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Versions: 00 01 02 03 04 05 06 07 08 09 rfc6896            Informational
Internet Engineering Task Force                               S. Barbato
Internet-Draft                                              S. Dorigotti
Intended status: Informational                           T. Fossati, Ed.
Expires: June 5, 2013                                          KoanLogic
                                                        December 2, 2012

                  SCS: Secure Cookie Sessions for HTTP


   This memo defines a generic URI and HTTP header friendly envelope for
   carrying symmetrically encrypted, authenticated, and origin-
   timestamped tokens.  It also describes one possible usage of such
   tokens via a simple protocol based on HTTP cookies.

   SCS use cases cover a wide spectrum of applications, ranging from
   distribution of authorized content via HTTP (e.g. with out-of-band
   signed URIs), to securing browser sessions with diskless embedded
   devices (e.g.  SOHO routers), or web servers with high availability
   or load balancing requirements that may want to delegate the handling
   of the application state to clients instead of using shared storage
   or forced peering.

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 June 5, 2013.

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

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   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.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Requirements Language  . . . . . . . . . . . . . . . . . . . .  4
   3.  SCS Protocol . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  SCS Cookie Description . . . . . . . . . . . . . . . . . .  5
       3.1.1.  ATIME  . . . . . . . . . . . . . . . . . . . . . . . .  6
       3.1.2.  DATA . . . . . . . . . . . . . . . . . . . . . . . . .  6
       3.1.3.  TID  . . . . . . . . . . . . . . . . . . . . . . . . .  7
       3.1.4.  IV . . . . . . . . . . . . . . . . . . . . . . . . . .  7
       3.1.5.  AUTHTAG  . . . . . . . . . . . . . . . . . . . . . . .  7
     3.2.  Crypto Transform . . . . . . . . . . . . . . . . . . . . .  8
       3.2.1.  Choice and Role of the Framing Symbol  . . . . . . . .  8
       3.2.2.  Cipher Set . . . . . . . . . . . . . . . . . . . . . .  9
       3.2.3.  Compression  . . . . . . . . . . . . . . . . . . . . .  9
       3.2.4.  Cookie Encoding  . . . . . . . . . . . . . . . . . . .  9
       3.2.5.  Outbound Transform . . . . . . . . . . . . . . . . . .  9
       3.2.6.  Inbound Transform  . . . . . . . . . . . . . . . . . . 10
     3.3.  PDU Exchange . . . . . . . . . . . . . . . . . . . . . . . 12
       3.3.1.  Cookie Attributes  . . . . . . . . . . . . . . . . . . 12  Expires  . . . . . . . . . . . . . . . . . . . . . 12  Max-Age  . . . . . . . . . . . . . . . . . . . . . 12  Domain . . . . . . . . . . . . . . . . . . . . . . 13  Secure . . . . . . . . . . . . . . . . . . . . . . 13  HttpOnly . . . . . . . . . . . . . . . . . . . . . 13
   4.  Key Management and Session State . . . . . . . . . . . . . . . 13
   5.  Cookie Size Considerations . . . . . . . . . . . . . . . . . . 14
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
     8.1.  Security of the Cryptographic Protocol . . . . . . . . . . 16
     8.2.  Impact of the SCS Cookie Model . . . . . . . . . . . . . . 16
       8.2.1.  Old cookie replay  . . . . . . . . . . . . . . . . . . 16
       8.2.2.  Cookie Deletion  . . . . . . . . . . . . . . . . . . . 18
       8.2.3.  Cookie Sharing or Theft  . . . . . . . . . . . . . . . 18
       8.2.4.  Session Fixation . . . . . . . . . . . . . . . . . . . 18
     8.3.  Advantages of SCS over Server-side Sessions  . . . . . . . 19
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 19
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 20
   Appendix A.  Examples  . . . . . . . . . . . . . . . . . . . . . . 21
     A.1.  No Compression . . . . . . . . . . . . . . . . . . . . . . 21
     A.2.  Use Compression  . . . . . . . . . . . . . . . . . . . . . 21
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22

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1.  Introduction

   This memo defines a generic URI and HTTP header friendly envelope for
   carrying symmetrically encrypted, authenticated, and origin-
   timestamped tokens.

   It is generic in that it does not force any specific format upon the
   authenticated information - which makes SCS tokens flexible, easy,
   and secure to use in many different scenarios.

   It is URI and HTTP header friendly, as it has been explicitly
   designed to be compatible with both the ABNF "token" syntax [RFC2616]
   (the one used for e.g.  Set-Cookie and Cookie headers), and the path
   or query syntax of HTTP URIs.

   This memo also describes one possible usage of such tokens via a
   simple protocol based on HTTP cookies that allows to establish
   "client mode" sessions.  This is not their sole possible usage, and
   while no other operational patterns are outlined here, it is expected
   that SCS tokens may be easily employed as a building block for other
   kind of HTTP based applications that need to carry in-band secured

   When SCS tokens are used to implement client mode cookie sessions,
   the SCS implementer must fully understand the security implications
   entailed by the act of delegating the whole application state to the
   client (browser).  In this regard, some hopefully useful security
   considerations have been collected in Section 8.2.  Please note that
   they may not cover all possible scenarios though, and must therefore
   be weighed carefully against the specific application threat model.

   An SCS server may be implemented within a web application by means of
   a user library that exposes the core SCS functionality and leaves
   explicit control over SCS tokens to the programmer, or transparently,
   for example by hiding a "diskless session" facility behind a generic
   session API abstraction.  SCS implementers are free to choose the
   model that best suits their needs.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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3.  SCS Protocol

   The SCS protocol defines:

   o  the SCS cookie structure and encoding (Section 3.1);

   o  the cryptographic transformations involved in SCS cookie creation
      and verification (Section 3.2);

   o  the HTTP-based PDU exchange that uses the Set-Cookie and Cookie
      HTTP headers (Section 3.3);

   o  the underlying key management model (Section 4).

   Note that the PDU is transmitted to the client as an opaque data
   block, hence no interpretation nor validation is necessary.  The
   single requirement for client-side support of SCS is cookie
   activation on the user agent.  The origin server is the sole actor
   involved in the PDU manipulation process, which greatly simplifies
   the crypto operations - especially key management, which is usually a
   pesky task.

   In the following sections we assume S to be one or more
   interchangeable HTTP server entities (e.g. a server pool in a load-
   balanced or high-availability environment) and C to be the client
   with a cookie-enabled browser, or any user agent with equivalent

3.1.  SCS Cookie Description

   S and C exchange a cookie (Section 3.3), whose cookie-value consists
   of a sequence of adjacent non-empty values, each of which is the 'URL
   and Filename safe' Base-64 encoding [RFC4648] of a specific SCS

   (Hereafter the encoded and raw versions of each SCS field are
   distinguished based on the presence, or lack thereof, of the 'e'
   prefix in their name, e.g. eATIME and ATIME.)

   Each SCS field is separated by its left and/or right sibling by means
   of the %x7c ASCII character (i.e. '|'), as follows:

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   scs-cookie        = scs-cookie-name "=" scs-cookie-value
   scs-cookie-name   = token
   scs-cookie-value  = eDATA "|" eATIME "|" eTID "|" eIV "|" eAUTHTAG
   eDATA             = 1*base64url-character
   eATIME            = 1*base64url-character
   eTID              = 1*base64url-character
   eIV               = 1*base64url-character
   eAUTHTAG          = 1*base64url-character

                                 Figure 1

   Confidentiality is limited to the application state information (i.e.
   the DATA field), while integrity and authentication apply to the
   entire cookie-value.

   The following subsections describe the syntax and semantics of each
   SCS cookie field.

3.1.1.  ATIME

   Absolute timestamp relating to the last read or write operation
   performed on session DATA, encoded as a HEX string holding the number
   of seconds since the UNIX epoch (i.e. since 00:00:00, Jan 1 1970.)

   This value is updated with each client contact and is used to
   identify expired sessions.  If the delta between the received ATIME
   value and the current time on S is larger than a predefined
   "session_max_age" (which is chosen by S as an application-level
   parameter), a session is considered to be no longer valid, and is
   therefore rejected.

   Such expiration error may be used to force user logout from an SCS
   cookie based session, or hooked in the web application logic to
   display an HTML form requiring re-validation of user credentials.

3.1.2.  DATA

   Block of encrypted and optionally compressed data, possibly
   containing the current session state.  Note that no restriction is
   imposed on the clear text structure: the protocol is completely
   agnostic as to inner data layout.

   Generally speaking, the plain text is the "normal" cookie that would
   have been exchanged by S and C if SCS had not been used.

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3.1.3.  TID

   This identifier is equivalent to a SPI in a Data Security SA
   [RFC3740]) and consists of an ASCII string that uniquely identifies
   the transform set (keys and algorithms) used to generate this SCS

   SCS assumes that a key-agreement/distribution mechanism exists for
   environments in which S consists of multiple servers, which provides
   a unique external identifier for each transform set shared amongst
   pool members.

   Such mechanism may safely downgrade to a periodic key-refresh if
   there is only one server in the pool and the key is generated in
   place - i.e. it is not handled by an external source.

   However, when many servers act concurrently upon the same pool, a
   more sophisticated protocol, whose specification is out of the scope
   of the present document, must be devised (ideally one that is able to
   handle key agreement for dynamic peer groups in a secure and
   efficient way, e.g.  [CLIQUES], [Steiner]).

3.1.4.  IV

   Initialization Vector used for the encryption algorithm (see
   Section 3.2).

   In order to avoid providing correlation information to a possible
   attacker with access to a sample of SCS cookies created using the
   same TID, the IV MUST be created randomly for each SCS cookie.

3.1.5.  AUTHTAG

   Authentication tag based on the plain string concatenation of the
   base64url encoded DATA, ATIME, TID and IV fields, framed by the "|"
   separator (see also the definition of the Box() function in
   Section 3.2):

   AUTHTAG = HMAC(base64url(DATA)  "|"
                  base64url(ATIME) "|"
                  base64url(TID)   "|"

   Note that, from a cryptographic point of view, the "|" character
   provides explicit authentication of the length of each supplied
   field, which results in a robust countermeasure against splicing

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3.2.  Crypto Transform

   SCS could potentially use any combination of primitives capable of
   performing authenticated encryption.  In practice an encrypt-then-mac
   approach [Kohno] with CBC-mode encryption and HMAC [RFC2104]
   authentication was chosen.

   The two algorithms MUST be associated with two independent keys.

   The following conventions will be used in the algorithm description
   (Section 3.2.5 and Section 3.2.6):

   o  Enc/Dec(): block encryption/decryption functions (Section 3.2.2);

   o  HMAC(): authentication function (Section 3.2.2);

   o  Comp/Uncomp(): compression/decompression functions
      (Section 3.2.3);

   o  e/d(): cookie value encoding/decoding functions (Section 3.2.4);

   o  RAND(): random number generator [RFC4086];

   o  Box(): string boxing function.  It takes an arbitrary number of
      base64url encoded strings and returns the string obtained by
      concatenating each of the inputs in the exact order in which they
      are listed, separated by the "|" char.  For example:

         Box("akxI", "MTM", "Hadvo") = "akxI|MTM|Hadvo".

3.2.1.  Choice and Role of the Framing Symbol

   Note that that the adoption of "|" as the framing symbol in the Box()
   function is arbitrary: any char allowed by the cookie-value ABNF in
   [RFC6265] is safe to be used as long it has empty intersection with
   the base64url alphabet.

   It is also worth noting that the role of the framing symbol, which
   provides an implicit length indicator for each of the atoms, is key
   to the correctness and security of SCS.

   This is especially relevant when the authentication tag is computed
   (see Section 3.1.5).  More specifically, the explicit inclusion of
   the framing symbol within the HMAC input seals the integrity of the
   blob as a whole together with each of its composing atoms in their
   exact position.

   This feature makes the protocol robust against attacks aimed at

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   disrupting the security of SCS PDUs by freely moving boundaries
   between adjacent atoms.

3.2.2.  Cipher Set

   Implementors MUST support at least the following algorithms:

   o  AES-CBC-128 for encryption [NIST-AES];

   o  HMAC-SHA1 with a 128 bit key for authenticity and integrity,

   which appear to be sufficiently secure in a broad range of use cases
   [Bellare], [RFC6194], are widely available, and can be implemented in
   a few kilobytes of memory, providing an extremely valuable feature
   for constrained devices.

   One should consider using larger cryptographic key lengths (192 or
   256 bit) according to the actual security and overall system
   performance requirements.

3.2.3.  Compression

   Compression, which may be useful or even necessary when handling
   large quantities of data, is not compulsory (in such case Comp/Uncomp
   are replaced by an identity matrix).  If this function is enabled,
   DEFLATE [RFC1951] format MUST be supported.

   Some advice to SCS users: compression should not be enabled when
   handling relatively short and entropic state such as pseudo random
   session identifiers.  Instead, large and quite regular state blobs
   could get a significant boost when compressed.

3.2.4.  Cookie Encoding

   SCS cookie values MUST be encoded using the URL and filename safe
   alphabet (i.e. base64url) defined in section 5 of Base-64 [RFC4648].
   This encoding is very wide-spread, falls smoothly into the encoding
   rules defined in Section 4.1.1 of [RFC6265], and can be safely used
   to supply SCS based authorization tokens within a URI (e.g. in a
   query string or straight into a path segment).

3.2.5.  Outbound Transform

   The output data transformation as seen by the server (the only actor
   which explicitly manipulates SCS cookies) is illustrated by the
   pseudo-code in Figure 2.

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         1.  IV := RAND()
         2.  ATIME := NOW
         3.  DATA := Enc(Comp(plain-text-cookie-value), IV)
         4.  AUTHTAG := HMAC(Box(e(DATA), e(ATIME), e(TID), e(IV)))

                                 Figure 2

   A new Initialization Vector is randomly picked (step 1.).  As
   previously mentioned (Section 3.1.4) this step is necessary to avoid
   providing correlation information to an attacker.

   A new ATIME value is taken as the current timestamp according to the
   server clock (step 2.).

   Since the only user of the ATIME field is the server, it is
   unnecessary for it to be synchronized with the client - though it
   needs to use a fairly stable clock.  However, if multiple servers are
   active in a load-balancing configuration, clocks SHOULD be
   synchronized to avoid errors in the calculation of session expiry.

   The plain text cookie value is then compressed (if needed) and
   encrypted by using the key-set identified by TID (step 3.).

   If the length of (compressed) state is not a multiple of the block
   size, its value MUST be filled with as many padding bytes of equal
   value as the pad length - as defined by the scheme given in Section
   6.3 of [RFC5652].

   Then the authentication tag, which encompasses each SCS field (along
   with lengths, and relative positions) is computed by HMAC'ing the
   "|"-separated concatenation of their base64url representations using
   the key-set identified by TID (step 4.).

   Finally the SCS cookie-value is created as follows:

      scs-cookie-value = Box(e(DATA), e(ATIME), e(TID), e(IV), e(tag))

3.2.6.  Inbound Transform

   The inbound transformation is described in Figure 3.  Each of the
   'e'-prefixed names shown has to be interpreted as the base64url
   encoded value of the corresponding SCS field.

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           0.  If (split_fields(scs-cookie-value) == ok)
           1.      tid' := d(eTID)
           2.      If (tid' is available)
           3.          tag' := d(eAUTHTAG)
           4.          tag := HMAC(Box(eDATA, eATIME, eTID, eIV))
           5.          If (tag = tag')
           6.              atime' := d(eATIME)
           7.              If (NOW - atime' <= session_max_age)
           8.                  iv' := d(eIV)
                               data' := d(eDATA)
           9.                  state := Uncomp(Dec(data', iv'))
           10.             Else discard PDU
           11.         Else discard PDU
           12.     Else discard PDU
           13. Else discard PDU

                                 Figure 3

   First of all, the inbound scs-cookie-value is broken into its
   component fields which MUST be exactly 5, and each at least of the
   minimum length specified in Figure 1 (step 0.).  In case any of these
   preliminary checks fails, the PDU is discarded (step 13.); else TID
   is decoded to allow key-set lookup (step 1.).

   If the cryptographic credentials (encryption and authentication
   algorithms and keys identified by TID) are unavailable (step 12.),
   the inbound SCS cookie is discarded since its value has no chance to
   be interpreted correctly.  This may happen for several reasons: e.g.,
   if a device without storage has been reset and loses the credentials
   stored in RAM, if a server pool node desynchronizes, or in case of a
   key compromise that forces the invalidation of all current TID's,

   When a valid key-set is found (step 2.), the AUTHTAG field is decoded
   (step 3.) and the (still) encoded DATA, ATIME, TID and IV fields are
   supplied to the primitive that computes the authentication tag (step

   If the tag computed using the local key-set matches the one carried
   by the supplied SCS cookie, we can be confident that the cookie
   carries authentic material; otherwise the SCS cookie is discarded
   (step 11.).

   Then the age of the SCS cookie (as deduced by ATIME field value and
   current time provided by the server clock) is decoded and compared to
   the maximum time-to-live defined by the session_max_age parameter.

   If the "age" check passes, the DATA and IV fields are finally decoded

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   (step 8.), so that the original plain text data can be extracted from
   the encrypted and optionally compressed blob (step 9.).

   Note that steps 5. and 7. allow any altered packets or expired
   sessions to be discarded, hence avoiding unnecessary state decryption
   and decompression.

3.3.  PDU Exchange

   SCS can be modeled in the same manner as a typical store-and-forward
   protocol, in which the endpoints are S, consisting of one or more
   HTTP servers, and the client C, an intermediate node used to
   "temporarily" store the data to be successively forwarded to S.

   In brief, S and C exchange an immutable cookie data block
   (Section 3.1): the state is stored on the client at the first hop and
   then restored on the server at the second, as in Figure 4.

     1.  dump-state:
         S --> C
             Set-Cookie: ANY_COOKIE_NAME=KrdPagFes_5ma-ZUluMsww|MTM0...
                Expires=...; Path=...; Domain=...;

     2.  restore-state:
         C --> S
             Cookie: ANY_COOKIE_NAME=KrdPagFes_5ma-ZUluMsww|MTM0...

                                 Figure 4

3.3.1.  Cookie Attributes

   In the following sub paragraphs a series of recommendations is
   provided in order to maximize SCS PDU fitness in the generic cookie
   ecosystem.  Expires

   If an SCS cookie includes an Expires attribute, then the attribute
   MUST be set to a value consistent with session_max_age.

   For maximum compatibility with existing user agents the timestamp
   value MUST be encoded in rfc1123-date format which requires a 4-digit
   year.  Max-Age

   Since not all UAs support this attribute, it MUST NOT be present in
   any SCS cookie.

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   SCS cookies MUST include a Domain attribute compatible with
   application usage.

   A trailing '.'  MUST NOT be present in order to minimize the
   possibility of a user agent ignoring the attribute value.  Secure

   This attribute MUST always be asserted when SCS sessions are carried
   over a TLS channel.  HttpOnly

   This attribute SHOULD always be asserted.

4.  Key Management and Session State

   This specification provides some common recommendations and practices
   relevant to cryptographic key management.

   In the following, the term 'key' references both encryption and HMAC

   o  The key SHOULD be generated securely following the randomness
      recommendations in [RFC4086];

   o  the key SHOULD only be used to generate and verify SCS PDUs;

   o  the key SHOULD be replaced regularly as well as any time the
      format of SCS PDUs or cryptographic algorithms changes.

   Furthermore, to preserve the validity of active HTTP sessions upon
   renewal of cryptographic credentials (whenever the value of TID
   changes), an SCS server MUST be capable of managing at least two
   transforms contemporarily: the currently instantiated one, and its

   Each transform set SHOULD be associated with an attribute pair:
   "refresh" and "expiry", which is used to identify the exposure limits
   (in terms of time or quantity of encrypted and/or authenticated
   bytes, etc) of related cryptographic material.

   In particular, the "refresh" attribute specifies the time limit for
   substitution of transform set T with new material T'.  From that
   moment onwards, and for an amount of time determined by "expiry", all

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   new sessions will be created using T', while the active T-protected
   ones go through a translation phase in which:

   o  the inbound transformation authenticates and decrypts/decompresses
      using T (identified by TID);

   o  the outbound transformation encrypts/compresses and authenticates
      using T'.

        T' {not valid yet} |---------------------|----------------
                           |  translation stage  |
        T  ----------------|---------------------| {no longer valid}
                         refresh         refresh + expiry

                                 Figure 5

   As shown in Figure 5, the duration of the HTTP session MUST fit
   within the lifetime of a given transform set (i.e. from creation time
   until "refresh" + "expiry").

   In practice, this should not be an obstacle because the longevity of
   the two entities (HTTP session and SCS transform set) should differ
   by one or two orders of magnitude.

   An SCS server may take this into account by determining the duration
   of a session adaptively according to the expected deletion time of
   the active T, or by setting the "expiry" value to at least the
   maximum lifetime allowed by an HTTP session.

   Since there is only one refresh attribute also in situations with
   more than one key (e.g. one for encryption and one for
   authentication) within the same T, the smallest value is chosen.

   It is critical for the correctness of the protocol that, in case
   multiple equivalent SCS servers are used in a pool, all of them share
   the same view of time (see also Section 3.2.5) and keying material.

   As far as the latter is concerned, SCS does not mandate the use of
   any specific key sharing mechanism, and will keep working correctly
   as long as the said mechanism is able to provide a single, coherent,
   view of the keys shared by pool members - while conforming to the
   recommendations given in this section.

5.  Cookie Size Considerations

   In general, SCS cookies are bigger than their plain text

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   counterparts.  This is due to the following reasons:

   o  inflation of the Base-64 encoding of state data (approx. 1.4 times
      the original size, including the encryption padding);

   o  the fixed size increment (approx. 80/90 bytes) caused by SCS
      fields and framing overhead.

   While the former is a price the user must always pay proportionally
   to the original data size, the latter is a fixed quantum, which can
   be huge on small amounts of data, but is quickly absorbed as soon as
   data becomes big enough.

   The following table compares byte lengths of SCS cookies (with a four
   byte TID) and corresponding plain text cookies in a worst case
   scenario, i.e. when no compression is in use (or applicable).

                                plain |  SCS
                                  11  |  128
                                 102  |  256
                                 285  |  512
                                 651  | 1024
                                1382  | 2048
                                2842  | 4096

   The largest uncompressed cookie value that can be safely supplied to
   SCS is about 2.8KB.

6.  Acknowledgements

   We would like to thank Jim Schaad, David Wagner, Lorenzo Cavallaro,
   Willy Tarreau, Tobias Gondrom, John Michener, Sean Turner, Barry
   Leiba, Robert Sparks, Stephen Farrell, Stewart Bryant, and Nevil
   Brownlee for their valuable feedback on this document.

7.  IANA Considerations

   This memo includes no request to IANA.

8.  Security Considerations

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8.1.  Security of the Cryptographic Protocol

   From a cryptographic architecture perspective, the described
   mechanism can be easily traced to an "encode then encrypt then MAC"
   scheme (Encode-then-EtM) as described in [Kohno].

   Given a "provably-secure" encryption scheme and MAC (as for the
   algorithms mandated in Section 3.2.2), Kohno et al.  [Kohno]
   demonstrate that their composition results in a secure authenticated
   encryption scheme.

8.2.  Impact of the SCS Cookie Model

   The fact that the server does not own the cookie it produces, gives
   rise to a series of consequences that must be clearly understood when
   one envisages the use of SCS as a cookie provider and validator for
   his/her application.

   In the following paragraphs, a set of different attack scenarios
   (together with corresponding countermeasures where applicable) are
   identified and analyzed.

8.2.1.  Old cookie replay

   SCS doesn't address replay of old cookie values.

   In fact, there is nothing that guarantees an SCS application about
   the client having returned the most recent version of the cookie.

   As with "server-side" sessions, if an attacker gains possession of a
   given user's cookies - via simple passive interception or another
   technique - he/she will always be able to restore the state of an
   intercepted session by representing the captured data to the server.

   The ATIME value along with the session_max_age configuration
   parameter allow SCS to mitigate the chances of an attack (by forcing
   a time window outside of which a given cookie is no longer valid),
   but cannot exclude it completely.

   A countermeasure against the "passive interception and replay"
   scenario can be applied at transport/network level using the anti-
   replay services provided by e.g., SSL/TLS [RFC5246] or IPsec

   A native solution is not in scope with the security properties
   inherent to an SCS cookie.  Hence, application wishing to be replay-
   resistant must put in place some ad hoc mechanism to prevent clients
   (both rogue and legitimate) from (a) being able to replay old cookies

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   as valid credentials and/or (b) getting any advantage by replaying

   In the following, some typical use cases are illustrated:

   o  Session inactivity timeout scenario (implicit invalidation): use
      the session_max_age parameter if a global setting is viable, else
      place an explicit TTL in the cookie (e.g.
      validity_period="start_time, duration") that can be verified by
      the application each time the Client presents the SCS cookie.

   o  Session voidance scenario (explicit invalidation): put a randomly
      chosen string into each SCS cookie (cid="$(random())") and keep a
      list of valid session cid's against which the SCS cookie presented
      by the client can be checked.  When a cookie needs to be
      invalidated, delete the corresponding cid from the list.  The
      described method has the drawback that, in case a non-permanent
      storage is used to archive valid cid's, a reboot/restart would
      invalidate all sessions (It can't be used when |S| > 1).

   o  One-shot transaction scenario (ephemeral): this is a variation on
      the previous theme when sessions are consumed within a single
      request/response.  Put a nonce="$(random())" within the state
      information and keep a list of not-yet-consumed nonces in RAM.
      Once the client presents its cookie credential, the embodied nonce
      is deleted from the list and will be therefore discarded whenever

   o  TLS binding scenario: the Server application must run on TLS, be
      able to extract information related to the current TLS session,
      and store it in the DATA field of the SCS cookie itself [RFC5056].
      The establishment of this secure channel binding prevents any
      third party from reusing the SCS cookie, and drops its value
      altogether after the TLS session is terminated - regardless of the
      lifetime of the cookie.  This approach suffers a scalability
      problem in that it requires each SCS session to be handled by the
      same client-server pair.  However, it provides a robust model and
      an affordable compromise when security of the session is
      exceptionally valuable (e.g. a user interacting with his/her
      online banking site).

   It is worth noting that in all but the latter scenario, if an
   attacker is able to use the cookie before the legitimate Client gets
   a chance to, then the impersonation attack will always succeed.

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8.2.2.  Cookie Deletion

   A direct, and important, consequence of the missing owner role in SCS
   is that a client could intentionally delete its cookie and return

   The application protocol has to be designed so there is no incentive
   to do so, for instance:

   o  it is safe for the cookie to represent some kind of positive
      capability - the possession of which increases the client's

   o  it is not safe to use the cookie to represent negative
      capabilities - where possession reduces the client's powers - or
      for revocation.

   Note that this behavior is not equivalent to cookie removal in the
   "server-side" cookie model, because in case of missing cookie backup
   by other parties (e.g. the application using SCS), the Client could
   simply make it disappear once and for all.

8.2.3.  Cookie Sharing or Theft

   Just like with plain cookies, SCS doesn't prevent sharing (both
   voluntary and illegitimate) of cookies between multiple clients.

   In the context of voluntary cookie sharing, using HTTPS only as a
   separate secure transport provider is useless: in fact, Client
   certificates are just as shareable as cookies.  Instead, using some
   form of secure channel binding (as illustrated in Section 8.2.1) may
   cancel this risk.

   The risk of theft could be mitigated by securing the wire (e.g. via
   HTTPS, IPsec, VPN, ...), thus reducing the opportunity of cookie
   stealing to a successful attack on the protocol endpoints.

   In order to reduce the attack window on stolen cookies, an
   application may choose to generate cookies whose lifetime is upper
   bounded by the browsing session lifetime (i.e. by not attaching an
   Expires attribute to them.)

8.2.4.  Session Fixation

   Session fixation vulnerabilities [Kolsec] are not addressed by SCS.

   A more sophisticated protocol involving active participation of the
   UA in the SCS cookie manipulation process would be needed: e.g. some

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   form of challange-response exchange initiated by the Server in the
   HTTP response and replied to by the UA in the next chained HTTP

   Unfortunately the present specification which is based on [RFC6265]
   sees the UA as a completely passive actor, whose role is to blindly
   paste the cookie value set by the Server.

   Nevertheless, the SCS cookies wrapping mechanism may be used in the
   future as a building block for a more robust HTTP state management

8.3.  Advantages of SCS over Server-side Sessions

   Note that all the above-mentioned vulnerabilities also apply to plain
   cookies, making SCS at least as secure, but there are a few good
   reasons to consider its security level enhanced.

   First of all, the confidentiality and authentication features
   provided by SCS protect the cookie-value which is normally plain text
   and tamperable.

   Furthermore, neither of the common vulnerabilities of server-side
   sessions (SID prediction and SID brute forcing) can be exploited when
   using SCS, unless the attacker possesses encryption and HMAC keys
   (both current ones and those relating to the previous set of

   More in general, no slicing nor altering operations can be done over
   an SCS PDU without controlling the cryptographic key-set.

9.  References

9.1.  Normative References

              "Advanced Encryption Standard (AES)", NIST, FIPS PUB 197,
              November 2001.

   [RFC1951]  Deutsch, P., "DEFLATE Compressed Data Format Specification
              version 1.3", RFC 1951, May 1996.

   [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

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              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.

   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, September 2009.

   [RFC6194]  Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
              Considerations for the SHA-0 and SHA-1 Message-Digest
              Algorithms", RFC 6194, March 2011.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              April 2011.

9.2.  Informative References

   [Bellare]  Bellare, M., "New Proofs for NMAC and HMAC: Security
              Without Collision-Resistance", 2006.

   [CLIQUES]  Steiner, M., Tsudik, G., and M. Waidner, "Cliques: A New
              Approach to Group Key Agreement", 1996.

   [Kohno]    Kohno, T., Palacio, A., and J. Black, "Building Secure
              Cryptographic Transforms, or How to Encrypt and MAC",

   [Kolsec]   Kolsec, M., "Session Fixation Vulnerability in Web-based
              Applications", 2002.

   [RFC3740]  Hardjono, T. and B. Weis, "The Multicast Group Security
              Architecture", RFC 3740, March 2004.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
              Channels", RFC 5056, November 2007.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

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   [Steiner]  Steiner, M., Tsudik, G., and M. Waidner, "Diffie-Hellman
              Key Distribution Extended to Group Communication", 1996.

Appendix A.  Examples

   The examples in this section have been created using the 'scs' test
   tool bundled with LibSCS, a free and opensource reference
   implementation of the SCS protocol that can be found at

A.1.  No Compression

   The following parameters:

   o  Plain text cookie: "a state string"

   o  AES-CBC-128 key: "123456789abcdef"

   o  HMAC-SHA1 key: "12345678901234567890"

   o  TID: "tid"

   o  ATIME: 1347265955

   o  IV:

   produce the following tokens:

   o  DATA: DqfW4SFqcjBXqSTvF2qnRA


   o  TID: OHU7M1cqdDQt

   o  IV: tL3lJPf2nUSFMN6dtVXJTw

   o  AUTHTAG: AznYHKga9mLL8ioi3If_1iy2KSA

A.2.  Use Compression

   The same parameters as above, except ATIME and IV:

   o  Plain text cookie: "a state string"

   o  AES-CBC-128 key: "123456789abcdef"

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   o  HMAC-SHA1 key: "12345678901234567890"

   o  TID: "tid"

   o  ATIME: 1347281709

   o  IV:

   produce the following tokens:

   o  DATA: PbE-ypmQ43M8LzKZ6fMwFg-COrLP2l-Bvgs


   o  TID: akxIKmhbMTE8

   o  IV: HadvoP8R15XjS_up_2X5xw

   o  AUTHTAG: A6qevPr-ugHQChlr_EiKYWPvpB0

   In both cases, the resulting SCS cookie is obtained via ordered
   concatenation of the produced tokens, as described in Section 3.1.

Authors' Addresses

   Stefano Barbato
   Via Marmolada, 4
   Vitorchiano (VT),   01030

   Email: tat@koanlogic.com

   Steven Dorigotti
   Via Maso della Pieve 25/C
   Bolzano,   39100

   Email: stewy@koanlogic.com

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   Thomas Fossati (editor)
   Via di Sabbiuno 11/5
   Bologna,   40136

   Phone: +39 051 644 82 68
   Email: tho@koanlogic.com

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