rfc2274.txt

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          }
   END

   The fields of this sequence are:

   - The msgAuthoritativeEngineID specifies the snmpEngineID of the
     authoritative SNMP engine involved in the exchange of the message.

   - The msgAuthoritativeEngineBoots specifies the snmpEngineBoots
     value at the authoritative SNMP engine involved in the exchange of
     the message.

   - The msgAuthoritativeEngineTime specifies the snmpEngineTime value
     at the authoritative SNMP engine involved in the exchange of the
     message.

   - The msgUserName specifies the user (principal) on whose behalf
     the message is being exchanged.






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   - The msgAuthenticationParameters are defined by the authentication
     protocol in use for the message, as defined by the
     usmUserAuthProtocol column in the user's entry in the usmUserTable.

   - The msgPrivacyParameters are defined by the privacy protocol in
     use for the message, as defined by the usmUserPrivProtocol column
     in the user's entry in the usmUserTable).

   See appendix A.4 for an example of the BER encoding of field
   msgSecurityParameters.

2.5.  Services provided by the User-based Security Model

   This section describes the services provided by the User-based
   Security Model with their inputs and outputs.

   The services are described as primitives of an abstract service
   interface and the inputs and outputs are described as abstract data
   elements as they are passed in these abstract service primitives.

2.5.1.  Services for Generating an Outgoing SNMP Message

   When the Message Processing (MP) Subsystem invokes the User-based
   Security module to secure an outgoing SNMP message, it must use the
   appropriate service as provided by the Security module.  These two
   services are provided:

   1) A service to generate a Request message. The abstract service
      primitive is:

      statusInformation =            -- success or errorIndication
        generateRequestMsg(
        IN   messageProcessingModel  -- typically, SNMP version
        IN   globalData              -- message header, admin data
        IN   maxMessageSize          -- of the sending SNMP entity
        IN   securityModel           -- for the outgoing message
        IN   securityEngineID        -- authoritative SNMP entity
        IN   securityName            -- on behalf of this principal
        IN   securityLevel           -- Level of Security requested
        IN   scopedPDU               -- message (plaintext) payload
        OUT  securityParameters      -- filled in by Security Module
        OUT  wholeMsg                -- complete generated message
        OUT  wholeMsgLength          -- length of generated message
             )

   2) A service to generate a Response message. The abstract service
      primitive is:




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      statusInformation =            -- success or errorIndication
        generateResponseMsg(
        IN   messageProcessingModel  -- typically, SNMP version
        IN   globalData              -- message header, admin data
        IN   maxMessageSize          -- of the sending SNMP entity
        IN   securityModel           -- for the outgoing message
        IN   securityEngineID        -- authoritative SNMP entity
        IN   securityName            -- on behalf of this principal
        IN   securityLevel           -- Level of Security requested
        IN   scopedPDU               -- message (plaintext) payload
        IN   securityStateReference  -- reference to security state
                                     -- information from original
                                     -- request
        OUT  securityParameters      -- filled in by Security Module
        OUT  wholeMsg                -- complete generated message
        OUT  wholeMsgLength          -- length of generated message
             )

   The abstract data elements passed as parameters in the abstract
   service primitives are as follows:

    statusInformation
      An indication of whether the encoding and securing of the message
      was successful.  If not it is an indication of the problem.
     essageProcessingModel
      The SNMP version number for the message to be generated.  This
      data is not used by the User-based Security module.
    globalData
      The message header (i.e., its administrative information). This
      data is not used by the User-based Security module.
    maxMessageSize
      The maximum message size as included in the message.  This data is
      not used by the User-based Security module.
    securityParameters
      These are the security parameters. They will be filled in by the
      User-based Security module.
    securityModel
      The securityModel in use. Should be User-based Security Model.
      This data is not used by the User-based Security module.
    securityName
      Together with the snmpEngineID it identifies a row in the
      usmUserTable that is to be used for securing the message.  The
      securityName has a format that is independent of the Security
      Model. In case of a response this parameter is ignored and the
      value from the cache is used.
    securityLevel
      The Level of Security from which the User-based Security module
      determines if the message needs to be protected from disclosure



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      and if the message needs to be authenticated.  In case of a
      response this parameter is ignored and the value from the cache is
      used.
    securityEngineID
      The snmpEngineID of the authoritative SNMP engine to which a
      Request message is to be sent. In case of a response it is implied
      to be the processing SNMP engine's snmpEngineID and so if it is
      specified, then it is ignored.
    scopedPDU
      The message payload.  The data is opaque as far as the User-based
      Security Model is concerned.
    securityStateReference
      A handle/reference to cachedSecurityData to be used when securing
      an outgoing Response message.  This is the exact same
      handle/reference as it was generated by the User-based Security
      module when processing the incoming Request message to which this
      is the Response message.
    wholeMsg
      The fully encoded and secured message ready for sending on the
      wire.
    wholeMsgLength
      The length of the encoded and secured message (wholeMsg).

   Upon completion of the process, the User-based Security module
   returns statusInformation. If the process was successful, the
   completed message with privacy and authentication applied if such was
   requested by the specified securityLevel is returned. If the process
   was not successful, then an errorIndication is returned.

2.5.2.  Services for Processing an Incoming SNMP Message

   When the Message Processing (MP) Subsystem invokes the User-based
   Security module to verify proper security of an incoming message, it
   must use the service provided for an incoming message. The abstract
   service primitive is:

   statusInformation =             -- errorIndication or success
                                   -- error counter OID/value if error
     processIncomingMsg(
     IN   messageProcessingModel   -- typically, SNMP version
     IN   maxMessageSize           -- of the sending SNMP entity
     IN   securityParameters       -- for the received message
     IN   securityModel            -- for the received message
     IN   securityLevel            -- Level of Security
     IN   wholeMsg                 -- as received on the wire
     IN   wholeMsgLength           -- length as received on the wire
     OUT  securityEngineID         -- authoritative SNMP entity
     OUT  securityName             -- identification of the principal



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     OUT  scopedPDU,               -- message (plaintext) payload
     OUT  maxSizeResponseScopedPDU -- maximum size of the Response PDU
     OUT  securityStateReference   -- reference to security state
          )                        -- information, needed for response

   The abstract data elements passed as parameters in the abstract
   service primitives are as follows:

    statusInformation
      An indication of whether the process was successful or not.  If
      not, then the statusInformation includes the OID and the value of
      the error counter that was incremented.
    messageProcessingModel
      The SNMP version number as received in the message.  This data is
      not used by the User-based Security module.
    maxMessageSize
      The maximum message size as included in the message.  The User-
      based Security module uses this value to calculate the
      maxSizeResponseScopedPDU.
    securityParameters
      These are the security parameters as received in the message.
    securityModel
      The securityModel in use.  Should be the User-based Security
      Model.  This data is not used by the User-based Security module.
    securityLevel
      The Level of Security from which the User-based Security module
      determines if the message needs to be protected from disclosure
      and if the message needs to be authenticated.
    wholeMsg
      The whole message as it was received.
    wholeMsgLength
      The length of the message as it was received (wholeMsg).
    securityEngineID
      The snmpEngineID that was extracted from the field
      msgAuthoritativeEngineID and that was used to lookup the secrets
      in the usmUserTable.
    securityName
      The security name representing the user on whose behalf the
      message was received.  The securityName has a format that is
      independent of the Security Model.
    scopedPDU
      The message payload.  The data is opaque as far as the User-based
      Security Model is concerned.
    maxSizeResponseScopedPDU
      The maximum size of a scopedPDU to be included in a possible
      Response message.  The User-base Security module calculates





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      this size based on the mms (as received in the message) and the
      space required for the message header (including the
      securityParameters) for such a Response message.
    securityStateReference
      A handle/reference to cachedSecurityData to be used when securing
      an outgoing Response message.  When the Message Processing
      Subsystem calls the User-based Security module to generate a
      response to this incoming message it must pass this
      handle/reference.

   Upon completion of the process, the User-based Security module
   returns statusInformation and, if the process was successful, the
   additional data elements for further processing of the message.  If
   the process was not successful, then an errorIndication, possibly
   with a OID and value pair of an error counter that was incremented.

2.6.  Key Localization Algorithm.

   A localized key is a secret key shared between a user U and one
   authoritative SNMP engine E.  Even though a user may have only one
   password and therefore one key for the whole network, the actual
   secrets shared between the user and each authoritative SNMP engine
   will be different. This is achieved by key localization [Localized-
   key].

   First, if a user uses a password, then the user's password is
   converted into a key Ku using one of the two algorithms described in
   Appendices A.2.1 and A.2.2.

   To convert key Ku into a localized key Kul of user U at the
   authoritative SNMP engine E, one appends the snmpEngineID of the
   authoritative SNMP engine to the key Ku and then appends the key Ku
   to the result, thus enveloping the snmpEngineID within the two copies
   of user's key Ku. Then one runs a secure hash function (which one
   depends on the authentication protocol defined for this user U at
   authoritative SNMP engine E; this document defines two authentication
   protocols with their associated algorithms based on MD5 and SHA). The
   output of the hash-function is the localized key Kul for user U at