Internet DRAFT - draft-tschofenig-suit-firmware-encryption
draft-tschofenig-suit-firmware-encryption
SUIT H. Tschofenig
Internet-Draft Arm Limited
Intended status: Standards Track R. Housley
Expires: December 2, 2021 Vigil Security
B. Moran
Arm Limited
May 31, 2021
Firmware Encryption with SUIT Manifests
draft-tschofenig-suit-firmware-encryption-01
Abstract
This document specifies a firmware update mechanism where the
firmware image is encrypted. This mechanism uses the IETF SUIT
manifest with key establishment provided by the hybrid public-key
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
firmware encryption.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
3. AES Key Wrap . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Hybrid Public-Key Encryption (HPKE) . . . . . . . . . . . . . 7
5. Complete Examples . . . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. Normative References . . . . . . . . . . . . . . . . . . 14
8.2. Informative References . . . . . . . . . . . . . . . . . 15
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
Vulnerabilities with Internet of Things (IoT) devices have raised the
need for a reliable and secure firmware update mechanism that is also
suitable for constrained devices. To protect firmware images the
SUIT manifest format was developed [I-D.ietf-suit-manifest]. The
SUIT manifest provides a bundle of metadata about the firmware for an
IoT device, where to find the firmware image, and the devices to
which it applies.
The SUIT information model [I-D.ietf-suit-information-model] details
the information that has to be offered by the SUIT manifest format.
In addition to offering protection against modification, which is
provided by a digital signature or a message authentication code, the
firmware image may also be afforded confidentiality using encryption.
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Encryption prevents third parties, including attackers, from gaining
access to the firmware image. For example, return-oriented
programming (ROP) requires intimate knowledge of the target firmware
and that encryption makes this approach much more difficult to
exploit. The SUIT manifest provides the data needed for authorized
recipients of the firmware image to decrypt it.
A symmetric cryptographic key is established for encryption and
decryption, and that key can be applied to a SUIT manifest, firmware
images, or personalization data, depending on the encryption choices
of the firmware author. This symmetric key can be established using
a variety of mechanisms; this document defines two approaches for use
with the IETF SUIT manifest. Key establishment can be provided by
the hybrid public-key encryption (HPKE) scheme or AES Key Wrap (AES-
KW) with a pre-shared key-encryption key. These choices reduce the
number of possible key establishment options for interoperability of
different SUIT manifest implementations. The document also offers a
number of examples for developers.
2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This document assumes familiarity with the IETF SUIT manifest
[I-D.ietf-suit-manifest] and the SUIT architecture [RFC9019].
The terms "recipient" and "firmware consumer" are used
interchangeably.
Additionally, the following abbreviations are used in this document:
- Key Wrap (KW), defined in RFC 3394 [RFC3394] for use with AES.
- Key-encryption key / key-encrypting key (KEK), a term defined in
RFC 1949 [RFC1949].
- Content-encryption key (CEK), a term defined in RFC 2630
[RFC2630].
- Hybrid Public Key Encryption (HPKE), defined in
[I-D.irtf-cfrg-hpke].
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3. AES Key Wrap
The AES Key Wrap (AES-KW) algorithm is described in RFC 3394
[RFC3394], and it can be used to encrypt a randomly generated
content-encryption key (CEK) with a pre-shared key-encryption key
(KEK). The COSE conventions for using AES-KW are specified in
Section 12.2.1 of [RFC8152]. The encrypted CEK is carried in the
COSE_recipient structure alongside the information needed for AES-KW.
The COSE_recipient structure, which is a substructure of the
COSE_Encrypt, contains the CEK encrypted by the KEK. When the
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
encrypted with many different KEKs if needed to reach all of the
authorized recipients.
Note that the AES-KW algorithm, as defined in Section 2.2.3.1 of
[RFC3394], does not have public parameters that vary on a per-
invocation basis. Hence, the protected structure in the
COSE_recipient is a byte string of zero length.
The COSE_Encrypt conveys information for encrypting the firmware
image, which includes information like the algorithm and the IV, even
though the firmware image is not embedded in the
COSE_Encrypt.ciphertext itself since it conveyed as detached content.
The CDDL for the COSE_Encrypt_Tagged structure is shown in Figure 1.
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COSE_Encrypt_Tagged = #6.96(COSE_Encrypt)
SUIT_Encryption_Info = COSE_Encrypt_Tagged
COSE_Encrypt = [
protected : bstr .cbor outer_header_map_protected,
unprotected : outer_header_map_unprotected,
ciphertext : null, ; because of detached ciphertext
recipients : [ + COSE_recipient ]
]
outer_header_map_protected =
{
1 => int, ; algorithm identifier
* label =values ; extension point
}
outer_header_map_unprotected =
{
5 => bstr, ; IV
* label =values ; extension point
}
COSE_recipient = [
protected : bstr .size 0,
unprotected : recipient_header_map,
ciphertext : bstr ; CEK encrypted with KEK
]
recipient_header_map =
{
1 => int, ; algorithm identifier
4 => bstr, ; key identifier
* label =values ; extension point
}
Figure 1: CDDL for AES Key Wrap-based Firmware Encryption
The COSE specification requires a consistent byte stream for the
authenticated data structure to be created, which is defined as shown
in Figure 2.
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Enc_structure = [
context : "Encrypt",
protected : empty_or_serialized_map,
external_aad : bstr
]
Figure 2: CDDL for Enc_structure Data Structure
As it can be seen in the CDDL in Figure 1, there are two protected
fields and the 'protected' field in the Enc_structure, see Figure 2,
refers to the outer protected field, not the protected field of the
COSE_recipient structure.
The value of the external_aad is set to null.
The following example illustrates the use of the AES-KW algorithm
with AES-128.
We use the following parameters in this example:
- IV: 0x26, 0x68, 0x23, 0x06, 0xd4, 0xfb, 0x28, 0xca, 0x01, 0xb4,
0x3b, 0x80
- KEK: "aaaaaaaaaaaaaaaa"
- KID: "kid-1"
- Plaintext Firmware: "This is a real firmware image."
- Firmware (hex):
546869732069732061207265616C206669726D7761726520696D6167652E
The COSE_Encrypt structure in hex format is (with a line break
inserted):
D8608443A10101A1054C26682306D4FB28CA01B43B80F68340A2012204456B69642D
315818AF09622B4F40F17930129D18D0CEA46F159C49E7F68B644D
The resulting COSE_Encrypt structure in a dignostic format is shown
in Figure 3.
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96(
[
// protected field with alg=AES-GCM-128
h'A10101',
{
// unprotected field with iv
5: h'26682306D4FB28CA01B43B80'
},
// null because of detached ciphertext
null,
[ // recipients array
h'', // protected field
{ // unprotected field
1: -3, // alg=A128KW
4: h'6B69642D31' // key id
},
// CEK encrypted with KEK
h'AF09622B4F40F17930129D18D0CEA46F159C49E7F68B644D'
]
]
)
Figure 3: COSE_Encrypt Example for AES Key Wrap
The CEK was "4C805F1587D624ED5E0DBB7A7F7FA7EB" and the encrypted
firmware was:
A8B6E61EF17FBAD1F1BF3235B3C64C06098EA512223260
F9425105F67F0FB6C92248AE289A025258F06C2AD70415
4. Hybrid Public-Key Encryption (HPKE)
Hybrid public-key encryption (HPKE) [I-D.irtf-cfrg-hpke] is a scheme
that provides public key encryption of arbitrary-sized plaintexts
given a recipient's public key.
For use with firmware encryption the scheme works as follows: The
firmware author uses HPKE, which internally utilizes a non-
interactive ephemeral-static Diffie-Hellman exchange to derive a
shared secret, which is then used to encrypt plaintext. In the
firmware encryption scenario, the plaintext passed to HPKE for
encryption is a randomly generated CEK. The output of the HPKE
operation is therefore the encrypted CEK along with HPKE encapsulated
key (i.e. the ephemeral ECDH public key of the author). The CEK is
then used to encrypt the firmware.
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Only the holder of recipient's private key can decapsulate the CEK to
decrypt the firmware. Key generation is influced by additional
parameters, such as identity information.
This approach allows us to have all recipients to use the same CEK to
encrypt the firmware image, in case there are multiple recipients, to
fulfill a requirement for the efficient distribution of firmware
images using a multicast or broadcast protocol.
The CDDL for the COSE_Encrypt structure as used with HPKE is shown in
Figure 4.
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COSE_Encrypt_Tagged = #6.96(COSE_Encrypt)
SUIT_Encryption_Info = COSE_Encrypt_Tagged
COSE_Encrypt = [
protected : bstr .cbor header_map, ; must contain alg
unprotected : header_map, ; must contain iv
ciphertext : null, ; because of detached ciphertext
recipients : [ + COSE_recipient_outer ]
]
COSE_recipient_outer = [
protected : bstr .size 0,
unprotected : header_map, ; must contain alg
ciphertext : bstr ; CEK encrypted based on HPKE algo
recipients : [ + COSE_recipient_inner ]
]
COSE_recipient_inner = [
protected : bstr .cbor header_map, ; must contain alg
unprotected : header_map, ; must contain kid,
ciphertext : bstr ; CEK encrypted based on HPKE algo
recipients : null
]
header_map = {
Generic_Headers,
* label =values,
}
Generic_Headers = (
? 1 => int, ; algorithm identifier
? 2 => crv, ; EC identifier
? 4 => bstr, ; key identifier
? 5 => bstr ; IV
)
Figure 4: CDDL for HPKE-based COSE_Encrypt Structure
The COSE_Encrypt structure in Figure 4 requires the encrypted CEK and
the ephemeral public key of the firmare author to be generated. This
is accomplished with the HPKE encryption function as shown in
Figure 5.
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CEK = random()
pkR = DeserializePublicKey(recipient_public_key)
info = "cose hpke" || 0x00 || COSE_KDF_Context
enc, context = SetupBaseS(pkR, info)
ciphertext = context.Seal(null, CEK)
Figure 5
Legend:
- The functions DeserializePublicKey(), SetupBaseS() and Seal() are
defined in HPKE [I-D.irtf-cfrg-hpke].
- CEK is a random byte sequence of keysize length whereby keysize
corresponds to the size of the indicated symmetric encryption
algorithm used for firmware encryption. For example, AES-128-GCM
requires a 16 byte key. The CEK would therefore be 16 bytes long.
- 'recipient_public_key' represents the public key of the recipient.
- 'info' is a data structure described below used as input to the
key derivation internal to the HPKE algorithm. In addition to the
constant prefix, the COSE_KDF_Context structure is used. The
COSE_KDF_Context is shown in Figure 6.
The result of the above-described operation is the encrypted CEK
(denoted as ciphertext) and the enc - the HPKE encapsulated key (i.e.
the ephemeral ECDH public key of the author).
PartyInfo = (
identity : bstr,
nonce : nil,
other : nil
)
COSE_KDF_Context = [
AlgorithmID : int,
PartyUInfo : [ PartyInfo ],
PartyVInfo : [ PartyInfo ],
SuppPubInfo : [
keyDataLength : uint,
protected : empty_or_serialized_map
],
]
Figure 6: COSE_KDF_Context Data Structure
Notes:
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- PartyUInfo.identity corresponds to the kid found in the
COSE_Sign_Tagged or COSE_Sign1_Tagged structure (when a digital
signature is used. When utilizing a MAC, then the kid is found in
the COSE_Mac_Tagged or COSE_Mac0_Tagged structure.
- PartyVInfo.identity corresponds to the kid used for the respective
recipient from the inner-most recipients array.
- The value in the AlgorithmID field corresponds to the alg
parameter in the protected structure in the inner-most recipients
array.
- keyDataLength is set to the number of bits of the desired output
value.
- protected refers to the protected structure of the inner-most
array.
The author encrypts the firmware using the CEK with the selected
algorithm.
The recipient decrypts the received ciphertext, i.e. the encrypted
CEK, using two input parameters:
- the private key skR corresponding to the public key pkR used by
the author when creating the manifest.
- the HPKE encapsulated key (i.e. ephemeral ECDH public key) created
by the author.
If the HPKE operation is successful, the recipient obtains the CEK
and can decrypt the firmware.
Figure 7 shows the HPKE computations performed by the recipient for
decryption.
info = "cose hpke" || 0x00 || COSE_KDF_Context
context = SetupBaseR(ciphertext, skR, info)
CEK = context.Open(null, ciphertext)
Figure 7
An example of the COSE_Encrypt structure using the HPKE scheme is
shown in Figure 8.
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96(
[
// protected field with alg=AES-GCM-128
h'A10101',
{ // unprotected field with iv
5: h'26682306D4FB28CA01B43B80'
},
// null because of detached ciphertext
null,
[ // COSE_recipient_outer
h'', // empty protected field
{ // unprotected field with ...
1: 1 // alg=A128GCM
},
// Encrypted CEK
h'FA55A50CF110908DA6443149F2C2062011A7D8333A72721A',
[ // COSE_recipient_inner
// protected field with alg HPKE/P-256+HKDF-256 (new)
h'A1013818',
{ // unprotected field with ...
// HPKE encapsulated key
-1: h'A4010220012158205F...979D51687187510C445',
// kid for recipient static ECDH public key
4: h'6B69642D31'
},
// empty ciphertext
null
]
]
]
)
Figure 8: COSE_Encrypt Example for HPKE
5. Complete Examples
TBD: Add example for complete manifest here (which also includes the
digital signature). TBD: Add multiple recipient example as well.
TBD: Add encryption of manifest (in addition of firmware encryption).
6. Security Considerations
The algorithms described in this document assume that the firmware
author
- has either shared a key-encryption key (KEK) with the firmware
consumer (for use with the AES-Key Wrap scheme), or
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- is in possession of the public key of the firmware consumer (for
use with HPKE).
Both cases require some upfront communication interaction, which is
not part of the SUIT manifest. This interaction is likely be
provided by a IoT device management solution, as described in
[RFC9019].
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
disclosure. Compromise of the KEK may result in the disclosure of
all key data protected with that KEK.
Since the CEK is randomly generated, it must be ensured that the
guidelines for random number generations are followed, see [RFC8937].
7. IANA Considerations
This document requests IANA to create new entries in the COSE
Algorithms registry established with [I-D.ietf-cose-rfc8152bis-algs].
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+-------------+-------+---------+------------+--------+---------------+
| 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 |
+-------------+-------+---------+------------+--------+---------------+
8. References
8.1. Normative References
[I-D.ietf-cose-rfc8152bis-algs]
August Cellars, "CBOR Object Signing and Encryption
(COSE): Initial Algorithms", draft-ietf-cose-rfc8152bis-
algs-12 (work in progress), September 2020.
[I-D.ietf-suit-manifest]
Arm Limited, Arm Limited, Fraunhofer SIT, and Inria, "A
Concise Binary Object Representation (CBOR)-based
Serialization Format for the Software Updates for Internet
of Things (SUIT) Manifest", draft-ietf-suit-manifest-12
(work in progress), February 2021.
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[I-D.irtf-cfrg-hpke]
Cisco, Inria, Inria, and Cloudflare, "Hybrid Public Key
Encryption", draft-irtf-cfrg-hpke-08 (work in progress),
February 2021.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3394] Schaad, J. and R. Housley, "Advanced Encryption Standard
(AES) Key Wrap Algorithm", RFC 3394, DOI 10.17487/RFC3394,
September 2002, <https://www.rfc-editor.org/info/rfc3394>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
8.2. Informative References
[I-D.ietf-suit-information-model]
Arm Limited, Arm Limited, and Fraunhofer SIT, "A Manifest
Information Model for Firmware Updates in IoT Devices",
draft-ietf-suit-information-model-11 (work in progress),
April 2021.
[RFC1949] Ballardie, A., "Scalable Multicast Key Distribution",
RFC 1949, DOI 10.17487/RFC1949, May 1996,
<https://www.rfc-editor.org/info/rfc1949>.
[RFC2630] Housley, R., "Cryptographic Message Syntax", RFC 2630,
DOI 10.17487/RFC2630, June 1999,
<https://www.rfc-editor.org/info/rfc2630>.
[RFC8937] Cremers, C., Garratt, L., Smyshlyaev, S., Sullivan, N.,
and C. Wood, "Randomness Improvements for Security
Protocols", RFC 8937, DOI 10.17487/RFC8937, October 2020,
<https://www.rfc-editor.org/info/rfc8937>.
[RFC9019] Moran, B., Tschofenig, H., Brown, D., and M. Meriac, "A
Firmware Update Architecture for Internet of Things",
RFC 9019, DOI 10.17487/RFC9019, April 2021,
<https://www.rfc-editor.org/info/rfc9019>.
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Appendix A. Acknowledgements
We would like to thank Henk Birkholz for his feedback on the CDDL
description in this document. Additionally, we would like to thank
Michael Richardson and Carsten Bormann for their review feedback.
Authors' Addresses
Hannes Tschofenig
Arm Limited
EMail: hannes.tschofenig@arm.com
Russ Housley
Vigil Security, LLC
EMail: housley@vigilsec.com
Brendan Moran
Arm Limited
EMail: Brendan.Moran@arm.com
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