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RFC 2522

 
 
 

Photuris: Session-Key Management Protocol

Part 2 of 3, p. 25 to 53
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4.  Value Exchange

   Initiator                            Responder
   =========                            =========
   Value_Request                  ->
      pick scheme
      offer value
      offer attributes
                                   <-   Value_Response
                                           offer value
                                           offer attributes

             [generate shared-secret from exchanged values]



4.0.1.  Send Value_Request

   The Initiator generates an appropriate Exchange-Value for the
   Scheme-Choice.  This Exchange-Value may be pre-calculated and used
   for multiple Responders.

   The IP Destination for the Responder is examined, and the attributes
   available between the parties are listed in the Offered-Attributes.

   The Initiator also starts a retransmission timer.  If no valid
   Value_Response arrives within the time limit, the same Value_Request
   is retransmitted for the remaining number of Retransmissions.

   When Retransmissions have been exceeded, if a Bad_Cookie or
   Resource_Limit message has been received during the exchange, the
   Initiator SHOULD begin the Photuris exchange again by sending a new
   Cookie_Request.

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4.0.2.  Receive Value_Request

   The Responder validates the Responder-Cookie, the Counter, the
   Scheme-Choice, the Exchange-Value, and the Offered-Attributes.

   -  When an invalid/expired Responder-Cookie is detected, a Bad_Cookie
      message is sent.

   -  When too many SPI values are already in use for this particular
      peer, or too many concurrent exchanges are in progress, or some
      other resource limit is reached, a Resource_Limit message is sent.

   -  When an invalid Scheme-Choice is detected, or the Exchange-Value
      is obviously defective, or the variable length Offered-Attributes
      do not match the UDP Length, the message is silently discarded;
      the implementation SHOULD log the occurance, and notify an
      operator as appropriate.

   When the message is valid, the Responder sets its Exchange timer to
   the Exchange TimeOut, and returns a Value_Response.

   The Responder keeps a copy of the incoming Value_Request cookie pair,
   and its Value_Response.  If a duplicate Value_Request is received, it
   merely resends its previous Value_Response, and takes no further
   action.


4.0.3.  Send Value_Response

   The Responder generates an appropriate Exchange-Value for the
   Scheme-Choice.  This Exchange-Value may be pre-calculated and used
   for multiple Initiators.

   The IP Source for the Initiator is examined, and the attributes
   available between the parties are listed in the Offered-Attributes.

   Implementation Notes:

      At this time, the Responder begins calculation of the shared-
      secret.  Calculation of the shared-secret is executed in parallel
      to minimize delay.

      This may take a substantial amount of time.  The implementor
      should ensure that retransmission is not blocked by this
      calculation.  This is not usually a problem, as retransmission
      timeouts typically exceed calculation time.

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4.0.4.  Receive Value_Response

   The Initiator validates the pair of Cookies, the Exchange-Value, and
   the Offered-Attributes.

   -  When an invalid/expired cookie is detected, the message is
      silently discarded.

   -  When the Exchange-Value is obviously defective, or the variable
      length Offered-Attributes do not match the UDP Length, the message
      is silently discarded; the implementation SHOULD log the
      occurance, and notify an operator as appropriate.

   -  Once a valid message has been received, later Value_Responses with
      both matching cookies are also silently discarded, until a new
      Cookie_Request is sent.

   When the message is valid, the Initiator begins its parallel
   computation of the shared-secret.

   When the Initiator completes computation, it sends an
   Identity_Request to the Responder.

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4.1.  Value_Request

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Initiator-Cookie                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Responder-Cookie                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Message    |    Counter    |         Scheme-Choice         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                   Initiator-Exchange-Value                    ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Initiator-Offered-Attributes ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-


   Initiator-Cookie  16 bytes.  Copied from the Cookie_Response.

   Responder-Cookie  16 bytes.  Copied from the Cookie_Response.

   Message          2

   Counter          1 byte.  Copied from the Cookie_Response.

   Scheme-Choice    2 bytes.  A value selected by the Initiator from the
                    list of Offered-Schemes in the Cookie_Response.

                    Only the Scheme is specified; the Size will match
                    the Initiator-Exchange-Value, and the Value(s) are
                    implicit.

   Initiator-Exchange-Value
                    Variable Precision Integer.  Provided by the
                    Initiator for calculating a shared-secret between
                    the parties.  The Value format is indicated by the
                    Scheme-Choice.

                    The field may be any integral number of bytes in
                    length, as indicated by its Size field.  It does not
                    require any particular alignment.  The 32-bit
                    alignment shown is for convenience in the
                    illustration.

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   Initiator-Offered-Attributes
                    4 or more bytes.  A list of Security Parameter
                    attributes supported by the Initiator.

                    The contents and usage of this list are further
                    described in "Offered Attributes List".  The end of
                    the list is indicated by the UDP Length.



4.2.  Value_Response

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Initiator-Cookie                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Responder-Cookie                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Message    |                    Reserved                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                   Responder-Exchange-Value                    ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Responder-Offered-Attributes ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-


   Initiator-Cookie  16 bytes.  Copied from the Value_Request.

   Responder-Cookie  16 bytes.  Copied from the Value_Request.

   Message          3

   Reserved         3 bytes.  For future use; MUST be set to zero when
                    transmitted, and MUST be ignored when received.

   Responder-Exchange-Value
                    Variable Precision Integer.  Provided by the
                    Responder for calculating a shared-secret between
                    the parties.  The Value format is indicated by the
                    current Scheme-Choice specified in the
                    Value_Request.

                    The field may be any integral number of bytes in

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                    length, as indicated by its Size field.  It does not
                    require any particular alignment.  The 32-bit
                    alignment shown is for convenience in the
                    illustration.

   Responder-Offered-Attributes
                    4 or more bytes.  A list of Security Parameter
                    attributes supported by the Responder.

                    The contents and usage of this list are further
                    described in "Offered Attributes List".  The end of
                    the list is indicated by the UDP Length.



4.3.  Offered Attribute List

   This list includes those attributes supported by the party that are
   available to the other party.  The attribute formats are specified in
   the "Basic Attributes".

   The list is composed of two or three sections: Identification-
   Attributes, Authentication-Attributes, and (optional) Encapsulation-
   Attributes.  Within each section, the attributes are ordered from
   most to least preferable.

   The first section of the list includes methods of identification.  An
   Identity-Choice is selected from this list.

   The second section of the list begins with "AH-Attributes" (#1).  It
   includes methods of authentication, and other operational types.

   The third section of the list begins with "ESP-Attributes" (#2).  It
   includes methods of authentication, compression, encryption, and
   other operational types.  When no Encapsulation-Attributes are
   offered, the "ESP-Attributes" attribute itself is omitted from the
   list.

   Attribute-Choices are selected from the latter two sections of the
   list.

   Support is required for the "MD5-IPMAC" (#5) attribute for both
   "Symmetric Identification" and "Authentication" and they SHOULD be
   present in every Offered-Attributes list.

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   Implementation Notes:

      For example,

         "MD5-IPMAC" (Symmetric Identification),
         "AH-Attributes",
         "MD5-IPMAC" (Authentication).

      Since the offer is made by the prospective SPI User (sender),
      order of preference likely reflects the capabilities and
      engineering tradeoffs of a particular implementation.

      However, the critical processing bottlenecks are frequently in the
      receiver.  The SPI Owner (receiver) may express its needs by
      choosing a less preferable attribute.

      The order may also be affected by operational policy and requested
      services for an application.  Such considerations are outside the
      scope of this document.

      The list may be divided into additional sections.  These sections
      will always follow the ESP-Attributes section, and are
      indistinguishable from unrecognized attributes.

      The authentication, compression, encryption and identification
      mechanisms chosen, as well as the encapsulation modes (if any),
      need not be the same in both directions.

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5.  Identification Exchange

   Initiator                            Responder
   =========                            =========
   Identity_Request               ->
      make SPI
      pick SPI attribute(s)
      identify self
      authenticate
      make privacy key(s)
      mask/encrypt message
                                   <-   Identity_Response
                                           make SPI
                                           pick SPI attribute(s)
                                           identify self
                                           authenticate
                                           make privacy key(s)
                                           mask/encrypt message

               [make SPI session-keys in each direction]

   The exchange of messages is ordered, although the formats and
   meanings of the messages are identical in each direction.  The
   messages are easily distinguished by the parties themselves, by
   examining the Message and Identification fields.

   Implementation Notes:

      The amount of time for the calculation may be dependent on the
      value of particular bits in secret values used in generating the
      shared-secret or identity verification.  To prevent analysis of
      these secret bits by recording the time for calculation, sending
      of the Identity_Messages SHOULD be delayed until the time expected
      for the longest calculation.  This will be different for different
      processor speeds, different algorithms, and different length
      variables.  Therefore, the method for estimating time is
      implementation dependent.

      Any authenticated and/or encrypted user datagrams received before
      the completion of identity verification can be placed on a queue
      pending completion of this step.  If verification succeeds, the
      queue is processed as though the datagrams had arrived subsequent
      to the verification.  If verification fails, the queue is
      discarded.

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5.0.1.  Send Identity_Request

   The Initiator chooses an appropriate Identification, the SPI and
   SPILT, a set of Attributes for the SPI, calculates the Verification,
   and masks the message using the Privacy-Method indicated by the
   current Scheme-Choice.

   The Initiator also starts a retransmission timer.  If no valid
   Identity_Response arrives within the time limit, its previous
   Identity_Request is retransmitted for the remaining number of
   Retransmissions.

   When Retransmissions have been exceeded, if a Bad_Cookie message has
   been received during the exchange, the Initiator SHOULD begin the
   Photuris exchange again by sending a new Cookie_Request.


5.0.2.  Receive Identity_Request

   The Responder validates the pair of Cookies, the Padding, the
   Identification, the Verification, and the Attribute-Choices.

   -  When an invalid/expired cookie is detected, a Bad_Cookie message
      is sent.

   -  After unmasking, when invalid Padding is detected, the variable
      length Attribute-Choices do not match the UDP Length, or an
      attribute was not in the Offered-Attributes, the message is
      silently discarded.

   -  When an invalid Identification is detected, or the message
      verification fails, a Verification_Failure message is sent.

   -  Whenever such a problem is detected, the Security Association is
      not established; the implementation SHOULD log the occurance, and
      notify an operator as appropriate.

   When the message is valid, the Responder sets its Exchange timer to
   the Exchange LifeTime (if this has not already been done for a
   previous exchange).  When its parallel computation of the shared-
   secret is complete, the Responder returns an Identity_Response.

   The Responder keeps a copy of the incoming Identity_Request values,
   and its Identity_Response.  If a duplicate Identity_Request is
   received, it merely resends its previous Identity_Response, and takes
   no further action.

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5.0.3.  Send Identity_Response

   The Responder chooses an appropriate Identification, the SPI and
   SPILT, a set of Attributes for the SPI, calculates the Verification,
   and masks the message using the Privacy-Method indicated by the
   current Scheme-Choice.

   The Responder calculates the SPI session-keys in both directions.

   At this time, the Responder begins the authentication and/or
   encryption of user datagrams.


5.0.4.  Receive Identity_Response

   The Initiator validates the pair of Cookies, the Padding, the
   Identification, the Verification, and the Attribute-Choices.

   -  When an invalid/expired cookie is detected, the message is
      silently discarded.

   -  After unmasking, when invalid Padding is detected, the variable
      length Attribute-Choices do not match the UDP Length, or an
      attribute was not in the Offered-Attributes, the message is
      silently discarded.

   -  When an invalid Identification is detected, or the message
      verification fails, a Verification_Failure message is sent.

   -  Whenever such a problem is detected, the Security Association is
      not established; the implementation SHOULD log the occurance, and
      notify an operator as appropriate.

   -  Once a valid message has been received, later Identity_Responses
      with both matching cookies are also silently discarded, until a
      new Cookie_Request is sent.

   When the message is valid, the Initiator sets its Exchange timer to
   the Exchange LifeTime (if this has not already been done for a
   previous exchange).

   The Initiator calculates the SPI session-keys in both directions.

   At this time, the Initiator begins the authentication and/or
   encryption of user datagrams.

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5.1.  Identity_Messages

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Initiator-Cookie                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Responder-Cookie                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Message    |                    LifeTime                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Security-Parameters-Index                   |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |        Identity-Choice        |                               |
   + + + + + + + + + + + + + + + + +                               +
   |                                                               |
   ~                        Identification                         ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                         Verification                          ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Attribute-Choices ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                                      ... Padding  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Initiator-Cookie  16 bytes.  Copied from the Value_Request.

   Responder-Cookie  16 bytes.  Copied from the Value_Request.

   Message          4 (Request) or 7 (Response)

   LifeTime         3 bytes.  The number of seconds remaining before the
                    indicated SPI expires.

                    When the SPI is zero, this field MUST be filled with
                    a random non-zero value.

   Security-Parameters-Index (SPI)
                    4 bytes.  The SPI to be used for incoming
                    communications.

                    When zero, indicates that no SPI is created in this

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                    direction.

   Identity-Choice  2 or more bytes.  An identity attribute is selected
                    from the list of Offered-Attributes sent by the
                    peer, and is used to calculate the Verification.

                    The field may be any integral number of bytes in
                    length, as indicated by its Length field.  It does
                    not require any particular alignment.  The 16-bit
                    alignment shown is for convenience in the
                    illustration.

   Identification   Variable Precision Integer, or alternative format
                    indicated by the Identity-Choice.  See the "Basic
                    Attributes" for details.

                    The field may be any integral number of bytes in
                    length.  It does not require any particular
                    alignment.  The 32-bit alignment shown is for
                    convenience in the illustration.

   Verification     Variable Precision Integer, or alternative format
                    indicated by the Identity-Choice.  The calculation
                    of the value is described in "Identity
                    Verification".

                    The field may be any integral number of bytes in
                    length.  It does not require any particular
                    alignment.  The 32-bit alignment shown is for
                    convenience in the illustration.

   Attribute-Choices
                    0 or more bytes.  When the SPI is non-zero, a list
                    of attributes selected from the list of Offered-
                    Attributes supported by the peer.

                    The contents and usage of this list are further
                    described in "Attribute Choices List".  The end of
                    the list is indicated by the UDP Length after
                    removing the Padding (UDP Length - last Padding
                    value).

   Padding          8 to 255 bytes.  This field is filled up to at least
                    a 128 byte boundary, measured from the beginning of
                    the message.  The number of pad bytes are chosen
                    randomly.

                    In addition, when a Privacy-Method indicated by the

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                    current Scheme-Choice requires the plaintext to be a
                    multiple of some number of bytes (the block size of
                    a block cipher), this field is adjusted as necessary
                    to the size required by the algorithm.

                    Self-Describing-Padding begins with the value 1.
                    Each byte contains the index of that byte.  Thus,
                    the final pad byte indicates the number of pad bytes
                    to remove.  For example, when the unpadded message
                    length is 120 bytes, the padding values might be 1,
                    2, 3, 4, 5, 6, 7, and 8.

   The portion of the message after the SPI field is masked using the
   Privacy-Method indicated by the current Scheme-Choice.

   The fields following the SPI are opaque.  That is, the values are set
   prior to masking (and optional encryption), and examined only after
   unmasking (and optional decryption).


5.2.  Attribute Choices List

   This list specifies the attributes of the SPI.  The attribute formats
   are specified in the "Basic Attributes".

   The list is composed of one or two sections: Authentication-
   Attributes, and/or Encapsulation-Attributes.

   When sending from the SPI User to the SPI Owner, the attributes are
   processed in the order listed.  For example,

      "ESP-Attributes",
      "Deflate" (Compression),
      "XOR" (Encryption),
      "DES-CBC" (Encryption),
      "XOR" (Encryption),
      "AH-Attributes",
      "AH-Sequence",
      "MD5-IPMAC" (Authentication),

   would result in ESP with compression and triple encryption (inside),
   and then AH authentication with sequence numbers (outside) of the ESP
   payload.

   The SPI Owner will naturally process the datagram in the reverse
   order.

   This ordering also affects the order of key generation.  Both SPI

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   Owner and SPI User generate the keys in the order listed.

   Implementation Notes:

      When choices are made from the list of Offered-Attributes, it is
      not required that any Security Association include every kind of
      offered attribute in any single SPI, or that a separate SPI be
      created for every offered attribute.

      Some kinds of attributes may be included more than once in a
      single SPI.  The set of allowable combinations of attributes are
      dependent on implementation and operational policy.  Such
      considerations are outside the scope of this document.

      The list may be divided into additional sections.  This can occur
      only when both parties recognize the affected attributes.

      The authentication, compression, encryption and identification
      mechanisms chosen, as well as the encapsulation modes (if any),
      need not be the same in both directions.


5.3.  Shared-Secret

   A shared-secret is used in a number of calculations.  Regardless of
   the internal representation of the shared-secret, when used in
   calculations it is in the same form as the Value part of a Variable
   Precision Integer:

    - most significant byte first.
    - bits used are right justified within byte boundaries.
    - any unused bits are in the most significant byte.
    - unused bits are zero filled.

   The shared-secret does not include a Size field.


5.4.  Identity Verification

   These messages are authenticated using the Identity-Choice.  The
   Verification value is calculated prior to masking (and optional
   encryption), and verified after unmasking (and optional decryption).

   The Identity-Choice authentication function is supplied with two
   input values:

    - the sender (SPI Owner) verification-key,
    - the data to be verified (as a concatenated sequence of bytes).

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   The resulting output value is stored in the Verification field.

   The Identity-Choice verification data consists of the following
   concatenated values:

    + the Initiator Cookie,
    + the Responder Cookie,
    + the Message, LifeTime and SPI fields,
    + the Identity-Choice and Identification,
    + the SPI User Identity Verification (response only),
    + the Attribute-Choices following the Verification field,
    + the Padding,
    + the SPI Owner TBV,
    + the SPI Owner Exchange-Value,
    + the SPI Owner Offered-Attributes,
    + the SPI User TBV,
    + the SPI User Exchange-Value,
    + the SPI User Offered-Attributes,
    + the Responder Offered-Schemes.

   The TBV (Three Byte Value) consists of the Counter and Scheme-Choice
   fields from the Value_Request, or the Reserved field from the
   Value_Response, immediately preceding the associated Exchange-Value.

   Note that the order of the Exchange-Value and Offered-Attributes
   fields is different in each direction, and the Identification and SPI
   fields are also likely to be different in each direction.  Note also
   that the SPI User Identity Verification (from the Identity_Request)
   is present only in the Identity_Response.

   If the verification fails, the users are notified, and a
   Verification_Failure message is sent, without adding any SPI.  On
   success, normal operation begins with the authentication and/or
   encryption of user datagrams.

   Implementation Notes:

      This is distinct from any authentication method specified for the
      SPI.

      The exact details of the Identification and verification-key
      included in the Verification calculation are dependent on the
      Identity-Choice, as described in the "Basic Attributes".

      Each party may wish to keep their own trusted databases, such as
      the Pretty Good Privacy (PGP) web of trust, and accept only those
      identities found there.  Failure to find the Identification in
      either an internal or external database results in the same

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      Verification_Failure message as failure of the verification
      computation.

      The Exchange-Value data includes both the Size and Value fields.
      The Offered-Attributes and Attribute-Choices data includes the
      Attribute, Length and Value fields.


5.5.  Privacy-Key Computation

   Identification Exchange messages are masked using the Privacy-Method
   indicated by the current Scheme-Choice.  Masking begins with the next
   field after the SPI, and continues to the end of the data indicated
   by the UDP Length, including the Padding.

   The Scheme-Choice specified Key-Generation-Function is used to create
   a special privacy-key for each message.  This function is calculated
   over the following concatenated values:

    + the SPI Owner Exchange-Value,
    + the SPI User Exchange-Value,
    + the Initiator Cookie,
    + the Responder Cookie,
    + the Message, LifeTime and SPI (or Reserved) fields,
    + the computed shared-secret.

   Since the order of the Exchange-Value fields is different in each
   direction, and the Message, LifeTime and SPI fields are also
   different in each direction, the resulting privacy-key will usually
   be different in each direction.

   When a larger number of keying-bits are needed than are available
   from one iteration of the specified Key-Generation-Function, more
   keying-bits are generated by duplicating the trailing shared-secret,
   and recalculating the function.  That is, the first iteration will
   have one trailing copy of the shared-secret, the second iteration
   will have two trailing copies of the shared-secret, and so forth.

   Implementation Notes:

      This is distinct from any encryption method specified for the SPI.

      The length of the Padding, and other details, are dependent on the
      Privacy-Method.  See the "Basic Privacy-Method" list for details.

      To avoid keeping the Exchange-Values in memory after the initial
      verification, it is often possible to pre-compute the function
      over the initial bytes of the concatenated data values for each

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      direction, and append the trailing copies of the shared-secret.

      The Exchange-Value data includes both the Size and Value fields.


5.6.  Session-Key Computation

   Each SPI has one or more session-keys.  These keys are generated
   based on the attributes of the SPI.  See the "Basic Attributes" for
   details.

   The Scheme-Choice specified Key-Generation-Function is used to create
   the SPI session-key for that particular attribute.  This function is
   calculated over the following concatenated values:

    + the Initiator Cookie,
    + the Responder Cookie,
    + the SPI Owner generation-key,
    + the SPI User generation-key,
    + the message Verification field,
    + the computed shared-secret.

   Since the order of the generation-keys is different in each
   direction, and the Verification field is also likely to be different
   in each direction, the resulting session-key will usually be
   different in each direction.

   When a larger number of keying-bits are needed than are available
   from one iteration of the specified Key-Generation-Function, more
   keying-bits are generated by duplicating the trailing shared-secret,
   and recalculating the function.  That is, the first iteration will
   have one trailing copy of the shared-secret, the second iteration
   will have two trailing copies of the shared-secret, and so forth.

   Implementation Notes:

      This is distinct from any privacy-key generated for the Photuris
      exchange.  Different initialization data is used, and iterations
      are maintained separately.

      The exact details of the Verification field and generation-keys
      that are included in the session-key calculation are dependent on
      the Identity-Choices, as described in the "Basic Attributes".

      To avoid keeping the generation-keys in memory after the initial
      verification, it is often possible to pre-compute the function
      over the initial bytes of the concatenated data values for each
      direction, and append the trailing copies of the shared-secret.

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      When both authentication and encryption attributes are used for
      the same SPI, there may be multiple session-keys associated with
      the same SPI.  These session-keys are generated in the order of
      the Attribute-Choices list.


6.  SPI Messages

   SPI User                             SPI Owner
   ========                             =========
   SPI_Needed                     ->
      list SPI attribute(s)
      make validity key
      authenticate
      make privacy key(s)
      mask/encrypt message
                                   <-   SPI_Update
                                           make SPI
                                           pick SPI attribute(s)
                                           make SPI session-key(s)
                                           make validity key
                                           authenticate
                                           make privacy key(s)
                                           mask/encrypt message

   The exchange of messages is not related to the Initiator and
   Responder.  Instead, either party may send one of these messages at
   any time.  The messages are easily distinguished by the parties.


6.0.1.  Send SPI_Needed

   At any time after completion of the Identification Exchange, either
   party can send SPI_Needed.  This message is sent when a prospective
   SPI User needs particular attributes for a datagram (such as
   confidentiality), and no current SPI has those attributes.

   The prospective SPI User selects from the intersection of attributes
   that both parties have previously offered, calculates the
   Verification, and masks the message using the Privacy-Method
   indicated by the current Scheme-Choice.

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6.0.2.  Receive SPI_Needed

   The potential SPI Owner validates the pair of Cookies, the Padding,
   the Verification, and the Attributes-Needed.

   -  When an invalid/expired cookie is detected, a Bad_Cookie message
      is sent.

   -  When too many SPI values are already in use for this particular
      peer, or some other resource limit is reached, a Resource_Limit
      message is sent.

   -  After unmasking, when invalid Padding is detected, the variable
      length Attributes-Needed do not match the UDP Length, or an
      attribute was not in the Offered-Attributes, the message is
      silently discarded.

   -  When the message verification fails, a Verification_Failure
      message is sent.

   -  Whenever such a problem is detected, the SPI is not established;
      the implementation SHOULD log the occurance, and notify an
      operator as appropriate.

   When the message is valid, the party SHOULD send SPI_Update with the
   necessary attributes.

   If an existing SPI has those attributes, that SPI is returned in the
   SPI_Update with the remaining SPILT.


6.0.3.  Send SPI_Update

   At any time after completion of the Identification Exchange, either
   party can send SPI_Update.  This message has effect in only one
   direction, from the SPI Owner to the SPI User.

   The SPI Owner chooses the SPI and SPILT, a set of Attributes for the
   SPI, calculates the Verification, and masks the message using the
   Privacy-Method indicated by the current Scheme-Choice.


6.0.4.  Receive SPI_Update

   The prospective SPI User validates the pair of Cookies, the Padding,
   the Verification, and the Attributes-Needed.

   -  When an invalid/expired cookie is detected, a Bad_Cookie message

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      is sent.

   -  After unmasking, when invalid Padding is detected, the variable
      length Attribute-Choices do not match the UDP Length, an attribute
      was not in the Offered-Attributes, or the message modifies an
      existing SPI, the message is silently discarded.

   -  When the message verification fails, a Verification_Failure
      message is sent.

   -  Whenever such a problem is detected, the SPI is not established;
      the implementation SHOULD log the occurance, and notify an
      operator as appropriate.

   When the message is valid, further actions are dependent on the value
   of the LifeTime field, as described later.


6.0.5.  Automated SPI_Updates

   Each SPI requires replacement under several circumstances:

   -  the volume of data processed (inhibiting probability
      cryptanalysis),

   -  exhaustion of available anti-replay Sequence Numbers,

   -  and expiration of the LifeTime.

   In general, a determination is made upon receipt of a datagram.  If
   the transform specific processing finds that refreshment is needed,
   an automated SPI_Update is triggered.

   In addition, automated SPI_Updates allow rapid SPI refreshment for
   high bandwidth applications in a high delay environment.  The update
   messages flow in the opposite direction from the primary traffic,
   conserving bandwidth and avoiding service interruption.

   When creating each SPI, the implementation MAY optionally set an
   Update TimeOut (UTO); by default, to half the value of the LifeTime
   (SPILT/2).  This time is highly dynamic, and adjustable to provide an
   automated SPI_Update long before transform specific processing.  If
   no new Photuris exchange occurs within the time limit, and the
   current exchange state has not expired, an automated SPI_Update is
   sent.

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6.1.  SPI_Needed

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Initiator-Cookie                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Responder-Cookie                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Message    |                  Reserved-LT                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Reserved-SPI                          |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                                                               |
   ~                         Verification                          ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Attributes-Needed ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                                      ... Padding  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Initiator-Cookie  16 bytes.  Copied from the Value_Request.

   Responder-Cookie  16 bytes.  Copied from the Value_Request.

   Message          8

   Reserved-LT      3 bytes.  For future use; MUST be filled with a
                    random non-zero value when transmitted, and MUST be
                    ignored when received.

   Reserved-SPI     4 bytes.  For future use; MUST be set to zero when
                    transmitted, and MUST be ignored when received.

   Verification     Variable Precision Integer, or other format
                    indicated by the current Scheme-Choice.  The
                    calculation of the value is described in "Validity
                    Verification".

                    The field may be any integral number of bytes in
                    length.  It does not require any particular
                    alignment.  The 32-bit alignment shown is for
                    convenience in the illustration.

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   Attributes-Needed
                    4 or more bytes.  A list of two or more attributes,
                    selected from the list of Offered-Attributes
                    supported by the peer.

                    The contents and usage of this list are as
                    previously described in "Attribute Choices List".
                    The end of the list is indicated by the UDP Length
                    after removing the Padding (UDP Length - last
                    Padding value).

   Padding          8 or more bytes.  The message is padded in the same
                    fashion specified for Identification Exchange
                    messages.

   The portion of the message after the SPI field is masked using the
   Privacy-Method indicated by the current Scheme-Choice.

   The fields following the SPI are opaque.  That is, the values are set
   prior to masking (and optional encryption), and examined only after
   unmasking (and optional decryption).

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6.2.  SPI_Update

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Initiator-Cookie                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Responder-Cookie                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Message    |                    LifeTime                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Security-Parameters-Index                   |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                                                               |
   ~                         Verification                          ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Attribute-Choices ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                                      ... Padding  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Initiator-Cookie  16 bytes.  Copied from the Value_Request.

   Responder-Cookie  16 bytes.  Copied from the Value_Request.

   Message          9

   LifeTime         3 bytes.  The number of seconds remaining before the
                    indicated SPI expires.  The value zero indicates
                    deletion of the indicated SPI.

   Security-Parameters-Index (SPI)
                    4 bytes.  The SPI to be used for incoming
                    communications.

                    This may be a new SPI value (for creation), or an
                    existing SPI value (for deletion).  The value zero
                    indicates special processing.

   Verification     Variable Precision Integer, or other format
                    indicated by the current Scheme-Choice.  The
                    calculation of the value is described in "Validity
                    Verification".

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                    The field may be any integral number of bytes in
                    length.  It does not require any particular
                    alignment.  The 32-bit alignment shown is for
                    convenience in the illustration.

   Attribute-Choices
                    0 or more bytes.  When the SPI and SPILT are non-
                    zero, a list of attributes selected from the list of
                    Offered-Attributes supported by the peer.

                    The contents and usage of this list are as
                    previously described in "Attribute Choices List".
                    The end of the list is indicated by the UDP Length
                    after removing the Padding (UDP Length - last
                    Padding value).

   Padding          8 or more bytes.  The message is padded in the same
                    fashion specified for Identification Exchange
                    messages.

   The portion of the message after the SPI field is masked using the
   Privacy-Method indicated by the current Scheme-Choice.

   The fields following the SPI are opaque.  That is, the values are set
   prior to masking (and optional encryption), and examined only after
   unmasking (and optional decryption).


6.2.1.  Creation

   When the LifeTime is non-zero, and the SPI is also non-zero, the
   SPI_Update can be used to create a new SPI.  When the SPI is zero,
   the SPI_Update is silently discarded.

   The new session-keys are calculated in the same fashion as the
   Identity_Messages.  Since the SPI value is always different than any
   previous SPI during the Exchange LifeTime of the shared-secret, the
   resulting session-keys will necessarily be different from all others
   used in the same direction.

   No retransmission timer is necessary.  Success is indicated by the
   peer use of the new SPI.

   Should all creation attempts fail, eventually the peer will find that
   all existing SPIs have expired, and will begin the Photuris exchange
   again by sending a new Cookie_Request.  When appropriate, this
   Cookie_Request MAY include a Responder-Cookie to retain previous
   party pairings.

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6.2.2.  Deletion

   When the LifeTime is zero, the SPI_Update can be used to delete a
   single existing SPI.  When the SPI is also zero, the SPI_Update will
   delete all existing SPIs related to this Security Association, and
   mark the Photuris exchange state as expired.  This is especially
   useful when the application that needed them terminates.

   No retransmission timer is necessary.  This message is advisory, to
   reduce the number of ICMP Security Failures messages.

   Should any deletion attempts fail, the peer will learn that the
   deleted SPIs are invalid through the normal ICMP Security Failures
   messages, and will initiate a Photuris exchange by sending a new
   Cookie_Request.


6.2.3.  Modification

   The SPI_Update cannot be used to modify existing SPIs, such as
   lengthen an existing SPI LifeTime, resurrect an expired SPI, or
   add/remove an Attribute-Choice.

   On receipt, such an otherwise valid message is silently discarded.


6.3.  Validity Verification

   These messages are authenticated using the Validity-Method indicated
   by the current Scheme-Choice.  The Verification value is calculated
   prior to masking (and optional encryption), and verified after
   unmasking (and optional decryption).

   The Validity-Method authentication function is supplied with two
   input values:

    - the sender (SPI Owner) verification-key,
    - the data to be verified (as a concatenated sequence of bytes).

   The resulting output value is stored in the Verification field.

   The Validity-Method verification data consists of the following
   concatenated values:

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    + the Initiator Cookie,
    + the Responder Cookie,
    + the Message, LifeTime and SPI (or Reserved) fields,
    + the SPI Owner Identity Verification,
    + the SPI User Identity Verification,
    + the Attribute-Choices following the Verification field,
    + the Padding.

   Note that the order of the Identity Verification fields (from the
   Identity_Messages) is different in each direction, and the Message,
   LifeTime and SPI fields are also likely to be different in each
   direction.

   If the verification fails, the users are notified, and a
   Verification_Failure message is sent, without adding or deleting any
   SPIs.  On success, normal operation begins with the authentication
   and/or encryption of user datagrams.

   Implementation Notes:

      This is distinct from any authentication method specified for the
      SPI.

      The Identity Verification data includes both the Size and Value
      fields.  The Attribute-Choices data includes the Attribute, Length
      and Value fields.


7.  Error Messages

   These messages are issued in response to Photuris state loss or other
   problems.  A message has effect in only one direction.  No
   retransmission timer is necessary.

   These messages are not masked.

   The receiver checks the Cookies for validity.  Special care MUST be
   taken that the Cookie pair in the Error Message actually match a pair
   currently in use, and that the protocol is currently in a state where
   such an Error Message might be expected.  Otherwise, these messages
   could provide an opportunity for a denial of service attack.  Invalid
   messages are silently discarded.

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7.1.  Bad_Cookie

   For the format of the 33 byte message, see "Header Format".  There
   are no additional fields.

   Initiator-Cookie  16 bytes.  Copied from the offending message.

   Responder-Cookie  16 bytes.  Copied from the offending message.

   Message          10

   This error message is sent when a Value_Request, Identity_Request,
   SPI_Needed, or SPI_Update is received, and the receiver specific
   Cookie is invalid or the associated exchange state has expired.

   During the Photuris exchange, when this error message is received, it
   has no immediate effect on the operation of the protocol phases.
   Later, when Retransmissions have been exceeded, and this error
   message has been received, the Initiator SHOULD begin the Photuris
   exchange again by sending a new Cookie_Request with the Responder-
   Cookie and Counter updated appropriately.

   When this error message is received in response to SPI_Needed, the
   exchange state SHOULD NOT be marked as expired, but the party SHOULD
   initiate a Photuris exchange by sending a new Cookie_Request.

   When this error message is received in response to SPI_Update, the
   exchange state SHOULD NOT be marked as expired, and no further action
   is taken.  A new exchange will be initiated later when needed by the
   peer to send authenticated and/or encrypted data.

   Existing SPIs are not deleted.  They expire normally, and are purged
   sometime later.


7.2.  Resource_Limit

   For the format of the 34 byte message, see "Cookie_Request".  There
   are no additional fields.

   Initiator-Cookie  16 bytes.  Copied from the offending message.

   Responder-Cookie  16 bytes.  Copied from the offending message.

                    Special processing is applied to a Cookie_Request.
                    When the offending message Responder-Cookie and
                    Counter were both zero, and an existing exchange has
                    not yet been purged, this field is replaced with the

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                    Responder-Cookie from the existing exchange.

   Message          11

   Counter          1 byte.  Copied from the offending message.

                    When zero, the Responder-Cookie indicates the
                    Initiator of a previous exchange, or no previous
                    exchange is specified.

                    When non-zero, the Responder-Cookie indicates the
                    Responder to a previous exchange.  This value is set
                    to the Counter from the corresponding
                    Cookie_Response.

   This error message is sent when a Cookie_Request, Value_Request or
   SPI_Needed is received, and too many SPI values are already in use
   for that peer, or some other Photuris resource is unavailable.

   During the Photuris exchange, when this error message is received in
   response to a Cookie_Request or Value_Request, the implementation
   SHOULD double the retransmission timeout (as usual) for sending
   another Cookie_Request or Value_Request.  Otherwise, it has no
   immediate effect on the operation of the protocol phases.  Later,
   when Retransmissions have been exceeded, and this error message has
   been received, the Initiator SHOULD begin the Photuris exchange again
   by sending a new Cookie_Request with the Responder-Cookie and Counter
   updated appropriately.

   When this error message is received in response to SPI_Needed, the
   implementation SHOULD NOT send another SPI_Needed until one of the
   existing SPIs associated with this exchange is deleted or has
   expired.


7.3.  Verification_Failure

   For the format of the 33 byte message, see "Header Format".  There
   are no additional fields.

   Initiator-Cookie  16 bytes.  Copied from the offending message.

   Responder-Cookie  16 bytes.  Copied from the offending message.

   Message          12

   This error message is sent when an Identity_Message, SPI_Needed or
   SPI_Update is received, and verification fails.

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   When this error message is received, the implementation SHOULD log
   the occurance, and notify an operator as appropriate.  However,
   receipt has no effect on the operation of the protocol.


7.4.  Message_Reject

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Initiator-Cookie                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Responder-Cookie                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Message    |  Bad-Message  |             Offset            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Initiator-Cookie  16 bytes.  Copied from the offending message.

   Responder-Cookie  16 bytes.  Copied from the offending message.

   Message          13

   Bad-Message      1 byte.  Indicates the Message number of the
                    offending message.

   Offset           2 bytes.  The number of bytes from the beginning of
                    the offending message where the unrecognized field
                    starts.  The minimum value is 32.

   This error message is sent when an optional Message type is received
   that is not supported, or an optional format of a supported Message
   is not recognized.

   When this error message is received, the implementation SHOULD log
   the occurance, and notify an operator as appropriate.  However,
   receipt has no effect on the operation of the protocol.


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