Encrypted Payloads in SUIT Manifests
draft-ietf-suit-firmware-encryption-21
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Active".
|
|
|---|---|---|---|
| Authors | Hannes Tschofenig , Russ Housley , Brendan Moran , David Brown , Ken Takayama | ||
| Last updated | 2024-11-21 (Latest revision 2024-10-21) | ||
| Replaces | draft-tschofenig-suit-firmware-encryption | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
ARTART Last Call review
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by Martin Thomson
Almost ready
SECDIR Last Call Review due 2024-08-02
Incomplete
IOTDIR Telechat Review due 2024-11-15
Requested
|
||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Associated WG milestone |
|
||
| Document shepherd | Akira Tsukamoto | ||
| Shepherd write-up | Show Last changed 2024-06-05 | ||
| IESG | IESG state | IESG Evaluation::Revised I-D Needed | |
| Consensus boilerplate | Yes | ||
| Telechat date |
(None)
Has a DISCUSS. Has enough positions to pass once DISCUSS positions are resolved. |
||
| Responsible AD | Deb Cooley | ||
| Send notices to | david.waltermire@nist.gov, akira.tsukamoto@gmail.com | ||
| IANA | IANA review state | IANA OK - Actions Needed |
draft-ietf-suit-firmware-encryption-21
SUIT H. Tschofenig
Internet-Draft H-BRS
Intended status: Standards Track R. Housley
Expires: 24 April 2025 Vigil Security
B. Moran
Arm Limited
D. Brown
Linaro
K. Takayama
SECOM CO., LTD.
21 October 2024
Encrypted Payloads in SUIT Manifests
draft-ietf-suit-firmware-encryption-21
Abstract
This document specifies techniques for encrypting software, firmware,
machine learning models, and personalization data by utilizing the
IETF SUIT manifest. Key agreement is provided by ephemeral-static
(ES) Diffie-Hellman (DH) and AES Key Wrap (AES-KW). ES-DH uses
public key cryptography while AES-KW uses a pre-shared key.
Encryption of the plaintext is accomplished with conventional
symmetric key cryptography.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 24 April 2025.
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
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 4
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Encryption Extensions . . . . . . . . . . . . . . . . . . . . 7
5. Extended Directives . . . . . . . . . . . . . . . . . . . . . 8
6. Content Key Distribution . . . . . . . . . . . . . . . . . . 10
6.1. Content Key Distribution with AES Key Wrap . . . . . . . 10
6.1.1. Introduction . . . . . . . . . . . . . . . . . . . . 10
6.1.2. Deployment Options . . . . . . . . . . . . . . . . . 11
6.1.3. CDDL . . . . . . . . . . . . . . . . . . . . . . . . 12
6.2. Content Key Distribution with Ephemeral-Static
Diffie-Hellman . . . . . . . . . . . . . . . . . . . . . 13
6.2.1. Introduction . . . . . . . . . . . . . . . . . . . . 13
6.2.2. Deployment Options . . . . . . . . . . . . . . . . . 13
6.2.3. CDDL . . . . . . . . . . . . . . . . . . . . . . . . 14
6.2.4. Context Information Structure . . . . . . . . . . . . 15
7. Content Encryption . . . . . . . . . . . . . . . . . . . . . 17
7.1. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.1.1. Introduction . . . . . . . . . . . . . . . . . . . . 19
7.1.2. AES-KW + AES-GCM Example . . . . . . . . . . . . . . 19
7.1.3. ECDH-ES+AES-KW + AES-GCM Example . . . . . . . . . . 20
7.2. AES-CTR . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.2.1. Introduction . . . . . . . . . . . . . . . . . . . . 22
7.2.2. AES-KW + AES-CTR Example . . . . . . . . . . . . . . 24
7.2.3. ECDH-ES+AES-KW + AES-CTR Example . . . . . . . . . . 25
7.3. AES-CBC . . . . . . . . . . . . . . . . . . . . . . . . . 26
7.3.1. Introduction . . . . . . . . . . . . . . . . . . . . 26
7.3.2. AES-KW + AES-CBC Example . . . . . . . . . . . . . . 27
7.3.3. ECDH-ES+AES-KW + AES-CBC Example . . . . . . . . . . 29
8. Integrity Check on Encrypted and Decrypted Payloads . . . . . 30
8.1. Validating Payload Integrity . . . . . . . . . . . . . . 31
8.1.1. Image Match after Decryption . . . . . . . . . . . . 31
8.1.2. Image Match before Decryption . . . . . . . . . . . . 31
8.1.3. Checking Authentication Tag while Decrypting . . . . 32
8.2. Payload Integrity Validation . . . . . . . . . . . . . . 32
9. Firmware Updates on IoT Devices with Flash Memory . . . . . . 34
10. Complete Examples . . . . . . . . . . . . . . . . . . . . . . 36
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10.1. AES Key Wrap Example with Write Directive . . . . . . . 37
10.2. AES Key Wrap Example with Fetch + Copy Directives . . . 39
10.3. ES-DH Example with Write + Copy Directives . . . . . . . 44
10.4. ES-DH Example with Dependency . . . . . . . . . . . . . 46
11. Operational Considerations . . . . . . . . . . . . . . . . . 51
12. Security Considerations . . . . . . . . . . . . . . . . . . . 52
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 54
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 54
14.1. Normative References . . . . . . . . . . . . . . . . . . 54
14.2. Informative References . . . . . . . . . . . . . . . . . 55
Appendix A. Full CDDL . . . . . . . . . . . . . . . . . . . . . 56
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 56
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 56
1. Introduction
Vulnerabilities in Internet of Things (IoT) devices have highlighted
the need for a reliable and secure firmware update mechanism,
especially for constrained devices. To protect firmware images, the
SUIT manifest format was developed [I-D.ietf-suit-manifest]. This
format provides a bundle of metadata, including the payload location,
applicable devices, and a security wrapper.
[RFC9124] outlines the necessary information provided by the SUIT
manifest format. In addition to protecting against modification via
digital signatures or message authentication codes, the format can
also offer confidentiality.
Encryption prevents third parties, including attackers, from
accessing the payload. Attackers often require detailed knowledge of
a binary, such as a firmware image, to launch successful attacks.
For instance, return-oriented programming (ROP) [ROP] requires access
to the binary, and encryption makes writing exploits significantly
more difficult. Beyond ensuring the confidentiality of the binary
itself, protecting the confidentiality of the source code (e.g., in
the case of open source software) may also be necessary to prevent
reverse engineering and reproduction of the firmware.
While the initial motivation for this document was firmware
encryption, the use of SUIT manifests has expanded to cover other
scenarios requiring integrity and confidentiality protection, such
as:
* software packages,
* personalization data,
* configuration data, and
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* machine learning models.
Thus, the term "payload" is used generically to refer to all these
objects.
The payload is encrypted using a symmetric content encryption key,
which can be established through various mechanisms. This document
defines two content key distribution methods for use with the IETF
SUIT manifest:
* Ephemeral-Static (ES) Diffie-Hellman (DH), and
* AES Key Wrap (AES-KW).
The first method relies on asymmetric cryptography, while the second
uses symmetric cryptography.
Our design aims to reduce the number of content key distribution
methods for payload encryption, thereby increasing interoperability
between different SUIT manifest parser implementations.
The goal of this specification is to protect payloads both during
end-to-end transport (from the distribution system to the device) and
at rest when stored on the device. Constrained devices often employ
eXecute In Place (XIP), a method of executing code directly from
flash memory rather than loading it into RAM. Many of these devices
lack hardware-based, on-the-fly decryption for code stored in flash
memory, which may require decrypting and storing firmware images in
on-chip flash before execution. However, we expect hardware-based,
on-the-fly decryption to become more common in the future, enhancing
confidentiality at rest.
2. 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 assumes familiarity with the IETF SUIT manifest
[I-D.ietf-suit-manifest], the SUIT information model [RFC9124], and
the SUIT architecture [RFC9019].
The following abbreviations are used in this document:
* Key Wrap (KW), defined in [RFC3394] (for use with AES)
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* Key-Encryption Key (KEK) [RFC3394]
* Content-Encryption Key (CEK) [RFC5652]
* Ephemeral-Static (ES) Diffie-Hellman (DH) [RFC9052]
* Authenticated Encryption with Associated Data (AEAD)
* Execute in Place (XIP)
The terms sender and recipient have the following meaning:
* Sender: Entity that sends an encrypted payload.
* Recipient: Entity that receives an encrypted payload.
Additionally, we introduce the term "distribution system" (or
distributor) to refer to an entity that knows the recipients of
payloads. It is important to note that the distribution system is
far more than a file server. For use of encryption, the distribution
system either knows the public key of the recipient (for ES-DH), or
the KEK (for AES-KW).
The author, which is responsible for creating the payload, does not
know the recipients. The authors may, for example, be a developer
building a firmware image.
The author and the distribution system are logical roles. In some
deployments these roles are separated in different physical entities
and in others they are co-located.
3. Architecture
[RFC9019] outlines the architecture for distributing payloads and
manifests from an author to devices. However, it does not cover
payload encryption in detail. This document extends that
architecture to support encryption, as illustrated in Figure 1.
To encrypt a payload, it is essential to know the recipient. For
AES-KW, the Key Encryption Key (KEK) must be known, and for ES-DH,
the sender needs access to the recipient's public key. This public
key and its associated parameters may be found in the recipient's
X.509 certificate [RFC5280]. For authentication and integrity
protection, recipients must be provisioned with a trust anchor when
the manifest is protected by a digital signature. If a MAC is used
for manifest protection, a symmetric key must be shared between the
recipient and the sender.
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With encryption, the author cannot simply create and sign a manifest
for the payload, as the recipients are often unknown. Therefore, the
author must collaborate with the distribution system. The degree of
this collaboration is discussed below.
The primary purpose of encryption is to protect against adversaries
along the path between the distribution system and the device. There
is also a risk that adversaries may extract the decrypted firmware
image from the device itself. Consequently, the device must be
safeguarded against physical attacks, though such countermeasures are
typically beyond the scope of IETF specifications.
Note: It is assumed that the communication between the author and the
distribution system is secure. For example, the author might upload
the manifest and firmware image to the distribution system using a
secure HTTPS REST API.
+----------+
| Device | +----------+
| 1 |<--+ | Author |
| | | +----------+
+----------+ | |
| | Payload +
| | Manifest
| v
+----------+ | +--------------+
| Device | | Payload + Manifest | Distribution |
| 2 |<--+------------------------| System |
| | | +--------------+
+----------+ |
|
... |
|
+----------+ |
| Device | |
| n |<--+
| |
+----------+
Figure 1: Architecture for the distribution of Encrypted Payloads.
When the author delegates encryption rights to the distributor, two
models are possible:
1. Replacing the COSE_Encrypt and Re-signing the Manifest: The
distributor replaces the COSE_Encrypt structure in the manifest
and then signs the manifest again. However, since the
COSE_Encrypt structure is within a signed container, this
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presents a challenge: replacing COSE_Encrypt alters the digest of
the manifest, thereby invalidating the signature. As a result,
the distributor must be able to sign the new manifest. If this
is the case, the distributor gains the authority to construct and
sign manifests, effectively allowing them to sign code and giving
them full control over the recipient. Distributors typically
perform re-encryption online to manage large numbers of devices
efficiently, which prevents air-gapping the signing operations.
This has implications for the recommendations in Section 4.3.17
of [RFC9124]. Despite these issues, this model represents the
current standard practice for IoT firmware updates.
2. Two-Layer Manifest System: The distributor creates a new manifest
that overrides the COSE_Encrypt using the dependency system
defined in [I-D.ietf-suit-trust-domains]. This method introduces
additional overhead, including one more signature verification,
one extra manifest, and the need for extra mechanisms on the
recipient side to handle dependency processing. While this adds
complexity, it also enhances security.
These two models offer different threat profiles for the distributor.
If the distributor is limited to encryption rights, an attacker who
breaches the distributor can only launch a limited attack by
encrypting a modified binary. However, recipients will detect the
attack during the image digest check and immediately revert to the
correct image.
It is RECOMMENDED that distributors adopt the two-layer manifest
approach to distribute content encryption keys without re-signing the
manifest, despite the added complexity and the increased number of
signature verifications required on the recipient side.
4. Encryption Extensions
This specification introduces a new extension to the SUIT_Parameters
structure.
The SUIT_Encryption_Info structure (referred to as suit-parameter-
encryption-info in Figure 2) contains the content key distribution
information. The details of the SUIT_Encryption_Info structure are
provided in Section 6.1 (for AES-KW) and Section 6.2 (for ES-DH).
Once a CEK is available, the steps outlined in Section 7 apply to
both content key distribution methods described in this section.
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The SUIT_Encryption_Info structure is carried within either the suit-
directive-override-parameters or suit-directive-set-parameters used
in the "Directive Write" and "Directive Copy" operations. An
implementation conforming to this specification must support these
two parameters.
Interoperability requirements for content key distribution methods
vary: since a device will typically support only one of the two
specified methods, the distribution system needs to know which method
is supported. Limiting a constrained device to a single content key
distribution method also helps reduce code size.
SUIT_Parameters //= (suit-parameter-encryption-info
=> bstr .cbor SUIT_Encryption_Info)
suit-parameter-encryption-info = TBD19
Figure 2: CDDL of the SUIT_Parameters Extension.
RFC Editor's Note (TBD19): The value for the suit-parameter-
encryption-info parameter is set to 19, as the proposed value.
5. Extended Directives
Here’s an improved version of your text:
This specification extends the following directives:
* Directive Write (suit-directive-write): Used to decrypt the
content specified by suit-parameter-content using suit-parameter-
encryption-info.
* Directive Copy (suit-directive-copy): Used to decrypt the content
of the component specified by suit-parameter-source-component
using suit-parameter-encryption-info.
Examples of these two directives are provided below, focusing on the
essential aspects. A complete example for AES Key Wrap with the
Fetch and Copy directives can be found in Section 10.2, while an
example illustrating the Write directive is shown in Section 10.1.
Figure 3 illustrates the Directive Write. The encrypted payload
specified by parameter-content, represented as h'EA1...CED' in the
example, is decrypted using the SUIT_Encryption_Info structure
referenced by parameter-encryption-info, i.e., h'D86...1F0'. The
resulting plaintext payload is then stored in component #0.
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/ directive-override-parameters / 20, {
/ parameter-content / 18: h'EA1...CED',
/ parameter-encryption-info / TBD19: h'D86...1F0'
},
/ directive-write / 18, 15
Figure 3: Example showing the extended suit-directive-write.
RFC Editor's Note (TBD19): The value for the parameter-encryption-
info parameter is set to 19, as the proposed value.
Figure 4 illustrates the Directive Copy. In this example the
encrypted payload is found at the URI indicated by the parameter-uri,
i.e. "http://example.com/encrypted.bin". The encrypted payload will
be downloaded and stored in component #1. Then, the information in
the SUIT_Encryption_Info structure referred to by parameter-
encryption-info, i.e. h'D86...1F0', will be used to decrypt the
content in component #1 and the resulting plaintext payload will be
stored into component #0.
/ directive-set-component-index / 12, 1,
/ directive-override-parameters / 20, {
/ parameter-uri / 21: "http://example.com/encrypted.bin",
},
/ directive-fetch / 21, 15,
/ directive-set-component-index / 12, 0,
/ directive-override-parameters / 20, {
/ parameter-encryption-info / TBD19: h'D86...1F0',
/ parameter-source-component / 22: 1
},
/ directive-copy / 22, 15
Figure 4: Example showing the extended suit-directive-copy.
RFC Editor's Note (TBD19): The value for the suit-parameter-
encryption-info parameter is set to 19, as the proposed value.
The payload to be encrypted may be detached and, in that case, it is
not covered by the digital signature or the MAC protecting the
manifest. (To be more precise, the suit-authentication-wrapper found
in the envelope contains a digest of the manifest in the SUIT Digest
Container.)
The lack of authentication and integrity protection of the payload is
particularly a concern when a cipher without integrity protection is
used.
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To provide authentication and integrity protection of the payload in
the detached payload case a SUIT Digest Container with the hash of
the encrypted and/or plaintext payload MUST be included in the
manifest. See suit-parameter-image-digest parameter in
Section 8.4.8.6 of [I-D.ietf-suit-manifest].
Once a CEK is available, the steps described in Section 7 are
applicable. These steps apply to both content key distribution
methods.
6. Content Key Distribution
The following sub-sections describe two content key distribution
methods: AES Key Wrap (AES-KW) and Ephemeral-Static Diffie-Hellman
(ES-DH). While many other methods are specified in the literature
and supported by COSE, AES-KW and ES-DH were chosen for their
widespread use in the market today. They were selected for their
maturity, differing security properties, and strong interoperability.
Both content key distribution methods require the CEKs to be randomly
generated. The guidelines for random number generation in [RFC8937]
MUST be followed.
When sending an encrypted payload to multiple recipients, various
deployment options are available. The following notation is used to
explain these options:
- KEK[R1, S] refers to a KEK shared between recipient R1 and
the sender S.
- CEK[R1, S] refers to a CEK shared between R1 and S.
- CEK[*, S] or KEK[*, S] are used when a single CEK or a single
KEK is shared with all authorized recipients by a given sender
S in a certain context.
- ENC(plaintext, k) refers to the encryption of plaintext with
a key k.
6.1. Content Key Distribution with AES Key Wrap
6.1.1. Introduction
The AES Key Wrap (AES-KW) algorithm, as described in [RFC3394], is
used to encrypt a randomly generated content-encryption key (CEK)
with a pre-shared key-encryption key (KEK). The COSE conventions for
using AES-KW are specified in Section 8.5.2 of [RFC9052] and in
Section 6.2.1 of [RFC9053]. The encrypted CEK is carried within the
COSE_recipient structure , which includes the necessary information
for AES-KW. The COSE_recipient structure, a substructure of
COSE_Encrypt, contains the CEK encrypted by the KEK.
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To ensure high security when using AES Key Wrap, it is important that
the KEK is of high entropy and that implementations protect the KEK
from disclosure. A compromised KEK could expose all data encrypted
with it, including binaries and configuration data.
The COSE_Encrypt structure conveys the information needed to encrypt
the payload, including details such as the algorithm and IV. Even
though the payload may be conveyed as detached content, the
encryption information is still embedded in the
COSE_Encrypt.ciphertext structure.
6.1.2. Deployment Options
There are three deployment options for use with AES Key Wrap for
payload encryption:
* If all recipients (typically of the same product family) share the
same KEK, a single COSE_recipient structure contains the encrypted
CEK. The sender executes the following steps:
1. Fetch KEK[*, S]
2. Generate CEK
3. ENC(CEK, KEK)
4. ENC(payload, CEK)
This deployment option is strongly discouraged. An attacker gaining
access to the KEK will be able to encrypt and send payloads to all
recipients configured to use this KEK.
* If recipients have different KEKs, then multiple COSE_recipient
structures are included but only a single CEK is used. Each
COSE_recipient structure contains the CEK encrypted with the KEKs
appropriate for a given recipient. The benefit of this approach
is that the payload is encrypted only once with a CEK while there
is no sharing of the KEK across recipients. Hence, authorized
recipients still use their individual KEK to decrypt the CEK and
to subsequently obtain the plaintext. The steps taken by the
sender are:
1. Generate CEK
2. for i=1 to n
{
2a. Fetch KEK[Ri, S]
2b. ENC(CEK, KEK[Ri, S])
}
3. ENC(payload, CEK)
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* The third option is to use different CEKs encrypted with KEKs of
authorized recipients. This approach is appropriate when no
benefits can be gained from encrypting and transmitting payloads
only once. Assume there are n recipients with their unique KEKs -
KEK[R1, S], ..., KEK[Rn, S] and unique CEKs. The sender needs to
execute the following steps:
1. for i=1 to n
{
1a. Fetch KEK[Ri, S]
1b. Generate CEK[Ri, S]
1c. ENC(CEK[Ri, S], KEK[Ri, S])
1d. ENC(payload, CEK[Ri, S])
2. }
6.1.3. CDDL
The CDDL for the AES-KW binary is shown in Figure 5.
empty_or_serialized_map and header_map are structures defined in
[RFC9052].
=============== NOTE: '\' line wrapping per RFC 8792 ================
SUIT_Encryption_Info_AESKW = #6.96([
protected : outer_header_map_protected,
unprotected : outer_header_map_unprotected,
ciphertext : bstr / nil,
recipients : [ + COSE_recipient_AESKW ]
])
outer_header_map_protected = empty_or_serialized_map
outer_header_map_unprotected = header_map
COSE_recipient_AESKW = [
protected : bstr .size 0 / bstr .cbor empty_map,
unprotected : recipient_header_unpr_map_aeskw,
ciphertext : bstr ; CEK encrypted with KEK
]
empty_map = {}
recipient_header_unpr_map_aeskw =
{
1 => int, ; algorithm identifier
? 4 => bstr, ; identifier of the KEK pre-shared with the \
recipient
* label => values ; extension point
}
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Figure 5: CDDL for AES-KW-based Content Key Distribution
Note that the AES-KW algorithm, as defined in Section 2.2.3.1 of
[RFC3394], does not have public parameters that vary on a per-
invocation basis. Hence, the protected header in the COSE_recipient
structure is a byte string of zero length.
6.2. Content Key Distribution with Ephemeral-Static Diffie-Hellman
6.2.1. Introduction
Ephemeral-Static Diffie-Hellman (ES-DH) is a public key encryption
scheme that enables encryption using the recipient's public key.
There are several variations of this scheme; this document adopts the
version specified in Section 8.5.5 of [RFC9052].
The structure is composed of two layers:
* Layer 0: Contains content encrypted with a Content Encryption Key
(CEK). The content may be provided separately.
* Layer 1: Uses the AES Key Wrap (AES-KW) algorithm to encrypt the
randomly generated CEK with a Key Encryption Key (KEK) derived via
ES-DH. The resulting symmetric key is processed through an HKDF-
based key derivation function.
This two-layer structure combines ES-DH with AES-KW and HKDF,
referred to as ECDH-ES + AES-KW. An example can be found in
Figure 10.
Another variant of the ES-DH algorithm, called ECDH-ES + HKDF, does
not utilize AES Key Wrap. However, this version is not covered in
this document.
6.2.2. Deployment Options
This approach supports only two deployment options, as it assumes
that each recipient is always equipped with a device-specific public/
private key pair.
* When a sender transmits a payload to multiple recipients, all
recipients receive the same encrypted payload, meaning the same
CEK is used to encrypt the content. For each recipient, a
separate COSE_recipient structure is used, which contains the CEK
encrypted with the recipient-specific KEK. To derive the KEK,
each COSE_recipient structure includes a COSE_recipient_inner
structure that carries the sender's ephemeral key and an
identifier for the recipient's public key.
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The steps taken by the sender are:
1. Generate CEK
2. for i=1 to n
{
2a. Generate KEK[Ri, S] using ES-DH
2b. ENC(CEK, KEK[Ri, S])
}
3. ENC(payload,CEK)
* The alternative is to encrypt the payload with a unique CEK for
each recipient, resulting in multiple manifests. This approach is
useful when payloads contain device-specific information. In this
case, the encryption operation becomes ENC(payload_i, CEK[Ri, S])
where each recipient Ri receives a unique CEK. Assume that
KEK[R1, S],..., KEK[Rn, S] have been generated for the recipients
using ES-DH. The sender must then follow these steps:
1. for i=1 to n
{
1a. Generate KEK[Ri, S] using ES-DH
1b. Generate CEK[Ri, S]
1c. ENC(CEK[Ri, S], KEK[Ri, S])
1d. ENC(payload, CEK[Ri, S])
}
6.2.3. CDDL
The CDDL for the ECDH-ES+AES-KW binary is provided in Figure 6. Only
the essential parameters are included. The structures
empty_or_serialized_map and header_map are defined in [RFC9052].
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SUIT_Encryption_Info_ESDH = #6.96([
protected : outer_header_map_protected,
unprotected : outer_header_map_unprotected,
ciphertext : bstr / nil,
recipients : [ + COSE_recipient_ESDH ]
])
outer_header_map_protected = empty_or_serialized_map
outer_header_map_unprotected = header_map
COSE_recipient_ESDH = [
protected : bstr .cbor recipient_header_map_esdh,
unprotected : recipient_header_unpr_map_esdh,
ciphertext : bstr ; CEK encrypted with KEK
]
recipient_header_map_esdh =
{
1 => int, ; algorithm identifier
* label => values ; extension point
}
recipient_header_unpr_map_esdh =
{
? 4 => bstr, ; identifier of the recipient public key
-1 => COSE_Key, ; ephemeral public key for the sender
* label => values ; extension point
}
Figure 6: CDDL for ES-DH-based Content Key Distribution
See Section 7 for a description on how to encrypt the payload.
6.2.4. Context Information Structure
The context information structure ensures that the derived keying
material is "bound" to the specific context of the transaction. This
specification reuses the structure defined in Section 5.2 of
[RFC9053], with modifications to fit the current use case.
The following elements are bound to the context:
* the protocol employing the key-derivation method,
* information about the utilized AES Key Wrap algorithm, and the key
length.
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* the protected header field, which contains the content key
encryption algorithm.
The sender and recipient identities are left empty.
The following fields in Figure 7 require an explanation:
* The COSE_KDF_Context.AlgorithmID field MUST contain the identifier
for the AES Key Wrap algorithm being used. This specification
uses the following values: A128KW (value -3), A192KW (value -4),
or A256KW (value -5)
* The COSE_KDF_Context.SuppPubInfo.keyDataLength field MUST specify
the key length, in bits, corresponding to the algorithm in the
AlgorithmID field. For A128KW the value is 128, for A192KW the
value is 192, and for A256KW the value 256.
* The COSE_KDF_Context.SuppPubInfo.other field captures the protocol
that uses the ES-DH content key distribution algorithm. It MUST
be set to the constant string "SUIT Payload Encryption".
* The COSE_KDF_Context.SuppPubInfo.protected field MUST contain the
serialized content of the recipient_header_map_esdh field, which
contains (among other elements) the identifier of the content key
distribution method.
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COSE_KDF_Context = [
AlgorithmID : int,
PartyUInfo : [ PartyInfoSender ],
PartyVInfo : [ PartyInfoRecipient ],
SuppPubInfo : [
keyDataLength : uint,
protected : bstr,
other: 'SUIT Payload Encryption'
],
? SuppPrivInfo : bstr
]
PartyInfoSender = (
identity : nil,
nonce : nil,
other : nil
)
PartyInfoRecipient = (
identity : nil,
nonce : nil,
other : nil
)
Figure 7: CDDL for COSE_KDF_Context Structure
The HKDF-based key derivation function MAY contain a salt value, as
described in Section 5.1 of [RFC9053]. This optional value
influences the key generation process, though this specification does
not require the use of a salt. If the salt is public and included in
the message, the "salt" algorithm header parameter MUST be used. The
salt adds extra randomness to the KDF context. When the salt is
transmitted via the "salt" algorithm header parameter, the receiver
MUST be capable of processing it and MUST pass it into the key
derivation function. For more details on salt usage, refer to
[RFC5869] and NIST SP800-56 [SP800-56].
Profiles of this specification MAY define an extended version of the
context information structure or MAY employ a different context
information structure.
7. Content Encryption
This section summarizes the steps involved in content encryption,
applicable to both content key distribution methods.
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When using AEAD ciphers, such as AES-GCM or ChaCha20/Poly1305, the
COSE specification requires a consistent byte stream to create the
authenticated data structure. This structure is illustrated in
Figure 8 and defined in Section 5.3 of [RFC9052].
Enc_structure = [
context : "Encrypt",
protected : empty_or_serialized_map,
external_aad : bstr
]
Figure 8: CDDL for Enc_structure Data Structure
This Enc_structure must be populated as follows:
* The protected field in the Enc_structure from Figure 8 refers to
the content of the protected field in the COSE_Encrypt structure.
* The value of external_aad MUST be set to a zero-length byte
string, represented as h'' in diagnostic notation and encoded as
0x40.
Some ciphers, such as AES-CTR and AES-CBC, provide confidentiality
without integrity protection (see [RFC9459]). For these ciphers, the
Enc_structure shown in Figure 8 cannot be used, as the Additional
Authenticated Data (AAD) byte string is only applicable to AEAD
ciphers. Therefore, the AAD structure is not passed to the API for
these ciphers, and the protected header in the SUIT_Encryption_Info
structure MUST be a zero-length byte string.
AES-CTR and AES-CBC are discussed in separate sub-sections below and
Figure 11 and Figure 14 use the following abbreviations:
* Pi = Plaintext blocks
* Ci = Ciphertext blocks
* E = Encryption function
* k = Symmetric key
* ⊕ = XOR operation
7.1. AES-GCM
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7.1.1. Introduction
AES-GCM is an AEAD cipher and provides confidentiality and integrity
protection.
Examples in this section use the following parameters:
* Algorithm for payload encryption: AES-GCM-128
- k: h'15F785B5C931414411B4B71373A9C0F7'
- IV: h'F14AAB9D81D51F7AD943FE87AF4F70CD'
* Plaintext: "This is a real firmware image."
- in hex:
546869732069732061207265616C206669726D7761726520696D6167652E
7.1.2. AES-KW + AES-GCM Example
This example uses the following parameters:
* Algorithm id for key wrap: A128KW
* KEK COSE_Key (Secret Key):
- kty: Symmetric
- k: 'aaaaaaaaaaaaaaaa'
- kid: 'kid-1'
The COSE_Encrypt structure, in hex format, is (with a line break
inserted):
D8608443A10101A1054CF14AAB9D81D51F7AD943FE87F6818340A2012204
456B69642D31581875603FFC9518D794713C8CA8A115A7FB32565A6D5953
4D62
The resulting COSE_Encrypt structure in a diagnostic format is shown
in Figure 9.
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96([
/ protected: / << {
/ alg / 1: 1 / A128GCM /
} >>,
/ unprotected: / {
/ IV / 5: h'F14AAB9D81D51F7AD943FE87'
},
/ payload: / null / detached ciphertext /,
/ recipients: / [
[
/ protected: / h'',
/ unprotected: / {
/ alg / 1: -3 / A128KW /,
/ kid / 4: 'kid-1'
},
/ payload: /
h'75603FFC9518D794713C8CA8A115A7FB32565A6D59534D62'
/ CEK encrypted with KEK /
]
]
])
Figure 9: COSE_Encrypt Example for AES Key Wrap
The encrypted payload (with a line feed added) was:
758C4B7BBAE2C4C1D462423E0F0DC3164FFA7B85BB94D4BD6D7ED26AB32F
EB063385D4D3465927EC82CB5E198A59
7.1.3. ECDH-ES+AES-KW + AES-GCM Example
This example uses the following parameters:
* Algorithm for content key distribution: ECDH-ES + A128KW
* KEK COSE_Key (Receiver's Private Key):
- kty: EC2
- crv: P-256
- x: h'5886CD61DD875862E5AAA820E7A15274C968A9BC96048DDCACE32F50C3
651BA3'
- y: h'9EED8125E932CD60C0EAD3650D0A485CF726D378D1B016ED4298B2961E
258F1B'
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- d: h'60FE6DD6D85D5740A5349B6F91267EEAC5BA81B8CB53EE249E4B4EB102
C476B3'
- kid: 'kid-2'
* KDF Context
- Algorithm ID: -3 (A128KW)
- SuppPubInfo
o keyDataLength: 128
o protected: { / alg / 1: -29 / ECDH-ES+A128KW / }
o other: 'SUIT Payload Encryption'
The COSE_Encrypt structure, in hex format, is (with a line break
inserted):
D8608443A10101A1054CF14AAB9D81D51F7AD943FE87F6818344A101381C
A120A40102200121582073024F415AA51529A66CCEFD88F3F62A734492FF
45F6AD37FD2888E73EAF19DA2258204005B48A6FD091AA6ABFE3CFBEEDE8
8B347E521D43405FDBD7D2CFF0EBC21B265818A06B8E6550F308712B1DF0
44B21B7D11D9B22792F1DE0997
The resulting COSE_Encrypt structure in a diagnostic format is shown
in Figure 10.
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96([
/ protected: / << {
/ alg / 1: 1 / A128GCM /
} >>,
/ unprotected: / {
/ IV / 5: h'F14AAB9D81D51F7AD943FE87'
},
/ payload: / null / detached ciphertext /,
/ recipients: / [
[
/ protected: / << {
/ alg / 1: -29 / ECDH-ES + A128KW /
} >>,
/ unprotected: / {
/ ephemeral key / -1: {
/ kty / 1: 2 / EC2 /,
/ crv / -1: 1 / P-256 /,
/ x / -2: h'73024F415AA51529A66CCEFD88F3F62A
734492FF45F6AD37FD2888E73EAF19DA',
/ y / -3: h'4005B48A6FD091AA6ABFE3CFBEEDE88B
347E521D43405FDBD7D2CFF0EBC21B26'
}
},
/ payload:
/ h'A06B8E6550F308712B1DF044B21B7D11D9B22792F1DE0997'
/ CEK encrypted with KEK /
]
]
])
Figure 10: COSE_Encrypt Example for ES-DH
The encrypted payload (with a line feed added) was:
758C4B7BBAE2C4C1D462423E0F0DC3164FFA7B85BB94D4BD6D7ED26AB32F
EB063385D4D3465927EC82CB5E198A59
7.2. AES-CTR
7.2.1. Introduction
AES-CTR is a non-AEAD cipher that provides confidentiality but lacks
integrity protection. Unlike AES-CBC, AES-CTR uses an IV per AES
operation, as shown in Figure 11. Hence, when an image is encrypted
using AES-CTR-128 or AES-CTR-256, the IV MUST start with zero (0) and
MUST be incremented by one for each 16-byte plaintext block within
the entire slot.
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Using the previous example with a slot size of 64 KiB, the sector
size 4096 bytes and the AES plaintext block size of 16 byte requires
IVs from 0 to 255 in the first sector and 16 * 256 IVs for the
remaining sectors in the slot.
In our example, we assume the slot size of a specific flash
controller on an IoT device is 64 KiB, the sector size 4096 bytes (4
KiB) and an AES plaintext block size of 16 bytes, the IVs range from
0 to 255 in the first sector, and 16 * 256 IVs are required for the
remaining sectors in the slot.
IV1 IV2
| |
| |
| |
+-------+ +-------+
| | | |
| | | |
k--| E | k--| E |
| | | |
+-------+ +-------+
| |
P1---⊕ P2---⊕
| |
| |
C1 C2
Figure 11: AES-CTR Operation
Note: The abbreviations shown in Figure 11 are described in
Section 7.
Examples in this section use the following parameters:
* Algorithm for payload encryption: AES-CTR-128
- k: h'261DE6165070FB8951EC5D7B92A065FE'
- IV: h'DAE613B2E0DC55F4322BE38BDBA9DC68'
* Plaintext: "This is a real firmware image."
- in hex:
546869732069732061207265616C206669726D7761726520696D6167652E
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7.2.2. AES-KW + AES-CTR Example
This example uses the following parameters:
* Algorithm id for key wrap: A128KW
* KEK COSE_Key (Secret Key):
- kty: Symmetric
- k: 'aaaaaaaaaaaaaaaa'
- kid: 'kid-1'
The COSE_Encrypt structure, in hex format, is (with a line break
inserted):
D8608440A20139FFFD0550DAE613B2E0DC55F4322BE38BDBA9DC68F68183
40A2012204456B69642D315818CE34035CE5C2E2666E46D4C131FC561DD1
90A6D26CFA1990
The resulting COSE_Encrypt structure in a diagnostic format is shown
in Figure 12.
96([
/ protected: / h'',
/ unprotected: / {
/ alg / 1: -65534 / A128CTR /,
/ IV / 5: h'DAE613B2E0DC55F4322BE38BDBA9DC68'
},
/ payload: / null / detached ciphertext /,
/ recipients: / [
[
/ protected: / h'',
/ unprotected: / {
/ alg / 1: -3 / A128KW /,
/ kid / 4: 'kid-1'
},
/ payload: /
h'CE34035CE5C2E2666E46D4C131FC561DD190A6D26CFA1990'
/ CEK encrypted with KEK /
]
]
])
Figure 12: COSE_Encrypt Example for AES Key Wrap
The encrypted payload (with a line feed added) was:
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2BB8DB522AE978246CC775C3B0241BD4B0333FFDD2DB70C7EE7A4966E3B7
7.2.3. ECDH-ES+AES-KW + AES-CTR Example
This example uses the following parameters:
* Algorithm for content key distribution: ECDH-ES + A128KW
* KEK COSE_Key (Receiver's Private Key):
- kty: EC2
- crv: P-256
- x: h'5886CD61DD875862E5AAA820E7A15274C968A9BC96048DDCACE32F50C3
651BA3'
- y: h'9EED8125E932CD60C0EAD3650D0A485CF726D378D1B016ED4298B2961E
258F1B'
- d: h'60FE6DD6D85D5740A5349B6F91267EEAC5BA81B8CB53EE249E4B4EB102
C476B3'
- kid: 'kid-2'
* KDF Context
- Algorithm ID: -3 (A128KW)
- SuppPubInfo
o keyDataLength: 128
o protected: { / alg / 1: -29 / ECDH-ES+A128KW / }
o other: 'SUIT Payload Encryption'
The COSE_Encrypt structure, in hex format, is (with a line break
inserted):
D8608440A20139FFFD0550DAE613B2E0DC55F4322BE38BDBA9DC68F68183
44A101381CA120A401022001215820EE0718F6B019C29CC611C18CEDE221
4066DDCEDC2F0DBEF873CB224C715C1174225820279F2A88E4AB9E2ED30C
0FCB69515B31B5D36725BFDB9AE02032ED4D5AB52CB85818E28B4502E4F5
151884A995405579006E9465C3E94E3E0808
The resulting COSE_Encrypt structure in a diagnostic format is shown
in Figure 13.
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=============== NOTE: '\' line wrapping per RFC 8792 ================
96([
/ protected: / h'',
/ unprotected: / {
/ alg / 1: -65534 / A128CTR /,
/ IV / 5: h'DAE613B2E0DC55F4322BE38BDBA9DC68'
},
/ payload: / null / detached ciphertext /,
/ recipients: / [
[
/ protected: / << {
/ alg / 1: -29 / ECDH-ES + A128KW /
} >>,
/ unprotected: / {
/ ephemeral key / -1: {
/ kty / 1: 2 / EC2 /,
/ crv / -1: 1 / P-256 /,
/ x / -2: h'EE0718F6B019C29CC611C18CEDE22140
66DDCEDC2F0DBEF873CB224C715C1174',
/ y / -3: h'279F2A88E4AB9E2ED30C0FCB69515B31
B5D36725BFDB9AE02032ED4D5AB52CB8'
}
},
/ payload: / h'\
E28B4502E4F5151884A995405579006E9465C3E94E3E0808'
/ CEK encrypted with KEK /
]
]
])
Figure 13: COSE_Encrypt Example for ES-DH
The encrypted payload (with a line feed added) was:
2BB8DB522AE978246CC775C3B0241BD4B0333FFDD2DB70C7EE7A4966E3B7
7.3. AES-CBC
7.3.1. Introduction
AES-CBC is a non-AEAD cipher that provides confidentiality but but
does not offer integrity protection. In AES-CBC, a single IV is used
to encrypt the firmware belonging to a single sector, as individual
AES blocks are chained together, as illustrated in Figure 14. The
numbering of sectors in a slot start with zero (0) and increase by
one with every sector till the end of the slot is reached. The IV
follows this numbering.
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For example, let us assume the slot size of a specific flash
controller on an IoT device is 64 KiB, the sector size 4096 bytes (4
KiB) and AES-128-CBC uses an AES-block size of 128 bit (16 bytes).
Hence, sector 0 needs 4096/16=256 AES-128-CBC operations using IV 0.
If the firmware image occupies the entire slot, it will contain 16
sectors, corresponding to IVs ranging from 0 to 15.
P1 P2
| |
IV---⊕ +--------⊕
| | |
| | |
+-------+ | +-------+
| | | | |
| | | | |
k--| E | | k--| E |
| | | | |
+-------+ | +-------+
| | |
+-----+ |
| |
| |
C1 C2
Figure 14: AES-CBC Operation
Note: The abbreviations shown in Figure 14 are described in
Section 7.
Examples in this section use the following parameters:
* Algorithm for payload encryption: AES-CBC-128
- k: h'627FCF0EA82C967D5ED8981EB325F303'
- IV: h'93702C81590F845D9EC866CCAC767BD1'
* Plaintext: "This is a real firmware image."
- in hex:
546869732069732061207265616C206669726D7761726520696D6167652E
7.3.2. AES-KW + AES-CBC Example
This example uses the following parameters:
* Algorithm id for key wrap: A128KW
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* KEK COSE_Key (Secret Key):
- kty: Symmetric
- k: 'aaaaaaaaaaaaaaaa'
- kid: 'kid-1'
The COSE_Encrypt structure, in hex format, is (with a line break
inserted):
D8608440A20139FFFA055093702C81590F845D9EC866CCAC767BD1F68183
40A2012204456B69642D315818E198FF269626EC43299D33586FC7B2646B
13292261160422
The resulting COSE_Encrypt structure in a diagnostic format is shown
in Figure 15.
96([
/ protected: / h'',
/ unprotected: / {
/ alg / 1: -65531 / A128CBC /,
/ IV / 5: h'93702C81590F845D9EC866CCAC767BD1'
},
/ payload: / null / detached ciphertext /,
/ recipients: / [
[
/ protected: / h'',
/ unprotected: / {
/ alg / 1: -3 / A128KW /,
/ kid / 4: 'kid-1'
},
/ payload: /
h'E198FF269626EC43299D33586FC7B2646B13292261160422'
/ CEK encrypted with KEK /
]
]
])
Figure 15: COSE_Encrypt Example for AES Key Wrap
The encrypted payload (with a line feed added) was:
9C09156CF4ACE0401086D98586E0B09FA5B5CF78F2BCCBF6C914DDB42BF0
E21E
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7.3.3. ECDH-ES+AES-KW + AES-CBC Example
This example uses the following parameters:
* Algorithm for content key distribution: ECDH-ES + A128KW
* KEK COSE_Key (Receiver's Private Key):
- kty: EC2
- crv: P-256
- x: h'5886CD61DD875862E5AAA820E7A15274C968A9BC96048DDCACE32F50C3
651BA3'
- y: h'9EED8125E932CD60C0EAD3650D0A485CF726D378D1B016ED4298B2961E
258F1B'
- d: h'60FE6DD6D85D5740A5349B6F91267EEAC5BA81B8CB53EE249E4B4EB102
C476B3'
- kid: 'kid-2'
* KDF Context
- Algorithm ID: -3 (A128KW)
- SuppPubInfo
o keyDataLength: 128
o protected: { / alg / 1: -29 / ECDH-ES+A128KW / }
o other: 'SUIT Payload Encryption'
The COSE_Encrypt structure, in hex format, is (with a line break
inserted):
D8608440A20139FFFA055093702C81590F845D9EC866CCAC767BD1F68183
44A101381CA120A40102200121582084C1768A1AFA92BAF27C3289A3FAF7
5E7511B27C654947BBD391E8A12EB8295B225820F32B0AECA4F8B6C51C15
5037B1FB726CAE3D0C77976864EF725DB2B30ABCE6F6581804928D52CD34
AC8CA0427CEED6D7C5D06A21B5F4F79CAA18
The resulting COSE_Encrypt structure in a diagnostic format is shown
in Figure 16.
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=============== NOTE: '\' line wrapping per RFC 8792 ================
96([
/ protected: / h'',
/ unprotected: / {
/ alg / 1: -65531 / A128CBC /,
/ IV / 5: h'93702C81590F845D9EC866CCAC767BD1'
},
/ payload: / null / detached ciphertext /,
/ recipients: / [
[
/ protected: / << {
/ alg / 1: -29 / ECDH-ES + A128KW /
} >>,
/ unprotected: / {
/ ephemeral key / -1: {
/ kty / 1: 2 / EC2 /,
/ crv / -1: 1 / P-256 /,
/ x / -2: h'84C1768A1AFA92BAF27C3289A3FAF75E
7511B27C654947BBD391E8A12EB8295B',
/ y / -3: h'F32B0AECA4F8B6C51C155037B1FB726C
AE3D0C77976864EF725DB2B30ABCE6F6'
}
},
/ payload: / h'\
04928D52CD34AC8CA0427CEED6D7C5D06A21B5F4F79CAA18'
/ CEK encrypted with KEK /
]
]
])
Figure 16: COSE_Encrypt Example for ES-DH
The encrypted payload (with a line feed added) was:
9C09156CF4ACE0401086D98586E0B09FA5B5CF78F2BCCBF6C914DDB42BF0
E21E
8. Integrity Check on Encrypted and Decrypted Payloads
In addition to suit-condition-image-match (see Section 8.4.9.2 of
[I-D.ietf-suit-manifest]), AEAD algorithms used for content
encryption provides another way to validate the integrity of
components. This section provides a guideline to construct secure
but not redundant SUIT Manifest for encrypted payloads.
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8.1. Validating Payload Integrity
This sub-section explains three ways to validate the integrity of
payloads.
8.1.1. Image Match after Decryption
The suit-condition-image-match on the plaintext payload is used after
decryption. An example command sequence is shown in Figure 17.
/ directive-set-component-index / 12, 1,
/ directive-override-parameters / 20, {
/ parameter-uri / 21: "http://example.com/encrypted.bin"
},
/ directive-fetch / 21, 15,
/ directive-set-component-index / 12, 0,
/ directive-override-parameters / 20, {
/ parameter-image-digest / 3: << {
/ algorithm-id: / -16 / SHA256 /,
/ digest-bytes: / h'3B1...92A' / digest of plaintext payload /
} >>,
/ parameter-image-size / 14: 30 / size of plaintext payload /,
/ parameter-encryption-info / TBD19: h'369...50F',
/ parameter-source-component / 22: 1
},
/ directive-copy / 22, 15,
/ condition-image-match / 3, 15 / check decrypted payload integrity /,
Figure 17: Check Image Match After Decryption
RFC Editor's Note (TBD19): The value for the suit-parameter-
encryption-info parameter is set to 19, as the proposed value.
8.1.2. Image Match before Decryption
The suit-condition-image-match can also be applied on encrypted
payloads before decryption takes place. An example command sequence
is shown in Figure 18.
This option mitigates battery exhaustion attacks discussed in
Section 12.
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/ directive-set-component-index / 12, 1,
/ directive-override-parameters / 20, {
/ parameter-image-digest / 3: << {
/ algorithm-id: / -16 / SHA256 /,
/ digest-bytes: / h'8B4...D34' / digest of encrypted payload /
} >>,
/ parameter-image-size / 14: 30 / size of encrypted payload /,
/ parameter-uri / 21: "http://example.com/encrypted.bin"
},
/ directive-fetch / 21, 15,
/ condition-image-match / 3, 15 / check decrypted payload integrity /,
/ directive-set-component-index / 12, 0,
/ directive-override-parameters / 20, {
/ parameter-encryption-info / TBD19: h'D86...1F0',
/ parameter-source-component / 22: 1
},
/ directive-copy / 22, 15,
Figure 18: Check Image Match Before Decryption
RFC Editor's Note (TBD19): The value for the suit-parameter-
encryption-info parameter is set to 19, as the proposed value.
8.1.3. Checking Authentication Tag while Decrypting
AEAD algorithms, such as AES-GCM and ChaCha20/Poly1305, verify the
integrity of the encrypted concent.
8.2. Payload Integrity Validation
This subsection offers guidelines for validating the integrity of
payloads within the SUIT manifest. The decision tree in Figure 19
illustrates the process for establishing payload integrity.
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+------------------------------------------------+
| Q1. Payload Delivery |
+-+--------------------------------------------+-+
| |
| in Content others |
| v
| +--------------------------------+
| | Q2. Mitigate Battery |
| | Exhaustion Attacks |
| +-+----------------------------+-+
| | |
| | No Yes |
| v |
| +-----------------+ |
| | Q3. AEAD cipher | |
| +-+-------------+-+ |
| | | |
| | Yes No | |
v v v v
.+------+. .-----+-----. .----------+.
| NOT | | AFTER | | BEFORE |
| Required | | Decryption | | Decryption |
'--------' '-----------' '-----------'
Figure 19: Decision Tree: Validating the Payload
There are three conditions to consider:
* Q1. How does the recipient receive the encrypted payload? If the
encrypted payload is part of an integrated payload, its integrity
is already validated by the suit-authentication-wrapper, so no
additional integrity check is necessary.
* Q2. Does the sender wish to mitigate battery exhaustion attacks?
If so, the encrypted payload must be validated before decryption.
* Q3. Is the payload encrypted with an AEAD cipher? If yes, no
additional integrity check is required, as the recipient verifies
the payload's integrity during decryption. If no, integrity
validation can occur either before or after decryption; however,
validating integrity before decryption is RECOMMENDED.
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9. Firmware Updates on IoT Devices with Flash Memory
Embedded devices come in many forms, and the market is both large and
fragmented. As a result, some implementations and deployments may
adopt firmware update procedures that differ from the descriptions
provided here. On a positive note, the SUIT manifest accommodates
various deployment scenarios, thanks to the "scripting" functionality
offered by its commands.
This section specifically addresses firmware images on
microcontrollers and does not pertain to generic software,
configuration data, or machine learning models. The differences
arise from two main aspects:
* Use of Flash Memory: Flash memory in microcontrollers is a type of
non-volatile memory that erases data in larger units called
blocks, pages, or sectors, and rewrites data at the byte level
(often 4 bytes) or larger units. Furthermore, flash memory is
segmented into different regions, storing the bootloader, various
versions of firmware images (in designated slots), and
configuration data. An example layout of a microcontroller flash
area is illustrated in Figure 20.
* Microcontroller Design: Code on microcontrollers typically cannot
be executed from arbitrary locations in flash memory without
additional software development and design efforts. Consequently,
developers often compile firmware so that the bootloader can
execute code from a specific location in flash memory, commonly
referred to as the "primary slot."
Once the encrypted firmware image is transferred to the device, it is
usually stored in a dedicated area known as the "secondary slot."
During the next boot, the bootloader detects the new firmware image
and begins decrypting it sector by sector, swapping it with the image
located in the primary slot. This method of swapping the newly
downloaded image with the previously valid one requires two slots,
allowing for a rollback if the new firmware fails to boot, thereby
enhancing the robustness of the firmware update process.
The swap occurs only after verifying the signature on the plaintext.
It is important to note that the plaintext firmware image is
available in the primary slot only after the swap is completed,
unless "dummy decrypt" is used to compute the hash over the plaintext
prior to executing the decryption during the swap. In this context,
dummy decryption refers to decrypting the firmware image in the
secondary slot sector by sector while computing a rolling hash over
the resulting plaintext (also sector by sector) without performing
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the swap operation. Although performance optimizations, such as
conveying hashes for each sector in the manifest rather than a hash
of the entire firmware image, are possible, these optimizations are
not detailed in this specification.
Without hardware-based, on-the-fly decryption, the image in the
primary slot is available in cleartext and may need to be re-
encrypted before copying it to the secondary slot. This step might
be necessary if the secondary slot has different access permissions
or is located in off-chip flash memory, which tends to be more
vulnerable to physical attacks.
+--------------------------------------------------+
| Bootloader |
+--------------------------------------------------+
| Primary Slot |
| (sector 1)|
|..................................................|
| |
| (sector 2)|
|..................................................|
| |
| (sector 3)|
|..................................................|
| |
| (sector 4)|
+--------------------------------------------------+
| Secondary Slot |
| (sector 1)|
|..................................................|
| |
| (sector 2)|
|..................................................|
| |
| (sector 3)|
|..................................................|
| |
| (sector 4)|
+--------------------------------------------------+
| Swap Area |
| |
+--------------------------------------------------+
| Configuration Data |
+--------------------------------------------------+
Figure 20: Example Flash Area Layout
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The ability to resume an interrupted firmware update is often
essential for unattended devices, including low-end, constrained IoT
devices. To meet this requirement, a firmware image must be divided
into sectors, with each sector encrypted individually using a cipher
that does not increase the size of the resulting ciphertext (i.e., by
avoiding the addition of an authentication tag after each encrypted
block).
If an update is aborted while the bootloader is decrypting the newly
received image and swapping the sectors, the bootloader can restart
from where it left off. This technique enhances robustness and
performance.
For this purpose, ciphers without integrity protection are employed
to encrypt the firmware image. It is crucial that integrity
protection for the firmware image is provided, and the suit-
parameter-image-digest, defined in Section 8.4.8.6 of
[I-D.ietf-suit-manifest], MUST be utilized.
[RFC9459] specifies the AES Counter (AES-CTR) mode and AES Cipher
Block Chaining (AES-CBC) ciphers, both of which do not provide
integrity protection. These ciphers are suitable for firmware
encryption in IoT devices. However, for many other scenarios
involving software packages, configuration information, or
personalization data, the use of AEAD ciphers is RECOMMENDED.
The following subsections offer additional information on the
selection of initialization vectors (IVs) for use with AES-CBC and
AES-CTR in the context of firmware encryption. An IV MUST NOT be
reused when the same key is employed. In this application, the IVs
are not random but are instead based on the slot/sector combination
in flash memory. The discussion assumes that the block size of AES
is significantly smaller than the sector size. Typically, flash
memory sectors are measured in KiB, necessitating the decryption of
multiple AES blocks to complete the decryption of an entire sector.
10. Complete Examples
The following manifests illustrate how to deliver an encrypted
payload along with its encryption information to devices.
In the AES-KW examples, HMAC-256 MACs are included, utilizing the
following secret key:
'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'
(616161... in hex, and its length is 32)
ES-DH examples are signed using the following ECDSA secp256r1 key:
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-----BEGIN PRIVATE KEY-----
MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQgApZYjZCUGLM50VBC
CjYStX+09jGmnyJPrpDLTz/hiXOhRANCAASEloEarguqq9JhVxie7NomvqqL8Rtv
P+bitWWchdvArTsfKktsCYExwKNtrNHXi9OB3N+wnAUtszmR23M4tKiW
-----END PRIVATE KEY-----
The corresponding public key can be used to verify these examples:
-----BEGIN PUBLIC KEY-----
MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEhJaBGq4LqqvSYVcYnuzaJr6qi/Eb
bz/m4rVlnIXbwK07HypLbAmBMcCjbazR14vTgdzfsJwFLbM5kdtzOLSolg==
-----END PUBLIC KEY-----
Each example uses SHA-256 as the digest function.
10.1. AES Key Wrap Example with Write Directive
The following SUIT manifest instructs a parser to authenticate the
manifest using COSE_Mac0 with HMAC256. It also directs the parser to
write and decrypt the encrypted payload into a component using the
suit-directive-write directive.
The SUIT manifest in diagnostic notation (with line breaks added for
clarity) is displayed below:
=============== NOTE: '\' line wrapping per RFC 8792 ================
/ 1/ / SUIT_Envelope_Tagged / 107({
/ 2/ / authentication-wrapper / 2: << [
/ 3/ << [
/ 4/ / digest-algorithm-id: / -16 / SHA256 /,
/ 5/ / digest-bytes: / h'037A5C325CE14078A0AADF007428EAC6
/ 6/ 59361AD9402A732410BDA542FAE94E2C'
/ 7/ ] >>,
/ 8/ << / COSE_Mac0_Tagged / 17([
/ 9/ / protected: / << {
/ 10/ / algorithm-id / 1: 5 / HMAC256 /
/ 11/ } >>,
/ 12/ / unprotected: / {},
/ 13/ / payload: / null,
/ 14/ / tag: / h'8D92599011C451A4C5FB69709FA6CA6C
/ 15/ 0F846D692BDBB3F624EC91F82F9F620A'
/ 16/ ]) >>
/ 17/ ] >>,
/ 18/ / manifest / 3: << {
/ 19/ / manifest-version / 1: 1,
/ 20/ / manifest-sequence-number / 2: 1,
/ 21/ / common / 3: << {
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/ 22/ / components / 2: [
/ 23/ ['plaintext-firmware']
/ 24/ ]
/ 25/ } >>,
/ 26/ / install / 20: << [
/ 27/ / fetch encrypted firmware /
/ 28/ / directive-override-parameters / 20, {
/ 29/ / parameter-content / 18:
/ 30/ h'758C4B7BBAE2C4C1D462423E0F0DC3164FFA7B85BB94D4
/ 31/ BD6D7ED26AB32FEB063385D4D3465927EC82CB5E198A59',
/ 32/ / parameter-encryption-info / 19: << 96([
/ 33/ / protected: / << {
/ 34/ / alg / 1: 1 / A128GCM /
/ 35/ } >>,
/ 36/ / unprotected: / {
/ 37/ / IV / 5: h'F14AAB9D81D51F7AD943FE87'
/ 38/ },
/ 39/ / payload: / null / detached ciphertext /,
/ 40/ / recipients: / [
/ 41/ [
/ 42/ / protected: / h'',
/ 43/ / unprotected: / {
/ 44/ / alg / 1: -3 / A128KW /,
/ 45/ / kid / 4: 'kid-1'
/ 46/ },
/ 47/ / payload: /
/ 48/ h'\
75603FFC9518D794713C8CA8A115A7FB32565A6D59534D62'
/ 49/ / CEK encrypted with KEK /
/ 50/ ]
/ 51/ ]
/ 52/ ]) >>
/ 53/ },
/ 54/
/ 55/ / decrypt encrypted firmware /
/ 56/ / directive-write / 18, 15
/ 57/ / consumes the SUIT_Encryption_Info above /
/ 58/ ] >>
/ 59/ } >>
/ 60/ })
In hex format, the SUIT manifest is:
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D86BA2025853825824822F5820037A5C325CE14078A0AADF007428EAC659
361AD9402A732410BDA542FAE94E2C582AD18443A10105A0F658208D9259
9011C451A4C5FB69709FA6CA6C0F846D692BDBB3F624EC91F82F9F620A03
5898A4010102010357A102818152706C61696E746578742D6669726D7761
72651458778414A212582E758C4B7BBAE2C4C1D462423E0F0DC3164FFA7B
85BB94D4BD6D7ED26AB32FEB063385D4D3465927EC82CB5E198A5913583E
D8608443A10101A1054CF14AAB9D81D51F7AD943FE87F6818340A2012204
456B69642D31581875603FFC9518D794713C8CA8A115A7FB32565A6D5953
4D62120F
10.2. AES Key Wrap Example with Fetch + Copy Directives
The following SUIT manifest instructs a parser to fetch and store the
encrypted payload. Subsequently, the payload is decrypted and copied
into another component using the suit-directive-copy directive. This
approach is particularly effective for constrained devices with
execute-in-place (XIP) flash memory.
The SUIT manifest in diagnostic notation (with line breaks added for
clarity) is displayed below:
=============== NOTE: '\' line wrapping per RFC 8792 ================
/ 1/ / SUIT_Envelope_Tagged / 107({
/ 2/ / authentication-wrapper / 2: << [
/ 3/ << [
/ 4/ / digest-algorithm-id: / -16 / SHA256 /,
/ 5/ / digest-bytes: / h'8814BC46089ACA6A863A7BA8393F9747
/ 6/ 589940EFA40641335EF86155598F06C3'
/ 7/ ] >>,
/ 8/ << / COSE_Mac0_Tagged / 17([
/ 9/ / protected: / << {
/ 10/ / algorithm-id / 1: 5 / HMAC256 /
/ 11/ } >>,
/ 12/ / unprotected: / {},
/ 13/ / payload: / null,
/ 14/ / tag: / h'B68572F6F0494FEAF390CE44B462F2A7
/ 15/ BDF73EF5DFE9FB8E12585A12F8F641AD'
/ 16/ ]) >>
/ 17/ ] >>,
/ 18/ / manifest / 3: << {
/ 19/ / manifest-version / 1: 1,
/ 20/ / manifest-sequence-number / 2: 1,
/ 21/ / common / 3: << {
/ 22/ / components / 2: [
/ 23/ ['plaintext-firmware'],
/ 24/ ['encrypted-firmware']
/ 25/ ]
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/ 26/ } >>,
/ 27/ / install / 20: << [
/ 28/ / fetch encrypted firmware /
/ 29/ / directive-set-component-index / 12, 1
/ 30/ / ['encrypted-firmware'] /,
/ 31/ / directive-override-parameters / 20, {
/ 32/ / parameter-image-size / 14: 46,
/ 33/ / parameter-uri / 21:
/ 34/ "https://example.com/encrypted-firmware"
/ 35/ },
/ 36/ / directive-fetch / 21, 15,
/ 37/
/ 38/ / decrypt encrypted firmware /
/ 39/ / directive-set-component-index / 12, 0
/ 40/ / ['plaintext-firmware'] /,
/ 41/ / directive-override-parameters / 20, {
/ 42/ / parameter-encryption-info / 19: << 96([
/ 43/ / protected: / << {
/ 44/ / alg / 1: 1 / A128GCM /
/ 45/ } >>,
/ 46/ / unprotected: / {
/ 47/ / IV / 5: h'F14AAB9D81D51F7AD943FE87'
/ 48/ },
/ 49/ / payload: / null / detached ciphertext /,
/ 50/ / recipients: / [
/ 51/ [
/ 52/ / protected: / h'',
/ 53/ / unprotected: / {
/ 54/ / alg / 1: -3 / A128KW /,
/ 55/ / kid / 4: 'kid-1'
/ 56/ },
/ 57/ / payload: /
/ 58/ h'\
75603FFC9518D794713C8CA8A115A7FB32565A6D59534D62'
/ 59/ / CEK encrypted with KEK /
/ 60/ ]
/ 61/ ]
/ 62/ ]) >>,
/ 63/ / parameter-source-component / 22: 1
/ 64/ / ['encrypted-firmware'] /
/ 65/ },
/ 66/ / directive-copy / 22, 15
/ 67/ / consumes the SUIT_Encryption_Info above /
/ 68/ ] >>
/ 69/ } >>
/ 70/ })
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The default storage area is defined by the component identifier (see
Section 8.4.5.1 of [I-D.ietf-suit-manifest]). In this example, the
component identifier for component #0 is ['plaintext-firmware'] and
the file path "/plaintext-firmware" is the expected location.
While parsing the manifest, the behavior of SUIT manifest processor
would be
* [L2-L17] authenticates the manifest part on [L18-L68]
* [L22-L25] gets two component identifiers; ['plaintext-firmware']
for component #0, and ['encrypted-firmware'] for component # 1
respectively
* [L29] sets current component index # 1 (the lasting directives
target ['encrypted-firmware'])
* [L33-L34] sets source uri parameter "https://example.com/
encrypted-firmware"
* [L36] fetches content from source uri into ['encrypted-firmware']
* [L39] sets current component index # 0 (the lasting directives
target ['plaintext-firmware'])
* [L42-L62] sets SUIT encryption info parameter
* [L63-L64] sets source component index parameter # 1
* [L66] decrypts component # 1 (source component index) and stores
the result into component # 0 (current component index)
The following attributes and features from the SUIT manifest
specification are used:
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+===============================+=========+===============+
| Attribute Name | Abb. | Manifest Ref. |
+===============================+=========+===============+
| component identifier | CI | Sec. 8.4.5.1 |
+-------------------------------+---------+---------------+
| (destination) component index | dst-CI | Sec. 8.4.10.1 |
+-------------------------------+---------+---------------+
| (destination) component slot | dst-CS | Sec. 8.4.8.8 |
| OPTIONAL param | | |
+-------------------------------+---------+---------------+
| (source) uri OPTIONAL | src-URI | Sec. 8.4.8.10 |
| parameter | | |
+-------------------------------+---------+---------------+
| source component index | src-CI | Sec. 8.4.8.11 |
| OPTIONAL parameter | | |
+-------------------------------+---------+---------------+
Table 1
The resulting state of SUIT manifest processor is shown in the
following table:
+==============+========================+========================+
| Abbreviation | Plaintext | Ciphertext |
+==============+========================+========================+
| CI | ['plaintext-firmware'] | ['encrypted-firmware'] |
+--------------+------------------------+------------------------+
| dst-CI | 0 | 1 |
+--------------+------------------------+------------------------+
| dst-CS | N/A | N/A |
+--------------+------------------------+------------------------+
| src-URI | N/A | "https://example.com/ |
| | | encrypted-firmware" |
+--------------+------------------------+------------------------+
| src-CI | 1 | N/A |
+--------------+------------------------+------------------------+
Table 2
In hex format, the SUIT manifest shown above is:
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D86BA2025853825824822F58208814BC46089ACA6A863A7BA8393F974758
9940EFA40641335EF86155598F06C3582AD18443A10105A0F65820B68572
F6F0494FEAF390CE44B462F2A7BDF73EF5DFE9FB8E12585A12F8F641AD03
58B2A40101020103582BA102828152706C61696E746578742D6669726D77
6172658152656E637279707465642D6669726D7761726514587C8C0C0114
A20E182E15782668747470733A2F2F6578616D706C652E636F6D2F656E63
7279707465642D6669726D77617265150F0C0014A213583ED8608443A101
01A1054CF14AAB9D81D51F7AD943FE87F6818340A2012204456B69642D31
581875603FFC9518D794713C8CA8A115A7FB32565A6D59534D621601160F
The previous example does not utilize storage slots. However, it is
possible to implement this functionality for devices that support
slots in flash memory. In the enhanced example below, we reference
the slots using [h'00'] and [h'01']. In this context, the component
identifier [h'00'] designates component slot #0.
=============== NOTE: '\' line wrapping per RFC 8792 ================
/ 1/ / SUIT_Envelope_Tagged / 107({
/ 2/ / authentication-wrapper / 2: << [
/ 3/ << [
/ 4/ / digest-algorithm-id: / -16 / SHA256 /,
/ 5/ / digest-bytes: / h'A6D2C13A0DEFD57A09FA65C24206A9A8
/ 6/ 747261E6EA017532B3DAB0419C42A2FC'
/ 7/ ] >>,
/ 8/ << / COSE_Mac0_Tagged / 17([
/ 9/ / protected: / << {
/ 10/ / algorithm-id / 1: 5 / HMAC256 /
/ 11/ } >>,
/ 12/ / unprotected: / {},
/ 13/ / payload: / null,
/ 14/ / tag: / h'DB350AC9603B0BBA3895B85019699363
/ 15/ 3F314A9066634B31BF62E596FD870434'
/ 16/ ]) >>
/ 17/ ] >>,
/ 18/ / manifest / 3: << {
/ 19/ / manifest-version / 1: 1,
/ 20/ / manifest-sequence-number / 2: 1,
/ 21/ / common / 3: << {
/ 22/ / components / 2: [
/ 23/ [h'00'],
/ 24/ [h'01']
/ 25/ ]
/ 26/ } >>,
/ 27/ / install / 20: << [
/ 28/ / fetch encrypted firmware /
/ 29/ / directive-set-component-index / 12, 1 / [h'01'] /,
/ 30/ / directive-override-parameters / 20, {
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/ 31/ / parameter-image-size / 14: 46,
/ 32/ / parameter-uri / 21:
/ 33/ "https://example.com/encrypted-firmware"
/ 34/ },
/ 35/ / directive-fetch / 21, 15,
/ 36/
/ 37/ / decrypt encrypted firmware /
/ 38/ / directive-set-component-index / 12, 0 / ['00'] /,
/ 39/ / directive-override-parameters / 20, {
/ 40/ / parameter-encryption-info / 19: << 96([
/ 41/ / protected: / << {
/ 42/ / alg / 1: 1 / A128GCM /
/ 43/ } >>,
/ 44/ / unprotected: / {
/ 45/ / IV / 5: h'F14AAB9D81D51F7AD943FE87'
/ 46/ },
/ 47/ / payload: / null / detached ciphertext /,
/ 48/ / recipients: / [
/ 49/ [
/ 50/ / protected: / h'',
/ 51/ / unprotected: / {
/ 52/ / alg / 1: -3 / A128KW /,
/ 53/ / kid / 4: 'kid-1'
/ 54/ },
/ 55/ / payload: /
/ 56/ h'\
75603FFC9518D794713C8CA8A115A7FB32565A6D59534D62'
/ 57/ / CEK encrypted with KEK /
/ 58/ ]
/ 59/ ]
/ 60/ ]) >>,
/ 61/ / parameter-source-component / 22: 1 / [h'01'] /
/ 62/ },
/ 63/ / directive-copy / 22, 15
/ 64/ / consumes the SUIT_Encryption_Info above /
/ 65/ ] >>
/ 66/ } >>
/ 67/ })
10.3. ES-DH Example with Write + Copy Directives
The following SUIT manifest instructs a parser to authenticate the
manifest using COSE_Sign1 with ES256. It also directs the parser to
write and decrypt the encrypted payload into a component via the
suit-directive-write directive.
The SUIT manifest in diagnostic notation (formatted with line breaks
for clarity) is presented below:
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=============== NOTE: '\' line wrapping per RFC 8792 ================
/ 1/ / SUIT_Envelope_Tagged / 107({
/ 2/ / authentication-wrapper / 2: << [
/ 3/ << [
/ 4/ / digest-algorithm-id: / -16 / SHA256 /,
/ 5/ / digest-bytes: / h'1DB69EF1477E9942815F29F78E09957B
/ 6/ 26B4ADD03902BDB3D1EDF3DA2075F593'
/ 7/ ] >>,
/ 8/ << / COSE_Sign1_Tagged / 18([
/ 9/ / protected: / << {
/ 10/ / algorithm-id / 1: -7 / ES256 /
/ 11/ } >>,
/ 12/ / unprotected: / {},
/ 13/ / payload: / null,
/ 14/ / signature: / h'CB4EADA6BEC17EEB22EB836FB2BF9136
/ 15/ A6EF733C11DAC955F543BBDCAA373B85
/ 16/ 9321BC77969917E4C70F049527607F4C
/ 17/ 32752D53E01346E96BFF4880B437DF64'
/ 18/ ]) >>
/ 19/ ] >>,
/ 20/ / manifest / 3: << {
/ 21/ / manifest-version / 1: 1,
/ 22/ / manifest-sequence-number / 2: 1,
/ 23/ / common / 3: << {
/ 24/ / components / 2: [
/ 25/ ['decrypted-firmware']
/ 26/ ]
/ 27/ } >>,
/ 28/ / install / 20: << [
/ 29/ / directive-set-component-index / 12, 0
/ 30/ / ['plaintext-firmware'] /,
/ 31/ / directive-override-parameters / 20, {
/ 32/ / parameter-content / 18:
/ 33/ h'758C4B7BBAE2C4C1D462423E0F0DC3164FFA7B85BB94D4
/ 34/ BD6D7ED26AB32FEB063385D4D3465927EC82CB5E198A59',
/ 35/ / parameter-encryption-info / 19: << 96([
/ 36/ / protected: / << {
/ 37/ / alg / 1: 1 / A128GCM /
/ 38/ } >>,
/ 39/ / unprotected: / {
/ 40/ / IV / 5: h'F14AAB9D81D51F7AD943FE87'
/ 41/ },
/ 42/ / payload: / null / detached ciphertext /,
/ 43/ / recipients: / [
/ 44/ [
/ 45/ / protected: / << {
/ 46/ / alg / 1: -29 / ECDH-ES + A128KW /
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/ 47/ } >>,
/ 48/ / unprotected: / {
/ 49/ / ephemeral key / -1: {
/ 50/ / kty / 1: 2 / EC2 /,
/ 51/ / crv / -1: 1 / P-256 /,
/ 52/ / x / -2: h'73024F415AA51529A66CCEFD88F3F62A
/ 53/ 734492FF45F6AD37FD2888E73EAF19DA\
',
/ 54/ / y / -3: h'4005B48A6FD091AA6ABFE3CFBEEDE88B
/ 55/ 347E521D43405FDBD7D2CFF0EBC21B26'
/ 56/ },
/ 57/ / kid / 4: 'kid-2'
/ 58/ },
/ 59/ / payload: /
/ 60/ h'\
A06B8E6550F308712B1DF044B21B7D11D9B22792F1DE0997'
/ 61/ / CEK encrypted with KEK /
/ 62/ ]
/ 63/ ]
/ 64/ ]) >>
/ 65/ },
/ 66/ / directive-write / 18, 15
/ 67/ / consumes the SUIT_Encryption_Info above /
/ 68/ ] >>
/ 69/ } >>
/ 70/ })
In hex format, the SUIT manifest is this: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10.4. ES-DH Example with Dependency
The following SUIT manifest requests a parser to resolve the
dependency.
The dependent manifest is signed with another key:
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-----BEGIN EC PRIVATE KEY-----
MHcCAQEEIIQa67e56m8CYL5zVaJFiLl30j0qxb8ray2DeUMqH+qYoAoGCCqGSM49
AwEHoUQDQgAEDpCKqPBm2x8ITgw2UsY5Ur2Z8qW9si+eATZ6rQOrpot32hvYrE8M
tJC6IQZIv3mrFk1JrTVR1x0xSydJ7kLSmg==
-----END EC PRIVATE KEY-----
The dependency manifest is embedded as an integrated-dependency and
referred to by the "#dependency-manifest" URI.
The SUIT manifest in diagnostic notation (with line breaks added for
readability) is shown here:
=============== NOTE: '\' line wrapping per RFC 8792 ================
/ 1/ / SUIT_Envelope_Tagged / 107({
/ 2/ / authentication-wrapper / 2: << [
/ 3/ << [
/ 4/ / digest-algorithm-id: / -16 / SHA256 /,
/ 5/ / digest-bytes: / h'A00CB6C85515C1EF471B50B542FACDD8
/ 6/ 8B71B3C7EA2A43DE13D32C4A99056FE9'
/ 7/ ] >>,
/ 8/ << / COSE_Sign1_Tagged / 18([
/ 9/ / protected: / << {
/ 10/ / algorithm-id / 1: -7 / ES256 /
/ 11/ } >>,
/ 12/ / unprotected: / {},
/ 13/ / payload: / null,
/ 14/ / signature: / h'421B30FE76DA848616D72FC1115EA610
/ 15/ 5578CB95DF9C6BEAD931105C9D555CF8
/ 16/ CD38C8FD68ACE43445D8D2CAE6391A99
/ 17/ 5A212487D92F8DAD789F65511AC61778'
/ 18/ ]) >>
/ 19/ ] >>,
/ 20/ / manifest / 3: << {
/ 21/ / manifest-version / 1: 1,
/ 22/ / manifest-sequence-number / 2: 1,
/ 23/ / common / 3: << {
/ 24/ / dependencies / 1: {
/ 25/ / component-index / 1: {
/ 26/ / dependency-prefix / 1: [
/ 27/ 'dependency-manifest.suit'
/ 28/ ]
/ 29/ }
/ 30/ },
/ 31/ / components / 2: [
/ 32/ ['decrypted-firmware']
/ 33/ ]
/ 34/ } >>,
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/ 35/ / manifest-component-id / 5: [
/ 36/ 'dependent-manifest.suit'
/ 37/ ],
/ 38/ / install / 20: << [
/ 39/ / NOTE: set SUIT_Encryption_Info /
/ 40/ / directive-set-component-index / 12, 0
/ 41/ / ['decrypted-firmware'] /,
/ 42/ / directive-override-parameters / 20, {
/ 43/ / parameter-content / 18:
/ 44/ h'758C4B7BBAE2C4C1D462423E0F0DC3164FFA7B85BB94D4
/ 45/ BD6D7ED26AB32FEB063385D4D3465927EC82CB5E198A59',
/ 46/ / parameter-encryption-info / 19: << 96([
/ 47/ / protected: / << {
/ 48/ / alg / 1: 1 / A128GCM /
/ 49/ } >>,
/ 50/ / unprotected: / {
/ 51/ / IV / 5: h'F14AAB9D81D51F7AD943FE87'
/ 52/ },
/ 53/ / payload: / null / detached ciphertext /,
/ 54/ / recipients: / [
/ 55/ [
/ 56/ / protected: / << {
/ 57/ / alg / 1: -29 / ECDH-ES + A128KW /
/ 58/ } >>,
/ 59/ / unprotected: / {
/ 60/ / ephemeral key / -1: {
/ 61/ / kty / 1: 2 / EC2 /,
/ 62/ / crv / -1: 1 / P-256 /,
/ 63/ / x / -2: h'73024F415AA51529A66CCEFD88F3F62A
/ 64/ 734492FF45F6AD37FD2888E73EAF19DA\
',
/ 65/ / y / -3: h'4005B48A6FD091AA6ABFE3CFBEEDE88B
/ 66/ 347E521D43405FDBD7D2CFF0EBC21B26'
/ 67/ },
/ 68/ / kid / 4: 'kid-2'
/ 69/ },
/ 70/ / payload: /
/ 71/ h'\
A06B8E6550F308712B1DF044B21B7D11D9B22792F1DE0997'
/ 72/ / CEK encrypted with KEK /
/ 73/ ]
/ 74/ ]
/ 75/ ]) >>
/ 76/ },
/ 77/
/ 78/ / NOTE: call dependency-manifest /
/ 79/ / directive-set-component-index / 12, 1
/ 80/ / ['dependenty-manifest.suit'] /,
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/ 81/ / directive-override-parameters / 20, {
/ 82/ / parameter-image-digest / 3: << [
/ 83/ / algorithm-id / -16 / SHA256 /,
/ 84/ / digest-bytes / h'4B15C90FBD776A820E7E733DF040D90B
/ 85/ 356B5C75982ECAECE8673818179BDF16'
/ 86/ ] >>,
/ 87/ / parameter-image-size / 14: 247,
/ 88/ / parameter-uri / 21: "#dependency-manifest"
/ 89/ },
/ 90/ / directive-fetch / 21, 15,
/ 91/ / condition-dependency-integrity / 7, 15,
/ 92/ / directive-process-dependency / 11, 15
/ 93/ ] >>
/ 94/ } >>,
/ 95/ "#dependency-manifest": <<
/ 96/ / SUIT_Envelope_Tagged / 107({
/ 97/ / authentication-wrapper / 2: << [
/ 98/ << [
/ 99/ / digest-algorithm-id: / -16 / SHA256 /,
/100/ / digest-bytes: / h'4B15C90FBD776A820E7E733DF040D90B
/101/ 356B5C75982ECAECE8673818179BDF16'
/102/ ] >>,
/103/ << / COSE_Sign1_Tagged / 18([
/104/ / protected: / << {
/105/ / algorithm-id / 1: -7 / ES256 /
/106/ } >>,
/107/ / unprotected: / {},
/108/ / payload: / null,
/109/ / signature: / h'2B1B9C4E44E52863A78F73DA2A935823
/110/ B28AEAE6A85CADAC4C4E3AABAAD56CBC
/111/ E5A47D288F86B54D0186657E972E748B
/112/ 48CDB1D420FBAC1285DCC978382F62CC'
/113/ ]) >>
/114/ ] >>,
/115/ / manifest / 3: << {
/116/ / manifest-version / 1: 1,
/117/ / manifest-sequence-number / 2: 1,
/118/ / common / 3: << {
/119/ / components / 2: [
/120/ ['decrypted-firmware']
/121/ ],
/122/ / shared-sequence / 4: << [
/123/ / directive-set-componnt-index / 12, 0
/124/ / ['decrypted-firmware'] /,
/125/ / directive-override-parameters / 20, {
/126/ / parameter-image-digest / 3: << [
/127/ / algorithm-id / -16 / SHA256 /,
/128/ / digest-bytes /
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/129/ h'36921488FE6680712F734E11F58D87EE
/130/ B66D4B21A8A1AD3441060814DA16D50F'
/131/ ] >>,
/132/ / parameter-image-size / 14: 30
/133/ }
/134/ ] >>
/135/ } >>,
/136/ / manifest-component-id / 5: [
/137/ 'dependency-manifest.suit'
/138/ ],
/139/ / validate / 7: << [
/140/ / condition-image-match / 3, 15
/141/ ] >>,
/142/ / install / 20: << [
/143/ / directive-set-component-index / 12, 0
/144/ / ['decrypted-firmware'] /,
/145/ / directive-write / 18, 15
/146/ / consumes the SUIT_Encryption_Info in dependent /,
/147/ / condition-image-match / 3, 15
/148/ / check the integrity of the decrypted payload /
/149/ ] >>
/150/ } >>
/151/ })
/152/ >>
/153/ })
In hex format, the SUIT manifest is this:
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D86BA3025873825824822F5820A00CB6C85515C1EF471B50B542FACDD88B
71B3C7EA2A43DE13D32C4A99056FE9584AD28443A10126A0F65840421B30
FE76DA848616D72FC1115EA6105578CB95DF9C6BEAD931105C9D555CF8CD
38C8FD68ACE43445D8D2CAE6391A995A212487D92F8DAD789F65511AC617
780359016CA501010201035837A201A101A101815818646570656E64656E
63792D6D616E69666573742E73756974028181526465637279707465642D
6669726D77617265058157646570656E64656E742D6D616E69666573742E
737569741459010F8E0C0014A212582E758C4B7BBAE2C4C1D462423E0F0D
C3164FFA7B85BB94D4BD6D7ED26AB32FEB063385D4D3465927EC82CB5E19
8A5913588CD8608443A10101A1054CF14AAB9D81D51F7AD943FE87F68183
44A101381CA220A40102200121582073024F415AA51529A66CCEFD88F3F6
2A734492FF45F6AD37FD2888E73EAF19DA2258204005B48A6FD091AA6ABF
E3CFBEEDE88B347E521D43405FDBD7D2CFF0EBC21B2604456B69642D3258
18A06B8E6550F308712B1DF044B21B7D11D9B22792F1DE09970C0114A303
5824822F58204B15C90FBD776A820E7E733DF040D90B356B5C75982ECAEC
E8673818179BDF160E18F7157423646570656E64656E63792D6D616E6966
657374150F070F0B0F7423646570656E64656E63792D6D616E6966657374
58F7D86BA2025873825824822F58204B15C90FBD776A820E7E733DF040D9
0B356B5C75982ECAECE8673818179BDF16584AD28443A10126A0F658402B
1B9C4E44E52863A78F73DA2A935823B28AEAE6A85CADAC4C4E3AABAAD56C
BCE5A47D288F86B54D0186657E972E748B48CDB1D420FBAC1285DCC97838
2F62CC03587BA601010201035849A2028181526465637279707465642D66
69726D7761726504582F840C0014A2035824822F582036921488FE668071
2F734E11F58D87EEB66D4B21A8A1AD3441060814DA16D50F0E181E058158
18646570656E64656E63792D6D616E69666573742E73756974074382030F
1447860C00120F030F
11. Operational Considerations
The algorithms outlined in this document assume that the party
responsible for payload encryption:
* shares a key-encryption key (KEK) with the recipient (for use with
the AES Key Wrap scheme), or
* possesses the recipient's public key (for use with ES-DH).
Both scenarios necessitate initial communication to distribute these
keys among the involved parties. This interaction can be facilitated
by a device management protocol, as described in [RFC9019], or may
occur earlier in the device lifecycle, such as during manufacturing
or commissioning. In addition to the keying material, key
identifiers and algorithm information must also be provisioned. This
specification does not impose any requirements on the structure of
the key identifier.
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In certain situations, third-party companies analyze binaries for
known security vulnerabilities. However, encrypted payloads hinder
this type of analysis. Consequently, these third-party companies
must either be granted access to the plaintext binary before
encryption or be authorized recipients of the encrypted payloads. In
either case, it is essential to explicitly consider these third
parties in the software supply chain when binary analysis is
required.
12. Security Considerations
This entire document focuses on security.
It is considered best security practice to use different keys for
different purposes. For instance, the key-encryption key (KEK)
utilized in an AES-KW-based content key distribution method for
encryption should be distinct from the long-term symmetric key
employed for authentication in a communication security protocol.
To further minimize the attack surface, it may be advantageous to use
different long-term keys for encrypting various types of payloads.
For example, KEK_1 could be used with an AES-KW content key
distribution method to encrypt a firmware image, while KEK_2 would
encrypt configuration data.
A significant portion of this document addresses content key
distribution, employing two methods: AES Key Wrap (AES-KW) and
Ephemeral-Static Diffie-Hellman (ES-DH). The table below summarizes
the main properties related to their deployment:
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+===============+=======================+============+============+
|Number ofLong- |Number |Use Case |Recommended?|
|TermKeys |ofContentEncryptionKeys| | |
| |(CEKs) | | |
+===============+=======================+============+============+
|Same key |Single |Legacy |No, bad |
|for all |CEK per |Usage |practice |
|devices |payload | | |
| |shared | | |
| |with all | | |
| |devies | | |
+---------------+-----------------------+------------+------------+
|One key |Single |Efficient |Yes |
|per device |CEK per |Payload | |
| |payload |Distribution| |
| |shared | | |
| |with all | | |
| |devies | | |
+---------------+-----------------------+------------+------------+
|One Key |One CEK |Point-to- |Yes |
|per device |per payload |Point | |
| |encryption |Payload | |
| |transaction |Distribution| |
| |per device | | |
+---------------+-----------------------+------------+------------+
Table 3
The use of firmware encryption in IoT devices introduces the risk of
a battery exhaustion attack. This attack exploits the high energy
cost of flash memory operations. To execute this attack, the
adversary must be able to swap detached payloads and trick the device
into processing an incorrect payload. Payload swapping is feasible
only if there is no communication security protocol between the
device and the distribution system or if the distribution system
itself has been compromised.
While the security features provided by the manifest can detect this
attack and prevent the device from booting with an incorrectly
supplied payload, the energy-intensive flash operations will have
already occurred. As a result, these operations can diminish the
lifespan of the devices, making battery-powered IoT devices
particularly susceptible to such attacks. For further discussion on
IoT devices using flash memory, see Section 9.
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Including the digest of the encrypted payload in the manifest enables
the device to detect a battery exhaustion attack before energy-
consuming decryption and flash memory copy or swap operations take
place.
13. IANA Considerations
IANA is asked to add the following value to the SUIT Parameters
registry established by Section 11.5 of [I-D.ietf-suit-manifest]:
Label Name Reference
-----------------------------------------
TBD19 Encryption Info Section 4
RFC Editor's Note (TBD19): The value for the Encryption Info
parameter is set to 19, as the proposed value.
14. References
14.1. Normative References
[I-D.ietf-suit-manifest]
Moran, B., Tschofenig, H., Birkholz, H., Zandberg, K., and
O. Rønningstad, "A Concise Binary Object Representation
(CBOR)-based Serialization Format for the Software Updates
for Internet of Things (SUIT) Manifest", Work in Progress,
Internet-Draft, draft-ietf-suit-manifest-28, 21 October
2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
suit-manifest-28>.
[I-D.ietf-suit-trust-domains]
Moran, B. and K. Takayama, "SUIT Manifest Extensions for
Multiple Trust Domains", Work in Progress, Internet-Draft,
draft-ietf-suit-trust-domains-08, 21 October 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-suit-
trust-domains-08>.
[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>.
[RFC3394] Schaad, J. and R. Housley, "Advanced Encryption Standard
(AES) Key Wrap Algorithm", RFC 3394, DOI 10.17487/RFC3394,
September 2002, <https://www.rfc-editor.org/rfc/rfc3394>.
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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/rfc/rfc8174>.
[RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", STD 96, RFC 9052,
DOI 10.17487/RFC9052, August 2022,
<https://www.rfc-editor.org/rfc/rfc9052>.
[RFC9053] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053,
August 2022, <https://www.rfc-editor.org/rfc/rfc9053>.
[RFC9459] Housley, R. and H. Tschofenig, "CBOR Object Signing and
Encryption (COSE): AES-CTR and AES-CBC", RFC 9459,
DOI 10.17487/RFC9459, September 2023,
<https://www.rfc-editor.org/rfc/rfc9459>.
14.2. Informative References
[iana-suit]
Internet Assigned Numbers Authority, "IANA SUIT Manifest
Registry", 2023, <TBD>.
[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>.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<https://www.rfc-editor.org/rfc/rfc5652>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/rfc/rfc5869>.
[RFC8937] Cremers, C., Garratt, L., Smyshlyaev, S., Sullivan, N.,
and C. Wood, "Randomness Improvements for Security
Protocols", RFC 8937, DOI 10.17487/RFC8937, October 2020,
<https://www.rfc-editor.org/rfc/rfc8937>.
[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>.
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[RFC9124] Moran, B., Tschofenig, H., and H. Birkholz, "A Manifest
Information Model for Firmware Updates in Internet of
Things (IoT) Devices", RFC 9124, DOI 10.17487/RFC9124,
January 2022, <https://www.rfc-editor.org/rfc/rfc9124>.
[ROP] Wikipedia, "Return-Oriented Programming", March 2023,
<https://en.wikipedia.org/wiki/Return-
oriented_programming>.
[SP800-56] NIST, "Recommendation for Pair-Wise Key Establishment
Schemes Using Discrete Logarithm Cryptography, NIST
Special Publication 800-56A Revision 3", April 2018,
<http://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-56Ar3.pdf>.
Appendix A. Full CDDL
The following CDDL must be appended to the SUIT Manifest CDDL. The
SUIT CDDL is defined in Appendix A of [I-D.ietf-suit-manifest]
SUIT_Encryption_Info = #6.96(COSE_Encrypt)
$$SUIT_Parameters //= (suit-parameter-encryption-info =>
bstr .cbor SUIT_Encryption_Info)
suit-parameter-encryption-info = 19
Acknowledgements
We would like to thank Henk Birkholz for his feedback on the CDDL
description in this document. Additionally, we would like to thank
Michael Richardson, Øyvind Rønningstad, Dave Thaler, Laurence
Lundblade, Christian Amsüss, Ruud Derwig, Martin Thomson and Carsten
Bormann for their review feedback. Finally, we would like to thank
Dick Brooks for making us aware of the challenges encryption imposes
on binary analysis.
Reviews from the IESG include Deb Cooley and Roman Danyliw.
Authors' Addresses
Hannes Tschofenig
University of Applied Sciences Bonn-Rhein-Sieg
Email: Hannes.Tschofenig@gmx.net
Russ Housley
Vigil Security, LLC
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Email: housley@vigilsec.com
Brendan Moran
Arm Limited
Email: Brendan.Moran@arm.com
David Brown
Linaro
Email: david.brown@linaro.org
Ken Takayama
SECOM CO., LTD.
Email: ken.takayama.ietf@gmail.com
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