--- 1/draft-ietf-ace-dtls-authorize-06.txt 2019-03-11 14:15:33.430906243 -0700 +++ 2/draft-ietf-ace-dtls-authorize-07.txt 2019-03-11 14:15:33.478907422 -0700 @@ -1,24 +1,24 @@ ACE Working Group S. Gerdes Internet-Draft O. Bergmann Intended status: Standards Track C. Bormann -Expires: September 1, 2019 Universitaet Bremen TZI +Expires: September 12, 2019 Universitaet Bremen TZI G. Selander Ericsson AB L. Seitz RISE SICS - February 28, 2019 + March 11, 2019 Datagram Transport Layer Security (DTLS) Profile for Authentication and Authorization for Constrained Environments (ACE) - draft-ietf-ace-dtls-authorize-06 + draft-ietf-ace-dtls-authorize-07 Abstract This specification defines a profile of the ACE framework that allows constrained servers to delegate client authentication and authorization. The protocol relies on DTLS 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 @@ -32,21 +32,21 @@ Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." - This Internet-Draft will expire on September 1, 2019. + This Internet-Draft will expire on September 12, 2019. Copyright Notice Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents @@ -59,31 +59,31 @@ Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 3 3. Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1. Communication between C and AS . . . . . . . . . . . . . 5 3.2. RawPublicKey Mode . . . . . . . . . . . . . . . . . . . . 6 3.2.1. DTLS Channel Setup Between C and RS . . . . . . . . . 7 3.3. PreSharedKey Mode . . . . . . . . . . . . . . . . . . . . 8 - 3.3.1. DTLS Channel Setup Between C and RS . . . . . . . . . 11 - 3.4. Resource Access . . . . . . . . . . . . . . . . . . . . . 12 - 4. Dynamic Update of Authorization Information . . . . . . . . . 13 - 5. Token Expiration . . . . . . . . . . . . . . . . . . . . . . 14 - 6. Security Considerations . . . . . . . . . . . . . . . . . . . 15 - 7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 15 - 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 - 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 16 - 9.1. Normative References . . . . . . . . . . . . . . . . . . 16 - 9.2. Informative References . . . . . . . . . . . . . . . . . 17 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 + 3.3.1. DTLS Channel Setup Between C and RS . . . . . . . . . 12 + 3.4. Resource Access . . . . . . . . . . . . . . . . . . . . . 13 + 4. Dynamic Update of Authorization Information . . . . . . . . . 14 + 5. Token Expiration . . . . . . . . . . . . . . . . . . . . . . 16 + 6. Security Considerations . . . . . . . . . . . . . . . . . . . 16 + 7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 17 + 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 + 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 + 9.1. Normative References . . . . . . . . . . . . . . . . . . 18 + 9.2. Informative References . . . . . . . . . . . . . . . . . 19 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20 1. Introduction 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 [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 @@ -113,75 +113,78 @@ 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 authz-info resource refers to the authz-info endpoint as - specified in [I-D.ietf-ace-oauth-authz]. + The authorization information (authz-info) resource refers to the + authorization information endpoint as specified in + [I-D.ietf-ace-oauth-authz]. 2. Protocol Overview 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 C can use CoAP over DTLS to retrieve an access token from the authorization server (AS) for a protected resource hosted on the resource server. This profile requires the client to retrieve an access token for protected resource(s) it wants to access on RS 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 --->] | | + | [---- Resource Request ------>]| | | | | - | [<----- AS Information --] | | + | [<-AS Request Creation Hints-] | | | | | - | --- Token Request ----------------------------> | + | ------- Token Request ----------------------------> | | | | - | <---------------------------- Access Token ----- | + | <---------------------------- Access Token --------- | | + Access Information | Figure 1: Retrieving an Access Token To determine the AS in charge of a resource hosted at the RS, C MAY send an initial Unauthorized Resource Request message to the RS. The - RS then denies the request and sends an AS information message - containing the address of its AS back to the client as specified in - Section 5.1.2 of [I-D.ietf-ace-oauth-authz]. + RS then denies the request and sends an AS Request Creation Hints + message containing the address of its AS 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 AS 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. C may have been registered at the AS 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 asymmetric cryptography 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]. If only - symmetric cryptography is used between the client and the resource - server, the access token MAY instead be transferred in the DTLS - ClientKeyExchange message (see Section 3.3.1). + 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). Figure 2 depicts the common protocol flow for the DTLS profile after the client C has retrieved the access token from the authorization server AS. C RS AS | [--- Access Token ------>] | | | | | | <== DTLS channel setup ==> | | | | | @@ -204,107 +207,132 @@ secured. Depending on the used CoAP security mode (see also Section 9 of [RFC7252], the Client-to-AS request, AS-to-Client response 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. 3.1. Communication between C and AS 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 C can request the access token, C and AS must - establish a secure communication channel. C must securely have - obtained keying material to communicate with AS, and C must securely - have received authorization information intended for C that states - that AS is authorized to provide keying material concerning RS to C. - Also, AS must securely have obtained keying material for C, and - obtained authorization rules approved by the resource owner (RO) - concerning C and RS that relate to this keying material. C and AS - must use their respective keying material for all exchanged messages. - How the security association between C and AS is established is not - part of this document. C and AS MUST ensure the confidentiality, - integrity and authenticity of all exchanged messages. + server. Before C can request the access token, C and AS MUST + establish a secure communication channel. C MUST securely have + obtained keying material to communicate with AS. Furthermore, C MUST + verify that AS is authorized to provide access tokens (including + authorization information) about RS to C. Also, AS MUST securely + have obtained keying material for C, and obtained authorization rules + approved by the resource owner (RO) concerning C and RS that relate + to this keying material. C and AS MUST use their respective keying + material for all exchanged messages. How the security association + between C and AS is bootstrapped is not part of this document. C and + AS MUST ensure the confidentiality, integrity and authenticity of all + exchanged messages. If C is constrained, C and AS should use DTLS to communicate with each other. But C and AS may also use other means to secure their - communication, e.g., TLS. The used security protocol must provide - confidentiality, integrity and authenticity, and enable the client to - determine if it is the intended recipient of a message, e.g., by - using an AEAD mechanism. C must also be able to determine if a - response from AS belongs to a certain request. Additionally, the - protocol must offer replay protection. + communication, e.g., TLS. The used security protocol MUST fulfill + the communication security requirements in Section 6.2 of + [I-D.ietf-ace-oauth-authz]. 3.2. RawPublicKey Mode After C and AS mutually authenticated each other and validated each other's authorization, C sends a token request to AS'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. To prove that the client is in possession of this key, C MUST use the same keying material that it uses to secure the communication with AS, e.g., the DTLS session. An example access token request from the client to the AS 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...' + "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. AS MUST check if the client that it communicates with is associated with the RPK in the cnf object before issuing an access token to it. If AS determines that the request is to be authorized according to the respective authorization rules, it generates an access token - response for C. The response SHOULD contain a "profile" parameter - with the value "coap_dtls" to indicate that this profile must be used - for communication between the client C and the resource server. The - response also contains an access token and an "rs_cnf" parameter - containing information about the public key that is used by the - resource server. AS MUST ascertain that the RPK specified in - "rs_cnf" belongs to the resource server that C wants to communicate - with. AS MUST protect the integrity of the token. If the access - token contains confidential data, AS MUST also protect the - confidentiality of the access token. + response for C. 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 C 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 and an "rs_cnf" parameter containing information + about the public key that is used by the resource server. AS MUST + ascertain that the RPK specified in "rs_cnf" belongs to the resource + server that C wants to communicate with. AS MUST protect the + integrity of the token. If the access token contains confidential + data, AS MUST also protect the confidentiality of the access token. C 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 C wants to communicate. + An example access token response from the AS to the client is + depicted in Figure 4. + + 2.01 Created + Content-Format: application/ace+cbor + Max-Age: 3600 + Payload: + { + "access_token" : "b64'SlAV32hkKG ... + (remainder of CWT omitted for brevity; + CWT contains clients 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 + 3.2.1. DTLS Channel Setup Between C and RS Before the client initiates the DTLS handshake with the resource server, C MUST send a "POST" request containing the new access token - to the authz-info resource hosted by the resource server. If this - operation yields a positive response, the client SHOULD proceed to - establish a new DTLS channel with the resource server. To use the - RawPublicKey mode, the client MUST specify the public key that AS - defined in the "cnf" field of the access token response in the - SubjectPublicKeyInfo structure in the DTLS handshake as specified in - [RFC7250]. + to the authz-info resource hosted by the resource server. After the + client + receives a confirmation that the RS has accepted the access token, it + SHOULD proceed to establish a new DTLS channel with the resource + server. To use the RawPublicKey mode, the client MUST specify the + public key that AS defined in the "cnf" field of the access token + response in the SubjectPublicKeyInfo structure in the DTLS handshake + as specified in [RFC7250]. 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] with the ed25519 curve (cf. [RFC8032], [RFC8422]). Note: According to [RFC7252], CoAP implementations MUST support the ciphersuite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 [RFC7251] and the NIST P-256 curve. As discussed in [RFC7748], new ECC curves have been defined recently that are considered superior to the so- called NIST curves. The curve that is mandatory to implement in @@ -348,123 +376,140 @@ that the access token is generated for the resource server that C wants to communicate with. Also, AS MUST protect the integrity of the access token. 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. AS adds a "cnf" parameter to the access information carrying a "COSE_Key" object that informs the client about the symmetric key - that is to be used between C and the resource server. + that is to be used between C and the resource server. The access + token MUST be bound to the same symmetric key by means of the cnf + claim. - An example access token response is illustrated in Figure 4. In this + 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, symmetric PoP-key + + An 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 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: 86400 + Payload: { - access_token: h'd08343a10... + "access_token" : h'd08343a10... (remainder of CWT omitted for brevity) - token_type: pop, - expires_in: 86400, - profile: coap_dtls, - cnf: { - COSE_Key: { - kty: symmetric, - alg: TLS_PSK_WITH_AES_128_CCM_8 - kid: h'3d027833fc6267ce', - k: h'73657373696f6e6b6579' + "token_type" : "pop", + "expires_in" : 86400, + "profile" : "coap_dtls", + "cnf" : { + "COSE_Key" : { + "kty" : "symmetric", + "kid" : h'3d027833fc6267ce', + "k" : h'73657373696f6e6b6579' } } } - Figure 4: Example Access Token Response + 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 shared with the client. If the access token carries a symmetric key, the access token MUST be encrypted using a "COSE_Encrypt0" structure. The AS MUST use the keying material shared with the RS to encrypt the token. + The "cnf" structure in the access token is provided in Figure 7. + + "cnf" : { + "COSE_Key" : { + "kty" : "symmetric", + "kid" : h'eIiOFCa9lObw' + } + } + + 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]). 4.00 Bad Request Content-Format: application/ace+cbor + Payload: { - error: invalid_request + "error" : "invalid_request" } - Figure 5: Example Access Token Response With Reject + 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 AS 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 AS 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 AS and resource server. + derived + from some other secret key shared between the AS 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 AS and + RS. + + 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 AS and resource server. In order to generate a new symmetric key to be used by client and resource server, the AS generates a key identifier and 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 AS includes the key identifier in - the "kid" parameter, see Figure 6. This key identifier enables the + the "kid" parameter, see Figure 7. This key identifier enables the resource server to calculate the keying material for the communication with the client from the access token using the key derivation key and following Section 11 of [RFC8152] with parameters as specified here. The KDF to be used needs to be defined by the application, for example HKDF-SHA-256. The key identifier picked by the AS needs to be unique for each access token where a unique symmetric key is required. The fields in the context information "COSE_KDF_Context" (Section 11.2 of [RFC8152]) have the following values: o AlgorithmID = "ACE-CoAP-DTLS-key-derivation" o PartyUInfo = PartyVInfo = ( null, null, null ) - o keyDataLength is a uint equal the length of the symmetric key - shared between C and RS in bits - + o keyDataLength needs to be defined by the application o protected MUST be a zero length bstr o other is a zero length bstr o SuppPrivInfo is omitted - The "cnf" structure in the access token is provided in Figure 6. - - cnf : { - COSE_Key : { - kty : symmetric, - alg : TLS_PSK_WITH_AES_128_CCM_8, - kid : h'eIiOFCa9lObw' - } - } - - Figure 6: Access Token without Keying Material - 3.3.1. DTLS Channel Setup Between C and RS When a client receives an access token response from an authorization server, C 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 C wants to communicate. C 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 @@ -570,52 +615,52 @@ 1. with response code 4.03 (Forbidden) when the resource URI specified in the request is not covered by the authorization information, and 2. with response code 4.05 (Method Not Allowed) when the resource URI specified in the request covered by the authorization information but not the requested action. The client cannot always know a priori if an Authorized Resource - Request will succeed. It must check the validity of its keying + Request will succeed. It MUST check the validity of its keying material before sending a request or processing a response. If the client repeatedly gets error responses containing AS 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 RS, i.e., the DTLS session SHOULD be kept alive until the associated access token has expired. 4. Dynamic Update of Authorization Information The client can 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 7 depicts the message flow where the C requests a new access + Figure 9 depicts the message flow where the C 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 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]. + 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. Note: By associating the access tokens with the identifier of an existing DTLS session, the authorization information can be @@ -632,79 +677,85 @@ | <---------------------------- New Access Token - | | + Access Information | | | | | --- Update /authz-info --> | | | New Access Token | | | | | | == Authorized Request ===> | | | | | | <=== Protected Resource == | | - Figure 7: Overview of Dynamic Update Operation + Figure 9: Overview of Dynamic Update Operation 5. Token Expiration DTLS sessions that have been established in accordance with this - profile are always tied to a specific set of access tokens. As these - tokens may become invalid at any time (either because the token has - expired or the responsible authorization server has revoked the - token), the session may become useless at some point. A resource - server therefore MUST terminate existing DTLS sessions after the last - valid access token for this session has been deleted. + profile are always tied to a specific access token. As this token + may become invalid at any time (e.g. because it has expired), the + session may become useless at some point. A resource server + therefore MUST terminate existing DTLS sessions after the access + token for this session has been deleted. 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 session. 6. Security Considerations 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 and privacy considerations from - section 6 and section 7 also apply to this profile. + approach, the general security considerations from section 6 also + apply to this profile. + + When using pre-shared keys provisioned by the AS, the security level + depends on the randomness of PSK, and the security of the TLS cipher + suite and key exchange algorithm. 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 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 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. + Developers SHOULD avoid using multiple access tokens for a client. 7. Privacy Considerations + 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 the client and the resource server already exists, - more detailed information may be included with an error response to + 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 + 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. 8. IANA Considerations The following registrations are done for the ACE OAuth Profile Registry following the procedure specified in @@ -733,22 +784,22 @@ [I-D.ietf-ace-cwt-proof-of-possession] Jones, M., Seitz, L., Selander, G., Erdtman, S., and H. Tschofenig, "Proof-of-Possession Key Semantics for CBOR Web Tokens (CWTs)", draft-ietf-ace-cwt-proof-of- possession-06 (work in progress), February 2019. [I-D.ietf-ace-oauth-authz] Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and H. Tschofenig, "Authentication and Authorization for Constrained Environments (ACE) using the OAuth 2.0 - Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-21 - (work in progress), February 2019. + Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-22 + (work in progress), March 2019. [I-D.ietf-ace-oauth-params] Seitz, L., "Additional OAuth Parameters for Authorization in Constrained Environments (ACE)", draft-ietf-ace-oauth- params-04 (work in progress), February 2019. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, .