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Network Working Group                                            F. Gont
Internet-Draft                                    SI6 Networks / UTN-FRH
Intended status: Best Current Practice                           I. Arce
Expires: September 1, 2018                                     Quarkslab
                                                       February 28, 2018


           On the Generation of Transient Numeric Identifiers
                  draft-gont-numeric-ids-generation-02

Abstract

   This document performs an analysis of the security and privacy
   implications of different types of "numeric identifiers" used in IETF
   protocols, and tries to categorize them based on their
   interoperability requirements and the associated failure severity
   when such requirements are not met.  Subsequently, it provides advice
   on possible algorithms that could be employed to satisfy the
   interoperability requirements of each identifier type, while
   minimizing the security and privacy implications, thus providing
   guidance to protocol designers and protocol implementers.  Finally,
   describes a number of algorithms that have been employed in real
   implementations to generate transient numeric identifiers and
   analyzes their security and privacy properties.

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
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   This Internet-Draft will expire on September 1, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may not be modified, and derivative works of it may not
   be created, and it may not be published except as an Internet-Draft.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Threat Model  . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Issues with the Specification of Identifiers  . . . . . . . .   5
   5.  Protocol Failure Severity . . . . . . . . . . . . . . . . . .   6
   6.  Categorizing Identifiers  . . . . . . . . . . . . . . . . . .   6
   7.  Common Algorithms for Identifier Generation . . . . . . . . .   9
     7.1.  Category #1: Uniqueness (soft failure)  . . . . . . . . .   9
     7.2.  Category #2: Uniqueness (hard failure)  . . . . . . . . .  10
     7.3.  Category #3: Uniqueness, constant within context (soft-
           failure)  . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.4.  Category #4: Uniqueness, monotonically increasing within
           context (hard failure)  . . . . . . . . . . . . . . . . .  12
   8.  Common Vulnerabilities Associated with Transient Numeric
       Identifiers . . . . . . . . . . . . . . . . . . . . . . . . .  17
     8.1.  Network Activity Correlation  . . . . . . . . . . . . . .  17
     8.2.  Information Leakage . . . . . . . . . . . . . . . . . . .  17
     8.3.  Exploitation of Semantics of Transient Numeric
           Identifiers . . . . . . . . . . . . . . . . . . . . . . .  19
     8.4.  Exploitation of Collisions of Transient Numeric
           Identifiers . . . . . . . . . . . . . . . . . . . . . . .  19
   9.  Vulnerability Analysis of Specific Transient Numeric
       Identifiers Categories  . . . . . . . . . . . . . . . . . . .  19
     9.1.  Category #1: Uniqueness (soft failure)  . . . . . . . . .  19
     9.2.  Category #2: Uniqueness (hard failure)  . . . . . . . . .  20
     9.3.  Category #3: Uniqueness, constant within context (soft
           failure)  . . . . . . . . . . . . . . . . . . . . . . . .  20
     9.4.  Category #4: Uniqueness, monotonically increasing within
           context (hard failure)  . . . . . . . . . . . . . . . . .  20
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  22
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  22
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22



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     13.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     13.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Appendix A.  Flawed Algorithms  . . . . . . . . . . . . . . . . .  27
     A.1.  Predictable Linear Identifiers Algorithm  . . . . . . . .  27
     A.2.  Random-Increments Algorithm . . . . . . . . . . . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30

1.  Introduction

   Network protocols employ a variety of numeric identifiers for
   different protocol entities, ranging from DNS Transaction IDs (TxIDs)
   to transport protocol numbers (e.g.  TCP ports) or IPv6 Interface
   Identifiers (IIDs).  These identifiers usually have specific
   properties that must be satisfied such that they do not result in
   negative interoperability implications (e.g. uniqueness during a
   specified period of time), and associated failure severities when
   such properties are not met, ranging from soft to hard failures.

   For more than 30 years, a large number of implementations of the TCP/
   IP protocol suite have been subject to a variety of attacks, with
   effects ranging from Denial of Service (DoS) or data injection, to
   information leakage that could be exploited for pervasive monitoring
   [RFC7528].  The root of these issues has been, in many cases, the
   poor selection of identifiers in such protocols, usually as a result
   of an insufficient or misleading specification.  While it is
   generally trivial to identify an algorithm that can satisfy the
   interoperability requirements for a given identifier, there exists
   practical evidence that doing so without negatively affecting the
   security and/or privacy properties of the aforementioned protocols is
   prone to error.

   For example, implementations have been subject to security and/or
   privacy issues resulting from:

   o  Predictable TCP sequence numbers

   o  Predictable transport protocol numbers

   o  Predictable IPv4 or IPv6 Fragment Identifiers

   o  Predictable IPv6 IIDs

   o  Predictable DNS TxIDs

   Recent history indicates that when new protocols are standardized or
   new protocol implementations are produced, the security and privacy
   properties of the associated identifiers tend to be overlooked and
   inappropriate algorithms to generate identifier values are either



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   suggested in the specification or selected by implementers.  As a
   result, we believe that advice in this area is warranted.

   This document contains a non-exhaustive survey of identifiers
   employed in various IETF protocols, and aims to categorize such
   identifiers based on their interoperability requirements, and the
   associated failure severity when such requirements are not met.
   Subsequently, it provides advice on possible algorithms that could be
   employed to satisfy the interoperability requirements of each
   category, while minimizing the associated security and privacy
   implications.  Finally, it analyzes several algorithms that have been
   employed in real implementations to meet such requirements and
   analyzes their security and privacy properties.

2.  Terminology

   Identifier:
      A data object in a protocol specification that can be used to
      definitely distinguish a protocol object (a datagram, network
      interface, transport protocol endpoint, session, etc) from all
      other objects of the same type, in a given context.  Identifiers
      are usually defined as a series of bits and represented using
      integer values.  We note that different identifiers may have
      additional requirements or properties depending on their specific
      use in a protocol.  We use the term "identifier" as a generic term
      to refer to any data object in a protocol specification that
      satisfies the identification property stated above.

   Failure Severity:
      The consequences of a failure to comply with the interoperability
      requirements of a given identifier.  Severity considers the worst
      potential consequence of a failure, determined by the system
      damage and/or time lost to repair the failure.  In this document
      we define two types of failure severity: "soft" and "hard".

   Hard Failure:
      A hard failure is a non-recoverable condition in which a protocol
      does not operate in the prescribed manner or it operates with
      excessive degradation of service.  For example, an established TCP
      connection that is aborted due to an error condition constitutes,
      from the point of view of the transport protocol, a hard failure,
      since it enters a state from which normal operation cannot be
      recovered.

   Soft Failure:
      A soft failure is a recoverable condition in which a protocol does
      not operate in the prescribed manner but normal operation can be
      resumed automatically in a short period of time.  For example, a



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      simple packet-loss event that is subsequently recovered with a
      retransmission can be considered a soft failure.

   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 RFC 2119 [RFC2119].

3.  Threat Model

   Throughout this document, we assume an attacker does not have
   physical or logical device to the device(s) being attacked.  We
   assume the attacker can simply send any traffic to the target
   devices, to e.g. sample identifiers employed by such devices.

4.  Issues with the Specification of Identifiers

   While assessing protocol specifications regarding the use of
   identifiers, we found that most of the issues discussed in this
   document arise as a result of one of the following:

   o  Protocol specifications which under-specify the requirements for
      their identifiers

   o  Protocol specifications that over-specify their identifiers

   o  Protocol implementations that simply fail to comply with the
      specified requirements

   A number of protocol implementations (too many of them) simply
   overlook the security and privacy implications of identifiers.
   Examples of them are the specification of TCP port numbers in
   [RFC0793], the specification of TCP sequence numbers in [RFC0793], or
   the specification of the DNS TxID in [RFC1035].

   On the other hand, there are a number of protocol specifications that
   over-specify some of their associated protocol identifiers.  For
   example, [RFC4291] essentially results in link-layer addresses being
   embedded in the IPv6 Interface Identifiers (IIDs) when the
   interoperability requirement of uniqueness could be achieved in other
   ways that do not result in negative security and privacy implications
   [RFC7721].  Similarly, [RFC2460] suggests the use of a global counter
   for the generation of Fragment Identification values, when the
   interoperability properties of uniqueness per {Src IP, Dst IP} could
   be achieved with other algorithms that do not result in negative
   security and privacy implications.

   Finally, there are protocol implementations that simply fail to
   comply with existing protocol specifications.  For example, some



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   popular operating systems (notably Microsoft Windows) still fail to
   implement randomization of transport protocol ephemeral ports, as
   specified in [RFC6056].

5.  Protocol Failure Severity

   Section 2 defines the concept of "Failure Severity" and two types of
   failures that we employ throughout this document: soft and hard.

   Our analysis of the severity of a failure is performed from the point
   of view of the protocol in question.  However, the corresponding
   severity on the upper application or protocol may not be the same as
   that of the protocol in question.  For example, a TCP connection that
   is aborted may or may not result in a hard failure of the upper
   application: if the upper application can establish a new TCP
   connection without any impact on the application, a hard failure at
   the TCP protocol may have no severity at the application level.  On
   the other hand, if a hard failure of a TCP connection results in
   excessive degradation of service at the application layer, it will
   also result in a hard failure at the application.

6.  Categorizing Identifiers

   This section includes a non-exhaustive survey of identifiers, and
   proposes a number of categories that can accommodate these
   identifiers based on their interoperability requirements and their
   failure modes (soft or hard)
























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   +------------+--------------------------------------+---------------+
   | Identifier |    Interoperability Requirements     |    Failure    |
   |            |                                      |    Severity   |
   +------------+--------------------------------------+---------------+
   | IPv6 Frag  |   Uniqueness (for IP address pair)   | Soft/Hard (1) |
   |     ID     |                                      |               |
   +------------+--------------------------------------+---------------+
   |  IPv6 IID  | Uniqueness (and constant within IPv6 |    Soft (3)   |
   |            |             prefix) (2)              |               |
   +------------+--------------------------------------+---------------+
   |  TCP SEQ   |       Monotonically-increasing       |    Hard (4)   |
   +------------+--------------------------------------+---------------+
   |  TCP eph.  |    Uniqueness (for connection ID)    |      Hard     |
   |    port    |                                      |               |
   +------------+--------------------------------------+---------------+
   | IPv6 Flow  |              Uniqueness              |    None (5)   |
   |     L.     |                                      |               |
   +------------+--------------------------------------+---------------+
   |  DNS TxID  |              Uniqueness              |    None (6)   |
   +------------+--------------------------------------+---------------+

                      Table 1: Survey of Identifiers

   Notes:

   (1)
      While a single collision of Fragment ID values would simply lead
      to a single packet drop (and hence a "soft" failure), repeated
      collisions at high data rates might trash the Fragment ID space,
      leading to a hard failure [RFC4963].

   (2)
      While the interoperability requirements are simply that the
      Interface ID results in a unique IPv6 address, for operational
      reasons it is typically desirable that the resulting IPv6 address
      (and hence the corresponding Interface ID) be constant within each
      network [I-D.ietf-6man-default-iids] [RFC7217].

   (3)
      While IPv6 Interface IDs must result in unique IPv6 addresses,
      IPv6 Duplicate Address Detection (DAD) [RFC4862] allows for the
      detection of duplicate Interface IDs/addresses, and hence such
      Interface ID collisions can be recovered.

   (4)
      In theory there are no interoperability requirements for TCP
      sequence numbers, since the TIME-WAIT state and TCP's "quiet time"
      take care of old segments from previous incarnations of the



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      connection.  However, a widespread optimization allows for a new
      incarnation of a previous connection to be created if the Initial
      Sequence Number (ISN) of the incoming SYN is larger than the last
      sequence number seen in that direction for the previous
      incarnation of the connection.  Thus, monotonically-increasing TCP
      sequence numbers allow for such optimization to work as expected
      [RFC6528].

   (5)
      The IPv6 Flow Label is typically employed for load sharing
      [RFC7098], along with the Source and Destination IPv6 addresses.
      Reuse of a Flow Label value for the same set {Source Address,
      Destination Address} would typically cause both flows to be
      multiplexed into the same link.  However, as long as this does not
      occur deterministically, it will not result in any negative
      implications.

   (6)
      DNS TxIDs are employed, together with the Source Address,
      Destination Address, Source Port, and Destination Port, to match
      DNS requests and responses.  However, since an implementation
      knows which DNS requests were sent for that set of {Source
      Address, Destination Address, Source Port, and Destination Port,
      DNS TxID}, a collision of TxID would result, if anything, in a
      small performance penalty (the response would be discarded when it
      is found that it does not answer the query sent in the
      corresponding DNS query).

   Based on the survey above, we can categorize identifiers as follows:

   +-----+---------------------------------------+---------------------+
   | Cat |                Category               |   Sample Proto IDs  |
   |  #  |                                       |                     |
   +-----+---------------------------------------+---------------------+
   |  1  |       Uniqueness (soft failure)       |  IPv6 Flow L., DNS  |
   |     |                                       |        TxIDs        |
   +-----+---------------------------------------+---------------------+
   |  2  |       Uniqueness (hard failure)       |  IPv6 Frag ID, TCP  |
   |     |                                       |    ephemeral port   |
   +-----+---------------------------------------+---------------------+
   |  3  |  Uniqueness, constant within context  |      IPv6 IIDs      |
   |     |             (soft failure)            |                     |
   +-----+---------------------------------------+---------------------+
   |  4  |  Uniqueness, monotonically increasing |       TCP ISN       |
   |     |     within context (hard failure)     |                     |
   +-----+---------------------------------------+---------------------+

                      Table 2: Identifier Categories



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   We note that Category #4 could be considered a generalized case of
   category #3, in which a monotonically increasing element is added to
   a constant (within context) element, such that the resulting
   identifiers are monotonically increasing within a specified context.
   That is, the same algorithm could be employed for both #3 and #4,
   given appropriate parameters.

7.  Common Algorithms for Identifier Generation

   The following subsections describe common algorithms found for
   Protocol ID generation for each of the categories above.

7.1.  Category #1: Uniqueness (soft failure)

7.1.1.  Simple Randomization Algorithm

       /* Ephemeral port selection function */
       id_range = max_id - min_id + 1;
       next_id = min_id + (random() % id_range);
       count = next_id;

       do {
           if(check_suitable_id(next_id))
               return next_id;

           if (next_id == max_id) {
               next_id = min_id;
           } else {
               next_id++;
           }

           count--;
       } while (count > 0);

       return ERROR;


   Note:
      random() is a function that returns a pseudo-random unsigned
      integer number of appropriate size.  Note that the output needs to
      be unpredictable, and typical implementations of POSIX random()
      function do not necessarily meet this requirement.  See [RFC4086]
      for randomness requirements for security.

      The function check_suitable_id() can check, when possible, whether
      this identifier is e.g. already in use.  When already used, this
      algorithm selects the next available protocol ID.




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      All the variables (in this and all the algorithms discussed in
      this document) are unsigned integers.

   This algorithm does not suffer from any of the issues discussed in
   Section 8.

7.1.2.  Another Simple Randomization Algorithm

   The following pseudo-code illustrates another algorithm for selecting
   a random identifier in which, in the event the identifier is found to
   be not suitable (e.g., already in use), another identifier is
   selected randomly:

       id_range = max_id - min_id + 1;
       next_id = min_id + (random() % id_range);
       count = id_range;

       do {
           if(check_suitable_id(next_id))
               return next_id;

           next_id = min_id + (random() % id_range);
           count--;
       } while (count > 0);

       return ERROR;


   This algorithm might be unable to select an identifier (i.e., return
   "ERROR") even if there are suitable identifiers available, when there
   are a large number of identifiers "in use".

   This algorithm does not suffer from any of the issues discussed in
   Section 8.

7.2.  Category #2: Uniqueness (hard failure)

   One of the most trivial approaches for achieving uniqueness for an
   identifier (with a hard failure mode) is to implement a linear
   function.  As a result, all of the algorithms described in
   Section 7.4 are of use for complying the requirements of this
   identifier category.

7.3.  Category #3: Uniqueness, constant within context (soft-failure)

   The goal of this algorithm is to produce identifiers that are
   constant for a given context, but that change when the aforementioned
   context changes.



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   Keeping one value for each possible "context" may in many cases be
   considered too onerous in terms of memory requirements.  As a
   workaround, the following algorithm employs a calculated technique
   (as opposed to keeping state in memory) to maintain the constant
   identifier for each given context.

   In the following algorithm, the function F() provides (statelessly) a
   constant identifier for each given context.

       /* Protocol ID selection function  */
       id_range = max_id - min_id + 1;

       counter = 0;

       do {
           offset = F(CONTEXT, counter, secret_key);
           next_id = min_id + (offset % id_range);

           if(check_suitable_id(next_id))
               return next_id;

           counter++;

       } while (counter <= MAX_RETRIES);

       return ERROR;


   The function F() provides a "per-CONTEXT" constant identifier for a
   given context. 'offset' may take any value within the storage type
   range since we are restricting the resulting identifier to be in the
   range [min_id, max_id] in a similar way as in the algorithm described
   in Section 7.1.1.  Collisions can be recovered by incrementing the
   'counter' variable and recomputing F().

   The function F() should be a cryptographic hash function like SHA-256
   [FIPS-SHS].  Note: MD5 [RFC1321] is considered unacceptable for F()
   [RFC6151].  CONTEXT is the concatenation of all the elements that
   define a given context.  For example, if this algorithm is expected
   to produce identifiers that are unique per network interface card
   (NIC) and SLAAC autoconfiguration prefix, the CONTEXT should be the
   concatenation of e.g. the interface index and the SLAAC
   autoconfiguration prefix (please see [RFC7217] for an implementation
   of this algorithm for the generation of IPv6 IIDs).

   The secret should be chosen to be as random as possible (see
   [RFC4086] for recommendations on choosing secrets).




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   For obvious reasons, the transient network identifiers generated with
   this algorithm allow for network activity correlation within
   "CONTEXT".  However, this is essentially a design goal of this
   category of transient network identifiers.

7.4.  Category #4: Uniqueness, monotonically increasing within context
      (hard failure)

7.4.1.  Per-context Counter Algorithm

   One possible way to achieve low identifier reuse frequency while
   still avoiding predictable sequences would be to employ a per-context
   counter, as opposed to a global counter.  Such an algorithm could be
   described as follows:

       /* Initialization at system boot time. Could be random */
       id_inc= 1;

       /* Identifier selection function */
       count = max_id - min_id + 1;

       if(lookup_counter(CONTEXT) == ERROR){
            create_counter(CONTEXT);
       }

       next_id= lookup_counter(CONTEXT);

       do {
           if (next_id == max_id) {
               next_id = min_id;
           }
           else {
               next_id = next_id + id_inc;
           }

           if (check_suitable_id(next_id)){
               store_counter(CONTEXT, next_id);
               return next_id;
           }

           count--;

       } while (count > 0);

       store_counter(CONTEXT, next_id);
       return ERROR;

   NOTE:



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      lookup_counter() returns the current counter for a given context,
      or an error condition if such a counter does not exist.

      create_counter() creates a counter for a given context, and
      initializes such counter to a random value.

      store_counter() saves (updates) the current counter for a given
      context.

      check_suitable_id() is a function that checks whether the
      resulting identifier is acceptable (e.g., whether its in use,
      etc.).

   Essentially, whenever a new identifier is to be selected, the
   algorithm checks whether there there is a counter for the
   corresponding context.  If there is, such counter is incremented to
   obtain the new identifier, and the new identifier updates the
   corresponding counter.  If there is no counter for such context, a
   new counter is created an initialized to a random value, and used as
   the new identifier.

   This algorithm produces a per-context counter, which results in one
   linear function for each context.  Since the origin of each "line" is
   a random value, the resulting values are unknown to an off-path
   attacker.

   This algorithm has the following drawbacks:

   o  If, as a result of resource management, the counter for a given
      context must be removed, the last identifier value used for that
      context will be lost.  Thus, if subsequently an identifier needs
      to be generated for such context, that counter will need to be
      recreated and reinitialized to random value, thus possibly leading
      to reuse/collistion of identifiers.

   o  If the identifiers are predictable by the destination system
      (e.g., the destination host represents the "context"), a
      vulnerable host might possibly leak to third parties the
      identifiers used by other hosts to send traffic to it (i.e., a
      vulnerable Host B could leak to Host C the identifier values that
      Host A is using to send packets to Host B).  Appendix A of
      [RFC7739] describes one possible scenario for such leakage in
      detail.

   Otherwise, the identifiers produced by this algorithm do not suffer
   from the other issues discussed in Section 8.





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7.4.2.  Simple Hash-Based Algorithm

   The goal of this algorithm is to produce monotonically-increasing
   sequences, with a randomized initial value, for each given context.
   For example, if the identifiers being generated must be unique for
   each {src IP, dst IP} set, then each possible combination of {src IP,
   dst IP} should have a corresponding "next_id" value.

   Keeping one value for each possible "context" may in many cases be
   considered too onerous in terms of memory requirements.  As a
   workaround, the following algorithm employs a calculated technique
   (as opposed to keeping state in memory) to maintain the random offset
   for each possible context.

   In the following algorithm, the function F() provides (statelessly) a
   random offset for each given context.

       /* Initialization at system boot time. Could be random. */
       counter = 0;

       /* Protocol ID selection function  */
       id_range = max_id - min_id + 1;
       offset = F(CONTEXT, secret_key);
       count = id_range;

       do {
           next_id = min_id +
                  (counter + offset) % id_range;

           counter++;

           if(check_suitable_id(next_id))
               return next_id;

           count--;

       } while (count > 0);

       return ERROR;


   The function F() provides a "per-CONTEXT" fixed offset within the
   identifier space.  Both the 'offset' and 'counter' variables may take
   any value within the storage type range since we are restricting the
   resulting identifier to be in the range [min_id, max_id] in a similar
   way as in the algorithm described in Section 7.1.1.  This allows us
   to simply increment the 'counter' variable and rely on the unsigned
   integer to wrap around.



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   The function F() should be a cryptographic hash function like SHA-256
   [FIPS-SHS].  Note: MD5 [RFC1321] is considered unacceptable for F()
   [RFC6151].  CONTEXT is the concatenation of all the elements that
   define a given context.  For example, if this algorithm is expected
   to produce identifiers that are monotonically-increasing for each set
   (Source IP Address, Destination IP Address), the CONTEXT should be
   the concatenation of these two values.

   The secret should be chosen to be as random as possible (see
   [RFC4086] for recommendations on choosing secrets).

   It should be noted that, since this algorithm uses a global counter
   ("counter") for selecting identifiers, this algorithm produces an
   information leakage (as described in Section 8.2).  For example, if
   an attacker could force a client to periodically establish a new TCP
   connection to an attacker-controlled machine (or through an attacker-
   observable routing path), the attacker could subtract consecutive
   source port values to obtain the number of outgoing TCP connections
   established globally by the target host within that time period (up
   to wrap-around issues and five-tuple collisions, of course).

7.4.3.  Double-Hash Algorithm

   A trade-off between maintaining a single global 'counter' variable
   and maintaining 2**N 'counter' variables (where N is the width of the
   result of F()) could be achieved as follows.  The system would keep
   an array of TABLE_LENGTH integers, which would provide a separation
   of the increment of the 'counter' variable.  This improvement could
   be incorporated into the algorithm from Section 7.4.2 as follows:






















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       /* Initialization at system boot time */
       for(i = 0; i < TABLE_LENGTH; i++)
           table[i] = random();

       id_inc = 1;


       /* Protocol ID selection function */
       id_range = max_id - min_id + 1;
       offset = F(CONTEXT, secret_key1);
       index = G(CONTEXT, secret_key2);
       count = id_range;

       do {
           next_id = min_id + (offset + table[index]) % id_range;
           table[index] = table[index] + id_inc;

           if(check_suitable_id(next_id))
               return next_id;

          count--;

       } while (count > 0);

       return ERROR;


   'table[]' could be initialized with random values, as indicated by
   the initialization code in pseudo-code above.  The function G()
   should be a cryptographic hash function.  It should use the same
   CONTEXT as F(), and a secret key value to compute a value between 0
   and (TABLE_LENGTH-1).  Alternatively, G() could take an "offset" as
   input, and perform the exclusive-or (XOR) operation between all the
   bytes in 'offset'.

   The array 'table[]' assures that successive identifiers for a given
   context will be monotonically-increasing.  However, the increments
   space is separated into TABLE_LENGTH different spaces, and thus
   identifier reuse frequency will be (probabilistically) lower than
   that of the algorithm in Section 7.4.2.  That is, the generation of
   identifier for one given context will not necessarily result in
   increments in the identifiers for other contexts.

   It is interesting to note that the size of 'table[]' does not limit
   the number of different identifier sequences, but rather separates
   the *increments* into TABLE_LENGTH different spaces.  The identifier
   sequence will result from adding the corresponding entry of 'table[]'




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   to the variable 'offset', which selects the actual identifier
   sequence (as in the algorithm from Section 7.4.2).

   An attacker can perform traffic analysis for any "increment space"
   (i.e., context) into which the attacker has "visibility" -- namely,
   the attacker can force a node to generate identifiers where G(offset)
   identifies the target "increment space".  However, the attacker's
   ability to perform traffic analysis is very reduced when compared to
   the predictable linear identifiers (described in Appendix A.1) and
   the hash-based identifiers (described in Section 7.4.2).
   Additionally, an implementation can further limit the attacker's
   ability to perform traffic analysis by further separating the
   increment space (that is, using a larger value for TABLE_LENGTH) and/
   or by randomizing the increments.

   Otherwise, this algorithm does not suffer from the issues discussed
   in Section 8.

8.  Common Vulnerabilities Associated with Transient Numeric Identifiers

8.1.  Network Activity Correlation

   An idetifier that is preditable or stable within a given context
   allows for network activity correlation within that context.

   For example, a stable IPv6 Interface Identifier allows for network
   activity to be correlated for the context in which that address is
   stable [RFC7721].  A stable-per-network (as in [RFC7217] allows for
   network activity correlation within a network, whereas a constant
   IPv6 Interface Identifier allows not only network activity
   correlation within the same network, but also across networks ("host
   tracking").

   Predictable transient numeric identifiers can also allow for network
   activity correlation.  For example, a node that generates TCP ISNs
   with a global counter will typically allow network activity
   correlation even as it roams across networks, since the communicating
   nodes could infer the identity of the node based on the TCP ISNs
   employed for subsequent communication instances.  Similarly, a node
   that generates predictable IPv6 Fragment Identification values could
   be subject to network activity correlation (see e.g.
   [Bellovin2002]).

8.2.  Information Leakage

   Transient numeric identifiers that are not randomized can leak out
   information to other communicating nodes.  For example, it is common
   to generate identifiers like:



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                ID = offset(CONTEXT_1) + linear(CONTEXT_2);

   This generic expression generates identifiers by adding a linear
   function to an offset.  The offset is constant within a given
   context, whereas linear() is a linear function for a given context
   (possibly different to that of offset()).  Identifiers generated with
   this expression will generally be predictable within CONTEXT_1.
   Thus, CONTEXT_1 essentially specifies e.g. the context within which
   network activity correlation is possible thanks to these identifiers.
   When CONTEXT_1 is "global" (e.g., offset() is simply a constant
   value), then all the corresponding transient numeric identifiers
   become predictable in all contexts.

      NOTE: If offset() has a global context and the specific value is
      known, the resulting identifiers may leak even more information.
      For example, the if Fragment Identification values are generated
      with the generic function above, and CONTEXT_1 is "global", then
      the corresponding identifiers will leak the number of fragmented
      datagrams sent for CONTEXT_2.  If both CONTEXT_1 and CONTEXT_2 are
      "global", then Fragment Identification values would be generated
      with a global counter (initialized to offset()), and thus each
      generated Fragment Identification value would leak the number of
      fragmented datagrams transmitted by the node since it was
      bootstrapped.

   On the other hand, linear() will be predictable within CONTEXT_2.
   The predictability of linear(), irrespective of the context and/or
   predictability of offset(), can leak out information that is of use
   to attackers.  For example, a node that selects ephemeral port
   numbers on as in_

                ehemeral_port = offset(Dest_IP) + linear()

   that is, with a per-destination offset, but global linear() function
   (e.g., a global counter), will leak information about the number of
   outgoing connections that have been issued between any two issued
   outgoing connections.  Similarly, a node that generates Fragment
   Identification values as in:

                Frag_ID = offset(Srd_IP, Dst_IP) + linear()

   will leak out information about the number of fragmented packets that
   have been transmitted between any two other transmitted fragmented
   packets.  The vulnerabilities described in [Sanfilippo1998a],
   [Sanfilippo1998b], and [Sanfilippo1999] are all associated with the
   use of a global linear() function (i.e., a global CONTEXT_2).





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8.3.  Exploitation of Semantics of Transient Numeric Identifiers

   Identifiers that are not semantically opaque tend to be more
   predictable than semantically-opaque identifiers.  For example, a MAC
   address contains an OUI (Organizationally-Unique Identifier) which
   identifies the vendor that manufactured the underlying network
   interface card.  This fact may be leveraged by an attacker meaning to
   "predict" MAC addresses, if he has some knowledge about the possible
   NIC vendor.

   [RFC7707] discusses a number of techniques to reduce the search space
   when performing IPv6 address-scanning attacks by leveraging the
   semantics of the IIDs produced by a number of IID-generation
   techniques.

8.4.  Exploitation of Collisions of Transient Numeric Identifiers

   In many cases, th collision of transient network identifiers can have
   a hard failure severity (or result in a hard failure severity if an
   attacker can cause multiple collisions deterministically, one after
   another).  For example, predictable Fragment Identification values
   open the door to Denial of Service (DoS) attacks (see e.g.
   [RFC5722].  Similarly, predictable TCP ISNs open the door to trivial
   connection-reset and data injection attacks (see e.g.
   [Joncheray1995]).

9.  Vulnerability Analysis of Specific Transient Numeric Identifiers
    Categories

   The following subsections analyze common vulnerabilities associated
   with the generation of identifiers for each of the categories
   identified in Section 6.

9.1.  Category #1: Uniqueness (soft failure)

   Possible vulnerabilities associated with identifiers of this category
   are:

   o  Use of trivial algorithms (e.g. global counters) that generate
      predictable identifiers

   o  Use of flawed PRNGs (please see e.g.  [Zalewski2001],
      [Zalewski2002] and [Klein2007])

   Since the only interoperability requirement for these identifiers is
   uniqueness, the obvious approach to generate them is to employ a
   PRNG.  An implementer should consult [RFC4086] regarding randomness




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   requirements for security, and consult relevant documentation when
   employing a PRNG provided by the underlying system.

   Use of algorithms other than PRNGs for generating identifiers of this
   category is discouraged.

9.2.  Category #2: Uniqueness (hard failure)

   As noted in Section 7.2 this category typically employs the same
   algorithms as Category #4, since a monotonically-increasing sequence
   tends to minimize the identifier reuse frequency.  Therefore, the
   vulnerability analysis of Section 9.4 applies to this case.

9.3.  Category #3: Uniqueness, constant within context (soft failure)

   There are two main vulnerabilities that may be associated with
   identifiers of this category:

   1.  Use algorithms or sources that result in predictable identifiers

   2.  Employing the same identifier across contexts in which constantcy
       is not required

   At times, an implementation or specification may be tempted to employ
   a source for the identifier which is known to provide unique values.
   However, while unique, the associated identifiers may have other
   properties such as being predictable or leaking information about the
   node in question.  For example, as noted in [RFC7721], embedding
   link-layer addresses for generating IPv6 IIDs not only results in
   predictable values, but also leaks information about the manufacturer
   of the network interface card.

   On the other hand, using an identifier across contexts where
   constantcy is not required can be leveraged for correlation of
   activities.  On of the most trivial examples of this is the use of
   IPv6 IIDs that are constant across networks (such as IIDs that embed
   the underlying link-layer address).

9.4.  Category #4: Uniqueness, monotonically increasing within context
      (hard failure)

   A simple way to generalize algorithms employed for generating
   identifiers of Category #4 would be as follows:








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       /* Identifier selection function */
       count = max_id - min_id + 1;

       do {
                   linear(CONTEXT_2)= linear(CONTEXT_2) + increment();
           next_id= offset(CONTEXT_1) + linear(CONTEXT_1);

           if(check_suitable_id(next_id))
              return next_id;

           count--;
       } while (count > 0);

       return ERROR;


   Essentially, an identifier (next_id) is generated by adding a linear
   function (linear()) to an offset value, which is unknown to the
   attacker, and constant for given context (CONTEXT_1).

   The following aspects of the algorithm should be considered:

   o  For the most part, it is the offset() function that results in
      identifiers that are unpredictable by an off-path attacker.  While
      the resulting sequence will be monotonically-increasing, the use
      of an offset value that is unknown to the attacker makes the
      resulting values unknown to the attacker.

   o  The most straightforward "stateless" implementation of offset
      would be that in which offset() is the result of a
      cryptographically-secure hash-function that takes the values that
      identify the context and a "secret" (not shown in the figure
      above) as arguments.

   o  Another possible (but stateful) approach would be to simply
      generate a random offset and store it in memory, and then look-up
      the corresponding context when a new identifier is to be selected.
      The algorithm in Section 7.4.1 is essentially an implementation of
      this type.

   o  The linear function is incremented according to increment().  In
      the most trivial case increment() could always return the constant
      "1".  But it could also possibly return small integers such the
      increments are randomized.

   Considering the generic algorithm illustrated above we can identify
   the following possible vulnerabilities:




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   o  If the offset value spans more than the necessary context,
      identifiers could be unnecessarily predictable by other parties,
      since the offset value would be unnecessarily leaked to them.  For
      example, an implementation that means to produce a per-destination
      counter but replaces offset() with a constant number (i.e.,
      employs a global counter), will unnecessarily result in
      predictable identifiers.

   o  The function linear() could be seen as representing the number of
      identifiers that have so far been generated for a given context
      (CONTEXT_2).  If linear() spans more than the necessary context,
      the "increments" could be leaked to other parties, thus disclosing
      information about the number of identifiers that have so far been
      generated.  For example, an implementation in which linear() is
      implemented as a single global counter will unnecessarily leak
      information the number of identifiers that have been produced.

   o  increment() determines how the linear() is incremented for each
      identifier that is selected.  In the most trivial case,
      increment() will return the integer "1".  However, an
      implementation may have increment() return a "small" integer value
      such that even if the current value employed by the generator is
      guessed (see Appendix A of [RFC7739]), the exact next identifier
      to be selected will be slightly harder to identify.

10.  IANA Considerations

   There are no IANA registries within this document.  The RFC-Editor
   can remove this section before publication of this document as an
   RFC.

11.  Security Considerations

   The entire document is about the security and privacy implications of
   identifiers.

12.  Acknowledgements

   The authors would like to thank (in alphabetical order) Steven
   Bellovin, Joseph Lorenzo Hall, Gre Norcie, and Martin Thomson, for
   providing valuable comments on earlier versions of this document.

13.  References








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13.1.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <https://www.rfc-editor.org/info/rfc2460>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/info/rfc4086>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <https://www.rfc-editor.org/info/rfc4862>.

   [RFC5722]  Krishnan, S., "Handling of Overlapping IPv6 Fragments",
              RFC 5722, DOI 10.17487/RFC5722, December 2009,
              <https://www.rfc-editor.org/info/rfc5722>.

   [RFC6528]  Gont, F. and S. Bellovin, "Defending against Sequence
              Number Attacks", RFC 6528, DOI 10.17487/RFC6528, February
              2012, <https://www.rfc-editor.org/info/rfc6528>.

   [RFC7217]  Gont, F., "A Method for Generating Semantically Opaque
              Interface Identifiers with IPv6 Stateless Address
              Autoconfiguration (SLAAC)", RFC 7217,
              DOI 10.17487/RFC7217, April 2014,
              <https://www.rfc-editor.org/info/rfc7217>.

13.2.  Informative References







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   [Bellovin1989]
              Bellovin, S., "Security Problems in the TCP/IP Protocol
              Suite", Computer Communications Review, vol. 19, no. 2,
              pp. 32-48, 1989,
              <https://www.cs.columbia.edu/~smb/papers/ipext.pdf>.

   [Bellovin2002]
              Bellovin, S., "A Technique for Counting NATted Hosts",
              IMW'02 Nov. 6-8, 2002, Marseille, France, 2002.

   [CERT2001]
              CERT, "CERT Advisory CA-2001-09: Statistical Weaknesses in
              TCP/IP Initial Sequence Numbers", 2001,
              <http://www.cert.org/advisories/CA-2001-09.html>.

   [CPNI-TCP]
              Gont, F., "Security Assessment of the Transmission Control
              Protocol (TCP)",  United Kingdom's Centre for the
              Protection of National Infrastructure (CPNI) Technical
              Report, 2009, <http://www.gont.com.ar/papers/
              tn-03-09-security-assessment-TCP.pdf>.

   [FIPS-SHS]
              FIPS, "Secure Hash Standard (SHS)",  Federal Information
              Processing Standards Publication 180-4, March 2012,
              <http://csrc.nist.gov/publications/fips/fips180-4/
              fips-180-4.pdf>.

   [Fyodor2004]
              Fyodor, "Idle scanning and related IP ID games", 2004,
              <http://www.insecure.org/nmap/idlescan.html>.

   [I-D.ietf-6man-default-iids]
              Gont, F., Cooper, A., Thaler, D., and S. LIU,
              "Recommendation on Stable IPv6 Interface Identifiers",
              draft-ietf-6man-default-iids-16 (work in progress),
              September 2016.

   [Joncheray1995]
              Joncheray, L., "A Simple Active Attack Against TCP", Proc.
              Fifth Usenix UNIX Security Symposium, 1995.

   [Klein2007]
              Klein, A., "OpenBSD DNS Cache Poisoning and Multiple O/S
              Predictable IP ID Vulnerability", 2007,
              <http://www.trusteer.com/files/OpenBSD_DNS_Cache_Poisoning
              _and_Multiple_OS_Predictable_IP_ID_Vulnerability.pdf>.




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   [Morris1985]
              Morris, R., "A Weakness in the 4.2BSD UNIX TCP/IP
              Software", CSTR 117, AT&T Bell Laboratories, Murray Hill,
              NJ, 1985,
              <https://pdos.csail.mit.edu/~rtm/papers/117.pdf>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              DOI 10.17487/RFC1321, April 1992,
              <https://www.rfc-editor.org/info/rfc1321>.

   [RFC4963]  Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
              Errors at High Data Rates", RFC 4963,
              DOI 10.17487/RFC4963, July 2007,
              <https://www.rfc-editor.org/info/rfc4963>.

   [RFC6056]  Larsen, M. and F. Gont, "Recommendations for Transport-
              Protocol Port Randomization", BCP 156, RFC 6056,
              DOI 10.17487/RFC6056, January 2011,
              <https://www.rfc-editor.org/info/rfc6056>.

   [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
              RFC 6151, DOI 10.17487/RFC6151, March 2011,
              <https://www.rfc-editor.org/info/rfc6151>.

   [RFC7098]  Carpenter, B., Jiang, S., and W. Tarreau, "Using the IPv6
              Flow Label for Load Balancing in Server Farms", RFC 7098,
              DOI 10.17487/RFC7098, January 2014,
              <https://www.rfc-editor.org/info/rfc7098>.

   [RFC7528]  Higgs, P. and J. Piesing, "A Uniform Resource Name (URN)
              Namespace for the Hybrid Broadcast Broadband TV (HbbTV)
              Association", RFC 7528, DOI 10.17487/RFC7528, April 2015,
              <https://www.rfc-editor.org/info/rfc7528>.

   [RFC7707]  Gont, F. and T. Chown, "Network Reconnaissance in IPv6
              Networks", RFC 7707, DOI 10.17487/RFC7707, March 2016,
              <https://www.rfc-editor.org/info/rfc7707>.

   [RFC7721]  Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              RFC 7721, DOI 10.17487/RFC7721, March 2016,
              <https://www.rfc-editor.org/info/rfc7721>.




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   [RFC7739]  Gont, F., "Security Implications of Predictable Fragment
              Identification Values", RFC 7739, DOI 10.17487/RFC7739,
              February 2016, <https://www.rfc-editor.org/info/rfc7739>.

   [Sanfilippo1998a]
              Sanfilippo, S., "about the ip header id", Post to Bugtraq
              mailing-list, Mon Dec 14 1998,
              <http://seclists.org/bugtraq/1998/Dec/48>.

   [Sanfilippo1998b]
              Sanfilippo, S., "Idle scan", Post to Bugtraq mailing-list,
              1998, <http://www.kyuzz.org/antirez/papers/dumbscan.html>.

   [Sanfilippo1999]
              Sanfilippo, S., "more ip id", Post to Bugtraq mailing-
              list, 1999,
              <http://www.kyuzz.org/antirez/papers/moreipid.html>.

   [Shimomura1995]
              Shimomura, T., "Technical details of the attack described
              by Markoff in NYT", Message posted in USENET's
              comp.security.misc newsgroup  Message-ID:
              <3g5gkl$5j1@ariel.sdsc.edu>, 1995,
              <http://www.gont.com.ar/docs/post-shimomura-usenet.txt>.

   [Silbersack2005]
              Silbersack, M., "Improving TCP/IP security through
              randomization without sacrificing interoperability",
              EuroBSDCon 2005 Conference, 2005,
              <http://citeseerx.ist.psu.edu/viewdoc/
              download?doi=10.1.1.91.4542&rep=rep1&type=pdf>.

   [USCERT2001]
              US-CERT, "US-CERT Vulnerability Note VU#498440: Multiple
              TCP/IP implementations may use statistically predictable
              initial sequence numbers", 2001,
              <http://www.kb.cert.org/vuls/id/498440>.

   [Zalewski2001]
              Zalewski, M., "Strange Attractors and TCP/IP Sequence
              Number Analysis", 2001,
              <http://lcamtuf.coredump.cx/oldtcp/tcpseq.html>.

   [Zalewski2002]
              Zalewski, M., "Strange Attractors and TCP/IP Sequence
              Number Analysis - One Year Later", 2001,
              <http://lcamtuf.coredump.cx/newtcp/>.




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Appendix A.  Flawed Algorithms

   The following subsections document algorithms with known negative
   security and privacy imlpications.

A.1.  Predictable Linear Identifiers Algorithm

   One of the most trivial ways to achieve uniqueness with a low
   identifier reuse frequency is to produce a linear sequence.

   For example, the following algorithm has been employed (see e.g.
   [Morris1985], [Shimomura1995], [Silbersack2005] and [CPNI-TCP]) in a
   number of operating systems for selecting IP fragment IDs, TCP
   ephemeral ports, etc.:

       /* Initialization at system boot time. Could be random */
       next_id = min_id;
       id_inc= 1;

       /* Identifier selection function */
       count = max_id - min_id + 1;

       do {
           if (next_id == max_id) {
               next_id = min_id;
           }
           else {
               next_id = next_id + id_inc;
           }

           if (check_suitable_id(next_id))
               return next_id;

           count--;

       } while (count > 0);

       return ERROR;

   Note:
      check_suitable_id() is a function that checks whether the
      resulting identifier is acceptable (e.g., whether its in use,
      etc.).

   For obvious reasons, this algorithm results in predicable sequences.
   If a global counter is used (such as "next_id" in the example above),
   a node that learns one protocol identifier can also learn or guess
   values employed by past and future protocol instances.  On the other



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   hand, when the value of increments is known (such as "1" in this
   case), an attacker can sample two values, and learn the number of
   identifiers that were generated in-between.

   Where identifier reuse would lead to a hard failure, one typical
   approach to generate unique identifiers (while minimizing the
   security and privacy implications of predictable identifiers) is to
   obfuscate the resulting protocol IDs by either:

   o  Replace the global counter with multiple counters (initialized to
      a random value)

   o  Randomizing the "increments"

   Avoiding global counters essentially means that learning one
   identifier for a given context (e.g., one TCP ephemeral port for a
   given {src IP, Dst IP, Dst Port}) is of no use for learning or
   guessing identifiers for a different context (e.g., TCP ephemeral
   ports that involve other peers).  However, this may imply keeping one
   additional variable/counter per context, which may be prohibitive in
   some environments.  The choice of id_inc has implications on both the
   security and privacy properties of the resulting identifiers, but
   also on the corresponding interoperability properties.  On one hand,
   minimizing the increments (as in "id_inc = 1" in our case) generally
   minimizes the identifier reuse frequency, albeit at increased
   predictability.  On the other hand, if the increments are randomized,
   predictability of the resulting identifiers is reduced, and the
   information leakage produced by global constant increments is
   mitigated.  However, using larger increments than necessary can
   result in an increased identifier reuse frequency.

A.2.  Random-Increments Algorithm

   This algorithm offers a middle ground between the algorithms that
   select ephemeral ports randomly (such as those described in
   Section 7.1.1 and Section 7.1.2), and those that offer obfuscation
   but no randomization (such as those described in Section 7.4.2 and
   Section 7.4.3).













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       /* Initialization code at system boot time. */
       next_id = random();        /* Initialization value */
       id_inc = 500;        /* Determines the trade-off */

       /* Identifier selection function */
       id_range = max_id - min_id + 1;

       count = id_range;

       do {
           /* Random increment */
           next_id = next_id + (random() % id_inc) + 1;

           /* Keep the identifier within acceptable range */
           next_id = min_id + (next_id % id_range);

           if(check_suitable_id(next_id))
              return next_id;

           count--;
       } while (count > 0);

       return ERROR;


   This algorithm aims at producing a monotonically increasing sequence
   of identifiers, while avoiding the use of fixed increments, which
   would lead to trivially predictable sequences.  The value "id_inc"
   allows for direct control of the trade-off between the level of
   obfuscation and the ID reuse frequency.  The smaller the value of
   "id_inc", the more similar this algorithm is to a predicable, global
   monotonically-increasing ID generation algorithm.  The larger the
   value of "id_inc", the more similar this algorithm is to the
   algorithm described in Section 7.1.1 of this document.

   When the identifiers wrap, there is the risk of collisions of
   identifiers (i.e., identifier reuse).  Therefore, "id_inc" should be
   selected according to the following criteria:

   o  It should maximize the wrapping time of the identifier space.

   o  It should minimize identifier reuse frequency.

   o  It should maximize obfuscation.

   Clearly, these are competing goals, and the decision of which value
   of "id_inc" to use is a trade-off.  Therefore, the value of "id_inc"




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   should be configurable so that system administrators can make the
   trade-off for themselves.

Authors' Addresses

   Fernando Gont
   SI6 Networks / UTN-FRH
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706
   Argentina

   Phone: +54 11 4650 8472
   Email: fgont@si6networks.com
   URI:   http://www.si6networks.com


   Ivan Arce
   Quarkslab

   Email: iarce@quarkslab.com
   URI:   https://www.quarkslab.com






























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