| ACE Working Group | S. Gerdes |
| Internet-Draft | O. Bergmann |
| Intended status: Standards Track | C. Bormann |
| Expires: December 20, 2020 | Universität Bremen TZI |
| G. Selander | |
| Ericsson AB | |
| L. Seitz | |
| Combitech | |
| June 18, 2020 |
Datagram Transport Layer Security (DTLS) Profile for Authentication and Authorization for Constrained Environments (ACE)
draft-ietf-ace-dtls-authorize-11
This specification defines a profile of the ACE framework that allows constrained servers to delegate client authentication and authorization. The protocol relies on DTLS version 1.2 for communication security between entities in a constrained network using either raw public keys or pre-shared keys. A resource-constrained server can use this protocol to delegate management of authorization information to a trusted host with less severe limitations regarding processing power and memory.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.
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This Internet-Draft will expire on December 20, 2020.
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This specification defines a profile of the ACE framework [I-D.ietf-ace-oauth-authz]. In this profile, a client and a resource server use CoAP [RFC7252] over DTLS version 1.2 [RFC6347] to communicate. The client obtains an access token, bound to a key (the proof-of-possession key), from an authorization server to prove its authorization to access protected resources hosted by the resource server. Also, the client and the resource server are provided by the authorization server with the necessary keying material to establish a DTLS session. The communication between client and authorization server may also be secured with DTLS. This specification supports DTLS with Raw Public Keys (RPK) [RFC7250] and with Pre-Shared Keys (PSK) [RFC4279].
The ACE framework requires that client and server mutually authenticate each other before any application data is exchanged. DTLS enables mutual authentication if both client and server prove their ability to use certain keying material in the DTLS handshake. The authorization server assists in this process on the server side by incorporating keying material (or information about keying material) into the access token, which is considered a “proof of possession” token.
In the RPK mode, the client proves that it can use the RPK bound to the token and the server shows that it can use a certain RPK.
The resource server needs access to the token in order to complete this exchange. For the RPK mode, the client must upload the access token to the resource server before initiating the handshake, as described in Section 5.8.1 of the ACE framework [I-D.ietf-ace-oauth-authz].
In the PSK mode, client and server show with the DTLS handshake that they can use the keying material that is bound to the access token. To transfer the access token from the client to the resource server, the psk_identity parameter in the DTLS PSK handshake may be used instead of uploading the token prior to the handshake.
As recommended in Section 5.8 of [I-D.ietf-ace-oauth-authz], this specification uses CBOR web tokens to convey claims within an access token issued by the server. While other formats could be used as well, those are out of scope for this document.
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 [I-D.ietf-ace-oauth-authz] and in [I-D.ietf-ace-oauth-params].
The authorization information (authz-info) resource refers to the authorization information endpoint as specified in [I-D.ietf-ace-oauth-authz]. The term claim is used in this document with the same semantics as in [I-D.ietf-ace-oauth-authz], i.e., it denotes information carried in the access token or returned from introspection.
The CoAP-DTLS profile for ACE specifies the transfer of authentication information and, if necessary, authorization information between the client (C) and the resource server (RS) during setup of a DTLS session for CoAP messaging. It also specifies how the client can use CoAP over DTLS to retrieve an access token from the authorization server (AS) for a protected resource hosted on the resource server. As specified in Section 6.7 of [I-D.ietf-ace-oauth-authz], use of DTLS for one or both of these interactions is completely independent
This profile requires the client to retrieve an access token for protected resource(s) it wants to access on the resource server as specified in [I-D.ietf-ace-oauth-authz]. Figure 1 shows the typical message flow in this scenario (messages in square brackets are optional):
C RS AS | [---- Resource Request ------>]| | | | | | [<-AS Request Creation Hints-] | | | | | | ------- Token Request ----------------------------> | | | | | <---------------------------- Access Token --------- | | + Access Information |
Figure 1: Retrieving an Access Token
To determine the authorization server in charge of a resource hosted at the resource server, the client can send an initial Unauthorized Resource Request message to the resource server. The resource server then denies the request and sends an AS Request Creation Hints message containing the address of its authorization server back to the client as specified in Section 5.1.2 of [I-D.ietf-ace-oauth-authz].
Once the client knows the authorization server’s address, it can send an access token request to the token endpoint at the authorization server as specified in [I-D.ietf-ace-oauth-authz]. As the access token request as well as the response may contain confidential data, the communication between the client and the authorization server must be confidentiality-protected and ensure authenticity. The client may have been registered at the authorization server via the OAuth 2.0 client registration mechanism as outlined in Section 5.3 of [I-D.ietf-ace-oauth-authz].
The access token returned by the authorization server can then be used by the client to establish a new DTLS session with the resource server. When the client intends to use an asymmetric proof-of-possession key in the DTLS handshake with the resource server, the client MUST upload the access token to the authz-info resource, i.e. the authz-info endpoint, on the resource server before starting the DTLS handshake, as described in Section 5.8.1 of [I-D.ietf-ace-oauth-authz]. In case the client uses a symmetric proof-of-possession key in the DTLS handshake, the procedure as above MAY be used, or alternatively, the access token MAY instead be transferred in the DTLS ClientKeyExchange message (see Section 3.3.1). In any case, DTLS MUST be used in a mode that provides replay protection.
Figure 2 depicts the common protocol flow for the DTLS profile after the client has retrieved the access token from the authorization server, AS.
C RS AS | [--- Access Token ------>] | | | | | | <== DTLS channel setup ==> | | | | | | == Authorized Request ===> | | | | | | <=== Protected Resource == | |
Figure 2: Protocol overview
The following sections specify how CoAP is used to interchange access-related data between the resource server, the client and the authorization server so that the authorization server can provide the client and the resource server with sufficient information to establish a secure channel, and convey authorization information specific for this communication relationship to the resource server.
Section 3.1 describes how the communication between the client (C) and the authorization server (AS) must be secured. Depending on the used CoAP security mode (see also Section 9 of [RFC7252], the Client-to-AS request, AS-to-Client response (see Section 5.6 of [I-D.ietf-ace-oauth-authz]) and DTLS session establishment carry slightly different information. Section 3.2 addresses the use of raw public keys while Section 3.3 defines how pre-shared keys are used in this profile.
To retrieve an access token for the resource that the client wants to access, the client requests an access token from the authorization server. Before the client can request the access token, the client and the authorization server MUST establish a secure communication channel. This profile assumes that the keying material to secure this communication channel has securely been obtained either by manual configuration or in an automated provisioning process. The following requirements in alignment with Section 6.5 of [I-D.ietf-ace-oauth-authz] therefore must be met:
The client and the authorization server MUST use their respective keying material for all exchanged messages. How the security association between the client and the authorization server is bootstrapped is not part of this document. The client and the authorization server must ensure the confidentiality, integrity and authenticity of all exchanged messages within the ACE protocol.
Section 6 specifies how communication with the authorization server is secured.
When the client and the resource server use RawPublicKey authentication, the procedure is as follows: After the client and the authorization server mutually authenticated each other and validated each other’s authorization, the client sends a token request to the authorization server’s token endpoint. The client MUST add a req_cnf object carrying either its raw public key or a unique identifier for a public key that it has previously made known to the authorization server. It is RECOMMENDED that the client uses DTLS with the same keying material to secure the communication with the authorization server, proving possession of the key as part of the token request. Other mechanisms for proving possession of the key may be defined in the future.
An example access token request from the client to the authorization server is depicted in Figure 3.
POST coaps://as.example.com/token
Content-Format: application/ace+cbor
Payload:
{
grant_type : client_credentials,
req_aud : "tempSensor4711",
req_cnf : {
COSE_Key : {
kty : EC2,
crv : P-256,
x : h'e866c35f4c3c81bb96a1...',
y : h'2e25556be097c8778a20...'
}
}
}
Figure 3: Access Token Request Example for RPK Mode
The example shows an access token request for the resource identified by the string “tempSensor4711” on the authorization server using a raw public key.
The authorization server MUST check if the client that it communicates with is associated with the RPK in the req_cnf parameter before issuing an access token to it. If the authorization server determines that the request is to be authorized according to the respective authorization rules, it generates an access token response for the client. The access token MUST be bound to the RPK of the client by means of the cnf claim.
The response MAY contain a profile parameter with the value coap_dtls to indicate that this profile MUST be used for communication between the client and the resource server. The profile may be specified out-of-band, in which case it does not have to be sent. The response also contains an access token with information for the resource server about the client’s public key. The authorization server MUST return in its response the parameter rs_cnf unless it is certain that the client already knows the public key of the resource server. The authorization server MUST ascertain that the RPK specified in rs_cnf belongs to the resource server that the client wants to communicate with. The authorization server MUST protect the integrity of the access token such that the resource server can detect unauthorized changes. If the access token contains confidential data, the authorization server MUST also protect the confidentiality of the access token.
The client MUST ascertain that the access token response belongs to a certain previously sent access token request, as the request may specify the resource server with which the client wants to communicate.
An example access token response from the authorization to the client is depicted in Figure 4. Here, the contents of the access_token claim have been truncated to improve readability. Caching proxies process the Max-Age option in the CoAP response which has a default value of 60 seconds (Section 5.6.1 of [RFC7252]). The authorization server SHOULD adjust the Max-Age option such that it does not exceed the expires_in parameter to avoid stale responses.
2.01 Created
Content-Format: application/ace+cbor
Max-Age: 3560
Payload:
{
access_token : b64'SlAV32hkKG...
(remainder of CWT omitted for brevity;
CWT contains the client's RPK in the cnf claim)',
expires_in : 3600,
rs_cnf : {
COSE_Key : {
kty : EC2,
crv : P-256,
x : h'd7cc072de2205bdc1537...',
y : h'f95e1d4b851a2cc80fff...'
}
}
}
Figure 4: Access Token Response Example for RPK Mode
Before the client initiates the DTLS handshake with the resource server, the client MUST send a POST request containing the obtained access token to the authz-info resource hosted by the resource server. After the client receives a confirmation that the resource server has accepted the access token, it SHOULD proceed to establish a new DTLS channel with the resource server. The client MUST use its correct public key in the DTLS handshake. If the authorization server has specified a cnf field in the access token response, the client MUST use this key. Otherwise, the client MUST use the public key that it specified in the req_cnf of the access token request. The client MUST specify this public key in the SubjectPublicKeyInfo structure of the DTLS handshake as described in [RFC7250].
To be consistent with [RFC7252] which allows for shortened MAC tags in constrained environments, an implementation that supports the RPK mode of this profile MUST at least support the ciphersuite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 [RFC7251]. As discussed in [RFC7748], new ECC curves have been defined recently that are considered superior to the so-called NIST curves. This specification therefore mandates implementation support for curve25519 (cf. [RFC8032], [RFC8422]) as this curve said to be efficient and less dangerous regarding implementation errors than the secp256r1 curve mandated in [RFC7252].
The resource server MUST check if the access token is still valid, if the resource server is the intended destination (i.e., the audience) of the token, and if the token was issued by an authorized authorization server. The access token is constructed by the authorization server such that the resource server can associate the access token with the Client’s public key. The cnf claim MUST contain either the client’s RPK or, if the key is already known by the resource server (e.g., from previous communication), a reference to this key. If the authorization server has no certain knowledge that the Client’s key is already known to the resource server, the Client’s public key MUST be included in the access token’s cnf parameter. If CBOR web tokens [RFC8392] are used (as recommended in [I-D.ietf-ace-oauth-authz]), keys MUST be encoded as specified in [RFC8747].
The raw public key used in the DTLS handshake with the client MUST belong to the resource server. If the resource server has several raw public keys, it needs to determine which key to use. The authorization server can help with this decision by including a cnf parameter in the access token that is associated with this communication. In this case, the resource server MUST use the information from the cnf field to select the proper keying material.
Thus, the handshake only finishes if the client and the resource server are able to use their respective keying material.
To retrieve an access token for the resource that the client wants to access, the client MAY include a cnf object carrying an identifier for a symmetric key in its access token request to the authorization server. This identifier can be used by the authorization server to determine the shared secret to construct the proof-of-possession token. The authorization server MUST check if the identifier refers to a symmetric key that was previously generated by the authorization server as a shared secret for the communication between this client and the resource server. If no such symmetric key was found, the authorization server MUST generate a new symmetric key that is returned in its response to the client.
The authorization server MUST determine the authorization rules for the client it communicates with as defined by the resource owner and generate the access token accordingly. If the authorization server authorizes the client, it returns an AS-to-Client response. If the profile parameter is present, it is set to coap_dtls. The authorization server MUST ascertain that the access token is generated for the resource server that the client wants to communicate with. Also, the authorization server MUST protect the integrity of the access token to ensure that the resource server can detect unauthorized changes. If the token contains confidential data such as the symmetric key, the confidentiality of the token MUST also be protected. Depending on the requested token type and algorithm in the access token request, the authorization server adds access Information to the response that provides the client with sufficient information to setup a DTLS channel with the resource server. The authorization server adds a cnf parameter to the access information carrying a COSE_Key object that informs the client about the shared secret that is to be used between the client and the resource server. To convey the same secret to the resource server, the authorization server can include it directly in the access token by means of the cnf claim or provide sufficient information to enable the resource server to derive the shared secret from the access token. As an alternative, the resource server MAY use token introspection to retrieve the keying material for this access token directly from the authorization server.
An example access token request for an access token with a symmetric proof-of-possession key is illustrated in Figure 5.
POST coaps://as.example.com/token
Content-Format: application/ace+cbor
Payload:
{
audience : "smokeSensor1807",
}
Figure 5: Example Access Token Request, (implicit) symmetric PoP-key
A corresponding example access token response is illustrated in Figure 6. In this example, the authorization server returns a 2.01 response containing a new access token (truncated to improve readability) and information for the client, including the symmetric key in the cnf claim. The information is transferred as a CBOR data structure as specified in [I-D.ietf-ace-oauth-authz].
2.01 Created
Content-Format: application/ace+cbor
Max-Age: 85800
Payload:
{
access_token : h'd08343a10...
(remainder of CWT omitted for brevity)
token_type : PoP,
expires_in : 86400,
profile : coap_dtls,
cnf : {
COSE_Key : {
kty : symmetric,
kid : h'3d027833fc6267ce',
k : h'73657373696f6e6b6579'
}
}
}
Figure 6: Example Access Token Response, symmetric PoP-key
The access token also comprises a cnf claim. This claim usually contains a COSE_Key object that carries either the symmetric key itself or a key identifier that can be used by the resource server to determine the secret key it shares with the client. If the access token carries a symmetric key, the access token MUST be encrypted using a COSE_Encrypt0 structure. The authorization server MUST use the keying material shared with the resource server to encrypt the token.
The cnf structure in the access token is provided in Figure 7.
cnf : {
COSE_Key : {
kty : symmetric,
kid : h'3d027833fc6267ce'
}
}
Figure 7: Access Token without Keying Material
A response that declines any operation on the requested resource is constructed according to Section 5.2 of [RFC6749], (cf. Section 5.6.3. of [I-D.ietf-ace-oauth-authz]). Figure 8 shows an example for a request that has been rejected due to invalid request parameters.
4.00 Bad Request
Content-Format: application/ace+cbor
Payload:
{
error : invalid_request
}
Figure 8: Example Access Token Response With Reject
The method for how the resource server determines the symmetric key from an access token containing only a key identifier is application-specific; the remainder of this section provides one example.
The authorization server and the resource server are assumed to share a key derivation key used to derive the symmetric key shared with the client from the key identifier in the access token. The key derivation key may be derived from some other secret key shared between the authorization server and the resource server. This key needs to be securely stored and processed in the same way as the key used to protect the communication between the authorization server and the resource server.
Knowledge of the symmetric key shared with the client must not reveal any information about the key derivation key or other secret keys shared between the authorization server and resource server.
In order to generate a new symmetric key to be used by client and resource server, the authorization server generates a new key identifier which MUST be unique among all key identifiers used by the authorization server for this resource server. The authorization server then uses the key derivation key shared with the resource server to derive the symmetric key as specified below. Instead of providing the keying material in the access token, the authorization server includes the key identifier in the kid parameter, see Figure 7. This key identifier enables the resource server to calculate the symmetric key used for the communication with the client using the key derivation key and a KDF to be defined by the application, for example HKDF-SHA-256. The key identifier picked by the authorization server needs to be unique for each access token where a unique symmetric key is required.
In this example, HKDF consists of the composition of the HKDF-Extract and HKDF-Expand steps [RFC5869]. The symmetric key is derived from the key identifier, the key derivation key and other data:
OKM = HKDF(salt, IKM, info, L),
where:
info = [
type : tstr,
L : uint,
access_token: map
]
where:
Use of a unique (per resource server) kid and the use of a key derivation IKM that is unique per authorization server/resource server pair as specified above will ensure that the derived key is not shared across multiple clients. However, to additionally provide variation in the derived key across different tokens used by the same client, it is additionally RECOMMENDED to include the “iat” claim and either the “exp” or “exi” claims in the access token.
When a client receives an access token response from an authorization server, the client MUST check if the access token response is bound to a certain previously sent access token request, as the request may specify the resource server with which the client wants to communicate.
The client checks if the payload of the access token response contains an access_token parameter and a cnf parameter. With this information the client can initiate the establishment of a new DTLS channel with a resource server. To use DTLS with pre-shared keys, the client follows the PSK key exchange algorithm specified in Section 2 of [RFC4279] using the key conveyed in the cnf parameter of the AS response as PSK when constructing the premaster secret. To be consistent with the recommendations in [RFC7252] a client is expected to offer at least the ciphersuite TLS_PSK_WITH_AES_128_CCM_8 [RFC6655] to the resource server.
In PreSharedKey mode, the knowledge of the shared secret by the client and the resource server is used for mutual authentication between both peers. Therefore, the resource server must be able to determine the shared secret from the access token. Following the general ACE authorization framework, the client can upload the access token to the resource server’s authz-info resource before starting the DTLS handshake. The client then needs to indicate during the DTLS handshake which previously uploaded access token it intends to use. To do so, it MUST create a COSE_Key structure with the kid that was conveyed in the rs_cnf claim in the token response from the authorization server and the key type symmetric. This structure then is included as the only element in the cnf structure that is used as value for psk_identity as shown in Figure 9.
{ cnf : {
COSE_Key : {
kty: symmetric,
kid : h'3d027833fc6267ce'
}
}
}
Figure 9: Access token containing a single kid parameter
As an alternative to the access token upload, the client can provide the most recent access token in the psk_identity field of the ClientKeyExchange message. To do so, the client MUST treat the contents of the access_token field from the AS-to-Client response as opaque data as specified in Section 4.2 of [RFC7925] and not perform any re-coding. This allows the resource server to retrieve the shared secret directly from the cnf claim of the access token.
If a resource server receives a ClientKeyExchange message that contains a psk_identity with a length greater than zero, it MUST parse the contents of the psk_identity field as CBOR data structure and process the contents as following:
If the contents of the psk_identity do not yield sufficient information to select a valid access token for the requesting client, the resource server aborts the DTLS handshake with an illegal_parameter alert.
When the resource server receives an access token, it MUST check if the access token is still valid, if the resource server is the intended destination (i.e., the audience of the token), and if the token was issued by an authorized authorization server. This specification assumes that the access token is a PoP token as described in [I-D.ietf-ace-oauth-authz] unless specifically stated otherwise. Therefore, the access token is bound to a symmetric PoP key that is used as shared secret between the client and the resource server. The resource server may use token introspection [RFC7662] on the access token to retrieve more information about the specific token. The use of introspection is out of scope for this specification.
While the client can retrieve the shared secret from the contents of the cnf parameter in the AS-to-Client response, the resource server uses the information contained in the cnf claim of the access token to determine the actual secret when no explicit kid was provided in the psk_identity field. If key derivation is used, the resource server uses the COSE_KDF_Context information as described above.
Once a DTLS channel has been established as described in Section 3.2 or Section 3.3, respectively, the client is authorized to access resources covered by the access token it has uploaded to the authz-info resource hosted by the resource server.
With the successful establishment of the DTLS channel, the client and the resource server have proven that they can use their respective keying material. An access token that is bound to the client’s keying material is associated with the channel. According to Section 5.8.1 of [I-D.ietf-ace-oauth-authz], there should be only one access token for each client. New access tokens issued by the authorization server are supposed to replace previously issued access tokens for the respective client. The resource server therefore must have a common understanding with the authorization server how access tokens are ordered.
Any request that the resource server receives on a DTLS channel that is tied to an access token via its keying material MUST be checked against the authorization rules that can be determined with the access token. The resource server MUST check for every request if the access token is still valid. If the token has expired, the resource server MUST remove it. Incoming CoAP requests that are not authorized with respect to any access token that is associated with the client MUST be rejected by the resource server with 4.01 response. The response SHOULD include AS Request Creation Hints as described in Section 5.1.1 of [I-D.ietf-ace-oauth-authz].
The resource server MUST only accept an incoming CoAP request as authorized if the following holds:
Incoming CoAP requests received on a secure DTLS channel that are not thus authorized MUST be rejected according to Section 5.8.2 of [I-D.ietf-ace-oauth-authz]
The client MUST ascertain that its keying material is still valid before sending a request or processing a response. If the client recently has updated the access token (see Section 4), it must be prepared that its request is still handled according to the previous authorization rules as there is no strict ordering between access token uploads and resource access messages. See also Section 7.2 for a discussion of access token processing.
If the client gets an error response containing AS Request Creation Hints (cf. Section 5.1.2 of [I-D.ietf-ace-oauth-authz] as response to its requests, it SHOULD request a new access token from the authorization server in order to continue communication with the resource server.
Unauthorized requests that have been received over a DTLS session SHOULD be treated as non-fatal by the resource server, i.e., the DTLS session SHOULD be kept alive until the associated access token has expired.
Resource servers must only use a new access token to update the authorization information for a DTLS session if the keying material that is bound to the token is the same that was used in the DTLS handshake. By associating the access tokens with the identifier of an existing DTLS session, the authorization information can be updated without changing the cryptographic keys for the DTLS communication between the client and the resource server, i.e. an existing session can be used with updated permissions.
The client can therefore update the authorization information stored at the resource server at any time without changing an established DTLS session. To do so, the client requests a new access token from the authorization server for the intended action on the respective resource and uploads this access token to the authz-info resource on the resource server.
Figure 10 depicts the message flow where the client requests a new access token after a security association between the client and the resource server has been established using this protocol. If the client wants to update the authorization information, the token request MUST specify the key identifier of the proof-of-possession key used for the existing DTLS channel between the client and the resource server in the kid parameter of the Client-to-AS request. The authorization server MUST verify that the specified kid denotes a valid verifier for a proof-of-possession token that has previously been issued to the requesting client. Otherwise, the Client-to-AS request MUST be declined with the error code unsupported_pop_key as defined in Section 5.6.3 of [I-D.ietf-ace-oauth-authz].
When the authorization server issues a new access token to update existing authorization information, it MUST include the specified kid parameter in this access token. A resource server MUST replace the authorization information of any existing DTLS session that is identified by this key identifier with the updated authorization information.
C RS AS | <===== DTLS channel =====> | | | + Access Token | | | | | | --- Token Request ----------------------------> | | | | | <---------------------------- New Access Token - | | + Access Information | | | | | --- Update /authz-info --> | | | New Access Token | | | | | | == Authorized Request ===> | | | | | | <=== Protected Resource == | |
Figure 10: Overview of Dynamic Update Operation
The resource server MUST delete access tokens that are no longer valid. DTLS associations that have been setup in accordance with this profile are always tied to specific tokens (which may be exchanged with a dynamic update as described in Section 4). As tokens may become invalid at any time (e.g., because they have expired), the association may become useless at some point. A resource server therefore MUST terminate existing DTLS association after the last access token associated with this association has expired.
As specified in Section 5.8.3 of [I-D.ietf-ace-oauth-authz], the resource server MUST notify the client with an error response with code 4.01 (Unauthorized) for any long running request before terminating the association.
As specified in the ACE framework (Sections 5.6 and 5.7 of [I-D.ietf-ace-oauth-authz]), the requesting entity (the resource server and/or the client) and the authorization server communicate via the token endpoint or introspection endpoint. The use of CoAP and DTLS for this communication is RECOMMENDED in this profile, other protocols (such as HTTP and TLS, or CoAP and OSCORE [RFC8613]) MAY be used instead.
How credentials (e.g., PSK, RPK, X.509 cert) for using DTLS with the authorization server are established is out of scope for this profile.
If other means of securing the communication with the authorization server are used, the communication security requirements from Section 6.2 of [I-D.ietf-ace-oauth-authz] remain applicable.
This document specifies a profile for the Authentication and Authorization for Constrained Environments (ACE) framework [I-D.ietf-ace-oauth-authz]. As it follows this framework’s general approach, the general security considerations from Section 6 of [I-D.ietf-ace-oauth-authz] also apply to this profile.
The authorization server must ascertain that the keying material for the client that it provides to the resource server actually is associated with this client. Malicious clients may hand over access tokens containing their own access permissions to other entities. This problem cannot be completely eliminated. Nevertheless, in RPK mode it should not be possible for clients to request access tokens for arbitrary public keys: if the client can cause the authorization server to issue a token for a public key without proving possession of the corresponding private key, this allows for identity misbinding attacks where the issued token is usable by an entity other than the intended one. The authorization server therefore at some point needs to validate that the client can actually use the private key corresponding to the client’s public key.
When using pre-shared keys provisioned by the authorization server, the security level depends on the randomness of PSK, and the security of the TLS cipher suite and key exchange algorithm. As this specification targets at constrained environments, message payloads exchanged between the client and the resource server are expected to be small and rare. CoAP [RFC7252] mandates the implementation of cipher suites with abbreviated, 8-byte tags for message integrity protection. For consistency, this profile requires implementation of the same cipher suites. For application scenarios where the cost of full-width authentication tags is low compared to the overall amount of data being transmitted, the use of cipher suites with 16-byte integrity protection tags is preferred.
The PSK mode of this profile offers a distribution mechanism to convey authorization tokens together with a shared secret to a client and a server. As this specification aims at constrained devices and uses CoAP [RFC7252] as transfer protocol, at least the ciphersuite TLS_PSK_WITH_AES_128_CCM_8 [RFC6655] should be supported. The access tokens and the corresponding shared secrets generated by the authorization server are expected to be sufficiently short-lived to provide similar forward-secrecy properties to using ephemeral Diffie-Hellman (DHE) key exchange mechanisms. For longer-lived access tokens, DHE ciphersuites should be used.
Constrained devices that use DTLS [RFC6347] are inherently vulnerable to Denial of Service (DoS) attacks as the handshake protocol requires creation of internal state within the device. This is specifically of concern where an adversary is able to intercept the initial cookie exchange and interject forged messages with a valid cookie to continue with the handshake. A similar issue exists with the unprotected authorization information endpoint where the resource server needs to keep valid access tokens until their expiry. Adversaries can fill up the constrained resource server’s internal storage for a very long time with interjected or otherwise retrieved valid access tokens. The protection of access tokens that are stored in the authorization information endpoint depends on the keying material that is used between the authorization server and the resource server: The resource server must ensure that it processes only access tokens that are encrypted and integrity-protected by an authorization server that is authorized to provide access tokens for the resource server.
To avoid the overhead of a repeated DTLS handshake, [RFC7925] recommends session resumption [RFC5077] to reuse session state from an earlier DTLS association and thus requires client side implementation. In this specification, the DTLS session is subject to the authorization rules denoted by the access token that was used for the initial setup of the DTLS association. Enabling session resumption would require the server to transfer the authorization information with the session state in an encrypted SessionTicket to the client. Assuming that the server uses long-lived keying material, this could open up attacks due to the lack of forward secrecy. Moreover, using this mechanism, a client can resume a DTLS session without proving the possession of the PoP key again. Therefore, the use of session resumption is NOT RECOMMENDED for resource servers.
Since renogiation of DTLS associations is prone to attacks as well, [RFC7925] requires clients to decline any renogiation attempt. A server that wants to initiate re-keying therefore SHOULD periodically force a full handshake.
The use of multiple access tokens for a single client increases the strain on the resource server as it must consider every access token and calculate the actual permissions of the client. Also, tokens may contradict each other which may lead the server to enforce wrong permissions. If one of the access tokens expires earlier than others, the resulting permissions may offer insufficient protection. Developers SHOULD avoid using multiple access tokens for a client.
Even when a single access token per client is used, an attacker could compromise the dynamic update mechanism for existing DTLS connections by delaying or reordering packets destined for the authz-info endpoint. Thus, the order in which operations occur at the resource server (and thus which authorization info is used to process a given client request) cannot be guaranteed. Especially in the presence of later-issued access tokens that reduce the client’s permissions from the initial access token, it is impossible to guarantee that the reduction in authorization will take effect prior to the expiration of the original token.
To communicate securely, the authorization server, the client and the resource server require certain information that must be exchanged outside the protocol flow described in this document. The authorization server must have obtained authorization information concerning the client and the resource server that is approved by the resource owner as well as corresponding keying material. The resource server must have received authorization information approved by the resource owner concerning its authorization managers and the respective keying material. The client must have obtained authorization information concerning the authorization server approved by its owner as well as the corresponding keying material. Also, the client’s owner must have approved of the client’s communication with the resource server. The client and the authorization server must have obtained a common understanding how this resource server is identified to ensure that the client obtains access token and keying material for the correct resource server. If the client is provided with a raw public key for the resource server, it must be ascertained to which resource server (which identifier and authorization information) the key is associated. All authorization information and keying material must be kept up to date.
This privacy considerations from Section 7 of the [I-D.ietf-ace-oauth-authz] apply also to this profile.
An unprotected response to an unauthorized request may disclose information about the resource server and/or its existing relationship with the client. It is advisable to include as little information as possible in an unencrypted response. When a DTLS session between an authenticated client and the resource server already exists, more detailed information MAY be included with an error response to provide the client with sufficient information to react on that particular error.
Also, unprotected requests to the resource server may reveal information about the client, e.g., which resources the client attempts to request or the data that the client wants to provide to the resource server. The client SHOULD NOT send confidential data in an unprotected request.
Note that some information might still leak after DTLS session is established, due to observable message sizes, the source, and the destination addresses.
The following registrations are done for the ACE OAuth Profile Registry following the procedure specified in [I-D.ietf-ace-oauth-authz].
Note to RFC Editor: Please replace all occurrences of “[RFC-XXXX]” with the RFC number of this specification and delete this paragraph.
Profile name: coap_dtls
Profile Description: Profile for delegating client authentication and authorization in a constrained environment by establishing a Datagram Transport Layer Security (DTLS) channel between resource-constrained nodes.
Profile ID: TBD (suggested: 1)
Change Controller: IESG
Reference: [RFC-XXXX]
Thanks to Jim Schaad for his contributions and reviews of this document. Special thanks to Ben Kaduk for his thorough review of this document.
Ludwig Seitz worked on this document as part of the CelticNext projects CyberWI, and CRITISEC with funding from Vinnova.