SUIT                                                       H. Tschofenig
Internet-Draft                                               Arm Limited
Intended status: Standards Track                              R. Housley
Expires: January 8, 13, 2022                                 Vigil Security
                                                                B. Moran
                                                             Arm Limited
                                                           July 07, 12, 2021

                Firmware Encryption with SUIT Manifests
                 draft-ietf-suit-firmware-encryption-00
                 draft-ietf-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.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  AES Key Wrap  . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.   5
   5.  Hybrid Public-Key Encryption (HPKE) . . . . . . . . . . . . .   7
   5.   9
   6.  Complete Examples . . . . . . . . . . . . . . . . . . . . . .  12
   6.  14
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   7.  15
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   8.  15
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  16
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     8.2.  16
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  15  17
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  16  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16  18

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.

   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 for and thereby help
   increase interoperability of between different SUIT manifest parser
   implementations.

   The document also offers contains 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

   In context of encryption, 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 4949 [RFC4949].

   -  Content-encryption key (CEK), a term defined in RFC 2630
      [RFC2630].

   -  Hybrid Public Key Encryption (HPKE), defined in
      [I-D.irtf-cfrg-hpke].

3.  Architecture

   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  +---+
    |          |
    +----------+

                Figure 1: Firmware Encryption Architecture.

4.  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,
   COSE_Encrypt structure, 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
   there are three options:

   -  If all of the authorized recipients have access to the KEK.

   However, KEK, a single
      COSE_recipient structure contains the encrypted CEK.

   -  If recipients have different KEKs, then the COSE_recipient
      structure can may contain the same CEK encrypted with many different
      KEKs.  The benefit of this approach is that the firmware image is
      encrypted only once with the CEK while the authorized recipients
      still need to use their individual KEKs if needed to reach all obtain the plaintext.

   -  The last option is to use different CEKs encrypted with KEKs of
      the authorized recipients.  This 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 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. 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
 }

         Figure 1: 2: 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. 3.

          Enc_structure = [
            context : "Encrypt",
            protected : empty_or_serialized_map,
            external_aad : bstr
          ]

              Figure 2: 3: CDDL for Enc_structure Data Structure

   As it can be seen in the CDDL shown in Figure 1, 2, there are two protected
   fields 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 2, 3, refers to the outer content of the protected field,
   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:

   -  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. 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'
           ]
       ]
   )

              Figure 3: 4: COSE_Encrypt Example for AES Key Wrap

   The CEK was "4C805F1587D624ED5E0DBB7A7F7FA7EB" and the encrypted
   firmware was:

   A8B6E61EF17FBAD1F1BF3235B3C64C06098EA512223260
   F9425105F67F0FB6C92248AE289A025258F06C2AD70415

4.

5.  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 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 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. 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
 )

           Figure 4: 5: CDDL for HPKE-based COSE_Encrypt Structure

   The COSE_Encrypt structure in Figure 4 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 5. 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)

                                 Figure 5 6

   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. 7.

   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: 7: COSE_KDF_Context Data Structure

   Notes:

   -  PartyUInfo.identity corresponds to the kid found in the
      COSE_Sign_Tagged or COSE_Sign1_Tagged structure (when a digital
      signature is used. 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 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)

                                 Figure 7 8

   An example of the COSE_Encrypt structure using the HPKE scheme is
   shown in Figure 8. 9.  It uses the following algorithm combination:

   -  AES-GCM-128 for encryption of the firmware image.

   -  AES-GCM-128 for encrytion of the CEK.

   -  Key Encapsulation Mechanism (KEM): NIST P-256

   -  Key Derivation Function (KDF): HKDF-SHA256

  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: 9: COSE_Encrypt Example for HPKE

5.

6.  Complete Examples

   TBD: Add example Example for complete manifest here (which also includes the
   digital signature).  TBD: Add multiple Multiple recipient example as well.  TBD: Add encryption
   Encryption of manifest (in addition of firmware encryption).

6.

7.  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

   -  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 a 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].

7.

   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.

8.  IANA Considerations

   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      |
 +-------------+-------+---------+------------+--------+---------------+

8.

9.  References

8.1.

9.1.  Normative References

   [I-D.ietf-cose-rfc8152bis-algs]
              August Cellars,
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Initial Algorithms", draft-ietf-cose-rfc8152bis-
              algs-12 draft-ietf-cose-rfc8152bis-algs-12
              (work in progress), September 2020.

   [I-D.ietf-suit-manifest]
              Arm Limited, Arm Limited, Fraunhofer SIT,
              Moran, B., Tschofenig, H., Birkholz, H., and Inria, K. Zandberg,
              "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.

   [I-D.irtf-cfrg-hpke]
              Cisco, Inria, Inria,
              Barnes, R. L., Bhargavan, K., Lipp, B., and Cloudflare, C. A. Wood,
              "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.

9.2.  Informative References

   [I-D.ietf-suit-information-model]
              Arm Limited, Arm Limited,
              Moran, B., Tschofenig, H., and Fraunhofer SIT, H. Birkholz, "A Manifest
              Information Model for Firmware Updates in IoT Devices",
              draft-ietf-suit-information-model-11 (work in progress),
              April 2021.

   [RFC2630]  Housley, R., "Cryptographic Message Syntax", RFC 2630,
              DOI 10.17487/RFC2630, June 1999,
              <https://www.rfc-editor.org/info/rfc2630>.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://www.rfc-editor.org/info/rfc4949>.

   [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>.

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

   EMail: hannes.tschofenig@arm.com

   Russ Housley
   Vigil Security, LLC

   EMail: housley@vigilsec.com

   Brendan Moran
   Arm Limited

   EMail: Brendan.Moran@arm.com