Internet DRAFT - draft-ietf-cose-hpke
draft-ietf-cose-hpke
COSE H. Tschofenig
Internet-Draft
Intended status: Standards Track B. Moran
Expires: 15 October 2023 Arm Limited
13 April 2023
Use of Hybrid Public-Key Encryption (HPKE) with CBOR Object Signing and
Encryption (COSE)
draft-ietf-cose-hpke-05
Abstract
This specification defines hybrid public-key encryption (HPKE) for
use with CBOR Object Signing and Encryption (COSE). HPKE offers a
variant of public-key encryption of arbitrary-sized plaintexts for a
recipient public key.
HPKE works for any combination of an asymmetric key encapsulation
mechanism (KEM), key derivation function (KDF), and authenticated
encryption with additional data (AEAD) function. Authentication for
HPKE in COSE is provided by COSE-native security mechanisms.
This document defines the use of the HPKE base mode with COSE. Other
modes are supported by HPKE but not by this specification.
Status of This Memo
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Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
3. HPKE for COSE . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1.1. Single Recipient / One Layer Structure . . . . . . . 5
3.1.2. Multiple Recipients / Two Layer Structure . . . . . . 6
4. HPKE Encryption and Decryption . . . . . . . . . . . . . . . 7
4.1. HPKE Encryption with SealBase . . . . . . . . . . . . . . 7
4.2. HPKE Decryption with OpenBase . . . . . . . . . . . . . . 8
4.3. AAD Parameter . . . . . . . . . . . . . . . . . . . . . . 8
4.3.1. AAD provided to HPKE for COSE_Encrypt0 . . . . . . . 9
4.3.2. AAD provided to HPKE for COSE_Encrypt at the Recipient
Layer . . . . . . . . . . . . . . . . . . . . . . . . 9
4.3.3. AAD provided to the AEAD cipher used for Content
Encryption at Layer 0 by COSE_Encrypt . . . . . . . . 10
4.4. Info Parameter . . . . . . . . . . . . . . . . . . . . . 10
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1. Single Recipient / One Layer Example . . . . . . . . . . 11
5.2. Multiple Recipients / Two Layer . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
7.1. COSE Algorithms Registry . . . . . . . . . . . . . . . . 14
7.2. COSE Header Algorithm Parameters . . . . . . . . . . . . 14
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. Normative References . . . . . . . . . . . . . . . . . . 15
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8.2. Informative References . . . . . . . . . . . . . . . . . 15
Appendix A. Contributors . . . . . . . . . . . . . . . . . . . . 15
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
Hybrid public-key encryption (HPKE) [RFC9180] is a scheme that
provides public key encryption of arbitrary-sized plaintexts given a
recipient's public key. HPKE utilizes a non-interactive ephemeral-
static Diffie-Hellman exchange to establish a shared secret. The
motivation for standardizing a public key encryption scheme is
explained in the introduction of [RFC9180].
The HPKE specification defines several features for use with public
key encryption and a subset of those features is applied to COSE
([RFC9052], [RFC9053]). Since COSE provides constructs for
authentication, those are not re-used from the HPKE specification.
This specification uses the "base" mode, as it is called in HPKE
specification language.
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 specification uses the following abbreviations and terms:
* Content-encryption key (CEK), a term defined in CMS [RFC2630].
* Hybrid Public Key Encryption (HPKE) is defined in [RFC9180].
* pkR is the public key of the recipient, as defined in [RFC9180].
* skR is the private key of the recipient, as defined in [RFC9180].
* Key Encapsulation Mechanism (KEM), see [RFC9180].
* Key Derivation Function (KDF), see [RFC9180].
* Authenticated Encryption with Associated Data (AEAD), see
[RFC9180].
* Additional Authenticated Data (AAD), see [RFC9180].
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3. HPKE for COSE
3.1. Overview
This specification supports two uses of HPKE in COSE, namely
* HPKE in a single recipient setup. This use cases uses a one layer
COSE structure. Section 3.1.1 provides the details.
* HPKE in a multiple recipient setup. This use case requires a two
layer COSE structure. Section 3.1.2 provides the details. While
it is possible to support the single recipient use case with a two
layer structure, the single layer setup is more efficient.
HPKE in Base mode requires little information to be provided by the
sender, namely
* algorithm information (KEM, KDF, and AEAD identifiers),
* an encapsulated key generated by the sender, and
* an identifier of the static recipient key.
In the subsections below we explain how this information is carried
inside the COSE_Encrypt0 and the COSE_Encrypt for the one layer and
the two layer structure, respectively.
In both cases a new structure is used to convey information about the
HPKE sender, namely the HPKE sender information structure
(HPKE_sender_info).
When the alg value is set to 'HPKE-v1-BASE', the HPKE_sender_info
structure MUST be present in the unprotected header parameter.
The CDDL grammar describing the HPKE_sender_info structure is:
HPKE_sender_info = [
kem_id : uint, ; kem identifier
kdf_id : uint, ; kdf identifier
aead_id : uint, ; aead identifier
enc : bstr, ; encapsulated key
]
The fields have the following meaning:
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+---------+----------------+------------+-------------------+
| Name | CBOR Type | Value | Description |
| | | Registry | |
+---------+----------------+------------+-------------------+
| kem_id | uint | HPKE | Identifier for |
| | | KEM IDs | the KEM |
| | | Registry | |
| | | | |
| kdf_id | uint | HPKE KDF | Identifier for |
| | | IDs | the KDF ID |
| | | | |
| aead_id | uint | HPKE AEAD | Identifier for |
| | | IDs | the AEAD ID |
| | | | |
| enc | bstr | | Encapsulated key |
| | | | defined by HPKE |
+---------+----------------+------------+-------------------+
Figure 1: HPKE_sender_info structure
kem_id: This parameter is used to identify the KEM. The registry for
KEM ids has been established with RFC 9180.
kdf_id: This parameter contains the KDF identifier. The registry
containing the KDF ids has been established with RFC 9180.
aead_id: This parameter contains the AEAD identifier. The registry
containing the AEAD ids has been established with RFC 9180.
enc: This parameter contains the encapsulated key, which is output of
the HPKE KEM.
3.1.1. Single Recipient / One Layer Structure
With the one layer structure the information carried inside the
COSE_recipient structure is embedded inside the COSE_Encrypt0.
HPKE is used to directly encrypt the plaintext. The resulting
ciphertext may be included in the COSE_Encrypt0 or may be detached.
If a payload is transported separately then it is called "detached
content". A nil CBOR object is placed in the location of the
ciphertext. See Section 5 of [RFC9052] for a description of detached
payloads.
The sender MUST set the alg parameter in the protected header, which
indicates the use of HPKE.
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The sender MUST place the HPKE_sender_info structure into the
unprotected header. Although the use of the kid parameter in
COSE_Encrypt0 is discouraged by RFC 9052, this specification allows
profiles of this specification to use the kid parameter (or other
parameters) to identify the static recipient public key used by the
sender. If the COSE_Encrypt0 contains the kid then the recipient may
use it to select the appropriate private key.
Figure 2 shows the COSE_Encrypt0 CDDL structure.
COSE_Encrypt0_Tagged = #6.16(COSE_Encrypt0)
; Layer 0
COSE_Encrypt0 = [
Headers,
ciphertext : bstr / nil,
]
Figure 2: CDDL for HPKE-based COSE_Encrypt0 Structure
The COSE_Encrypt0 MAY be tagged or untagged.
An example is shown in Section 5.1.
3.1.2. Multiple Recipients / Two Layer Structure
With the two layer structure the HPKE information is conveyed in the
COSE_recipient structure, i.e. one COSE_recipient structure per
recipient.
In this approach the following layers are involved:
* Layer 0 (corresponding to the COSE_Encrypt structure) contains the
content (plaintext) encrypted with the CEK. This ciphertext MAY
be detached. If not detached, then it is included in the
COSE_Encrypt structure.
* Layer 1 (corresponding to a recipient structure) contains
parameters needed for HPKE to generate a shared secret used to
encrypt the CEK. This layer conveys the encrypted CEK in the
encCEK structure. The protected header MUST contain the HPKE alg
parameter and the unprotected header MUST contain the
HPKE_sender_info structure. The unprotected header MAY contain
the kid parameter to identify the static recipient public key the
sender has been using with HPKE.
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This two-layer structure is used to encrypt content that can also be
shared with multiple parties at the expense of a single additional
encryption operation. As stated above, the specification uses a CEK
to encrypt the content at layer 0. For example, the content
encrypted at layer 0 may be a firmware image. The same encrypted
firmware image may need to be sent to many recipients; however, each
recipient uses their own private key to obtain the CEK.
The COSE_recipient structure, shown in Figure 3, is repeated for each
recipient.
COSE_Encrypt_Tagged = #6.96(COSE_Encrypt)
/ Layer 0 /
COSE_Encrypt = [
Headers,
ciphertext : bstr / nil,
recipients : + COSE_recipient
]
/ Layer 1 /
COSE_recipient = [
protected : bstr .cbor header_map,
unprotected : header_map,
encCEK : bstr,
]
header_map = {
Generic_Headers,
* label => values,
}
Figure 3: CDDL for HPKE-based COSE_Encrypt Structure
The COSE_Encrypt MAY be tagged or untagged.
An example is shown in Section 5.2.
4. HPKE Encryption and Decryption
4.1. HPKE Encryption with SealBase
The SealBase(pkR, info, aad, pt) function is used to encrypt a
plaintext pt to a recipient's public key (pkR).
Two cases of plaintext need to be distinguished:
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* For use in COSE_Encrypt, the plaintext "pt" passed into
SealBase is the CEK. The CEK is a random byte sequence of length
appropriate for the encryption algorithm selected in layer 0. For
example, AES-128-GCM requires a 16 byte key and the CEK would
therefore be 16 bytes long.
* In case of COSE_Encrypt0, the plaintext "pt" passed into SealBase
is the content to be encrypted. Hence, there is no intermediate
layer utilizing a CEK.
The "aad" and the "info" parameters are described in Section 4.3 and
Section 4.4, respectively.
If SealBase() is successful, it will output a ciphertext "ct" and an
encapsulated key "enc".
4.2. HPKE Decryption with OpenBase
The recipient will use the OpenBase(enc, skR, info, aad, ct) function
with the "enc" and the "ct" parameters received from the sender. The
"aad" and the "info" parameters are assumed to be constructed from
the context and described in Section 4.3 and Section 4.4,
respectively.
The OpenBase function will, if successful, decrypt "ct". When
decrypted, the result will be either the CEK (when COSE_Encrypt is
used), or the content (if COSE_Encrypt0 is used). The CEK is the
symmetric key used to decrypt the ciphertext at layer 0.
4.3. AAD Parameter
HPKE requires an "aad" parameter to be provided to the SealBase and
OpenBase functions. Note that there are three types of additional
authenticated data used by this specification:
* AAD provided to HPKE for COSE_Encrypt0.
* AAD provided to HPKE for COSE_Encrypt at the recipient layer.
* AAD provided to the AEAD cipher used for content encryption at
layer 0 by COSE_Encrypt.
We describe the three variants in the subsections below.
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4.3.1. AAD provided to HPKE for COSE_Encrypt0
When COSE_Encrypt0 is used then there is no separate AEAD function at
the content encryption layer provided by COSE natively and HPKE
offers this functionality.
The "aad" parameter provided to the SealBase and OpenBase functions
is constructed as follows:
Enc_structure = [
context : "Encrypt0",
protected : empty_or_serialized_map,
external_aad : bstr
]
The protected field in the Enc_structure contains the protected
attributes from the COSE_Encrypt0 structure at layer 0, encoded in a
bstr type.
The external_aad field in the Enc_structure contains the Externally
Supplied Data described in Section 4.3 and Section 5.3 in RFC 9052.
If this field is not supplied, it defaults to a zero-length byte
string.
4.3.2. AAD provided to HPKE for COSE_Encrypt at the Recipient Layer
The AAD used at the recipient layer re-uses Enc_structure from
[RFC9052] and populates it with the following content:
Enc_structure = [
context : "Enc_Recipient",
protected : empty_or_serialized_map,
external_aad : bstr
]
The protected field in the Enc_structure contains the protected
attributes from the COSE_recipient structure at layer 1, encoded in a
bstr type.
The external_aad field in the Enc_structure contains the Externally
Supplied Data described in Section 4.3 and Section 5.3 in RFC 9052.
In the COSE_Encrypt case this AAD information is also input to the
AAD at layer 0, if an AEAD cipher is used at layer 0. If this field
is not supplied, it defaults to a zero-length byte string.
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4.3.3. AAD provided to the AEAD cipher used for Content Encryption at
Layer 0 by COSE_Encrypt
The construction of AAD is defined in Section 5.3 of [RFC9052] (see
Enc_structure structure).
4.4. Info Parameter
The HPKE specification defines the "info" parameter as a context
information structure that is used to ensure that the derived keying
material is "bound" to the context of the transaction.
This section provides a suggestion for constructing the info
structure, when used with SealBase() and OpenBase(). HPKE leaves the
info parameter for these two functions as optional. Application
profiles of this specification MAY populate the fields of the
COSE_KDF_Context structure or MAY use a different structure as input
to the "info" parameter. If no content for the "info" parameter is
not supplied, it defaults to a zero-length byte string.
This specification re-uses the context information structure defined
in [RFC9053] as a foundation for the info structure. This payload
becomes the content of the info parameter for the HPKE functions,
when utilized. For better readability of this specification the
COSE_KDF_Context structure is repeated in Figure 4.
PartyInfo = (
identity : bstr / nil,
nonce : bstr / int / nil,
other : bstr / nil
)
COSE_KDF_Context = [
AlgorithmID : int / tstr,
PartyUInfo : [ PartyInfo ],
PartyVInfo : [ PartyInfo ],
SuppPubInfo : [
keyDataLength : uint,
protected : empty_or_serialized_map,
? other : bstr
],
? SuppPrivInfo : bstr
]
Figure 4: COSE_KDF_Context Data Structure as 'info' Parameter for
HPKE
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5. Examples
5.1. Single Recipient / One Layer Example
This example assumes that a sender wants to communicate an encrypted
payload to a single recipient in the most efficient way.
An example of the COSE_Encrypt0 structure using the HPKE scheme is
shown in Figure 5. Line breaks and comments have been inserted for
better readability.
It uses the following algorithm combination: - KEM: DHKEM(P-256,
HKDF-SHA256) - KDF: HKDF-SHA256 - AEAD: AES-128-GCM
// payload: "This is the content", aad: ""
//
16([
h'a10120', // alg = HPKE-v1-BASE
{
4: h'3031', // kid
-4: [ // HPKE_sender_info
16, // kem = DHKEM(P-256, HKDF-SHA256)
1, // kdf = HKDF-SHA256
1, // aead = AES-128-GCM
h'048c6f75e463a773082f3cb0d3a701348a578c67
80aba658646682a9af7291dfc277ec93c3d58707
818286c1097825457338dc3dcaff367e2951342e
9db30dc0e7', // enc
],
},
/ encrypted plaintext /
h'ee22206308e478c279b94bb071f3a5fbbac412a6effe34195f7
c4169d7d8e81666d8be13',
])
Figure 5: COSE_Encrypt0 Example for HPKE
5.2. Multiple Recipients / Two Layer
In this example we assume that a sender wants to transmit a payload
to two recipients using the two-layer structure. Note that it is
possible to send two single-layer payloads, although it will be less
efficient.
An example of the COSE_Encrypt structure using the HPKE scheme is
shown in Figure 6. Line breaks and comments have been inserted for
better readability.
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It uses the following algorithm combination:
* At layer 0 AES-128-GCM is used for encryption of the plaintext
"This is the content.". In our example, the ciphertext is
detached.
* At the recipient structure at layer 1, DHKEM(P-256, HKDF-SHA256)
(as the KEM), with AES-128-GCM (as the AEAD) and HKDF-SHA256 (as
the KDF) is used.
The algorithm selection is based on the registry of the values
offered by the alg parameters (see Section 7).
// plaintext: "This is the content.", aad: ""
96_0([
h'a10101', // alg = AES-128-GCM (1)
{5: h'67303696a1cc2b6a64867096'}, // iv
h'', // detached ciphertext
[
[
h'a10120', // alg = HPKE-v1-BASE (-1 #TBD)
{
4: h'3031', // kid
-4: [ // HPKE_sender_info
16, // kem = DHKEM(P-256, HKDF-SHA256)
1, // kdf = HKDF-SHA256
1, // aead = AES-128-GCM
/ enc output /
h'0421ccd1b00dd958d77e10399c
97530fcbb91a1dc71cb3bf41d9
9fd39f22918505c973816ecbca
6de507c4073d05cceff73e0d35
f60e2373e09a9433be9e95e53c',
],
},
// ciphertext containing encrypted CEK
h'bb2f1433546c55fb38d6f23f5cd95e1d72eb4
c129b99a165cd5a28bd75859c10939b7e4d',
],
[
h'a10120', // alg = HPKE-v1-BASE (-1 #TBD)
{
4: h'313233', // kid
-4: [ // HPKE_sender_info
16, // kem = DHKEM(P-256, HKDF-SHA256)
1, // kdf = HKDF-SHA256
1, // aead = AES-128-GCM
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/ enc output /
h'6de507c4073d05cceff73e0d35
f60e2373e09a9433be9e95e53c
9fd39f22918505c973816ecbca
6de507c4073d05cceff73e0d35
f60e2373e09a9433be9e95e53c',
],
},
// ciphertext containing encrypted CEK
h'c4169d7d8e81666d8be13bb2f1433546c55fb
c129b99a165cd5a28bd75859c10939b7e4d',
]
],
])
Figure 6: COSE_Encrypt Example for HPKE
To offer authentication of the sender the payload in Figure 6 is
signed with a COSE_Sign1 wrapper, which is shown in Figure 7. The
payload in Figure 7 corresponds to the content shown in Figure 6.
18(
[
/ protected / h'a10126' / {
\ alg \ 1:-7 \ ECDSA 256 \
} / ,
/ unprotected / {
/ kid / 4:'sender@example.com'
},
/ payload / h'AA19...B80C',
/ signature / h'E3B8...25B8'
]
)
Figure 7: COSE_Encrypt Example for HPKE
6. Security Considerations
This specification is based on HPKE and the security considerations
of HPKE [RFC9180] are therefore applicable also to this
specification.
HPKE assumes the sender is in possession of the public key of the
recipient and HPKE COSE makes the same assumptions. Hence, some form
of public key distribution mechanism is assumed to exist.
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HPKE relies on a source of randomness to be available on the device.
Additionally, with the two layer structure the CEK is randomly
generated and the it MUST be ensured that the guidelines for random
number generations are followed.
The COSE_Encrypt structure MUST be authenticated using COSE
constructs like COSE_Sign, COSE_Sign1, COSE_MAC, or COSE_MAC0.
When COSE_Encrypt or COSE_Encrypt0 is used with a detached ciphertext
then the subsequently applied integrity protection via COSE_Sign,
COSE_Sign1, COSE_MAC, or COSE_MAC0 does not cover this detached
ciphertext. Implementers MUST ensure that the detached ciphertext
also experiences integrity protection. This is, for example, the
case when an AEAD cipher is used to produce the detached ciphertext
but may not be guaranteed by non-AEAD ciphers.
7. IANA Considerations
This document requests IANA to add new values to the 'COSE
Algorithms' and to the 'COSE Header Algorithm Parameters' registries
in the 'Standards Action With Expert Review category.
7.1. COSE Algorithms Registry
* Name: HPKE-v1-BASE
* Value: TBD1 (Assumed: -1)
* Description: HPKE in version 1 in base mode for use with COSE
* Capabilities: [kty]
* Change Controller: IESG
* Reference: [[TBD: This RFC]]
* Recommended: Yes
7.2. COSE Header Algorithm Parameters
* Name: HPKE_sender_info
* Label: TBD2 (Assumed: -4)
* Value type: HPKE_sender_info
* Value Registry: N/A
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* Description: HPKE Sender Information structure for the Base mode.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[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>.
[RFC9180] Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid
Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180,
February 2022, <https://www.rfc-editor.org/rfc/rfc9180>.
8.2. Informative References
[RFC2630] Housley, R., "Cryptographic Message Syntax", RFC 2630,
DOI 10.17487/RFC2630, June 1999,
<https://www.rfc-editor.org/rfc/rfc2630>.
[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>.
Appendix A. Contributors
We would like thank the following individuals for their contributions
to the design of embedding the HPKE output into the COSE structure
following a long and lively mailing list discussion.
* Daisuke Ajitomi
* Richard Barnes
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Internet-Draft COSE HPKE April 2023
* Ilari Liusvaara
Finally, we would like to thank Russ Housley for his contributions to
the draft as a co-author of initial versions.
Appendix B. Acknowledgements
We would like to thank John Mattsson, Mike Prorock, Michael
Richardson, Goeran Selander, Laurence Lundblade and Orie Steele for
their review feedback.
Authors' Addresses
Hannes Tschofenig
Email: hannes.tschofenig@gmx.net
Brendan Moran
Arm Limited
Email: Brendan.Moran@arm.com
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