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RFC 2264


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User-based Security Model (USM) for version 3 of the Simple Network Management Protocol (SNMPv3)

Part 1 of 3, p. 1 to 21
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Network Working Group                                      U. Blumenthal
Request for Comments: 2264                     IBM T. J. Watson Research
Category: Standards Track                                      B. Wijnen
                                               IBM T. J. Watson Research
                                                            January 1998


          User-based Security Model (USM) for version 3 of the
              Simple Network Management Protocol (SNMPv3)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1997).  All Rights Reserved.

Abstract

   This document describes the User-based Security Model (USM) for SNMP
   version 3 for use in the SNMP architecture [RFC2261].  It defines the
   Elements of Procedure for providing SNMP message level security.
   This document also includes a MIB for remotely monitoring/managing
   the configuration parameters for this Security Model.

Table of Contents

 1.  Introduction                                                      3
 1.1.  Threats                                                         4
 1.2.  Goals and Constraints                                           5
 1.3.  Security Services                                               6
 1.4.  Module Organization                                             7
 1.4.1.  Timeliness Module                                             7
 1.4.2.  Authentication Protocol                                       8
 1.4.3.  Privacy Protocol                                              8
 1.5.  Protection against Message Replay, Delay and Redirection        8
 1.5.1.  Authoritative SNMP engine                                     8
 1.5.2.  Mechanisms                                                    8
 1.6.  Abstract Service Interfaces.                                   10
 1.6.1.  User-based Security Model Primitives for Authentication      11
 1.6.2.  User-based Security Model Primitives for Privacy             11
 2.  Elements of the Model                                            12
 2.1.  User-based Security Model Users                                12

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 2.2.  Replay Protection                                              13
 2.2.1.  msgAuthoritativeEngineID                                     13
 2.2.2.  msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime   14
 2.2.3.  Time Window                                                  15
 2.3.  Time Synchronization                                           15
 2.4.  SNMP Messages Using this Security Model                        16
 2.5.  Services provided by the User-based Security Model             17
 2.5.1.  Services for Generating an Outgoing SNMP Message             17
 2.5.2.  Services for Processing an Incoming SNMP Message             19
 2.6.  Key Localization Algorithm.                                    21
 3.  Elements of Procedure                                            21
 3.1.  Generating an Outgoing SNMP Message                            22
 3.2.  Processing an Incoming SNMP Message                            25
 4.  Discovery                                                        30
 5.  Definitions                                                      31
 6.  HMAC-MD5-96 Authentication Protocol                              45
 6.1.  Mechanisms                                                     45
 6.1.1.  Digest Authentication Mechanism                              46
 6.2.  Elements of the Digest Authentication Protocol                 46
 6.2.1.  Users                                                        46
 6.2.2.  msgAuthoritativeEngineID                                     47
 6.2.3.  SNMP Messages Using this Authentication Protocol             47
 6.2.4.  Services provided by the HMAC-MD5-96 Authentication Module   47
 6.2.4.1.  Services for Generating an Outgoing SNMP Message           47
 6.2.4.2.  Services for Processing an Incoming SNMP Message           48
 6.3.  Elements of Procedure                                          49
 6.3.1.  Processing an Outgoing Message                               49
 6.3.2.  Processing an Incoming Message                               50
 7.  HMAC-SHA-96 Authentication Protocol                              51
 7.1.  Mechanisms                                                     51
 7.1.1.  Digest Authentication Mechanism                              51
 7.2.  Elements of the HMAC-SHA-96 Authentication Protocol            52
 7.2.1.  Users                                                        52
 7.2.2.  msgAuthoritativeEngineID                                     52
 7.2.3.  SNMP Messages Using this Authentication Protocol             53
 7.2.4.  Services provided by the HMAC-SHA-96 Authentication Module   53
 7.2.4.1.  Services for Generating an Outgoing SNMP Message           53
 7.2.4.2.  Services for Processing an Incoming SNMP Message           54
 7.3.  Elements of Procedure                                          54
 7.3.1.  Processing an Outgoing Message                               55
 7.3.2.  Processing an Incoming Message                               55
 8.  CBC-DES Symmetric Encryption Protocol                            56
 8.1.  Mechanisms                                                     56
 8.1.1.  Symmetric Encryption Protocol                                57
 8.1.1.1.  DES key and Initialization Vector.                         57
 8.1.1.2.  Data Encryption.                                           58
 8.1.1.3.  Data Decryption                                            59
 8.2.  Elements of the DES Privacy Protocol                           59

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 8.2.1.  Users                                                        59
 8.2.2.  msgAuthoritativeEngineID                                     59
 8.2.3.  SNMP Messages Using this Privacy Protocol                    60
 8.2.4.  Services provided by the DES Privacy Module                  60
 8.2.4.1.  Services for Encrypting Outgoing Data                      60
 8.2.4.2.  Services for Decrypting Incoming Data                      61
 8.3.  Elements of Procedure.                                         61
 8.3.1.  Processing an Outgoing Message                               61
 8.3.2.  Processing an Incoming Message                               62
 9.  Intellectual Property                                            62
 10. Acknowledgements                                                 63
 11. Security Considerations                                          64
 11.1. Recommended Practices                                          64
 11.2. Defining Users                                                 66
 11.3. Conformance                                                    67
 12. References                                                       67
 13. Editors' Addresses                                               69
 A.1.  SNMP engine Installation Parameters                            70
 A.2.  Password to Key Algorithm                                      71
 A.2.1.  Password to Key Sample Code for MD5                          71
 A.2.2.  Password to Key Sample Code for SHA                          72
 A.3.  Password to Key Sample Results                                 73
 A.3.1.  Password to Key Sample Results using MD5                     73
 A.3.2.  Password to Key Sample Results using SHA                     74
 A.4.  Sample encoding of msgSecurityParameters                       74
 B.  Full Copyright Statement                                         76

1.  Introduction

   The Architecture for describing Internet Management Frameworks
   [RFC2261] describes that an SNMP engine is composed of:

     1) a Dispatcher
     2) a Message Processing Subsystem,
     3) a Security Subsystem, and
     4) an Access Control Subsystem.

   Applications make use of the services of these subsystems.

   It is important to understand the SNMP architecture and the
   terminology of the architecture to understand where the Security
   Model described in this document fits into the architecture and
   interacts with other subsystems within the architecture.  The reader
   is expected to have read and understood the description of the SNMP
   architecture, as defined in [RFC2261].

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   This memo [RFC2264] describes the User-based Security Model as it is
   used within the SNMP Architecture.  The main idea is that we use the
   traditional concept of a user (identified by a userName) with which
   to associate security information.

   This memo describes the use of HMAC-MD5-96 and HMAC-SHA-96 as the
   authentication protocols and the use of CBC-DES as the privacy
   protocol. The User-based Security Model however allows for other such
   protocols to be used instead of or concurrent with these protocols.
   Therefore, the description of HMAC-MD5-96, HMAC-SHA-96 and CBC-DES
   are in separate sections to reflect their self-contained nature and
   to indicate that they can be replaced or supplemented in the future.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

1.1.  Threats

   Several of the classical threats to network protocols are applicable
   to the network management problem and therefore would be applicable
   to any SNMP Security Model.  Other threats are not applicable to the
   network management problem.  This section discusses principal
   threats, secondary threats, and threats which are of lesser
   importance.

   The principal threats against which this SNMP Security Model should
   provide protection are:

   - Modification of Information
     The modification threat is the danger that some unauthorized entity
     may alter in-transit SNMP messages generated on behalf of an
     authorized user in such a way as to effect unauthorized management
     operations, including falsifying the value of an object.

   - Masquerade
     The masquerade threat is the danger that management operations not
     authorized for some user may be attempted by assuming the identity
     of another user that has the appropriate authorizations.

   Two secondary threats are also identified.  The Security Model
   defined in this memo provides limited protection against:

   - Disclosure
     The disclosure threat is the danger of eavesdropping on the
     exchanges between managed agents and a management station.
     Protecting against this threat may be required as a matter of local
     policy.

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   - Message Stream Modification
     The SNMP protocol is typically based upon a connection-less
     transport service which may operate over any sub-network service.
     The re-ordering, delay or replay of messages can and does occur
     through the natural operation of many such sub-network services.
     The message stream modification threat is the danger that messages
     may be maliciously re-ordered, delayed or replayed to an extent
     which is greater than can occur through the natural operation of a
     sub-network service, in order to effect unauthorized management
     operations.

   There are at least two threats that an SNMP Security Model need not
   protect against.  The security protocols defined in this memo do not
   provide protection against:

   - Denial of Service
     This SNMP Security Model does not attempt to address the broad
     range of attacks by which service on behalf of authorized users is
     denied.  Indeed, such denial-of-service attacks are in many cases
     indistinguishable from the type of network failures with which any
     viable network management protocol must cope as a matter of course.
   - Traffic Analysis
     This SNMP Security Model does not attempt to address traffic
     analysis attacks.  Indeed, many traffic patterns are predictable -
     devices may be managed on a regular basis by a relatively small
     number of management applications - and therefore there is no
     significant advantage afforded by protecting against traffic
     analysis.

1.2.  Goals and Constraints

   Based on the foregoing account of threats in the SNMP network
   management environment, the goals of this SNMP Security Model are as
   follows.

   1) Provide for verification that each received SNMP message has
      not been modified during its transmission through the network.

   2) Provide for verification of the identity of the user on whose
      behalf a received SNMP message claims to have been generated.

   3) Provide for detection of received SNMP messages, which request
      or contain management information, whose time of generation was
      not recent.

   4) Provide, when necessary, that the contents of each received
      SNMP message are protected from disclosure.

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   In addition to the principal goal of supporting secure network
   management, the design of this SNMP Security Model is also influenced
   by the following constraints:

   1) When the requirements of effective management in times of
      network stress are inconsistent with those of security, the design
      should prefer the former.

   2) Neither the security protocol nor its underlying security
      mechanisms should depend upon the ready availability of other
      network services (e.g., Network Time Protocol (NTP) or key
      management protocols).

   3) A security mechanism should entail no changes to the basic
      SNMP network management philosophy.

1.3.  Security Services

   The security services necessary to support the goals of this SNMP
   Security Model are as follows:

   - Data Integrity
     is the provision of the property that data has not been altered or
     destroyed in an unauthorized manner, nor have data sequences been
     altered to an extent greater than can occur non-maliciously.

   - Data Origin Authentication
     is the provision of the property that the claimed identity of the
     user on whose behalf received data was originated is corroborated.

   - Data Confidentiality
     is the provision of the property that information is not made
     available or disclosed to unauthorized individuals, entities, or
     processes.

   - Message timeliness and limited replay protection
     is the provision of the property that a message whose generation
     time is outside of a specified time window is not accepted.  Note
     that message reordering is not dealt with and can occur in normal
     conditions too.

   For the protocols specified in this memo, it is not possible to
   assure the specific originator of a received SNMP message; rather, it
   is the user on whose behalf the message was originated that is
   authenticated.

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   For these protocols, it not possible to obtain data integrity without
   data origin authentication, nor is it possible to obtain data origin
   authentication without data integrity.  Further, there is no
   provision for data confidentiality without both data integrity and
   data origin authentication.

   The security protocols used in this memo are considered acceptably
   secure at the time of writing.  However, the procedures allow for new
   authentication and privacy methods to be specified at a future time
   if the need arises.

1.4.  Module Organization

   The security protocols defined in this memo are split in three
   different modules and each has its specific responsibilities such
   that together they realize the goals and security services described
   above:

   - The authentication module MUST provide for:

     - Data Integrity,

     - Data Origin Authentication

   - The timeliness module MUST provide for:

     - Protection against message delay or replay (to an extent
       greater than can occur through normal operation)

     The privacy module MUST provide for

     - Protection against disclosure of the message payload.

   The timeliness module is fixed for the User-based Security Model
   while there is provision for multiple authentication and/or privacy
   modules, each of which implements a specific authentication or
   privacy protocol respectively.

1.4.1.  Timeliness Module

   Section 3 (Elements of Procedure) uses the timeliness values in an
   SNMP message to do timeliness checking.  The timeliness check is only
   performed if authentication is applied to the message.  Since the
   complete message is checked for integrity, we can assume that the
   timeliness values in a message that passes the authentication module
   are trustworthy.

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1.4.2.  Authentication Protocol

   Section 6 describes the HMAC-MD5-96 authentication protocol which is
   the first authentication protocol that MUST be supported with the
   User-based Security Model.  Section 7 describes the HMAC-SHA-96
   authentication protocol which is another authentication protocol that
   SHOULD be supported with the User-based Security Model.  In the
   future additional or replacement authentication protocols may be
   defined as new needs arise.

   The User-based Security Model prescribes that, if authentication is
   used, then the complete message is checked for integrity in the
   authentication module.

   For a message to be authenticated, it needs to pass authentication
   check by the authentication module and the timeliness check which is
   a fixed part of this User-based Security model.

1.4.3.  Privacy Protocol

   Section 8 describes the CBC-DES Symmetric Encryption Protocol which
   is the first privacy protocol to be used with the User-based Security
   Model.  In the future additional or replacement privacy protocols may
   be defined as new needs arise.

   The User-based Security Model prescribes that the scopedPDU is
   protected from disclosure when a message is sent with privacy.

   The User-based Security Model also prescribes that a message needs to
   be authenticated if privacy is in use.

1.5.  Protection against Message Replay, Delay and Redirection

1.5.1.  Authoritative SNMP engine

   In order to protect against message replay, delay and redirection,
   one of the SNMP engines involved in each communication is designated
   to be the authoritative SNMP engine.  When an SNMP message contains a
   payload which expects a response (for example a Get, GetNext,
   GetBulk, Set or Inform PDU), then the receiver of such messages is
   authoritative.  When an SNMP message contains a payload which does
   not expect a response (for example an SNMPv2-Trap, Response or Report
   PDU), then the sender of such a message is authoritative.

1.5.2.  Mechanisms

   The following mechanisms are used:

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   1) To protect against the threat of message delay or replay (to an
      extent greater than can occur through normal operation), a set of
      timeliness indicators (for the authoritative SNMP engine) are
      included in each message generated.  An SNMP engine evaluates the
      timeliness indicators to determine if a received message is
      recent.  An SNMP engine may evaluate the timeliness indicators to
      ensure that a received message is at least as recent as the last
      message it received from the same source.  A non-authoritative
      SNMP engine uses received authentic messages to advance its notion
      of the timeliness indicators at the remote authoritative source.

      An SNMP engine MUST also use a mechanism to match incoming
      Responses to outstanding Requests and it MUST drop any Responses
      that do not match an outstanding request. For example, a msgID can
      be inserted in every message to cater for this functionality.

      These mechanisms provide for the detection of authenticated
      messages whose time of generation was not recent.

      This protection against the threat of message delay or replay does
      not imply nor provide any protection against unauthorized deletion
      or suppression of messages.  Also, an SNMP engine may not be able
      to detect message reordering if all the messages involved are sent
      within the Time Window interval.  Other mechanisms defined
      independently of the security protocol can also be used to detect
      the re-ordering replay, deletion, or suppression of messages
      containing Set operations (e.g., the MIB variable snmpSetSerialNo
      [RFC1907]).

   2) Verification that a message sent to/from one authoritative SNMP
      engine cannot be replayed to/as-if-from another authoritative SNMP
      engine.

      Included in each message is an identifier unique to the
      authoritative SNMP engine associated with the sender or intended
      recipient of the message.

      A Report, Response or Trap message sent by an authoritative SNMP
      engine to one non-authoritative SNMP engine can potentially be
      replayed to another non-authoritative SNMP engine. The latter
      non-authoritative SNMP engine might (if it knows about the same
      userName with the same secrets at the authoritative SNMP engine)
      as a result update its notion of timeliness indicators of the
      authoritative SNMP engine, but that is not considered a threat.
      In this case, A Report or Response message will be discarded by
      the Message Processing Model, because there should not be an
      outstanding Request message. A Trap will possibly be accepted.
      Again, that is not considered a threat, because the communication

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      was authenticated and timely. It is as if the authoritative SNMP
      engine was configured to start sending Traps to the second SNMP
      engine, which theoretically can happen without the knowledge of
      the second SNMP engine anyway. Anyway, the second SNMP engine may
      not expect to receive this Trap, but is allowed to see the
      management information contained in it.

   3) Detection of messages which were not recently generated.

      A set of time indicators are included in the message, indicating
      the time of generation.  Messages without recent time indicators
      are not considered authentic.  In addition, an SNMP engine MUST
      drop any Responses that do not match an outstanding request. This
      however is the responsibility of the Message Processing Model.

   This memo allows the same user to be defined on multiple SNMP
   engines.  Each SNMP engine maintains a value, snmpEngineID, which
   uniquely identifies the SNMP engine.  This value is included in each
   message sent to/from the SNMP engine that is authoritative (see
   section 1.5.1).  On receipt of a message, an authoritative SNMP
   engine checks the value to ensure that it is the intended recipient,
   and a non-authoritative SNMP engine uses the value to ensure that the
   message is processed using the correct state information.

   Each SNMP engine maintains two values, snmpEngineBoots and
   snmpEngineTime, which taken together provide an indication of time at
   that SNMP engine.  Both of these values are included in an
   authenticated message sent to/received from that SNMP engine.  On
   receipt, the values are checked to ensure that the indicated
   timeliness value is within a Time Window of the current time.  The
   Time Window represents an administrative upper bound on acceptable
   delivery delay for protocol messages.

   For an SNMP engine to generate a message which an authoritative SNMP
   engine will accept as authentic, and to verify that a message
   received from that authoritative SNMP engine is authentic, such an
   SNMP engine must first achieve timeliness synchronization with the
   authoritative SNMP engine. See section 2.3.

1.6.  Abstract Service Interfaces.

   Abstract service interfaces have been defined to describe the
   conceptual interfaces between the various subsystems within an SNMP
   entity. Similarly a set of abstract service interfaces have been
   defined within the User-based Security Model (USM) to describe the
   conceptual interfaces between the generic USM services and the self-
   contained authentication and privacy services.

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   These abstract service interfaces are defined by a set of primitives
   that define the services provided and the abstract data elements that
   must be passed when the services are invoked. This section lists the
   primitives that have been defined for the User-based Security Model.

1.6.1.  User-based Security Model Primitives for Authentication

   The User-based Security Model provides the following internal
   primitives to pass data back and forth between the Security Model
   itself and the authentication service:

   statusInformation =
     authenticateOutgoingMsg(
     IN   authKey                   -- secret key for authentication
     IN   wholeMsg                  -- unauthenticated complete message
     OUT  authenticatedWholeMsg     -- complete authenticated message
          )

   statusInformation =
     authenticateIncomingMsg(
     IN   authKey                   -- secret key for authentication
     IN   authParameters            -- as received on the wire
     IN   wholeMsg                  -- as received on the wire
     OUT  authenticatedWholeMsg     -- complete authenticated message
          )

1.6.2.  User-based Security Model Primitives for Privacy

   The User-based Security Model provides the following internal
   primitives to pass data back and forth between the Security Model
   itself and the privacy service:

   statusInformation =
     encryptData(
     IN    encryptKey               -- secret key for encryption
     IN    dataToEncrypt            -- data to encrypt (scopedPDU)
     OUT   encryptedData            -- encrypted data (encryptedPDU)
     OUT   privParameters           -- filled in by service provider
           )

   statusInformation =
     decryptData(
     IN    decryptKey               -- secret key for decrypting
     IN    privParameters           -- as received on the wire
     IN    encryptedData            -- encrypted data (encryptedPDU)
     OUT   decryptedData            -- decrypted data (scopedPDU)
              )

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2.  Elements of the Model

   This section contains definitions required to realize the security
   model defined by this memo.

2.1.  User-based Security Model Users

   Management operations using this Security Model make use of a defined
   set of user identities.  For any user on whose behalf management
   operations are authorized at a particular SNMP engine, that SNMP
   engine must have knowledge of that user.  An SNMP engine that wishes
   to communicate with another SNMP engine must also have knowledge of a
   user known to that engine, including knowledge of the applicable
   attributes of that user.

   A user and its attributes are defined as follows:

   userName
     A string representing the name of the user.

   securityName
     A human-readable string representing the user in a format that is
     Security Model independent.

   authProtocol
     An indication of whether messages sent on behalf of this user can
     be authenticated, and if so, the type of authentication protocol
     which is used.  Two such protocols are defined in this memo:
       - the HMAC-MD5-96 authentication protocol.
       - the HMAC-SHA-96 authentication protocol.

   authKey
     If messages sent on behalf of this user can be authenticated,
     the (private) authentication key for use with the authentication
     protocol.  Note that a user's authentication key will normally
     be different at different authoritative SNMP engines. The authKey
     is not accessible via SNMP. The length requirements of the authKey
     are defined by the authProtocol in use.

   authKeyChange and authOwnKeyChange
     The only way to remotely update the authentication key.  Does
     that in a secure manner, so that the update can be completed
     without the need to employ privacy protection.

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   privProtocol
     An indication of whether messages sent on behalf of this user
     can be protected from disclosure, and if so, the type of privacy
     protocol which is used.  One such protocol is defined in this
     memo: the CBC-DES Symmetric Encryption Protocol.

   privKey
     If messages sent on behalf of this user can be en/decrypted,
     the (private) privacy key for use with the privacy protocol.
     Note that a user's privacy key will normally be different at
     different authoritative SNMP engines. The privKey is not
     accessible via SNMP. The length requirements of the privKey are
     defined by the privProtocol in use.

   privKeyChange and privOwnKeyChange
     The only way to remotely update the encryption key. Does that
     in a secure manner, so that the update can be completed without
     the need to employ privacy protection.

2.2.  Replay Protection

   Each SNMP engine maintains three objects:

   - snmpEngineID, which (at least within an administrative domain)
     uniquely and unambiguously identifies an SNMP engine.

   - snmpEngineBoots, which is a count of the number of times the
     SNMP engine has re-booted/re-initialized since snmpEngineID
     was last configured; and,

   - snmpEngineTime, which is the number of seconds since the
     snmpEngineBoots counter was last incremented.

   Each SNMP engine is always authoritative with respect to these
   objects in its own SNMP entity.  It is the responsibility of a
   non-authoritative SNMP engine to synchronize with the
   authoritative SNMP engine, as appropriate.

   An authoritative SNMP engine is required to maintain the values of
   its snmpEngineID and snmpEngineBoots in non-volatile storage.

2.2.1.  msgAuthoritativeEngineID

   The msgAuthoritativeEngineID value contained in an authenticated
   message is used to defeat attacks in which messages from one SNMP
   engine to another SNMP engine are replayed to a different SNMP
   engine. It represents the snmpEngineID at the authoritative SNMP
   engine involved in the exchange of the message.

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   When an authoritative SNMP engine is first installed, it sets its
   local value of snmpEngineID according to a enterprise-specific
   algorithm (see the definition of the Textual Convention for
   SnmpEngineID in the SNMP Architecture document [RFC2261]).

2.2.2.  msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime

   The msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime
   values contained in an authenticated message are used to defeat
   attacks in which messages are replayed when they are no longer
   valid.  They represent the snmpEngineBoots and snmpEngineTime
   values at the authoritative SNMP engine involved in the exchange
   of the message.

   Through use of snmpEngineBoots and snmpEngineTime, there is no
   requirement for an SNMP engine to have a non-volatile clock which
   ticks (i.e., increases with the passage of time) even when the
   SNMP engine is powered off.  Rather, each time an SNMP engine
   re-boots, it retrieves, increments, and then stores snmpEngineBoots
   in non-volatile storage, and resets snmpEngineTime to zero.

   When an SNMP engine is first installed, it sets its local values
   of snmpEngineBoots and snmpEngineTime to zero.  If snmpEngineTime
   ever reaches its maximum value (2147483647), then snmpEngineBoots
   is incremented as if the SNMP engine has re-booted and
   snmpEngineTime is reset to zero and starts incrementing again.

   Each time an authoritative SNMP engine re-boots, any SNMP engines
   holding that authoritative SNMP engine's values of snmpEngineBoots
   and snmpEngineTime need to re-synchronize prior to sending
   correctly authenticated messages to that authoritative SNMP engine
   (see Section 2.3 for (re-)synchronization procedures).  Note,
   however, that the procedures do provide for a notification to be
   accepted as authentic by a receiving SNMP engine, when sent by an
   authoritative SNMP engine which has re-booted since the receiving
   SNMP engine last (re-)synchronized.

   If an authoritative SNMP engine is ever unable to determine its
   latest snmpEngineBoots value, then it must set its snmpEngineBoots
   value to 2147483647.

   Whenever the local value of snmpEngineBoots has the value 2147483647
   it latches at that value and an authenticated message always causes
   an notInTimeWindow authentication failure.

   In order to reset an SNMP engine whose snmpEngineBoots value has
   reached the value 2147483647, manual intervention is required.
   The engine must be physically visited and re-configured, either

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   with a new snmpEngineID value, or with new secret values for the
   authentication and privacy protocols of all users known to that
   SNMP engine. Note that even if an SNMP engine re-boots once a second
   that it would still take approximately 68 years before the max value
   of 2147483647 would be reached.

2.2.3.  Time Window

   The Time Window is a value that specifies the window of time in
   which a message generated on behalf of any user is valid.  This
   memo specifies that the same value of the Time Window, 150 seconds,
   is used for all users.

2.3.  Time Synchronization

   Time synchronization, required by a non-authoritative SNMP engine
   in order to proceed with authentic communications, has occurred
   when the non-authoritative SNMP engine has obtained a local notion
   of the authoritative SNMP engine's values of snmpEngineBoots and
   snmpEngineTime from the authoritative SNMP engine.  These values
   must be (and remain) within the authoritative SNMP engine's Time
   Window.  So the local notion of the authoritative SNMP engine's
   values must be kept loosely synchronized with the values stored
   at the authoritative SNMP engine.  In addition to keeping a local
   copy of snmpEngineBoots and snmpEngineTime from the authoritative
   SNMP engine, a non-authoritative SNMP engine must also keep one
   local variable, latestReceivedEngineTime.  This value records the
   highest value of snmpEngineTime that was received by the
   non-authoritative SNMP engine from the authoritative SNMP engine
   and is used to eliminate the possibility of replaying messages
   that would prevent the non-authoritative SNMP engine's notion of
   the snmpEngineTime from advancing.

   A non-authoritative SNMP engine must keep local notions of these
   values for each authoritative SNMP engine with which it wishes to
   communicate.  Since each authoritative SNMP engine is uniquely
   and unambiguously identified by its value of snmpEngineID, the
   non-authoritative SNMP engine may use this value as a key in
   order to cache its local notions of these values.

   Time synchronization occurs as part of the procedures of receiving
   an SNMP message (Section 3.2, step 7b). As such, no explicit time
   synchronization procedure is required by a non-authoritative SNMP
   engine.  Note, that whenever the local value of snmpEngineID is
   changed (e.g., through discovery) or when secure communications
   are first established with an authoritative SNMP engine, the local

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   values of snmpEngineBoots and latestReceivedEngineTime should be
   set to zero.  This will cause the time synchronization to occur
   when the next authentic message is received.

2.4.  SNMP Messages Using this Security Model

   The syntax of an SNMP message using this Security Model adheres
   to the message format defined in the version-specific Message
   Processing Model document (for example [RFC2262]).

   The field msgSecurityParameters in SNMPv3 messages has a data type
   of OCTET STRING.  Its value is the BER serialization of the
   following ASN.1 sequence:

   USMSecurityParametersSyntax DEFINITIONS IMPLICIT TAGS ::= BEGIN

      UsmSecurityParameters ::=
          SEQUENCE {
           -- global User-based security parameters
              msgAuthoritativeEngineID     OCTET STRING,
              msgAuthoritativeEngineBoots  INTEGER (0..2147483647),
              msgAuthoritativeEngineTime   INTEGER (0..2147483647),
              msgUserName                  OCTET STRING (SIZE(1..32)),
           -- authentication protocol specific parameters
              msgAuthenticationParameters  OCTET STRING,
           -- privacy protocol specific parameters
              msgPrivacyParameters         OCTET STRING
          }
   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
   the authoritative SNMP engine E.



(page 21 continued on part 2)

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