| Internet-Draft | Software Encryption | September 2022 |
| Tschofenig, et al. | Expires 24 March 2023 | [Page] |
This document specifies techniques for encrypting software, firmware and personalization data by utilizing the IETF SUIT manifest. Key establishment is provided by hybrid public-key encryption (HPKE) and AES Key Wrap (AES-KW). HPKE uses public key cryptography while AES-KW uses a pre-shared key-encryption key. Encryption of the plaintext is accomplished with conventional symmetric key cryptography.¶
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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 [RFC9124] 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 binary. Hackers typically need intimate knowledge of the target firmware to mount their attacks. For example, return-oriented programming (ROP) requires access to the binary and encryption makes it much more difficult to write exploits.¶
The SUIT manifest provides the data needed for authorized recipients of the firmware image to decrypt it. The firmware image is encrypted using a symmetric key. This 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.¶
A symmetric key can be established using a variety of mechanisms; this document defines two approaches for use with the IETF SUIT manifest, namely:¶
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.¶
The original motivating use case of this document was to encrypt firmware. However, SUIT manifests may require other payloads than firmware images to experience confidentiality protection using encryption, for example software, personalization data, configuration data, or machine learning models. While the term firmware is used throughout the document, plaintext other than firmware images may get encrypted using the described mechanism. Hence, the terms firmware (image), software and plaintext are used interchangeably.¶
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], the SUIT information model [RFC9124] and the SUIT architecture [RFC9019].¶
The terms sender and recipient are defined in [RFC9180] and have the following meaning:¶
Additionally, the following abbreviations are used in this document:¶
[RFC9019] describes the architecture for distributing firmware images and manifests from the author to the firmware consumer. It does not, however, detail the use of encrypted firmware images.¶
This document enhances the SUIT architecture to include firmware encryption. Figure 1 shows the distribution system, which represents the firmware server and the device management infrastructure. The distribution system is aware of the individual devices to which a firmware update has to be delivered.¶
+----------+
| |
| Author |
| |
+----------+ +----------+
| Device |---+ |
|(Firmware | | | Firmware +
| Consumer)| | | Manifest
+----------+ | |
| |
| +--------------+
| | |
+----------+ | Firmware + Manifest | Distribution |
| Device |---+------------------------| System |
|(Firmware | | | |
| Consumer)| | | |
+----------+ | +--------------+
|
|
+----------+ |
| Device +---+
|(Firmware |
| Consumer)|
+----------+
Firmware encryption requires the sender to know the firmware consumers and the respective credentials used by the key distribution mechanism. For AES-KW the KEK needs to be known and, in case of HPKE, the sender needs to be in possession of the public key of the recipient.¶
The firmware author may have knowledge about all devices that need to receive an encrypted firmware image but in most cases this is not likely. The distribution system certainly has the knowledge about the recipients to perform firmware encryption.¶
To offer confidentiality protection for firmware images two deployment variants need to be supported:¶
Irrespectively of the two variants, the key distribution data (in the form of the COSE_Encrypt structure) is included in the SUIT envelope rather than in the SUIT manifest since the manifest will be digitally signed (or MACed) by the firmware author.¶
Since the SUIT envelope is not protected cryptographically an adversary could modify the COSE_Encrypt structure. For example, if the attacker alters the key distribution data then a recipient will decrypt the firmware image with an incorrect key. This will lead to expending energy and flash cycles until the failure is detected. To mitigate this attack, the optional suit-cek-verification parameter is added to the manifest. Since the manifest is protected by a digital signature (or a MAC), an adversary cannot successfully modify this value. This parameter allows the recipient to verify whether the CEK has successfully been derived.¶
Details about the changes to the envelope and the manifest can be found in the next section.¶
This specification introduces two extensions to the SUIT envelope and the manifest structure, as motivated in Section 3.¶
The SUIT envelope is enhanced with a key exchange payload, which is carried inside the suit-protection-wrappers parameter, see Figure 2. One or multiple SUIT_Encryption_Info payload(s) are carried within the suit-protection-wrappers parameter. The content of the SUIT_Encryption_Info payload is explained in Section 5 (for AES-KW) and in Section 6 (for HPKE). When the encryption capability is used, the suit-protection-wrappers parameter MUST be included in the envelope.¶
SUIT_Envelope_Tagged = #6.107(SUIT_Envelope)
SUIT_Envelope = {
suit-authentication-wrapper => bstr .cbor SUIT_Authentication,
suit-manifest => bstr .cbor SUIT_Manifest,
SUIT_Severable_Manifest_Members,
suit-protection-wrappers => bstr .cbor {
*(int/str) => [+ SUIT_Encryption_Info]
}
* SUIT_Integrated_Payload,
* SUIT_Integrated_Dependency,
* $$SUIT_Envelope_Extensions,
* (int => bstr)
}
The manifest is extended with a CEK verification parameter (called suit-cek-verification), see Figure 3. This parameter is optional and is utilized in environments where battery exhaustion attacks are a concern. Details about the CEK verification can be found in Section 7.¶
SUIT_Manifest = {
suit-manifest-version => 1,
suit-manifest-sequence-number => uint,
suit-common => bstr .cbor SUIT_Common,
? suit-reference-uri => tstr,
SUIT_Severable_Members_Choice,
SUIT_Unseverable_Members,
* $$SUIT_Manifest_Extensions,
}
SUIT_Parameters //= (suit-parameter-cek-verification => bstr)
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. We use the following notation KEK(R1,S) is the KEK shared between recipient R1 and the sender S. Likewise, CEK(R1,S) is shared between R1 and S. If a single CEK or a single KEK is shared with all authorized recipients R by a given sender S in a certain context then we use CEK(,S) or KEK(,S), respectively. The notation ENC(plaintext, key) refers to the encryption of plaintext with a given key.¶
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 4.¶
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 5.¶
Enc_structure = [
context : "Encrypt",
protected : empty_or_serialized_map,
external_aad : bstr
]
As shown in Figure 4, 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 5, refers to the content of the protected field from the COSE_Encrypt structure.¶
The value of the external_aad MUST be 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 diagnostic format is shown in Figure 6.¶
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, in hex format, was "4C805F1587D624ED5E0DBB7A7F7FA7EB" and the encrypted firmware (with a line feed added) was:¶
A8B6E61EF17FBAD1F1BF3235B3C64C06098EA512223260 F9425105F67F0FB6C92248AE289A025258F06C2AD70415¶
Hybrid public-key encryption (HPKE) [RFC9180] 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: HPKE, which internally utilizes a non-interactive ephemeral-static Diffie-Hellman exchange to derive a shared secret, is used to encrypt a CEK. This CEK is subsequently used to encrypt the firmware image. Hence, the plaintext passed to HPKE is the randomly generated CEK. The output of the HPKE SealBase function is therefore the encrypted CEK along with HPKE encapsulated key (i.e., the ephemeral ECDH public key).¶
Only the holder of recipient's private key can decapsulate the CEK to decrypt the firmware. Key generation in HPKE is influenced by additional parameters, such as identity information.¶
This approach allows all recipients to use the same CEK to decrypt 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.¶
[I-D.ietf-cose-hpke] defines the use of HPKE with COSE and provides examples.¶
The suit-cek-verification parameter contains a byte string resulting from the encryption of 8 bytes of 0xA5 using the CEK with a nonce of all zeros and empty additional data using the cipher algorithm and mode also used to encrypt the plaintext. The same nonce used for CEK verification MUST NOT be used to encrypt plaintext with the same CEK.¶
As explained in Section 3, the suit-cek-verification parameter is optional to implement and optional to use. When used, it reduces the risk of a battery exhaustion attack against the IoT device.¶
The ability to restart an interrupted firmware update is often a requirement for low-end IoT devices. To fulfill this requirement it is necessary to chunk a larger firmware image into blocks and to encrypt each block individually using a cipher that does not increase the size of the resulting ciphertext (i.e., by not adding an authentication tag after each encrypted block).¶
When the encrypted firmware image has been transferred to the device, it will typically be stored in a staging area. Then, the bootloader starts decrypting the downloaded image block-by-block and swaps it with the currently valid image. Note that the currently valid image is available in cleartext and hence it has to be re-encrypted before copying it to the staging area.¶
This approach of swapping the newly downloaded image with the previously valid image is often referred as A/B approach. A/B refers to the two storage areas, sometimes called slots, involved. Two slots are used to allow the update to be reversed in case the newly obtained firmware image fails to boot. This approach adds robustness to the firmware update procedure.¶
When an update gets aborted while the bootloader is decrypting the newly obtained image and swapping the blocks, the bootloader can restart where it left off. This technique again offers robustness.¶
To accomplish this functionality, ciphers without integrity protection are used to encrypt the firmware image. Integrity protection for the firmware image is, however, important and therefore the image digest defined in [I-D.ietf-suit-manifest] MUST be used.¶
[I-D.housley-cose-aes-ctr-and-cbc] registers several ciphers that do not offer integrity protection.¶
[[Editor's Note: Add examples for a complete manifest here (including a digital signature), multiple recipients, encryption of manifests (in comparison to firmware images).]]¶
The algorithms described in this document assume that the party performing the firmware encryption¶
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 generation in [RFC8937] are followed.¶
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 asks IANA to register new values into the COSE algorithm registry. The values are listed in [iana-algo].¶
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, Dave Thaler, 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.¶