]>

hannes.tschofenig@gmx.net

Vigil Security, LLC

housley@vigilsec.com

Arm Limited

Brendan.Moran@arm.com

Linaro

david.brown@linaro.org

SECOM CO., LTD.

ken.takayama.ietf@gmail.com

Security SUIT Internet-Draft 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.

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 . It provides a bundle of metadata, including where to find the payload, the devices to which it applies and a security wrapper. 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) requires access to the binary and encryption makes it much more difficult to write exploits. Beside confidentiality of the binary, confidentiality of the sources (e.g. in case of open source software) may be required as well to prevent reverse engineering and/or reproduction of the binary firmware. 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: software packages, personalization data, configuration data, and machine learning models. 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: Ephemeral-Static (ES) Diffie-Hellman (DH), and AES Key Wrap (AES-KW). The former method relies on asymmetric key cryptography while the latter uses symmetric key cryptography. Our design aims to reduce the number of content key distribution methods for use with payload encryption and thereby increase interoperability between different SUIT manifest parser implementations. The goal of this specification is to protect payloads during end-to-end transport, and at rest when stored on a device. Constrained devices often make use of XIP, which is a method of executing code directly from flash memory rather than copying it into RAM. Since many of these devices today do not offer hardware-based, on-the-fly decryption of code stored in flash memory, it may be necessary to decrypt and store firmware images in on-chip flash before code can be executed. We do, however, expect that hardware-based, on-the-fly decryption will become more common in the future, which will improve confidentiality at rest.

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 when, and only when, they appear in all capitals, as shown here. This document assumes familiarity with the IETF SUIT manifest , the SUIT information model , and the SUIT architecture . The following abbreviations are used in this document: Key Wrap (KW), defined in (for use with AES) Key-Encryption Key (KEK) Content-Encryption Key (CEK) Ephemeral-Static (ES) Diffie-Hellman (DH) 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 authors 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.

Architecture 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 and shows it graphically. To encrypt a payload it is necessary to know the recipient. 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 . 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 it typically does not know the recipients. Hence, the author has to collaborate with the distribution system. The varying degree of collaboration is discussed below.

The author has several deployment options, namely: The author, as the sender, obtains information about the recipients and their keys from the distribution system. There are proprietary as well as standardized device management solutions available providing this functionality, as discussed in . Then, it performs the necessary steps to encrypt the payload. As a last step it creates one or more manifests. The device(s) perform decryption and act as recipients. The author treats the distribution system as the initial recipient. The author typically uses REST APIs or web user interfaces to interact with the distribution system. Then, the distribution system decrypts and re-encrypts the payload for consumption by the device (or the devices). Delegating the task of re-encrypting the payload to the distribution system offers flexibility when the number of devices that need to receive encrypted payloads changes dynamically or when updates to KEKs or recipient public keys are necessary. As a downside, the author needs to trust the distribution system with performing the re-encryption of the payload. If the author delegates encryption rights to the distributor two models are possible: 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 . This model nevertheless represent the current state of firmware updates for IoT devices. The distributor uses a two-layer manifest system. More precisely, the distributor constructs a new manifest that overrides the COSE_Encrypt using the dependency system defined in . 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. This extra complexity offers extra security. 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 implement the two-layer manifest approach 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.

Encryption Extensions This specification introduces a new extension to the SUIT_Parameters structure. The SUIT_Encryption_Info structure (called suit-parameter-encryption-info in ) contains the content key distribution information. The content of the SUIT_Encryption_Info structure is explained in (for AES-KW) and in (for ES-DH). Once a CEK is available, the steps described in 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.

bstr .cbor SUIT_Encryption_Info) suit-parameter-encryption-info = 19 ]]>

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

Extended Directives This specification extends these directives: Directive Write (suit-directive-write) to decrypt the content specified by suit-parameter-content with suit-parameter-encryption-info. Directive Copy (suit-directive-copy) to decrypt the content of the component specified by suit-parameter-source-component with suit-parameter-encryption-info. Examples of the two directives are shown below. 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.

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.

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

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. AES-KW and ES-DH cover the popular methods used in the market today and they were selected due to their maturity, different security properties, and because of their interoperability properties. The two content key distribution methods require the CEKs to be randomly generated. The guidelines for random number generation in MUST be followed. When an encrypted payload is sent to multiple recipients, there are different deployment options. To explain these options we use the following notation:

Content Key Distribution with AES Key Wrap

Introduction The AES Key Wrap (AES-KW) algorithm is described in , 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 and in Section 6.2.1 of . The encrypted CEK is carried in the COSE_recipient structure alongside the information needed for AES-KW. The COSE_recipient structure, which is a substructure of the COSE_Encrypt 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.

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:

This deployment option is strongly 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:

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) and unique CEKs. The sender needs to execute the following steps:

CDDL The CDDL for the AES-KW binary is shown in . empty_or_serialized_map and header_map are structures defined in .

int, ; algorithm identifier ? 4 => bstr, ; identifier of the KEK pre-shared with the recipient * label => values ; extension point } ]]>

Note that the AES-KW algorithm, as defined in Section 2.2.3.1 of , does not have public parameters that vary on a per-invocation basis. Hence, the protected header in the COSE_recipient structure is a byte string of zero length.

Content Key Distribution with Ephemeral-Static Diffie-Hellman

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 . 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, and it is called ECDH-ES + AES-KW. An example is given in . There exists another version of ES-DH algorithm, namely ECDH-ES + HKDF, which does not use AES Key Wrap. It is not specified in this document.

Deployment Options There are only two deployment options with this approach since 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 and all recipients receive the same encrypted payload, i.e. the same CEK is used to encrypt the payload. 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:

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:

CDDL The CDDL for the ECDH-ES+AES-KW binary is shown in . Only the minimum number of parameters is shown. empty_or_serialized_map and header_map are structures defined in .

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 } ]]>

See for a description on how to encrypt the payload.

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 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 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 -3), 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.

The HKDF-based key derivation function MAY contain a salt value, as described in Section 5.1 of . 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 and NIST SP800-56 . Profiles of this specification MAY specify an extended version of the context information structure or MAY utilize a different context information structure.

Content Encryption This section summarizes the steps taken for content encryption, which applies to both content key distribution methods. For use with AEAD ciphers, such as AES-GCM and ChaCha20/Poly1305, the COSE specification requires a consistent byte stream for the authenticated data structure to be created. This structure is shown in and is defined in Section 5.3 of .

This Enc_structure needs to be populated as follows: The protected field in the Enc_structure from 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. Some ciphers provide confidentiality witout integrity protection, such as AES-CTR and AES-CBC (see ). For these ciphers the Enc_structure, shown in , MUST NOT be used because the Additional Authenticated Data (AAD) byte string is only consumable by AEAD ciphers. Hence, the AAD structure is not supplied to the API of those ciphers and the protected header in the SUIT_Encryption_Info structure MUST be a zero-length byte string.

AES-GCM

Introduction AES-GCM is an AEAD cipher and provides confidentiality and integrity protection. Examples in this section use the following parameters: Algorithm for payload encryption: AES-GCM-128 k: h'15F785B5C931414411B4B71373A9C0F7' IV: h'F14AAB9D81D51F7AD943FE87AF4F70CD' Plaintext: "This is a real firmware image." in hex: 546869732069732061207265616C206669726D7761726520696D6167652E

AES-KW + AES-GCM Example This example uses the following parameters: Algorithm id for key wrap: A128KW KEK COSE_Key (Secret Key): kty: Symmetric k: 'aaaaaaaaaaaaaaaa' kid: 'kid-1' The COSE_Encrypt structure, in hex format, is (with a line break inserted):

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

>, / unprotected: / { / IV / 5: h'F14AAB9D81D51F7AD943FE87AF4F70CD' }, / payload: / null / detached ciphertext /, / recipients: / [ [ / protected: / h'', / unprotected: / { / alg / 1: -3 / A128KW /, / kid / 4: 'kid-1' }, / payload: / h'75603FFC9518D794713C8CA8A115A7FB32565A6D59534D62' / CEK encrypted with KEK / ] ] ]) ]]>

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

ECDH-ES+AES-KW + AES-GCM Example This example uses the following parameters: Algorithm for content key distribution: ECDH-ES + A128KW KEK COSE_Key (Receiver's Private Key): kty: EC2 crv: P-256 x: h'5886CD61DD875862E5AAA820E7A15274C968A9BC96048DDCACE32F50C3651BA3' y: h'9EED8125E932CD60C0EAD3650D0A485CF726D378D1B016ED4298B2961E258F1B' d: h'60FE6DD6D85D5740A5349B6F91267EEAC5BA81B8CB53EE249E4B4EB102C476B3' kid: 'kid-2' KDF Context Algorithm ID: 1 (A128GCM) SuppPubInfo keyDataLength: 128 protected = << { / alg / 1: -29 / ECDH-ES+A128KW / } >> other = 'SUIT Payload Encryption' The COSE_Encrypt structure, in hex format, is (with a line break inserted):

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

>, / unprotected: / { / IV / 5: h'F14AAB9D81D51F7AD943FE87AF4F70CD' }, / 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'38876D8B4552E6BC9484A3F06E3646B3 0AEFF51B95583CFFA0B5776D52734942', / y / -3: h'34577AB5DD17276BB6BF15AA46531037 1557AFF61FAC5BA5A1EFF46698DD8B7B' } }, / payload: / h'C36BF2E8843246F0E148DBA607375204A040D8B19629B2B5' / CEK encrypted with KEK / ] ] ]) ]]>

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

AES-CTR

Introduction AES-CTR is a non-AEAD cipher, provides confidentiality but no integrity protection. Unlike AES-CBC, AES-CTR uses an IV per AES operation, as shown in . 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.

Examples in this section use the following parameters: Algorithm for payload encryption: AES-CTR-128 k: h'261DE6165070FB8951EC5D7B92A065FE' IV: h'DAE613B2E0DC55F4322BE38BDBA9DC68' Plaintext: "This is a real firmware image." in hex: 546869732069732061207265616C206669726D7761726520696D6167652E

AES-KW + AES-CTR Example This example uses the following parameters: Algorithm id for key wrap: A128KW KEK COSE_Key (Secret Key): kty: Symmetric k: 'aaaaaaaaaaaaaaaa' kid: 'kid-1' The COSE_Encrypt structure, in hex format, is (with a line break inserted):

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

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

ECDH-ES+AES-KW + AES-CTR Example This example uses the following parameters: Algorithm for content key distribution: ECDH-ES + A128KW KEK COSE_Key (Receiver's Private Key): kty: EC2 crv: P-256 x: h'5886CD61DD875862E5AAA820E7A15274C968A9BC96048DDCACE32F50C3651BA3' y: h'9EED8125E932CD60C0EAD3650D0A485CF726D378D1B016ED4298B2961E258F1B' d: h'60FE6DD6D85D5740A5349B6F91267EEAC5BA81B8CB53EE249E4B4EB102C476B3' kid: 'kid-2' KDF Context ALgorithm ID: -3 (A128KW) SuppPubInfo keyDataLength: 128 protected = << { / alg / 1: -3 / A128KW / } >> other = 'SUIT Payload Encryption' The COSE_Encrypt structure, in hex format, is (with a line break inserted):

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

>, / unprotected: / { / ephemeral key / -1: { / kty / 1: 2 / EC2 /, / crv / -1: 1 / P-256 /, / x / -2: h'50364E4DF3F5E8749D98E4378C04FAFE643B6ACEE7138382D83F768C7186FB85', / y / -3: h'99E6C96BEF3952B12EF83921B1749475D767284AA42D74D8923C137B01EDF5A0' } }, / payload: / h'E8599DCEE4944EECA9781D3ECDE3D9C34E1C9FCE8906617F' / CEK encrypted with KEK / ] ] ]) ]]>

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

AES-CBC

Introduction AES-CBC is a non-AEAD cipher, provides confidentiality but no integrity protection. 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 . 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.

Examples in this section use the following parameters: Algorithm for payload encryption: AES-CTR-128 k: h'627FCF0EA82C967D5ED8981EB325F303' IV: h'93702C81590F845D9EC866CCAC767BD1' Plaintext: "This is a real firmware image." in hex: 546869732069732061207265616C206669726D7761726520696D6167652E

AES-KW + AES-CBC Example This example uses the following parameters: Algorithm id for key wrap: A128KW KEK COSE_Key (Secret Key): kty: Symmetric k: 'aaaaaaaaaaaaaaaa' kid: 'kid-1' The COSE_Encrypt structure, in hex format, is (with a line break inserted):

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

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

ECDH-ES+AES-KW + AES-CBC Example This example uses the following parameters: Algorithm for content key distribution: ECDH-ES + A128KW KEK COSE_Key (Receiver's Private Key): kty: EC2 crv: P-256 x: h'5886CD61DD875862E5AAA820E7A15274C968A9BC96048DDCACE32F50C3651BA3' y: h'9EED8125E932CD60C0EAD3650D0A485CF726D378D1B016ED4298B2961E258F1B' d: h'60FE6DD6D85D5740A5349B6F91267EEAC5BA81B8CB53EE249E4B4EB102C476B3' kid: 'kid-2' KDF Context Algorithm ID: -65531 (A128CBC) SuppPubInfo keyDataLength: 128 protected = h'' other = 'SUIT Payload Encryption' The COSE_Encrypt structure, in hex format, is (with a line break inserted):

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

>, / unprotected: / { / ephemeral key / -1: { / kty / 1: 2 / EC2 /, / crv / -1: 1 / P-256 /, / x / -2: h'BC6A2DCD5025C8C0F7A5D120EB3E458CA722F8FB94BD56A24709CB15A8697489', / y / -3: h'10136574F673511540FE2A8589A7EDA372CB7B1AF94A8E1B4B94F6BDBD98AA18' } }, / payload: / h'AC8CDFB5426422298FCF235EB5F24D9E4C44C1689167473A' / CEK encrypted with KEK / ] ] ]) ]]>

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

Integrity Check on Encrypted and Decrypted Payloads In addition to suit-condition-image-match (Section 8.4.9.2 of ), 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.

Validating Payload Integrity This sub-section explains three ways to validate the integrity of payloads.

Image Match after Decryption The suit-condition-image-match on the plaintext payload is used after decryption. An example command sequence is shown in .

>, / parameter-image-size / 14: 30 / size of plaintext payload /, / parameter-encryption-info / 19: h'369...50F', / parameter-source-component / 22: 1 }, / directive-copy / 22, 15, / condition-image-match / 3, 15 / integrity check on decrypted payload /, ]]>

Image Match before Decryption The suit-condition-image-match can also be applied on encrypted payloads before decryption takes place. An example command sequence is shown in . This option mitigates battery exhaustion attacks discussed in .

>, / 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 / integrity check on encrypted payload /, / directive-set-component-index / 12, 0, / directive-override-parameters / 20, { / parameter-encryption-info / 19: h'D86...1F0', / parameter-source-component / 22: 1 }, / directive-copy / 22, 15, ]]>

Checking Authentication Tag while Decryption AEAD algorithms, such as AES-GCM and ChaCha20/Poly1305, verify the integrity of the encrypted concent.

Payload Integrity in SUIT Manifest This sub-section provides a guideline to decide how to validate the integrity of the payloads with SUIT Manifest. illustrates a classification tree to decide how to establish payload integrity.

There are three conditions: Q1. How does the recipient get the encrypted payload? If the encrypted payload is an integrated payload, its integrity is already validated with the suit-authentication-wrapper, so additional integrity check is not required. Q2. Does the sender want to mitigate battery exhaustion attacks? If yes, the encrypted payload has to be validated before decryption. Q3. Is the payload encrypted with an AEAD cipher? If yes, the additional integrity check is not required because the recipient validates the integrity of the payload while decrypting it. If no, validating its integrity must take place either before or after decryption.

Firmware Updates on IoT Devices with Flash Memory 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 differently 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. This section is specific to firmware images on microcontrollers and does not apply to generic software, configuration data, and machine learning models. The differences are the result of two aspects: Use of flash memory: Flash memory on microcontrollers is a type of non-volatile memory that erases data in larger 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. shows an example layout of a microcontroller flash area. Microcontroller Design: Code on microcontrollers typically cannot be executed from an arbitrary place in flash memory without extra software development and design efforts. Hence, developers often compile firmware such that the bootloader can execute the code from a specific location in flash memory. Often, the location where the to-be-booted firmware image is found is called "primary slot". When the encrypted firmware image has been transferred to the device, it will typically be stored in a dedicated area called the "secondary slot". At the next boot, the bootloader will recognize a new firmware image and will start decrypting the downloaded image sector-by-sector and will swap it with the image found in the primary slot. 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 adds robustness to the firmware update procedure. 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. Without hardware-based, on-the-fly decryption the image in the 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 it is located in off-chip flash memory. Off-chip flash memory tends to be more vulnerable to physical attacks.

The ability to restart an interrupted firmware update is often a requirement for unattended devices and the same is true for low-end, constrained 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 , MUST be used. 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 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.

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:

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

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

Each example uses SHA-256 as the digest function.

AES Key Wrap Example with Write Directive The following SUIT manifest requests a parser to authenticate the manifest with COSE_Mac0 HMAC256, to write and 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:

>, << / COSE_Mac0_Tagged / 17([ / protected: / << { / algorithm-id / 1: 5 / HMAC256 / } >>, / unprotected: / {}, / payload: / null, / tag: / h'3B70571169B0FEE5E6220BF86E5E973F 7F32875495908EDAA91EC994BCA44B29' ]) >> ] >>, / 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'2F59C3A34D9570FB99A5382E66466A3221A8AD85CE508B A306FB431A60EFA5AAAA078355070205A4B196832DF17F', / parameter-encryption-info / 19: << 96([ / protected: / << { / alg / 1: 1 / AES-GCM-128 / } >>, / unprotected: / { / IV / 5: h'F14AAB9D81D51F7AD943FE87AF4F70CD' }, / payload: / null / detached ciphertext /, / recipients: / [ [ / protected: / << { } >>, / unprotected: / { / alg / 1: -3 / A128KW /, / kid / 4: 'kid-1' }, / payload: / h'75603FFC9518D794713C8CA8A115A7FB32565A6D59534D62' / 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:

AES Key Wrap Example with Fetch + Copy Directives The following SUIT manifest requests a parser to fetch the encrypted payload and to store 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 XIP flash memory. The SUIT manifest in diagnostic notation (with line breaks added for readability) is shown here:

>, << / COSE_Mac0_Tagged / 17([ / protected: / << { / algorithm-id / 1: 5 / HMAC256 / } >>, / unprotected: / {}, / payload: / null, / tag: / h'93B4B774A5D0421ED6FB5EBF890A284C DAC7816CBC048BF47EE7FA7FF3BC02C3' ]) >> ] >>, / 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'F14AAB9D81D51F7AD943FE87AF4F70CD' }, / payload: / null / detached ciphertext /, / recipients: / [ [ / protected: / << { } >>, / unprotected: / { / alg / 1: -3 / A128KW /, / kid / 4: 'kid-1' }, / payload: / h'75603FFC9518D794713C8CA8A115A7FB32565A6D59534D62' / 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:

ES-DH Example with Write + Copy Directives The following SUIT manifest requests a parser to authenticate the manifest with COSE_Sign1 ES256, to write and 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:

>, << / COSE_Sign1_Tagged / 18([ / protected: / << { / algorithm-id / 1: -7 / ES256 / } >>, / unprotected: / {}, / payload: / null, / signature: / h'65E59AAB8A35BDE9547458316D1C769F FB2CEA304C9FB6151E5C8A88A002A292 C5B8C63C81B5AC0AE31948B610834E12 CBDBB2753EA221544B6733076A92EE20' ]) >> ] >>, / 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'344FA2D5AD2F43F6F363DA6FF2C337FE69E33E3D63714D 23985BF02499EB0E8B231D45C378245DA3611C160CC511', / parameter-encryption-info / 19: << 96([ / protected: / << { / alg / 1: 1 / AES-GCM-128 / } >>, / unprotected: / { / 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'FF6E266DABAF51B7207569E31CF72646 183E94CEE64FCDC8695AD9A505AEFDEA', / y / -3: h'5FBC4A29844450B3AC22AB30C7F7004B B59D8BD60D7997734A9FA0124B650895' }, / kid / 4: 'kid-2' }, / payload: / h'B0E21628283F3E409F8158D8FFCA567F340E379AC39E49C9' / CEK encrypted with KEK / ] ] ]) >> }, / directive-write / 18, 15 / consumes the SUIT_Encryption_Info above / ] >> } >> }) ]]>

In hex format, the SUIT manifest is this:

ES-DH Example with Dependency The following SUIT manifest requests a parser to resolve the dependency. The dependent manifest is signed with another key: ~~~ -----BEGIN EC PRIVATE KEY----- MHcCAQEEIIQa67e56m8CYL5zVaJFiLl30j0qxb8ray2DeUMqH+qYoAoGCCqGSM49 AwEHoUQDQgAEDpCKqPBm2x8ITgw2UsY5Ur2Z8qW9si+eATZ6rQOrpot32hvYrE8M tJC6IQZIv3mrFk1JrTVR1x0xSydJ7kLSmg== -----END EC PRIVATE KEY----- ~~~ The dependency manifest is embedded as an integrated-dependency and referred to by the "#dependency-manifest" URI. The SUIT manifest in diagnostic notation (with line breaks added for readability) is shown here:

>, << / COSE_Sign1_Tagged / 18([ / protected: / << { / algorithm-id / 1: -7 / ES256 / } >>, / unprotected: / {}, / payload: / null, / signature: / h'DF493BDBF167EFFB40593C5910D33B66 429721467DF05800EA66A88B91729CD5 1007981F151FC324745FF43E6F75AAF5 197DD5EC4AA6BCEFCE43E4B1E35C948E' ]) >> ] >>, / 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'344FA2D5AD2F43F6F363DA6FF2C337FE69E33E3D63714D 23985BF02499EB0E8B231D45C378245DA3611C160CC511', / parameter-encryption-info / 19: << 96([ / protected: / << { / alg / 1: 1 / AES-GCM-128 / } >>, / unprotected: / { / 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'FF6E266DABAF51B7207569E31CF72646 183E94CEE64FCDC8695AD9A505AEFDEA', / y / -3: h'5FBC4A29844450B3AC22AB30C7F7004B B59D8BD60D7997734A9FA0124B650895' }, / kid / 4: 'kid-2' }, / payload: / h'B0E21628283F3E409F8158D8FFCA567F340E379AC39E49C9' / 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:

Operational Considerations The algorithms described in this document assume that the party performing payload encryption shares a key-encryption key (KEK) with the recipient (for use with the AES Key Wrap scheme), or is in possession of the public key of the recipient (for use with ES-DH). 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 , 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.

Security Considerations This entire document is about security. It is good security practise to use different 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 in a communication security protocol. To further reduce the attack surface it may be beneficial use different long-term keys for the encryption of different types of 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. A large part of this document is focused on the content key distribution and two methods are utilized, namely AES Key Wrap (AES-KW) and Ephemeral-Static Diffie-Hellman (ES-DH). In this table we summarize the main properties with respect to their deployment:

The use of firmware encryption with IoT devices introduces an battery exhaustion attack. This attack utilizes the fact that flash memory operations are energy-expensive. To perform this attacker, the adversary needs to be able to swap detached payloads and force the device to process a wrong payload. Swapping the payloads is only possible when there is no communication security protocol in place between the device and the distribution system or when the distribution system itself is compromised. The security features provided by the manifest will detect this attack and the device will not boot the incorrectly provided payload. However, at this time the energy-expensive flash operations have already been performed. Consequently, these operations may reduce the lifetime of devices and battery powered IoT devices are particularly vulnerable to such an attack. See for further discussion about IoT devices using flash memory. Including the digest of the encrypted payload in the manifest allows the device to detect a battery exhaustion attack before energy consuming decryption and flash memory copy or swap operations took place. When battery exhaustion attacks are not a concern, it is adequate to use the digest of the plaintet payload instead.

IANA Considerations IANA is asked to add the following value to the SUIT Parameters registry established by Section 11.5 of :

[Editor's Note: TBD19: Proposed 19]

In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need to be able to define basic security services for this data format. This document defines the CBOR Object Signing and Encryption (COSE) protocol. This specification describes how to create and process signatures, message authentication codes, and encryption using CBOR for serialization. This specification additionally describes how to represent cryptographic keys using CBOR. This document, along with RFC 9053, obsoletes RFC 8152. Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need to be able to define basic security services for this data format. This document defines a set of algorithms that can be used with the CBOR Object Signing and Encryption (COSE) protocol (RFC 9052). This document, along with RFC 9052, obsoletes RFC 8152. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Arm Limited Fraunhofer SIT Inria Nordic Semiconductor This specification describes the format of a manifest. A manifest is a bundle of metadata about code/data obtained by a recipient (chiefly the firmware for an IoT device), where to find the code/data, the devices to which it applies, and cryptographic information protecting the manifest. Software updates and Trusted Invocation both tend to use sequences of common operations, so the manifest encodes those sequences of operations, rather than declaring the metadata. The Concise Binary Object Representation (CBOR) data format is designed for small code size and small message size. CBOR Object Signing and Encryption (COSE) is specified in RFC 9052 to provide basic security services using the CBOR data format. This document specifies the conventions for using AES-CTR and AES-CBC as content encryption algorithms with COSE. Arm Limited SECOM CO., LTD. This specification describes extensions to the SUIT Manifest format (as defined in [I-D.ietf-suit-manifest]) for use in deployments with multiple trust domains. A device has more than one trust domain when it enables delegation of different rights to mutually distrusting entities for use for different purposes or Components in the context of firmware or software update. Vulnerabilities in Internet of Things (IoT) devices have raised the need for a reliable and secure firmware update mechanism suitable for devices with resource constraints. Incorporating such an update mechanism is a fundamental requirement for fixing vulnerabilities, but it also enables other important capabilities such as updating configuration settings and adding new functionality. In addition to the definition of terminology and an architecture, this document provides the motivation for the standardization of a manifest format as a transport-agnostic means for describing and protecting firmware updates. Vulnerabilities with Internet of Things (IoT) devices have raised the need for a reliable and secure firmware update mechanism that is also suitable for constrained devices. Ensuring that devices function and remain secure over their service lifetime requires such an update mechanism to fix vulnerabilities, update configuration settings, and add new functionality. One component of such a firmware update is a concise and machine-processable metadata document, or manifest, that describes the firmware image(s) and offers appropriate protection. This document describes the information that must be present in the manifest. Randomness is a crucial ingredient for Transport Layer Security (TLS) and related security protocols. Weak or predictable "cryptographically secure" pseudorandom number generators (CSPRNGs) can be abused or exploited for malicious purposes. An initial entropy source that seeds a CSPRNG might be weak or broken as well, which can also lead to critical and systemic security problems. This document describes a way for security protocol implementations to augment their CSPRNGs using long-term private keys. This improves randomness from broken or otherwise subverted CSPRNGs. This document is a product of the Crypto Forum Research Group (CFRG) in the IRTF. This document describes the Cryptographic Message Syntax (CMS). This syntax is used to digitally sign, digest, authenticate, or encrypt arbitrary message content. [STANDARDS-TRACK] This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK] This document specifies a simple Hashed Message Authentication Code (HMAC)-based key derivation function (HKDF), which can be used as a building block in various protocols and applications. The key derivation function (KDF) is intended to support a wide range of applications and requirements, and is conservative in its use of cryptographic hash functions. This document is not an Internet Standards Track specification; it is published for informational purposes. Internet Assigned Numbers Authority Wikipedia NIST

Full CDDL The following CDDL must be appended to the SUIT Manifest CDDL. The SUIT CDDL is defined in Appendix A of

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, Ruud Derwig, 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.