SUIT                                                       H. Tschofenig
Internet-Draft                                               Arm Limited
Intended status: Standards Track                              R. Housley
Expires: 8 September 2022                                 Vigil Security
                                                                B. Moran
                                                             Arm Limited
                                                            7 March 2022

                Firmware Encryption with SUIT Manifests


   This document specifies a firmware update mechanism where the
   firmware image is encrypted.  Firmware encryption uses the IETF SUIT
   manifest with key establishment provided by the hybrid public-key
   encryption (HPKE) scheme and the AES Key Wrap (AES-KW) with a pre-
   shared key-encryption key.  Encryption of the firmware image is
   encrypted using AES-GCM or AES-CCM.

Status of This Memo

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   extracted from this document must include Revised BSD License text as
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   3
   3.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  SUIT Envelope and SUIT Manifest . . . . . . . . . . . . . . .   6
   5.  AES Key Wrap  . . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Hybrid Public-Key Encryption (HPKE) . . . . . . . . . . . . .  11
   7.  CEK Verification  . . . . . . . . . . . . . . . . . . . . . .  13
   8.  Complete Examples . . . . . . . . . . . . . . . . . . . . . .  13
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     11.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

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.

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   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:

   *  hybrid public-key encryption (HPKE), and

   *  AES Key Wrap (AES-KW) using a pre-shared key-encryption key (KEK).

   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.

2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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 [I-D.irtf-cfrg-hpke]
   and have the following meaning:

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   *  Sender: Role of entity which sends an encrypted message.

   *  Recipient: Role of entity which receives an encrypted message.

   Additionally, the following abbreviations are used in this document:

   *  Key Wrap (KW), defined in RFC 3394 [RFC3394] for use with AES.

   *  Key-encryption key (KEK), a term defined in RFC 4949 [RFC4949].

   *  Content-encryption key (CEK), a term defined in RFC 2630

   *  Hybrid Public Key Encryption (HPKE), defined in

3.  Architecture

   [RFC9019] describes 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.

   This document enhances the SUIT architecutre 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.

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                                              |          |
                                              |  Author  |
                                              |          |
    +----------+                              +----------+
    |  Device  |---+                               |
    |(Firmware |   |                               | Firmware +
    | Consumer)|   |                               | Manifest
    +----------+   |                               |
                   |                               |
                   |                        +--------------+
                   |                        |              |
    +----------+   |  Firmware + Manifest   | Distribution |
    |  Device  |---+------------------------|    System    |
    |(Firmware |   |                        |              |
    | Consumer)|   |                        |              |
    +----------+   |                        +--------------+
    +----------+   |
    |  Device  +---+
    |(Firmware |
    | Consumer)|

                Figure 1: Firmware Encryption Architecture.

   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

   The firmware author may have knowledge about all devices that need to
   receive an encrypted firmware image but in most cases this will not
   be 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:

   *  The firmware author acts as the sender and the recipient is the
      firmware consumer (or the firmware consumers).

   *  The firmware author encrypts the firmware image with the
      distribution system as the initial recipient.  Then, the
      distribution system decrypts and re-encrypts the firmware image
      towards the firmware consumer(s).  Delegating the task of re-

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      encrypting the firmware image to the distribution system offers
      flexiblity when the number of devices that need to receive
      encrypted firmware images changes dynamically or when updates to
      KEKs or recipient public keys are necessary.  As a downside, the
      author needs to trust the distribution system with performing the
      re-encryption of the firmware image.

   Irrespectively of the two variants, the key distribution data (in
   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.

4.  SUIT Envelope and SUIT Manifest

   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

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   SUIT_Envelope_Tagged = #6.107(SUIT_Envelope)
   SUIT_Envelope = {
     suit-authentication-wrapper => bstr .cbor SUIT_Authentication,
     suit-manifest  => bstr .cbor SUIT_Manifest,
     suit-protection-wrappers => bstr .cbor {
         *(int/str) => [+ SUIT_Encryption_Info]
     * SUIT_Integrated_Payload,
     * SUIT_Integrated_Dependency,
     * $$SUIT_Envelope_Extensions,
     * (int => bstr)

                       Figure 2: SUIT Envelope CDDL.

   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-cek-verification       => bstr,
       * $$SUIT_Manifest_Extensions,

                       Figure 3: SUIT Manifest CDDL.

5.  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 structure, contains the CEK encrypted by the KEK.

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   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.

   *  If all authorized recipients have access to the KEK, a single
      COSE_recipient structure contains the encrypted CEK.  This means
      KEK(*,S) ENC(CEK,KEK), and ENC(firmware,CEK).

   *  If recipients have different KEKs, then multiple COSE_recipient
      structures are included but only a single CEK is used.  Each
      COSE_recipient structure contains the CEK encrypted with the KEKs
      appropriate for the recipient.  In short, KEK_1(R1, S),...,
      KEK_n(Rn, S), ENC(CEK, KEK_i) for i=1 to n, and ENC(firmware,CEK).
      The benefit of this approach is that the firmware image is
      encrypted only once with a CEK while there is no sharing of the
      KEK accross recipients.  Hence, authorized recipients still use
      their individual KEKs to decrypt the CEK and to subsequently
      obtain the plaintext firmware.

   *  The third option is to use different CEKs encrypted with KEKs of
      the authorized recipients.  Assume there are KEK_1(R1, S),...,
      KEK_n(Rn, S), and for i=1 to n the following computations need to
      be made: ENC(CEK_i, KEK_i) and ENC(firmware,CEK_i).  This approach
      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 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.

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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 4: CDDL for AES Key Wrap Encryption

   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

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              Figure 5: CDDL for Enc_structure Data Structure

   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:

   *  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):

   The COSE_Encrypt structure, in hex format, is (with a line break


   The resulting COSE_Encrypt structure in a dignostic format is shown
   in Figure 6.

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           / protected field with alg=AES-GCM-128 /
              / unprotected field with iv /
              5: h'26682306D4FB28CA01B43B80'
           / null because of detached ciphertext /
           [ / recipients array /
              h'', / protected field /
              {    / unprotected field /
                 1: -3,            / alg=A128KW /
                 4: h'6B69642D31'  / key id /
              / CEK encrypted with KEK /

              Figure 6: COSE_Encrypt Example for AES Key Wrap

   The CEK, in hex format, was "4C805F1587D624ED5E0DBB7A7F7FA7EB" and
   the encrypted firmware (with a line feed added) was:


6.  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: 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

   Only the holder of recipient's private key can decapsulate the CEK to
   decrypt the firmware.  Key generation in HPKE is influced by
   additional parameters, such as identity information.

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   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.

   [I-D.ietf-cose-hpke] defines the use of HPKE with COSE.

   An example of the COSE_Encrypt structure using the HPKE scheme is
   shown in Figure 7.  It uses the following algorithm combination:

   *  AES-GCM-128 for encryption of the (detached) firmware image (at
      layer 0).

   *  AES-GCM-128 for encryption of the CEK in layer 1 as well as ECDH
      with NIST P-256 and HKDF-SHA256 as a Key Encapsulation Mechanism

       / protected header with alg=AES-GCM-128 /
       / unprotected header with nonce /
       {5: h'938b528516193cc7123ff037809f4c2a'},
       / detached ciphertext /
       / recipient structure /
           / protected field with alg for HPKE /
           / unprotected header /
               / ephemeral public key with x / y coodinate /
               -1: h'a401022001215820a596f2ca8d159c04942308ca90
                /  kid for recipient static ECDH public key /
                4: h'6b69642d32',
           / encrypted CEK /

                  Figure 7: COSE_Encrypt Example for HPKE

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7.  CEK Verification

   The suit-cek-verification parameter contains a byte string resulting
   from the encryption of 8 bytes of 0xA5 using the CEK.

   [[Editor's Note: Guidance about the selection of an IV needs to be
   added here.]]

8.  Complete Examples

   [[Editor's Note: Add examples for a complete manifest here (including
   a digital signature), multiple recipients, encryption of manifests
   (in comparison to firmware images).]]

9.  Security Considerations

   The algorithms described in this document assume that the party
   performing the firmware encryption

   *  shares 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 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.

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10.  IANA Considerations

   This document does not require any actions by IANA.

11.  References

11.1.  Normative References

              Tschofenig, H., Housley, R., and B. Moran, "Use of Hybrid
              Public-Key Encryption (HPKE) with CBOR Object Signing and
              Encryption (COSE)", Work in Progress, Internet-Draft,
              draft-ietf-cose-hpke-00, 10 January 2022,

              Moran, B., Tschofenig, H., Birkholz, H., and K. Zandberg,
              "A Concise Binary Object Representation (CBOR)-based
              Serialization Format for the Software Updates for Internet
              of Things (SUIT) Manifest", Work in Progress, Internet-
              Draft, draft-ietf-suit-manifest-16, 25 October 2021,

              Barnes, R. L., Bhargavan, K., Lipp, B., and C. A. Wood,
              "Hybrid Public Key Encryption", Work in Progress,
              Internet-Draft, draft-irtf-cfrg-hpke-12, 2 September 2021,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3394]  Schaad, J. and R. Housley, "Advanced Encryption Standard
              (AES) Key Wrap Algorithm", RFC 3394, DOI 10.17487/RFC3394,
              September 2002, <>.

   [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              RFC 8152, DOI 10.17487/RFC8152, July 2017,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

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11.2.  Informative References

   [RFC2630]  Housley, R., "Cryptographic Message Syntax", RFC 2630,
              DOI 10.17487/RFC2630, June 1999,

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,

   [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,

   [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,

   [RFC9124]  Moran, B., Tschofenig, H., and H. Birkholz, "A Manifest
              Information Model for Firmware Updates in Internet of
              Things (IoT) Devices", RFC 9124, DOI 10.17487/RFC9124,
              January 2022, <>.

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.
   Finally, we would like to thank Dick Brooks for making us aware of
   the challenges firmware encryption imposes on binary analysis.

Authors' Addresses

   Hannes Tschofenig
   Arm Limited

   Russ Housley
   Vigil Security, LLC

   Brendan Moran
   Arm Limited

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