Internet Engineering Task Force P. M. Hallam-Baker Internet-Draft Comodo Group Inc. Intended status: Standards Track May 14, 2013 Expires: November 15, 2013 HTTP Session Management draft-hallambaker-httpsession-01 Abstract The HTTP Session Management Mechanism provides a mean of securely establishing a persistent authentication session between a HTTP client and server that does not rely on the presentation of a confidential bearer token. The Session Management Mechanism is intended to provide a replacement for the existing HTTP State Management Mechanism (Cookies) for this purpose. This document defines the HTTP Accept-Session, Set-Session and Session headers and specifies their use to establish symmetric authentication keys and their use to authenticate and verify specific parts of an HTTP message. Other means by which keys used to authenticate the messages are established are outside the scope of this document. 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 November 15, 2013. Copyright Notice Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved. Hallam-Baker Expires November 15, 2013 [Page 1] Internet-Draft HTTP Session Management May 2013 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 . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Relationship to Other Authentication Technologies . . . . 4 1.2. Example: Web Browser User Authentication . . . . . . . . 5 1.3. Use in Web Services . . . . . . . . . . . . . . . . . . . 7 2. Session Context . . . . . . . . . . . . . . . . . . . . . . . 8 2.1. Fixed Session Context . . . . . . . . . . . . . . . . . . 8 2.1.1. Id: Identifier . . . . . . . . . . . . . . . . . . . 8 2.1.2. MAC: Message Authentication Code Algorithm . . . . . 9 2.1.3. Key: Authentication Key . . . . . . . . . . . . . . . 9 2.1.4. Scope Attributes . . . . . . . . . . . . . . . . . . 9 2.1.5. Replay Attacks . . . . . . . . . . . . . . . . . . . 10 2.1.5.1. Request Replay Attack . . . . . . . . . . . . . . 10 2.1.5.2. Response Replay Attack . . . . . . . . . . . . . 11 2.1.6. Direction . . . . . . . . . . . . . . . . . . . . . . 11 2.1.7. TLS Binding (Fixed) . . . . . . . . . . . . . . . . . 11 2.1.8. Domain: String . . . . . . . . . . . . . . . . . . . 11 2.2. Session Context State Attributes . . . . . . . . . . . . 12 2.2.1. Expiry time: Max-Age . . . . . . . . . . . . . . . . 12 2.2.2. Now: Time Offset (Time) . . . . . . . . . . . . . . . 12 2.2.2.1. Now: Last Now (Time) . . . . . . . . . . . . . . 12 2.2.3. Count: Last Count (Count) . . . . . . . . . . . . . . 12 2.2.4. Nonce: Last Nonce (Nonce) . . . . . . . . . . . . . . 13 3. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.1. Accept-Session Header . . . . . . . . . . . . . . . . . . 13 3.2. Set-Session Header . . . . . . . . . . . . . . . . . . . 14 3.3. Session Header . . . . . . . . . . . . . . . . . . . . . 15 3.3.1. Value=[Binary] (required) . . . . . . . . . . . . . . 16 3.3.2. Nonce=[Binary] . . . . . . . . . . . . . . . . . . . 16 3.3.3. Stream=[Decimal] . . . . . . . . . . . . . . . . . . 16 3.3.3.1. Count=[Decimal] . . . . . . . . . . . . . . . . . 16 3.3.3.2. Time=[NTime] . . . . . . . . . . . . . . . . . . 16 3.3.3.3. Attribute tlsu=[value] . . . . . . . . . . . . . 16 3.3.3.4. Attribute tlss=[value] . . . . . . . . . . . . . 16 3.3.4. Preparing the Input to the Authentication Algorithm . 16 4. Processing . . . . . . . . . . . . . . . . . . . . . . . . . 17 4.1. Calculating the Authentication Value . . . . . . . . . . 17 Hallam-Baker Expires November 15, 2013 [Page 2] Internet-Draft HTTP Session Management May 2013 4.1.1. Start line . . . . . . . . . . . . . . . . . . . . . 17 4.1.2. Canonical Headers . . . . . . . . . . . . . . . . . . 17 4.1.3. Message Content . . . . . . . . . . . . . . . . . . . 17 4.2. Generating a Session Header . . . . . . . . . . . . . . . 18 4.3. Verifying a HTTP Message under a Session Context . . . . 18 5. Security Considerations . . . . . . . . . . . . . . . . . . . 18 5.1. Data outside the specified scope is not authenticated . . 18 5.2. Truncated Hash Algorithms . . . . . . . . . . . . . . . . 18 5.3. Randomness of Secret Keys and nonces . . . . . . . . . . 18 5.4. Weak Ciphers . . . . . . . . . . . . . . . . . . . . . . 19 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 7.1. Normative References . . . . . . . . . . . . . . . . . . 19 7.2. Non Normative References . . . . . . . . . . . . . . . . 20 Appendix A. Session Identifier Encoding . . . . . . . . . . . . 20 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20 1. Introduction The HTTP State Management Mechanism 'Cookies'[RFC6265] was intended to allow HTTP [RFC2616] servers to let servers maintain a stateful session over the mostly stateless HTTP protocol. While the exchange of static tokens is an acceptable mechanism for maintaining state, use of static tokens as bearer tokens for authentication is not. Such tokens are not bound to any part of the message they purport to authenticate and may be disclosed to intermediaries including HTTP proxies and caches. While use of TLS transport provides a confidentiality enhancement for HTTP content, recent research [CRIME], [BEAST] demonstrates that relying on a transport or network layer to protect the confidentiality of a bearer authentication token is fundamentaly unsound. The interaction of HTTP header compression mechanisms and a Turing complete active code mechanism under control of the attacker produces a threat model in which the capabilities afforded the attacker far exceed the capabilities that it is sensible to expect a protocol design to resist. The HTTP Accept-Session, Set-Session and Session headers provide a simple and effective means of maintaining a HTTP authentication session without passing static authentication data in either direction after the authentication session has been established. The design of the Set-Session and Session headers permit them to be used as a replacement for the Set-Cookie and Cookie headers in situations where they are supported by both the client and the server and ensure correct behavior by intermediaries conformant to the HTTP specification. Hallam-Baker Expires November 15, 2013 [Page 3] Internet-Draft HTTP Session Management May 2013 A HTTP authentication session MAY be established inband by means of the Set-Session header. The Set-Session header specifies a unique identifier for the session and determines the session parameters including the cryptographic algorithm and shared key. Applications SHOULD make use of cryptographic enhancements to protect the confidentiality of a session context established using the Set- Session header. Clients and Servers MAY support other means of establishing a HTTP authentication session. For example in a federated authentication scheme such as SAML, Kerberos or OpenID, the authentication session might be provided by a third party. Once the HTTP authentication session is established, a Session header is added to HTTP requests and/or responses as directed by the session context. The session header specifies the session identifier and an authentication value calculated over portions of the HTTP message and other attributes to which it is bound as directed by the corresponding session context. The bound attributes and portions of the HTTP message cannot then be changed without invalidating the authentication value. The use of bound attributes permits protection against TLS channel rebinding and/or HTTP message replay attacks. The portions of a HTTP message to which it is desirable to bind an authentication session depend on the situation. Binding the authentication session to the message content prevents modification of the content but imposes more constraints on implementations than binding to the message start line. Interactions with intermediaries and in particular intermediarries that are not fully compliant with the HTTP specification also raise concerns Web browsers are typically coded to be tollerant of such implementation defects and operate despite unauthorized modification of content by caches and other intermediaries. The prefered behavior of a Web Service client in such situations is likely to be to abort the transaction rather than risk continuing with corrupted data. 1.1. Relationship to Other Authentication Technologies The term 'user authentication' is commonly used in three separate contexts; credential management, credential presentation and session continuation: Credential Management describes the means by which credentials are created, issued and revoked. Hallam-Baker Expires November 15, 2013 [Page 4] Internet-Draft HTTP Session Management May 2013 Credential Presentation describes the means by which a party demonstrates holdership of a credential to establish an authentication session. Session Continuation describes the means by which a party demonstrates that a particular transaction is taking place within the context of a particular authentication session. The HTTP Session Management Mechanism is designed to support only Session Continuation and to compliment existing and future mechanisms for Credential management and Credential Presentation. While a session continuation mechanism is not in itself a solution to the problem of user authentication, the provision of a robust session continuation mechanism that does not depend on a bearer token addresses the most challenging problem facing the designers of SAML, OpenID and OAUTH. 1.2. Example: Web Browser User Authentication The principal mechanism for user authentication in use today is to present a HTML form in which the user enters their username and password. This approach has many known defects that are outside the scope of this document. These include the risk of impersonation of the Web site causing the user to enter their username and password into a form controlled by the attack and the strong likelihood that the user will use the same password across multiple sites. The client indicates that it supports the session header by including one or more Accept-Session headers in the request transfering the username and password values. The Accept-Session header specifies the scope and replay binding options that the client offers to support. [NB: These examples are not yet generated from running code and are for illustrative purposes only] POST /login.php HTTP/1.1 Host: example.com Cache-Control: no-store Content-Type: application/x-www-form-urlencoded Content-Length: 29 Accept-Session: Start=required Request=required Content=optional Time=required username=skroob&password=1234 Hallam-Baker Expires November 15, 2013 [Page 5] Internet-Draft HTTP Session Management May 2013 If the browser does not specify a Accept-Session header the server MAY reject the connection request entirely or fallback to the traditional Cookie mechanism as determined by site policy. If the service accepts the offer of session management support, it includes a Set-session Header in the response specifying the session context: HTTP/1.1 201 OK Content-Length: 35 Set-Session: Id=TUMnorO0SjHHS7D2uFcGlRYJ0Hd3eibwe0ogptoNMQuCYmCHfHAJcJlyvi j8WoXDglTSOkctnmoBzl8W0NLSlcgSyZcmsAyoWs8y1Rn2ZlO2WBgoWrFIOqPa4 oB29dgs/ei6ieINZtmvXNCm2NUkWA== Key=7eb219188339135ba51e8715f3900bfb974995e145d6e490e4addbbdb26f4bb4 Alg=HMAC-SHA256 Start=True Request=True Time=True Now=745531 Domain=example.com Max-Age=31536000

Authenticated

In this case the server avoids the need to track per client state by using a time based mechanism to avoid replay attacks and storing the state associated with the client session as encrypted data within the session identifier. The scope of the content binding is limited to the start line and the timer to be used for replay attack prevention has an offset 745531 seconds in the past. Once the session has been established, the client MUST include a Session header in subsequent HTTP requests made to the specified DNS domains. GET /status.php HTTP/1.1 Host: example.com Cache-Control: no-store Content-Type: application/x-www-form-urlencoded Content-Length: 29 Session: Id=TUMnorO0SjHHS7D2uFcGlRYJ0Hd3eibwe0ogptoNMQuCYmCHfHAJcJlyvi j8WoXDglTSOkctnmoBzl8W0NLSlcgSyZcmsAyoWs8y1Rn2ZlO2WBgoWrFIOqPa4 oB29dgs/ei6ieINZtmvXNCm2NUkWA== Value=cjkMkfnnYP8JYWZAbRLvtpqImmOK3rsrOT1XcvAgHDk=; Now=745533 In this case the session scope does not specify responses and so the response does not require an Session header but a server MAY provide one so as to specify updated values for the replay attack prevention Hallam-Baker Expires November 15, 2013 [Page 6] Internet-Draft HTTP Session Management May 2013 attributes Now and/or Count. Whenever a Session header is present the Id and Value attributes MUST be specified and correct: HTTP/1.1 201 OK Content-Length: 35 Session: Id=TUMnorO0SjHHS7D2uFcGlRYJ0Hd3eibwe0ogptoNMQuCYmCHfHAJcJlyvi j8WoXDglTSOkctnmoBzl8W0NLSlcgSyZcmsAyoWs8y1Rn2ZlO2WBgoWrFIOqPa4 oB29dgs/ei6ieINZtmvXNCm2NUkWA== Value=cjkMkfnnYP8JYWZAbRLvtpqImmOK3rsrOT1XcvAgHDk=; Now=745532

Shield is Closed

In this particular instance the clock at the server is running behind that of the client requiring the timer offset value to be decreased by one second. To ensure that the replay attack protection values only increase or stay the same, the client uses the last value of the old time offset until the new time offset value has superceded it. The Web Browser MAY terminate the session by simply deleting the session context information from its store preventing reuse. A client MAY inform the server that the session context is about to be deleted by including a Session header with the Deleted attribute: HEAD /status.php HTTP/1.1 Host: example.com Session: Id=TUMnorO0SjHHS7D2uFcGlRYJ0Hd3eibwe0ogptoNMQuCYmCHfHAJcJlyvi j8WoXDglTSOkctnmoBzl8W0NLSlcgSyZcmsAyoWs8y1Rn2ZlO2WBgoWrFIOqPa4 oB29dgs/ei6ieINZtmvXNCm2NUkWA== Value=cjkMkfnnYP8JYWZAbRLvtpqImmOK3rsrOT1XcvAgHDk=; Deleted A server may inform the client that the session has been terminated by including a Session header with the Deleted attribute in the response. 1.3. Use in Web Services Use of HTTP Session Managment simplifies implemenatation of Web Services. Using the SOAP [TBS] approach a Web Service message is encoded in XML [TBS], wrapped in a SOAP envelope and a WS-Security [TBS] header with an XML Signature [TBS] attached. The whole package is then attached to a HTTP message as a content payload. Hallam-Baker Expires November 15, 2013 [Page 7] Internet-Draft HTTP Session Management May 2013 This approach involves a considerable degree of complexity and in most cases does nothing more than attach authentication data to a message. Carrying the authentication value as a HTTP header typically eliminates the need for the SOAP and WS-Security layers entirely. Use of session management in Web Services presents different requirements and constraints. In the case of an entirely new Web Service with no deployment history, there is no need to consider support for legacy code at all, eliminating one of the principal constraints governing use of new HTTP protocol features in Web Browsers. A single HTTP message MAY have multiple Session headers. This facilitates support for multi-party transactions in which A submits a transaction to B who countersigns it and passes it to C who is required to chek that she has proof of agreement by both A and B. Use of the Session header permits the developer to isolate integrity and authentication checks to a single point of control, as is advised by best security practice. The security monitor examines a HTTP message, verifies that the required integrity data is present and correct and only passes the payload on for processing by the Web Service itself if and only if the verification checks have been passed. 2. Session Context The processing of the Session header is determined by the session context which consists of a set of fixed attributes that remain constant for the lifetime of the session and state attributes that are updated as Session headers are generated and verified. 2.1. Fixed Session Context The fixed session context elements are set when the session is established and remain constant for the lifetime of the session. The values specified can only be changed by establishing a new session which MUST have a different session identifier. 2.1.1. Id: Identifier The session identifier is a statistically unique sequence of binary data which SHOULD be unique, MUST be statistically unique, SHOULD be less than 512 octets in length and MUST NOT be longer than 4096 octets in length. Hallam-Baker Expires November 15, 2013 [Page 8] Internet-Draft HTTP Session Management May 2013 Servers MAY avoid the need to maintain per-session server side state by encoding the some or all of the fixed session context parameters in to the identifier. Servers MUST ensure that appropriate cryptographic enhancements are employed to authenticate the sessikon context and protect the confidentiality of the authentication key. The scheme used to construct the session identifiers used in the examples is described in Appendix A 2.1.2. MAC: Message Authentication Code Algorithm The message authentication algorithm to be used to calculate the authentication value. HMAC construction [RFC2104] HMAC-SHA256-128 HMAC using the SHA-1 algorithm with the output truncated to the first 64 bits. HMAC-SHA512-256 HMAC using the SHA-1 algorithm with the output truncated to the first 256 bits. HMAC-SHA2-256-128 HMAC using the SHA-2 algorithm with the output truncated to the first 128 bits. HMAC-SHA2-512-256 HMAC using the SHA-2 algorithm with the output truncated to the first 256 bits. CMAC Construction [RFC4493] CMAC-AES128-64 The AES algorithm employed in CMAC mode with a 128 bit key and the output truncated to the first 64 bits. CMAC-AES128 The AES algorithm employed in CMAC mode with a 128 bit key and the entire output. 2.1.3. Key: Authentication Key The cryptographic key to be used to calculate the authentication value. 2.1.4. Scope Attributes The scope attributes specify which parts of the message are authenticated. The scope is specified by the start, header and content attributes. The order in which the scope attributes are specified in the HTTP Set-Session header is immaterial. The scope is always constructed in Hallam-Baker Expires November 15, 2013 [Page 9] Internet-Draft HTTP Session Management May 2013 the same order as the elements occur in a HTTP message, i.e. start, dummy headers and content. Content: Boolean If set true, the specified scope includes the message body. The content transfer encoding (e.g. chunked) is ignored for the purpose of determining the content. ContentDigest: Label If a message digest algorithm is specified the authentication scope MAY be calculated indirectly by first calculating a Message Digest value over the content and using the resulting value in place of the actual content value to calculate the Message Authentication Code value. Start: Boolean If set true, the specified scope includes the message start line. This being the request Line in the case of a request and the status line in the case of a response. 2.1.5. Replay Attacks Preventing replay attacks in HTTP requests and responses poses considerably different challenges. Since a HTTP response is always immediately preceded by a request, return of a request nonce is sufficient to prevent a response replay attack. This approach is stateless and does not require client or server to store state information. Since the HTTP protocol requires that certain methods be idempotent, the HTTP protocol does not lend itself to preventing request replay attacks in the same fashion. Request replay MAY be prevented by use of counter techniques or mitigated by limiting request replay to a particular time window. 2.1.5.1. Request Replay Attack Two mechanisms for preventing or mitigating request replay attacks are specified: Counter: Boolean Counter based mechanisms are supported by the count attribute. The value of a counter MUST increase for successive transactions within the same transaction stream. Concurrency MAY be supported by specifying multiple streams but this requires a separate counter state to be maintained for each transaction stream. Time: Boolean Time based approaches are supported by the time attribute. If the value of the time attribute falls within the permitted acceptance window, the message MAY be accepted. Otherwise the message MUST be rejected. Hallam-Baker Expires November 15, 2013 [Page 10] Internet-Draft HTTP Session Management May 2013 Using a time based approach avoids the need to maintain state at either the client or server. The principal disadvantage of this approach being that the mechanism only protects against a replay attack within a specific time. Another disadvantage to the time based approach is that it relies on the sender and receiver maintaining a tollerably close time synchronization over the duration of the transaction and for the latency introduced by the communication path being tollerably small. Neither method is entirely satisfactory. The counter mechanism requires that the client and server both maintain state and the time based mechanism only prevents request replay attack outside a specified time interval. For Web Services that require a stronger assurance that request replay attack cannot succeed (e.g. payment transactions) without maintaining server side state, such controls should be provided by the Web Service protocol rather than relying on the HTTP session continuation mechanism. For example, the Web Services protocol might define a two phase interaction in which the client requested a server nonce in the first phase to be returned in the second phase. 2.1.5.2. Response Replay Attack If a HTTP Session header in a request specifies a nonce value, the corresponding Session header in the response (if present) MUST specify the same nonce value. 2.1.6. Direction A session MAY be defined to apply to requests only, responses only or to both requests and responses. Request: Boolean This session context applies to requests. Response: Boolean This session context applies to responses. 2.1.7. TLS Binding (Fixed) The TLS binding attribute specifies whether TLS channel binding is to be used. 2.1.8. Domain: String Hallam-Baker Expires November 15, 2013 [Page 11] Internet-Draft HTTP Session Management May 2013 The DNS Domain(s) to which the session context applies. The syntax and semantics of the Domain attribute are identical to those of the Domain attribute of the Cookie header defined in []. 2.2. Session Context State Attributes 2.2.1. Expiry time: Max-Age The time at which the session expires. To avoid the need for the client or server to have access to a realtime clock, Set-Session and Session headers specify the expiry time as the remaining lifetime of the session from the instant the header is generated in seconds. A server MAY update the value Max-Age value to extend the lifetime of the session before expiry by specifying a new value for Max-Age in the Session header. 2.2.2. Now: Time Offset (Time) The Time Offset value is used to calculate the value of the Now attribute in the session header and is only required when the Time replay protection mechanism is in use. To avoid the need for clients or servers to have access to a reference time source, time values used to protect against replay attack are specified relative to an arbitrary epoch start time specified by the server. The Time Offset value is the difference between the time epoch specified by the server and the local time according to the machine. A server MAY use the same epoch start time for all clients or use a different epoch for each one. 2.2.2.1. Now: Last Now (Time) If the local clock at the client runs faster or slower than that of the server, a timing discrepancy emerges over time. A client SHOULD and a server MAY correct for such inaccuracies by noting the value of the now attribute specified by the other party and adjusting the local time offset value accordingly provided that the mechanism employed to do so ensures that the values of the now attribute in a HTTP message is never less than the value specified in a previous header. Recording the value of the last value of Now specified in a header permits this condition to be met. 2.2.3. Count: Last Count (Count) Hallam-Baker Expires November 15, 2013 [Page 12] Internet-Draft HTTP Session Management May 2013 If counter based replay attack prevention is in use the client and server MUST maintain a record of the last value of the counter for each concurrent stream active within the session. 2.2.4. Nonce: Last Nonce (Nonce) If nonce based replay attack prevention is in use, the parties MUST maintain a record of the last nonce value so as to be able to return it when necessary. In most circumstances the nonce value is used immediately and need not be stored. 3. Syntax The Accept-Session, Set-Session and Session headers use the following common syntax elements Label [ alpha (alpha | '-')* ] Binary [Base 64 encoding of a binary value] Offer [ "Optional" | "Required" | "Refused" ] DTime [Decimal time value from current time] Decimal [Decimal numeric value] 3.1. Accept-Session Header The Accept-Session header is used to negotiate the establishment of an authentication context. When used in a request the Accept-Session header specifies a set of acceptable parameters for the session context. MAC=[Label(,Label)*] The message authentication algorithms the client is willing to support. Content=[Offer] Offers or requires the inclusion of the message content in the authentication scope. ContentDigest=[Offer] Offers or requires the inclusion of the message content by means of a content digest in the authentication scope. Start=[Offer] Offers or requires the inclusion of the message start line in the authentication scope. Hallam-Baker Expires November 15, 2013 [Page 13] Internet-Draft HTTP Session Management May 2013 Request=[Offer] Offers or requires the use of a Session header in a request message. Response=[Offer] Offers or requires the use of a Session header in a response message. TLSU=[Offer] Offers or requires the use of tls-unique TLS chanel binding as specified in [RFC5929]. TLSE=[Offer] Offers or requires the use of tls-server-end-point TLS chanel binding as specified in [RFC5929]. Nonce=[Offer] Offers or requires the use of the nonce response replay attack prevention mechanism. Counter=[Offer] Offers or requires the use of the counter request replay attack prevention mechanism. Time=[Optional | Required] Offers or requires the use of the time request replay attack prevention mechanism. When used by the client to offer the use of an authentication session, all header attributes are optional. Note however that even though it is permissable for a client to offer an empty Accept- Session header, doing so does not allow a valid session context to be established as the server is required to specify at least an authentication scope and MAC algorithm from amongst those offered by the client. 3.2. Set-Session Header The Set-Session Header is specified in a response to accept an offer of using the session continuation mechanism made by specifing accept- session in the corresponding request. The features specified in the Set-Session header MUST be consistent with the features offered in the corresponding request. Id=[Binary] The session context identifier in base64 encoding. Key=[Binary] The cryptographic key to be used to calculate the authentication value in base64 encoding. MAC=[Label] The message authentication algorithm to be used to calculate the authentication value as defined in [RFC5698] . Content Specifies the inclusion of the message content in the authentication scope. Hallam-Baker Expires November 15, 2013 [Page 14] Internet-Draft HTTP Session Management May 2013 ContentDigest Specifies the inclusion of the message content by means of a content digest in the authentication scope. Start Specifies the inclusion of the message start line in the authentication scope. Request Specifies the use of a Session header in a request message. Response Specifies the use of a Session header in a response message. TLSBinging Specifies the use of TLS Binding [Need to think this through further] Counter=[Decimal] Specifies the use of the counter replay attack prevention mechanism. The value of the attribute specifies the maximum number of permitted streams. Time=[NTime] Specifies the use of the time replay attack prevention mechanism and the current value of the time value in seconds. Servers SHOULD NOT use a time offset from a fixed epoch (e.g. 32 bit UNIX epoch). Max-Age=[NTime] Specifies the number of seconds in which the session parameters expire measured from the time at which the request was issued. A Set-Session header MUST contain the following elements: Id Key MAC At least one Scope attribute offered by the client At least one direction attribute A Max-Age value 3.3. Session Header The Session header has the tag 'Session' and takes a sequence of attribute values as follows: [Insert ABNF here] Hallam-Baker Expires November 15, 2013 [Page 15] Internet-Draft HTTP Session Management May 2013 The session context identifier as in base64 encoding. 3.3.1. Value=[Binary] (required) The value attribute specifies the value resulting from applying the authentication context and nonce (if present) to the specified scope. 3.3.2. Nonce=[Binary] The nonce attribute MAY be specified in a request. If a request specifies a nonce attribute, the corresponding response MUST specify a nonce attribute with the same value. 3.3.3. Stream=[Decimal] The Stream attribute MUST NOT be specified in a request unless the counter attribute is specified in the session context and the value of the stream count is less than the number of permitted streams. 3.3.3.1. Count=[Decimal] The Count attribute MUST NOT be specified in a request unless the counter attribute is specified in the session context. The value of the count attribute MUST be greater than the value of the count attribute in all previous requests under the specified session with the same stream attribute. 3.3.3.2. Time=[NTime] Specifies a time value to be used in combination with the specified authentication context. The format of the time value is determined by the authentication context. 3.3.3.3. Attribute tlsu=[value] Specifies the TLS unique channel binding as specified in [RFC5929]. 3.3.3.4. Attribute tlss=[value] Specifies the TLS server end point channel binding as specified in [RFC5929]. 3.3.4. Preparing the Input to the Authentication Algorithm [Should specify how the content scope is assembles and how the replay attack attributes are included within it.] Hallam-Baker Expires November 15, 2013 [Page 16] Internet-Draft HTTP Session Management May 2013 4. Processing 4.1. Calculating the Authentication Value The input to the MAC algorithm is the concatenation of the following values. The Start Line Is included if and only if the value of the start attribute of the session context is true. The Canonical HTTP Headers Are always included. The Message Content Is included if and only if the value of the content attribute of the session context is true. 4.1.1. Start line The Start line is the HTTP start line including the final CRLF. Example: 4.1.2. Canonical Headers The canonical form of the header(s) specified for inclusion in the authentication scope by the session context sorted into alphabetical order. At present only the Session header is specified and MUST always be included. The canonical Session header contains all the attributes of the Session header to be added to the HTTP message with the exception of the Value attribute. Attributes MUST be specified in alphabetical order. Example: 4.1.3. Message Content If the Content-Digest parameter of the session context is empty the Message content value is the actual value of the message content ignoring any transfer encoding but after any content-encoding has taken place. Hallam-Baker Expires November 15, 2013 [Page 17] Internet-Draft HTTP Session Management May 2013 If the Content-Digest parameter of the session context specifies at least one Message Digest algorithm, the sender MAY chose to calculate the authentication value over the actual value of the content as specified above or first apply one of the specified message digest algorithms to the actual value of the message content as specified above and then calculate the authentication value over the resulting digest value. Example: 4.2. Generating a Session Header Generating a Session Header requires the following steps to be performed: The Session header parameters are calculated according to the session context. If necessary, the session context is updated to reflect new values of relevant replay attack prevention attributes. The authentication value is calculated over the specified scope. The Session header is added to the HTTP headers. 4.3. Verifying a HTTP Message under a Session Context Verifying messages follows the same approach as generation. The verifier calculates the authentication value over the input values as specified in the session context. If the resulting authentication value matches that specified by the sender, the authentication succeeds and fails otherwise. 5. Security Considerations 5.1. Data outside the specified scope is not authenticated The integrity check only extends to the portions of the message that are within the specified scope. 5.2. Truncated Hash Algorithms If the authentication context permits the use of a truncated MAC, it MUST specify the minimum length of the MAC after truncation and verifiers MUST reject MAC values shorter than that length as invalid. 5.3. Randomness of Secret Keys and nonces Hallam-Baker Expires November 15, 2013 [Page 18] Internet-Draft HTTP Session Management May 2013 The security of any cryptographic protocol relies on the difficulty of guessing secret keys. Secret keys and nonces SHOULD be generated using a mechanism that ensures that the range of possible values is sufficiently large to prevent 'brute force' guessing attacks. For more information see [RFC4086]. 5.4. Weak Ciphers Specification of the cryptographic algorithms used to construct the Integrity header value is implicit in the authentication context identifier and thus outside the scope of this specification. 6. IANA Considerations Add the 'Accept-Session', 'Set-Session' and 'Session' headers to the list of provisional HTTP headers. Add the HMAC algorithm entries to the RFC 5698 regitry http:// www.iana.org/assignments/dssc/dssc.xml [Upgrade if/when this becomes an RFC] Create a registry for Session Header attributes. The initial contents of the registry to be: [Stuff from rest of document.] 7. References 7.1. Normative References [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. [RFC2965] Kristol, D. M. and L. Montulli, "HTTP State Management Mechanism", RFC 2965, October 2000. [RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005. Hallam-Baker Expires November 15, 2013 [Page 19] Internet-Draft HTTP Session Management May 2013 [RFC4493] Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The AES-CMAC Algorithm", RFC 4493, June 2006. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008. [RFC5698] Kunz, T., Okunick, S., and U. Pordesch, "Data Structure for the Security Suitability of Cryptographic Algorithms (DSSC)", RFC 5698, November 2009. [RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings for TLS", RFC 5929, July 2010. 7.2. Non Normative References [BEAST] , "TBS", , , . [CRIME] , "TBS", , , . [RFC3275] Eastlake, D., Reagle, J., and D. Solo, "(Extensible Markup Language) XML-Signature Syntax and Processing", RFC 3275, March 2002. [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos Network Authentication Service (V5)", RFC 4120, July 2005. [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, September 2009. [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, April 2011. Appendix A. Session Identifier Encoding Author's Address Phillip Hallam-Baker Comodo Group Inc. Email: philliph@comodo.com Hallam-Baker Expires November 15, 2013 [Page 20]