draft-ietf-cose-hpke-00.txt   draft-ietf-cose-hpke-01.txt 
COSE H. Tschofenig COSE H. Tschofenig
Internet-Draft Arm Limited Internet-Draft Arm Limited
Intended status: Standards Track R. Housley Intended status: Standards Track R. Housley
Expires: 14 July 2022 Vigil Security Expires: 8 September 2022 Vigil Security
B. Moran B. Moran
Arm Limited Arm Limited
10 January 2022 7 March 2022
Use of Hybrid Public-Key Encryption (HPKE) with CBOR Object Signing and Use of Hybrid Public-Key Encryption (HPKE) with CBOR Object Signing and
Encryption (COSE) Encryption (COSE)
draft-ietf-cose-hpke-00 draft-ietf-cose-hpke-01
Abstract Abstract
This specification defines hybrid public-key encryption (HPKE) for This specification defines hybrid public-key encryption (HPKE) for
use with CBOR Object Signing and Encryption (COSE). 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) encryption function.
Authentication for HPKE in COSE is provided by COSE-native security
mechanisms.
Status of This Memo Status of This Memo
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3 2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
3. HPKE for COSE . . . . . . . . . . . . . . . . . . . . . . . . 3 3. HPKE for COSE . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 3 3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 3
3.2. HPKE Encryption with SealBase . . . . . . . . . . . . . . 4 3.2. HPKE Encryption with SealBase . . . . . . . . . . . . . . 4
3.3. HPKE Decryption with Open . . . . . . . . . . . . . . . . 5 3.3. HPKE Decryption with OpenBase . . . . . . . . . . . . . . 5
3.4. Info Structure . . . . . . . . . . . . . . . . . . . . . 5 3.4. Info Structure . . . . . . . . . . . . . . . . . . . . . 5
4. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 6.1. HPKE/P-256+HKDF-256 and AES-128-GCM . . . . . . . . . . . 8
6.2. HPKE/P-512+HKDF-512 and AES-256-GCM . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 9 7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 9 7.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 9 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction 1. Introduction
Hybrid public-key encryption (HPKE) [I-D.irtf-cfrg-hpke] is a scheme Hybrid public-key encryption (HPKE) [I-D.irtf-cfrg-hpke] is a scheme
that provides public key encryption of arbitrary-sized plaintexts that provides public key encryption of arbitrary-sized plaintexts
given a recipient's public key. HPKE utilizes a non-interactive given a recipient's public key. HPKE utilizes a non-interactive
ephemeral-static Diffie-Hellman exchange to establish a shared ephemeral-static Diffie-Hellman exchange to establish a shared
secret, which is then used to encrypt plaintext. secret. The motivation for standardizing a public key encryption
scheme is explained in the introduction of [I-D.irtf-cfrg-hpke].
The HPKE specification defines several features for use with public The HPKE specification defines several features for use with public
key encryption and a subset of those features is applied to COSE key encryption and a subset of those features is applied to COSE
[RFC8152]. Since COSE provides constructs for authenticcation, those [RFC8152]. Since COSE provides constructs for authentication, those
are not re-used from the HPKE specification. This specification uses are not re-used from the HPKE specification. This specification uses
the "base" mode (as it is called in HPKE specification language). the "base" mode, as it is called in HPKE specification language.
2. Conventions and Terminology 2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in "OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
This specification uses the following abbreviations and terms: - This specification uses the following abbreviations and terms: -
Content-encryption key (CEK), a term defined in RFC 2630 [RFC2630]. - Content-encryption key (CEK), a term defined in CMS [RFC2630]. -
Hybrid Public Key Encryption (HPKE) is defined in Hybrid Public Key Encryption (HPKE) is defined in
[I-D.irtf-cfrg-hpke]. - pkR is the public key of the recipient, as [I-D.irtf-cfrg-hpke]. - pkR is the public key of the recipient, as
defined in [I-D.irtf-cfrg-hpke]. - skR is the private key of the defined in [I-D.irtf-cfrg-hpke]. - skR is the private key of the
recipient, as defined in [I-D.irtf-cfrg-hpke]. recipient, as defined in [I-D.irtf-cfrg-hpke].
3. HPKE for COSE 3. HPKE for COSE
3.1. Overview 3.1. Overview
The CDDL for the COSE_Encrypt structure, as used with this The CDDL for the COSE_Encrypt structure, as used with this
specification, is shown in Figure 1. The structures referenced below specification, is shown in Figure 1.
are found in the CDDL.
HPKE, when used with COSE, follows a three layer structure: HPKE, when used with COSE, follows a two layer structure:
* Layer 0 (corresponding to the COSE_Encrypt structure) contains * Layer 0 (corresponding to the COSE_Encrypt structure) contains
content encrypted with the CEK. This ciphertext may be detached. content (plaintext) encrypted with the CEK. This ciphertext may
If not detached, then it is included in the COSE_Encrypt be detached. If not detached, then it is included in the
structure. COSE_Encrypt structure.
* Layer 1 (see COSE_recipient_outer structure) includes the
encrypted CEK.
* Layer 2 (in the COSE_recipient_inner structure) contains * Layer 1 (corresponding to a recipient structure) contains
parameters needed for HPKE to generate a shared secret used to parameters needed for HPKE to generate a shared secret used to
encrypt the CEK from layer 1. encrypt the CEK. This layer includes the encrypted CEK.
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 is a firmware image.
The same ciphertext firmware image is processed by all of the
recipients; however, each recipient uses their own private key to
obtain the CEK.
COSE_Encrypt_Tagged = #6.96(COSE_Encrypt) COSE_Encrypt_Tagged = #6.96(COSE_Encrypt)
SUIT_Encryption_Info = COSE_Encrypt_Tagged HPKE_Encryption_Info = COSE_Encrypt_Tagged
; Layer 0 ; Layer 0
COSE_Encrypt = [ COSE_Encrypt = [
Headers, Headers,
ciphertext : bstr / nil, ciphertext : bstr / nil,
recipients : [+COSE_recipient_outer] recipients : + COSE_recipient
] ]
; Layer 1 ; Layer 1
COSE_recipient_outer = [ COSE_recipient = [
protected : bstr .size 0, protected : bstr .cbor header_map, ; must contain alg parameter
unprotected : header_map, ; must contain alg
encCEK : bstr, ; CEK encrypted with HPKE-derived shared secret
recipients : [ + COSE_recipient_inner ]
]
; Layer 2
COSE_recipient_inner = [
protected : bstr .cbor header_map, ; must contain HPKE alg
unprotected : header_map, ; must contain kid and ephemeral public key unprotected : header_map, ; must contain kid and ephemeral public key
empty : null, encCEK : bstr, ; CEK encrypted with HPKE-derived shared secret
empty : null
] ]
header_map = { header_map = {
Generic_Headers, Generic_Headers,
* label =values, * label =values,
} }
Figure 1: CDDL for HPKE-based COSE_Encrypt Structure Figure 1: CDDL for HPKE-based COSE_Encrypt Structure
The COSE_recipient_outer structure shown in Figure 1 includes the The COSE_recipient structure shown in Figure 1 is repeated for each
encrypted CEK (in the encCEK structure) and the COSE_recipient_inner recipient, and it includes the encrypted CEK as well as the sender-
structure, also shown in Figure 1, contains the ephemeral public key generated ephemeral public key in the unprotected header structure.
(in the unprotected structure).
3.2. HPKE Encryption with SealBase 3.2. HPKE Encryption with SealBase
The SealBase(pkR, info, aad, pt) function is used to encrypt a The SealBase(pkR, info, aad, pt) function is used to encrypt a
plaintext pt to a recipient's public key (pkR). For use in this plaintext pt to a recipient's public key (pkR).
specification, the plaintext "pt" passed into the SealBase is the
CEK. The CEK is a random byte sequence of length appropriate for the IMPORTANT: For use in this specification, the plaintext "pt" passed
encryption algorithm selected in layer 0. For example, AES-128-GCM into the SealBase is the CEK. The CEK is a random byte sequence of
requires a 16 byte key and the CEK would therefore be 16 bytes long. 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.
The "info" parameter can be used to influence the generation of keys The "info" parameter can be used to influence the generation of keys
and the "aad" parameter provides additional authenticated data to the and the "aad" parameter provides additional authenticated data to the
AEAD algorithm in use. If successful, SealBase() will output a AEAD algorithm in use. This specification does not mandate the use
ciphertext "ct" and an encapsulated key "enc". The content of enc is of the info and the aad parameters.
the ephemeral public key.
If SealBase() is successful, it will output a ciphertext "ct" and an
encapsulated key "enc". The content of enc is the ephemeral public
key.
The content of the info parameter is based on the 'COSE_KDF_Context' The content of the info parameter is based on the 'COSE_KDF_Context'
structure, which is detailed in Figure 2. structure, which is detailed in Figure 2.
3.3. HPKE Decryption with Open 3.3. HPKE Decryption with OpenBase
The recipient will use the OpenBase(enc, skR, info, aad, ct) function The recipient will use the OpenBase(enc, skR, info, aad, ct) function
with the enc and ct parameters received from the sender. The "aad" with the enc and ct parameters received from the sender. The "aad"
and the "info" parameters are obtained via the context of the usage. and the "info" parameters are obtained via the context of the usage.
The OpenBase function will, if successful, decrypt "ct". When The OpenBase function will, if successful, decrypt "ct". When
decrypted, the result will be the CEK. The CK is the symmetric key decrypted, the result will be the CEK. The CK is the symmetric key
used to decrypt the ciphertext in the COSE_Encrypt structure. used to decrypt the ciphertext in layer 0 of the COSE_Encrypt
structure.
3.4. Info Structure 3.4. Info Structure
This section provides a suggestion for constructing the info
structure, when used with SealBase() and OpenBase(). Note that the
use of the aad and the info structures for these two functions is
optional. Profiles of this specification may require their use and
may define different info structure.
This specification re-uses the context information structure defined This specification re-uses the context information structure defined
in [RFC8152] for use with the HPKE algorithm. This payload becomes in [RFC8152] as a foundation for the info structure. This payload
the content of the info parameter for the HPKE functions. For better becomes the content of the info parameter for the HPKE functions,
readability of this specification the COSE_KDF_Context structure is when utilized. For better readability of this specification the
repeated in Figure 2. COSE_KDF_Context structure is repeated in Figure 2.
PartyInfo = ( PartyInfo = (
identity : bstr / nil, identity : bstr / nil,
nonce : bstr / int / nil, nonce : bstr / int / nil,
other : bstr / nil other : bstr / nil
) )
COSE_KDF_Context = [ COSE_KDF_Context = [
AlgorithmID : int / tstr, AlgorithmID : int / tstr,
PartyUInfo : [ PartyInfo ], PartyUInfo : [ PartyInfo ],
skipping to change at page 6, line 5 skipping to change at page 6, line 25
SuppPubInfo : [ SuppPubInfo : [
keyDataLength : uint, keyDataLength : uint,
protected : empty_or_serialized_map, protected : empty_or_serialized_map,
? other : bstr ? other : bstr
], ],
? SuppPrivInfo : bstr ? SuppPrivInfo : bstr
] ]
Figure 2: COSE_KDF_Context Data Structure for info parameter Figure 2: COSE_KDF_Context Data Structure for info parameter
Since this specification may be used in a number of different The fields in Figure 2 are populated as follows:
deployment environments flexibility for populating the fields in the
COSE_KDF_Context structure is provided.
For better interoperability, the following recommended settings are
provided:
* PartyUInfo.identity corresponds to the kid found in the * PartyUInfo.identity corresponds to the kid found in the
COSE_Sign_Tagged or COSE_Sign1_Tagged structure (when a digital COSE_Sign_Tagged or COSE_Sign1_Tagged structure (when a digital
signature is used). When utilizing a MAC, then the kid is found signature is used). When utilizing a MAC, then the kid is found
in the COSE_Mac_Tagged or COSE_Mac0_Tagged structure. in the COSE_Mac_Tagged or COSE_Mac0_Tagged structure.
* PartyVInfo.identity corresponds to the kid used for the respective * PartyVInfo.identity corresponds to the kid used for the respective
recipient from the inner-most recipients array. recipient from the inner-most recipients array.
* The value in the AlgorithmID field corresponds to the alg * The value in the AlgorithmID field corresponds to the alg
parameter in the protected structure in the inner-most recipients parameter in the unprotected header structure of the recipient
array. structure.
* keyDataLength is set to the number of bits of the desired output * keyDataLength is set to the number of bits of the desired output
value. value.
* protected refers to the protected structure of the inner-most * protected refers to the protected structure of the inner-most
array. array.
4. Example 4. Example
An example of the COSE_Encrypt structure using the HPKE scheme is An example of the COSE_Encrypt structure using the HPKE scheme is
shown in Figure 3. It uses the following algorithm combination: shown in Figure 3. Line breaks and comments have been inserted for
better readability. It uses the following algorithm combination:
* AES-GCM-128 for encryption of detached ciphertext. * AES-GCM-128 for encryption of detached ciphertext in layer 0.
* AES-GCM-128 for encryption of the CEK. * AES-GCM-128 for encryption of the CEK in layer 1 as well as ECDH
with NIST P-256 and HKDF-SHA256 as a Key Encapsulation Mechanism
(KEM).
* Key Encapsulation Mechanism (KEM): NIST P-256 The algorithm selection is based on the registry of the values
offered by the alg parameters.
* Key Derivation Function (KDF): HKDF-SHA256 96_0([
96( / protected header with alg=AES-GCM-128 /
h'a10101',
/ unprotected header with nonce /
{5: h'938b528516193cc7123ff037809f4c2a'},
/ detached ciphertext /
null,
/ recipient structure /
[ [
// protected field with alg=AES-GCM-128 / protected field with alg for HPKE /
h'A10101', h'a1013863',
{ // unprotected field with iv / unprotected header /
5: h'26682306D4FB28CA01B43B80' {
/ ephemeral public key with x / y coodinate /
-1: h'a401022001215820a596f2ca8d159c04942308ca90
cfbfca65b108ca127df8fe191a063d00d7c5172258
20aef47a45d6d6c572e7bd1b9f3e69b50ad3875c68
f6da0caaa90c675df4162c39',
/ kid for recipient static ECDH public key /
4: h'6b69642d32',
}, },
// null because of detached ciphertext / encrypted CEK /
null, h'9aba6fa44e9b2cef9d646614dcda670dbdb31a3b9d37c7a
[ // COSE_recipient_outer 65b099a8152533062',
h'', // empty protected field ],
{ // unprotected field with ... ])
1: 1 // alg=A128GCM
},
// Encrypted CEK
h'FA55A50CF110908DA6443149F2C2062011A7D8333A72721A',
/ recipients / [ // COSE_recipient_inner
[
/ protected / h'a1013818' / {
\ alg \ 1:TBD1 \ HPKE/P-256+HKDF-256 \
} / ,
/ unprotected / {
// HPKE encapsulated key
/ ephemeral / -1:{
/ kty / 1:2,
/ crv / -1:1,
/ x / -2:h'98f50a4ff6c05861c8...90bbf91d6280',
/ y / -3:true
},
// kid for recipient static ECDH public key
/ kid / 4:'meriadoc.brandybuck@buckland.example'
},
// empty ciphertext
/ ciphertext / h''
]
]
]
]
)
Figure 3: COSE_Encrypt Example for HPKE Figure 3: COSE_Encrypt Example for HPKE
Note that the COSE_Sign1 wrapper outside the COSE_Encrypt structure
is not shown in the example above.
5. Security Considerations 5. Security Considerations
This specification is based on HPKE and the security considerations This specification is based on HPKE and the security considerations
of HPKE [I-D.irtf-cfrg-hpke] are therefore applicable also to this of HPKE [I-D.irtf-cfrg-hpke] are therefore applicable also to this
specification. specification.
HPKE assumes that the sender is in possession of the public key of HPKE assumes the sender is in possession of the public key of the
the recipient. A system using HPKE COSE has to assume the same recipient and HPKE COSE makes the same assumptions. Some form of
assumptions and public key distribution mechanism is assumed to public key distribution mechanism is assumed to exist.
exist.
Since the CEK is randomly generated it must be ensured that the Since the CEK is randomly generated it must be ensured that the
guidelines for random number generations are followed, see [RFC8937]. guidelines for random number generations are followed, see [RFC8937].
The SUIT_Encryption_Info structure shown in this document does not The COSE_Encrypt structure must be authenticated using COSE
provide authentication. Hence, the SUIT_Encryption_Info structure constructs like COSE_Sign, or COSE_Sign1.
has to be used in combination with other COSE constructs, such as the
COSE_Sign or COSE_Sign1.
6. IANA Considerations 6. IANA Considerations
This document requests IANA to create new entries in the COSE This document requests IANA to add new values to the COSE Algorithms
Algorithms registry established with [RFC8152]. registry defined in [RFC8152] (in the Standards Action With Expert
Review category):
+-------------+-------+---------+------------+--------+---------------+ 6.1. HPKE/P-256+HKDF-256 and AES-128-GCM
| Name | Value | KDF | Ephemeral- | Key | Description |
| | | | Static | Wrap | |
+-------------+-------+---------+------------+--------+---------------+
| HPKE/P-256+ | TBD1 | HKDF - | yes | none | HPKE with |
| HKDF-256 | | SHA-256 | | | ECDH-ES |
| | | | | | (P-256) + |
| | | | | | HKDF-256 |
+-------------+-------+---------+------------+--------+---------------+
| HPKE/P-384+ | TBD2 | HKDF - | yes | none | HPKE with |
| HKDF-SHA384 | | SHA-384 | | | ECDH-ES |
| | | | | | (P-384) + |
| | | | | | HKDF-384 |
+-------------+-------+---------+------------+--------+---------------+
| HPKE/P-521+ | TBD3 | HKDF - | yes | none | HPKE with |
| HKDF-SHA521 | | SHA-521 | | | ECDH-ES |
| | | | | | (P-521) + |
| | | | | | HKDF-521 |
+-------------+-------+---------+------------+--------+---------------+
| HPKE | TBD4 | HKDF - | yes | none | HPKE with |
| X25519 + | | SHA-256 | | | ECDH-ES |
| HKDF-SHA256 | | | | | (X25519) + |
| | | | | | HKDF-256 |
+-------------+-------+---------+------------+--------+---------------+
| HPKE | TBD4 | HKDF - | yes | none | HPKE with |
| X448 + | | SHA-512 | | | ECDH-ES |
| HKDF-SHA512 | | | | | (X448) + |
| | | | | | HKDF-512 |
+-------------+-------+---------+------------+--------+---------------+
7. References * Name: HPKE_P256_HKDF256_AES128_GCM
* Value: TBD1
* Description: HPKE/P-256+HKDF-256 and AES-128-GCM
* Capabilities: [kty]
* Change Controller: IESG
* Reference: [[TBD: This RFC]]
* Recommended: Yes
6.2. HPKE/P-512+HKDF-512 and AES-256-GCM
* Name: HPKE_P521_HKDF512_AES256_GCM
* Value: TBD2
* Description: HPKE/P-512+HKDF-512 and AES-256-GCM
* Capabilities: [kty]
* Change Controller: IESG
* Reference: [[TBD: This RFC]]
* Recommended: Yes
TBD: More values to be added.
7. References
7.1. Normative References 7.1. Normative References
[I-D.irtf-cfrg-hpke] [I-D.irtf-cfrg-hpke]
Barnes, R. L., Bhargavan, K., Lipp, B., and C. A. Wood, Barnes, R. L., Bhargavan, K., Lipp, B., and C. A. Wood,
"Hybrid Public Key Encryption", Work in Progress, "Hybrid Public Key Encryption", Work in Progress,
Internet-Draft, draft-irtf-cfrg-hpke-12, 2 September 2021, Internet-Draft, draft-irtf-cfrg-hpke-12, 2 September 2021,
<https://www.ietf.org/archive/id/draft-irtf-cfrg-hpke- <https://www.ietf.org/archive/id/draft-irtf-cfrg-hpke-
12.txt>. 12.txt>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
skipping to change at page 9, line 42 skipping to change at page 9, line 39
DOI 10.17487/RFC2630, June 1999, DOI 10.17487/RFC2630, June 1999,
<https://www.rfc-editor.org/info/rfc2630>. <https://www.rfc-editor.org/info/rfc2630>.
[RFC8937] Cremers, C., Garratt, L., Smyshlyaev, S., Sullivan, N., [RFC8937] Cremers, C., Garratt, L., Smyshlyaev, S., Sullivan, N.,
and C. Wood, "Randomness Improvements for Security and C. Wood, "Randomness Improvements for Security
Protocols", RFC 8937, DOI 10.17487/RFC8937, October 2020, Protocols", RFC 8937, DOI 10.17487/RFC8937, October 2020,
<https://www.rfc-editor.org/info/rfc8937>. <https://www.rfc-editor.org/info/rfc8937>.
Appendix A. Acknowledgements Appendix A. Acknowledgements
TBD: Add your name here. We would like to thank Goeran Selander, John Mattsson and Ilari
Liusvaara for their review feedback.
Authors' Addresses Authors' Addresses
Hannes Tschofenig Hannes Tschofenig
Arm Limited Arm Limited
Email: hannes.tschofenig@arm.com Email: hannes.tschofenig@arm.com
Russ Housley Russ Housley
Vigil Security, LLC Vigil Security, LLC
Email: housley@vigilsec.com Email: housley@vigilsec.com
Brendan Moran Brendan Moran
Arm Limited Arm Limited
Email: Brendan.Moran@arm.com Email: Brendan.Moran@arm.com
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