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Add OpenDICE cwt rendering (#84)
Add rendering of CBOR web token (CWT) with OpenDICE claims.
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‎CMakeLists.txt‎

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@@ -95,6 +95,7 @@ set(LIBNAT20_SOURCES
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src/core/asn1.c
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src/core/cbor.c
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src/core/cose.c
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src/core/cwt.c
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src/core/functionality.c
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src/core/oid.c
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src/core/stream.c
@@ -112,6 +113,7 @@ set(LIBNAT20_PUB_HEADERS
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include/nat20/cbor.h
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include/nat20/constants.h
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include/nat20/cose.h
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include/nat20/cwt.h
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include/nat20/crypto.h
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include/nat20/endian.h
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include/nat20/error.h
@@ -150,6 +152,7 @@ set(LIBNAT20_TEST_SOURCES
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src/core/test/asn1.cpp
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src/core/test/cbor.cpp
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src/core/test/cose.cpp
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src/core/test/cwt.cpp
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src/core/test/functionality.cpp
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src/core/test/oid.cpp
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src/core/test/stream.cpp

‎include/nat20/cwt.h‎

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/*
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* Copyright 2026 Aurora Operations, Inc.
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*
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* SPDX-License-Identifier: Apache-2.0 OR GPL-2.0
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*
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* This work is dual licensed.
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* You may use it under Apache-2.0 or GPL-2.0 at your option.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*
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* OR
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License
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* as published by the Free Software Foundation; either version 2
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* of the License, or (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, see
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* <https://www.gnu.org/licenses/>.
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*/
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#pragma once
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#include <nat20/cose.h>
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#include <nat20/crypto.h>
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#include <nat20/open_dice.h>
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#include <nat20/stream.h>
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#include <nat20/types.h>
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#ifdef __cplusplus
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extern "C" {
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#endif
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/** @file */
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/**
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* @defgroup open_dice_cwt_labels Open DICE CWT Label Constants
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*
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* These constants are defined in the OpenDice specification and represent
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* the labels used in the Open DICE CWT structure.
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*
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* [OpenDICE specification]
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* (https://pigweed.googlesource.com/open-dice/+/HEAD/docs/specification.md#cbor-cdi-certificates)
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*
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* @{
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*/
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/** Label for code hash. */
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#define N20_OPEN_DICE_CWT_LABEL_CODE_HASH (-4670545)
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/** Label for code descriptor. */
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#define N20_OPEN_DICE_CWT_LABEL_CODE_DESCRIPTOR (-4670546)
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/** Label for configuration hash. */
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#define N20_OPEN_DICE_CWT_LABEL_CONFIGURATION_HASH (-4670547)
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/** Label for configuration descriptor. */
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#define N20_OPEN_DICE_CWT_LABEL_CONFIGURATION_DESCRIPTOR (-4670548)
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/** Label for authority hash. */
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#define N20_OPEN_DICE_CWT_LABEL_AUTHORITY_HASH (-4670549)
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/** Label for authority descriptor. */
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#define N20_OPEN_DICE_CWT_LABEL_AUTHORITY_DESCRIPTOR (-4670550)
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/** Label for DICE mode. */
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#define N20_OPEN_DICE_CWT_LABEL_MODE (-4670551)
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/** Label for subject public key. */
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#define N20_OPEN_DICE_CWT_LABEL_SUBJECT_PUBLIC_KEY (-4670552)
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/** Label for key usage. */
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#define N20_OPEN_DICE_CWT_LABEL_KEY_USAGE (-4670553)
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/** Label for profile name. */
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#define N20_OPEN_DICE_CWT_LABEL_PROFILE (-4670554)
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/** @} */
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/**
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* @defgroup cwt_labels CBOR Web Token (CWT) Label Constants
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*
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* These constants are defined in the CWT specification and represent
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* the labels used in the CWT structure.
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*
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* [CWT specification](https://tools.ietf.org/html/rfc8392)
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*
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* @{
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*/
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#define N20_CWT_LABEL_ISSUER (1) /**< Label for issuer. */
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#define N20_CWT_LABEL_SUBJECT (2) /**< Label for subject. */
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/** @} */
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/**
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* @brief Convert Open DICE public key information to COSE key format.
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*
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* If either argument is NULL this function is a no-op.
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*
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* If the @p key_info->algorithm is not supported, @p cose_key will be left unmodified.
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*
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* Supported algorithms are Ed25519, P-256, and P-384 which are expressed
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* by @ref n20_crypto_key_type_ed25519_e, @ref n20_crypto_key_type_secp256r1_e,
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* and @ref n20_crypto_key_type_secp384r1_e respectively.
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*
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* The function sets the algorithm ID to @ref n20_cose_algorithm_id_eddsa_e,
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* @ref n20_cose_algorithm_id_es256_e, or @ref n20_cose_algorithm_id_es384_e
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* respectively. Note that these values have been deprecated according to [1]
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* but are still in use.
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*
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* The function also populates the public key from the the supplied buffer
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* in @p key_info->key.
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*
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* In the case of P-256 and P-384, the function assumes that the key buffer
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* is formatted as an uncompressed EC point. If the buffer size is odd it
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* is assumed that the first byte is the format header `0x04` which gets ignored.
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* The point is then split into its X and Y coordinates and stored in
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* @p cose_key->x and @p cose_key->y respectively. @p cose_key->d is set to an
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* empty slice.
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*
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* In the case of ED25519, the function expects the key buffer to be in the
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* standard format for EdDSA keys and stores in @p cose_key->x.
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* @p cose_key->y and @p cose_key->d are set to empty slices.
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*
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* Buffers are not copied. Both @p key_info and @p cose_key point to the
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* same underlying buffers. No ownership transfer is implied. The buffers
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* must remain valid for the life time of both structures.
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*
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* [1] https://www.iana.org/assignments/cose/cose.xhtml
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*
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* @param cose_key Pointer to the COSE key structure to populate.
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* @param key_info Pointer to the Open DICE public key information.
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*/
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extern void n20_cwt_key_info_to_cose(n20_cose_key_t *const cose_key,
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n20_open_dice_public_key_info_t const *const key_info);
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/**
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* @brief Render a CBOR web token (CWT) structure.
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*
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* This function encodes a CWT (RFC8392) structure into a CBOR representation
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* including the issuer and subject claims, the subject public key and the OpenDICE
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* claims as described in the OpenDICE specification [1].
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*
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* The function never fails, however, the stream needs to be checked
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* for buffer @ref n20_stream_has_buffer_overflow (or write position overflow
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* @ref n20_stream_has_write_position_overflow if the to-be-rendered size is measured)
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* after the call to ensure that it was rendered to the buffer successfully.
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*
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* The function is not responsible for managing the lifetime of the buffers
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* used by the input parameters. The caller must ensure that these buffers
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* remain valid for the duration of the function call.
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*
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* The function does not sanitize the input and may not produce a valid CWT.
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* E.g. if the key info is missing, or if the key algorithm is not supported.
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* the public key field will be rendered as a CBOR null value
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* @sa n20_cwt_key_info_to_cose.
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*
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* [1] https://pigweed.googlesource.com/open-dice/+/HEAD/docs/specification.md
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*
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* @param s The stream to write the CBOR representation to.
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* @param cert_info The CWT structure to render.
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*/
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extern void n20_open_dice_cwt_write(n20_stream_t *const s,
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n20_open_dice_cert_info_t const *const cert_info);
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#ifdef __cplusplus
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}
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#endif

‎src/core/cwt.c‎

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/*
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* Copyright 2026 Aurora Operations, Inc.
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*
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* SPDX-License-Identifier: Apache-2.0 OR GPL-2.0
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*
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* This work is dual licensed.
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* You may use it under Apache-2.0 or GPL-2.0 at your option.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*
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* OR
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License
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* as published by the Free Software Foundation; either version 2
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* of the License, or (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, see
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* <https://www.gnu.org/licenses/>.
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*/
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#include <nat20/cbor.h>
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#include <nat20/cose.h>
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#include <nat20/crypto.h>
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#include <nat20/cwt.h>
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#include <nat20/error.h>
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#include <nat20/stream.h>
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#include <nat20/types.h>
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static void n20_open_dice_write_name_as_hex(n20_stream_t *const s, n20_slice_t const name) {
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if (name.size != 0 && name.buffer == NULL) {
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n20_cbor_write_null(s);
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return;
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}
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for (size_t i = 0; i < name.size; ++i) {
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uint8_t byte = name.buffer[name.size - (i + 1)];
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uint8_t hex[2] = {(byte >> 4) + '0', (byte & 0x0f) + '0'};
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if (hex[0] > '9') {
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hex[0] += 39; // Convert to 'a' - 'f'
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}
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if (hex[1] > '9') {
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hex[1] += 39; // Convert to 'a' - 'f'
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}
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n20_stream_prepend(s, hex, sizeof(hex));
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}
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n20_cbor_write_header(s, n20_cbor_type_string_e, name.size * 2);
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}
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void n20_cwt_key_info_to_cose(n20_cose_key_t *const cose_key,
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n20_open_dice_public_key_info_t const *const key_info) {
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if (key_info == NULL || cose_key == NULL) {
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return;
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}
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switch (key_info->algorithm) {
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case n20_crypto_key_type_ed25519_e:
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cose_key->algorithm_id = n20_cose_algorithm_id_eddsa_e;
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cose_key->x = key_info->key;
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cose_key->y = N20_SLICE_NULL;
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cose_key->d = N20_SLICE_NULL;
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return;
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case n20_crypto_key_type_secp256r1_e:
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cose_key->algorithm_id = n20_cose_algorithm_id_es256_e;
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break;
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case n20_crypto_key_type_secp384r1_e:
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cose_key->algorithm_id = n20_cose_algorithm_id_es384_e;
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break;
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default:
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/* Unsupported algorithm -> don't touch the COSE key structure. */
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return;
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}
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/* For NIST curves P-256 and P-384 the key is split into
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* X and Y coordinates. */
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size_t coordinate_size = key_info->key.size / 2;
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/* If the key size is odd skip the first byte to swallow
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* the format header. */
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cose_key->x.buffer = key_info->key.buffer + (key_info->key.size & 1);
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cose_key->x.size = coordinate_size;
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cose_key->y.buffer = cose_key->x.buffer + coordinate_size;
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cose_key->y.size = coordinate_size;
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cose_key->d = N20_SLICE_NULL;
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}
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void n20_open_dice_cwt_write(n20_stream_t *const s, n20_open_dice_cert_info_t const *const cwt) {
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uint32_t pairs = 0;
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// Write Profile Name
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if (cwt->open_dice_input.profile_name.size > 0) {
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n20_cbor_write_text_string(s, cwt->open_dice_input.profile_name);
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n20_cbor_write_int(s, N20_OPEN_DICE_CWT_LABEL_PROFILE);
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pairs++;
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}
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// Write Key Usage
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n20_cbor_write_byte_string(
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s,
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(n20_slice_t){.buffer = (uint8_t *)cwt->key_usage,
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.size = (cwt->key_usage[1] != 0 ? 2 : (cwt->key_usage[0] != 0))});
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n20_cbor_write_int(s, N20_OPEN_DICE_CWT_LABEL_KEY_USAGE);
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++pairs; // Key Usage
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size_t mark = n20_stream_byte_count(s);
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n20_cose_key_t subject_public_key = {0};
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n20_cose_key_ops_set(&subject_public_key.key_ops, n20_cose_key_op_verify_e);
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n20_cwt_key_info_to_cose(&subject_public_key, &cwt->subject_public_key);
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// Subject public key
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n20_cose_write_key(s, &subject_public_key);
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n20_cbor_write_header(s, n20_cbor_type_bytes_e, n20_stream_byte_count(s) - mark);
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n20_cbor_write_int(s, N20_OPEN_DICE_CWT_LABEL_SUBJECT_PUBLIC_KEY);
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++pairs;
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// Convert mode to uint8_t
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uint8_t mode = (uint8_t)cwt->open_dice_input.mode;
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n20_cbor_write_byte_string(s, (n20_slice_t){.buffer = &mode, .size = 1});
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n20_cbor_write_int(s, N20_OPEN_DICE_CWT_LABEL_MODE);
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++pairs;
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if (cwt->open_dice_input.authority_descriptor.size > 0) {
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n20_cbor_write_byte_string(s, cwt->open_dice_input.authority_descriptor);
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n20_cbor_write_int(s, N20_OPEN_DICE_CWT_LABEL_AUTHORITY_DESCRIPTOR);
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++pairs;
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}
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if (cwt->open_dice_input.authority_hash.size > 0) {
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n20_cbor_write_byte_string(s, cwt->open_dice_input.authority_hash);
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n20_cbor_write_int(s, N20_OPEN_DICE_CWT_LABEL_AUTHORITY_HASH);
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++pairs;
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}
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if (cwt->open_dice_input.configuration_descriptor.size > 0) {
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n20_cbor_write_byte_string(s, cwt->open_dice_input.configuration_descriptor);
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n20_cbor_write_int(s, N20_OPEN_DICE_CWT_LABEL_CONFIGURATION_DESCRIPTOR);
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++pairs;
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}
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if (cwt->open_dice_input.configuration_hash.size > 0) {
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n20_cbor_write_byte_string(s, cwt->open_dice_input.configuration_hash);
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n20_cbor_write_int(s, N20_OPEN_DICE_CWT_LABEL_CONFIGURATION_HASH);
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++pairs;
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}
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if (cwt->open_dice_input.code_descriptor.size > 0) {
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n20_cbor_write_byte_string(s, cwt->open_dice_input.code_descriptor);
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n20_cbor_write_int(s, N20_OPEN_DICE_CWT_LABEL_CODE_DESCRIPTOR);
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++pairs;
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}
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if (cwt->open_dice_input.code_hash.size > 0) {
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n20_cbor_write_byte_string(s, cwt->open_dice_input.code_hash);
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n20_cbor_write_int(s, N20_OPEN_DICE_CWT_LABEL_CODE_HASH);
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++pairs;
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}
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if (cwt->subject.size > 0) {
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n20_open_dice_write_name_as_hex(s, cwt->subject);
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n20_cbor_write_int(s, N20_CWT_LABEL_SUBJECT);
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++pairs;
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}
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if (cwt->issuer.size > 0) {
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n20_open_dice_write_name_as_hex(s, cwt->issuer);
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n20_cbor_write_int(s, N20_CWT_LABEL_ISSUER);
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++pairs;
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}
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// Write the map header with the number of pairs.
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n20_cbor_write_map_header(s, pairs);
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}

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