| Internet-Draft | Firmware Encryption | July 2021 |
| Tschofenig, et al. | Expires 13 January 2022 | [Page] |
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.¶
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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.¶
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 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 and thereby help increase interoperability between different SUIT manifest parser implementations.¶
The document also contains a number of examples for developers.¶
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].¶
In context of encryption, the terms "recipient" and "firmware consumer" are used interchangeably.¶
Additionally, the following abbreviations are used in this document:¶
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 +---+
| |
+----------+
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 structure, contains the CEK encrypted by the KEK.¶
When the firmware image is encrypted for use by multiple recipients, there are three options:¶
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 2.¶
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
}
The COSE specification requires a consistent byte stream for the authenticated data structure to be created, which is shown in Figure 3.¶
Enc_structure = [
context : "Encrypt",
protected : empty_or_serialized_map,
external_aad : bstr
]
As shown in Figure 2, there are two protected fields: one protected field in the COSE_Encrypt structure and a second one in the COSE_recipient structure. The 'protected' field in the 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 following example illustrates the use of the AES-KW algorithm with AES-128.¶
We use the following parameters in this example:¶
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 4.¶
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'
]
]
)
The CEK was "4C805F1587D624ED5E0DBB7A7F7FA7EB" and the encrypted firmware was:¶
A8B6E61EF17FBAD1F1BF3235B3C64C06098EA512223260 F9425105F67F0FB6C92248AE289A025258F06C2AD70415¶
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 the 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.¶
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 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 5.¶
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
)
The COSE_Encrypt structure in Figure 5 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 6.¶
CEK = random()
pkR = DeserializePublicKey(recipient_public_key)
info = "cose hpke" || 0x00 || COSE_KDF_Context
enc, context = SetupBaseS(pkR, info)
ciphertext = context.Seal(null, CEK)
Legend:¶
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
],
]
Notes:¶
The author encrypts the firmware using the CEK with the selected algorithm.¶
The recipient decrypts the encrypted CEK, using two input parameters:¶
If the HPKE operation is successful, the recipient obtains the CEK and can decrypt the firmware.¶
Figure 8 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)
An example of the COSE_Encrypt structure using the HPKE scheme is shown in Figure 9. It uses the following algorithm combination:¶
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
]
]
]
)
TBD: Example for complete manifest here (which also includes the digital signature). TBD: Multiple recipient example as well. TBD: Encryption of manifest (in addition of firmware encryption).¶
The algorithms described in this document assume that the firmware author¶
Both cases require some upfront communication interaction, which is not part of the SUIT manifest. This interaction is likely provided by an 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].¶
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.¶
This document requests IANA to create new entries in the COSE Algorithms registry established with [I-D.ietf-cose-rfc8152bis-algs].¶
+-------------+-------+---------+------------+--------+---------------+ | 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 | +-------------+-------+---------+------------+--------+---------------+¶
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. Finally, we would like to thank Dick Brooks for making us aware of the challenges firmware encryption imposes on binary analysis.¶