Encrypted Payloads in SUIT Manifests

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 10 June 2025.¶

Copyright Notice

Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved.¶

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶

Table of Contents

1. Introduction

Vulnerabilities in Internet of Things (IoT) devices have highlighted the need for a reliable and secure firmware update mechanism, especially for constrained devices. To protect firmware images, the SUIT manifest format was developed [I-D.ietf-suit-manifest]. A manifest is a bundle of metadata about the firmware for an IoT device, where to find the firmware, and the devices to which it applies. [RFC9124] outlines the necessary information a SUIT manifest has to provide. In addition to protecting against modification via digital signatures or message authentication codes, the format can also offer confidentiality.¶

Encryption prevents third parties, including attackers, from accessing the payload. Attackers often require detailed knowledge of a binary, such as a firmware image, to launch successful attacks. For instance, return-oriented programming (ROP) [ROP] requires access to the binary, and encryption makes writing exploits significantly more difficult. Beyond ensuring the confidentiality of the binary itself, protecting the confidentiality of the source code will also be necessary to prevent reverse engineering and reproduction of the firmware.¶

While the initial motivation for this document was firmware encryption, the use of SUIT manifests has expanded to cover other scenarios requiring integrity and confidentiality protection, such as:¶

• software packages,¶

• personalization data,¶

• configuration data, and¶

• machine learning models.¶

Thus, the term "payload" is used generically to refer to all these objects.¶

The payload is encrypted using a symmetric content encryption key, which can be established through various mechanisms. This document defines two content key distribution methods for use with the IETF SUIT manifest:¶

• Ephemeral-Static (ES) Diffie-Hellman (DH), and¶

• AES Key Wrap (AES-KW).¶

The first method relies on asymmetric cryptography, while the second uses symmetric cryptography.¶

Our design aims to reduce the number of content key distribution methods for payload encryption, thereby increasing interoperability between different SUIT manifest parser implementations.¶

The goal of this specification is to protect payloads both during end-to-end transport (from the distribution system to the device) and at rest when stored on the device. Constrained devices often employ eXecute In Place (XIP), a method of executing code directly from flash memory rather than loading it into RAM. Many of these devices lack hardware-based, on-the-fly decryption for code stored in flash memory, which may require decrypting and storing firmware images in on-chip flash before execution. However, we expect hardware-based, on-the-fly decryption to become more common in the future, enhancing confidentiality at rest.¶

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:¶

• Key Wrap (KW), defined in [RFC3394] (for use with AES)¶

• Key-Encryption Key (KEK) [RFC3394]¶

• Content-Encryption Key (CEK) [RFC5652]¶

• Ephemeral-Static (ES) Diffie-Hellman (DH) [RFC9052]¶

• Authenticated Encryption with Associated Data (AEAD)¶

• Execute in Place (XIP)¶

The terms sender and recipient have the following meaning:¶

• Sender: Entity that sends an encrypted payload.¶

• Recipient: Entity that receives an encrypted payload.¶

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 may, for example, be a developer building a firmware image.¶

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] outlines the architecture for distributing payloads and manifests from an author to devices. However, it does not cover payload encryption in detail. This document extends that architecture to support encryption, as illustrated in Figure 1.¶

To encrypt a payload, it is essential to know the recipient. For AES-KW, the Key Encryption Key (KEK) must be known, and for ES-DH, the sender needs access to the recipient's public key. This public key and its associated parameters may be found in the recipient's X.509 certificate [RFC5280]. For authentication and integrity protection, recipients must be provisioned with a trust anchor when the manifest is protected by a digital signature. If a MAC is used for manifest protection, a symmetric key must be shared between the recipient and the sender.¶

With encryption, the author cannot simply create and sign a manifest for the payload, as the recipients are often unknown. Therefore, the author must collaborate with the distribution system. The degree of this collaboration is discussed below.¶

The primary purpose of encryption is to protect against adversaries along the path between the distribution system and the device. There is also a risk that adversaries may extract the decrypted firmware image from the device itself. Consequently, the device must be safeguarded against physical attacks, though such countermeasures are typically beyond the scope of IETF specifications.¶

Note: It is assumed that the communication between the author and the distribution system is secure. For example, the author might upload the manifest and firmware image to the distribution system using a secure HTTPS REST API.¶

When the author delegates encryption rights to the distributor, two models are possible:¶

1. Replacing the COSE_Encrypt and Re-signing the Manifest: The distributor replaces the COSE_Encrypt structure in the manifest and then signs the manifest again. However, since the COSE_Encrypt structure is within a signed container, this presents a challenge: replacing COSE_Encrypt alters the digest of the manifest, thereby invalidating the signature. As a result, the distributor must be able to sign the new manifest. If this is the case, the distributor gains the authority to construct and sign manifests, effectively allowing them to sign code and giving them full control over the recipient. Distributors typically perform re-encryption online to manage large numbers of devices efficiently, which prevents air-gapping the signing operations. This has implications for the recommendations in Section 4.3.17 of [RFC9124]. Despite these issues, this model represents the current standard practice for IoT firmware updates.¶

2. Two-Layer Manifest System: The distributor creates a new manifest that overrides the COSE_Encrypt using the dependency system defined in [I-D.ietf-suit-trust-domains]. This method introduces additional overhead, including one more signature verification, one extra manifest, and the need for extra mechanisms on the recipient side to handle dependency processing. While this adds complexity, it also enhances security.¶

These two models offer different threat profiles for the distributor. If the distributor is limited to encryption rights, an attacker who breaches the distributor can only launch a limited attack by encrypting a modified binary. However, recipients will detect the attack during the image digest check and immediately revert to the correct image.¶

It is RECOMMENDED that distributors adopt the two-layer manifest approach to distribute content encryption keys without re-signing the manifest, despite the added complexity and the increased number of signature verifications required on the recipient side.¶

4. Encryption Extensions

This specification introduces a new extension to the SUIT_Parameters structure.¶

The SUIT_Encryption_Info structure (referred to as suit-parameter-encryption-info in Figure 2) contains the content key distribution information. The details of the SUIT_Encryption_Info structure are provided in Section 6.1 (for AES-KW) and Section 6.2 (for ES-DH).¶

Once a CEK is available, the steps outlined in Section 7 apply to both content key distribution methods described in this section.¶

The SUIT_Encryption_Info structure is carried within either the suit-directive-override-parameters or suit-directive-set-parameters used in the "Directive Write" and "Directive Copy" operations. An implementation conforming to this specification must support these two parameters.¶

Interoperability requirements for content key distribution methods vary: since a device will typically support only one of the two specified methods, the distribution system needs to know which method is supported. Limiting a constrained device to a single content key distribution method also helps reduce code size.¶

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

suit-parameter-encryption-info = TBD19

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 the following directives:¶

• Directive Write (suit-directive-write): Used to decrypt the content specified by suit-parameter-content using suit-parameter-encryption-info.¶

• Directive Copy (suit-directive-copy): Used to decrypt the content of the component specified by suit-parameter-source-component using suit-parameter-encryption-info.¶

Examples of these two directives are provided below, focusing on the essential aspects. A complete example for AES Key Wrap with the Fetch and Copy directives can be found in Section 10.2, while an example illustrating the Write directive is shown in Section 10.1.¶

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

/ directive-override-parameters / 20, {
  / parameter-content / 18: h'EA1...CED',
  / parameter-encryption-info / TBD19: h'D86...1F0'
},
/ directive-write / 18, 15

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

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 referred to by 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-encryption-info / TBD19: h'D86...1F0',
  / parameter-source-component / 22: 1
},
/ directive-copy / 22, 15

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

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 7 are applicable. These steps apply to both content key distribution methods.¶

6. Content Key Distribution

The following sub-sections describe two content key distribution methods: AES Key Wrap (AES-KW) and Ephemeral-Static Diffie-Hellman (ES-DH). While many other methods are specified in the literature and supported by COSE, AES-KW and ES-DH were chosen for their widespread use in the market today. They were selected for their maturity, differing security properties, and strong interoperability.¶

Both content key distribution methods require the CEKs to be randomly generated. The guidelines for random number generation in [RFC8937] MUST be followed.¶

When sending an encrypted payload to multiple recipients, various deployment options are available. The following notation is used to explain these options:¶

  - KEK[R1, S] refers to a KEK shared between recipient R1 and
    the sender S.
  - 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.

     1. Fetch KEK[*, S]
     2. Generate CEK
     3. ENC(CEK, KEK)
     4. ENC(payload, CEK)

    1.  Generate CEK
    2.  for i=1 to n
        {
    2a.    Fetch KEK[Ri, S]
    2b.    ENC(CEK, KEK[Ri, S])
        }
    3.  ENC(payload, CEK)

    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.  }

=============== NOTE: '\' line wrapping per RFC 8792 ================

SUIT_Encryption_Info_AESKW = #6.96([
  protected   : outer_header_map_protected,
  unprotected : outer_header_map_unprotected,
  ciphertext  : bstr / nil,
  recipients  : [ + COSE_recipient_AESKW ]
])

outer_header_map_protected = empty_or_serialized_map
outer_header_map_unprotected = header_map

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
}

    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)

    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])
        }

SUIT_Encryption_Info_ESDH = #6.96([
  protected   : outer_header_map_protected,
  unprotected : outer_header_map_unprotected,
  ciphertext  : bstr / nil,
  recipients  : [ + COSE_recipient_ESDH ]
])

outer_header_map_protected = empty_or_serialized_map
outer_header_map_unprotected = header_map

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

COSE_KDF_Context = [
    AlgorithmID : int,
    PartyUInfo : [ PartyInfoSender ],
    PartyVInfo : [ PartyInfoRecipient ],
    SuppPubInfo : [
        keyDataLength : uint,
        protected : bstr,
        other: 'SUIT Payload Encryption'
    ],
    ? SuppPrivInfo : bstr
]

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

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

7. Content Encryption

This section summarizes the steps involved in content encryption, applicable to both content key distribution methods.¶

When using AEAD ciphers, such as AES-GCM or ChaCha20/Poly1305, the COSE specification requires a consistent byte stream to create the authenticated data structure. This structure is illustrated in Figure 8 and defined in Section 5.3 of [RFC9052].¶

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

This Enc_structure must be populated as follows:¶

• The protected field in the Enc_structure from Figure 8 refers to the content of the protected field in the COSE_Encrypt structure.¶

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

Some ciphers, such as AES-CTR and AES-CBC, provide confidentiality without integrity protection (see [RFC9459]). For these ciphers, the Enc_structure shown in Figure 8 cannot be used, as the Additional Authenticated Data (AAD) byte string is only applicable to AEAD ciphers. Therefore, the AAD structure is not passed to the API for these ciphers, and the protected header in the SUIT_Encryption_Info structure MUST be a zero-length byte string.¶

AES-CTR and AES-CBC are discussed in separate sub-sections below and Figure 11 and Figure 14 use the following abbreviations:¶

• Pi = Plaintext blocks¶

• Ci = Ciphertext blocks¶

• E = Encryption function¶

• k = Symmetric key¶

• ⊕ = XOR operation¶

D8608443A10101A1054CF14AAB9D81D51F7AD943FE87F6818340A2012204
456B69642D31581875603FFC9518D794713C8CA8A115A7FB32565A6D5953
4D62

96([
  / protected: / << {
    / alg / 1: 1 / A128GCM /
  } >>,
  / unprotected: / {
    / IV / 5: h'F14AAB9D81D51F7AD943FE87'
  },
  / payload: / null / detached ciphertext /,
  / recipients: / [
    [
      / protected: / h'',
      / unprotected: / {
        / alg / 1: -3 / A128KW /,
        / kid / 4: 'kid-1'
      },
      / payload: /
        h'75603FFC9518D794713C8CA8A115A7FB32565A6D59534D62'
        / CEK encrypted with KEK /
    ]
  ]
])

758C4B7BBAE2C4C1D462423E0F0DC3164FFA7B85BB94D4BD6D7ED26AB32F
EB063385D4D3465927EC82CB5E198A59

D8608443A10101A1054CF14AAB9D81D51F7AD943FE87F6818344A101381C
A120A40102200121582073024F415AA51529A66CCEFD88F3F62A734492FF
45F6AD37FD2888E73EAF19DA2258204005B48A6FD091AA6ABFE3CFBEEDE8
8B347E521D43405FDBD7D2CFF0EBC21B265818A06B8E6550F308712B1DF0
44B21B7D11D9B22792F1DE0997

96([
  / protected: / << {
    / alg / 1: 1 / A128GCM /
  } >>,
  / unprotected: / {
    / IV / 5: h'F14AAB9D81D51F7AD943FE87'
  },
  / 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'73024F415AA51529A66CCEFD88F3F62A
                      734492FF45F6AD37FD2888E73EAF19DA',
          / y / -3: h'4005B48A6FD091AA6ABFE3CFBEEDE88B
                      347E521D43405FDBD7D2CFF0EBC21B26'
        }
      },
      / payload:
        / h'A06B8E6550F308712B1DF044B21B7D11D9B22792F1DE0997'
        / CEK encrypted with KEK /
    ]
  ]
])

758C4B7BBAE2C4C1D462423E0F0DC3164FFA7B85BB94D4BD6D7ED26AB32F
EB063385D4D3465927EC82CB5E198A59

D8608440A20139FFFD0550DAE613B2E0DC55F4322BE38BDBA9DC68F68183
40A2012204456B69642D315818CE34035CE5C2E2666E46D4C131FC561DD1
90A6D26CFA1990

96([
  / protected: / h'',
  / unprotected: / {
    / alg / 1: -65534 / A128CTR /,
    / IV / 5: h'DAE613B2E0DC55F4322BE38BDBA9DC68'
  },
  / payload: / null / detached ciphertext /,
  / recipients: / [
    [
      / protected: / h'',
      / unprotected: / {
        / alg / 1: -3 / A128KW /,
        / kid / 4: 'kid-1'
      },
      / payload: /
        h'CE34035CE5C2E2666E46D4C131FC561DD190A6D26CFA1990'
        / CEK encrypted with KEK /
    ]
  ]
])

2BB8DB522AE978246CC775C3B0241BD4B0333FFDD2DB70C7EE7A4966E3B7

D8608440A20139FFFD0550DAE613B2E0DC55F4322BE38BDBA9DC68F68183
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4066DDCEDC2F0DBEF873CB224C715C1174225820279F2A88E4AB9E2ED30C
0FCB69515B31B5D36725BFDB9AE02032ED4D5AB52CB85818E28B4502E4F5
151884A995405579006E9465C3E94E3E0808

=============== NOTE: '\' line wrapping per RFC 8792 ================

96([
  / protected: / h'',
  / unprotected: / {
    / alg / 1: -65534 / A128CTR /,
    / IV / 5: h'DAE613B2E0DC55F4322BE38BDBA9DC68'
  },
  / 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'EE0718F6B019C29CC611C18CEDE22140
                      66DDCEDC2F0DBEF873CB224C715C1174',
          / y / -3: h'279F2A88E4AB9E2ED30C0FCB69515B31
                      B5D36725BFDB9AE02032ED4D5AB52CB8'
        }
      },
      / payload: / h'\
                    E28B4502E4F5151884A995405579006E9465C3E94E3E0808'
        / CEK encrypted with KEK /
    ]
  ]
])

2BB8DB522AE978246CC775C3B0241BD4B0333FFDD2DB70C7EE7A4966E3B7

D8608440A20139FFFA055093702C81590F845D9EC866CCAC767BD1F68183
40A2012204456B69642D315818E198FF269626EC43299D33586FC7B2646B
13292261160422

96([
  / protected: / h'',
  / unprotected: / {
    / alg / 1: -65531 / A128CBC /,
    / IV / 5: h'93702C81590F845D9EC866CCAC767BD1'
  },
  / payload: / null / detached ciphertext /,
  / recipients: / [
    [
      / protected: / h'',
      / unprotected: / {
        / alg / 1: -3 / A128KW /,
        / kid / 4: 'kid-1'
      },
      / payload: /
        h'E198FF269626EC43299D33586FC7B2646B13292261160422'
        / CEK encrypted with KEK /
    ]
  ]
])

9C09156CF4ACE0401086D98586E0B09FA5B5CF78F2BCCBF6C914DDB42BF0
E21E

D8608440A20139FFFA055093702C81590F845D9EC866CCAC767BD1F68183
44A101381CA120A40102200121582084C1768A1AFA92BAF27C3289A3FAF7
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5037B1FB726CAE3D0C77976864EF725DB2B30ABCE6F6581804928D52CD34
AC8CA0427CEED6D7C5D06A21B5F4F79CAA18

=============== NOTE: '\' line wrapping per RFC 8792 ================

96([
  / protected: / h'',
  / unprotected: / {
    / alg / 1: -65531 / A128CBC /,
    / IV / 5: h'93702C81590F845D9EC866CCAC767BD1'
  },
  / 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'84C1768A1AFA92BAF27C3289A3FAF75E
                      7511B27C654947BBD391E8A12EB8295B',
          / y / -3: h'F32B0AECA4F8B6C51C155037B1FB726C
                      AE3D0C77976864EF725DB2B30ABCE6F6'
        }
      },
      / payload: / h'\
                    04928D52CD34AC8CA0427CEED6D7C5D06A21B5F4F79CAA18'
        / CEK encrypted with KEK /
    ]
  ]
])

9C09156CF4ACE0401086D98586E0B09FA5B5CF78F2BCCBF6C914DDB42BF0
E21E

8. Integrity Check on Encrypted and Decrypted Payloads

In addition to suit-condition-image-match (see Section 8.4.9.2 of [I-D.ietf-suit-manifest]), AEAD algorithms used for content encryption provides another way to validate the integrity of components. This section provides a guideline to construct secure but not redundant SUIT Manifest for encrypted payloads.¶

/ 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-image-digest / 3: << {
    / algorithm-id: / -16 / SHA256 /,
    / digest-bytes: / h'3B1...92A' / digest of plaintext payload /
  } >>,
  / parameter-image-size / 14: 30 / size of plaintext payload /,
  / parameter-encryption-info / TBD19: h'369...50F',
  / parameter-source-component / 22: 1
},
/ directive-copy / 22, 15,
/ condition-image-match / 3, 15 / check decrypted payload integrity /,

/ directive-set-component-index / 12, 1,
/ directive-override-parameters / 20, {
  / parameter-image-digest / 3: << {
    / algorithm-id: / -16 / SHA256 /,
    / digest-bytes: / h'8B4...D34' / digest of encrypted payload /
  } >>,
  / parameter-image-size / 14: 30 / size of encrypted payload /,
  / parameter-uri / 21: "http://example.com/encrypted.bin"
},
/ directive-fetch / 21, 15,
/ condition-image-match / 3, 15 / check decrypted payload integrity /,

/ directive-set-component-index / 12, 0,
/ directive-override-parameters / 20, {
  / parameter-encryption-info / TBD19: h'D86...1F0',
  / parameter-source-component / 22: 1
},
/ directive-copy / 22, 15,

9. Firmware Updates on IoT Devices with Flash Memory

Embedded devices come in many forms, and the market is both large and fragmented. As a result, some implementations and deployments may adopt firmware update procedures that differ from the descriptions provided here. On a positive note, the SUIT manifest accommodates various deployment scenarios, thanks to the "scripting" functionality offered by its commands.¶

This section specifically addresses firmware images on microcontrollers and does not pertain to generic software, configuration data, or machine learning models. The differences arise from two main aspects:¶

• Use of Flash Memory: Flash memory in microcontrollers is a type of non-volatile memory that erases data in larger units called blocks, pages, or sectors, and rewrites data at the byte level (often 4 bytes) or larger units. Furthermore, flash memory is segmented into different regions, storing the bootloader, various versions of firmware images (in designated slots), and configuration data. An example layout of a microcontroller flash area is illustrated in Figure 20.¶

• Microcontroller Design: Code on microcontrollers typically cannot be executed from arbitrary locations in flash memory without additional software development and design efforts. Consequently, developers often compile firmware so that the bootloader can execute code from a specific location in flash memory, commonly referred to as the "primary slot."¶

Once the encrypted firmware image is transferred to the device, it is usually stored in a dedicated area known as the "secondary slot."¶

During the next boot, the bootloader detects the new firmware image and begins decrypting it sector by sector, swapping it with the image located in the primary slot. This method of swapping the newly downloaded image with the previously valid one requires two slots, allowing for a rollback if the new firmware fails to boot, thereby enhancing the robustness of the firmware update process.¶

The swap occurs only after verifying the signature on the plaintext. It is important to note that the plaintext firmware image is available in the primary slot only after the swap is completed, unless "dummy decrypt" is used to compute the hash over the plaintext prior to executing the decryption during the swap. In this context, dummy decryption refers to decrypting the firmware image in the secondary slot sector by sector while computing a rolling hash over the resulting plaintext (also sector by sector) without performing the swap operation. Although performance optimizations, such as conveying hashes for each sector in the manifest rather than a hash of the entire firmware image, are possible, these optimizations are not detailed in this specification.¶

Without hardware-based, on-the-fly decryption, the image in the primary slot is available in cleartext and may need to be re-encrypted before copying it to the secondary slot. This step might be necessary if the secondary slot has different access permissions or is located in off-chip flash memory, which tends to be more vulnerable to physical attacks.¶

The ability to resume an interrupted firmware update is often essential for unattended devices, including low-end, constrained IoT devices. To meet this requirement, a firmware image must be divided into sectors, with each sector encrypted individually using a cipher that does not increase the size of the resulting ciphertext (i.e., by avoiding the addition of an authentication tag after each encrypted block).¶

If an update is aborted while the bootloader is decrypting the newly received image and swapping the sectors, the bootloader can restart from where it left off. This technique enhances robustness and performance.¶

For this purpose, ciphers without integrity protection are employed to encrypt the firmware image. It is crucial that integrity protection for the firmware image is provided, and the suit-parameter-image-digest, defined in Section 8.4.8.6 of [I-D.ietf-suit-manifest], MUST be utilized.¶

[RFC9459] specifies the AES Counter (AES-CTR) mode and AES Cipher Block Chaining (AES-CBC) ciphers, both of which do not provide integrity protection. These ciphers are suitable for firmware encryption in IoT devices. However, for many other scenarios involving software packages, configuration information, or personalization data, the use of AEAD ciphers is RECOMMENDED.¶

The following subsections offer additional information on the selection of initialization vectors (IVs) for use with AES-CBC and AES-CTR in the context of firmware encryption. An IV MUST NOT be reused when the same key is employed. In this application, the IVs are not random but are instead based on the slot/sector combination in flash memory. The discussion assumes that the block size of AES is significantly smaller than the sector size. Typically, flash memory sectors are measured in KiB, necessitating the decryption of multiple AES blocks to complete the decryption of an entire sector.¶

10. Complete Examples

The following manifests illustrate how to deliver an encrypted payload along with its encryption information to devices.¶

In the AES-KW examples, HMAC-256 MACs are included, utilizing the following secret key:¶

  'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'
  (616161... 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.¶

=============== NOTE: '\' line wrapping per RFC 8792 ================

/  1/ / SUIT_Envelope_Tagged / 107({
/  2/   / authentication-wrapper / 2: << [
/  3/     << [
/  4/       / digest-algorithm-id: / -16 / SHA256 /,
/  5/       / digest-bytes: / h'037A5C325CE14078A0AADF007428EAC6
/  6/                           59361AD9402A732410BDA542FAE94E2C'
/  7/     ] >>,
/  8/     << / COSE_Mac0_Tagged / 17([
/  9/       / protected: / << {
/ 10/         / algorithm-id / 1: 5 / HMAC256 /
/ 11/       } >>,
/ 12/       / unprotected: / {},
/ 13/       / payload: / null,
/ 14/       / tag: / h'8D92599011C451A4C5FB69709FA6CA6C
/ 15/                  0F846D692BDBB3F624EC91F82F9F620A'
/ 16/     ]) >>
/ 17/   ] >>,
/ 18/   / manifest / 3: << {
/ 19/     / manifest-version / 1: 1,
/ 20/     / manifest-sequence-number / 2: 1,
/ 21/     / common / 3: << {
/ 22/       / components / 2: [
/ 23/         ['plaintext-firmware']
/ 24/       ]
/ 25/     } >>,
/ 26/     / install / 20: << [
/ 27/       / fetch encrypted firmware /
/ 28/       / directive-override-parameters / 20, {
/ 29/         / parameter-content / 18:
/ 30/           h'758C4B7BBAE2C4C1D462423E0F0DC3164FFA7B85BB94D4
/ 31/             BD6D7ED26AB32FEB063385D4D3465927EC82CB5E198A59',
/ 32/         / parameter-encryption-info / 19: << 96([
/ 33/           / protected: / << {
/ 34/             / alg / 1: 1 / A128GCM /
/ 35/           } >>,
/ 36/           / unprotected: / {
/ 37/             / IV / 5: h'F14AAB9D81D51F7AD943FE87'
/ 38/           },
/ 39/           / payload: / null / detached ciphertext /,
/ 40/           / recipients: / [
/ 41/             [
/ 42/               / protected: / h'',
/ 43/               / unprotected: / {
/ 44/                 / alg / 1: -3 / A128KW /,
/ 45/                 / kid / 4: 'kid-1'
/ 46/               },
/ 47/               / payload: /
/ 48/               h'\
                    75603FFC9518D794713C8CA8A115A7FB32565A6D59534D62'
/ 49/                 / CEK encrypted with KEK /
/ 50/             ]
/ 51/           ]
/ 52/         ]) >>
/ 53/       },
/ 54/
/ 55/       / decrypt encrypted firmware /
/ 56/       / directive-write / 18, 15
/ 57/         / consumes the SUIT_Encryption_Info above /
/ 58/     ] >>
/ 59/   } >>
/ 60/ })
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=============== NOTE: '\' line wrapping per RFC 8792 ================

/  1/ / SUIT_Envelope_Tagged / 107({
/  2/   / authentication-wrapper / 2: << [
/  3/     << [
/  4/       / digest-algorithm-id: / -16 / SHA256 /,
/  5/       / digest-bytes: / h'8814BC46089ACA6A863A7BA8393F9747
/  6/                           589940EFA40641335EF86155598F06C3'
/  7/     ] >>,
/  8/     << / COSE_Mac0_Tagged / 17([
/  9/       / protected: / << {
/ 10/         / algorithm-id / 1: 5 / HMAC256 /
/ 11/       } >>,
/ 12/       / unprotected: / {},
/ 13/       / payload: / null,
/ 14/       / tag: / h'B68572F6F0494FEAF390CE44B462F2A7
/ 15/                  BDF73EF5DFE9FB8E12585A12F8F641AD'
/ 16/     ]) >>
/ 17/   ] >>,
/ 18/   / manifest / 3: << {
/ 19/     / manifest-version / 1: 1,
/ 20/     / manifest-sequence-number / 2: 1,
/ 21/     / common / 3: << {
/ 22/       / components / 2: [
/ 23/         ['plaintext-firmware'],
/ 24/         ['encrypted-firmware']
/ 25/       ]
/ 26/     } >>,
/ 27/     / install / 20: << [
/ 28/       / fetch encrypted firmware /
/ 29/       / directive-set-component-index / 12, 1
/ 30/         / ['encrypted-firmware'] /,
/ 31/       / directive-override-parameters / 20, {
/ 32/         / parameter-image-size / 14: 46,
/ 33/         / parameter-uri / 21:
/ 34/           "https://example.com/encrypted-firmware"
/ 35/       },
/ 36/       / directive-fetch / 21, 15,
/ 37/
/ 38/       / decrypt encrypted firmware /
/ 39/       / directive-set-component-index / 12, 0
/ 40/         / ['plaintext-firmware'] /,
/ 41/       / directive-override-parameters / 20, {
/ 42/         / parameter-encryption-info / 19: << 96([
/ 43/           / protected: / << {
/ 44/             / alg / 1: 1 / A128GCM /
/ 45/           } >>,
/ 46/           / unprotected: / {
/ 47/             / IV / 5: h'F14AAB9D81D51F7AD943FE87'
/ 48/           },
/ 49/           / payload: / null / detached ciphertext /,
/ 50/           / recipients: / [
/ 51/             [
/ 52/               / protected: / h'',
/ 53/               / unprotected: / {
/ 54/                 / alg / 1: -3 / A128KW /,
/ 55/                 / kid / 4: 'kid-1'
/ 56/               },
/ 57/               / payload: /
/ 58/               h'\
                    75603FFC9518D794713C8CA8A115A7FB32565A6D59534D62'
/ 59/                 / CEK encrypted with KEK /
/ 60/             ]
/ 61/           ]
/ 62/         ]) >>,
/ 63/         / parameter-source-component / 22: 1
/ 64/           / ['encrypted-firmware'] /
/ 65/       },
/ 66/       / directive-copy / 22, 15
/ 67/         / consumes the SUIT_Encryption_Info above /
/ 68/     ] >>
/ 69/   } >>
/ 70/ })

D86BA2025853825824822F58208814BC46089ACA6A863A7BA8393F974758
9940EFA40641335EF86155598F06C3582AD18443A10105A0F65820B68572
F6F0494FEAF390CE44B462F2A7BDF73EF5DFE9FB8E12585A12F8F641AD03
58B2A40101020103582BA102828152706C61696E746578742D6669726D77
6172658152656E637279707465642D6669726D7761726514587C8C0C0114
A20E182E15782668747470733A2F2F6578616D706C652E636F6D2F656E63
7279707465642D6669726D77617265150F0C0014A213583ED8608443A101
01A1054CF14AAB9D81D51F7AD943FE87F6818340A2012204456B69642D31
581875603FFC9518D794713C8CA8A115A7FB32565A6D59534D621601160F

=============== NOTE: '\' line wrapping per RFC 8792 ================

/  1/ / SUIT_Envelope_Tagged / 107({
/  2/   / authentication-wrapper / 2: << [
/  3/     << [
/  4/       / digest-algorithm-id: / -16 / SHA256 /,
/  5/       / digest-bytes: / h'A6D2C13A0DEFD57A09FA65C24206A9A8
/  6/                           747261E6EA017532B3DAB0419C42A2FC'
/  7/     ] >>,
/  8/     << / COSE_Mac0_Tagged / 17([
/  9/       / protected: / << {
/ 10/         / algorithm-id / 1: 5 / HMAC256 /
/ 11/       } >>,
/ 12/       / unprotected: / {},
/ 13/       / payload: / null,
/ 14/       / tag: / h'DB350AC9603B0BBA3895B85019699363
/ 15/                  3F314A9066634B31BF62E596FD870434'
/ 16/     ]) >>
/ 17/   ] >>,
/ 18/   / manifest / 3: << {
/ 19/     / manifest-version / 1: 1,
/ 20/     / manifest-sequence-number / 2: 1,
/ 21/     / common / 3: << {
/ 22/       / components / 2: [
/ 23/         [h'00'],
/ 24/         [h'01']
/ 25/       ]
/ 26/     } >>,
/ 27/     / install / 20: << [
/ 28/       / fetch encrypted firmware /
/ 29/       / directive-set-component-index / 12, 1 / [h'01'] /,
/ 30/       / directive-override-parameters / 20, {
/ 31/         / parameter-image-size / 14: 46,
/ 32/         / parameter-uri / 21:
/ 33/           "https://example.com/encrypted-firmware"
/ 34/       },
/ 35/       / directive-fetch / 21, 15,
/ 36/
/ 37/       / decrypt encrypted firmware /
/ 38/       / directive-set-component-index / 12, 0 / ['00'] /,
/ 39/       / directive-override-parameters / 20, {
/ 40/         / parameter-encryption-info / 19: << 96([
/ 41/           / protected: / << {
/ 42/             / alg / 1: 1 / A128GCM /
/ 43/           } >>,
/ 44/           / unprotected: / {
/ 45/             / IV / 5: h'F14AAB9D81D51F7AD943FE87'
/ 46/           },
/ 47/           / payload: / null / detached ciphertext /,
/ 48/           / recipients: / [
/ 49/             [
/ 50/               / protected: / h'',
/ 51/               / unprotected: / {
/ 52/                 / alg / 1: -3 / A128KW /,
/ 53/                 / kid / 4: 'kid-1'
/ 54/               },
/ 55/               / payload: /
/ 56/               h'\
                    75603FFC9518D794713C8CA8A115A7FB32565A6D59534D62'
/ 57/                 / CEK encrypted with KEK /
/ 58/             ]
/ 59/           ]
/ 60/         ]) >>,
/ 61/         / parameter-source-component / 22: 1 / [h'01'] /
/ 62/       },
/ 63/       / directive-copy / 22, 15
/ 64/         / consumes the SUIT_Encryption_Info above /
/ 65/     ] >>
/ 66/   } >>
/ 67/ })

=============== NOTE: '\' line wrapping per RFC 8792 ================

/  1/ / SUIT_Envelope_Tagged / 107({
/  2/   / authentication-wrapper / 2: << [
/  3/     << [
/  4/       / digest-algorithm-id: / -16 / SHA256 /,
/  5/       / digest-bytes: / h'1DB69EF1477E9942815F29F78E09957B
/  6/                           26B4ADD03902BDB3D1EDF3DA2075F593'
/  7/     ] >>,
/  8/     << / COSE_Sign1_Tagged / 18([
/  9/       / protected: / << {
/ 10/         / algorithm-id / 1: -7 / ES256 /
/ 11/       } >>,
/ 12/       / unprotected: / {},
/ 13/       / payload: / null,
/ 14/       / signature: / h'CB4EADA6BEC17EEB22EB836FB2BF9136
/ 15/                        A6EF733C11DAC955F543BBDCAA373B85
/ 16/                        9321BC77969917E4C70F049527607F4C
/ 17/                        32752D53E01346E96BFF4880B437DF64'
/ 18/     ]) >>
/ 19/   ] >>,
/ 20/   / manifest / 3: << {
/ 21/     / manifest-version / 1: 1,
/ 22/     / manifest-sequence-number / 2: 1,
/ 23/     / common / 3: << {
/ 24/       / components / 2: [
/ 25/         ['decrypted-firmware']
/ 26/       ]
/ 27/     } >>,
/ 28/     / install / 20: << [
/ 29/       / directive-set-component-index / 12, 0
/ 30/         / ['plaintext-firmware'] /,
/ 31/       / directive-override-parameters / 20, {
/ 32/         / parameter-content / 18:
/ 33/           h'758C4B7BBAE2C4C1D462423E0F0DC3164FFA7B85BB94D4
/ 34/             BD6D7ED26AB32FEB063385D4D3465927EC82CB5E198A59',
/ 35/         / parameter-encryption-info / 19: << 96([
/ 36/           / protected: / << {
/ 37/             / alg / 1: 1 / A128GCM /
/ 38/           } >>,
/ 39/           / unprotected: / {
/ 40/             / IV / 5: h'F14AAB9D81D51F7AD943FE87'
/ 41/           },
/ 42/           / payload: / null / detached ciphertext /,
/ 43/           / recipients: / [
/ 44/             [
/ 45/               / protected: / << {
/ 46/                 / alg / 1: -29 / ECDH-ES + A128KW /
/ 47/               } >>,
/ 48/               / unprotected: / {
/ 49/                 / ephemeral key / -1: {
/ 50/                   / kty / 1: 2 / EC2 /,
/ 51/                   / crv / -1: 1 / P-256 /,
/ 52/                   / x / -2: h'73024F415AA51529A66CCEFD88F3F62A
/ 53/                               734492FF45F6AD37FD2888E73EAF19DA\
                                                                   ',
/ 54/                   / y / -3: h'4005B48A6FD091AA6ABFE3CFBEEDE88B
/ 55/                               347E521D43405FDBD7D2CFF0EBC21B26'
/ 56/                 },
/ 57/                 / kid / 4: 'kid-2'
/ 58/               },
/ 59/               / payload: /
/ 60/               h'\
                    A06B8E6550F308712B1DF044B21B7D11D9B22792F1DE0997'
/ 61/                 / CEK encrypted with KEK /
/ 62/             ]
/ 63/           ]
/ 64/         ]) >>
/ 65/       },
/ 66/       / directive-write / 18, 15
/ 67/         / consumes the SUIT_Encryption_Info above /
/ 68/     ] >>
/ 69/   } >>
/ 70/ })
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-----BEGIN EC PRIVATE KEY-----
MHcCAQEEIIQa67e56m8CYL5zVaJFiLl30j0qxb8ray2DeUMqH+qYoAoGCCqGSM49
AwEHoUQDQgAEDpCKqPBm2x8ITgw2UsY5Ur2Z8qW9si+eATZ6rQOrpot32hvYrE8M
tJC6IQZIv3mrFk1JrTVR1x0xSydJ7kLSmg==
-----END EC PRIVATE KEY-----

=============== NOTE: '\' line wrapping per RFC 8792 ================

/  1/ / SUIT_Envelope_Tagged / 107({
/  2/   / authentication-wrapper / 2: << [
/  3/     << [
/  4/       / digest-algorithm-id: / -16 / SHA256 /,
/  5/       / digest-bytes: / h'A00CB6C85515C1EF471B50B542FACDD8
/  6/                           8B71B3C7EA2A43DE13D32C4A99056FE9'
/  7/     ] >>,
/  8/     << / COSE_Sign1_Tagged / 18([
/  9/       / protected: / << {
/ 10/         / algorithm-id / 1: -7 / ES256 /
/ 11/       } >>,
/ 12/       / unprotected: / {},
/ 13/       / payload: / null,
/ 14/       / signature: / h'421B30FE76DA848616D72FC1115EA610
/ 15/                        5578CB95DF9C6BEAD931105C9D555CF8
/ 16/                        CD38C8FD68ACE43445D8D2CAE6391A99
/ 17/                        5A212487D92F8DAD789F65511AC61778'
/ 18/     ]) >>
/ 19/   ] >>,
/ 20/   / manifest / 3: << {
/ 21/     / manifest-version / 1: 1,
/ 22/     / manifest-sequence-number / 2: 1,
/ 23/     / common / 3: << {
/ 24/       / dependencies / 1: {
/ 25/         / component-index / 1: {
/ 26/           / dependency-prefix / 1: [
/ 27/              'dependency-manifest.suit'
/ 28/           ]
/ 29/         }
/ 30/       },
/ 31/       / components / 2: [
/ 32/         ['decrypted-firmware']
/ 33/       ]
/ 34/     } >>,
/ 35/     / manifest-component-id / 5: [
/ 36/       'dependent-manifest.suit'
/ 37/     ],
/ 38/     / install / 20: << [
/ 39/       / NOTE: set SUIT_Encryption_Info /
/ 40/       / directive-set-component-index / 12, 0
/ 41/         / ['decrypted-firmware'] /,
/ 42/       / directive-override-parameters / 20, {
/ 43/         / parameter-content / 18:
/ 44/           h'758C4B7BBAE2C4C1D462423E0F0DC3164FFA7B85BB94D4
/ 45/             BD6D7ED26AB32FEB063385D4D3465927EC82CB5E198A59',
/ 46/         / parameter-encryption-info / 19: << 96([
/ 47/           / protected: / << {
/ 48/             / alg / 1: 1 / A128GCM /
/ 49/           } >>,
/ 50/           / unprotected: / {
/ 51/             / IV / 5: h'F14AAB9D81D51F7AD943FE87'
/ 52/           },
/ 53/           / payload: / null / detached ciphertext /,
/ 54/           / recipients: / [
/ 55/             [
/ 56/               / protected: / << {
/ 57/                 / alg / 1: -29 / ECDH-ES + A128KW /
/ 58/               } >>,
/ 59/               / unprotected: / {
/ 60/                 / ephemeral key / -1: {
/ 61/                   / kty / 1: 2 / EC2 /,
/ 62/                   / crv / -1: 1 / P-256 /,
/ 63/                   / x / -2: h'73024F415AA51529A66CCEFD88F3F62A
/ 64/                               734492FF45F6AD37FD2888E73EAF19DA\
                                                                   ',
/ 65/                   / y / -3: h'4005B48A6FD091AA6ABFE3CFBEEDE88B
/ 66/                               347E521D43405FDBD7D2CFF0EBC21B26'
/ 67/                 },
/ 68/                 / kid / 4: 'kid-2'
/ 69/               },
/ 70/               / payload: /
/ 71/               h'\
                    A06B8E6550F308712B1DF044B21B7D11D9B22792F1DE0997'
/ 72/                 / CEK encrypted with KEK /
/ 73/             ]
/ 74/           ]
/ 75/         ]) >>
/ 76/       },
/ 77/
/ 78/       / NOTE: call dependency-manifest /
/ 79/       / directive-set-component-index / 12, 1
/ 80/         / ['dependenty-manifest.suit'] /,
/ 81/       / directive-override-parameters / 20, {
/ 82/         / parameter-image-digest / 3: << [
/ 83/           / algorithm-id / -16 / SHA256 /,
/ 84/           / digest-bytes / h'4B15C90FBD776A820E7E733DF040D90B
/ 85/                              356B5C75982ECAECE8673818179BDF16'
/ 86/         ] >>,
/ 87/         / parameter-image-size / 14: 247,
/ 88/         / parameter-uri / 21: "#dependency-manifest"
/ 89/       },
/ 90/       / directive-fetch / 21, 15,
/ 91/       / condition-dependency-integrity / 7, 15,
/ 92/       / directive-process-dependency / 11, 15
/ 93/     ] >>
/ 94/   } >>,
/ 95/   "#dependency-manifest": <<
/ 96/     / SUIT_Envelope_Tagged / 107({
/ 97/       / authentication-wrapper / 2: << [
/ 98/         << [
/ 99/           / digest-algorithm-id: / -16 / SHA256 /,
/100/           / digest-bytes: / h'4B15C90FBD776A820E7E733DF040D90B
/101/                               356B5C75982ECAECE8673818179BDF16'
/102/         ] >>,
/103/         << / COSE_Sign1_Tagged / 18([
/104/           / protected: / << {
/105/             / algorithm-id / 1: -7 / ES256 /
/106/           } >>,
/107/           / unprotected: / {},
/108/           / payload: / null,
/109/           / signature: / h'2B1B9C4E44E52863A78F73DA2A935823
/110/                            B28AEAE6A85CADAC4C4E3AABAAD56CBC
/111/                            E5A47D288F86B54D0186657E972E748B
/112/                            48CDB1D420FBAC1285DCC978382F62CC'
/113/         ]) >>
/114/       ] >>,
/115/       / manifest / 3: << {
/116/         / manifest-version / 1: 1,
/117/         / manifest-sequence-number / 2: 1,
/118/         / common / 3: << {
/119/           / components / 2: [
/120/             ['decrypted-firmware']
/121/           ],
/122/           / shared-sequence / 4: << [
/123/             / directive-set-componnt-index / 12, 0
/124/               / ['decrypted-firmware'] /,
/125/             / directive-override-parameters / 20, {
/126/               / parameter-image-digest / 3: << [
/127/                 / algorithm-id / -16 / SHA256 /,
/128/                 / digest-bytes /
/129/                   h'36921488FE6680712F734E11F58D87EE
/130/                     B66D4B21A8A1AD3441060814DA16D50F'
/131/               ] >>,
/132/               / parameter-image-size / 14: 30
/133/             }
/134/           ] >>
/135/         } >>,
/136/         / manifest-component-id / 5: [
/137/           'dependency-manifest.suit'
/138/         ],
/139/         / validate / 7: << [
/140/           / condition-image-match / 3, 15
/141/         ] >>,
/142/         / install / 20: << [
/143/           / directive-set-component-index / 12, 0
/144/             / ['decrypted-firmware'] /,
/145/           / directive-write / 18, 15
/146/             / consumes the SUIT_Encryption_Info in dependent /,
/147/           / condition-image-match / 3, 15
/148/             / check the integrity of the decrypted payload /
/149/         ] >>
/150/       } >>
/151/     })
/152/   >>
/153/ })

D86BA3025873825824822F5820A00CB6C85515C1EF471B50B542FACDD88B
71B3C7EA2A43DE13D32C4A99056FE9584AD28443A10126A0F65840421B30
FE76DA848616D72FC1115EA6105578CB95DF9C6BEAD931105C9D555CF8CD
38C8FD68ACE43445D8D2CAE6391A995A212487D92F8DAD789F65511AC617
780359016CA501010201035837A201A101A101815818646570656E64656E
63792D6D616E69666573742E73756974028181526465637279707465642D
6669726D77617265058157646570656E64656E742D6D616E69666573742E
737569741459010F8E0C0014A212582E758C4B7BBAE2C4C1D462423E0F0D
C3164FFA7B85BB94D4BD6D7ED26AB32FEB063385D4D3465927EC82CB5E19
8A5913588CD8608443A10101A1054CF14AAB9D81D51F7AD943FE87F68183
44A101381CA220A40102200121582073024F415AA51529A66CCEFD88F3F6
2A734492FF45F6AD37FD2888E73EAF19DA2258204005B48A6FD091AA6ABF
E3CFBEEDE88B347E521D43405FDBD7D2CFF0EBC21B2604456B69642D3258
18A06B8E6550F308712B1DF044B21B7D11D9B22792F1DE09970C0114A303
5824822F58204B15C90FBD776A820E7E733DF040D90B356B5C75982ECAEC
E8673818179BDF160E18F7157423646570656E64656E63792D6D616E6966
657374150F070F0B0F7423646570656E64656E63792D6D616E6966657374
58F7D86BA2025873825824822F58204B15C90FBD776A820E7E733DF040D9
0B356B5C75982ECAECE8673818179BDF16584AD28443A10126A0F658402B
1B9C4E44E52863A78F73DA2A935823B28AEAE6A85CADAC4C4E3AABAAD56C
BCE5A47D288F86B54D0186657E972E748B48CDB1D420FBAC1285DCC97838
2F62CC03587BA601010201035849A2028181526465637279707465642D66
69726D7761726504582F840C0014A2035824822F582036921488FE668071
2F734E11F58D87EEB66D4B21A8A1AD3441060814DA16D50F0E181E058158
18646570656E64656E63792D6D616E69666573742E73756974074382030F
1447860C00120F030F

11. Operational Considerations

The algorithms outlined in this document assume that the party responsible for payload encryption:¶

• shares a key-encryption key (KEK) with the recipient (for use with the AES Key Wrap scheme), or¶

• possesses the recipient's public key (for use with ES-DH).¶

Both scenarios necessitate initial communication to distribute these keys among the involved parties. This interaction can be facilitated by a device management protocol, as described in [RFC9019], or may occur earlier in the device lifecycle, such as during manufacturing or commissioning. In addition to the keying material, key identifiers and algorithm information must also be provisioned. This specification does not impose any requirements on the structure of the key identifier.¶

In certain situations, third-party companies analyze binaries for known security vulnerabilities. However, encrypted payloads hinder this type of analysis. Consequently, these third-party companies must either be granted access to the plaintext binary before encryption or be authorized recipients of the encrypted payloads. In either case, it is essential to explicitly consider these third parties in the software supply chain when binary analysis is required.¶

12. Security Considerations

This entire document focuses on security.¶

It is considered best security practice to use different keys for different purposes. For instance, the key-encryption key (KEK) utilized in an AES-KW-based content key distribution method for encryption should be distinct from the long-term symmetric key employed for authentication in a communication security protocol.¶

To further minimize the attack surface, it may be advantageous to use different long-term keys for encrypting various types of payloads. For example, KEK_1 could be used with an AES-KW content key distribution method to encrypt a firmware image, while KEK_2 would encrypt configuration data.¶

A significant portion of this document addresses content key distribution, employing two methods: AES Key Wrap (AES-KW) and Ephemeral-Static Diffie-Hellman (ES-DH). The table below summarizes the main properties related to their deployment:¶

The use of firmware encryption in IoT devices introduces the risk of a battery exhaustion attack. This attack exploits the high energy cost of flash memory operations. To execute this attack, the adversary must be able to swap detached payloads and trick the device into processing an incorrect payload. Payload swapping is feasible only if there is no communication security protocol between the device and the distribution system or if the distribution system itself has been compromised.¶

While the security features provided by the manifest can detect this attack and prevent the device from booting with an incorrectly supplied payload, the energy-intensive flash operations will have already occurred. As a result, these operations can diminish the lifespan of the devices, making battery-powered IoT devices particularly susceptible to such attacks. For further discussion on IoT devices using flash memory, see Section 9.¶

Including the digest of the encrypted payload in the manifest enables the device to detect a battery exhaustion attack before energy-consuming decryption and flash memory copy or swap operations take place.¶

13. 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

RFC Editor's Note (TBD19): The value for the Encryption Info parameter is set to 19, as the proposed value.¶

14. References

Appendix 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]¶

SUIT_Encryption_Info = #6.96(COSE_Encrypt)

$$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, Dick Brooks, Øyvind Rønningstad, Dave Thaler, Laurence Lundblade, Christian Amsüss, Ruud Derwig, Martin Thomson. Kris Kwiatkowski, Suresh Krishnan and Carsten Bormann for their review feedback.¶

We would like to thank the IESG, in particular Deb Cooley, Éric Vyncke and Roman Danyliw, for their help to improve the quality of this document.¶

Authors' Addresses