< draft-ietf-suit-firmware-encryption-00.txt		draft-ietf-suit-firmware-encryption-01.txt >
	SUIT H. Tschofenig		SUIT H. Tschofenig
	Internet-Draft Arm Limited		Internet-Draft Arm Limited
	Intended status: Standards Track R. Housley		Intended status: Standards Track R. Housley
	Expires: January 8, 2022 Vigil Security		Expires: January 13, 2022 Vigil Security
	B. Moran		B. Moran
	Arm Limited		Arm Limited
	July 07, 2021		July 12, 2021
	Firmware Encryption with SUIT Manifests		Firmware Encryption with SUIT Manifests
	draft-ietf-suit-firmware-encryption-00		draft-ietf-suit-firmware-encryption-01
	Abstract		Abstract
	This document specifies a firmware update mechanism where the		This document specifies a firmware update mechanism where the
	firmware image is encrypted. This mechanism uses the IETF SUIT		firmware image is encrypted. This mechanism uses the IETF SUIT
	manifest with key establishment provided by the hybrid public-key		manifest with key establishment provided by the hybrid public-key
	encryption (HPKE) scheme or AES Key Wrap (AES-KW) with a pre-shared		encryption (HPKE) scheme or AES Key Wrap (AES-KW) with a pre-shared
	key-encryption key. In either case, AES-GCM or AES-CCM is used for		key-encryption key. In either case, AES-GCM or AES-CCM is used for
	firmware encryption.		firmware encryption.
	skipping to change at page 1, line 38 ¶		skipping to change at page 1, line 38 ¶
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at https://datatracker.ietf.org/drafts/current/.		Drafts is at https://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on January 8, 2022.		This Internet-Draft will expire on January 13, 2022.
	Copyright Notice		Copyright Notice
	Copyright (c) 2021 IETF Trust and the persons identified as the		Copyright (c) 2021 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(https://trustee.ietf.org/license-info) in effect on the date of		(https://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	skipping to change at page 2, line 25 ¶		skipping to change at page 2, line 25 ¶
	the copyright in such materials, this document may not be modified		the copyright in such materials, this document may not be modified
	outside the IETF Standards Process, and derivative works of it may		outside the IETF Standards Process, and derivative works of it may
	not be created outside the IETF Standards Process, except to format		not be created outside the IETF Standards Process, except to format
	it for publication as an RFC or to translate it into languages other		it for publication as an RFC or to translate it into languages other
	than English.		than English.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
	2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3		2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
	3. AES Key Wrap . . . . . . . . . . . . . . . . . . . . . . . . 4		3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 4
	4. Hybrid Public-Key Encryption (HPKE) . . . . . . . . . . . . . 7		4. AES Key Wrap . . . . . . . . . . . . . . . . . . . . . . . . 5
	5. Complete Examples . . . . . . . . . . . . . . . . . . . . . . 12		5. Hybrid Public-Key Encryption (HPKE) . . . . . . . . . . . . . 9
	6. Security Considerations . . . . . . . . . . . . . . . . . . . 12		6. Complete Examples . . . . . . . . . . . . . . . . . . . . . . 14
	7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13		7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
	8. References . . . . . . . . . . . . . . . . . . . . . . . . . 14		8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
	8.1. Normative References . . . . . . . . . . . . . . . . . . 14		9. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
	8.2. Informative References . . . . . . . . . . . . . . . . . 15		9.1. Normative References . . . . . . . . . . . . . . . . . . 16
	Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 16		9.2. Informative References . . . . . . . . . . . . . . . . . 17
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16		Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 18
			Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
	1. Introduction		1. Introduction
	Vulnerabilities with Internet of Things (IoT) devices have raised the		Vulnerabilities with Internet of Things (IoT) devices have raised the
	need for a reliable and secure firmware update mechanism that is also		need for a reliable and secure firmware update mechanism that is also
	suitable for constrained devices. To protect firmware images the		suitable for constrained devices. To protect firmware images the
	SUIT manifest format was developed [I-D.ietf-suit-manifest]. The		SUIT manifest format was developed [I-D.ietf-suit-manifest]. The
	SUIT manifest provides a bundle of metadata about the firmware for an		SUIT manifest provides a bundle of metadata about the firmware for an
	IoT device, where to find the firmware image, and the devices to		IoT device, where to find the firmware image, and the devices to
	which it applies.		which it applies.
	skipping to change at page 3, line 20 ¶		skipping to change at page 3, line 20 ¶
	of the firmware image to decrypt it.		of the firmware image to decrypt it.
	A symmetric cryptographic key is established for encryption and		A symmetric cryptographic key is established for encryption and
	decryption, and that key can be applied to a SUIT manifest, firmware		decryption, and that key can be applied to a SUIT manifest, firmware
	images, or personalization data, depending on the encryption choices		images, or personalization data, depending on the encryption choices
	of the firmware author. This symmetric key can be established using		of the firmware author. This symmetric key can be established using
	a variety of mechanisms; this document defines two approaches for use		a variety of mechanisms; this document defines two approaches for use
	with the IETF SUIT manifest. Key establishment can be provided by		with the IETF SUIT manifest. Key establishment can be provided by
	the hybrid public-key encryption (HPKE) scheme or AES Key Wrap (AES-		the hybrid public-key encryption (HPKE) scheme or AES Key Wrap (AES-
	KW) with a pre-shared key-encryption key. These choices reduce the		KW) with a pre-shared key-encryption key. These choices reduce the
	number of possible key establishment options for interoperability of		number of possible key establishment options and thereby help
	different SUIT manifest implementations. The document also offers a		increase interoperability between different SUIT manifest parser
	number of examples for developers.		implementations.
			The document also contains a number of examples for developers.
	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 document assumes familiarity with the IETF SUIT manifest		This document assumes familiarity with the IETF SUIT manifest
	[I-D.ietf-suit-manifest] and the SUIT architecture [RFC9019].		[I-D.ietf-suit-manifest] and the SUIT architecture [RFC9019].
	The terms "recipient" and "firmware consumer" are used		In context of encryption, the terms "recipient" and "firmware
	interchangeably.		consumer" are used interchangeably.
	Additionally, the following abbreviations are used in this document:		Additionally, the following abbreviations are used in this document:
	- Key Wrap (KW), defined in RFC 3394 [RFC3394] for use with AES.		- Key Wrap (KW), defined in RFC 3394 [RFC3394] for use with AES.
	- Key-encryption key / key-encrypting key (KEK), a term defined in		- Key-encryption key / key-encrypting key (KEK), a term defined in
	RFC 4949 [RFC4949].		RFC 4949 [RFC4949].
	- Content-encryption key (CEK), a term defined in RFC 2630		- Content-encryption key (CEK), a term defined in RFC 2630
	[RFC2630].		[RFC2630].
	- Hybrid Public Key Encryption (HPKE), defined in		- Hybrid Public Key Encryption (HPKE), defined in
	[I-D.irtf-cfrg-hpke].		[I-D.irtf-cfrg-hpke].
	3. AES Key Wrap		3. Architecture
			Figure 1 in [RFC9019] shows the architecture for distributing
			firmware images and manifests from the author to the firmware
			consumer. It does, however, not detail the use of encrypted firmware
			images. Figure 1 therefore focuses on those aspects. The firmware
			server and the device management infrastructure is represented by the
			distribution system, which is aware of the individual devices a
			firmware update has to be delivered to.
			Firmware encryption requires the party doing the encryption to know
			either the KEK (in case of AES-KW) or the public key of the recipient
			(in case of HPKE). The firmware author may have knowledge about all
			the devices but in most cases this will not be likely. Hence, it is
			the responsibility of the distribution system to perform the firmware
			encryption.
			Since including the COSE_Encrypt structure in the manifest
			invalidates a a digital signature or a MAC added by the author, this
			structure needs to be added to the envelope by the distribution
			system. This approach offers flexiblity when the number of devices
			that need to receive encrypted firmware images changes dynamically or
			when the updates to KEKs or recipient public keys are necessary. As
			a downside, the author needs to trust the distribution system with
			performing the encryption of the plaintext firmware image.
			+----------+
			| |
			| Author |
			| |
			+----------+ +----------+
			| | |
			| Device |---+ | Firmware +
			| | | | Manifest
			+----------+ | |
			| |
			| +--------------+
			+----------+ | | |
			| | | Firmware + Manifest | Distribution |
			| Device |---+------------------------| System |
			| | | | |
			+----------+ | +--------------+
			|
			|
			+----------+ |
			| | |
			| Device +---+
			| |
			+----------+
			Figure 1: Firmware Encryption Architecture.
			4. AES Key Wrap
	The AES Key Wrap (AES-KW) algorithm is described in RFC 3394		The AES Key Wrap (AES-KW) algorithm is described in RFC 3394
	[RFC3394], and it can be used to encrypt a randomly generated		[RFC3394], and it can be used to encrypt a randomly generated
	content-encryption key (CEK) with a pre-shared key-encryption key		content-encryption key (CEK) with a pre-shared key-encryption key
	(KEK). The COSE conventions for using AES-KW are specified in		(KEK). The COSE conventions for using AES-KW are specified in
	Section 12.2.1 of [RFC8152]. The encrypted CEK is carried in the		Section 12.2.1 of [RFC8152]. The encrypted CEK is carried in the
	COSE_recipient structure alongside the information needed for AES-KW.		COSE_recipient structure alongside the information needed for AES-KW.
	The COSE_recipient structure, which is a substructure of the		The COSE_recipient structure, which is a substructure of the
	COSE_Encrypt, contains the CEK encrypted by the KEK. When the		COSE_Encrypt structure, contains the CEK encrypted by the KEK.
	firmware image is encrypted for use by multiple recipients, the
	COSE_recipient structure will contain one encrypted CEK if all of the
	authorized recipients have access to the KEK.
	However, the COSE_recipient structure can contain the same CEK		When the firmware image is encrypted for use by multiple recipients,
	encrypted with many different KEKs if needed to reach all of the		there are three options:
	authorized recipients.
			- If all of authorized recipients have access to the KEK, a single
			COSE_recipient structure contains the encrypted CEK.
			- If recipients have different KEKs, then the COSE_recipient
			structure may contain the same CEK encrypted with many different
			KEKs. The benefit of this approach is that the firmware image is
			encrypted only once with the CEK while the authorized recipients
			still need to use their individual KEKs to obtain the plaintext.
			- The last option is to use different CEKs encrypted with KEKs of
			the authorized recipients. This is appropriate when no benefits
			can be gained from encrypting and transmitting firmware images
			only once. For example, firmware images may contain information
			unique to a device instance.
	Note that the AES-KW algorithm, as defined in Section 2.2.3.1 of		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-		[RFC3394], does not have public parameters that vary on a per-
	invocation basis. Hence, the protected structure in the		invocation basis. Hence, the protected structure in the
	COSE_recipient is a byte string of zero length.		COSE_recipient is a byte string of zero length.
	The COSE_Encrypt conveys information for encrypting the firmware		The COSE_Encrypt conveys information for encrypting the firmware
	image, which includes information like the algorithm and the IV, even		image, which includes information like the algorithm and the IV, even
	though the firmware image is not embedded in the		though the firmware image is not embedded in the
	COSE_Encrypt.ciphertext itself since it conveyed as detached content.		COSE_Encrypt.ciphertext itself since it conveyed as detached content.
	The CDDL for the COSE_Encrypt_Tagged structure is shown in Figure 1.		The CDDL for the COSE_Encrypt_Tagged structure is shown in Figure 2.
	COSE_Encrypt_Tagged = #6.96(COSE_Encrypt)		COSE_Encrypt_Tagged = #6.96(COSE_Encrypt)
	SUIT_Encryption_Info = COSE_Encrypt_Tagged		SUIT_Encryption_Info = COSE_Encrypt_Tagged
	COSE_Encrypt = [		COSE_Encrypt = [
	protected : bstr .cbor outer_header_map_protected,		protected : bstr .cbor outer_header_map_protected,
	unprotected : outer_header_map_unprotected,		unprotected : outer_header_map_unprotected,
	ciphertext : null, ; because of detached ciphertext		ciphertext : null, ; because of detached ciphertext
	recipients : [ + COSE_recipient ]		recipients : [ + COSE_recipient ]
	skipping to change at page 5, line 41 ¶		skipping to change at page 7, line 41 ¶
	ciphertext : bstr ; CEK encrypted with KEK		ciphertext : bstr ; CEK encrypted with KEK
	]		]
	recipient_header_map =		recipient_header_map =
	{		{
	1 => int, ; algorithm identifier		1 => int, ; algorithm identifier
	4 => bstr, ; key identifier		4 => bstr, ; key identifier
	* label =values ; extension point		* label =values ; extension point
	}		}
	Figure 1: CDDL for AES Key Wrap-based Firmware Encryption		Figure 2: CDDL for AES Key Wrap-based Firmware Encryption
	The COSE specification requires a consistent byte stream for the		The COSE specification requires a consistent byte stream for the
	authenticated data structure to be created, which is defined as shown		authenticated data structure to be created, which is shown in
	in Figure 2.		Figure 3.
	Enc_structure = [		Enc_structure = [
	context : "Encrypt",		context : "Encrypt",
	protected : empty_or_serialized_map,		protected : empty_or_serialized_map,
	external_aad : bstr		external_aad : bstr
	]		]
	Figure 2: CDDL for Enc_structure Data Structure		Figure 3: CDDL for Enc_structure Data Structure
	As it can be seen in the CDDL in Figure 1, there are two protected		As shown in Figure 2, there are two protected fields: one protected
	fields and the 'protected' field in the Enc_structure, see Figure 2,		field in the COSE_Encrypt structure and a second one in the
	refers to the outer protected field, not the protected field of the		COSE_recipient structure. The 'protected' field in the
	COSE_recipient structure.		Enc_structure, see Figure 3, refers to the content of the protected
			field from the COSE_Encrypt structure, not to the protected field of
			the COSE_recipient structure.
	The value of the external_aad is set to null.		The value of the external_aad is set to null.
	The following example illustrates the use of the AES-KW algorithm		The following example illustrates the use of the AES-KW algorithm
	with AES-128.		with AES-128.
	We use the following parameters in this example:		We use the following parameters in this example:
	- IV: 0x26, 0x68, 0x23, 0x06, 0xd4, 0xfb, 0x28, 0xca, 0x01, 0xb4,		- IV: 0x26, 0x68, 0x23, 0x06, 0xd4, 0xfb, 0x28, 0xca, 0x01, 0xb4,
	0x3b, 0x80		0x3b, 0x80
	skipping to change at page 6, line 44 ¶		skipping to change at page 8, line 46 ¶
	- Firmware (hex):		- Firmware (hex):
	546869732069732061207265616C206669726D7761726520696D6167652E		546869732069732061207265616C206669726D7761726520696D6167652E
	The COSE_Encrypt structure in hex format is (with a line break		The COSE_Encrypt structure in hex format is (with a line break
	inserted):		inserted):
	D8608443A10101A1054C26682306D4FB28CA01B43B80F68340A2012204456B69642D		D8608443A10101A1054C26682306D4FB28CA01B43B80F68340A2012204456B69642D
	315818AF09622B4F40F17930129D18D0CEA46F159C49E7F68B644D		315818AF09622B4F40F17930129D18D0CEA46F159C49E7F68B644D
	The resulting COSE_Encrypt structure in a dignostic format is shown		The resulting COSE_Encrypt structure in a dignostic format is shown
	in Figure 3.		in Figure 4.
	96(		96(
	[		[
	// protected field with alg=AES-GCM-128		// protected field with alg=AES-GCM-128
	h'A10101',		h'A10101',
	{		{
	// unprotected field with iv		// unprotected field with iv
	5: h'26682306D4FB28CA01B43B80'		5: h'26682306D4FB28CA01B43B80'
	},		},
	// null because of detached ciphertext		// null because of detached ciphertext
	skipping to change at page 7, line 27 ¶		skipping to change at page 9, line 27 ¶
	{ // unprotected field		{ // unprotected field
	1: -3, // alg=A128KW		1: -3, // alg=A128KW
	4: h'6B69642D31' // key id		4: h'6B69642D31' // key id
	},		},
	// CEK encrypted with KEK		// CEK encrypted with KEK
	h'AF09622B4F40F17930129D18D0CEA46F159C49E7F68B644D'		h'AF09622B4F40F17930129D18D0CEA46F159C49E7F68B644D'
	]		]
	]		]
	)		)
	Figure 3: COSE_Encrypt Example for AES Key Wrap		Figure 4: COSE_Encrypt Example for AES Key Wrap
	The CEK was "4C805F1587D624ED5E0DBB7A7F7FA7EB" and the encrypted		The CEK was "4C805F1587D624ED5E0DBB7A7F7FA7EB" and the encrypted
	firmware was:		firmware was:
	A8B6E61EF17FBAD1F1BF3235B3C64C06098EA512223260		A8B6E61EF17FBAD1F1BF3235B3C64C06098EA512223260
	F9425105F67F0FB6C92248AE289A025258F06C2AD70415		F9425105F67F0FB6C92248AE289A025258F06C2AD70415
	4. Hybrid Public-Key Encryption (HPKE)		5. Hybrid Public-Key Encryption (HPKE)
	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.		given a recipient's public key.
	For use with firmware encryption the scheme works as follows: The		For use with firmware encryption the scheme works as follows: The
	firmware author uses HPKE, which internally utilizes a non-		firmware author uses HPKE, which internally utilizes a non-
	interactive ephemeral-static Diffie-Hellman exchange to derive a		interactive ephemeral-static Diffie-Hellman exchange to derive a
	shared secret, which is then used to encrypt plaintext. In the		shared secret, which is then used to encrypt plaintext.
	firmware encryption scenario, the plaintext passed to HPKE for
	encryption is a randomly generated CEK. The output of the HPKE		In the firmware encryption scenario, the plaintext passed to HPKE for
			encryption is the randomly generated CEK. The output of the HPKE
	operation is therefore the encrypted CEK along with HPKE encapsulated		operation is therefore the encrypted CEK along with HPKE encapsulated
	key (i.e. the ephemeral ECDH public key of the author). The CEK is		key (i.e. the ephemeral ECDH public key of the author). The CEK is
	then used to encrypt the firmware.		then used to encrypt the firmware.
	Only the holder of recipient's private key can decapsulate the CEK to		Only the holder of recipient's private key can decapsulate the CEK to
	decrypt the firmware. Key generation is influced by additional		decrypt the firmware. Key generation is influced by additional
	parameters, such as identity information.		parameters, such as identity information.
	This approach allows us to have all recipients to use the same CEK to		This approach allows all recipients to use the same CEK to encrypt
	encrypt the firmware image, in case there are multiple recipients, to		the firmware image, in case there are multiple recipients, to fulfill
	fulfill a requirement for the efficient distribution of firmware		a requirement for the efficient distribution of firmware images using
	images using a multicast or broadcast protocol.		a multicast or broadcast protocol.
	The CDDL for the COSE_Encrypt structure as used with HPKE is shown in		The CDDL for the COSE_Encrypt structure as used with HPKE is shown in
	Figure 4.		Figure 5.
	COSE_Encrypt_Tagged = #6.96(COSE_Encrypt)		COSE_Encrypt_Tagged = #6.96(COSE_Encrypt)
	SUIT_Encryption_Info = COSE_Encrypt_Tagged		SUIT_Encryption_Info = COSE_Encrypt_Tagged
	COSE_Encrypt = [		COSE_Encrypt = [
	protected : bstr .cbor header_map, ; must contain alg		protected : bstr .cbor header_map, ; must contain alg
	unprotected : header_map, ; must contain iv		unprotected : header_map, ; must contain iv
	ciphertext : null, ; because of detached ciphertext		ciphertext : null, ; because of detached ciphertext
	recipients : [ + COSE_recipient_outer ]		recipients : [ + COSE_recipient_outer ]
	skipping to change at page 9, line 42 ¶		skipping to change at page 11, line 42 ¶
	* label =values,		* label =values,
	}		}
	Generic_Headers = (		Generic_Headers = (
	? 1 => int, ; algorithm identifier		? 1 => int, ; algorithm identifier
	? 2 => crv, ; EC identifier		? 2 => crv, ; EC identifier
	? 4 => bstr, ; key identifier		? 4 => bstr, ; key identifier
	? 5 => bstr ; IV		? 5 => bstr ; IV
	)		)
	Figure 4: CDDL for HPKE-based COSE_Encrypt Structure		Figure 5: CDDL for HPKE-based COSE_Encrypt Structure
	The COSE_Encrypt structure in Figure 4 requires the encrypted CEK and		The COSE_Encrypt structure in Figure 5 requires the encrypted CEK and
	the ephemeral public key of the firmare author to be generated. This		the ephemeral public key of the firmare author to be generated. This
	is accomplished with the HPKE encryption function as shown in		is accomplished with the HPKE encryption function as shown in
	Figure 5.		Figure 6.
	CEK = random()		CEK = random()
	pkR = DeserializePublicKey(recipient_public_key)		pkR = DeserializePublicKey(recipient_public_key)
	info = "cose hpke" || 0x00 || COSE_KDF_Context		info = "cose hpke" || 0x00 || COSE_KDF_Context
	enc, context = SetupBaseS(pkR, info)		enc, context = SetupBaseS(pkR, info)
	ciphertext = context.Seal(null, CEK)		ciphertext = context.Seal(null, CEK)
	Figure 5		Figure 6
	Legend:		Legend:
	- The functions DeserializePublicKey(), SetupBaseS() and Seal() are		- The functions DeserializePublicKey(), SetupBaseS() and Seal() are
	defined in HPKE [I-D.irtf-cfrg-hpke].		defined in HPKE [I-D.irtf-cfrg-hpke].
	- CEK is a random byte sequence of keysize length whereby keysize		- CEK is a random byte sequence of keysize length whereby keysize
	corresponds to the size of the indicated symmetric encryption		corresponds to the size of the indicated symmetric encryption
	algorithm used for firmware encryption. For example, AES-128-GCM		algorithm used for firmware encryption. For example, AES-128-GCM
	requires a 16 byte key. The CEK would therefore be 16 bytes long.		requires a 16 byte key. The CEK would therefore be 16 bytes long.
	- 'recipient_public_key' represents the public key of the recipient.		- 'recipient_public_key' represents the public key of the recipient.
	- 'info' is a data structure described below used as input to the		- 'info' is a data structure described below used as input to the
	key derivation internal to the HPKE algorithm. In addition to the		key derivation internal to the HPKE algorithm. In addition to the
	constant prefix, the COSE_KDF_Context structure is used. The		constant prefix, the COSE_KDF_Context structure is used. The
	COSE_KDF_Context is shown in Figure 6.		COSE_KDF_Context is shown in Figure 7.
	The result of the above-described operation is the encrypted CEK		The result of the above-described operation is the encrypted CEK
	(denoted as ciphertext) and the enc - the HPKE encapsulated key (i.e.		(denoted as ciphertext) and the enc - the HPKE encapsulated key (i.e.
	the ephemeral ECDH public key of the author).		the ephemeral ECDH public key of the author).
	PartyInfo = (		PartyInfo = (
	identity : bstr,		identity : bstr,
	nonce : nil,		nonce : nil,
	other : nil		other : nil
	)		)
	skipping to change at page 10, line 50 ¶		skipping to change at page 12, line 50 ¶
	COSE_KDF_Context = [		COSE_KDF_Context = [
	AlgorithmID : int,		AlgorithmID : int,
	PartyUInfo : [ PartyInfo ],		PartyUInfo : [ PartyInfo ],
	PartyVInfo : [ PartyInfo ],		PartyVInfo : [ PartyInfo ],
	SuppPubInfo : [		SuppPubInfo : [
	keyDataLength : uint,		keyDataLength : uint,
	protected : empty_or_serialized_map		protected : empty_or_serialized_map
	],		],
	]		]
	Figure 6: COSE_KDF_Context Data Structure		Figure 7: COSE_KDF_Context Data Structure
	Notes:		Notes:
	- 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 in		signature is used). When utilizing a MAC, then the kid is found
	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 protected structure in the inner-most recipients
	array.		array.
	- 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.
	The author encrypts the firmware using the CEK with the selected		The author encrypts the firmware using the CEK with the selected
	algorithm.		algorithm.
	The recipient decrypts the received ciphertext, i.e. the encrypted		The recipient decrypts the encrypted CEK, using two input parameters:
	CEK, using two input parameters:
	- the private key skR corresponding to the public key pkR used by		- the private key skR corresponding to the public key pkR used by
	the author when creating the manifest.		the author when creating the manifest.
	- the HPKE encapsulated key (i.e. ephemeral ECDH public key) created		- the HPKE encapsulated key (i.e. ephemeral ECDH public key) created
	by the author.		by the author.
	If the HPKE operation is successful, the recipient obtains the CEK		If the HPKE operation is successful, the recipient obtains the CEK
	and can decrypt the firmware.		and can decrypt the firmware.
	Figure 7 shows the HPKE computations performed by the recipient for		Figure 8 shows the HPKE computations performed by the recipient for
	decryption.		decryption.
	info = "cose hpke" || 0x00 || COSE_KDF_Context		info = "cose hpke" || 0x00 || COSE_KDF_Context
	context = SetupBaseR(ciphertext, skR, info)		context = SetupBaseR(ciphertext, skR, info)
	CEK = context.Open(null, ciphertext)		CEK = context.Open(null, ciphertext)
	Figure 7		Figure 8
	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 8.		shown in Figure 9. It uses the following algorithm combination:
			- AES-GCM-128 for encryption of the firmware image.
			- AES-GCM-128 for encrytion of the CEK.
			- Key Encapsulation Mechanism (KEM): NIST P-256
			- Key Derivation Function (KDF): HKDF-SHA256
	96(		96(
	[		[
	// protected field with alg=AES-GCM-128		// protected field with alg=AES-GCM-128
	h'A10101',		h'A10101',
	{ // unprotected field with iv		{ // unprotected field with iv
	5: h'26682306D4FB28CA01B43B80'		5: h'26682306D4FB28CA01B43B80'
	},		},
	// null because of detached ciphertext		// null because of detached ciphertext
	null,		null,
	skipping to change at page 12, line 37 ¶		skipping to change at page 14, line 41 ¶
	// kid for recipient static ECDH public key		// kid for recipient static ECDH public key
	4: h'6B69642D31'		4: h'6B69642D31'
	},		},
	// empty ciphertext		// empty ciphertext
	null		null
	]		]
	]		]
	]		]
	)		)
	Figure 8: COSE_Encrypt Example for HPKE		Figure 9: COSE_Encrypt Example for HPKE
	5. Complete Examples		6. Complete Examples
	TBD: Add example for complete manifest here (which also includes the		TBD: Example for complete manifest here (which also includes the
	digital signature). TBD: Add multiple recipient example as well.		digital signature). TBD: Multiple recipient example as well. TBD:
	TBD: Add encryption of manifest (in addition of firmware encryption).		Encryption of manifest (in addition of firmware encryption).
	6. Security Considerations		7. Security Considerations
	The algorithms described in this document assume that the firmware		The algorithms described in this document assume that the firmware
	author		author
	- has either shared a key-encryption key (KEK) with the firmware		- has either shared a key-encryption key (KEK) with the firmware
	consumer (for use with the AES-Key Wrap scheme), or		consumer (for use with the AES-Key Wrap scheme), or
	- is in possession of the public key of the firmware consumer (for		- is in possession of the public key of the firmware consumer (for
	use with HPKE).		use with HPKE).
	Both cases require some upfront communication interaction, which is		Both cases require some upfront communication interaction, which is
	not part of the SUIT manifest. This interaction is likely provided		not part of the SUIT manifest. This interaction is likely provided
	by a IoT device management solution, as described in [RFC9019].		by an IoT device management solution, as described in [RFC9019].
	For AES-Key Wrap to provide high security it is important that the		For AES-Key Wrap to provide high security it is important that the
	KEK is of high entropy, and that implementations protect the KEK from		KEK is of high entropy, and that implementations protect the KEK from
	disclosure. Compromise of the KEK may result in the disclosure of		disclosure. Compromise of the KEK may result in the disclosure of
	all key data protected with that KEK.		all key data protected with that KEK.
	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].
	7. IANA Considerations		In some cases third party companies analyse binaries for known
			security vulnerabilities. With encrypted firmware images this type
			of analysis is prevented. Consequently, these third party companies
			either need to be given access to the plaintext binary before
			encryption or they need to become authorized recipients of the
			encrypted firmware images. In either case, it is necessary to
			explicitly consider those third parties in the software supply chain
			when such a binary analysis is desired.
			8. IANA Considerations
	This document requests IANA to create new entries in the COSE		This document requests IANA to create new entries in the COSE
	Algorithms registry established with [I-D.ietf-cose-rfc8152bis-algs].		Algorithms registry established with [I-D.ietf-cose-rfc8152bis-algs].
	+-------------+-------+---------+------------+--------+---------------+		+-------------+-------+---------+------------+--------+---------------+
	| Name | Value | KDF | Ephemeral- | Key | Description |		| Name | Value | KDF | Ephemeral- | Key | Description |
	| | | | Static | Wrap | |		| | | | Static | Wrap | |
	+-------------+-------+---------+------------+--------+---------------+		+-------------+-------+---------+------------+--------+---------------+
	| HPKE/P-256+ | TBD1 | HKDF - | yes | none | HPKE with |		| HPKE/P-256+ | TBD1 | HKDF - | yes | none | HPKE with |
	| HKDF-256 | | SHA-256 | | | ECDH-ES |		| HKDF-256 | | SHA-256 | | | ECDH-ES |
	skipping to change at page 14, line 35 ¶		skipping to change at page 16, line 35 ¶
	| X25519 + | | SHA-256 | | | ECDH-ES |		| X25519 + | | SHA-256 | | | ECDH-ES |
	| HKDF-SHA256 | | | | | (X25519) + |		| HKDF-SHA256 | | | | | (X25519) + |
	| | | | | | HKDF-256 |		| | | | | | HKDF-256 |
	+-------------+-------+---------+------------+--------+---------------+		+-------------+-------+---------+------------+--------+---------------+
	| HPKE | TBD4 | HKDF - | yes | none | HPKE with |		| HPKE | TBD4 | HKDF - | yes | none | HPKE with |
	| X448 + | | SHA-512 | | | ECDH-ES |		| X448 + | | SHA-512 | | | ECDH-ES |
	| HKDF-SHA512 | | | | | (X448) + |		| HKDF-SHA512 | | | | | (X448) + |
	| | | | | | HKDF-512 |		| | | | | | HKDF-512 |
	+-------------+-------+---------+------------+--------+---------------+		+-------------+-------+---------+------------+--------+---------------+
	8. References		9. References
	8.1. Normative References		9.1. Normative References
	[I-D.ietf-cose-rfc8152bis-algs]		[I-D.ietf-cose-rfc8152bis-algs]
	August Cellars, "CBOR Object Signing and Encryption		Schaad, J., "CBOR Object Signing and Encryption (COSE):
	(COSE): Initial Algorithms", draft-ietf-cose-rfc8152bis-		Initial Algorithms", draft-ietf-cose-rfc8152bis-algs-12
	algs-12 (work in progress), September 2020.		(work in progress), September 2020.
	[I-D.ietf-suit-manifest]		[I-D.ietf-suit-manifest]
	Arm Limited, Arm Limited, Fraunhofer SIT, and Inria, "A		Moran, B., Tschofenig, H., Birkholz, H., and K. Zandberg,
	Concise Binary Object Representation (CBOR)-based		"A Concise Binary Object Representation (CBOR)-based
	Serialization Format for the Software Updates for Internet		Serialization Format for the Software Updates for Internet
	of Things (SUIT) Manifest", draft-ietf-suit-manifest-12		of Things (SUIT) Manifest", draft-ietf-suit-manifest-12
	(work in progress), February 2021.		(work in progress), February 2021.
	[I-D.irtf-cfrg-hpke]		[I-D.irtf-cfrg-hpke]
	Cisco, Inria, Inria, and Cloudflare, "Hybrid Public Key		Barnes, R. L., Bhargavan, K., Lipp, B., and C. A. Wood,
	Encryption", draft-irtf-cfrg-hpke-08 (work in progress),		"Hybrid Public Key Encryption", draft-irtf-cfrg-hpke-08
	February 2021.		(work in progress), February 2021.
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,		Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997,		DOI 10.17487/RFC2119, March 1997,
	<https://www.rfc-editor.org/info/rfc2119>.		<https://www.rfc-editor.org/info/rfc2119>.
	[RFC3394] Schaad, J. and R. Housley, "Advanced Encryption Standard		[RFC3394] Schaad, J. and R. Housley, "Advanced Encryption Standard
	(AES) Key Wrap Algorithm", RFC 3394, DOI 10.17487/RFC3394,		(AES) Key Wrap Algorithm", RFC 3394, DOI 10.17487/RFC3394,
	September 2002, <https://www.rfc-editor.org/info/rfc3394>.		September 2002, <https://www.rfc-editor.org/info/rfc3394>.
	[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",		[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
	RFC 8152, DOI 10.17487/RFC8152, July 2017,		RFC 8152, DOI 10.17487/RFC8152, July 2017,
	<https://www.rfc-editor.org/info/rfc8152>.		<https://www.rfc-editor.org/info/rfc8152>.
	[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC		[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
	2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,		2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
	May 2017, <https://www.rfc-editor.org/info/rfc8174>.		May 2017, <https://www.rfc-editor.org/info/rfc8174>.
	8.2. Informative References		9.2. Informative References
	[I-D.ietf-suit-information-model]		[I-D.ietf-suit-information-model]
	Arm Limited, Arm Limited, and Fraunhofer SIT, "A Manifest		Moran, B., Tschofenig, H., and H. Birkholz, "A Manifest
	Information Model for Firmware Updates in IoT Devices",		Information Model for Firmware Updates in IoT Devices",
	draft-ietf-suit-information-model-11 (work in progress),		draft-ietf-suit-information-model-11 (work in progress),
	April 2021.		April 2021.
	[RFC2630] Housley, R., "Cryptographic Message Syntax", RFC 2630,		[RFC2630] Housley, R., "Cryptographic Message Syntax", RFC 2630,
	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>.
	[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",		[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
	FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,		FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
	skipping to change at page 16, line 10 ¶		skipping to change at page 18, line 10 ¶
	[RFC9019] Moran, B., Tschofenig, H., Brown, D., and M. Meriac, "A		[RFC9019] Moran, B., Tschofenig, H., Brown, D., and M. Meriac, "A
	Firmware Update Architecture for Internet of Things",		Firmware Update Architecture for Internet of Things",
	RFC 9019, DOI 10.17487/RFC9019, April 2021,		RFC 9019, DOI 10.17487/RFC9019, April 2021,
	<https://www.rfc-editor.org/info/rfc9019>.		<https://www.rfc-editor.org/info/rfc9019>.
	Appendix A. Acknowledgements		Appendix A. Acknowledgements
	We would like to thank Henk Birkholz for his feedback on the CDDL		We would like to thank Henk Birkholz for his feedback on the CDDL
	description in this document. Additionally, we would like to thank		description in this document. Additionally, we would like to thank
	Michael Richardson and Carsten Bormann for their review feedback.		Michael Richardson and Carsten Bormann for their review feedback.
			Finally, we would like to thank Dick Brooks for making us aware of
			the challenges firmware encryption imposes on binary analysis.
	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
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