1AR recomments to encode information about a
Trusted Platform Module (TPM), if present, in the HardwareModuleName.
This specification does not follow this recommendation.
15.4.2. Authority Key Identifier
The Authority Key Identifier extension MUST be set, MUST NOT be
marked critical, and MUST contain the subjectKeyIdentifier of the CA
that issued this certificate.
15.4.3. Subject Key Identifier
The Subject Key Identifier SHOULD NOT be included in end-entity
certificates. If it is included, then the Subject Key Identifier
extension MUST NOT be marked critical, and MUST contain the key
identifier of the public key contained in the subject public key info
field.
[Editor's Note: Should we harden the requirement and state: "The
Subject Key Identifier MUST NOT be included in end-entity
certificates."]
15.4.4. Key Usage
The key usage extension MUST be set and MUST be marked as critical.
For signature verification keys the digitialSignature key usage
purpose MUST be specified. Other key usages are set according to the
intended usage of the key.
If enrollment of new certificates uses server-side key generation,
encrypted delivery of the private key is required. In such cases the
key usage keyEncipherment or keyAgreement MUST be set because the
encrypted delivery of the newly generated key involves encryption or
agreement of a symmetric key. On-device key generation is, however,
the preferred approach.
The extendedKeyUsage MUST be present and contain at least one of id-
kp-serverAuth or id-kp-clientAuth.
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16. Certificate Overhead
In a public key-based key exchange, certificates and public keys are
a major contributor to the size of the overall handshake. For
example, in a regular TLS 1.3 handshake with minimal ECC certificates
and no subordinate CA utilizing the secp256r1 curve with mutual
authentication, around 40% of the entire handshake payload is
consumed by the two exchanged certificates.
Hence, it is not surprising that there is a strong desire to reduce
the size of certificates and certificate chains. This has lead to
various standardization efforts. Below is a brief summary of what
options an implementer has to reduce the bandwidth requirements of a
public key-based key exchange. Note that many of the standardized
extensions are not readily available in TLS/DTLS stacks since
optimizations typically get implemented last.
* Use elliptic curve cryptography (ECC) instead of RSA-based
certificate due to the smaller certificate size. This document
recommends the use of elliptic curve cryptography only.
* Avoid deep and complex CA hierarchies to reduce the number of
subordinate CA certificates that need to be transmitted and
processed. See [I-D.irtf-t2trg-taxonomy-manufacturer-anchors] for
a discussion about CA hierarchies.
* Pay attention to the amount of information conveyed inside
certificates.
* Use session resumption to reduce the number of times a full
handshake is needed. Use Connection IDs [RFC9146], when possible,
to enable long-lasting connections.
* Use the TLS cached info [RFC7924] extension to avoid sending
certificates with every full handshake.
* Use client certificate URLs [RFC6066] instead of full certificates
for clients. When applications perform TLS client authentication
via DNS-Based Authentication of Named Entities (DANE) TLSA records
then the [I-D.ietf-dance-tls-clientid] specification may be used
to reduce the packets on the wire. Note: The term "TLSA" does not
stand for anything; it is just the name of the RRtype, as
explained in [RFC6698].
* Use certificate compression as defined in [RFC8879].
* Use alternative certificate formats, where possible, such as raw
public keys [RFC7250] or CBOR-encoded certificates
[I-D.ietf-cose-cbor-encoded-cert].
The use of certificate handles, as introduced in cTLS
[I-D.ietf-tls-ctls], is a form of caching or compressing certificates
as well.
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Whether to utilize any of the above extensions or a combination of
them depends on the anticipated deployment environment, the
availability of code, and the constraints imposed by already deployed
infrastructure (e.g., CA infrastructure, tool support).
17. Ciphersuites
According to Section 4.5.3 of [DTLS13], the use of AES-CCM with
8-octet authentication tags (CCM_8) is considered unsuitable for
general use with DTLS. This is because it has low integrity limits
(i.e., high sensitivity to forgeries) which makes endpoints that
negotiate ciphersuites based on such AEAD vulnerable to a trivial DoS
attack. See also Sections 5.3 and 5.4 of [I-D.irtf-cfrg-aead-limits]
for further discussion on this topic, as well as references to the
analysis supporting these conclusions.
Specifically, [DTLS13] warns that:
> TLS_AES_128_CCM_8_SHA256 MUST NOT be used in DTLS without additional
> safeguards against forgery. Implementations MUST set usage limits for
> AEAD_AES_128_CCM_8 based on an understanding of any additional forgery
> protections that are used.
Since all the ciphersuites required by [RFC7925] and [CoAP] rely on
CCM_8, there is no alternate ciphersuite available for applications
that aim to eliminate the security and availability threats related
to CCM_8 while retaining interoperability with the larger ecosystem.
In order to ameliorate the situation, this document RECOMMENDS that
implementations support the following two ciphersuites:
* TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256
* TLS_ECDHE_ECDSA_WITH_AES_128_CCM
and offer them as their first choice. These ciphersuites provide
confidentiality and integrity limits that are considered acceptable
in the most general settings. For the details on the exact bounds of
both ciphersuites see Section 4.5.3 of [DTLS13]. Note that the GCM-
based ciphersuite offers superior interoperability with cloud
services at the cost of a slight increase in the wire and peak RAM
footprints.
When the GCM-based ciphersuite is used with TLS 1.2, the
recommendations in Section 7.2.1 of [RFC9325] related to
deterministic nonce generation apply. In addition, the integrity
limits on key usage detailed in Section 4.4 of [RFC9325] also apply.
Table 1 summarizes the recommendations regarding ciphersuites:
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+=========================================+=============+
| Ciphersuite | Requirement |
+=========================================+=============+
| TLS_AES_128_CCM_8_SHA256 | MUST- |
+-----------------------------------------+-------------+
| TLS_ECDHE_ECDSA_WITH_AES_128_CCM | SHOULD+ |
+-----------------------------------------+-------------+
| TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 | SHOULD+ |
+-----------------------------------------+-------------+
Table 1: Ciphersuite requirements
18. Fault Attacks on Deterministic Signature Schemes
A number of passive side-channel attacks as well as active fault-
injection attacks (e.g., [Ambrose2017]) have been demonstrated to be
successful in allowing a malicious third party to gain information
about the signing key if a fully deterministic signature scheme
(e.g., [RFC6979] ECDSA or EdDSA [RFC8032]) is used.
Most of these attacks assume physical access to the device and are
therefore especially relevant to smart cards as well as IoT
deployments with poor or non-existent physical security.
In this security model, it is recommended to combine both randomness
and determinism, for example, as described in
[I-D.irtf-cfrg-det-sigs-with-noise].
19. Open Issues
A list of open issues can be found at https://github.com/thomas-
fossati/draft-tls13-iot/issues
20. Security Considerations
This entire document is about security.
21. IANA Considerations
This document makes no requests to IANA.
22. References
22.1. Normative References
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[DTLS13] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/rfc/rfc9147>.
[I-D.ietf-tls-dtls-rrc]
Tschofenig, H., Kraus, A., and T. Fossati, "Return
Routability Check for DTLS 1.2 and DTLS 1.3", Work in
Progress, Internet-Draft, draft-ietf-tls-dtls-rrc-10, 9
October 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-tls-dtls-rrc-10>.
[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>.
[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>.
[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>.
[RFC7925] Tschofenig, H., Ed. and T. Fossati, "Transport Layer
Security (TLS) / Datagram Transport Layer Security (DTLS)
Profiles for the Internet of Things", RFC 7925,
DOI 10.17487/RFC7925, July 2016,
<https://www.rfc-editor.org/rfc/rfc7925>.
[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>.
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[RFC8221] Wouters, P., Migault, D., Mattsson, J., Nir, Y., and T.
Kivinen, "Cryptographic Algorithm Implementation
Requirements and Usage Guidance for Encapsulating Security
Payload (ESP) and Authentication Header (AH)", RFC 8221,
DOI 10.17487/RFC8221, October 2017,
<https://www.rfc-editor.org/rfc/rfc8221>.
[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/rfc/rfc8446>.
[RFC8449] Thomson, M., "Record Size Limit Extension for TLS",
RFC 8449, DOI 10.17487/RFC8449, August 2018,
<https://www.rfc-editor.org/rfc/rfc8449>.
[RFC9258] Benjamin, D. and C. A. Wood, "Importing External Pre-
Shared Keys (PSKs) for TLS 1.3", RFC 9258,
DOI 10.17487/RFC9258, July 2022,
<https://www.rfc-editor.org/rfc/rfc9258>.
[RFC9325] Sheffer, Y., Saint-Andre, P., and T. Fossati,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
2022, <https://www.rfc-editor.org/rfc/rfc9325>.
[TLS13] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/rfc/rfc8446>.
22.2. Informative References
[ADD] IETF, "Adaptive DNS Discovery (add) Working Group",
September 2023,
<https://datatracker.ietf.org/wg/add/about/>.
[Ambrose2017]
Ambrose, C., Bos, J. W., Fay, B., Joye, M., Lochter, M.,
and B. Murray, "Differential Attacks on Deterministic
Signatures", 2017, <https://eprint.iacr.org/2017/975.pdf>.
[CoAP] 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/rfc/rfc7252>.
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[FDO] FIDO Alliance, "FIDO Device Onboard Specification 1.1",
April 2022, <https://fidoalliance.org/specifications/
download-iot-specifications/>.
[I-D.ietf-cose-cbor-encoded-cert]
Mattsson, J. P., Selander, G., Raza, S., Höglund, J., and
M. Furuhed, "CBOR Encoded X.509 Certificates (C509
Certificates)", Work in Progress, Internet-Draft, draft-
ietf-cose-cbor-encoded-cert-07, 20 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-cose-
cbor-encoded-cert-07>.
[I-D.ietf-dance-tls-clientid]
Huque, S. and V. Dukhovni, "TLS Extension for DANE Client
Identity", Work in Progress, Internet-Draft, draft-ietf-
dance-tls-clientid-03, 8 January 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-dance-
tls-clientid-03>.
[I-D.ietf-tls-ctls]
Rescorla, E., Barnes, R., Tschofenig, H., and B. M.
Schwartz, "Compact TLS 1.3", Work in Progress, Internet-
Draft, draft-ietf-tls-ctls-09, 23 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
ctls-09>.
[I-D.ietf-tls-esni]
Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
Encrypted Client Hello", Work in Progress, Internet-Draft,
draft-ietf-tls-esni-17, 9 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
esni-17>.
[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-07, 31 May 2023,
<https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
aead-limits-07>.
[I-D.irtf-cfrg-det-sigs-with-noise]
Mattsson, J. P., Thormarker, E., and S. Ruohomaa, "Hedged
ECDSA and EdDSA Signatures", Work in Progress, Internet-
Draft, draft-irtf-cfrg-det-sigs-with-noise-02, 1 March
2024, <https://datatracker.ietf.org/doc/html/draft-irtf-
cfrg-det-sigs-with-noise-02>.
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[I-D.irtf-cfrg-hpke]
Barnes, R., Bhargavan, K., Lipp, B., and C. A. Wood,
"Hybrid Public Key Encryption", Work in Progress,
Internet-Draft, draft-irtf-cfrg-hpke-12, 2 September 2021,
<https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
hpke-12>.
[I-D.irtf-t2trg-taxonomy-manufacturer-anchors]
Richardson, M., "A Taxonomy of operational security
considerations for manufacturer installed keys and Trust
Anchors", Work in Progress, Internet-Draft, draft-irtf-
t2trg-taxonomy-manufacturer-anchors-03, 30 January 2024,
<https://datatracker.ietf.org/doc/html/draft-irtf-t2trg-
taxonomy-manufacturer-anchors-03>.
[LwM2M] OMA SpecWorks, "Lightweight Machine to Machine (LwM2M)
V.1.2.1 Technical Specification: Transport Bindings",
December 2022,
<https://openmobilealliance.org/release/LightweightM2M/
V1_2_1-20221209-A/>.
[RFC4279] Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key
Ciphersuites for Transport Layer Security (TLS)",
RFC 4279, DOI 10.17487/RFC4279, December 2005,
<https://www.rfc-editor.org/rfc/rfc4279>.
[RFC5216] Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
Authentication Protocol", RFC 5216, DOI 10.17487/RFC5216,
March 2008, <https://www.rfc-editor.org/rfc/rfc5216>.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
<https://www.rfc-editor.org/rfc/rfc5746>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/rfc/rfc6066>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
2012, <https://www.rfc-editor.org/rfc/rfc6698>.
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[RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature
Algorithm (DSA) and Elliptic Curve Digital Signature
Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
2013, <https://www.rfc-editor.org/rfc/rfc6979>.
[RFC7030] Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
"Enrollment over Secure Transport", RFC 7030,
DOI 10.17487/RFC7030, October 2013,
<https://www.rfc-editor.org/rfc/rfc7030>.
[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
June 2014, <https://www.rfc-editor.org/rfc/rfc7250>.
[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/rfc/rfc7858>.
[RFC7924] Santesson, S. and H. Tschofenig, "Transport Layer Security
(TLS) Cached Information Extension", RFC 7924,
DOI 10.17487/RFC7924, July 2016,
<https://www.rfc-editor.org/rfc/rfc7924>.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
<https://www.rfc-editor.org/rfc/rfc8032>.
[RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
Transport Layer Security (DTLS)", RFC 8094,
DOI 10.17487/RFC8094, February 2017,
<https://www.rfc-editor.org/rfc/rfc8094>.
[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/rfc/rfc8484>.
[RFC8879] Ghedini, A. and V. Vasiliev, "TLS Certificate
Compression", RFC 8879, DOI 10.17487/RFC8879, December
2020, <https://www.rfc-editor.org/rfc/rfc8879>.
[RFC8995] Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
and K. Watsen, "Bootstrapping Remote Secure Key
Infrastructure (BRSKI)", RFC 8995, DOI 10.17487/RFC8995,
May 2021, <https://www.rfc-editor.org/rfc/rfc8995>.
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[RFC9019] Moran, B., Tschofenig, H., Brown, D., and M. Meriac, "A
Firmware Update Architecture for Internet of Things",
RFC 9019, DOI 10.17487/RFC9019, April 2021,
<https://www.rfc-editor.org/rfc/rfc9019>.
[RFC9146] Rescorla, E., Ed., Tschofenig, H., Ed., Fossati, T., and
A. Kraus, "Connection Identifier for DTLS 1.2", RFC 9146,
DOI 10.17487/RFC9146, March 2022,
<https://www.rfc-editor.org/rfc/rfc9146>.
[RFC9190] Preuß Mattsson, J. and M. Sethi, "EAP-TLS 1.3: Using the
Extensible Authentication Protocol with TLS 1.3",
RFC 9190, DOI 10.17487/RFC9190, February 2022,
<https://www.rfc-editor.org/rfc/rfc9190>.
[RFC9261] Sullivan, N., "Exported Authenticators in TLS", RFC 9261,
DOI 10.17487/RFC9261, July 2022,
<https://www.rfc-editor.org/rfc/rfc9261>.
[RFC9483] Brockhaus, H., von Oheimb, D., and S. Fries, "Lightweight
Certificate Management Protocol (CMP) Profile", RFC 9483,
DOI 10.17487/RFC9483, November 2023,
<https://www.rfc-editor.org/rfc/rfc9483>.
[_8021AR] IEEE, "IEEE Standard for Local and metropolitan area
networks – Secure Device Identity, IEEE 802.1AR-2018",
August 2018, <https://1.ieee802.org/security/802-1ar>.
Acknowledgments
We would like to thank Ben Kaduk, Hendrik Brockhaus, and John
Mattsson.
Contributors
Juliusz Sosinowicz
Achim Kraus
Authors' Addresses
Hannes Tschofenig
Email: Hannes.Tschofenig@gmx.net
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Thomas Fossati
Linaro
Email: Thomas.Fossati@linaro.com
Michael Richardson
Sandelman Software Works
Email: mcr+ietf@sandelman.ca
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