Internet-Draft Encrypted Payloads in SUIT Manifests September 2023
Tschofenig, et al. Expires 14 March 2024 [Page]
Workgroup:
SUIT
Internet-Draft:
draft-ietf-suit-firmware-encryption-17
Published:
Intended Status:
Standards Track
Expires:
Authors:
H. Tschofenig
R. Housley
Vigil Security
B. Moran
Arm Limited
D. Brown
Linaro
K. Takayama
SECOM CO., LTD.

Encrypted Payloads in SUIT Manifests

Abstract

This document specifies techniques for encrypting software, firmware, machine learning models, and personalization data by utilizing the IETF SUIT manifest. Key agreement is provided by ephemeral-static (ES) Diffie-Hellman (DH) and AES Key Wrap (AES-KW). ES-DH uses public key cryptography while AES-KW uses a pre-shared key. Encryption of the plaintext is accomplished with conventional symmetric key cryptography.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 14 March 2024.

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Table of Contents

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]. It provides a bundle of metadata, including where to find the payload, the devices to which it applies and a security wrapper.

[RFC9124] details the information that has to be provided by the SUIT manifest format. In addition to offering protection against modification, via a digital signature or a message authentication code, confidentiality may also be afforded.

Encryption prevents third parties, including attackers, from gaining access to the payload. Attackers typically need intimate knowledge of a binary, such as a firmware image, to mount their attacks. For example, return-oriented programming (ROP) [ROP] requires access to the binary and encryption makes it much more difficult to write exploits.

While the original motivating use case of this document was firmware encryption, the use of SUIT manifests has been extended to other use cases requiring integrity and confidentiality protection, such as:

Hence, we use the term payload to generically refer to all those objects.

The payload is encrypted using a symmetric content encryption key, which can be established using a variety of mechanisms; this document defines two content key distribution methods for use with the IETF SUIT manifest, namely:

The former method relies on asymmetric key cryptography while the latter uses symmetric key cryptography.

Our goal was to reduce the number of content key distribution methods for use with payload encryption and thereby increase interoperability between different SUIT manifest parser implementations.

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], the SUIT information model [RFC9124], and the SUIT architecture [RFC9019].

The following abbreviations are used in this document:

The terms sender and recipient have the following meaning:

Additionally, we introduce the term "distribution system" (or distributor) to refer to an entity that knows the recipients of payloads. It is important to note that the distribution system is far more than a file server. For use of encryption, the distribution system either knows the public key of the recipient (for ES-DH), or the KEK (for AES-KW).

The author, which is responsible for creating the payload, does not know the recipients.

The author and the distribution system are logical roles. In some deployments these roles are separated in different physical entities and in others they are co-located.

3. Architecture

[RFC9019] describes the architecture for distributing payloads and manifests from an author to devices. It does, however, not detail the use of payload encryption. This document enhances the architecture to support encryption.

Figure 1 shows the distribution system, which represents a file server and the device management infrastructure.

The sender (author) needs to know the recipient (device) to use encryption. For AES-KW, the KEK needs to be known and, in case of ES-DH, the sender needs to be in possession of the public key of the recipient. The public key and parameters may be in the recipient's X.509 certificate [RFC5280]. For authentication of the sender and for integrity protection the recipients must be provisioned with a trust anchor when a manifest is protected using a digital signature. When a MAC is used to protect the manifest then a symmetric key must be shared by the recipient and the sender.

With encryption, the author cannot just create a manifest for the payload and sign it, since the subsequent encryption step by the distribution system would invalidate the signature over the manifest. (The content key distribution information is embedded inside the COSE_Encrypt structure, which is included in the SUIT manifest.) Hence, the author has to collaborate with the distribution system. The varying degree of collaboration is discussed below.

 +----------+
 |  Device  |                              +----------+
 |    1     |<--+                          |  Author  |
 |          |   |                          +----------+
 +----------+   |                               |
                |                               | Payload +
                |                               | Manifest
                |                               v
 +----------+   |                        +--------------+
 |  Device  |   |  Payload + Manifest    | Distribution |
 |    2     |<--+------------------------|    System    |
 |          |   |                        +--------------+
 +----------+   |
                |
      ...       |
                |
 +----------+   |
 |  Device  |   |
 |    n     |<--+
 |          |
 +----------+
Figure 1: Architecture for the distribution of Encrypted Payloads.

The author has several deployment options, namely:

If the author delegates encryption rights to the distributor two models are possible:

  1. The distributor replaces the COSE_Encrypt in the manifest and then signs the manifest again. However, the COSE_Encrypt structure is contained within a signed container, which presents a problem: replacing the COSE_Encrypt with a new one will cause the digest of the manifest to change, thereby changing the signature. This means that the distributor must be able to sign the new manifest. If this is the case, then the distributor gains the ability to construct and sign manifests, which allows the distributor the authority to sign code, effectively presenting the distributor with full control over the recipient. Because distributors typically perform their re-encryption online in order to handle a large number of devices in a timely fashion, it is not possible to air-gap the distributor's signing operations. This impacts the recommendations in Section 4.3.17 of [RFC9124].
  2. The distributor uses a two-manifest system. More precisely, the distributor constructs a new manifest that overrides the COSE_Encrypt using the dependency system defined in [I-D.ietf-suit-trust-domains]. This incurs additional overhead: one additional signature verification and one additional manifest, as well as the additional machinery in the recipient needed for dependency processing.

These two models also present different threat profiles for the distributor. If the distributor only has encryption rights, then an attacker who breaches the distributor can only mount a limited attack: they can encrypt a modified binary, but the recipients will identify the attack as soon as they perform the required image digest check and revert back to a correct image immediately.

It is RECOMMENDED that distributors are implemented using a two-manifest system in order to distribute content encryption keys without requiring re-signing of the manifest, despite the increase in complexity and greater number of signature verifications that this imposes on the recipient.

4. Encryption Extensions

This specification introduces a new extension to the SUIT_Parameters structure.

The SUIT_Encryption_Info structure (called suit-parameter-encryption-info in Figure 2) contains the content key distribution information. The content of the SUIT_Encryption_Info structure is explained in Section 6.1 (for AES-KW) and in Section 6.2 (for ES-DH).

Once a CEK is available, the steps described in Section 6.3 are applicable. These steps apply to both content key distribution methods described in this section.

The SUIT_Encryption_Info structure is either carried inside the suit-directive-override-parameters or the suit-directive-set-parameters parameters used in the "Directive Write" and "Directive Copy" directives. An implementation claiming conformance with this specification must implement support for these two parameters. Since a device will typically only support one of the content key distribution methods, the distribution system needs to know which of two specified methods wis supported. Mandating only a single content key distribution method for a constrained device also reduces the code size.

SUIT_Parameters //= (suit-parameter-encryption-info
    => bstr .cbor SUIT_Encryption_Info)

suit-parameter-encryption-info = 19
Figure 2: CDDL of the SUIT_Parameters Extension.

RFC Editor's Note (TBD19): The value for the suit-parameter-encryption-info parameter is set to 19, as the proposed value.]

5. Extended Directives

This specification extends these directives:

Examples of the two directives are shown below.

Figure 3 illustrates the Directive Write. The encrypted payload specified with parameter-content, namely h'EA1...CED' in the example, is decrypted using the SUIT_Encryption_Info structure referred to by parameter-encryption-info, i.e., h'D86...1F0'. The resulting plaintext payload is stored into component #0.

/ directive-override-parameters / 20, {
  / parameter-content / 18: h'EA1...CED',
  / parameter-encryption-info / 19: h'D86...1F0'
},
/ directive-write / 18, 15
Figure 3: Example showing the extended suit-directive-write.

Figure 4 illustrates the Directive Copy. In this example the encrypted payload is found at the URI indicated by the parameter-uri, i.e. "http://example.com/encrypted.bin". The encrypted payload will be downloaded and stored in component #1. Then, the information in the SUIT_Encryption_Info structure of the parameter-encryption-info, i.e. h'D86...1F0', will be used to decrypt the content in component #1 and the resulting plaintext payload will be stored into component #0.

/ directive-set-component-index / 12, 1,
/ directive-override-parameters / 20, {
  / parameter-uri / 21: "http://example.com/encrypted.bin",
},
/ directive-fetch / 21, 15,
/ directive-set-component-index / 12, 0,
/ directive-override-parameters / 20, {
  / parameter-source-component / 22: 1,
  / parameter-encryption-info / 19: h'D86...1F0'
},
/ directive-copy / 22, 15
Figure 4: Example showing the extended suit-directive-copy.

The payload to be encrypted may be detached and, in that case, it is not covered by the digital signature or the MAC protecting the manifest. (To be more precise, the suit-authentication-wrapper found in the envelope contains a digest of the manifest in the SUIT Digest Container.)

The lack of authentication and integrity protection of the payload is particularly a concern when a cipher without integrity protection is used.

To provide authentication and integrity protection of the payload in the detached payload case a SUIT Digest Container with the hash of the encrypted and/or plaintext payload MUST be included in the manifest. See suit-parameter-image-digest parameter in Section 8.4.8.6 of [I-D.ietf-suit-manifest].

Once a CEK is available, the steps described in Section 6.3 are applicable. These steps apply to both content key distribution methods.

6. Content Key Distribution

The sub-sections below describe two content key distribution methods, namely AES Key Wrap (AES-KW) and Ephemeral-Static Diffie-Hellman (ES-DH). Many other methods are specified in the literature, and even supported by COSE. New methods can be added via enhancements to this specification. The two specified methods were selected to their maturity, different security properties, and to ensure interoperability in deployments.

The two content key distribution methods require the CEKs to be randomly generated. It must be ensured that the guidelines for random number generation in [RFC8937] are followed.

When an encrypted payload is sent to multiple recipients, there are different deployment options. To explain these options we use the following notation:

   - KEK(R1, S) refers to a KEK shared between recipient R1 and
     the sender S. The KEK, as a concept, is used by AES Key Wrap
     but not by ES-DH.
   - CEK(R1, S) refers to a CEK shared between R1 and S.
   - CEK(*, S) or KEK(*, S) are used when a single CEK or a single
     KEK is shared with all authorized recipients by a given sender
     S in a certain context.
   - ENC(plaintext, k) refers to the encryption of plaintext with
     a key k.

6.1. Content Key Distribution with AES Key Wrap

6.1.1. Introduction

The AES Key Wrap (AES-KW) algorithm is described in [RFC3394], and 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 8.5.2 of [RFC9052] and in Section 6.2.1 of [RFC9053]. 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.

To provide high security for AES Key Wrap, 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 data protected with that KEK, including binaries, and configuration data.

The COSE_Encrypt structure conveys information for encrypting the payload, which includes information like the algorithm and the IV, even though the payload may not be embedded in the COSE_Encrypt.ciphertext if it is conveyed as detached content.

6.1.2. Deployment Options

There are three deployment options for use with AES Key Wrap for payload encryption:

  • If all recipients (typically of the same product family) share the same KEK, a single COSE_recipient structure contains the encrypted CEK. The sender executes the following steps: 
     1. Fetch KEK(*, S)
     2. Generate CEK
     3. ENC(CEK, KEK)
     4. ENC(payload, CEK)

This deployment option is stronly discouraged. An attacker gaining access to the KEK will be able to encrypt and send payloads to all recipients configured to use this KEK.

  • 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 a given recipient. The benefit of this approach is that the payload is encrypted only once with a CEK while there is no sharing of the KEK across recipients. Hence, authorized recipients still use their individual KEK to decrypt the CEK and to subsequently obtain the plaintext. The steps taken by the sender are: 
    1.  Generate CEK
    2.  for i=1 to n
        {
    2a.    Fetch KEK(Ri, S)
    2b.    ENC(CEK, KEK(Ri, S))
        }
    3.  ENC(payload, CEK)
  • The third option is to use different CEKs encrypted with KEKs of authorized recipients. This approach is appropriate when no benefits can be gained from encrypting and transmitting payloads only once. Assume there are n recipients with their unique KEKs - KEK(R1, S), ..., KEK(Rn, S). The sender needs to execute the following steps: 
    1.  for i=1 to n
        {
    1a.    Fetch KEK(Ri, S)
    1b.    Generate CEK(Ri, S)
    1c.    ENC(CEK(Ri, S), KEK(Ri, S))
    1d.    ENC(payload, CEK(Ri, S))
    2.  }

6.1.3. CDDL

The CDDL for the COSE_Encrypt_Tagged structure is shown in Figure 5. empty_or_serialized_map and header_map are structures defined in [RFC9052].

outer_header_map_protected = empty_or_serialized_map
outer_header_map_unprotected = header_map

SUIT_Encryption_Info_AESKW = [
  protected   : bstr .cbor outer_header_map_protected,
  unprotected : outer_header_map_unprotected,
  ciphertext  : bstr / nil,
  recipients  : [ + COSE_recipient_AESKW .within COSE_recipient ]
]

COSE_recipient_AESKW = [
  protected   : bstr .size 0 / bstr .cbor empty_map,
  unprotected : recipient_header_unpr_map_aeskw,
  ciphertext  : bstr        ; CEK encrypted with KEK
]

empty_map = {}

recipient_header_unpr_map_aeskw =
{
    1 => int,         ; algorithm identifier
  ? 4 => bstr,        ; identifier of the KEK pre-shared with the recipient
  * label => values   ; extension point
}
Figure 5: CDDL for AES-KW-based Content Key Distribution

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 header in the COSE_recipient structure is a byte string of zero length.

6.1.4. Example

This example uses the following parameters:

  • Algorithm for authentication: COSE_Mac0 with HMAC-256
  • Algorithm for payload encryption: AES-GCM-128
  • Algorithm id for key wrap: A128KW
  • IV: h'93702C81590F845D9EC866CCAC767BD1'
  • KEK: 'aaaaaaaaaaaaaaaa'
  • KID: 'kid-1'
  • Plaintext (txt): "This is a real firmware image." (in hex): 546869732069732061207265616C206669726D7761726520696D6167652E

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

D8608443A10101A1055093702C81590F845D9EC866CCAC767BD1F6818341A0A2012204456B69642D
315818CA0DF4EE01796D12FE379165602E0AC2476391A4EF122A64

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

96([
  / protected: / << {
    / alg / 1: 1 / AES-GCM-128 /
  } >>,
  / unprotected: / {
    / IV / 5: h'93702C81590F845D9EC866CCAC767BD1'
  },
  / payload: / null / detached ciphertext /,
  / recipients: / [
    [
      / protected: / << {
      } >>,
      / unprotected: / {
        / alg / 1: -3 / A128KW /,
        / kid / 4: 'kid-1'
      },
      / payload: / h'CA0DF4EE01796D12FE379165602E0AC2476391A4EF122A64'
        / CEK encrypted with KEK /
    ]
  ]
])
Figure 6: COSE_Encrypt Example for AES Key Wrap

The encrypted payload (with a line feed added) was:

9890D8DC740A2E82C2BEA9BAB13E0BFA0FB4EB2BA3C0BCA4B23A0D660C5B3038F8634933921B3C2D
1A84EE6C2779

6.2. Content Key Distribution with Ephemeral-Static Diffie-Hellman

6.2.1. Introduction

Ephemeral-Static Diffie-Hellman (ES-DH) is a scheme that provides public key encryption given a recipient's public key. There are multiple variants of this scheme; this document re-uses the variant specified in Section 8.5.5 of [RFC9052].

The following two layer structure is used:

  • Layer 0: Has a content encrypted with the CEK. The content may be detached.
  • Layer 1: Uses the AES Key Wrap algorithm to encrypt the randomly generated CEK with the KEK derived with ES-DH, whereby the resulting symmetric key is fed into the HKDF-based key derivation function.

As a result, the two layers combine ES-DH with AES-KW and HKDF. An example is given in Figure 9.

6.2.2. Deployment Options

There are two deployment options with this approach. We assume that recipients are always configured with a device-unique public / private key pair.

  • A sender wants to transmit a payload to multiple recipients. All recipients shall receive the same encrypted payload, i.e. the same CEK is used. One COSE_recipient structure per recipient is used and it contains the CEK encrypted with the KEK. To generate the KEK each COSE_recipient structure contains a COSE_recipient_inner structure to carry the sender's ephemeral key and an identifier for the recipients public key.

The steps taken by the sender are:

    1.  Generate CEK
    2.  for i=1 to n
        {
    2a.     Generate KEK(Ri, S) using ES-DH
    2b.     ENC(CEK, KEK(Ri, S))
        }
    3.  ENC(payload,CEK)
  • The alternative is to encrypt a payload with a different CEK for each recipient. This results in n-manifests. This approach is useful when payloads contain information unique to a device. The encryption operation then effectively becomes ENC(payload_i, CEK(Ri, S)). Assume that KEK(R1, S),..., KEK(Rn, S) have been generated for the different recipients using ES-DH. The following steps need to be made by the sender: 
    1.  for i=1 to n
        {
    1a.     Generate KEK(Ri, S) using ES-DH
    1b.     Generate CEK(Ri, S)
    1c.     ENC(CEK(Ri, S), KEK(Ri, S))
    1d.     ENC(payload, CEK(Ri, S))
        }

6.2.3. CDDL

The CDDL for the COSE_Encrypt_Tagged structure is shown in Figure 7. Only the minimum number of parameters is shown. empty_or_serialized_map and header_map are structures defined in [RFC9052].

outer_header_map_protected = empty_or_serialized_map
outer_header_map_unprotected = header_map

SUIT_Encryption_Info_ESDH = [
  protected   : bstr .cbor outer_header_map_protected,
  unprotected : outer_header_map_unprotected,
  ciphertext  : bstr / nil,
  recipients  : [ + COSE_recipient_ESDH .within COSE_recipient ]
]

COSE_recipient_ESDH = [
  protected   : bstr .cbor recipient_header_map_esdh,
  unprotected : recipient_header_unpr_map_esdh,
  ciphertext  : bstr        ; CEK encrypted with KEK
]

recipient_header_map_esdh =
{
    1 => int,         ; algorithm identifier
  * label => values   ; extension point
}

recipient_header_unpr_map_esdh =
{
   -1 => COSE_Key,    ; ephemeral public key for the sender
  ? 4 => bstr,        ; identifier of the recipient public key
  * label => values   ; extension point
}
Figure 7: CDDL for ES-DH-based Content Key Distribution

See Section 6.3 for a description on how to encrypt the payload.

6.2.4. Context Information Structure

The context information structure is used to ensure that the derived keying material is "bound" to the context of the transaction. This specification re-uses the structure defined in Section 5.2 of [RFC9053] and tailors it accordingly.

The following information elements are bound to the context:

  • the protocol employing the key-derivation method,
  • information about the utilized AES Key Wrap algorithm, and the key length.
  • the protected header field, which contains the content key encryption algorithm.

The sender and recipient identities are left empty.

The following fields in Figure 8 require an explanation:

  • The COSE_KDF_Context.AlgorithmID field MUST contain the algorithm identifier for AES Key Wrap algorithm utilized. This specification uses the following values: A128KW (value -4), A192KW (value -4), or A256KW (value -5)
  • The COSE_KDF_Context.SuppPubInfo.keyDataLength field MUST contain the key length of the algorithm in the COSE_KDF_Context.AlgorithmID field expressed as the number of bits. For A128KW the value is 128, for A192KW the value is 192, and for A256KW the value 256.
  • The COSE_KDF_Context.SuppPubInfo.other field captures the protocol in which the ES-DH content key distribution algorithm is used and MUST be set to the constant string "SUIT Payload Encryption".
  • The COSE_KDF_Context.SuppPubInfo.protected field MUST contain the serialized content of the recipient_header_map_esdh field, which contains (among other fields) the identifier of the content key distribution method. 
PartyInfoSender = (
    identity : nil,
    nonce : nil,
    other : nil
)

PartyInfoRecipient = (
    identity : nil,
    nonce : nil,
    other : nil
)

COSE_KDF_Context = [
    AlgorithmID : int,
    PartyUInfo : [ PartyInfoSender ],
    PartyVInfo : [ PartyInfoRecipient ],
    SuppPubInfo : [
        keyDataLength : uint,
        protected : bstr .cbor recipient_header_map_esdh,
        other: bstr "SUIT Payload Encryption"
    ],
    SuppPrivInfo : bstr .size 0
]
Figure 8: CDDL for COSE_KDF_Context Structure

The HKDF-based key derivation function MAY contain a salt value, as described in Section 5.1 of [RFC9053]. This optional value is used to influence the key generation process. This specification does not mandate the use of a salt value. If the salt is public and carried in the message, then the "salt" algorithm header parameter MUST be used. The purpose of the salt is to provide extra randomness in the KDF context. If the salt is sent in the 'salt' algorithm header parameter, then the receiver MUST be able to process the salt and MUST pass it into the key derivation function. For more information about the salt, see [RFC5869] and NIST SP800-56 [SP800-56].

Profiles of this specification MAY specify an extended version of the context information structure or MAY utilize a different context information structure.

6.2.5. Example

This example uses the following parameters:

  • Algorithm for payload encryption: AES-GCM-128
  • IV: h'3517CE3E78AC2BF3D1CDFDAF955E8600'
  • Algorithm for content key distribution: ECDH-ES + A128KW
  • SuppPubInfo.other = 'SUIT Payload Encryption'
  • KID: 'kid-2'
  • Plaintext: "This is a real firmware image."
  • Plaintext (in hex encoding): 546869732069732061207265616C206669726D7761726520696D6167652E

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

D8608443A10101A105501485CADEC69C011B5CC3B0BE0F2B3801F6818344
A101381CA220A4010220012158203982C1863015824881D1A6C9059332BE
281C9613D7F7462D43EE520D20FE132F2258205EB51EF8AD7C1DA6972948
77BBA291DC5AEE20FEE887D8A173BAD7FAFF091E5C04456B69642D325818
DF698D95D3BF3EA7CCC655A7A5609BEF206E208A46D66D91

The resulting COSE_Encrypt structure in a diagnostic format is shown in Figure 9. Note that the COSE_Encrypt structure also needs to protected by a COSE_Sign1, which is not shown below.

96([
  / protected: / << {
    / alg / 1: 1 / AES-GCM-128 /
  } >>,
  / unprotected: / {
    / IV / 5: h'1485CADEC69C011B5CC3B0BE0F2B3801'
  },
  / payload: / null / detached ciphertext /,
  / recipients: / [
    [
      / protected: / << {
        / alg / 1: -29 / ECDH-ES + A128KW /
      } >>,
      / unprotected: / {
        / ephemeral key / -1: {
          / kty / 1: 2 / EC2 /,
          / crv / -1: 1 / P-256 /,
          / x / -2: h'3982C1863015824881D1A6C9059332BE281C9613D7F7462D43EE520D20FE132F',
          / y / -3: h'5EB51EF8AD7C1DA697294877BBA291DC5AEE20FEE887D8A173BAD7FAFF091E5C'
        },
        / kid / 4: 'kid-2'
      },
      / payload: / h'DF698D95D3BF3EA7CCC655A7A5609BEF206E208A46D66D91'
        / CEK encrypted with KEK /
    ]
  ]
])
Figure 9: COSE_Encrypt Example for ES-DH

The encrypted payload (with a line feed added) was:

C7C4583D2763F3ECCF09FD1EB34EC9296426899510DAF3098E849C8B4F8F
5364638B309447D6B6393B899F8F0AEE

6.3. Content Encryption

This section summarizes the steps taken for content encryption, which applies to both content key distribution methods.

For use with AEAD ciphers, the COSE specification requires a consistent byte stream for the authenticated data structure to be created. This structure is shown in Figure 10 and is defined in Section 5.3 of [RFC9052].

 Enc_structure = [
   context : "Encrypt",
   protected : empty_or_serialized_map,
   external_aad : bstr
 ]
Figure 10: CDDL for Enc_structure Data Structure

This Enc_structure needs to be populated as follows:

The protected field in the Enc_structure from Figure 10 refers to the content of the protected field from the COSE_Encrypt structure.

The value of the external_aad MUST be set to a zero-length byte string, i.e., h'' in diagnostic notation and encoded as 0x40.

For use with ciphers that do not provide integrity protection, such as AES-CTR and AES-CBC (see [I-D.ietf-cose-aes-ctr-and-cbc]), the Enc_structure shown in Figure 10 MUST NOT be used because the Enc_structure represents the Additional Authenticated Data (AAD) byte string consumable only by AEAD ciphers. Hence, the Additional Authenticated Data structure is not supplied to the API of the cipher. The protected header in the SUIT_Encryption_Info_AESKW or SUIT_Encryption_Info_ESDH structure MUST be a zero-length byte string, respectively.

7. Firmware Updates on IoT Devices with Flash Memory

Note: This section is specific to firmware images and does not apply to generic software, configuration data, and machine learning models.

Flash memory on microcontrollers is a type of non-volatile memory that erases data in units called blocks, pages, or sectors and re-writes data at the byte level (often 4-bytes) or larger units. Flash memory is furthermore segmented into different memory regions, which store the bootloader, different versions of firmware images (in so-called slots), and configuration data. Figure 11 shows an example layout of a microcontroller flash area. The primary slot typically contains the firmware image to be executed by the bootloader, which is a common deployment on devices that do not offer the concept of position independent code. Position independent code is not a feature frequently found in real-time operating systems used on microcontrollers. There are many flavors of embedded devices, the market is large and fragmented. Hence, it is likely that some implementations and deployments implement their firmware update procedure different than described below. On a positive note, the SUIT manifest allows different deployment scenarios to be supported easily thanks to the "scripting" functionality offered by the commands.

When the encrypted firmware image has been transferred to the device, it will typically be stored in a staging area, in the secondary slot in our example.

At the next boot, the bootloader will recognize a new firmware image in the secondary slot and will start decrypting the downloaded image sector-by-sector and will swap it with the image found in the primary slot.

The swap will only take place after the signature on the plaintext is verified. Note that the plaintext firmware image is available in the primary slot only after the swap has been completed, unless "dummy decrypt" is used to compute the hash over the plaintext prior to executing the decrypt operation during a swap. Dummy decryption here refers to the decryption of the firmware image found in the secondary slot sector-by-sector and computing a rolling hash over the resulting plaintext firmware image (also sector-by-sector) without performing the swap operation. While there are performance optimizations possible, such as conveying hashes for each sector in the manifest rather than a hash of the entire firmware image, such optimizations are not described in this specification.

This approach of swapping the newly downloaded image with the previously valid image requires two slots 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.

Since the image in primary slot is available in cleartext, it may need to be re-encrypted before copying it to the secondary slot. This may be necessary when the secondary slot has different access permissions or when the staging area is located in off-chip flash memory and is therefore more vulnerable to physical attacks. Note that this description assumes that the processor does not execute encrypted memory by using on-the-fly decryption in hardware.

+--------------------------------------------------+
| Bootloader                                       |
+--------------------------------------------------+
| Primary Slot                                     |
|                                        (sector 1)|
|..................................................|
|                                                  |
|                                        (sector 2)|
|..................................................|
|                                                  |
|                                        (sector 3)|
|..................................................|
|                                                  |
|                                        (sector 4)|
+--------------------------------------------------+
| Secondary Slot                                   |
|                                        (sector 1)|
|..................................................|
|                                                  |
|                                        (sector 2)|
|..................................................|
|                                                  |
|                                        (sector 3)|
|..................................................|
|                                                  |
|                                        (sector 4)|
+--------------------------------------------------+
| Swap Area                                        |
|                                                  |
+--------------------------------------------------+
| Configuration Data                               |
+--------------------------------------------------+
Figure 11: Example Flash Area Layout

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 firmware image into sectors and to encrypt each sector 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 an update gets aborted while the bootloader is decrypting the newly obtained image and swapping the sectors, the bootloader can restart where it left off. This technique offers robustness and better performance.

For this purpose, ciphers without integrity protection are used to encrypt the firmware image. Integrity protection of the firmware image MUST be provided and the suit-parameter-image-digest, defined in Section 8.4.8.6 of [I-D.ietf-suit-manifest], MUST be used.

[I-D.ietf-cose-aes-ctr-and-cbc] registers AES Counter (AES-CTR) mode and AES Cipher Block Chaining (AES-CBC) ciphers that do not offer integrity protection. These ciphers are useful for use cases that require firmware encryption on IoT devices. For many other use cases where software packages, configuration information or personalization data need to be encrypted, the use of Authenticated Encryption with Associated Data (AEAD) ciphers is RECOMMENDED.

The following sub-sections provide further information about the initialization vector (IV) selection for use with AES-CBC and AES-CTR in the firmware encryption context. An IV MUST NOT be re-used when the same key is used. For this application, the IVs are not random but rather based on the slot/sector-combination in flash memory. The text below assumes that the block-size of AES is (much) smaller than the sector size. The typical sector-size of flash memory is in the order of KiB. Hence, multiple AES blocks need to be decrypted until an entire sector is completed.

7.1. AES-CBC

In AES-CBC, a single IV is used for encryption of firmware belonging to a single sector, since individual AES blocks are chained together, as shown in Figure 12. The numbering of sectors in a slot MUST start with zero (0) and MUST increase by one with every sector till the end of the slot is reached. The IV follows this numbering.

For example, let us assume the slot size of a specific flash controller on an IoT device is 64 KiB, the sector size 4096 bytes (4 KiB) and AES-128-CBC uses an AES-block size of 128 bit (16 bytes). Hence, sector 0 needs 4096/16=256 AES-128-CBC operations using IV 0. If the firmware image fills the entire slot, then that slot contains 16 sectors, i.e. IVs ranging from 0 to 15.

       P1              P2
        |              |
   IV--(+)    +-------(+)
        |     |        |
        |     |        |
    +-------+ |    +-------+
    |       | |    |       |
    |       | |    |       |
 k--|  E    | | k--|  E    |
    |       | |    |       |
    +-------+ |    +-------+
        |     |        |
        +-----+        |
        |              |
        |              |
        C1             C2

Legend:
  Pi = Plaintext blocks
  Ci = Ciphertext blocks
  E = Encryption function
  k = Symmetric key
  (+) = XOR operation
Figure 12: AES-CBC Operation

7.2. AES-CTR

Unlike AES-CBC, AES-CTR uses an IV per AES operation, as shown in Figure 13. Hence, when an image is encrypted using AES-CTR-128 or AES-CTR-256, the IV MUST start with zero (0) and MUST be incremented by one for each 16-byte plaintext block within the entire slot.

Using the previous example with a slot size of 64 KiB, the sector size 4096 bytes and the AES plaintext block size of 16 byte requires IVs from 0 to 255 in the first sector and 16 * 256 IVs for the remaining sectors in the slot.

         IV1            IV2
          |              |
          |              |
          |              |
      +-------+      +-------+
      |       |      |       |
      |       |      |       |
   k--|  E    |   k--|  E    |
      |       |      |       |
      +-------+      +-------+
          |              |
     P1--(+)        P2--(+)
          |              |
          |              |
          C1             C2

Legend:
  See previous diagram.
Figure 13: AES-CTR Operation

7.3. Battery Exhaustion Attacks

The use of flash memory opens up for another attack. An attacker may swap detached payloads and thereby force the device to process a wrong payload. While this attack will be detected, a device may have performed energy-expensive flash operations already. These operations may reduce the lifetime of devices when they are battery powered Iot devices. See Section 7 for further discussion about IoT devices using flash memory.

Including the digest of the encrypted payload allows the device to detect a battery exhaustion attack before energy consuming decryption and flash operations took place. Including the digest of the plaintext payload is adequate when battery exhaustion attacks are not a concern.

8. Complete Examples

The following manifests exemplify how to deliver encrypted payload and its encryption info to devices.

HMAC-256 MAC are added in AES-KW examples using the following secret key:

  'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'
  (0x616161... in hex, and its length is 32)

ES-DH examples are signed using the following ECDSA secp256r1 key:

-----BEGIN PRIVATE KEY-----
MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQgApZYjZCUGLM50VBC
CjYStX+09jGmnyJPrpDLTz/hiXOhRANCAASEloEarguqq9JhVxie7NomvqqL8Rtv
P+bitWWchdvArTsfKktsCYExwKNtrNHXi9OB3N+wnAUtszmR23M4tKiW
-----END PRIVATE KEY-----

The corresponding public key can be used to verify these examples:

-----BEGIN PUBLIC KEY-----
MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEhJaBGq4LqqvSYVcYnuzaJr6qi/Eb
bz/m4rVlnIXbwK07HypLbAmBMcCjbazR14vTgdzfsJwFLbM5kdtzOLSolg==
-----END PUBLIC KEY-----

Each example uses SHA-256 as the digest function.

8.1. AES Key Wrap Example with Write Directive

The following SUIT manifest requests a parser to authenticate the manifest with COSE_Mac0 HMAC256, and to write and to decrypt the encrypted payload into a component with the suit-directive-write directive.

The SUIT manifest in diagnostic notation (with line breaks added for readability) is shown here:

/ SUIT_Envelope_Tagged / 107({
  / authentication-wrapper / 2: << [
    << [
      / digest-algorithm-id: / -16 / SHA256 /,
      / digest-bytes: / h'6117ACE74E900E65C560D609BBC34C90
                          E4A33E340B12444D525C427E4AD9FF10'
    ] >>,
    << / COSE_Mac0_Tagged / 17([
      / protected: / << {
        / algorithm-id / 1: 5 / HMAC256 /
      } >>,
      / unprotected: / {},
      / payload: / null,
      / tag: / h'02B4BCA870F62D6A351C13A52B622D2C
                 3779D43823A2FD20C08147A8A337A391'
    ]) >>
  ] >>,
  / manifest / 3: << {
    / manifest-version / 1: 1,
    / manifest-sequence-number / 2: 1,
    / common / 3: << {
      / components / 2: [
        ['plaintext-firmware']
      ]
    } >>,
    / install / 17: << [
      / fetch encrypted firmware /
      / directive-override-parameters / 20, {
        / parameter-content / 18:
          h'9890D8DC740A2E82C2BEA9BAB13E0BFA0FB4EB2BA3C0BC
            A4B23A0D660C5B3038F8634933921B3C2D1A84EE6C2779',
        / parameter-encryption-info / 19: << 96([
          / protected: / << {
            / alg / 1: 1 / AES-GCM-128 /
          } >>,
          / unprotected: / {
            / IV / 5: h'93702C81590F845D9EC866CCAC767BD1'
          },
          / payload: / null / detached ciphertext /,
          / recipients: / [
            [
              / protected: / << {
              } >>,
              / unprotected: / {
                / alg / 1: -3 / A128KW /,
                / kid / 4: 'kid-1'
              },
              / payload: /
                h'CA0DF4EE01796D12FE379165602E0AC2476391A4EF122A64'
                / CEK encrypted with KEK /
            ]
          ]
        ]) >>
      },

      / decrypt encrypted firmware /
      / directive-write / 18, 15
        / consumes the SUIT_Encryption_Info above /
    ] >>
  } >>
})

In hex format, the SUIT manifest is this:

D86BA2025853825824822F58206117ACE74E900E65C560D609BBC34C90E4
A33E340B12444D525C427E4AD9FF10582AD18443A10105A0F6582002B4BC
A870F62D6A351C13A52B622D2C3779D43823A2FD20C08147A8A337A39103
589DA4010102010357A102818152706C61696E746578742D6669726D7761
726511587C8414A212582E9890D8DC740A2E82C2BEA9BAB13E0BFA0FB4EB
2BA3C0BCA4B23A0D660C5B3038F8634933921B3C2D1A84EE6C2779135843
D8608443A10101A1055093702C81590F845D9EC866CCAC767BD1F6818341
A0A2012204456B69642D315818CA0DF4EE01796D12FE379165602E0AC247
6391A4EF122A64120F

8.2. AES Key Wrap Example with Fetch + Copy Directives

The following SUIT manifest requests a parser to fetch the encrypted payload and to stores it. Then, the payload is decrypted and stored into another component with the suit-directive-copy directive. This approach works well on constrained devices with execute-in-place flash memory.

The SUIT manifest in diagnostic notation (with line breaks added for readability) is shown here:

/ SUIT_Envelope_Tagged / 107({
  / authentication-wrapper / 2: << [
    << [
      / digest-algorithm-id: / -16 / SHA256 /,
      / digest-bytes: / h'9300AD376FE65C505593C82B78F14299
                          BD0125A477720C044AD0552ABD27AAF6'
    ] >>,
    << / COSE_Mac0_Tagged / 17([
      / protected: / << {
        / algorithm-id / 1: 5 / HMAC256 /
      } >>,
      / unprotected: / {},
      / payload: / null,
      / tag: / h'F48509F3027EEAA2C40473212C3A12F2
                 5A9C8BE6699E5E7936816836E91A9003'
    ]) >>
  ] >>,
  / manifest / 3: << {
    / manifest-version / 1: 1,
    / manifest-sequence-number / 2: 1,
    / common / 3: << {
      / components / 2: [
        ['plaintext-firmware'],
        ['encrypted-firmware']
      ]
    } >>,
    / install / 17: << [
      / fetch encrypted firmware /
      / directive-set-component-index / 12, 1 / ['encrypted-firmware'] /,
      / directive-override-parameters / 20, {
        / parameter-image-size / 14: 46,
        / parameter-uri / 21: "https://example.com/encrypted-firmware"
      },
      / directive-fetch / 21, 15,

      / decrypt encrypted firmware /
      / directive-set-component-index / 12, 0 / ['plaintext-firmware'] /,
      / directive-override-parameters / 20, {
        / parameter-encryption-info / 19: << 96([
          / protected: / << {
            / alg / 1: 1 / AES-GCM-128 /
          } >>,
          / unprotected: / {
            / IV / 5: h'93702C81590F845D9EC866CCAC767BD1'
          },
          / payload: / null / detached ciphertext /,
          / recipients: / [
            [
              / protected: / << {
              } >>,
              / unprotected: / {
                / alg / 1: -3 / A128KW /,
                / kid / 4: 'kid-1'
              },
              / payload: /
                h'CA0DF4EE01796D12FE379165602E0AC2476391A4EF122A64'
                / CEK encrypted with KEK /
            ]
          ]
        ]) >>,
        / parameter-source-component / 22: 1 / ['encrypted-firmware'] /
      },
      / directive-copy / 22, 15 / consumes the SUIT_Encryption_Info above /
    ] >>
  } >>
})

In hex format, the SUIT manifest is this:

D86BA2025853825824822F58209300AD376FE65C505593C82B78F14299BD
0125A477720C044AD0552ABD27AAF6582AD18443A10105A0F65820F48509
F3027EEAA2C40473212C3A12F25A9C8BE6699E5E7936816836E91A900303
58B7A40101020103582BA102828152706C61696E746578742D6669726D77
6172658152656E637279707465642D6669726D776172651158818C0C0114
A20E182E15782668747470733A2F2F6578616D706C652E636F6D2F656E63
7279707465642D6669726D77617265150F0C0014A2135843D8608443A101
01A1055093702C81590F845D9EC866CCAC767BD1F6818341A0A201220445
6B69642D315818CA0DF4EE01796D12FE379165602E0AC2476391A4EF122A
641601160F

8.3. ES-DH Example with Write + Copy Directives

The following SUIT manifest requests a parser to authenticate the manifest with COSE_Sign1 ES256, and to write and to decrypt the encrypted payload into a component with the suit-directive-write directive.

The SUIT manifest in diagnostic notation (with line breaks added for readability) is shown here:

/ SUIT_Envelope_Tagged / 107({
  / authentication-wrapper / 2: << [
    << [
      / digest-algorithm-id: / -16 / SHA256 /,
      / digest-bytes: / h'AA45EE17A2345F8161926980949C9CB3
                          0EB928BE302A1198B5F298434472DDA1'
    ] >>,
    << / COSE_Sign1_Tagged / 18([
      / protected: / << {
        / algorithm-id / 1: -7 / ES256 /
      } >>,
      / unprotected: / {},
      / payload: / null,
      / signature: /
        h'30CA0FF4223B0FBD9084B453624A4284
          6F5BE7B724CBBEF33C334F3C89699A7B
          1C0B2D97805A6F45707125EEBC51A807
          560064EA38498E48F33743DD29561B08'
    ]) >>
  ] >>,
  / manifest / 3: << {
    / manifest-version / 1: 1,
    / manifest-sequence-number / 2: 1,
    / common / 3: << {
      / components / 2: [
        ['decrypted-firmware']
      ]
    } >>,
    / install / 17: << [
      / directive-set-component-index / 12, 0 /
        ['plaintext-firmware'] /,
      / directive-override-parameters / 20, {
        / parameter-content / 18:
          h'C7C4583D2763F3ECCF09FD1EB34EC9296426899510DAF3
            098E849C8B4F8F5364638B309447D6B6393B899F8F0AEE',
        / parameter-encryption-info / 19: << 96([
          / protected: / << {
            / alg / 1: 1 / AES-GCM-128 /
          } >>,
          / unprotected: / {
            / IV / 5: h'1485CADEC69C011B5CC3B0BE0F2B3801'
          },
          / payload: / null / detached ciphertext /,
          / recipients: / [
            [
              / protected: / << {
                / alg / 1: -29 / ECDH-ES + A128KW /
              } >>,
              / unprotected: / {
                / ephemeral key / -1: {
                  / kty / 1: 2 / EC2 /,
                  / crv / -1: 1 / P-256 /,
                  / x / -2: h'3982C1863015824881D1A6C9059332BE
                              281C9613D7F7462D43EE520D20FE132F',
                  / y / -3: h'5EB51EF8AD7C1DA697294877BBA291DC
                              5AEE20FEE887D8A173BAD7FAFF091E5C'
                },
                / kid / 4: 'kid-2'
              },
              / payload: /
                h'DF698D95D3BF3EA7CCC655A7A5609BEF206E208A46D66D91'
                / CEK encrypted with KEK /
            ]
          ]
        ]) >>
      },
      / directive-write / 18, 15
        / consumes the SUIT_Encryption_Info above /
    ] >>
  } >>
})

In hex format, the SUIT manifest is this:

D86BA2025873825824822F5820AA45EE17A2345F8161926980949C9CB30E
B928BE302A1198B5F298434472DDA1584AD28443A10126A0F6584030CA0F
F4223B0FBD9084B453624A42846F5BE7B724CBBEF33C334F3C89699A7B1C
0B2D97805A6F45707125EEBC51A807560064EA38498E48F33743DD29561B
080358ECA4010102010357A1028181526465637279707465642D6669726D
776172651158CB860C0014A212582EC7C4583D2763F3ECCF09FD1EB34EC9
296426899510DAF3098E849C8B4F8F5364638B309447D6B6393B899F8F0A
EE135890D8608443A10101A105501485CADEC69C011B5CC3B0BE0F2B3801
F6818344A101381CA220A4010220012158203982C1863015824881D1A6C9
059332BE281C9613D7F7462D43EE520D20FE132F2258205EB51EF8AD7C1D
A697294877BBA291DC5AEE20FEE887D8A173BAD7FAFF091E5C04456B6964
2D325818DF698D95D3BF3EA7CCC655A7A5609BEF206E208A46D66D91120F

8.4. ES-DH Example with Dependency

The following SUIT manifest requests a parser to resolve the delegation chain and dependency respectively. The parser validates the COSE_Key in the suit-delegation section using the key above, and then dynamically trusts it. The dependency manifest is embedded as an integrated-dependency and referred by uri "#dependency-manifest" .

The SUIT manifest in diagnostic notation (with line breaks added for readability) is shown here:

/ SUIT_Envelope_Tagged / 107({
  / delegation / 1: << [
    [
      / NOTE: signed by trust anchor /
      << 18([
        / protected: / << {
          / alg / 1: -7 / ES256 /
        } >>,
        / unprotected / {
        },
        / payload: / << {
          / cnf / 8: {
            / NOTE: public key of delegated authority /
            / COSE_Key / 1: {
              / kty / 1: 2 / EC2 /,
              / crv / -1: 1 / P-256 /,
              / x / -2: h'0E908AA8F066DB1F084E0C3652C63952
                          BD99F2A5BDB22F9E01367AAD03ABA68B',
              / y / -3: h'77DA1BD8AC4F0CB490BA210648BF79AB
                          164D49AD3551D71D314B2749EE42D29A'
            }
          }
        } >>,
        / signature: /
          h'FB2D5ACF66B9C8573CE92E13BFB8D113
            F798715CC10B5A0010B11925C155E724
            5A64E131073B87AC50CAC71650A21315
            B82D06CA2298CD1A95519AAE4C4B5315'
      ]) >>
    ]
  ] >>,
  / authentication-wrapper / 2: << [
    << [
      / digest-algorithm-id: / -16 / SHA256 /,
      / digest-bytes: /
        h'5D7F604EE23212C615CB83246F17BD06
          4F237CEA31170183AFCD52D08EE4F58B'
    ] >>,
    << / COSE_Sign1_Tagged / 18([
      / protected: / << {
        / algorithm-id / 1: -7 / ES256 /
      } >>,
      / unprotected: / {},
      / payload: / null,
      / signature:
        / h'F5B14132C023ACBFD0BEC7954A63E94D
            2B2795B303FD6FB4031A5FD016353D72
            17BDC2AA6F0EF04A628B452F0ACD5A10
            EEDA04E5AD0B766B3C30838B4581B5B4'
    ]) >>
  ] >>,
  / manifest / 3: << {
    / manifest-version / 1: 1,
    / manifest-sequence-number / 2: 1,
    / common / 3: << {
      / dependencies / 1: {
        / component-index / 1: {
          / dependency-prefix / 1: [
             'dependency-manifest.suit'
          ]
        }
      },
      / components / 2: [
        ['decrypted-firmware']
      ]
    } >>,
    / manifest-component-id / 5: [
      'dependent-manifest.suit'
    ],
    / install / 17: << [
      / NOTE: set SUIT_Encryption_Info /
      / directive-set-component-index / 12, 0 / ['decrypted-firmware'] /,
      / directive-override-parameters / 20, {
        / parameter-content / 18:
          h'C7C4583D2763F3ECCF09FD1EB34EC9296426899510DAF3
            098E849C8B4F8F5364638B309447D6B6393B899F8F0AEE',
        / parameter-encryption-info / 19: << 96([
          / protected: / << {
            / alg / 1: 1 / AES-GCM-128 /
          } >>,
          / unprotected: / {
            / IV / 5: h'1485CADEC69C011B5CC3B0BE0F2B3801'
          },
          / payload: / null / detached ciphertext /,
          / recipients: / [
            [
              / protected: / << {
                / alg / 1: -29 / ECDH-ES + A128KW /
              } >>,
              / unprotected: / {
                / ephemeral key / -1: {
                  / kty / 1: 2 / EC2 /,
                  / crv / -1: 1 / P-256 /,
                  / x / -2: h'3982C1863015824881D1A6C9059332BE
                              281C9613D7F7462D43EE520D20FE132F',
                  / y / -3: h'5EB51EF8AD7C1DA697294877BBA291DC
                              5AEE20FEE887D8A173BAD7FAFF091E5C'
                },
                / kid / 4: 'kid-2'
              },
              / payload: /
                h'DF698D95D3BF3EA7CCC655A7A5609BEF206E208A46D66D91'
                / CEK encrypted with KEK /
            ]
          ]
        ]) >>
      },

      / NOTE: call dependency-manifest /
      / directive-set-component-index / 12, 1
        / ['dependenty-manifest.suit'] /,
      / directive-override-parameters / 20, {
        / parameter-image-digest / 3: << [
          / algorithm-id / -16 / SHA256 /,
          / digest-bytes / h'1051324059C5193317CAC9A099BBC0B6
                             AFB56184C04277F566A3A4131F4A1C25'
        ] >>,
        / parameter-image-size / 14: 247,
        / parameter-uri / 21: "#dependency-manifest"
      },
      / directive-fetch / 21, 15,
      / condition-dependency-integrity / 7, 15,
      / directive-process-dependency / 11, 15
    ] >>
  } >>,
  "#dependency-manifest": <<
    / SUIT_Envelope_Tagged / 107({
      / authentication-wrapper / 2: << [
        << [
          / digest-algorithm-id: / -16 / SHA256 /,
          / digest-bytes: / h'1051324059C5193317CAC9A099BBC0B6
                              AFB56184C04277F566A3A4131F4A1C25'
        ] >>,
        << / COSE_Sign1_Tagged / 18([
          / protected: / << {
            / algorithm-id / 1: -7 / ES256 /
          } >>,
          / unprotected: / {},
          / payload: / null,
          / signature: /
            h'55990F3745DC4F200FF946643A6DE30D
              DCE57B080B7D68DE9896D8190B9A63E2
              D60E7C3D9693B67221AA6D07BBF0AB45
              314C236827A242C22B5E688DDC467269'
        ]) >>
      ] >>,
      / manifest / 3: << {
        / manifest-version / 1: 1,
        / manifest-sequence-number / 2: 1,
        / common / 3: << {
          / components / 2: [
            ['decrypted-firmware']
          ],
          / shared-sequence / 4: << [
            / directive-set-componnt-index / 12, 0
              / ['decrypted-firmware'] /,
            / directive-override-parameters / 20, {
              / parameter-image-digest / 3: << [
                / algorithm-id / -16 / SHA256 /,
                / digest-bytes / h'36921488FE6680712F734E11F58D87EE
                                   B66D4B21A8A1AD3441060814DA16D50F'
              ] >>,
              / parameter-image-size / 14: 30
            }
          ] >>
        } >>,
        / manifest-component-id / 5: [
          'dependency-manifest.suit'
        ],
        / validate / 7: << [
          / condition-image-match / 3, 15
        ] >>,
        / install / 17: << [
          / directive-set-component-index / 12, 0
            / ['decrypted-firmware'] /,
          / directive-write / 18, 15
            / consumes the SUIT_Encryption_Info set by dependent /,
          / condition-image-match / 3, 15
            / check the integrity of the decrypted payload /
        ] >>
      } >>
    })
  >>
})

In hex format, the SUIT manifest is this:

D86BA401589E8181589AD28443A10126A0584FA108A101A4010220012158
200E908AA8F066DB1F084E0C3652C63952BD99F2A5BDB22F9E01367AAD03
ABA68B22582077DA1BD8AC4F0CB490BA210648BF79AB164D49AD3551D71D
314B2749EE42D29A5840FB2D5ACF66B9C8573CE92E13BFB8D113F798715C
C10B5A0010B11925C155E7245A64E131073B87AC50CAC71650A21315B82D
06CA2298CD1A95519AAE4C4B5315025873825824822F58205D7F604EE232
12C615CB83246F17BD064F237CEA31170183AFCD52D08EE4F58B584AD284
43A10126A0F65840F5B14132C023ACBFD0BEC7954A63E94D2B2795B303FD
6FB4031A5FD016353D7217BDC2AA6F0EF04A628B452F0ACD5A10EEDA04E5
AD0B766B3C30838B4581B5B403590170A501010201035837A201A101A101
815818646570656E64656E63792D6D616E69666573742E73756974028181
526465637279707465642D6669726D77617265058157646570656E64656E
742D6D616E69666573742E73756974115901138E0C0014A212582EC7C458
3D2763F3ECCF09FD1EB34EC9296426899510DAF3098E849C8B4F8F536463
8B309447D6B6393B899F8F0AEE135890D8608443A10101A105501485CADE
C69C011B5CC3B0BE0F2B3801F6818344A101381CA220A401022001215820
3982C1863015824881D1A6C9059332BE281C9613D7F7462D43EE520D20FE
132F2258205EB51EF8AD7C1DA697294877BBA291DC5AEE20FEE887D8A173
BAD7FAFF091E5C04456B69642D325818DF698D95D3BF3EA7CCC655A7A560
9BEF206E208A46D66D910C0114A3035824822F58201051324059C5193317
CAC9A099BBC0B6AFB56184C04277F566A3A4131F4A1C250E18F715742364
6570656E64656E63792D6D616E6966657374150F070F0B0F742364657065
6E64656E63792D6D616E696665737458F7D86BA2025873825824822F5820
1051324059C5193317CAC9A099BBC0B6AFB56184C04277F566A3A4131F4A
1C25584AD28443A10126A0F6584055990F3745DC4F200FF946643A6DE30D
DCE57B080B7D68DE9896D8190B9A63E2D60E7C3D9693B67221AA6D07BBF0
AB45314C236827A242C22B5E688DDC46726903587BA601010201035849A2
028181526465637279707465642D6669726D7761726504582F840C0014A2
035824822F582036921488FE6680712F734E11F58D87EEB66D4B21A8A1AD
3441060814DA16D50F0E181E05815818646570656E64656E63792D6D616E
69666573742E73756974074382030F1147860C00120F030F

9. Operational Considerations

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

Both cases require some upfront communication interaction to distribute these keys to the involved communication parties. This interaction may be provided by a device management protocol, as described in [RFC9019], or may be executed earlier in the lifecycle of the device, for example during manufacturing or during commissioning. In addition to the keying material key identifiers and algorithm information need to be provisioned. This specification places no requirements on the structure of the key identifier.

In some cases third party companies analyse binaries for known security vulnerabilities. With encrypted payloads, 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 payloads. In either case, it is necessary to explicitly consider those third parties in the software supply chain when such a binary analysis is desired.

10. Security Considerations

This entire document is about security.

Note that it is good security practise to use different long-term keys for different purpose. For example, the KEK used with an AES-KW-based content key distribution method for encryption should be different from the long-term symmetric key used for authentication and integrity protection when uses with COSE_Mac0.

The design of this specification allows to use different long-term keys for encrypting payloads. For example, KEK_1 may be used with an AES-KW content key distribution method to encrypt a firmware image while KEK_2 would be used to encrypt configuration data. This approach reduces the attack surface since permissions of authors to these long-term keys may vary based on their privileges.

11. IANA Considerations

IANA is asked to add the following value to the SUIT Parameters registry established by Section 11.5 of [I-D.ietf-suit-manifest]:

Label      Name                 Reference
-----------------------------------------
TBD19      Encryption Info      Section 4

[Editor's Note: TBD19: Proposed 19]

12. References

12.1. Normative References

[I-D.ietf-cose-aes-ctr-and-cbc]
Housley, R. and H. Tschofenig, "CBOR Object Signing and Encryption (COSE): AES-CTR and AES-CBC", Work in Progress, Internet-Draft, draft-ietf-cose-aes-ctr-and-cbc-06, , <https://datatracker.ietf.org/doc/html/draft-ietf-cose-aes-ctr-and-cbc-06>.
[I-D.ietf-suit-manifest]
Moran, B., Tschofenig, H., Birkholz, H., Zandberg, K., and O. Rønningstad, "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-23, , <https://datatracker.ietf.org/doc/html/draft-ietf-suit-manifest-23>.
[I-D.ietf-suit-trust-domains]
Moran, B. and K. Takayama, "SUIT Manifest Extensions for Multiple Trust Domains", Work in Progress, Internet-Draft, draft-ietf-suit-trust-domains-05, , <https://datatracker.ietf.org/doc/html/draft-ietf-suit-trust-domains-05>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/rfc/rfc2119>.
[RFC3394]
Schaad, J. and R. Housley, "Advanced Encryption Standard (AES) Key Wrap Algorithm", RFC 3394, DOI 10.17487/RFC3394, , <https://www.rfc-editor.org/rfc/rfc3394>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC9052]
Schaad, J., "CBOR Object Signing and Encryption (COSE): Structures and Process", STD 96, RFC 9052, DOI 10.17487/RFC9052, , <https://www.rfc-editor.org/rfc/rfc9052>.
[RFC9053]
Schaad, J., "CBOR Object Signing and Encryption (COSE): Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053, , <https://www.rfc-editor.org/rfc/rfc9053>.

12.2. Informative References

[iana-suit]
Internet Assigned Numbers Authority, "IANA SUIT Manifest Registry", , <TBD>.
[RFC5280]
Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, , <https://www.rfc-editor.org/rfc/rfc5280>.
[RFC5652]
Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, DOI 10.17487/RFC5652, , <https://www.rfc-editor.org/rfc/rfc5652>.
[RFC5869]
Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand Key Derivation Function (HKDF)", RFC 5869, DOI 10.17487/RFC5869, , <https://www.rfc-editor.org/rfc/rfc5869>.
[RFC8937]
Cremers, C., Garratt, L., Smyshlyaev, S., Sullivan, N., and C. Wood, "Randomness Improvements for Security Protocols", RFC 8937, DOI 10.17487/RFC8937, , <https://www.rfc-editor.org/rfc/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, , <https://www.rfc-editor.org/rfc/rfc9019>.
[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, , <https://www.rfc-editor.org/rfc/rfc9124>.
[ROP]
Wikipedia, "Return-Oriented Programming", , <https://en.wikipedia.org/wiki/Return-oriented_programming>.
[SP800-56]
NIST, "Recommendation for Pair-Wise Key Establishment Schemes Using Discrete Logarithm Cryptography, NIST Special Publication 800-56A Revision 3", , <http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-56Ar3.pdf>.

Appendix A. A. Full CDDL

The following CDDL must be appended to the SUIT Manifest CDDL. The SUIT CDDL is defined in Appendix A of [I-D.ietf-suit-manifest]

; Define SUIT_Encryption_Info_* as a subset of COSE_Encrypt

SUIT_Encryption_Info = #6.96(
    SUIT_Encryption_Info_AESKW .within COSE_Encrypt /
    SUIT_Encryption_Info_ESDH .within COSE_Encrypt)

SUIT_Encryption_Info_AESKW = [
  protected   : bstr .cbor outer_header_map_protected,
  unprotected : outer_header_map_unprotected,
  ciphertext  : bstr / nil,
  recipients  : [ + COSE_recipient_AESKW .within COSE_recipient ]
]

COSE_recipient_AESKW = [
  protected   : bstr .size 0 / bstr .cbor empty_map,
  unprotected : recipient_header_unpr_map_aeskw,
  ciphertext  : bstr        ; CEK encrypted with KEK
]
empty_map = {}

recipient_header_unpr_map_aeskw =
{
    1 => int,         ; algorithm identifier
  ? 4 => bstr,        ; identifier of the recipient public key
  * label => values   ; extension point
}

SUIT_Encryption_Info_ESDH = [
  protected   : bstr .cbor outer_header_map_protected,
  unprotected : outer_header_map_unprotected,
  ciphertext  : bstr / nil,
  recipients  : [ + COSE_recipient_ESDH .within COSE_recipient ]
]

COSE_recipient_ESDH = [
  protected   : bstr .cbor recipient_header_map_esdh,
  unprotected : recipient_header_unpr_map_esdh,
  ciphertext  : bstr        ; CEK encrypted with KEK
]

recipient_header_map_esdh =
{
    1 => int,         ; algorithm identifier
  * label => values   ; extension point
}

recipient_header_unpr_map_esdh =
{
   -1 => COSE_Key,    ; ephemeral public key for the sender
  ? 4 => bstr,        ; identifier of the recipient public key
  * label => values   ; extension point
}

; common definitions
outer_header_map_protected =
{
    1 => int,         ; algorithm identifier
  * label => values   ; extension point
}

outer_header_map_unprotected =
{
    5 => bstr,        ; IV
  * label => values   ; extension point
}


; Extends SUIT Manifest

$$SUIT_Parameters //= (suit-parameter-encryption-info =>
    bstr .cbor SUIT_Encryption_Info)

suit-parameter-encryption-info = 19

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, Øyvind Rønningstad, Dave Thaler, Laurence Lundblade, Christian Amsüss, and Carsten Bormann for their review feedback. Finally, we would like to thank Dick Brooks for making us aware of the challenges encryption imposes on binary analysis.

Authors' Addresses

Hannes Tschofenig
Russ Housley
Vigil Security, LLC
Brendan Moran
Arm Limited
David Brown
Linaro
Ken Takayama
SECOM CO., LTD.