Group Object Security for Constrained RESTful Environments (Group OSCORE)

1.1. 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.¶

Readers are expected to be familiar with the terms and concepts described in CoAP [RFC7252], including "endpoint", "client", "server", "sender", and "recipient"; group communication for CoAP [I-D.ietf-core-groupcomm-bis]; Observe [RFC7641]; Concise Binary Object Representation (CBOR) [RFC8949]; Concise Data Definition Language (CDDL) [RFC8610]; COSE [RFC9052][RFC9053] and related countersignatures [RFC9338].¶

Readers are also expected to be familiar with the terms and concepts for protection and processing of CoAP messages through OSCORE, such as "Security Context" and "Master Secret", defined in [RFC8613].¶

Terminology for constrained environments, such as "constrained device" and "constrained-node network", is defined in [RFC7228].¶

This document refers also to the following terminology.¶

• Keying material: data that is necessary to establish and maintain secure communication among endpoints. This includes, for instance, keys and IVs [RFC4949].¶

• Authentication credential: information associated with an entity, including that entity's public key and parameters associated with the public key. Examples of formats of authentication credentials are CBOR Web Tokens (CWTs) and CWT Claims Sets (CCSs) [RFC8392], X.509 certificates [RFC5280], and C509 certificates [I-D.ietf-cose-cbor-encoded-cert]. Further details about authentication credentials are provided in Section 2.4.¶

• Group: a set of endpoints that share group keying material and security parameters (Common Context, see Section 2). That is, unless otherwise specified, the term group used in this document refers to a "security group" (see Section 2.1 of [I-D.ietf-core-groupcomm-bis]), not to be confused with "CoAP group" or "application group".¶

• Group Manager: an entity responsible for a group, required neither to be an actual group member nor to take part in the group communication. The operations of the Group Manager are defined in Section 12 and its responsibilities are listed in Appendix D.¶

• Silent server: a member of a group that performs only group mode processing on incoming requests and never sends responses protected with Group OSCORE. For CoAP group communications, requests are normally sent without necessarily expecting a response. A silent server may send unprotected responses, as error responses reporting a Group OSCORE error.¶

• Group Identifier (Gid): identifier assigned to the group, unique within the set of groups of a given Group Manager. The Gid value changes every time the group is rekeyed (see Section 12.2).¶

• Birth Gid: with respect to a group member, the Gid obtained by that group member upon (re-)joining the group.¶

• Key Generation Number: an integer value identifying the current version of the keying material used in a group.¶

• Source authentication: evidence that a received message in the group originated from a specific identified group member. This also provides assurance that the message was not tampered with by anyone, be it a different legitimate group member or an endpoint which is not a group member.¶

• Group request: a CoAP request message sent by a client in the group to servers in that group.¶

• Long exchange: an exchange of messages associated with a request that is a group request and/or an Observe request [RFC7641].¶

  In either case, multiple responses can follow from the same server to the request associated with the long exchange, even if the request is not an Observe request (see Section 3.1.6 of [I-D.ietf-core-groupcomm-bis]). The client terminates a long exchange when freeing up the CoAP Token value used for the associated request, for which no further responses will be accepted afterwards.¶

2.1. Common Context

The following sections specify how the Common Context is used and extended compared to [RFC8613]. The Common Context may be acquired from the Group Manager (see Section 12).¶

2.2. Sender Context and Recipient Context

The Sender ID SHALL be unique for each endpoint in a group with a certain triplet (Master Secret, Master Salt, Group Identifier), see Section 3.3 of [RFC8613].¶

The maximum length of a Sender ID in bytes equals L minus 6, where L is determined as follows.¶

• If only one among the AEAD Algorithm and the Group Encryption Algorithm is set (see Section 2.1.1 and Section 2.1.7), then L is the nonce length for the set algorithm.¶

• If both the AEAD Algorithm and the Group Encryption Algorithm are set, then L is the smallest nonce length among those of the two algorithms.¶

With the exception of the authentication credential of the sender endpoint, a receiver endpoint can derive a complete Security Context from a received Group OSCORE message and the Common Context (see Section 2.3).¶

The authentication credentials in the Recipient Contexts can be retrieved from the Group Manager (see Section 12) upon joining the group. An authentication credential can alternatively be acquired from the Group Manager at a later time, for example the first time a message is received from a particular endpoint in the group (see Section 7.2 and Section 7.4).¶

For severely constrained devices, it may be infeasible to simultaneously handle the ongoing processing of a recently received message in parallel with the retrieval of the sender endpoint's authentication credential. Such devices can be configured to drop a received message for which there is no (complete) Recipient Context, and retrieve the sender endpoint's authentication credential in order to have it available to verify subsequent messages from that endpoint.¶

An endpoint may admit a maximum number of Recipient Contexts for a same Security Context, e.g., due to memory limitations. After reaching that limit, the endpoint has to delete a current Recipient Context to install a new one (see Section 2.6.1.2). It is up to the application to define the maximum number of Recipient Contexts for a same Security Context as well as policies for deleting Recipient Contexts.¶

2.3. Establishment of Security Context Parameters

OSCORE defines the derivation of Sender Context and Recipient Context (specifically, of Sender/Recipient Keys) and of the Common IV, from a set of input parameters (see Section 3.2 of [RFC8613]).¶

The derivation of Sender/Recipient Keys and of the Common IV defined in OSCORE applies also to Group OSCORE, with the following modifications compared to Section 3.2.1 of [RFC8613].¶

• If Group Encryption Algorithm in the Common Context is set (see Section 2.1.7), then the 'alg_aead' element of the 'info' array MUST specify Group Encryption Algorithm from the Common Context as a CBOR integer or text string, consistently with the "Value" field in the "COSE Algorithms" Registry for this algorithm.¶

• If Group Encryption Algorithm in the Common Context is not set, then the 'alg_aead' element of the 'info' array MUST specify AEAD Algorithm from the Common Context (see Section 2.1.1), as per Section 5.4 of [RFC8613].¶

• When deriving the Common IV, the 'L' element of the 'info' array MUST specify the length of the Common IV in bytes, which is determined as defined in Section 2.1.4.¶

2.4. Authentication Credentials

The authentication credentials of the endpoints in a group MUST be encoded according to the format used in the group, as indicated by the Authentication Credential Format parameter in the Common Context (see Section 2.1.5). The authentication credential of the Group Manager SHOULD be encoded according to that same format, in order to limit the number of formats that the group members have to support and handle, unless it is infeasible or impractical for the particular realization or instance of the Group Manager to have an own authentication credential encoded in that same format.¶

The format of authentication credentials MUST provide the public key and a comprehensive set of information related to the public key algorithm, including, e.g., the used elliptic curve (when applicable). If Group Encryption Algorithm in the Common Context is not set (see Section 2.1.7), then the public key algorithm is the Pairwise Key Agreement Algorithm used in the group (see Section 2.1.10), else the Signature Algorithm used in the group (see Section 2.1.8).¶

Examples of formats of authentication credentials are CBOR Web Tokens (CWTs) and CWT Claims Sets (CCSs) [RFC8392], X.509 certificates [RFC5280], and C509 certificates [I-D.ietf-cose-cbor-encoded-cert].¶

If the authentication credentials are X.509 certificates or C509 certificates, the public key algorithm is identified by the "algorithm" field of the "SubjectPublicKeyInfo" structure, and by the "subjectPublicKeyAlgorithm" element, respectively.¶

If authentication credentials are CBOR Web Tokens (CWTs) or CWT Claims Sets (CCSs), then a COSE Key structure and its "kty" and "crv" parameters identify the types of pertinent public key algorithms. For example: the pair ("crv" = X25519, "kty" = OKP) indicates that the public key is meant to be used with X25519 ECDH key agreement; the pair ("crv" = Ed25519, "kty" = OKP) indicates that the public key is meant to be used with the signature algorithm EdDSA; the pair ("crv" = P-256, "kty" = EC2) indicates that the public key is meant to be used with the signature algorithm ECDSA and/or with P-256 ECDH key agreement.¶

Authentication credentials are used to derive pairwise keys (see Section 2.5.1) and are included in the external additional authenticated data when processing messages (see Section 3.4). In both these cases, an endpoint in a group MUST treat authentication credentials as opaque data, i.e., by considering the same binary representation made available to other endpoints in the group, possibly through a designated trusted source (e.g., the Group Manager).¶

For example, an X.509 certificate is provided as its direct binary serialization. If C509 certificates or CWTs are used as authentication credentials, each is provided as the binary serialization of a (possibly tagged) CBOR array. If CCSs are used as authentication credentials, each is provided as the binary serialization of a (possibly tagged) CBOR map.¶

If authentication credentials are CWTs or CCSs, then the untagged CWT or CCS associated with an entity is stored in the Security Context and used as authentication credential for that entity.¶

If authentication credentials are X.509 / C509 certificates, CWTs, or CCSs and the authentication credential associated with an entity is provided within a chain or a bag, then only the end-entity certificate or end-entity untagged CWT / CCS is stored in the Security Context and used as authentication credential for that entity.¶

Storing whole authentication credentials rather than only a subset of those may result in a non-negligible storage overhead. On the other hand, it also ensures that authentication credentials are correctly used in a simple, flexible and non-error-prone way, also taking into account future credential formats as entirely new or extending existing ones. In particular, it is ensured that:¶

• When used to derive pairwise keys and when included in the external additional authenticated data, authentication credentials can also specify possible metadata and parameters related to the included public key. Besides the public key algorithm, these comprise other relevant pieces of information such as key usage, expiration time, issuer, and subject.¶

• All endpoints using another endpoint's authentication credential use exactly the same binary serialization, as obtained and distributed by the credential provider (e.g., the Group Manager), and as originally crafted by the credential issuer. In turn, this does not require to define and maintain canonical subsets of authentication credentials and their corresponding encoding, and spares endpoints from error-prone re-encoding operations.¶

Depending on the particular deployment and the intended group size, limiting the storage overhead of endpoints in a group can be an incentive for system/network administrators to prefer using a compact format of authentication credentials in the first place.¶

2.5. Pairwise Keys

In certain Elliptic Curve Cryptographic schemes, it is possible to use public/private key pairs with both signature operations (ECDSA or EdDSA) and key agreement operations (ECDH). This section specifies the derivation of "pairwise keys" for use in the pairwise mode defined in Section 8.¶

Group OSCORE keys used for both signature operations and key agreement operations MUST be used only for purposes related to Group OSCORE. These include the processing of messages with Group OSCORE, as well as performing proof of possession of private keys, e.g., upon joining a group through the Group Manager (see Section 12).¶

Pairwise Sender Key    = HKDF(Sender Key, IKM-Sender, info, L)
Pairwise Recipient Key = HKDF(Recipient Key, IKM-Recipient, info, L)

with

IKM-Sender    = Sender Auth Cred | Recipient Auth Cred | Shared Secret
IKM-Recipient = Recipient Auth Cred | Sender Auth Cred | Shared Secret

2.6. Update of Security Context

It is RECOMMENDED that the long-term part of the Security Context is stored in non-volatile memory, or that it can otherwise be reliably accessed throughout the operation of the group, e.g., after device reboots. However, also data in the long-term part of the Security Context may need to be updated, for example due to scheduled key renewal, new or re-joining members in the group, or the fact that the endpoint changes Sender ID (see Section 2.6.3).¶

The data in the varying part of the Security Context are updated by the endpoint when executing the security protocol, but may be lost (see Section 2.6.1) or become outdated by exhaustion of Sender Sequence Numbers (see Section 2.6.2).¶

3.1. Countersignature

When protecting a message in group mode, the 'unprotected' field MUST additionally include the following parameter:¶

• Countersignature0 version 2: its value is set to the countersignature of the COSE object.¶

  The countersignature is computed by the sender as described in Sections 3.2 and 3.3 of [RFC9338], by using its private key and according to the Signature Algorithm in the Security Context.¶

  In particular, the Countersign_structure contains the context text string "CounterSignature0", the external_aad as defined in Section 3.4 of this document, and the ciphertext of the COSE object as payload.¶

3.2. The 'kid' and 'kid context' parameters

The value of the 'kid' parameter in the 'unprotected' field of response messages MUST be set to the Sender ID of the endpoint transmitting the message, if the request was protected in group mode. That is, unlike in [RFC8613], the 'kid' parameter is always present in responses to a request that was protected in group mode.¶

The value of the 'kid context' parameter in the 'unprotected' field of request messages MUST be set to the ID Context, i.e., the Group Identifier value (Gid) of the group. That is, unlike in [RFC8613], the 'kid context' parameter is always present in requests.¶

3.3. Nonce Computation

The nonce is constructed like in OSCORE, with the difference that Step 4 in Section 5.2 of [RFC8613] is replaced with:¶

4. and then XOR with X bytes from the Common IV's start, where X is the length in bytes of the nonce.¶

For example, if X = 7 and the Common IV is 0x00112233445566778899aabbcc (13 bytes), then the bytes to XOR are 0x00112233445566 (7 bytes).¶

The constructed nonce is used both by the AEAD Algorithm (see Section 2.1.1) and by the Group Encryption Algorithm (see Section 2.1.7), independent of whether they are AEAD or plain encryption algorithms. Algorithms that do not use a nonce are not supported, as per Section 2.1.7.¶

3.4. Additional Authenticated Data

The external_aad of the Additional Authenticated Data (AAD) is different compared to OSCORE [RFC8613] and is defined in this section.¶

The same external_aad structure is used in group mode and pairwise mode for encryption/decryption (see Section 5.3 of [RFC9052]), as well as in group mode for computing and verifying the countersignature (see Sections 3.2 and 3.3 of [RFC9338]).¶

In particular, the external_aad includes also the Signature Algorithm, the Group Encryption Algorithm, the Pairwise Key Agreement Algorithm, the value of the 'kid context' in the COSE object of the request, the OSCORE Option of the protected message, the sender's authentication credential, and the Group Manager's authentication credential.¶

The external_aad SHALL be a CBOR array wrapped in a bstr object as defined below, following the notation of [RFC8610]:¶

external_aad = bstr .cbor aad_array

aad_array = [
   oscore_version : uint,
   algorithms : [alg_aead : int / tstr / null,
                 alg_group_enc : int / tstr / null,
                 alg_signature : int / tstr / null,
                 alg_pairwise_key_agreement : int / tstr / null],
   request_kid : bstr,
   request_piv : bstr,
   options : bstr,
   request_kid_context : bstr,
   OSCORE_option : bstr,
   sender_cred : bstr,
   gm_cred : bstr
]

Compared with Section 5.4 of [RFC8613], the aad_array has the following differences.¶

• The 'algorithms' array is extended as follows.¶

  The parameter 'alg_aead' MUST be set to the CBOR simple value null (0xf6) if the parameter AEAD Algorithm is not set in the Common Context of the Security Context used (see Section 2.1.1). Otherwise, regardless of whether the endpoint supports the pairwise mode or not, this parameter MUST specify AEAD Algorithm from the Common Context (see Section 2.1.1) as per Section 5.4 of [RFC8613].¶

  Furthermore, the 'algorithms' array additionally includes:¶

  • 'alg_group_enc', which specifies Group Encryption Algorithm from the Common Context of the Security Context used (see Section 2.1.7). This parameter MUST be set to the CBOR simple value null (0xf6) if the parameter Group Encryption Algorithm in the Common Context is not set. Otherwise, regardless of whether the endpoint supports the group mode or not, this parameter MUST specify Group Encryption Algorithm as a CBOR integer or text string, consistently with the "Value" field in the "COSE Algorithms" Registry for this algorithm.¶

  • 'alg_signature', which specifies Signature Algorithm from the Common Context of the Security Context used (see Section 2.1.8). This parameter MUST be set to the CBOR simple value null (0xf6) if the parameter Signature Algorithm in the Common Context is not set. Otherwise, regardless of whether the endpoint supports the group mode or not, this parameter MUST specify Signature Algorithm as a CBOR integer or text string, consistently with the "Value" field in the "COSE Algorithms" Registry for this algorithm.¶

  • 'alg_pairwise_key_agreement', which specifies Pairwise Key Agreement Algorithm from the Common Context of the Security Context used (see Section 2.1.10). This parameter MUST be set to the CBOR simple value null (0xf6) if Pairwise Key Agreement Algorithm in the Common Context is not set. Otherwise, regardless of whether the endpoint supports the pairwise mode or not, this parameter MUST specify Pairwise Key Agreement Algorithm as a CBOR integer or text string, consistently with the "Value" field in the "COSE Algorithms" Registry for this algorithm.¶

• The new element 'request_kid_context' contains the value of the 'kid context' in the COSE object of the request (see Section 3.2).¶

  This enables endpoints to safely keep a long exchange active beyond a possible change of Gid (i.e., of ID Context), following a group rekeying (see Section 12.2). In fact, it ensures that every response within a long exchange cryptographically matches with only one request (i.e., the request associated with that long exchange), rather than with multiple requests that were protected with different keying material but share the same 'request_kid' and 'request_piv' values.¶

• The new element 'OSCORE_option', containing the value of the OSCORE Option present in the protected message, encoded as a binary string. This prevents the attack described in Section 14.7 when using the group mode, as further explained in Section 14.7.2.¶

  Note for implementation: this construction requires the OSCORE Option of the message to be generated and finalized before computing the ciphertext of the COSE_Encrypt0 object (when using the group mode or the pairwise mode) and before calculating the countersignature (when using the group mode). Also, the aad_array needs to be large enough to contain the largest possible OSCORE Option.¶

• The new element 'sender_cred', containing the sender's authentication credential. This parameter MUST be set to a CBOR byte string, which encodes the sender's authentication credential in its original binary representation made available to other endpoints in the group (see Section 2.4).¶

• The new element 'gm_cred', containing the Group Manager's authentication credential. This parameter MUST be set to a CBOR byte string, which encodes the Group Manager's authentication credential in its original binary representation made available to other endpoints in the group (see Section 2.4). This prevents the attack described in Section 14.8.¶

4.1. Encoding of the OSCORE Option Value and Group OSCORE Payload

The OSCORE header compression defined in Section 6 of [RFC8613] is used for compactly encoding the COSE_Encrypt0 object specified in Section 3 of this document, with the following differences.¶

• When the Group OSCORE message is protected in group mode, the message payload SHALL encode the ciphertext of the COSE object, concatenated with the encrypted countersignature of the COSE object. That is:¶

  • The plain, original countersignature of the COSE object, namely SIGNATURE, is specified in the "Countersignature0 version 2" parameter within the 'unprotected' field of the COSE object (see Section 3.1).¶

  • The encrypted countersignature, namely ENC_SIGNATURE, is computed as¶

    ENC_SIGNATURE = SIGNATURE XOR KEYSTREAM¶

    where KEYSTREAM is derived as per Section 4.2.¶

• When the Group OSCORE message is protected in pairwise mode, the message payload SHALL encode the ciphertext of the COSE object.¶

• This document defines the usage of the sixth least significant bit, called "Group Flag", in the first byte of the OSCORE Option containing the OSCORE flag bits. This flag bit is specified in Section 15.1.¶

• The Group Flag MUST be set to 1 if the Group OSCORE message is protected using the group mode (see Section 7).¶

• The Group Flag MUST be set to 0 if the Group OSCORE message is protected using the pairwise mode (see Section 8). The Group Flag MUST also be set to 0 for ordinary OSCORE messages processed according to [RFC8613].¶

4.2. Keystream Derivation for Countersignature Encryption

The following defines how an endpoint derives the keystream KEYSTREAM, used to encrypt/decrypt the countersignature of an outgoing/incoming message M protected in group mode.¶

The keystream SHALL be derived as follows, by using the HKDF Algorithm from the Common Context (see Section 3.2 of [RFC8613]), which consists of composing the HKDF-Extract and HKDF-Expand steps [RFC5869].¶

KEYSTREAM = HKDF(salt, IKM, info, L)¶

The input parameters of HKDF are as follows.¶

• salt takes as value the Partial IV (PIV) used to protect M. Note that, if M is a response, salt takes as value either: i) the fresh Partial IV generated by the server and included in the response; or ii) the same Partial IV of the request generated by the client and not included in the response.¶

• IKM is the Signature Encryption Key from the Common Context (see Section 2.1.9).¶

• info is the serialization of a CBOR array with the structure defined below, following the notation of [RFC8610]:¶

   info = [
     id : bstr,
     id_context : bstr,
     type : bool,
     L : uint
   ]

where:¶

• id is the Sender ID of the endpoint that generated PIV.¶

• id_context is the ID Context (Gid) used when protecting M.¶

  Note that, in the case of group rekeying, a server might use a different Gid when protecting a response, compared to the Gid that it used to verify (that the client used to protect) the request, see Section 7.3.¶

• type is the CBOR simple value true (0xf5) if M is a request, or the CBOR simple value false (0xf4) otherwise.¶

• L is the size of the countersignature, as per Signature Algorithm from the Common Context (see Section 2.1.8), in bytes.¶

4.3. Examples of Compressed COSE Objects

This section covers a list of OSCORE Header Compression examples of Group OSCORE used in group mode (see Section 4.3.1) or in pairwise mode (see Section 4.3.2).¶

The examples assume that the COSE_Encrypt0 object is set (which means the CoAP message and cryptographic material is known). Note that the examples do not include the full CoAP unprotected message or the full Security Context, but only the input necessary to the compression mechanism, i.e., the COSE_Encrypt0 object. The output is the compressed COSE object as defined in Section 4.1 and divided into two parts, since the object is transported in two CoAP fields: OSCORE Option and payload.¶

The examples assume that the plaintext (see Section 5.3 of [RFC8613]) is 6 bytes long, and that the AEAD tag is 8 bytes long, hence resulting in a ciphertext which is 14 bytes long. When using the group mode, the COSE_Countersignature0 byte string as described in Section 3 is assumed to be 64 bytes long.¶

   [
     / protected / h'',
     / unprotected / {
       / kid /                           4 : h'25',
       / Partial IV /                    6 : h'05',
       / kid context /                  10 : h'44616c',
       / Countersignature0 version 2 /  12 : h'66e6d9b0
       db009f3e105a673f8855611726caed57f530f8cae9d0b168
       513ab949fedc3e80a96ebe94ba08d3f8d3bf83487458e2ab
       4c2f936ff78b50e33c885e35'
     },
     / ciphertext / h'aea0155667924dff8a24e4cb35b9'
   ]

   Flag byte: 0b00111001 = 0x39 (1 byte)

   Option Value: 0x39 05 03 44 61 6c 25 (7 bytes)

   Payload: 0xaea0155667924dff8a24e4cb35b9 de9e ... f1
   (14 bytes + size of the encrypted countersignature)

   [
     / protected / h'',
     / unprotected / {
       / kid /                           4 : h'52',
       / Countersignature0 version 2 /  12 : h'f5b659b8
       24487eb349c5c5c8a3fe401784cade2892725438e8be0fab
       daa2867ee6d29f68edb0818e50ebf98c28b923d0205f5162
       e73662e27c1a3ec562a49b80'
     },
     / ciphertext / h'60b035059d9ef5667c5a0710823b'
   ]

   Flag byte: 0b00101000 = 0x28 (1 byte)

   Option Value: 0x28 52 (2 bytes)

   Payload: 0x60b035059d9ef5667c5a0710823b ca1e ... b3
   (14 bytes + size of the encrypted countersignature)

   [
     / protected / h'',
     / unprotected / {
       / kid /           4 : h'25',
       / Partial IV /    6 : h'05',
       / kid context /  10 : h'44616c'
     },
     / ciphertext / h'aea0155667924dff8a24e4cb35b9'
   ]

   Flag byte: 0b00011001 = 0x19 (1 byte)

   Option Value: 0x19 05 03 44 61 6c 25 (7 bytes)

   Payload: 0xaea0155667924dff8a24e4cb35b9 (14 bytes)

   [
     / protected / h'',
     / unprotected / {},
     / ciphertext / h'60b035059d9ef5667c5a0710823b'
   ]

   Flag byte: 0b00000000 = 0x00 (1 byte)

   Option Value: 0x (0 bytes)

   Payload: 0x60b035059d9ef5667c5a0710823b (14 bytes)

5.1. Supporting Multiple Responses in Long Exchanges

For each of its ongoing long exchanges, a client maintains one Response Number for each different server. Then, separately for each server, the client uses the associated Response Number to perform ordering and replay protection of responses received from that server within that long exchange (see Section 5.3.1).¶

That is, the Response Number has the same purpose that the Notification Number has in OSCORE (see Section 4.1.3.5.2 of [RFC8613]), but a client uses it for handling any response from the associated server within a long exchange.¶

Group OSCORE allows a long exchange to remain active, even if the group is rekeyed (thus changing the ID Context) or the client obtains a new Sender ID.¶

As defined in Section 7, this is achieved by the client and server(s) storing the 'kid' and 'kid context' used in the original request, throughout the whole duration of the long exchange.¶

Upon leaving the group or before re-joining the group, a group member MUST terminate all the ongoing long exchanges that it has started in the group as a client. This frees up the CoAP Token associated with the corresponding request.¶

5.2. Freshness

If the application requires freshness, e.g., according to time- or event-based policies (see Section 2.5.1 of [RFC9175]), a server can use the approach in Section 9 as a variant of the Challenge-Response procedure based on the Echo Option [RFC9175] before delivering request messages from a client to the application.¶

Like in OSCORE [RFC8613], assuming an honest server, the message binding guarantees that a response is not older than the request it replies to. Therefore, building on Section 7.3 of [RFC8613], the following properties hold for Group OSCORE.¶

• The freshness of a response can be assessed if it is received soon after the request.¶

  For responses within a long exchange, this assessment gets weaker with time. If such responses are Observe notifications [RFC7641], it is RECOMMENDED that the client regularly re-register the observation.¶

  If the request was neither a group request nor an Observe request, there is at most a single valid response and only from one, individually targeted server in the group. Thus, freshness can be assessed depending on when the request was sent.¶

• It is not guaranteed that a misbehaving server did not create the response before receiving the request, i.e., Group OSCORE does not verify server aliveness.¶

• For requests and responses, the received Partial IV allows a recipient to determine the relative order of requests or responses.¶

5.3. Replay Protection

Like in OSCORE [RFC8613], the replay protection relies on the Partial IV of incoming messages. A server updates the Replay Window of its Recipient Contexts based on the Partial IV values in received request messages, which correspond to the Sender Sequence Numbers of the clients. Note that there can be large jumps in these Sender Sequence Number values, for example when a client exchanges unicast messages with other servers. The operation of validating the Partial IV and performing replay protection MUST be atomic. Section 2.6.1 and Section 2.6.3.2 describe the update of Replay Windows after the loss of data from the Security Context and the retrieving of new Security Context parameters.¶

The protection from replay of requests is performed as per Section 7.4 of [RFC8613], separately for each client and by leveraging the Replay Window in the corresponding Recipient Context. The protection from replay of responses in a long exchange is performed as defined in Section 5.3.1 of this document.¶

7.1. Protecting the Request

When using the group mode to protect a request, a client proceeds as described in Section 8.1 of [RFC8613] with the following modifications.¶

• In Step 2, the Additional Authenticated Data is modified as described in Section 3 of this document.¶

• In Step 4, the encryption of the COSE object is modified as described in Section 3 of this document. The encoding of the compressed COSE object is modified as described in Section 4 of this document. In particular, the Group Flag MUST be set to 1. The Group Encryption Algorithm from the Common Context MUST be used.¶

• In Step 5, the countersignature is computed and the format of the OSCORE message is modified as described in Section 3 and Section 4 of this document. In particular, the payload of the Group OSCORE message includes also the encrypted countersignature.¶

In addition, the following applies when sending a request that establishes a long exchange.¶

• If the client intends to keep the long exchange active beyond a possible change of Sender ID, the client MUST store the value of the 'kid' parameter from the request, and retain it until the long exchange is terminated. Even in case the client is individually rekeyed and receives a new Sender ID from the Group Manager (see Section 2.6.3.1), the client MUST NOT update the stored 'kid' parameter value associated with the long exchange and the corresponding request.¶

• If the client intends to keep the long exchange active beyond a possible change of ID Context following a group rekeying (see Section 12.2), then the following applies.¶

  • The client MUST store the value of the 'kid context' parameter from the request, and retain it until the long exchange is terminated. Upon establishing a new Security Context with a new Gid as ID Context (see Section 2.6.3.2), the client MUST NOT update the stored 'kid context' parameter value associated with the long exchange and the corresponding request.¶

  • The client MUST store an invariant identifier of the group, which is immutable even if the Security Context of the group is re-established. For example, this invariant identifier can be the "group name" in [I-D.ietf-ace-key-groupcomm-oscore], where it is used for joining the group and retrieving the current group keying material from the Group Manager.¶

    After a group rekeying, the client might have missed both the rekeying messages and the servers' first responses that are protected with the new Security Context and include the new ID Context (Gid) in the 'kid context' parameter (see Section 7.3). In such a case, while still not knowing the new ID Context (Gid) used in the group, the client is able to retrieve the current group keying material from the Group Manager, using the invariant identifier to unambiguously refer to the group.¶

7.2. Verifying the Request

Upon receiving a protected request with the Group Flag set to 1, following the procedure in Section 6, a server proceeds as described in Section 8.2 of [RFC8613] with the following modifications.¶

• In Step 2, the decoding of the compressed COSE object follows Section 4 of this document. In particular:¶

  • If the server discards the request due to not retrieving a Security Context associated with the OSCORE group, the server MAY respond with a 4.01 (Unauthorized) error message. When doing so, the server MAY set an Outer Max-Age Option with value zero, and MAY include a descriptive string as diagnostic payload.¶

  • If the received 'kid context' matches an existing ID Context (Gid) but the received 'kid' does not match any Recipient ID in this Security Context, then the server MAY create a new Recipient Context for this Recipient ID and initialize it according to Section 3 of [RFC8613], and also retrieve the authentication credential associated with the Recipient ID to be stored in the new Recipient Context. Such a configuration is application specific. If the application does not specify dynamic derivation of new Recipient Contexts, then the server SHALL stop processing the request.¶

• In Step 4, the Additional Authenticated Data is modified as described in Section 3 of this document.¶

• In Step 6, the server also verifies the countersignature by using the public key from the client's authentication credential stored in the associated Recipient Context. In particular:¶

  • If the server does not have the public key of the client yet, the server MUST stop processing the request and MAY respond with a 5.03 (Service Unavailable) response. The response MAY include a Max-Age Option, indicating to the client the number of seconds after which to retry. If the Max-Age Option is not present, Section 5.10.5 of [RFC7252] specifies a default retry time of 60 seconds.¶

  • The signature verification as defined below SHOULD be performed before decrypting the COSE object. An exception applies to implementations that cannot perform the two steps in this order. Those implementations MUST ensure that no access to the plaintext is possible before a successful signature verification and MUST prevent any possible leak of time-related information that can yield side-channel attacks.¶

  • The server retrieves the encrypted countersignature ENC_SIGNATURE from the message payload, and computes the original countersignature SIGNATURE as¶

    SIGNATURE = ENC_SIGNATURE XOR KEYSTREAM¶

    where KEYSTREAM is derived as per Section 4.2.¶

  • The server verifies the original countersignature SIGNATURE as described in Sections 3.2 and 3.3 of [RFC9338] by using the client's public key and according to the Signature Algorithm in the Security Context.¶

    In particular, the Countersign_structure contains the context text string "CounterSignature0", the external_aad as defined in Section 3.4 of this document, and the ciphertext of the COSE object as payload.¶

  • If the signature verification fails, the server SHALL stop processing the request, SHALL NOT update the Replay Window, and MAY respond with a 4.00 (Bad Request) response. Such a response MAY include an Outer Max-Age Option with value zero, and its diagnostic payload MAY contain a string, which, if present, MUST be "Decryption failed" as if the decryption of the COSE object had failed.¶

  • When decrypting the COSE object using the Recipient Key, the Group Encryption Algorithm from the Common Context MUST be used.¶

• Additionally, if the used Recipient Context was created upon receiving this request and the message is not verified successfully, the server MAY delete that Recipient Context. When this behavior is specified by the application, it mitigates attacks that aim at overloading the server's storage.¶

  If the server deletes the used Recipient Context in this particular circumstance, then this deletion does not require the server to initialize as invalid the Replay Window of any new Recipient Context created later within the Security Context (see Section 2.6.1.2).¶

A server SHOULD NOT process a request if the received Recipient ID ('kid') is equal to its own Sender ID in its own Sender Context. However, in some applications the server can prepare a request to be sent to itself (e.g., see [I-D.ietf-core-observe-multicast-notifications]), in which case such requests would be expected.¶

In addition, the following applies if the request establishes a long exchange and the server intends to reply with multiple responses.¶

• The server MUST store the value of the 'kid' parameter from the request, and retain it until the last response has been sent. The server MUST NOT update the stored value of the 'kid' parameter associated with the request, even if the client is individually rekeyed and starts using a new Sender ID received from the Group Manager (see Section 2.6.3.1).¶

• The server MUST store the value of the 'kid context' parameter from the request, and retain it until the last response has been sent, i.e., beyond a possible change of ID Context following a group rekeying (see Section 12.2). That is, upon establishing a new Security Context with a new Gid as ID Context (see Section 2.6.3.2), the server MUST NOT update the stored value of a 'kid context' parameter associated with the request.¶

7.3. Protecting the Response

When using the group mode to protect a response, a server proceeds as described in Section 8.3 of [RFC8613] with the following modifications.¶

Note that the server always protects a response with the Sender Context from its latest Security Context, and that establishing a new Security Context resets the Sender Sequence Number to 0 (see Section 2.6.3.1 and Section 2.6.3.2).¶

• In Step 2, the Additional Authenticated Data is modified as described in Section 3 of this document.¶

  In addition, the following applies if the server intends to reply with multiple responses within the long exchange established by the corresponding request.¶

  • The server MUST use the stored value of the 'kid' parameter from the request (see Section 7.2), as value for the 'request_kid' parameter in the external_aad (see Section 3.4).¶

  • The server MUST use the stored value of the 'kid context' parameter from the request (see Section 7.2), as value for the 'request_kid_context' parameter in the external_aad (see Section 3.4).¶

• In Step 3, if either of the following conditions holds, the server MUST include its Sender Sequence Number as Partial IV in the response and use it to build the nonce to protect the response. This prevents the server from reusing the nonce from the request together with the same encryption key.¶

  • The response is not the first response that the server sends to the request.¶

  • The server is using a different Security Context for the response than the one that was used to verify the request (see Section 12.2).¶

• In Step 4, the encryption of the COSE object is modified as described in Section 3 of this document. The encoding of the compressed COSE object is modified as described in Section 4 of this document. In particular, the Group Flag MUST be set to 1. The Group Encryption Algorithm from the Common Context MUST be used.¶

  In addition, the following applies.¶

  • If the server is using a different ID Context (Gid) for the response than the one that was used to verify the request (see Section 12.2) and this is the first response from the server to that request, then the new ID Context MUST be included in the 'kid context' parameter of the response.¶

  • The server may be replying to a request that was protected with an old Security Context. After completing the establishment of a new Security Context, the server MUST protect all the responses to that request with the Sender Context of the new Security Context.¶

    For each ongoing long exchange, the server can help the client to synchronize, by including also the 'kid context' parameter in responses following a group rekeying, with value set to the ID Context (Gid) of the new Security Context.¶

    If there is a known upper limit to the duration of a group rekeying, the server SHOULD include the 'kid context' parameter during that time. Otherwise, the server SHOULD include it until the Max-Age has expired for the last response sent before the installation of the new Security Context.¶

  • The server can obtain a new Sender ID from the Group Manager when individually rekeyed (see Section 2.6.3.1) or when re-joining the group. In such a case, the server can help the client to synchronize by including the 'kid' parameter in a response protected in group mode even when the request was protected in pairwise mode (see Section 8.3).¶

    That is, when responding to a request protected in pairwise mode, the server SHOULD include the 'kid' parameter in a response protected in group mode, if it is replying to that client for the first time since the assignment of its new Sender ID.¶

• In Step 5, the countersignature is computed and the format of the OSCORE message is modified as described in Section 3 and Section 4 of this document. In particular the payload of the Group OSCORE message includes also the encrypted countersignature (see Section 4.1).¶

7.4. Verifying the Response

Upon receiving a protected response with the Group Flag set to 1, following the procedure in Section 6, a client proceeds as described in Section 8.4 of [RFC8613] with the modifications described in this section.¶

Note that a client may receive a response protected with a Security Context different from the one used to protect the corresponding request, and that, upon the establishment of a new Security Context, the client re-initializes its Replay Windows in its Recipient Contexts (see Section 12.2).¶

• In Step 2, the decoding of the compressed COSE object is modified as described in Section 4 of this document. In particular, a 'kid' may not be present if the response is a reply to a request protected in pairwise mode. In such a case, the client assumes the response 'kid' to be the Recipient ID for the server for which the request protected in pairwise mode was intended.¶

  If the response 'kid context' matches an existing ID Context (Gid) but the received/assumed 'kid' does not match any Recipient ID in this Security Context, then the client MAY create a new Recipient Context for this Recipient ID and initialize it according to Section 3 of [RFC8613], and also retrieve the authentication credential associated with the Recipient ID to be stored in the new Recipient Context. If the application does not specify dynamic derivation of new Recipient Contexts, then the client SHALL stop processing the response.¶

• In Step 3, the Additional Authenticated Data is modified as described in Section 3 of this document.¶

  In addition, the following applies if the client processes a response to a request within a long exchange.¶

  • The client MUST use the stored value of the 'kid' parameter from the request (see Section 7.1), as value for the 'request_kid' parameter in the external_aad (see Section 3.4).¶

  • The client MUST use the stored value of the 'kid context' parameter from the request (see Section 7.1), as value for the 'request_kid_context' parameter in the external_aad (see Section 3.4).¶

  This ensures that, throughout a long exchange, the client can correctly verify the received responses, even if the client is individually rekeyed and starts using a new Sender ID received from the Group Manager (see Section 2.6.3.1), as well as when it installs a new Security Context with a new ID Context (Gid) following a group rekeying (see Section 12.2).¶

• In Step 5, the client also verifies the countersignature by using the public key from the server's authentication credential stored in the associated Recipient Context. In particular:¶

  • The signature verification as defined below SHOULD be performed before decrypting the COSE object. An exception applies to implementations that cannot perform the two steps in this order. Those implementations MUST ensure that no access to the plaintext is possible before a successful signature verification and MUST prevent any possible leak of time-related information that can yield side-channel attacks.¶

  • The client retrieves the encrypted countersignature ENC_SIGNATURE from the message payload, and computes the original countersignature SIGNATURE as¶

    SIGNATURE = ENC_SIGNATURE XOR KEYSTREAM¶

    where KEYSTREAM is derived as per Section 4.2.¶

    The client verifies the original countersignature SIGNATURE.¶

  • If the verification of the countersignature fails, the client: i) SHALL stop processing the response; and ii) SHALL NOT update the Response Number associated with the server.¶

  • After a successful verification of the countersignature, the client performs also the following actions in case the request was protected in pairwise mode (see Section 8.3).¶

    • If the 'kid' parameter is present in the response, the client checks whether this received 'kid' is equal to the expected 'kid', i.e., the known Recipient ID for the server for which the request was intended.¶

    • If the 'kid' parameter is not present in the response, the client checks whether the server that has sent the response is the same one for which the request was intended. This can be done by checking that the public key used to verify the countersignature of the response is equal to the public key included in the authentication credential Recipient Auth Cred, which was taken as input to derive the Pairwise Sender Key used for protecting the request (see Section 2.5.1).¶

    In either case, if the client determines that the response has come from a different server than the expected one, then the client: i) SHALL discard the response and SHALL NOT deliver it to the application; ii) SHALL NOT update the Response Number associated with the server.¶

    Otherwise, the client hereafter considers the received 'kid' as the current Recipient ID for the server.¶

• In Step 5, when decrypting the COSE object using the Recipient Key, the Group Encryption Algorithm from the Common Context MUST be used.¶

  In addition, the client performs the following actions if the response is received within a long exchange.¶

  • The ordering and the replay protection of responses received from the server during the long exchange are performed as per Section 5.3.1 of this document, by using the Response Number associated with that server within that long exchange. In case of unsuccessful decryption and verification of a response, the client SHALL NOT update the Response Number associated with the server.¶

  • When receiving the first valid response from the server within the long exchange, the client MUST store the kid "kid1" of that server for that long exchange. If the 'kid' field is included in the OSCORE Option of the response, its value specifies "kid1". If the request was protected in pairwise mode (see Section 8.3), the 'kid' field may not be present in the OSCORE Option of the response (see Section 3.2). In this case, the client assumes "kid1" to be the Recipient ID for the server for which the request was intended.¶

  • When receiving another valid response to the same request from the same server - which can be identified and recognized through the same public key used to verify the countersignature and included in the server's authentication credential - the client determines the kid "kid2" of the server as above for "kid1", and MUST check whether "kid2" is equal to the stored "kid1".¶

    If "kid1" and "kid2" are different, the client SHOULD NOT accept the response as valid to be delivered to the application, and SHOULD NOT update the Response Number associated with the server. Exceptions can apply as the client can retain the information required to order the responses, or if the client application does not require response ordering altogether. Servers MUST NOT rely on clients tolerating this, unless it was explicitly agreed on (e.g., as part of the group's setup).¶

  Note that, if "kid2" is different from "kid1" and the 'kid' field is omitted from the response - which is possible if the request was protected in pairwise mode - then the client will compute a wrong keystream to decrypt the countersignature (i.e., by using "kid1" rather than "kid2" in the 'id' field of the 'info' array in Section 4.2), thus subsequently failing to verify the countersignature and discarding the response.¶

  This ensures that the client remains able to correctly perform the ordering and replay protection of responses within a long exchange, even if the server legitimately starts using a new Sender ID, as received from the Group Manager when individually rekeyed (see Section 2.6.3.1) or when re-joining the group.¶

• In Step 8, if the used Recipient Context was created upon receiving this response and the message is not verified successfully, the client MAY delete that Recipient Context. When this behavior is specified by the application, it mitigates attacks that aim at overloading the client's storage.¶

  If the client deletes the used Recipient Context in this particular circumstance, then this deletion does not require the client to initialize as invalid the Replay Window of any new Recipient Context created later within the Security Context (see Section 2.6.1.2).¶

7.5. External Signature Checkers

When a message is protected in group mode, it is possible for designated external signature checkers to verify the countersignature of the message. For example, an external signature checker can be an intermediary gateway that intercepts messages protected in group mode and ensures that they reach the intended recipients only if it successfully verifies their countersignatures.¶

Since they do not join a group as members, external signature checkers need to retrieve from the Group Manager the authentication credentials of group members and other selected group data, such as the current Signature Encryption Key (see Section 2.1.9). Section 12.3 describes how the Group Manager supports signature checkers.¶

When receiving a message protected in group mode, a signature checker proceeds as follows.¶

• The signature checker retrieves the encrypted countersignature ENC_SIGNATURE from the message payload, and computes the original countersignature SIGNATURE as¶

  SIGNATURE = ENC_SIGNATURE XOR KEYSTREAM¶

  where KEYSTREAM is derived as per Section 4.2.¶

• The signature checker verifies the original countersignature SIGNATURE, by using the public key of the sender endpoint as included in that endpoint's authentication credential. The signature checker determines the right authentication credential based on the ID Context (Gid) and the Sender ID of the sender endpoint.¶

Note that the following applies when attempting to verify the countersignature of a response message.¶

• The response may not include a Partial IV and/or an ID Context. In such a case, the signature checker considers the same values from the corresponding request, i.e., the request matching with the response by CoAP Token value.¶

• The response may not include a Sender ID. This can happen when the response protected in group mode matches a request protected in pairwise mode (see Section 8.1), with a case in point provided by [I-D.ietf-core-cacheable-oscore]. In such a case, the signature checker needs to use other means (e.g., source addressing information of the server endpoint) to identify the correct authentication credential including the public key to use for verifying the countersignature of the response.¶

The particular actions following a successful or unsuccessful verification of the countersignature are application specific and out of the scope of this document.¶

8.1. Pre-Conditions

In order to protect an outgoing message in pairwise mode, the sender needs to know the authentication credential and the Recipient ID for the recipient endpoint, as stored in the Recipient Context associated with that endpoint (see Section 2.5.4).¶

Typically, the sender endpoint sends the message protected in pairwise mode over unicast, so that the message is delivered only to the intended recipient endpoint for which it is protected. This requires the sender to know the individual address of that recipient endpoint, which the sender may not know at any given point in time. For instance, right after having joined the group, a client may know the authentication credential and Recipient ID for a given server, but not the addressing information required to reach it with an individual, one-to-one request.¶

In order to make addressing information of individual endpoints available, servers in the group MAY expose a resource to which a client can send a request targeting a set of servers, identified by their 'kid' values specified in the request payload, or implicitly if the request is sent in pairwise mode. Further details of such an interface are out of scope for this document.¶

8.2. Main Differences from OSCORE

The pairwise mode protects messages between two members of a group, essentially following [RFC8613] but with the following notable differences.¶

• The 'kid' and 'kid context' parameters of the COSE object are used as defined in Section 3.2 of this document.¶

• The external_aad defined in Section 3.4 of this document is used for the encryption and decryption process.¶

• The Pairwise Sender/Recipient Keys used as Sender/Recipient keys are derived as defined in Section 2.5 of this document.¶

8.3. Protecting the Request

When using the pairwise mode to protect a request, a client SHALL proceed as described in Section 8.1 of [RFC8613] with the differences summarized in Section 8.2 of this document.¶

Furthermore, when sending a request that establishes a long exchange, what is specified in Section 7.1 of this document holds, with respect to storing the value of the 'kid' and 'kid context' parameters, and to storing an invariant identifier of the group.¶

8.4. Verifying the Request

Upon receiving a protected request with the Group Flag set to 0, following the procedure in Section 6, a server SHALL proceed as described in Section 8.2 of [RFC8613] with the differences summarized in Section 8.2 of this document. The following differences also apply.¶

• If the server discards the request due to not retrieving a Security Context associated with the OSCORE group or to not supporting the pairwise mode, the server MAY respond with a 4.01 (Unauthorized) error message or a 4.02 (Bad Option) error message, respectively. When doing so, the server MAY set an Outer Max-Age Option with value zero, and MAY include a descriptive string as diagnostic payload.¶

• If a new Recipient Context is created for this Recipient ID, new Pairwise Sender/Recipient Keys are also derived (see Section 2.5.1). The new Pairwise Sender/Recipient Keys are deleted if the Recipient Context is deleted as a result of the message not being successfully verified.¶

• What is specified in Section 7.2 of this document holds with respect to the following points.¶

  • The possible, dynamic creation and configuration of a Recipient Context upon receiving the request.¶

  • The possible deletion of a Recipient Context created upon receiving the request, in case the request is not verified successfully.¶

  • The rule about processing the request where the received Recipient ID ('kid') is equal to the server's Sender ID.¶

  • The storing of the value of the 'kid' and 'kid context' parameters from the request, if the server intends to reply with multiple responses within the long exchange established by the request.¶

8.5. Protecting the Response

When using the pairwise mode to protect a response, a server SHALL proceed as described in Section 8.3 of [RFC8613] with the differences summarized in Section 8.2 of this document. The following differences also apply.¶

• What is specified in Section 7.3 of this document holds with respect to the following points.¶

  • The protection of a response when using a different Security Context than the one used to verify the corresponding request (see Section 12.2). That is, the server always protects a response with the Sender Context from its latest Security Context, and establishing a new Security Context resets the Sender Sequence Number to 0 (see Section 2.6.3.1 and Section 2.6.3.2).¶

  • The use of the stored value of the 'kid' and 'kid context' parameters, if the server intends to reply with multiple responses within the long exchange established by the request.¶

  • The rules for the inclusion of the server's Sender Sequence Number as Partial IV in a response, as used to build the nonce to protect the response.¶

  • The rules for the inclusion of the ID Context (Gid) in the 'kid context' parameter of a response, if the ID Context used for the response differs from the one used to verify the request (see Section 12.2), also for helping the client to synchronize.¶

  • The rules for the inclusion of the Sender ID in the 'kid' parameter of a response to a request that was protected in pairwise mode, if the server has obtained a new Sender ID from the Group Manager when individually rekeyed (see Section 2.6.3.1), thus helping the client to synchronize.¶

8.6. Verifying the Response

Upon receiving a protected response with the Group Flag set to 0, following the procedure in Section 6, a client SHALL proceed as described in Section 8.4 of [RFC8613] with the differences summarized in Section 8.2 of this document. The following differences also apply.¶

• The client may receive a response protected with a Security Context different from the one used to protect the corresponding request. Also, upon the establishment of a new Security Context, the client re-initializes its Replay Windows in its Recipient Contexts (see Section 2.2).¶

• The same as described in Section 7.4 holds with respect to handling the 'kid' parameter of the response, when received as a reply to a request protected in pairwise mode. The client can also in this case check whether the replying server is the expected one, by relying on the server's public key. However, since the response is protected in pairwise mode, the public key is not used for verifying a countersignature as in Section 7.4. Instead, the expected server's authentication credential - namely Recipient Auth Cred and including the server's public key - was taken as input to derive the Pairwise Recipient Key used to decrypt and verify the response (see Section 2.5.1).¶

• If a new Recipient Context is created for this Recipient ID, new Pairwise Sender/Recipient Keys are also derived (see Section 2.5.1). The new Pairwise Sender/Recipient Keys are deleted if the Recipient Context is deleted as a result of the message not being successfully verified.¶

• What is specified in Section 7.4 of this document holds with respect to the following points.¶

  • The possible, dynamic creation and configuration of a Recipient Context upon receiving the response.¶

  • The use of the stored value of the 'kid' and 'kid context' parameters, when processing a response received within a long exchange.¶

  • The performing of ordering and replay protection for responses received within a long exchange.¶

  • The possible deletion of a Recipient Context created upon receiving the response, in case the response is not verified successfully.¶

12.1. Set-up of New Endpoints

From the Group Manager, an endpoint acquires group data such as the Gid and OSCORE input parameters including its own Sender ID, with which it can derive the Sender Context.¶

When joining the group or later on as a group member, an endpoint can also retrieve from the Group Manager the authentication credential of the Group Manager as well as the authentication credential and other information associated with other members of the group, with which it can derive the corresponding Recipient Context. An application can configure a group member to asynchronously retrieve information about Recipient Contexts, e.g., by Observing [RFC7641] a resource at the Group Manager to get updates on the group membership.¶

Upon endpoints' joining, the Group Manager collects their authentication credentials and MUST verify proof of possession of the respective private key. As an example, such proof of possession is possible to achieve during the join process provided by the realization of Group Manager specified in [I-D.ietf-ace-key-groupcomm-oscore]. Together with the requested authentication credentials of other group members, the Group Manager MUST provide the joining endpoints with the Sender ID of the associated group members and the current Key Generation Number in the group (see Section 12.2).¶

An endpoint may join a group, for example, by explicitly interacting with the responsible Group Manager, or by being configured with some tool performing the tasks of the Group Manager. When becoming members of a group, endpoints are not required to know how many and what endpoints are in the same group.¶

Communications that the Group Manager has with joining endpoints and group members MUST be secured. Specific details on how to secure such communications are out of the scope of this document.¶

The Group Manager MUST verify that the joining endpoint is authorized to join the group. To this end, the Group Manager can directly authorize the joining endpoint, or expect it to provide authorization evidence previously obtained from a trusted entity. Further details about the authorization of joining endpoints are out of the scope of this document.¶

In case of successful authorization check, the Group Manager provides the joining endpoint with the keying material to initialize the Security Context. The actual provisioning of keying material and parameters to the joining endpoint is out of the scope of this document.¶

12.2. Management of Group Keying Material

In order to establish a new Security Context for a group, the Group Manager MUST generate and assign to the group a new Group Identifier (Gid) and a new value for the Master Secret parameter. When doing so, a new value for the Master Salt parameter MAY also be generated and assigned to the group. When establishing the new Security Context, the Group Manager SHOULD preserve the current value of the Sender ID of each group member in order to ensure an efficient key rollover. Exceptions can apply if there are compelling reasons for making available again some of the Sender ID values currently used.¶

The specific group key management scheme used to distribute new keying material is out of the scope of this document. A simple group key management scheme is defined in [I-D.ietf-ace-key-groupcomm-oscore]. Different group key management schemes rely on different approaches to compose and deliver rekeying messages, i.e., individually targeting single recipients, or targeting multiple recipients at once (e.g., over UDP/IP multicast), or a combination of the two approaches. As long as it is viable for the specific rekeying message to be delivered and it is supported by the intended message recipient(s), using a reliable transport to deliver a rekeying message should be preferred, as it reduces chances of group members missing a rekeying instance.¶

Irrespective of the transport used being reliable or unreliable, appropriate congestion control MUST be enforced. If the key distribution traffic uses CoAP over UDP or over other unreliable transports, mechanisms for enforcing congestion control are specified in Section 4.7 of [RFC7252] and in Section 3.6 of [I-D.ietf-core-groupcomm-bis] for the case of group communication (e.g., over UDP/IP multicast). If, irrespective of using CoAP, the key distribution traffic relies on alternative setups with unreliable transports, one can rely on general congestion-control mechanisms such as DCCP [RFC4340], or on dedicated congestion control mechanisms for the transport specifically used (e.g., those defined in [RFC9002] for QUIC [RFC9000]).¶

The set of group members should not be assumed as fixed, i.e., the group membership is subject to changes, possibly on a frequent basis.¶

The Group Manager MUST rekey the group without undue delay when one or more endpoints leave the group. An endpoint may leave the group at own initiative, or may be evicted from the group by the Group Manager, e.g., in case the endpoint is compromised, or is suspected to be compromised (as determined by the Group Manager through its own means or based on information that it obtains from a trusted source such as an Intrusion Detection System or an issuer of authentication credentials). In either case, rekeying the group excludes such endpoints from future communications in the group, and thus preserves forward security. If a network node is compromised or suspected to be compromised, the Group Manager MUST evict from the group all the endpoints hosted by that node that are members of the group and rekey the group accordingly.¶

If required by the application, the Group Manager MUST also rekey the group when one or more new joining endpoints are added to the group, thus preserving backward security.¶

The Group Manager MAY also rekey the group for other reasons, e.g., according to an application-specific rekeying period or scheduling.¶

Separately for each group, the value of the Key Generation Number increases by one each time the Group Manager distributes new keying material to that group (see below).¶

The establishment of the new Security Context for the group takes the following steps.¶

1. The Group Manager MUST increment the Key Generation Number for the group by 1. It is up to the Group Manager what actions to take when a wrap-around of the Key Generation Number is detected.¶

2. The Group Manager MUST build a set of stale Sender IDs including:¶

   • The Sender IDs that, during the current Gid, were both assigned to an endpoint and subsequently relinquished (see Section 2.6.3.1).¶

   • The current Sender IDs of the group members that the upcoming group rekeying aims to exclude from future group communications, if any.¶

3. The Group Manager rekeys the group, by distributing:¶

   • The new keying material, i.e., the new Master Secret, the new Gid and (optionally) the new Master Salt.¶

   • The new Key Generation Number from Step 1.¶

   • The set of stale Sender IDs from Step 2.¶

   Further information may be distributed, depending on the specific group key management scheme used in the group.¶

When receiving the new group keying material, a group member considers the received stale Sender IDs and performs the following actions.¶

• The group member MUST remove every authentication credential associated with a stale Sender ID from its list of group members' authentication credentials used in the group.¶

• The group member MUST delete each of its Recipient Contexts used in the group whose corresponding Recipient ID is a stale Sen