Arm Limited

hannes.tschofenig@arm.com

Vigil Security, LLC

housley@vigilsec.com

Arm Limited

Brendan.Moran@arm.com

Security SUIT Internet-Draft 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.

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 . 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 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 confidentiality protected using encryption. Encryption prevents third parties, including attackers, from gaining access to the firmware image. This might be done to protect confidential information or prevent discovery of vulnerabilities through reverse engineering. 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.

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 when, and only when, they appear in all capitals, as shown here. This document assumes familiarity with the IETF SUIT manifest and the SUIT architecture . The terms “recipient” and “firmware consumer” are used interchangeably.

The AES Key Wrap (AES-KW) algorithm is described in RFC 3394 , 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 . 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 , 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 .

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 defined as shown in .

As it can be seen in the CDDL in , there are two protected fields and the ‘protected’ field in the Enc_structure, see , 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):

The resulting COSE_Encrypt structure in a dignostic format is shown in .

The CEK was “4C805F1587D624ED5E0DBB7A7F7FA7EB” and the encrypted firmware was:

Hybrid public-key encryption (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. 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 .

int, ; algorithm identifier ? 2 => crv, ; EC identifier ? 4 => bstr, ; key identifier ? 5 => bstr ; IV ) ]]>

The COSE_Encrypt structure in 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 here:

Legend: The functions DeserializePublicKey(), SetupBaseS() and Seal() are defined in 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 . 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).

Notes: 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. The following text shows the HPKE operations performed by the recipient:

An example of the COSE_Encrypt structure using the HPKE scheme is shown in .

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).

This entire document is about security. 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 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 document requests IANA to create new entries in the COSE Algorithms registry established with .

Arm Limited Arm Limited Fraunhofer SIT Inria This specification describes the format of a manifest. A manifest is a bundle of metadata about code/data obtained by a recipient (chiefly the firmware for an IoT device), where to find the that code/data, the devices to which it applies, and cryptographic information protecting the manifest. Software updates and Trusted Invocation both tend to use sequences of common operations, so the manifest encodes those sequences of operations, rather than declaring the metadata. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need for the ability to have basic security services defined for this data format. This document defines the CBOR Object Signing and Encryption (COSE) protocol. This specification describes how to create and process signatures, message authentication codes, and encryption using CBOR for serialization. This specification additionally describes how to represent cryptographic keys using CBOR. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Cisco Inria Inria Cloudflare This document describes a scheme for hybrid public-key encryption (HPKE). This scheme provides authenticated public key encryption of arbitrary-sized plaintexts for a recipient public key. HPKE works for any combination of an asymmetric key encapsulation mechanism (KEM), key derivation function (KDF), and authenticated encryption with additional data (AEAD) encryption function. We provide instantiations of the scheme using widely used and efficient primitives, such as Elliptic Curve Diffie-Hellman key agreement, HKDF, and SHA2. This document is a product of the Crypto Forum Research Group (CFRG) in the IRTF. August Cellars Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need for the ability to have basic security services defined for this data format. THis document defines a set of algorithms that can be used with the CBOR Object Signing and Encryption (COSE) protocol RFC XXXX. Vulnerabilities in Internet of Things (IoT) devices have raised the need for a reliable and secure firmware update mechanism suitable for devices with resource constraints. Incorporating such an update mechanism is a fundamental requirement for fixing vulnerabilities, but it also enables other important capabilities such as updating configuration settings and adding new functionality.In addition to the definition of terminology and an architecture, this document provides the motivation for the standardization of a manifest format as a transport-agnostic means for describing and protecting firmware updates. Arm Limited Arm Limited Fraunhofer SIT 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. Ensuring that devices function and remain secure over their service life requires such an update mechanism to fix vulnerabilities, to update configuration settings, as well as adding new functionality. One component of such a firmware update is a concise and machine- processable meta-data document, or manifest, that describes the firmware image(s) and offers appropriate protection. This document describes the information that must be present in the manifest.

We would like to thank Henk Birkholz for his feedback on the CDDL description in this document.