Network Working Group M. Miller Internet-Draft Cisco Systems, Inc. Intended status: Standards Track February 29, 2012 Expires: August 14, 2012 End-to-End Object Encryption for the Extensible Messaging and Presence Protocol (XMPP) draft-miller-xmpp-e2e-00 Abstract This document defines a method of end-to-end object encryption for the Extensible Messaging and Presence Protocol (XMPP). 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 http://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 August 14, 2012. Copyright Notice Copyright (c) 2012 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 (http://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 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2 3. Encrypting XMPP Stanzas . . . . . . . . . . . . . . . . . . . 2 3.1. Prerequisites . . . . . . . . . . . . . . . . . . . . . . 3 3.2. Process . . . . . . . . . . . . . . . . . . . . . . . . . 3 Miller std [Page 1] Internet-Draft XMPP E2E February 2012 3.3. Example Securing a Message . . . . . . . . . . . . . . . . 4 4. Requesting Content Keys . . . . . . . . . . . . . . . . . . . 6 4.1. Request Process . . . . . . . . . . . . . . . . . . . . . 6 4.2. Accept Process . . . . . . . . . . . . . . . . . . . . . . 7 4.3. Error Conditions . . . . . . . . . . . . . . . . . . . . . 8 4.4. Example of Successful Key Request . . . . . . . . . . . . 8 5. Handling of Inbound Encrypted Stanzas . . . . . . . . . . . . 10 5.1. Protocol Not Understood . . . . . . . . . . . . . . . . . 11 5.2. Insufficient Information . . . . . . . . . . . . . . . . . 11 5.3. Failed Decryption . . . . . . . . . . . . . . . . . . . . 12 5.4. Timestamp Not Acceptable . . . . . . . . . . . . . . . . . 12 5.5. Successful Decryption . . . . . . . . . . . . . . . . . . 13 6. Inclusion and Checking of Timestamps . . . . . . . . . . . . . 13 7. Interaction with Stanza Semantics . . . . . . . . . . . . . . 14 8. Mandatory-to-Implement Cryptographic Algorithms . . . . . . . 14 9. Security Considerations . . . . . . . . . . . . . . . . . . . 14 9.1. Storage of Encrypted Stanzas . . . . . . . . . . . . . . . 14 9.2. Re-use of Content Encryption Keys . . . . . . . . . . . . 14 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 10.1. XML Namespace Name for e2e Data in XMPP . . . . . . . . . 14 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15 11.1. Normative References . . . . . . . . . . . . . . . . . . 15 11.2. Informative References . . . . . . . . . . . . . . . . . 16 Appendix A. Schema for urn:ietf:params:xml:ns:xmpp-e2e:1 . . . . . 16 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18 1. Introduction End-to-end encryption of traffic sent over the Extensible Messaging and Presence Protocol [RFC6120] is a desirable goal. Requirements and a threat analysis for XMPP encryption are provided in [E2E-REQ]. Many possible approaches to meet those (or similar) requirements have been proposed over the years, including methods based on PGP, S/MIME, SIGMA, and TLS. Most proposals have not been able to support multiple end-points for a given recipient. As more devices support XMPP, it becomes more desirable to allow an entity to communicate with another in a more secure manner, regardless of the number of agents th entity is employing. This document specifies an approach for encrypting communications between two entities which each might have multiple end-points. 2. Terminology This document inherits terminology defined in [RFC6120]. Security-related terms are to be understood in the sense defined in [SECTERMS]. The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [KEYWORDS]. 3. Encrypting XMPP Stanzas Miller std [Page 2] Internet-Draft XMPP E2E February 2012 The process that a sending agent follows for securing stanzas is the same regardless of the form of stanza (i.e., , , or ). 3.1. Prerequisites First, the sending agent prepares and retains the following: o The JID of the sender (i.e. its own JID). This SHOULD be the bare JID (localpart@domainpart). o The JID of the recipient. This SHOULD be the bare JID (localpart@domainpart). o A random Content Encryption Key (CEK). The CEK MUST have a length at least equal to that of the required encryption keys and MUST be generated randomly. See [RFC4086] for considerations on generating random values. o A CEK identifier (CID). The CID MUST be unique for a given (sender, recipient, CEK) tuple, and MUST NOT be derived from CEK itself. 3.2. Process For a given plaintext stanza (S), the sending agent performs the following: 1. Notes the current UTC date and time N when this stanza is constructed, formatted as described under [timestamps]. 2. Constructs a forwarding envelope P using a element qualified by the "urn:xmpp:forward:0" namespace (as defined in [MSG-FWD]) as follows: * The child element qualified by the "urn:xmpp:delay" namespace (as defined in [DELAY]) with the attribute 'stamp' set to the UTC date and time value N * The plaintext stanza S 3. Generates any additional unprotected block cipher factors (IV); e.g. initialization vector, nonce, and/or associated authentication data. A sending agent MUST ensure that no two sets of factors are used with the same CEK, and SHOULD NOT reuse such factors for other stanzas. 4. Constructs a partial [JOSE-JWE] header (H) with the following information: * The property 'enc' indicating the algorithm used to encrypt the content. Miller std [Page 3] Internet-Draft XMPP E2E February 2012 * The property 'iv' indicating the initialization vector, if required by the algorithm. NOTE: this header is intentionally lacking required attributes. The receiving agent is responsible for assembling a final, valid JWE header. 5. Convert the stanza to a UTF-8 encoded string (P'), optionallly removing line breaks and other insignificant whitespace between elements and attributes, i.e. P' = UTF8-encode(P). We call P' a "stanza-string" because for purposes of encryption and decryption it is treated not as XML but as an opaque string (this avoids the need for complex canonicalization of the XML input). 6. Encrypts P' using the intended block cipher, i.e. T = block- encrypt(CEK, IV, P'). 7. Constructs an element qualified by the "urn:ietf:params:xml:ns:xmpp-e2e:1" namespace as follows: * The attribute 'id' set to the identifier value CID. * The child element
qualified by the "urn:ietf:params:xml:ns:xmpp-e2e:1" namespace and with XML character data as the base64url-encoded form of H. * The child element qualified by the "urn:ietf:params:xml:ns :xmpp-e2e:1" namespace and with XML character data as the base64url-encoded form of T. 8. Sends the element as the payload of a stanza that SHOULD match the stanza from step 1 in kind (e.g., ), type (e.g., "chat"), and addressing (e.g. to="romeo@montague.net" from="juliet@capulet.net/balcony"). If the original stanza (S) has a value for the "id" attribute, this stanza MUST NOT use the same value for its "id" attribute. 3.3. Example Securing a Message NOTE: unless otherwise indicated, all line breaks are included for readability. The sending agent begins with the plaintext version of the stanza 'S': Miller std [Page 4] Internet-Draft XMPP E2E February 2012 35740be5-b5a4-4c4e-962a-a03b14ed92f4 But to be frank, and give it thee again. And yet I wish but for the thing I have. My bounty is as boundless as the sea, My love as deep; the more I give to thee, The more I have, for both are infinite. and the following prerequisites: o Sender JID as "juliet@capulet.lit" o Recipient JID as "romeo@montegue.lit" o Content encryption key 'CEK' as (base64 encoded) "-ElMo6FndEkMxWP3TIkp1dDfVKqmqAAgrlcvVnUVpOc=" o CEK identifier CID as "835c92a8-94cd-4e96-b3f3-b2e75a438f92" The sending agent performs steps 1 and 2 to generate the envelope: 35740be5-b5a4-4c4e-962a-a03b14ed92f4 But to be frank, and give it thee again. And yet I wish but for the thing I have. My bounty is as boundless as the sea, My love as deep; the more I give to thee, The more I have, for both are infinite. Then the sending agent performs steps 3 and 4 to generate the [JOSE- JWE] header: { "enc":"A256CCM", "iv":"B7waCj2vF_sLaJfe-1GHrA==", "adata":"1492-05-12T20:07:37.012Z", "msize":"16" Miller std [Page 5] Internet-Draft XMPP E2E February 2012 } Then the sending agent performs steps 5 and 6 to generate the protected content: 7LlMXd-qqPAQ_LZm6u9AR2csyDgT09z5DWdn8K5GLr_qbWRDKw2ufZrmO9YZ-jHl 1IDeXeQ9azbNNViv8gpa-prDYkXOo3QoqYOJiA0RAkPU-UjN41wqqVvV62gad_OB Dd9q2xsNnK1PI5frIGTCZSexOIeSD3EcP0cDI_0MzMEKqpVnPbYQDkWQNrtxPs2b lE15KcQXbHVxA9rEz7y0a-ITXruV_fOXGftkRVDuiF1yVh2xNRPa-TQxDegZh1D_ u_c2mwPLO6ED_1ZlvL7075_VL0DT01YGtDDQeyzrQWnQNEBJ4G5jFpCyqtCszbgx 9kjWjxLYNLLGxbMOtwF45OLCd8JFUIQAHoLeKp4aIWr5yp7aATX8dKvQm5_TFICt nLonaM1e5mRPnRgg5zNeMErx6FFkqowOI1h7hVl_QHF8Ofoy99CmWKiVT3Nq5ngL 74Xm4CtiJMHnAEn2Q-10-fWuIHIEA0u4GwBaXo0ToBw4uCM4ZhG1SFSKyCKkjXmr 7TL-5jwSuuQHX6efS8Hhi7fujmqw9VXekeubsY2btl5put0SWT8_0S8ZBDMjrkXj y7iI5NUOhQMms3ou1r01NZdzNVKcqiX5q3z1eB1FLWmGymnnj_gE_HalWUL0HoqL 93Fr1nfFLNhXLCZYhZ7By6T9NN8omp4ZYE92HMpPzgo-eCGP Then the sending agent performs step 7 and sends the following:
eyJlbmMiOiJBMjU2Q0NNIiwiaXYiOiJCN3dhQ2oydkZfc0xhSmZlLTFHSHJBPT0i LCJhZGF0YSI6IjQ5Mi0wNS0xMlQyMDowNzozNy4wMTJaIiwibXNpemUiOiIxNiJ9
7LlMXd-qqPAQ_LZm6u9AR2csyDgT09z5DWdn8K5GLr_qbWRDKw2ufZrmO9YZ-jHl 1IDeXeQ9azbNNViv8gpa-prDYkXOo3QoqYOJiA0RAkPU-UjN41wqqVvV62gad_OB Dd9q2xsNnK1PI5frIGTCZSexOIeSD3EcP0cDI_0MzMEKqpVnPbYQDkWQNrtxPs2b lE15KcQXbHVxA9rEz7y0a-ITXruV_fOXGftkRVDuiF1yVh2xNRPa-TQxDegZh1D_ u_c2mwPLO6ED_1ZlvL7075_VL0DT01YGtDDQeyzrQWnQNEBJ4G5jFpCyqtCszbgx 9kjWjxLYNLLGxbMOtwF45OLCd8JFUIQAHoLeKp4aIWr5yp7aATX8dKvQm5_TFICt nLonaM1e5mRPnRgg5zNeMErx6FFkqowOI1h7hVl_QHF8Ofoy99CmWKiVT3Nq5ngL 74Xm4CtiJMHnAEn2Q-10-fWuIHIEA0u4GwBaXo0ToBw4uCM4ZhG1SFSKyCKkjXmr 7TL-5jwSuuQHX6efS8Hhi7fujmqw9VXekeubsY2btl5put0SWT8_0S8ZBDMjrkXj y7iI5NUOhQMms3ou1r01NZdzNVKcqiX5q3z1eB1FLWmGymnnj_gE_HalWUL0HoqL 93Fr1nfFLNhXLCZYhZ7By6T9NN8omp4ZYE92HMpPzgo-eCGP 4. Requesting Content Keys A receiving agent might not have the conetnt encryption key to decrypt the stanza. Miller std [Page 6] Internet-Draft XMPP E2E February 2012 4.1. Request Process Before a CEK can be requested, the receiving agent MUST have at least one public key for which it also has the private key. To request a CEK, the receiving agent performs the following: 1. Constructs a [JOSE-JWK] container object (KS), containing information about each public key the requesting agent wishes to use. Each key SHOULD include a value for the property 'kid' which uniquely identifies it within the context of all provided keys. Each key MUST include a value for the property 'kid' if any two keys use the same algorithm. 2. Constructs a element qualified by the "urn:ietf:params:xml:ns:xmpp-e2e:1" namespace as follows: * The attribute 'id' set to the CEK identifier value CID. * The child element qualified by the "urn:ietf:params:xml:ns :xmpp-e2e:1" namespace and with XML character data as the base64url-encoded form of KS. 3. Sends the element as the payload of an stanza with the attribute 'type' set to "get", the attribute 'to' set to the full JID of the original encrypted stanza's sender, and the attribute 'id' set to an opaque string value the receiving agent uses to track the response. 4.2. Accept Process If the sending agent approves the request, it performs the following: 1. Chooses a key (PK) from the list provided via KS, and notes its identifier value 'kid'. 2. Constructs a parial [JOSE-JWE] header (H) as follows: * The property 'alg' set to the cryptographic algorithm for PK, which is used to secure the content encryption key CEK. * The property 'kid' set to the identifier matching PK. NOTE: this header is intentionally lacking required attributes. The receiving agent is responsible for assembling a final, valid JWE header. 3. Encrypts the content encryption key CEK using the key PK, i.e. CEK' = pki-encrypt(PK, CEK). 4. Constructs a element qualified by the "urn:ietf:params:xml:ns:xmpp-e2e:1" namespace as follows: Miller std [Page 7] Internet-Draft XMPP E2E February 2012 * The attribute 'id' set to the CEK identifier CID. * The child element
qualified by the "urn:ietf:params:xml:ns:xmpp-e2e:1" namespace and with XML cahracter data as the base64url-encoded form of H. * The child element qualified by the "urn:ietf:params:xml:ns :xmpp-e2e:1" namespace and with XML character data as the base64url-encoded form of CEK'. 5. Sends the element as the payload of an stanza with the attribute 'type' set to "result", the attribute 'to' set to the full JID from the request 's 'from' attribute, and the attribute 'id' set to the value of the request 's 'id' attribute. 4.3. Error Conditions If the sending agent does not approve of the request, it sends an stanza of type "error" and containing the reason for denying the request: o : the key request is made by an entity that is not authorized to decrypt stanzas from the sending agent and/or for the indicated CID. o : the requested CID is no longer valid. o : the key request did not contain any keys the sending agent understands. 4.4. Example of Successful Key Request NOTE: unless otherwise indicated, all line breaks are included for readability. To begin a key request, the receiving agent performs step 1 from request process [keyreq-process-request] to generate the [JOSE-JWK]: [{ "alg":"RSA", "mod":"AL7ano5DBdNYfKChGh3xxGDtAtozO3iEm20rzbNEjSLWFjuhm46SXzeJXV8Nr6V UXcrJ8FN85OqNv0g9GAh9VoHriNECtZJzEw3TUl8edntXOAU8IZQPIh7rrU1pqQ anrJ9RjWZqgd42LCTpFQWMAibZAfQD2gb5NIrZUbYs4wn9Rx52ru5xkrcrIYLvP rK8Yv3fKwuU0y0gf809Yunz_H8A5aHplzrQtFOuW8uT_tWR8Wn_2w_H6Y6bOoME oHZGSjCQCNGzlmVrjKadp7amtMCwezXcW9iI63qIP7ObLAKEolw1XaLMEm6d-mT _tZZwpNo_r2eKm4WhYOW2bYLfoME=", "exp":"AQAB", "kid":"romeo@montegue.lit/garden" }] Miller std [Page 8] Internet-Draft XMPP E2E February 2012 Then the receiving agent performs step 2 to generate the : W3siYWxnIjoiUlNBIiwibW9kIjoiQUw3YW5vNURCZE5ZZktDaEdoM3h4R0R0QXR vek8zaUVtMjByemJORWpTTFdGanVobTQ2U1h6ZUpYVjhOcjZWVVhjcko4Rk44NU 9xTnYwZzlHQWg5Vm9IcmlORUN0Wkp6RXczVFVsOGVkbnRYT0FVOElaUVBJaDdyc lUxcHFRYW5ySjlSaldacWdkNDJMQ1RwRlFXTUFpYlpBZlFEMmdiNU5JclpVYllz NHduOVJ4NTJydTV4a3JjcklZTHZQcks4WXYzZkt3dVUweTBnZjgwOVl1bnpfSDh BNWFIcGx6clF0Rk91Vzh1VF90V1I4V25fMndfSDZZNmJPb01Fb0haR1NqQ1FDTk d6bG1WcmpLYWRwN2FtdE1Dd2V6WGNXOWlJNjNxSVA3T2JMQUtFb2x3MVhhTE1Fb TZkLW1UX3RaWndwTm9fcjJlS200V2hZT1cyYllMZm9NRT0iLCJleHAiOiJBUUFC Iiwia2lkIjoicm9tZW9AbW9udGVndWUubGl0L2dhcmRlbiJ9XQ== Then the receiving agent performs step 3 and sends the following: W3siYWxnIjoiUlNBIiwibW9kIjoiQUw3YW5vNURCZE5ZZktDaEdoM3h4R0R0QXR vek8zaUVtMjByemJORWpTTFdGanVobTQ2U1h6ZUpYVjhOcjZWVVhjcko4Rk44NU 9xTnYwZzlHQWg5Vm9IcmlORUN0Wkp6RXczVFVsOGVkbnRYT0FVOElaUVBJaDdyc lUxcHFRYW5ySjlSaldacWdkNDJMQ1RwRlFXTUFpYlpBZlFEMmdiNU5JclpVYllz NHduOVJ4NTJydTV4a3JjcklZTHZQcks4WXYzZkt3dVUweTBnZjgwOVl1bnpfSDh BNWFIcGx6clF0Rk91Vzh1VF90V1I4V25fMndfSDZZNmJPb01Fb0haR1NqQ1FDTk d6bG1WcmpLYWRwN2FtdE1Dd2V6WGNXOWlJNjNxSVA3T2JMQUtFb2x3MVhhTE1Fb TZkLW1UX3RaWndwTm9fcjJlS200V2hZT1cyYllMZm9NRT0iLCJleHAiOiJBUUFC Iiwia2lkIjoicm9tZW9AbW9udGVndWUubGl0L2dhcmRlbiJ9XQ== If the sending agent accepts this key request, it performs steps 1 and 2 from accept process [keyreq-process-accept] to generate the partial [JOSE-JWE] header: { "alg":"RSA-OEAP", "kid":"romeo@montegue.lit/garden" } Then the sending agent performs step 3 to generate the encrypted CEK: Miller std [Page 9] Internet-Draft XMPP E2E February 2012 DCKrJpLd8XYze7joNKsyvpzGD4MffVQF7apQMkPK8_vKx4J3xEqWB3tXxp6oxJF 4YHla4SFiMp44LccMEd6JchPp_23OXrBo8i7Fwo8jVH6QoQa71T_1rQj6CWQ1eF vXiU4Qmp52F-gjuUhlKfe3qccdlFp3kLy8Mpq5mDdq6deOq0_qusqr8QTqrT5LN IgsXYzrdiDdKx8KSKpI4FBB_H1x-A1C97DfJk9s5H7GzKVm2BRWVXWjJfO4lckE Z45ZJkA3CKq86Yth3YBetoScPVLhoRoO2qqBg4gR9dDO5mXnOgPslVfXWvegexH AyhHmJZsj3yV_pW1Rlb5jn5J4Rw== Then the sending agent performs step 4 to generate the response:
eyJhbGciOiJSU0EtT0VBUCIsImtpZCI6InJvbWVvQG1vbnRlZ3VlLmxpc3QvZ2F yZGVuIn0=
DCKrJpLd8XYze7joNKsyvpzGD4MffVQF7apQMkPK8_vKx4J3xEqWB3tXxp6oxJF 4YHla4SFiMp44LccMEd6JchPp_23OXrBo8i7Fwo8jVH6QoQa71T_1rQj6CWQ1eF vXiU4Qmp52F-gjuUhlKfe3qccdlFp3kLy8Mpq5mDdq6deOq0_qusqr8QTqrT5LN IgsXYzrdiDdKx8KSKpI4FBB_H1x-A1C97DfJk9s5H7GzKVm2BRWVXWjJfO4lckE Z45ZJkA3CKq86Yth3YBetoScPVLhoRoO2qqBg4gR9dDO5mXnOgPslVfXWvegexH AyhHmJZsj3yV_pW1Rlb5jn5J4Rw== Then the sending agent performs step 5 and sends the following:
eyJhbGciOiJSU0EtT0VBUCIsImtpZCI6InJvbWVvQG1vbnRlZ3VlLmxpc3QvZ2F yZGVuIn0=
DCKrJpLd8XYze7joNKsyvpzGD4MffVQF7apQMkPK8_vKx4J3xEqWB3tXxp6oxJF 4YHla4SFiMp44LccMEd6JchPp_23OXrBo8i7Fwo8jVH6QoQa71T_1rQj6CWQ1eF vXiU4Qmp52F-gjuUhlKfe3qccdlFp3kLy8Mpq5mDdq6deOq0_qusqr8QTqrT5LN IgsXYzrdiDdKx8KSKpI4FBB_H1x-A1C97DfJk9s5H7GzKVm2BRWVXWjJfO4lckE Z45ZJkA3CKq86Yth3YBetoScPVLhoRoO2qqBg4gR9dDO5mXnOgPslVfXWvegexH AyhHmJZsj3yV_pW1Rlb5jn5J4Rw== 5. Handling of Inbound Encrypted Stanzas Several scenarios are possible when an entity receives an encrypted stanza: Miller std [Page 10] Internet-Draft XMPP E2E February 2012 o The receiving agent does not understand the protocol. o The receiving agent understands the protocol but does not have enough infomration to decrypt the payload. o The receiving agent understands the protocol but is unable to decrypt the payload. o The receiving agent understands the protocol and is able to decrypt the payload, but the timestamps fail the checks specified under [timestamps]. o The receiving agent understands the protocol and is able to decrypt the payload (success case). 5.1. Protocol Not Understood In the case where the receiving agent does not understand the protocol, it MUST do one and only one of the following: (1) ignore the extension, (2) ignore the entire stanza, or (3) return a error to the sender, as described in [RFC6120]. NOTE: If the inbound stanza is an , the receiving agent MUST return an error to the sending agent, to comply with the exchanging of IQ stanzas in [RFC6121]. 5.2. Insufficient Information In the case where the protocol is understood but the receiving agent does not have enough information to decrypt the payload, it SHOULD request the additional information as described in [keyreq]. If the key request fails, or the receiving agent could not otherwise determine the additional information, it MAY return a error to the sending agent (as described in [RFC6120]), optionally supplemented by an application-specific error condition element of :
[XML character data]
[XML character data]
 Miller std [Page 11] Internet-Draft XMPP E2E February 2012 In addition to returning an error, the receiving agent SHOULD NOT present the stanza to the intended recipient (human or application) and SHOULD provide some explicit alternate processing of the stanza (which MAY be to display a message informing the recipient that it has received a stanza that cannot be decrypted). 5.3. Failed Decryption In the case where the protocol is understood but the receiving agent is unable to decrypt the payload, the receiving agent SHOULD return a error to the sending agent (as described in [RFC6120]), optionally supplemented by an application-specific error condition element of (previously defined in [RFC3923]):
[XML character data]
[XML character data]
 In addition to returning an error, the receiving agent SHOULD NOT present the stanza to the intended recipient (human or application) and SHOULD provide some explicit alternate processing of the stanza (which MAY be to display a message informing the recipient that it has received a stanza that cannot be decrypted). 5.4. Timestamp Not Acceptable In Case #4, the receiving agent MAY return a error to the sender (as described in [RFC6120]), optionally supplemented by an application-specific error condition element of (previously defined in [RFC3923]): Miller std [Page 12] Internet-Draft XMPP E2E February 2012
[XML character data]
[XML character data] 5.5. Successful Decryption In the case where the protocol is understood and the receiving agent successfully decrypted the payload, it MUST NOT return a stanza error. If the payload is an of type "get" or "set", and the response is an error, the receiving agent MUST send the encrypted response in an of type "result", to prevent exposing information about the payload. 6. Inclusion and Checking of Timestamps Timestamps are included to help prevent replay attacks. All timestamps MUST conform to [DATETIME] and be presented as UTC with no offset, and SHOULD include the seconds and fractions of a second to three digits. Absent a local adjustment to the sending agent's perceived time or the underlying clock time, the sending agent MUST ensure that the timestamps it sends to the receiver increase monotonically (if necessary by incrementing the seconds fraction in the timestamp if the clock returns the same time for multiple requests). The following rules apply to the receiving agent: o It MUST verify that the timestamp received is within five minutes of the current time, except as described below for offline messages. o It SHOULD verify that the timestamp received is greater than any timestamp received in the last 10 minutes which passed the previous check. o If any of the foregoing checks fails, the timestamp SHOULD be presented to the receiving entity (human or application) marked as "old timestamp", "future timestamp", or "decreasing timestamp", and the receiving entity MAY return a stanza error to the sender. Miller std [Page 13] Internet-Draft XMPP E2E February 2012 The foregoing timestamp checks assume that the recipient is online when the message is received. However, if the recipient is offline then the server will probably store the message for delivery when the recipient is next online (offline storage does not apply to or stanzas, only stanzas). As described in [OFFLINE], when sending an offline message to the recipient, the server SHOULD include delayed delivery data as specified in [DELAY] so that the recipient knows that this is an offline message and also knows the original time of receipt at the server. In this case, the recipient SHOULD verify that the timestamp received in the encrypted message is within five minutes of the time stamped by the recipient's server in the element. 7. Interaction with Stanza Semantics The following limitations and caveats apply: o Undirected stanzas SHOULD NOT be encrypted. Such stanzas are delivered to anyone the sender has authorized, and can generate a large volume of key requests. o Stanzas directed to multiplexing services (e.g. multi-user chat) SHOULD NOT be encrypted, unless the sender has established an acceptable trust relationship with the multiplexing service. 8. Mandatory-to-Implement Cryptographic Algorithms All algorithms that MUST be implemented for [JOSE-JWE] also MUST be implemented for this specification. 9. Security Considerations 9.1. Storage of Encrypted Stanzas The recipient's server might store any stanzas received until the recipient is next available; this duration could be anywhere from a few minutes to several months. 9.2. Re-use of Content Encryption Keys A sender SHOULD NOT use the same CEK for stanzas intended for different recipients. A sender MAY re-use a CEK for several stanzas to the same recpient. In this case, the CID remains the same, but MUST generate a new IV (and other data) for each encrypted stanza. The sender SHOULD periodically generate a new CEK; however, this specification does not mandate any specific algorithms or processes. 10. IANA Considerations 10.1. XML Namespace Name for e2e Data in XMPP Miller std [Page 14] Internet-Draft XMPP E2E February 2012 A URN sub-namespace of encrypted content for the Extensible Messaging and Presence Protocol (XMPP) is defined as follows. URI: urn:ietf:params:xml:ns:xmpp-e2e:1 Specification: RFC XXXX Description: This is an XML namespace name of encrypted content for the Extensible Messaging and Presence Protocol as defined by RFC XXXX. Registrant Contact: IESG, 11. References 11.1. Normative References [BASE64] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, October 2006. [CMS-AES] Schaad, J., "Use of the Advanced Encryption Standard (AES) Encryption Algorithm in Cryptographic Message Syntax (CMS)", RFC 3565, July 2003. [DATETIME] Saint-Andre, P., "XMPP Date and Time Profiles", XSF XEP 0082, May 2003. [DELAY] Saint-Andre, P., "Delayed Delivery", XSF XEP 0203, September 2009. [E2E-REQ] Saint-Andre, P, "Requirements for End-to-End Encryption in the Extensible Messaging and Presence Protocol (XMPP)", Internet-Draft draft-saintandre-xmpp-e2e-requirements-01, March 2010. [MSG-FWD] Wild, M. and K. Smith, "Message Forwarding", XSF XEP 0297, July 2011. [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [HMAC] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms (SHA and HMAC-SHA)", RFC 4634, July 2006. [JOSE-JWA] Jones, M., "JSON Web Algorithms (JWA)", Internet-Draft draft-ietf-jose-json-web-algorithms-00, January 2012. [JOSE-JWE] Jones, M., Rescola, E. and J. Hildebrand, "JSON Web Encryption (JWE)", Internet-Draft draft-ietf-jose-json- web-encryption-00, January 2012. [JOSE-JWK] Jones, M., "JSON Web Key (JWK)", Internet-Draft draft- ietf-jose-json-web-key-00, January 2012. Miller std [Page 15] Internet-Draft XMPP E2E February 2012 [PKIX] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R. and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, May 2008. [SECTERMS] Shirey, R., "Internet Security Glossary, Version 2", RFC 4949, August 2007. [X509-ALGO] Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, February 2003. 11.2. Informative References [OFFLINE] Saint-Andre, P., "Best Practices for Handling Offline Messages", XSF XEP 0160, January 2006. [RFC3923] Saint-Andre, P., "End-to-End Signing and Object Encryption for the Extensible Messaging and Presence Protocol (XMPP)", RFC 3923, October 2004. [RFC4086] Eastlake, D., Schiller, J. and S. Crocker, "Randomness Requirements for Security", RFC 4086, June 2005. [RFC6120] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Core", RFC 6121, March 2011. [RFC6121] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence", RFC 6121, March 2011. Appendix A. Schema for urn:ietf:params:xml:ns:xmpp-e2e:1 The following XML schema is descriptive, not normative. Miller std [Page 16] Internet-Draft XMPP E2E February 2012 Miller std [Page 17] Internet-Draft XMPP E2E February 2012 Author's Address Matthew Miller Cisco Systems, Inc. 1899 Wyknoop Street, Suite 600 Denver, CO 80202 USA Phone: +1-303-308-3204 Email: mamille2@cisco.com Miller std [Page 18]