ACE Working Group                                              S. GerdesInternet-Draft                                               O. BergmannIntended status: Standards Track                              C. BormannExpires: October 14, 2019                        Universitaet Bremen TZI                                                             G. Selander                                                             Ericsson AB                                                                L. Seitz                                                                    RISE                                                          April 12, 2019Datagram Transport Layer Security (DTLS) Profile for Authentication and            Authorization for Constrained Environments (ACE)                    draft-ietf-ace-dtls-authorize-08Abstract   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   power and memory.Status of This Memo   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/.   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 October 14, 2019.Copyright Notice   Copyright (c) 2019 IETF Trust and the persons identified as the   document authors.  All rights reserved.Gerdes, et al.          Expires October 14, 2019                [Page 1]Internet-Draft                  CoAP-DTLS                     April 2019   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   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.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 . . . . . . . . .  12     3.4.  Resource Access . . . . . . . . . . . . . . . . . . . . .  13   4.  Dynamic Update of Authorization Information . . . . . . . . .  14   5.  Token Expiration  . . . . . . . . . . . . . . . . . . . . . .  16   6.  Secure Communication with AS  . . . . . . . . . . . . . . . .  16   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  16   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  17   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18     10.1.  Normative References . . . . . . . . . . . . . . . . . .  18     10.2.  Informative References . . . . . . . . . . . . . . . . .  19   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  201.  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   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].Gerdes, et al.          Expires October 14, 2019                [Page 2]Internet-Draft                  CoAP-DTLS                     April 2019   The DTLS handshake requires the client and server to prove that they   can use certain keying material.  In the RPK mode, the client proves   with the DTLS handshake that it can use the RPK bound to the token   and the server shows that it can use a certain RPK.  The access token   must be presented to the resource server.  For the RPK mode, the   access token needs to be uploaded to the resource server before the   handshake is initiated, 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.1.1.  Terminology   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and   "OPTIONAL" in this document are to be interpreted as described in BCP   14 [RFC2119] [RFC8174] when, and only when, they appear in all   capitals, as shown here.   Readers are expected to be familiar with the terms and concepts   described in [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].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):Gerdes, et al.          Expires October 14, 2019                [Page 3]Internet-Draft                  CoAP-DTLS                     April 2019      C                                RS                   AS      | [---- Resource Request ------>]|                     |      |                                |                     |      | [<-AS Request Creation Hints-] |                     |      |                                |                     |      | ------- Token Request  ----------------------------> |      |                                |                     |      | <---------------------------- 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 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 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.Gerdes, et al.          Expires October 14, 2019                [Page 4]Internet-Draft                  CoAP-DTLS                     April 2019      C                            RS                   AS      | [--- Access Token ------>] |                     |      |                            |                     |      | <== DTLS channel setup ==> |                     |      |                            |                     |      | == Authorized Request ===> |                     |      |                            |                     |      | <=== Protected Resource == |                     |                        Figure 2: Protocol overview3.  Protocol Flow   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 C and 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 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.  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.   Section Section 6 specifies how communication with the AS is secured.Gerdes, et al.          Expires October 14, 2019                [Page 5]Internet-Draft                  CoAP-DTLS                     April 20193.2.  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...'          }        }      }            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 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 MUSTGerdes, et al.          Expires October 14, 2019                [Page 6]Internet-Draft                  CoAP-DTLS                     April 2019   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 Mode3.2.1.  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.  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].Gerdes, et al.          Expires October 14, 2019                [Page 7]Internet-Draft                  CoAP-DTLS                     April 2019   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      this specification is said to be efficient and less dangerous      regarding implementation errors than the secp256r1 curve mandated      in [RFC7252].   RS MUST check if the access token is still valid, if RS is the   intended destination, i.e., the audience, of the token, and if the   token was issued by an authorized AS.  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 C'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 [I-D.ietf-ace-cwt-proof-of-possession].  RS MUST use the   keying material in the handshake that AS specified in the rs_cnf   parameter in the access token.  Thus, the handshake only finishes if   C and RS are able to use their respective keying material.3.3.  PreSharedKey Mode   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.  AS MUST check if the identifier refers to a symmetric key   that was previously generated by AS as a shared secret for the   communication between this client and the resource server.   The authorization server MUST determine the authorization rules for   the C it communicates with as defined by RO 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".  AS MUST ascertain   that the access token is generated for the resource server that C   wants to communicate with.  Also, AS MUST protect the integrity ofGerdes, et al.          Expires October 14, 2019                [Page 8]Internet-Draft                  CoAP-DTLS                     April 2019   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.  The access   token MUST be bound to the same symmetric key by means of the cnf   claim.   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].Gerdes, et al.          Expires October 14, 2019                [Page 9]Internet-Draft                  CoAP-DTLS                     April 2019      2.01 Created      Content-Format: application/ace+cbor      Max-Age: 86400      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 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'6549694f464361396c4f6277'     }   }              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]).Gerdes, et al.          Expires October 14, 2019               [Page 10]Internet-Draft                  CoAP-DTLS                     April 2019       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 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.  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 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 needs to be defined by the applicationGerdes, et al.          Expires October 14, 2019               [Page 11]Internet-Draft                  CoAP-DTLS                     April 2019   o  protected MUST be a zero length bstr   o  other is a zero length bstr   o  SuppPrivInfo is omitted3.3.1.  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   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.   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.  Alternatively, the client MAY 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 and not perform any re-coding.   Note:  As stated in Section 4.2 of [RFC7925], the PSK identity should      be treated as binary data in the Internet of Things space and not      assumed to have a human-readable form of any sort.   If a resource server receives a ClientKeyExchange message that   contains a "psk_identity" with a length greater zero, it uses the   contents as index for its key store (i.e., treat the contents as key   identifier).  The resource server MUST check if it has one or more   access tokens that are associated with the specified key.   If no key with a matching identifier is found, the resource server   MAY process the contents of the "psk_identity" field as access token   that is stored with the authorization information endpoint, before   continuing the DTLS handshake.  If the contents of the "psk_identity"   do not yield a valid access token for the requesting client, the DTLSGerdes, et al.          Expires October 14, 2019               [Page 12]Internet-Draft                  CoAP-DTLS                     April 2019   session setup is terminated with an "illegal_parameter" DTLS alert   message.   Note1:  As a resource server cannot provide a client with a      meaningful PSK identity hint in response to the client's      ClientHello message, the resource server SHOULD NOT send a      ServerKeyExchange message.   Note2:  According to [RFC7252], CoAP implementations MUST support the      ciphersuite TLS_PSK_WITH_AES_128_CCM_8 [RFC6655].  A client is      therefore expected to offer at least this ciphersuite to the      resource server.   When RS receives an access token, RS MUST check if the access token   is still valid, if RS is the intended destination, i.e., the audience   of the token, and if the token was issued by an authorized AS.  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.   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 RS uses the   "COSE_KDF_Context" information as described above.3.4.  Resource Access   Once a DTLS channel has been established as described in Section 3.2   and 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, C and RS 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.  Any request that the resource server receives on   this channel MUST be checked against these authorization rules.  RS   MUST check for every request if the access token is still valid.   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 as described in Section 5.1.1   of [I-D.ietf-ace-oauth-authz].Gerdes, et al.          Expires October 14, 2019               [Page 13]Internet-Draft                  CoAP-DTLS                     April 2019   The resource server SHOULD treat an incoming CoAP request as   authorized if the following holds:   1.  The message was received on a secure channel that has been       established using the procedure defined in this document.   2.  The authorization information tied to the sending client is       valid.   3.  The request is destined for the resource server.   4.  The resource URI specified in the request is covered by the       authorization information.   5.  The request method is an authorized action on the resource with       respect to the authorization information.   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]   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   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 respectiveGerdes, et al.          Expires October 14, 2019               [Page 14]Internet-Draft                  CoAP-DTLS                     April 2019   resource and uploads this access token to the authz-info resource on   the resource server.   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 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      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.      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 9: Overview of Dynamic Update OperationGerdes, et al.          Expires October 14, 2019               [Page 15]Internet-Draft                  CoAP-DTLS                     April 20195.  Token Expiration   DTLS sessions that have been established in accordance with this   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.  Secure Communication with AS   As specified in the ACE framework (sections 5.6 and 5.7 of   [I-D.ietf-ace-oauth-authz]), the requesting entity (RS and/or client)   and the AS 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) MAY be used instead.   How credentials (e.g., PSK, RPK, X.509 cert) for using DTLS with the   AS are established is out of scope for this profile.   If other means of securing the communication with the AS are used,   the security protocol MUST fulfill the communication security   requirements in Section 6.2 of [I-D.ietf-ace-oauth-authz].7.  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 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 theGerdes, et al.          Expires October 14, 2019               [Page 16]Internet-Draft                  CoAP-DTLS                     April 2019   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.8.  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   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.9.  IANA Considerations   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_dtlsGerdes, et al.          Expires October 14, 2019               [Page 17]Internet-Draft                  CoAP-DTLS                     April 2019   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: 1   Change Controller: IESG   Reference: [RFC-XXXX]10.  References10.1.  Normative References   [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-24              (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-05 (work in progress), March 2019.   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels", BCP 14, RFC 2119,              DOI 10.17487/RFC2119, March 1997,              <https://www.rfc-editor.org/info/rfc2119>.   [RFC4279]  Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key              Ciphersuites for Transport Layer Security (TLS)",              RFC 4279, DOI 10.17487/RFC4279, December 2005,              <https://www.rfc-editor.org/info/rfc4279>.   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,              January 2012, <https://www.rfc-editor.org/info/rfc6347>.Gerdes, et al.          Expires October 14, 2019               [Page 18]Internet-Draft                  CoAP-DTLS                     April 2019   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",              RFC 6749, DOI 10.17487/RFC6749, October 2012,              <https://www.rfc-editor.org/info/rfc6749>.   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained              Application Protocol (CoAP)", RFC 7252,              DOI 10.17487/RFC7252, June 2014,              <https://www.rfc-editor.org/info/rfc7252>.   [RFC7925]  Tschofenig, H., Ed. and T. Fossati, "Transport Layer              Security (TLS) / Datagram Transport Layer Security (DTLS)              Profiles for the Internet of Things", RFC 7925,              DOI 10.17487/RFC7925, July 2016,              <https://www.rfc-editor.org/info/rfc7925>.   [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",              RFC 8152, DOI 10.17487/RFC8152, July 2017,              <https://www.rfc-editor.org/info/rfc8152>.   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,              May 2017, <https://www.rfc-editor.org/info/rfc8174>.10.2.  Informative References   [RFC6655]  McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for              Transport Layer Security (TLS)", RFC 6655,              DOI 10.17487/RFC6655, July 2012,              <https://www.rfc-editor.org/info/rfc6655>.   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,              Weiler, S., and T. Kivinen, "Using Raw Public Keys in              Transport Layer Security (TLS) and Datagram Transport              Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,              June 2014, <https://www.rfc-editor.org/info/rfc7250>.   [RFC7251]  McGrew, D., Bailey, D., Campagna, M., and R. Dugal, "AES-              CCM Elliptic Curve Cryptography (ECC) Cipher Suites for              TLS", RFC 7251, DOI 10.17487/RFC7251, June 2014,              <https://www.rfc-editor.org/info/rfc7251>.   [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves              for Security", RFC 7748, DOI 10.17487/RFC7748, January              2016, <https://www.rfc-editor.org/info/rfc7748>.Gerdes, et al.          Expires October 14, 2019               [Page 19]Internet-Draft                  CoAP-DTLS                     April 2019   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital              Signature Algorithm (EdDSA)", RFC 8032,              DOI 10.17487/RFC8032, January 2017,              <https://www.rfc-editor.org/info/rfc8032>.   [RFC8392]  Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,              "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,              May 2018, <https://www.rfc-editor.org/info/rfc8392>.   [RFC8422]  Nir, Y., Josefsson, S., and M. Pegourie-Gonnard, "Elliptic              Curve Cryptography (ECC) Cipher Suites for Transport Layer              Security (TLS) Versions 1.2 and Earlier", RFC 8422,              DOI 10.17487/RFC8422, August 2018,              <https://www.rfc-editor.org/info/rfc8422>.Authors' Addresses   Stefanie Gerdes   Universitaet Bremen TZI   Postfach 330440   Bremen  D-28359   Germany   Phone: +49-421-218-63906   Email: gerdes@tzi.org   Olaf Bergmann   Universitaet Bremen TZI   Postfach 330440   Bremen  D-28359   Germany   Phone: +49-421-218-63904   Email: bergmann@tzi.org   Carsten Bormann   Universitaet Bremen TZI   Postfach 330440   Bremen  D-28359   Germany   Phone: +49-421-218-63921   Email: cabo@tzi.orgGerdes, et al.          Expires October 14, 2019               [Page 20]Internet-Draft                  CoAP-DTLS                     April 2019   Goeran Selander   Ericsson AB   Email: goran.selander@ericsson.com   Ludwig Seitz   RISE   Scheelevaegen 17   Lund  223 70   Sweden   Email: ludwig.seitz@ri.seGerdes, et al.          Expires October 14, 2019               [Page 21]