Internet-Draft | OSCORE Profile of ACE | October 2020 |
Palombini, et al. | Expires 30 April 2021 | [Page] |
This memo specifies a profile for the Authentication and Authorization for Constrained Environments (ACE) framework. It utilizes Object Security for Constrained RESTful Environments (OSCORE) to provide communication security and proof-of-possession for a key owned by the client and bound to an OAuth 2.0 access token.¶
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This memo specifies a profile of the ACE framework [I-D.ietf-ace-oauth-authz]. In this profile, a client and a resource server use the Constrained Application Protocol (CoAP) [RFC7252] to communicate. The client uses an access token, bound to a symmetric key (the proof-of-possession key) to authorize its access to the resource server. Note that this profile uses a symmetric-crypto-based scheme, where the symmetric secret is used as input material for keying material derivation. In order to provide communication security and proof of possession, the client and resource server use Object Security for Constrained RESTful Environments (OSCORE) [RFC8613]. Note that the proof of possession is not done by a dedicated protocol element, but rather occurs after the first OSCORE exchange.¶
OSCORE specifies how to use CBOR Object Signing and Encryption (COSE) [RFC8152] to secure CoAP messages. Note that OSCORE can be used to secure CoAP messages, as well as HTTP and combinations of HTTP and CoAP; a profile of ACE similar to the one described in this document, with the difference of using HTTP instead of CoAP as communication protocol, could be specified analogously to this one.¶
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.¶
Certain security-related terms such as "authentication", "authorization", "confidentiality", "(data) integrity", "message authentication code", and "verify" are taken from [RFC4949].¶
RESTful terminology follows HTTP [RFC7231].¶
Terminology for entities in the architecture is defined in OAuth 2.0 [RFC6749], such as client (C), resource server (RS), and authorization server (AS). It is assumed in this document that a given resource on a specific RS is associated to a unique AS.¶
Concise Binary Object Representation (CBOR) [I-D.ietf-cbor-7049bis] and Concise Data Definition Language (CDDL) [RFC8610] are used in this specification. CDDL predefined type names, especially bstr for CBOR byte strings and tstr for CBOR text strings, are used extensively in the document.¶
Note that the term "endpoint" is used here, as in [I-D.ietf-ace-oauth-authz], following its OAuth definition, which is to denote resources such as token and introspect at the AS and authz-info at the RS. The CoAP [RFC7252] definition, which is "An entity participating in the CoAP protocol" is not used in this memo.¶
This section gives an overview of how to use the ACE Framework [I-D.ietf-ace-oauth-authz] to secure the communication between a client and a resource server using OSCORE [RFC8613]. The parameters needed by the client to negotiate the use of this profile with the authorization server, as well as the OSCORE setup process, are described in detail in the following sections.¶
The RS maintains a collection of OSCORE Security Contexts with associated authorization information for all the clients that it is communicating with. The authorization information is maintained as policy that is used as input to processing requests from those clients.¶
This profile requires a client to retrieve an access token from the AS for the resource it wants to access on an RS, by sending an access token request to the token endpoint, as specified in section 5.6 of [I-D.ietf-ace-oauth-authz]. The access token request and response MUST be confidentiality-protected and ensure authenticity. This profile RECOMMENDS the use of OSCORE between client and AS, but other protocols (such as TLS or DTLS) can be used as well.¶
Once the client has retrieved the access token, it generates a nonce N1. The client also generates its OSCORE Recipient ID (see Section 3.1 of [RFC8613]), ID1, for use with the keying material associated to the RS. The client posts the token, N1 and its Recipient ID to the RS using the authz-info endpoint and mechanisms specified in section 5.8 of [I-D.ietf-ace-oauth-authz] and Content-Format = application/ace+cbor. When using this profile, the communication with the authz-info endpoint is not protected, except for update of access rights.¶
If the access token is valid, the RS replies to this request with a 2.01 (Created) response with Content-Format = application/ace+cbor, which contains a nonce N2 and its newly generated OSCORE Recipient ID, ID2, for use with the keying material associated to the client. Moreover, the server concatenates the input salt received in the token, N1, and N2 to obtain the Master Salt of the OSCORE Security Context (see section 3 of [RFC8613]). The RS then derives the complete Security Context associated with the received token from the Master Salt, the OSCORE Recipient ID generated by the client (set as its OSCORE Sender ID), its own OSCORE Recipient ID, plus the parameters received in the access token from the AS, following section 3.2 of [RFC8613].¶
In a similar way, after receiving the nonce N2, the client concatenates the input salt, N1 and N2 to obtain the Master Salt of the OSCORE Security Context. The client then derives the complete Security Context from the Master Salt, the OSCORE Recipient ID generated by the RS (set as its OSCORE Sender ID), its own OSCORE Recipient ID, plus the parameters received from the AS.¶
Finally, the client sends a request protected with OSCORE to the RS. If the request verifies, the server stores the complete Security Context state that is ready for use in protecting messages, and uses it in the response, and in further communications with the client, until token expiration. This Security Context is discarded when a token (whether the same or different) is used to successfully derive a new Security Context for that client.¶
The use of random nonces during the exchange prevents the reuse of an Authenticated Encryption with Associated Data (AEAD) nonces/key pair for two different messages. Two-time pad might otherwise occur when client and RS derive a new Security Context from an existing (non-expired) access token, as might occur when either party has just rebooted. Instead, by using random nonces as part of the Master Salt, the request to the authz-info endpoint posting the same token results in a different Security Context, by OSCORE construction, since even though the Master Secret, Sender ID and Recipient ID are the same, the Master Salt is different (see Section 3.2.1 of [RFC8613]). Therefore, the main requirement for the nonces is that they have a good amount of randomness. If random nonces were not used, a node re-using a non-expired old token would be susceptible to on-path attackers provoking the creation of OSCORE messages using old AEAD keys and nonces.¶
After the whole message exchange has taken place, the client can contact the AS to request an update of its access rights, sending a similar request to the token endpoint that also includes an identifier so that the AS can find the correct OSCORE security material it has previously shared with the client. This specific identifier, encoded as a byte string, is assigned by the AS to be unique in the sets of its OSCORE Security Contexts, and is not used as input material to derive the full OSCORE Security Context.¶
An overview of the profile flow for the OSCORE profile is given in Figure 1. The names of messages coincide with those of [I-D.ietf-ace-oauth-authz] when applicable.¶
C RS AS | | | | ----- POST /token ----------------------------> | | | | | <---------------------------- Access Token ----- | | + Access Information | | ---- POST /authz-info ---> | | | (access_token, N1, ID1) | | | | | | <- 2.01 Created (N2, ID2)- | | | | | /Sec Context /Sec Context | derivation/ derivation/ | | | | | ---- OSCORE Request -----> | | | | | | /proof-of-possession | | Sec Context storage/ | | | | | <--- OSCORE Response ----- | | | | | /proof-of-possession | | Sec Context storage/ | | | | | | ---- OSCORE Request -----> | | | | | | <--- OSCORE Response ----- | | | ... | |
The following subsections describe the details of the POST request and response to the token endpoint between client and AS. Section 3.2 of [RFC8613] defines how to derive a Security Context based on a shared master secret and a set of other parameters, established between client and server, which the client receives from the AS in this exchange. The proof-of-possession key (pop-key) included in the response from the AS MUST be used as master secret in OSCORE.¶
The client-to-AS request is specified in Section 5.6.1 of [I-D.ietf-ace-oauth-authz].¶
The client must send this POST request to the token endpoint over a secure channel that guarantees authentication, message integrity and confidentiality (see Section 5).¶
An example of such a request, with payload in CBOR diagnostic notation without the tag and value abbreviations is reported in Figure 2¶
Header: POST (Code=0.02) Uri-Host: "as.example.com" Uri-Path: "token" Content-Format: "application/ace+cbor" Payload: { "req_aud" : "tempSensor4711", "scope" : "read" }
If the client wants to update its access rights without changing an existing OSCORE Security Context, it MUST include in its POST request to the token endpoint a req_cnf object. kid field carrying a CBOR byte string containing the OSCORE_Input_Material Identifier (assigned as discussed in Section 3.2). This identifier, together with other information such as audience (see Section 5.6.1 of [I-D.ietf-ace-oauth-authz]), can be used by the AS to determine the shared secret bound to the proof-of-possession token and therefore MUST identify a symmetric key that was previously generated by the AS as a shared secret for the communication between the client and the RS. The AS MUST verify that the received value identifies a proof-of-possession key that has previously been issued to the requesting client. If that is not the case, the Client-to-AS request MUST be declined with the error code 'invalid_request' as defined in Section 5.6.3 of [I-D.ietf-ace-oauth-authz].¶
An example of such a request, with payload in CBOR diagnostic notation without the tag and value abbreviations is reported in Figure 3¶
Header: POST (Code=0.02) Uri-Host: "as.example.com" Uri-Path: "token" Content-Format: "application/ace+cbor" Payload: { "req_aud" : "tempSensor4711", "scope" : "write", "req_cnf" : { "kid" : h'01' }
After verifying the POST request to the token endpoint and that the client is authorized to obtain an access token corresponding to its access token request, the AS responds as defined in section 5.6.2 of [I-D.ietf-ace-oauth-authz]. If the client request was invalid, or not authorized, the AS returns an error response as described in section 5.6.3 of [I-D.ietf-ace-oauth-authz].¶
The AS can signal that the use of OSCORE is REQUIRED for a specific access token by including the "profile" parameter with the value "coap_oscore" in the access token response. This means that the client MUST use OSCORE towards all resource servers for which this access token is valid, and follow Section 4.3 to derive the security context to run OSCORE. Usually it is assumed that constrained devices will be pre-configured with the necessary profile, so that this kind of profile negotiation can be omitted.¶
Moreover, the AS MUST send the following data:¶
Additionally, the AS MAY send the following data, in the same response.¶
This data is transported in the the OSCORE_Input_Material. The OSCORE_Input_Material is a CBOR map object, defined in Section 3.2.1. This object is transported in the 'cnf' parameter of the access token response as defined in Section 3.2 of [I-D.ietf-ace-oauth-params], as the value of a field named 'osc', registered in Section 9.5 and Section 9.6.¶
The AS MAY assign an identifier to the context (context identifier). This identifier is used as ID Context in the OSCORE context as described in section 3.1 of [RFC8613]. If assigned, this parameters MUST be communicated as the 'contextId' field in the OSCORE_Input_Material. The applications needs to consider that this identifier is sent in the clear and may reveal information about the endpoints, as mentioned in section 12.8 of [RFC8613].¶
The master secret and the identifier of the OSCORE_Input_Material MUST be communicated as the 'ms' and 'id' field in the 'osc' field in the 'cnf' parameter of the access token response. If included, the AEAD algorithm is sent in the 'alg' parameter in the OSCORE_Input_Material; the HKDF algorithm in the 'hkdf' parameter of the OSCORE_Input_Material; a salt in the 'salt' parameter of the OSCORE_Input_Material; and the OSCORE version in the 'version' parameter of the OSCORE_Input_Material.¶
The same parameters MUST be included in the claims associated with the access token. This profile RECOMMENDS the use of CBOR web token (CWT) as specified in [RFC8392]. If the token is a CWT, the same OSCORE_Input_Material structure defined above MUST be placed in the 'osc' field of the 'cnf' claim of this token.¶
The AS MUST send different OSCORE_Input_Material (and therefore different access tokens) to different authorized clients, in order for the RS to differentiate between clients.¶
Figure 4 shows an example of an AS response, with payload in CBOR diagnostic notation without the tag and value abbreviations. The access token has been truncated for readability.¶
Header: Created (Code=2.01) Content-Type: "application/ace+cbor" Payload: { "access_token" : h'8343a1010aa2044c53 ... (remainder of access token (CWT) omitted for brevity)', "profile" : "coap_oscore", "expires_in" : "3600", "cnf" : { "osc" : { "id" : h'01', "ms" : h'f9af838368e353e78888e1426bd94e6f' } } }
Figure 5 shows an example CWT Claims Set, including the relevant OSCORE parameters in the 'cnf' claim, in CBOR diagnostic notation without tag and value abbreviations.¶
{ "aud" : "tempSensorInLivingRoom", "iat" : "1360189224", "exp" : "1360289224", "scope" : "temperature_g firmware_p", "cnf" : { "osc" : { "ms" : h'f9af838368e353e78888e1426bd94e6f', "id" : h'01' } } }
The same CWT Claims Set as in Figure 5, using the value abbreviations defined in [I-D.ietf-ace-oauth-authz] and [RFC8747] and encoded in CBOR is shown in Figure 6. The bytes in hexadecimal are reported in the first column, while their corresponding CBOR meaning is reported after the '#' sign on the second column, for easiness of readability.¶
NOTE TO THE RFC EDITOR: before publishing, it should be checked (and in case fixed) that the values used below (which are not yet registered) are the final values registered in IANA.¶
A5 # map(5) 63 # text(3) 617564 # "aud" 76 # text(22) 74656D7053656E736F72496E4C6976696E67526F6F6D # "tempSensorInLivingRoom" 63 # text(3) 696174 # "iat" 6A # text(10) 31333630313839323234 # "1360189224" 63 # text(3) 657870 # "exp" 6A # text(10) 31333630323839323234 # "1360289224" 65 # text(5) 73636F7065 # "scope" 78 18 # text(24) 74656D70657261747572655F67206669726D776172655F70 # "temperature_g firmware_p" 63 # text(3) 636E66 # "cnf" A1 # map(1) 63 # text(3) 6F7363 # "osc" A2 # map(2) 62 # text(2) 6D73 # "ms" 50 # bytes(16) F9AF838368E353E78888E1426BD94E6F # "\xF9\xAF\x83\x83h\xE3S\xE7 \x88\x88\xE1Bk\xD9No" 62 # text(2) 6964 # "id" 41 # bytes(1) 01 # "\x01"
If the client has requested an update to its access rights using the same OSCORE Security Context, which is valid and authorized, the AS MUST omit the 'cnf' parameter in the response, and MUST carry the OSCORE Input Material identifier in the 'kid' field in the 'cnf' parameter of the token. This identifier needs to be included in the token in order for the RS to identify the correct OSCORE Input Material.¶
Figure 7 shows an example of such an AS response, with payload in CBOR diagnostic notation without the tag and value abbreviations. The access token has been truncated for readability.¶
Header: Created (Code=2.01) Content-Type: "application/ace+cbor" Payload: { "access_token" : h'8343a1010aa2044c53 ... (remainder of access token (CWT) omitted for brevity)', "profile" : "coap_oscore", "expires_in" : "3600" }
Figure 8 shows an example CWT Claims Set, containing the necessary OSCORE parameters in the 'cnf' claim for update of access rights, in CBOR diagnostic notation without tag and value abbreviations.¶
{ "aud" : "tempSensorInLivingRoom", "iat" : "1360189224", "exp" : "1360289224", "scope" : "temperature_h", "cnf" : { "kid" : h'01' } }
An OSCORE_Input_Material is an object that represents the input material to derive an OSCORE Security Context, i.e., the local set of information elements necessary to carry out the cryptographic operations in OSCORE (Section 3.1 of [RFC8613]). In particular, the OSCORE_Input_Material is defined to be serialized and transported between nodes, as specified by this document, but can also be used by other specifications if needed. The OSCORE_Input_Material can either be encoded as a JSON object or as a CBOR map. The set of common parameters that can appear in an OSCORE_Input_Material can be found in the IANA "OSCORE Security Context Parameters" registry (Section 9.4), defined for extensibility, and is specified below. All parameters are optional. Table 1 provides a summary of the OSCORE_Input_Material parameters defined in this section.¶
name | CBOR label | CBOR type | registry | description |
---|---|---|---|---|
version | 0 | unsigned integer | OSCORE Version | |
ms | 1 | byte string | OSCORE Master Secret value | |
id | 2 | byte string | OSCORE Input Material Identifier | |
hkdf | 3 | text string / integer | [COSE.Algorithms] Values (HMAC-based) | OSCORE HKDF value |
alg | 4 | text string / integer | [COSE.Algorithms] Values (AEAD) | OSCORE AEAD Algorithm value |
salt | 5 | byte string | an input to OSCORE Master Salt value | |
contextId | 6 | byte string | OSCORE ID Context value |
An example of JSON OSCORE_Input_Material is given in Figure 9.¶
"osc" : { "alg" : "AES-CCM-16-64-128", "id" : b64'AQ==' "ms" : b64'+a+Dg2jjU+eIiOFCa9lObw' }
The CDDL grammar describing the CBOR OSCORE_Input_Material is:¶
OSCORE_Input_Material = { ? 0 => int, ; version ? 1 => bstr, ; ms ? 2 => bstr, ; id ? 3 => tstr / int, ; hkdf ? 4 => tstr / int, ; alg ? 5 => bstr, ; salt ? 6 => bstr, ; contextId * int / tstr => any }¶
The following subsections describe the details of the POST request and response to the authz-info endpoint between client and RS. The client generates a nonce N1 and an identifier ID1 unique in the sets of its own Recipient IDs, and posts them together with the token that includes the materials (e.g., OSCORE parameters) received from the AS to the RS. The RS then generates a nonce N2 and an identifier ID2 unique in the sets of its own Recipient IDs, and uses Section 3.2 of [RFC8613] to derive a security context based on a shared master secret, the two nonces and the two identifiers, established between client and server. The nonces and identifiers are encoded as CBOR byte string if CBOR is used, and as Base64 string if JSON is used. This security context is used to protect all future communication between client and RS using OSCORE, as long as the access token is valid.¶
Note that the RS and client authenticates themselves by generating the shared OSCORE Security Context using the pop-key as master secret. An attacker posting a valid token to the RS will not be able to generate a valid OSCORE context and thus not be able to prove possession of the pop-key. Additionally, the mutual authentication is only achieved after the client has successfully verified the response from the RS.¶
The client MUST generate a nonce value very unlikely to have been previously used with the same input keying material. This profile RECOMMENDS to use a 64-bit long random number as nonce's value. The client MUST store the nonce N1 as long as the response from the RS is not received and the access token related to it is still valid.¶
The client generates its own Recipient ID, ID1, for the OSCORE Security Context that it is establishing with the RS. By generating its own Recipient ID, the client makes sure that it does not collide with any of its Recipient IDs.¶
The client MUST use CoAP and the Authorization Information resource as described in section 5.8.1 of [I-D.ietf-ace-oauth-authz] to transport the token, N1 and ID1 to the RS.¶
Note that the use of the payload and the Content-Format is different from what is described in section 5.8.1 of [I-D.ietf-ace-oauth-authz], which only transports the token without any CBOR wrapping. In this profile, the client MUST wrap the token and N1 in a CBOR map. The client MUST use the Content-Format "application/ace+cbor" defined in section 8.14 of [I-D.ietf-ace-oauth-authz]. The client MUST include the access token using the "access_token" parameter, N1 using the 'nonce1' parameter defined in Section 4.1.1, and ID1 using the 'ace_client_recipientid' parameter defined in Section 4.1.2.¶
The communication with the authz-info endpoint does not have to be protected, except for the update of access rights case described below.¶
Note that a client may be required to re-POST the access token in order to complete a request, since an RS may delete a stored access token (and associated Security Context) at any time, for example due to all storage space being consumed. This situation is detected by the client when it receives an AS Request Creation Hints response. Reposting the same access token will result in deriving a new OSCORE Security Context to be used with the RS, as different nonces will be used.¶
Figure 10 shows an example of the request sent from the client to the RS, with payload in CBOR diagnostic notation without the tag and value abbreviations. The access token has been truncated for readability.¶
Header: POST (Code=0.02) Uri-Host: "rs.example.com" Uri-Path: "authz-info" Content-Format: "application/ace+cbor" Payload: { "access_token": h'8343a1010aa2044c53 ... (remainder of access token (CWT) omitted for brevity)', "nonce1": h'018a278f7faab55a', "ace_client_recipientid" : h'1645' }
If the client has already posted a valid token, has already established a security association with the RS, and wants to update its access rights, the client can do so by posting the new token (retrieved from the AS and containing the update of access rights) to the /authz-info endpoint. The client MUST protect the request using the OSCORE Security Context established during the first token exchange. The client MUST only send the access token in the payload, no nonce or identifier are sent. After proper verification (see Section 4.2), the RS will replace the old token with the new one, maintaining the same Security Context.¶
This parameter MUST be sent from the client to the RS, together with the access token, if the ace profile used is coap_oscore. The parameter is encoded as a byte string for CBOR-based interactions, and as a string (Base64 encoded binary) for JSON-based interactions. This parameter is registered in Section 9.2.¶
This parameter MUST be sent from the client to the RS, together with the access token, if the ace profile used is coap_oscore. The parameter is encoded as a byte string for CBOR-based interactions, and as a string (Base64 encoded binary) for JSON-based interactions. This parameter is registered in Section 9.2.¶
The RS MUST follow the procedures defined in section 5.8.1 of [I-D.ietf-ace-oauth-authz]: the RS must verify the validity of the token. If the token is valid, the RS must respond to the POST request with 2.01 (Created). If the token is valid but is associated to claims that the RS cannot process (e.g., an unknown scope), or if any of the expected parameters is missing (e.g., any of the mandatory parameters from the AS or the identifier), or if any parameters received in the 'osc' is unrecognized, the RS must respond with an error response code equivalent to the CoAP code 4.00 (Bad Request). In the latter two cases, the RS may provide additional information in the error response, in order to clarify what went wrong. The RS may make an introspection request (see Section 5.7.1 of [I-D.ietf-ace-oauth-authz]) to validate the token before responding to the POST request to the authz-info endpoint.¶
Additionally, the RS MUST generate a nonce N2 very unlikely to have been previously used with the same input keying material, and its own Recipient ID, ID2. The RS makes sure that ID2 does not collide with any of its Recipient IDs. The RS MUST ensure that ID2 is different from the ace_client_recipientid. The RS sends N2 and ID2 within the 2.01 (Created) response. The payload of the 2.01 (Created) response MUST be a CBOR map containing the 'nonce2' parameter defined in Section 4.2.1, set to N2, and the 'ace_server_recipientid' parameter defined in Section 4.2.2, set to ID2. This profile RECOMMENDS to use a 64-bit long random number as nonce's value. The RS MUST use the Content-Format "application/ace+cbor" defined in section 8.14 of [I-D.ietf-ace-oauth-authz].¶
Figure 11 shows an example of the response sent from the RS to the client, with payload in CBOR diagnostic notation without the tag and value abbreviations.¶
Header: Created (Code=2.01) Content-Format: "application/ace+cbor" Payload: { "nonce2": h'25a8991cd700ac01', "ace_server_recipientid" : h'0000' }
As specified in section 5.8.3 of [I-D.ietf-ace-oauth-authz], the RS must notify the client with an error response with code 4.01 (Unauthorized) for any long running request before terminating the session, when the access token expires.¶
If the RS receives the token in a OSCORE protected message, it means that the client is requesting an update of access rights. The RS MUST discard any nonce and identifiers in the request, if any was sent. The RS MUST check that the "kid" of the "cnf" parameter of the new access token matches the OSCORE Input Material of the context used to protect the message. If that is the case, the RS MUST discard the old token and associate the new token to the Security Context identified by the "kid" value in the "cnf" parameter. The RS MUST respond with a 2.01 (Created) response protected with the same Security Context, with no payload. If any verification fails, the RS MUST respond with a 4.01 (Unauthorized) error response.¶
As specified in section 5.8.1 of [I-D.ietf-ace-oauth-authz], when receiving an updated access token with updated authorization information from the client (see Section 3.1), it is recommended that the RS overwrites the previous token, that is only the latest authorization information in the token received by the RS is valid. This simplifies the process needed by the RS to keep track of authorization information for a given client.¶
This parameter MUST be sent from the RS to the client if the ace profile used is coap_oscore. The parameter is encoded as a byte string for CBOR-based interactions, and as a string (Base64 encoded binary) for JSON-based interactions. This parameter is registered in Section 9.2¶
This parameter MUST be sent from the RS to the client if the ace profile used is coap_oscore. The parameter is encoded as a byte string for CBOR-based interactions, and as a string (Base64 encoded binary) for JSON-based interactions. This parameter is registered in Section 9.2¶
Once receiving the 2.01 (Created) response from the RS, following the POST request to authz-info endpoint, the client MUST extract the bstr nonce N2 from the 'nonce2' parameter in the CBOR map in the payload of the response. Then, the client MUST set the Master Salt of the Security Context created to communicate with the RS to the concatenation of salt, N1, and N2, in this order: Master Salt = salt | N1 | N2, where | denotes byte string concatenation, where salt is the CBOR byte string received from the AS in Section 3.2, and where N1 and N2 are the two nonces encoded as CBOR byte strings. An example of Master Salt construction using CBOR encoding is given in Figure 12.¶
N1, N2 and input salt expressed in CBOR diagnostic notation: nonce1 = h'018a278f7faab55a' nonce2 = h'25a8991cd700ac01' input salt = h'f9af838368e353e78888e1426bd94e6f' N1, N2 and input salt as CBOR encoded byte strings: nonce1 = 0x48018a278f7faab55a nonce2 = 0x4825a8991cd700ac01 input salt = 0x50f9af838368e353e78888e1426bd94e6f Master Salt = 0x50 f9af838368e353e78888e1426bd94e6f 48 018a278f7faab55a 48 25a8991cd700ac01
If JSON is used instead of CBOR, the Master Salt of the Security Context is the Base64 encoding of the concatenation of the same parameters, each of them prefixed by their size, encoded in 1 byte. When using JSON, the nonces and input salt have a maximum size of 255 bytes. An example of Master Salt construction using Base64 encoding is given in Figure 13.¶
N1, N2 and input salt values: nonce1 = 0x018a278f7faab55a (8 bytes) nonce2 = 0x25a8991cd700ac01 (8 bytes) input salt = 0xf9af838368e353e78888e1426bd94e6f (16 bytes) Input to Base64 encoding: 0x10 f9af838368e353e78888e1426bd94e6f 08 018a278f7faab55a 08 25a8991cd700ac01 Master Salt = b64'EPmvg4No41PniIjhQmvZTm8IAYonj3+qtVoIJaiZHNcArAE='
The client MUST set the Sender ID to the ace_server_recipientid received in Section 4.2, and the Recipient ID to the ace_client_recipientid sent in Section 4.1. The client MUST set the Master Secret from the parameter received from the AS in Section 3.2. The client MUST set the AEAD Algorithm, ID Context, HKDF, and OSCORE Version from the parameters received from the AS in Section 3.2, if present. In case an optional parameter is omitted, the default value SHALL be used as described in sections 3.2 and 5.4 of [RFC8613]. After that, the client MUST derive the complete Security Context following section 3.2.1 of [RFC8613]. From this point on, the client MUST use this Security Context to communicate with the RS when accessing the resources as specified by the authorization information.¶
If any of the expected parameters is missing (e.g., any of the mandatory parameters from the AS or the RS), or if ace_client_recipientid equals ace_server_recipientid, then the client MUST stop the exchange, and MUST NOT derive the Security Context. The client MAY restart the exchange, to get the correct security material.¶
The client then uses this Security Context to send requests to RS using OSCORE.¶
After sending the 2.01 (Created) response, the RS MUST set the Master Salt of the Security Context created to communicate with the client to the concatenation of salt, N1, and N2, in the same way described above. An example of Master Salt construction using CBOR encoding is given in Figure 12 and using Base64 encoding is given in Figure 13. The RS MUST set the Sender ID from the ace_client_recipientid received in Section 4.1, and the Recipient ID from the ace_server_recipientid sent in Section 4.2. The RS MUST set the Master Secret from the parameter, received from the AS and forwarded by the client in the access token in Section 4.1 after validation of the token as specified in Section 4.2. The RS MUST set the AEAD Algorithm, ID Context, HKDF, and OSCORE Version from the parameters received from the AS and forwarded by the client in the access token in Section 4.1 after validation of the token as specified in Section 4.2, if present. In case an optional parameter is omitted, the default value SHALL be used as described in sections 3.2 and 5.4 of [RFC8613]. After that, the RS MUST derive the complete Security Context following section 3.2.1 of [RFC8613], and MUST associate this Security Context with the authorization information from the access token.¶
The RS then uses this Security Context to verify requests and send responses to C using OSCORE. If OSCORE verification fails, error responses are used, as specified in section 8 of [RFC8613]. Additionally, if OSCORE verification succeeds, the verification of access rights is performed as described in section Section 4.4. The RS MUST NOT use the Security Context after the related token has expired, and MUST respond with a unprotected 4.01 (Unauthorized) error message to requests received that correspond to a Security Context with an expired token.¶
Note that the ID Context can be assigned by the AS, communicated and set in both the RS and client after the exchange specified in this profile is executed. Subsequently, client and RS can update their ID Context by running a mechanism such as the one defined in Appendix B.2 of [RFC8613] if they both support it and are configured to do so. In that case, the ID Context in the OSCORE Security Context will not match the "contextId" parameter of the corresponding OSCORE_Input_Material. Running Appendix B.2 results in the keying material in the Security Contexts of client and RS being updated; this same result can also be achieved by the client reposting the access token as described in Section 4.1, but without updating the ID Context.¶
The RS MUST follow the procedures defined in section 5.8.2 of [I-D.ietf-ace-oauth-authz]: if an RS receives an OSCORE-protected request from a client, then the RS processes it according to [RFC8613]. If OSCORE verification succeeds, and the target resource requires authorization, the RS retrieves the authorization information using the access token associated to the Security Context. The RS then must verify that the authorization information covers the resource and the action requested.¶
As specified in the ACE framework (section 5.7 of [I-D.ietf-ace-oauth-authz]), the requesting entity (RS and/or client) and the AS communicates via the introspection or token endpoint. The use of CoAP and OSCORE ([RFC8613]) for this communication is RECOMMENDED in this profile, other protocols (such as HTTP and DTLS or TLS) MAY be used instead.¶
If OSCORE is used, the requesting entity and the AS are expected to have pre-established security contexts in place. How these security contexts are established is out of scope for this profile. Furthermore the requesting entity and the AS communicate through the introspection endpoint as specified in section 5.7 of [I-D.ietf-ace-oauth-authz] and through the token endpoint as specified in section 5.6 of [I-D.ietf-ace-oauth-authz].¶
There are a number of scenarios where a client or RS needs to discard the OSCORE security context, and acquire a new one.¶
The client MUST discard the current Security Context associated with an RS when:¶
The RS MUST discard the current Security Context associated with a client when:¶
Whenever one more access token is successfully posted to the RS, and a new Security Context is derived between the client and RS, messages in transit that were protected with the previous Security Context might not pass verification, as the old context is discarded. That means that messages sent shortly before the client posts one more access token to the RS might not successfully reach the destination. Analogously, implementations may want to cancel CoAP observations at the RS registered before the Security Context is replaced, or conversely they will need to implement a mechanism to ensure that those observation are to be protected with the newly derived Security Context.¶
This document specifies a profile for the Authentication and Authorization for Constrained Environments (ACE) framework [I-D.ietf-ace-oauth-authz]. Thus the general security considerations from the framework also apply to this profile.¶
Furthermore the general security considerations of OSCORE [RFC8613] also apply to this specific use of the OSCORE protocol.¶
As previously stated, the proof-of-possession in this profile is performed by both parties verifying that they have established the same Security Context, as specified in Section 4.3, which means that both the OSCORE request and OSCORE response pass verification. RS authentication requires both that the client trusts the AS and that the OSCORE response from the RS pass verification.¶
OSCORE is designed to secure point-to-point communication, providing a secure binding between the request and the response(s). Thus the basic OSCORE protocol is not intended for use in point-to-multipoint communication (e.g., multicast, publish-subscribe). Implementers of this profile should make sure that their usecase corresponds to the expected use of OSCORE, to prevent weakening the security assurances provided by OSCORE.¶
Since the use of nonces in the exchange guarantees uniqueness of AEAD keys and nonces, it is REQUIRED that nonces are not reused with the same input keying material even in case of re-boots. This document RECOMMENDS the use of 64 bit random nonces. Considering the birthday paradox, the average collision for each nonce will happen after 2^32 messages, which is considerably more token provisionings than expected for intended applications. If applications use something else, such as a counter, they need to guarantee that reboot and loss of state on either node does not provoke re-use. If that is not guaranteed, nodes are susceptible to re-use of AEAD (nonces, keys) pairs, especially since an on-path attacker can cause the client to use an arbitrary nonce for Security Context establishment by replaying client-to-server messages.¶
This profile recommends that the RS maintains a single access token for a client. 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 indicating different or disjoint permissions from each other 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.¶
If a single OSCORE_Input_Material is used with multiple RSs, the RSs can impersonate C to one of the other RS, and impersonate another RS to the client. If a master secret is used with several clients, the Cs can impersonate RS to one of the other C. Similarly if symmetric keys are used to integrity protect the token between AS and RS and the token can be used with multiple RSs, the RSs can impersonate AS to one of the other RS. If the token key is used for any other communication between the RSs and AS, the RSs can impersonate each other to the AS.¶
This document specifies a profile for the Authentication and Authorization for Constrained Environments (ACE) framework [I-D.ietf-ace-oauth-authz]. Thus the general privacy considerations from the framework also apply to this profile.¶
As this document uses OSCORE, thus the privacy considerations from [RFC8613] apply here as well.¶
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 an OSCORE Security Context already exists between the client and the resource server, more detailed information may be included.¶
The token is sent in the clear to the authz-info endpoint, so if a client uses the same single token from multiple locations with multiple Resource Servers, it can risk being tracked by the token's value even when the access token is encrypted.¶
The nonces exchanged in the request and response to the authz-info endpoint are also sent in the clear, so using random nonces is best for privacy (as opposed to, e.g., a counter, that might leak some information about the client).¶
The identifiers used in OSCORE, negotiated between client and RS are privacy sensitive (see Section 12.8 of [RFC8613]), and could reveal information about the client, or may be used for correlating requests from one client.¶
Note that some information might still leak after OSCORE is established, due to observable message sizes, the source, and the destination addresses.¶
Note to RFC Editor: Please replace all occurrences of "[[this specification]]" with the RFC number of this specification and delete this paragraph.¶
The following registration is done for the ACE Profile Registry following the procedure specified in section 8.8 of [I-D.ietf-ace-oauth-authz]:¶
The following registrations are done for the OAuth Parameters Registry following the procedure specified in section 11.2 of [RFC6749]:¶
The following registrations are done for the OAuth Parameters CBOR Mappings Registry following the procedure specified in section 8.10 of [I-D.ietf-ace-oauth-authz]:¶
It is requested that IANA create a new registry entitled "OSCORE Security Context Parameters" registry. The registry is to be created as Expert Review Required. Guidelines for the experts is provided Section 9.7. It should be noted that in addition to the expert review, some portions of the registry require a specification, potentially on standards track, be supplied as well.¶
The columns of the registry are:¶
This registry will be initially populated by the values in Table 1. The specification column for all of these entries will be this document and [RFC8613].¶
The following registration is done for the CWT Confirmation Methods Registry following the procedure specified in section 7.2.1 of [RFC8747]:¶
The following registration is done for the JWT Confirmation Methods Registry following the procedure specified in section 6.2.1 of [RFC7800]:¶
The IANA registry established in this document is defined to use the Expert Review registration policy. This section gives some general guidelines for what the experts should be looking for, but they are being designated as experts for a reason so they should be given substantial latitude.¶
Expert reviewers should take into consideration the following points:¶
This section lists the specifications on this profile based on the requirements on the framework, as requested in Appendix C of [I-D.ietf-ace-oauth-authz].¶
The authors wish to thank Jim Schaad and Marco Tiloca for the input on this memo. Special thanks to the responsible area director Benjamin Kaduk for his extensive review and contributed text. Ludwig Seitz worked on this document as part of the CelticNext projects CyberWI, and CRITISEC with funding from Vinnova.¶