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Versions: 00 01 02 03 04 05 06 07 08 09 RFC 6896

Internet Engineering Task Force                               S. Barbato
Internet-Draft                                              S. Dorigotti
Intended status: Informational                           T. Fossati, Ed.
Expires: June 22, 2012                                         KoanLogic
                                                       December 20, 2011


                  SCS: Secure Cookie Sessions for HTTP
                draft-secure-cookie-session-protocol-03

Abstract

   This document provides an overview of SCS, a small cryptographic
   protocol layered on top of the HTTP cookie facility, that allows its
   users to produce and consume authenticated and encrypted cookies, as
   opposed to usual cookies, which are un-authenticated and sent in
   clear text.

   An interesting property, rising naturally from the given
   confidentiality and authentication properties, is that by using SCS
   cookies, it is possible to avoid storing the session state material
   on the server side altogether.  In fact, an SCS cookie presented by
   the user agent to the origin server can always be validated (i.e.
   possibly recognized as self-produced, untampered material) and, as
   such, be used to safely restore application state.

   Hence, typical use cases may include devices with little or no
   storage offering some functionality via an HTTP interface, as well as
   web applications with high availability or load balancing
   requirements which would prefer to handle application state without
   the need to synchronize the pool through shared storage or peering.

   Nevertheless, its security properties allow SCS to be used whenever
   the privacy and integrity of cookies is a concern, by paying an
   affordable price in terms of increased cookie size, additional CPU
   clock cycles needed by the symmetric key encryption and HMAC
   algorithms, and related key management, which can be made a nearly
   transparent task.

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/.



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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on June 22, 2012.

Copyright Notice

   Copyright (c) 2011 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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   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  . . . . . . . . . . . . . . . . . . . .  5
   3.  SCS Protocol . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  SCS Cookie Description . . . . . . . . . . . . . . . . . .  5
       3.1.1.  ATIME  . . . . . . . . . . . . . . . . . . . . . . . .  6
       3.1.2.  DATA . . . . . . . . . . . . . . . . . . . . . . . . .  6
       3.1.3.  TID  . . . . . . . . . . . . . . . . . . . . . . . . .  6
       3.1.4.  IV . . . . . . . . . . . . . . . . . . . . . . . . . .  7
       3.1.5.  AUTHTAG  . . . . . . . . . . . . . . . . . . . . . . .  7
     3.2.  Crypto Transform . . . . . . . . . . . . . . . . . . . . .  7
       3.2.1.  Cipher Set . . . . . . . . . . . . . . . . . . . . . .  8
       3.2.2.  Compression  . . . . . . . . . . . . . . . . . . . . .  8
       3.2.3.  Cookie Encoding  . . . . . . . . . . . . . . . . . . .  8
       3.2.4.  Outbound Transform . . . . . . . . . . . . . . . . . .  8
       3.2.5.  Inbound Transform  . . . . . . . . . . . . . . . . . .  9
     3.3.  PDU Exchange . . . . . . . . . . . . . . . . . . . . . . . 11
       3.3.1.  Cookie Attributes  . . . . . . . . . . . . . . . . . . 11
         3.3.1.1.  Expires  . . . . . . . . . . . . . . . . . . . . . 11
         3.3.1.2.  Max-Age  . . . . . . . . . . . . . . . . . . . . . 11
         3.3.1.3.  Domain . . . . . . . . . . . . . . . . . . . . . . 12
         3.3.1.4.  Secure . . . . . . . . . . . . . . . . . . . . . . 12
   4.  Key Management and Session State . . . . . . . . . . . . . . . 12
   5.  Cookie Size Considerations . . . . . . . . . . . . . . . . . . 13
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
     8.1.  Security of the Cryptographic Protocol . . . . . . . . . . 14
     8.2.  Impact of the SCS Cookie Model . . . . . . . . . . . . . . 14
       8.2.1.  Old cookie replay  . . . . . . . . . . . . . . . . . . 15
       8.2.2.  Cookie Deletion  . . . . . . . . . . . . . . . . . . . 16
       8.2.3.  Cookie Sharing or Theft  . . . . . . . . . . . . . . . 16
       8.2.4.  Session Fixation . . . . . . . . . . . . . . . . . . . 17
     8.3.  Advantages of SCS over Server-side Sessions  . . . . . . . 17
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 17
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 18
   Appendix A.  Examples  . . . . . . . . . . . . . . . . . . . . . . 18
     A.1.  No Compression . . . . . . . . . . . . . . . . . . . . . . 19
     A.2.  Use Compression  . . . . . . . . . . . . . . . . . . . . . 19
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20









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1.  Introduction

   SCS is a small cryptographic protocol layered on top of the HTTP
   cookie facility [RFC6265], that allows its users to produce and
   consume authenticated and encrypted cookies, as opposed to usual
   cookies, which are un-authenticated and sent in clear text.

   By having a non-tamperable proof of authorship attached, each SCS
   cookie can always be validated by the originator, making it possible
   for a server to handle clients' session state without the need to
   store it locally.  In fact, an SCS enabled server could completely
   delegate the application state storage to the client (e.g. a web
   browser) and use it, in all respects, as a remote storage device.
   The result of the cryptographic transformations applied to state data
   can be used to ensure that its information authenticity and
   confidentiality attributes are the same as if they were stored
   privately on server-side.

   The no-storage requirement, which is the key design constraint of
   SCS, makes it an ideal candidate in the following settings:

   a.  devices with little or no storage -- typically embedded devices
       which provide functionality such as software updates,
       configuration, device monitoring, etc. via an HTTP interface;

   b.  web applications with high availability or load balancing
       requirements, which may delegate handling of the application
       state to clients instead of using shared storage or forced
       peering, to enhance overall parallelism.

   It is worth noting that a peculiar difference between SCS, when used
   in strict no-storage mode, and usual "server-side" cookie sessions
   arises as soon as we carefully consider the roles of the playing
   entities.  In the "server-side" model, the server acts a triple role
   as the "generator", the "owner", and the "verifier" of cookie
   credentials.  Instead, a server implementing SCS in no-storage mode,
   acts the "generator" and "verifier" roles only -- the "owner" being
   inapplicable for obvious reasons.

   In all respects, the Server grants the custody of the generated
   cookie to the Client, whose trust model needs to be taken into
   consideration when designing applications that use SCS this way.  The
   consequences of such discrepancy (e.g. deliberate deletion of a
   cookie, explicit privilege revocation, etc.) will be analyzed in
   Section 8.2.

   An SCS server can be implemented within a web application by means of
   a user library that exposes the core SCS functionality and leaves



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   explicit control over SCS cookies to the programmer, or
   transparently, by hiding, for example, a "diskless session" facility
   behind a generic session API abstraction.  SCS implementers are free
   to choose the model that best suites their needs.


2.  Requirements Language

   The 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 [RFC2119].


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 (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 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
   capabilities.

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
   Base-64 encoding of a specific SCS field, 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 = DATA "|" ATIME "|" TID "|" IV "|" AUTHTAG
   DATA             = 4*base64-character
   ATIME            = 4*base64-character
   TID              = 4*base64-character
   IV               = 4*base64-character
   AUTHTAG          = 4*base64-character

   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 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 an expiration error may be used to force user logout from an SCS
   cookie based session, or hooked in the web application logics to the
   display of a HTML form asking 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 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 wasn't used.

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



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   cookie.

   SCS assumes that a key-agreement/distribution mechanism exists for
   environments in which S consists of multiple servers (a simple key-
   refresh in case |S|=1), which provides a unique external identifier
   for each transform set shared amongst pool members.

3.1.4.  IV

   Initialization Vector used for the encryption algorithm
   (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 concatenation of DATA, ATIME, TID and
   IV fields encoded in Base-64, framed by the "|" separator:

   AUTHTAG = HMAC(base64(DATA)  || "|" ||
                  base64(ATIME) || "|" ||
                  base64(TID)   || "|" ||
                  base64(IV))

   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
   attacks.

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 3.2.4 and Section 3.2.5):

   o  Enc/Dec(): block encryption/decryption functions (Section 3.2.1);

   o  HMAC(): authentication function (Section 3.2.1);





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   o  Comp/Uncomp(): compression/decompression functions
      (Section 3.2.2);

   o  e/d(): cookie value encoding/decoding functions (Section 3.2.3);

   o  ||: concatenation operator, i.e. "|" char;

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

3.2.1.  Cipher Set

   Implementors MUST support at least the following algorithms:

   o  AES-CBC-128 for encryption;

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

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

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

3.2.2.  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.

3.2.3.  Cookie Encoding

   Base-64 [RFC4648] is used for encoding/decoding of SCS cookie values.
   It is very wide-spread, and falls smoothly into the encoding rules
   defined in Section 4.1.1 of [RFC6265].

3.2.4.  Outbound Transform

   The output data transformation as seen by the server (the only actor
   which explicitly manipulates SCS cookies) is illustrated by the



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   pseudo-code in Figure 1.

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

                                 Figure 1

   A new Initialization Vector is randomly picked (step 1.).  As
   previouslty 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 be 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 keyset 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 in the scheme of 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 base64 representations using the
   keyset identified by TID (step 4.).

   Finally the SCS cookie-value is created as follows:

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

3.2.5.  Inbound Transform

   The inbound transformation is described in Figure 2.









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           0.  split_fields(scs-cookie-value)
           1.  tid' := d(e(TID))
           2.  If (tid' is available)
           3.      tag' := d(e(AUTHTAG))
           4.      tag := HMAC(e(DATA)||e(ATIME)||e(TID)||e(IV))
           5.      If (tag = tag')
           6.          atime' := d(e(ATIME))
           7.          If (NOW - atime' <= session_max_age)
           8.              iv' := d(e(IV))
                           data' := d(e(DATA))
           9.              state := Uncomp(Dec(data', iv'))
           10.         Else discard PDU
           11.     Else discard PDU
           12. Else discard PDU

                                 Figure 2

   First of all, the inbound scs-cookie-value is broken into its
   component fields (step 0.), and TID is decoded to allow keyset 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 as 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,
   etc.

   When a valid keyset 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
   4.).

   If the tag computed using the local keyset 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.

   In case the "age" check is passed, the DATA and IV fields are finally
   decoded (step 8.), so that the original plain text data can be
   extracted from the encrypted and optionally compressed blob (step
   9.).




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   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 3.

    1.  dump-state:
        S --> C
            Set-Cookie: ANY_COOKIE_NAME=BO2zHC0tRg76axnguyuK5g==|MTI5...
               Expires=...; Path=...; Domain=...;

    2.  restore-state:
        C --> S
            Cookie: ANY_COOKIE_NAME=BO2zHC0tRg76axnguyuK5g==|MTI5...

                                 Figure 3

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.

3.3.1.1.  Expires

   SCS cookies MUST include an Expires attribute which shall 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.

3.3.1.2.  Max-Age

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






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3.3.1.3.  Domain

   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.

3.3.1.4.  Secure

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


4.  Key Management and Session State

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

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

   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
   predecessor.

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





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   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 4

   As shown in Figure 4, 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.


5.  Cookie Size Considerations

   In general, SCS cookies are bigger than their plain text
   counterparts.  This is due to a couple of different factors:

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

   o  the fixed size increment (approx. 80/90 bytes) due to 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.




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   The following table compares byte lengths of SCS cookies (with a four
   bytes' 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 David Wagner and Lorenzo Cavallaro for their
   valuable feedback on this document.


7.  IANA Considerations

   This memo includes no request to IANA.


8.  Security Considerations

8.1.  Security of the Cryptographic Protocol

   From a cryptographic architecture perspective, the described
   mechanism can be easily traced to an Encode-then-EtM scheme described
   in [Kohno].

   Given a "provably-secure" encryption scheme and MAC (as for the
   algorithms mandated in Section 3.2.1), 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.




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   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
   [RFC4301].

   Anyway, a generic solution is still out of scope: an SCS 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 as valid credentials and/or (b)
   getting any advantage by replaying them.

   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).



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   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
      replayed.

   It may be noteworthy that despite the chances of preventing replay in
   some well established circumstances by using aforementioned
   mechanisms, if the attacker is able to use the cookie before the
   legitimate client gets a chance to, then the impersonation attack
   will always succeed.

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
   nothing.

   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
      powers;

   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 is useless:
   Client certificates are just as shareable as cookies, hence
   equivalently to the "server-side" cookie model, there seems to be no
   way to prevent this threat.

   The theft could be mitigated by securing the wire (e.g. via HTTPS,
   IPsec, VPN, ...), thus reducing the opportunity of cookie stealing to



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   a successful attack on the protocol endpoints.

8.2.4.  Session Fixation

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

   A more sophisticated protocol involving an active participation by
   the UA in the SCS cookie manipulation would be needed: e.g. some form
   of challange-response exchange initiated by the Server on the HTTP
   response and replied by the UA on the next chained HTTP request.

   Unfortunately the present specification which bases on [RFC6265] sees
   the UA as a completely passive character, 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
   protocol.

8.3.  Advantages of SCS over Server-side Sessions

   Note that all the abovementioned 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 protects the cookie-value which is normally plain
   text and tamperable.

   Furthermore, none of the common vulnerabilities of server-side
   sessions (SID prediction, 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
   credentials).

   More generally no slicing nor altering operations can be done over an
   SCS PDU without controlling the cryptographic keyset.


9.  References

9.1.  Normative References

   [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,



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              February 1997.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [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.

   [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.

   [Kohno]    Kohno, T., Palacio, A., and J. Black, "Building Secure
              Cryptographic Transforms, or How to Encrypt and MAC",
              2003.

   [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.

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


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
   <http://github.com/koanlogic/libscs>.






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A.1.  No Compression

   The following parameters:

   o  Plain text cookie: "a state string"

   o  AES-CBC-128 key: "cipher key"

   o  HMAC-SHA1 key: "hmac key"

   o  TID: "tid"

   o  ATIME: 1323898800

   o  IV:
      \xd1\x02\xfc\xca\xbf\x05\x03\xb1\xf4\x4f\x1f\xfd\x6d\x12\x5c\x66

   produce the following tokens:

   o  DATA: GJRz3N0cuPKTumCqjtVjgw==

   o  ATIME: MTMyMzg5ODgwMA==

   o  TID: dGlk

   o  IV: 0QL8yr8FA7H0Tx/9bRJcZg==

   o  AUTHTAG: ktKOYXnTjrCzXgxGH//dWXUZAJ8=

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: "cipher key"

   o  HMAC-SHA1 key: "hmac key"

   o  TID: "tid"

   o  ATIME: 1323899388

   o  IV:
      \x72\x6f\x00\x2e\x4c\xf3\x6d\xfd\xf1\x1f\x92\xcf\x12\x8e\xe7\x8b

   produce the following tokens:




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   o  DATA: XaLWZDoFmv9vYF8wYYYxeXtCkUYAwbzpCfBWBzAy3Y8=

   o  ATIME: MTMyMzg5OTM4OA==

   o  TID: dGlk

   o  IV: cm8ALkzzbf3xH5LPEo7niw==

   o  AUTHTAG: K/rig5ZxGz/aGPQkyAb8JRMcTUY=

   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
   KoanLogic
   Via Marmolada, 4
   Vitorchiano (VT),   01030
   Italy

   Email: tat@koanlogic.com


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

   Email: stewy@koanlogic.com


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

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









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