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Versions: 00 01 02 03 RFC 2264

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

                            1 August 1997


                            U. Blumenthal
                       IBM T. J. Watson Research
                           uri@watson.ibm.com

                              B. Wijnen
                       IBM T. J. Watson Research
                          wijnen@vnet.ibm.com


                     <draft-ietf-snmpv3-usm-01.txt>


                          Status of this Memo

This document is an Internet-Draft.  Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups.  Note that other groups may also distribute
working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet- Drafts as reference
material or to cite them other than as ``work in progress.''

To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet- Drafts Shadow
Directories on ds.internic.net (US East Coast), nic.nordu.net (Europe),
ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).


                          Abstract

This document describes the User-based Security Model (USM) for SNMP
version 3 for use in the SNMP architecture [SNMP-ARCH].  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.














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0.  Issues and Change Log

0.1.  Resolved Issues
      - Is it OK to use MD5 for KeyChange Algorithm ??
        We changed the TC so that we use the user's authentication
        protocol (usmUserAuthProtocol) instead of fixing it to MD5
      - Improve acknowledgements and sync it up with other documents
        Resolved by Russ.
      - Should the USM define checking such that a received Response
        messages used the same or better securityLevel than the Request
        message that this is a response to.
        In section 3.1 step 9, we return a completed outgoing message
        to the calling module (Message Processing). We believe it is
        the Message Processing Subsystem that should cache information
        about outgoing messages regarding msgID and such so that a
        possible Response Message can be mapped to an outstanding
        request. At the same time that piece of code can then ensure
        that the same securityModel and the same (or better??)
        securityLevel has been used for the Response Message. So in
        step 9 we do not save any cachedSecurityData for outgoing
        messages.
        Resolution. Statements have been added to state that this is
        the responsibility of the calling Message Processing Model.
        The v3MP has been changed to indeed do the check.
      - At an authoritative SNMP engine we must be able to determine
        if the msgAuthoritativeEngineID value used for Requests is the
        local snmpEngineID. However, to do so we'd have to peek into
        either the message or into the PDU. That is not clean.
        Resolution. The USM must pass the securityEngineID that was
        used for security purposes (i.e. the value extracted from
        the msgAuthoritativeEngineID) so that the MP can check it
        when it determines that it is a Request. The MP document has
        been updated to make the check.
      - An authenticated incoming message at a non-authoritative SNMP
        engine always allowed the notion snmpEngineTime to be updated
        (as long as it was not older than what we had). This allowed
        intruders to slow down our notion of time dramatically.
        Resolution. Check the value not only to be greater than
        latestReceivedSnmpEngineTime but also against out notion of
        time to ensure it is not older than 150 seconds. This will
        have a side-effect in that if an authoritative engine ever
        gets more than 150 seconds behind, then the authoritative end
        MUST do explicit time synchronization.
      - Should we use Integers or OIDs for identifying protocols
        Resolution. Consensus is to use OID.

0.2.  Change Log

   [version 2.2]
     - formatting
         - pagination



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   [version 2.1] - August 1 version
     - Changed max size for usmUserName from 16 to 32.
       For SnmpAdminString 16 is rather short.
     - incorporate comments by Uri.
     - Update References
     - Update acknowledgement section
     - remove expectResponse parameter. It was a bad idea
     - renamed snmpEngineID parameter to securityEngineID to
       reduce confusion with other uses of term snmpEngineID
     - added an OUT parameter securityEngineID to processIncomingMsg
       primitive, so that MP can check if correct snmpEngineID was
       used for Request messages.
     - added some more considerations about redirected Traps.
     - state that MP is responsible for matching a Response or Report
       to an outstanding Request and discard it if none found.
     - Discussion about msgID and request-id changed into an example,
       because it is SNMPv3 MP specific and the MP MUST handle it.
     - Spell check
     - SMICng MIB check
     - Paginate
     - Post to I-D repository and SNMPv3 mailing list as:
       <draft-ietf-snmpv3-usm-01.txt>

   [version 2.0] - July 28 version
     - Address comments by Juergen
       - fix typos and editorial changes
     - Other changes
       - adapt to new (synchronous) abstract service primitives
       - adapt to new field names in the messages
       - adapt to new parameter names for abstract data type
     - Address Dave Perkins comments
       - modify definition of usmSecurityParameters
     - Address Dave Levi's issue on checking msgAuthoritativeEngineID
       properly to local snmpEngineID for incoming Request Messages
     - Address Uri's issue about slow-down of non-authoritative
       engine's notion of time at remote SNMP engine.
     - Address Uri's issue about redirection of (authenticated) Traps.
       Describe it is not a problem because they come from the
       authoritative SNMP engine.
     - Address comments by Arnoud Zwemmer about the fact that only
       Report PDUs can slow down timeliness notion

   [version 1.8]
     - Add reference to RFC2119 about use of SHOULD and MUST
     - paginate and generate table of contents
     - posted as I-D <draft-ietf-snmpv3-usm-00.txt> on 15 July 1997

   [version 1.7]
     - Changed the KeyChange description so it allows for other
       hash algorithms instead of MD5. If in the future the MD5 gets
       replaced by another Authentication -- algorithm, then it seems



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       we also need to use that new algorithm to -- calculate the
       digest during KeyChange.
     - Updated the password to key code fragment to cater for the
       variable length of the snmpEngineID.
     - Added issue on cacheing of data on outgoing messages and one
       on required review of timeliness handling.

   [version 1.4 - version 1.6]
     - Editorial changes because of internal review by authors
     - Adapt to latest list of Primitive names and parameters
     - Change USEC to USM
     - Changes based on comments from Jeff Case.
     - Checked MIB with SMICng

   [version 1.3]
     - Too many changes have taken place, so marking it was skipped
       The most important changes are listed here.
       However, changes that just split text on different lines
       and changes like different capitalization of words/terms
       has not been listed. Also changes to fit new terms and such
       have not been listed.
     - Split/Join some lines to ensure we stay within 72 columns
       as required by RFC guidelines.
     - Addressed Dave Perkins comments:
       1) Section 1.3, last paragraph's, timeliness was left off. -done
       2) Section 1.5.1, the operations need to be made general, since
          additional one may be added later.  - done
       3) Section 1.5.2, the field "request-id" is used throughout
          this section when it should be field "msgID"  - done
       4) The document must allow the value of engineID in the
          security to be a zero length string. There are several
          places that are affected by this change. An actual value is
          never needed, since secrets are never the same on different
          agents (see your paper).  - done
       5) Last sentence of description for object usmUserCloneFrom is
          not correct, since the object has a OID data type - done
     - Removed groupName from usmUserTable.
       Now done in Access Control as agreed at 2nd interim
     - Stats counters back in this document as agreed at 2nd interim
     - Use AutonomousType for usmUserPrivProtocol and
       usmUserAuthProtocol. Also use OBJECT-IDENTITY for the
       protocol OIDs (John Flick).
     - Changed "SNMPv3 engine" to "SNMP engine" at various places
     - added appendix with sample encoding of securityParameters
     - cleanup elements of procedure to use consistent terms
     - fix up some problems in elements of procedure
     - Do not use IMPLIED on usmUserTable as agreed at 2nd interim.
       For one thing, SNMPv1 cannot handle it.
     - cleanup section 2.3 and 3.3 step 7b based on comments by
       Dave Levi.




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   [version 1.2]
     - changed (simplified) time sync in section 3 item 7.
     - added usmUserMiId
     - cleaned up text
     - defined IV "salt" generation
     - removed Statistics counters (now in MPC) and Report PDU
       generation (now in MPC)
     - Removed auth and DES MIBs which are now merged into
       User-based Security MIB
     - specified where cachedSecurityData needs to be discarded
     - added abstract service interface definitions
     - removed section on error reporting (is MPC responsibility)
     - removed auth/priv protocol definitions, they are in ARCH now
     - removed MIB definitions for snmpEngineID, Time, Boots. They
       are in ARCH now.

   [version 1.1]
     - removed <securityCookie>.
     - added <securityIdentity>, <securityCachedData>.
     - added abstract function interface description of
       inter-module communications.
     - modified IV generation process to accommodate messages produced
       faster than one-per-second (still open).
     - always update the clock regardless of whether incoming message
       was Report or not (if the message was properly authenticated
       and its time-stamp is ahead of our notion of their clock).

   [version 1.0]
     - first version posted to the SNMPv3 editor's mailing list.
     - based on v2adv slides, v2adv items and issues list and on
       RFC1910 and SNMPv2u and SNMPv2* documents.
     - various iterations were done by the authors via private email.






















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1.  Introduction

   The Architecture for describing Internet Management Frameworks
   [SNMP-ARCH] 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 [SNMP-ARCH].

   This memo [SNMP-USM] 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 Keyed-MD5 as the authentication
   protocol 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 Keyed-MD5 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.

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.




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   - 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.

   - 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



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      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.

   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.



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     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.

   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



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   the complete message is checked for integrity, we can assume that
   the timeliness values in a message that passes the authentication
   module are trustworthy.

1.4.2.  Authentication Protocol

   Section 6 describes the Keyed-MD5 authentication protocol which
   is the first authentication protocol to be used 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 7 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
      was authenticated and timely. It is as if the authoritative SNMP
      engine was configured to start sending Traps to the second SNMP



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      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.

















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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.  One such protocol is defined in this memo: the
     Digest 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.

   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.

   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 DES-based 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.



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     Note that a user's privacy key will normally be different at
     different authoritative SNMP engines. The privKey is not
     accessible via SNMP.

   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.

   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 [SNMP-ARCH]).

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



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   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 0xffffffff.

   Whenever the local value of snmpEngineBoots has the value
   0xffffffff, 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 0xffffffff, manual intervention is required.
   The engine must be physically visited and re-configured, either
   with a new snmpEngineID value, or with new secret values for the
   authentication and privacy protocols of all users known to that
   SNMP engine.

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



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   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 long as an authoritative
   SNMP engine does not lag behind more than 150 seconds, no explicit
   time synchronization procedure is required by a non-authoritative
   SNMP engine. If such a situation occurs, then the non-authoritative
   SNMP engine must issue an unauthenticated SNMP Get request to
   obtain the value for snmpEngineTime. It must then try an
   authenticated SNMP Get request (using the just retrieved value for
   msgAuthoritativeEngineTime) to ensure that the value it received
   was indeed correct.

   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 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.













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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 [SNMP-MP]).

   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:

      UsmSecurityParameters ::=
          SEQUENCE {
           -- global User-based security parameters
              msgAuthoritativeEngineID     OCTET STRING
              msgAuthoritativeEngineBoots  Unsigned32 (0..4294967295),
              msgAuthoritativeEngineTime   Unsigned32 (0..2147483647),
              msgUserName                  OCTET STRING (SIZE(1..16)),
           -- 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.

   - 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.




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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:

      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



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        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.
    messageProcessingModel
      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 cash is used.
    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.
      In case of a response this parameter is ignored and the value
      from the cash 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



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      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. 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
     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.



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    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
      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.










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3.  Elements of Procedure

   This section describes the security related procedures followed by
   an SNMP engine when processing SNMP messages according to the
   User-based Security Model.

3.1.  Generating an Outgoing SNMP Message

   This section describes the procedure followed by an SNMP engine
   whenever it generates a message containing a management operation
   (like a request, a response, a notification, or a report) on
   behalf of a user, with a particular securityLevel.

   1)  a) If any securityStateReference is passed (Response message),
          then information concerning the user is extracted from the
          cachedSecurityData.  The securityEngineID and the
          securityLevel are extracted from the cachedSecurityData.
          The cachedSecurityData can now be discarded.

          Otherwise,

       b) based on the securityName, information concerning the
          user at the destination snmpEngineID, specified by the
          securityEngineID, is extracted from the Local Configuration
          Datastore (LCD, usmUserTable). If information about the user
          is absent from the LCD, then an error indication
          (unknownSecurityName) is returned to the calling module.

   2)  If the securityLevel specifies that the message is to be
       protected from disclosure, but the user does not support both
       an authentication and a privacy protocol then the message
       cannot be sent.  An error indication (unsupportedSecurityLevel)
       is returned to the calling module.

   3)  If the securityLevel specifies that the message is to be
       authenticated, but the user does not support an authentication
       protocol, then the message cannot be sent. An error indication
       (unsupportedSecurityLevel) is returned to the calling module.

   4)  a) If the securityLevel specifies that the message is to be
          protected from disclosure, then the octet sequence
          representing the serialized scopedPDU is encrypted according
          to the user's privacy protocol. To do so a call is made to
          the privacy module that implements the user's privacy
          protocol according to the abstract primitive:

          statusInformation =       -- success or failure
            encryptData(
            IN    encryptKey        -- user's privKey
            IN    dataToEncrypt     -- serialized scopedPDU
            OUT   encryptedData     -- serialized encryptedPDU



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            OUT   privParameters    -- serialized privacy parameters
                  )

          statusInformation
            indicates if the encryption process was successful or not.
          encryptKey
            the user's private privKey is the secret key that can be
            used by the encryption algorithm.
          dataToEncrypt
            the serialized scopedPDU is the data that to be encrypted.
          encryptedData
            the encryptedPDU represents the encrypted scopedPDU,
            encoded as an OCTET STRING.
          privParameters
            the privacy parameters, encoded as an OCTET STRING.

          If the privacy module returns failure, then the message
          cannot be sent and an error indication (encryptionFailure)
          is returned to the calling module.

          If the privacy module returns success, then the returned
          privParameters are put into the msgPrivacyParameters field of
          the securityParameters and the encryptedPDU serves as the
          payload of the message being prepared.

          Otherwise,

       b) If the securityLevel specifies that the message is not to be
          protected from disclosure, then the NULL string is encoded
          as an OCTET STRING and put into the msgPrivacyParameters
          field of the securityParameters and the plaintext scopedPDU
          serves as the payload of the message being prepared.

   5)  The snmpEngineID is encoded as an OCTET STRING into the
       msgAuthoritativeEngineID field of the securityParameters.
       Note that an empty (zero length) snmpEngineID is OK for a
       Request message, because that will cause the remote
       (authoritative) SNMP engine to return a Report PDU with the
       proper snmpEngineID included in the privParameters of that
       returned Report PDU.

   6)  a) If the securityLevel specifies that the message is to be
          authenticated, then the current values of snmpEngineBoots
          and snmpEngineTime corresponding to the snmpEngineID from
          the LCD are used.

          Otherwise,

       b) If this is a Response message, then the current value of
          snmpEngineBoots and snmpEngineTime corresponding to the
          local snmpEngineID from the LCD are used.



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          Otherwise,

       c) If this is a Request message, then a zero value is used
          for both snmpEngineBoots and snmpEngineTime. This zero
          value gets used if snmpEngineID is empty.

       The values are encoded as Unsigned32 into the
       msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime
       fields of the securityParameters.

   7)  The userName is encoded as an OCTET STRING into the msgUserName
       field of the securityParameters.

   8)  a) If the securityLevel specifies that the message is to be
          authenticated, the message is authenticated according to the
          user's authentication protocol. To do so a call is made to
          the authentication module that implements the user's
          authentication protocol according to the abstract service
          primitive:

          statusInformation =
            authenticateOutgoingMsg(
            IN  authKey               -- the user's authKey
            IN  wholeMsg              -- unauthenticated message
            OUT authenticatedWholeMsg -- authenticated complete message
                )

          statusInformation
            indicates if authentication was successful or not.
          authKey
            the user's private authKey is the secret key that can be
            used by the authentication algorithm.
          wholeMsg
            the complete serialized message to be authenticated.
          authenticatedWholeMsg
            the same as the input given to the authenticateOutgoingMsg
            service, but with msgAuthenticationParameters properly
            filled in.

          If the authentication module returns failure, then the
          message cannot be sent and an error indication
          (authenticationFailure) is returned to the calling module.

          If the authentication module returns success, then the
          msgAuthenticationParameters field is put into the
          securityParameters and the authenticatedWholeMsg represents
          the serialization of the authenticated message being
          prepared.

          Otherwise,



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       b) If the securityLevel specifies that the message is not to
          be authenticated then the NULL string is encoded as an
          OCTET STRING into the msgAuthenticationParameters field of
          the securityParameters.  The wholeMsg is now serialized and
          then represents the unauthenticated message being prepared.

   9)  The completed message with its length is returned to the
       calling module with the statusInformation set to success.

3.2.  Processing an Incoming SNMP Message

   This section describes the procedure followed by an SNMP engine
   whenever it receives a message containing a management operation
   on behalf of a user, with a particular securityLevel.

   To simplify the elements of procedure, the release of state
   information is not always explicitly specified. As a general rule,
   if state information is available when a message gets discarded,
   the state information should also be released.
   Also, when an error indication with an OID and value for an
   incremented counter is returned, then the available information
   (like securityStateReference) must be passed back to the caller
   so it can generate a Report PDU.

   1)  If the received securityParameters is not the serialization
       (according to the conventions of [RFC1906]) of an OCTET STRING
       formatted according to the UsmSecurityParameters defined in
       section 2.4, then the snmpInASNParseErrs counter [RFC1907] is
       incremented, and an error indication (parseError) is returned
       to the calling module.
       Note that we return without the OID and value of the incremented
       counter, because in this case there is not enough information
       to generate a Report PDU.

   2)  The values of the security parameter fields are extracted from
       the securityParameters. The securityEngineID to be returned to
       the caller is the value of the msgAuthoritativeEngineID field.
       The cachedSecurityData is prepared and a securityStateReference
       is prepared to reference this data. Values to be cached are:

           msgUserName
           securityEngineID
           securityLevel

   3)  If the value of the msgAuthoritativeEngineID field in the
       securityParameters is unknown then:

       a) a non-authoritative SNMP engine that performs discovery may
          optionally create a new entry in its Local Configuration
          Datastore (LCD) and continue processing;



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          or

       b) the usmStatsUnknownEngineIDs counter is incremented, and
          an error indication (unknownEngineID) together with the
          OID and value of the incremented counter is returned to
          the calling module.

   4)  Information about the value of the msgUserName and
       msgAuthoritativeEngineID fields is extracted from the Local
       Configuration Datastore (LCD, usmUserTable).  If no information
       is available for the user, then the usmStatsUnknownUserNames
       counter is incremented and an error indication
       (unknownSecurityName) together with the OID and value of the
       incremented counter is returned to the calling module.

   5)  If the information about the user indicates that it does not
       support the securityLevel requested by the caller, then the
       usmStatsUnsupportedSecLevels counter is incremented and an
       error indication (unsupportedSecurityLevel) together with the
       OID and value of the incremented counter is returned to the
       calling module.

   6)  If the securityLevel specifies that the message is to be
       authenticated, then the message is authenticated according to
       the user's authentication protocol. To do so a call is made
       to the authentication module that implements the user's
       authentication protocol according to the abstract service
       primitive:

       statusInformation =          -- success or failure
         authenticateIncomingMsg(
         IN   authKey               -- the user's authKey
         IN   authParameters        -- as received on the wire
         IN   wholeMsg              -- as received on the wire
         OUT  authenticatedWholeMsg -- checked for authentication
                 )

       statusInformation
         indicates if authentication was successful or not.
       authKey
         the user's private authKey is the secret key that can be
         used by the authentication algorithm.
       wholeMsg
         the complete serialized message to be authenticated.
       authenticatedWholeMsg
         the same as the input given to the authenticateIncomingMsg
         service, but after authentication has been checked.

       If the authentication module returns failure, then the message
       cannot be trusted, so the usmStatsWrongDigests counter is



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       incremented and an error indication (authenticationFailure)
       together with the OID and value of the incremented counter is
       returned to the calling module.

       If the authentication module returns success, then the message
       is authentic and can be trusted so processing continues.

   7)  If the securityLevel indicates an authenticated message, then
       the local values of snmpEngineBoots and snmpEngineTime
       corresponding to the value of the msgAuthoritativeEngineID
       field are extracted from the Local Configuration Datastore.

       a) If the extracted value of msgAuthoritativeEngineID is the
          same as the value of snmpEngineID of the processing SNMP
          engine (meaning this is the authoritative SNMP engine),
          then if any of the following conditions is true, then the
          message is considered to be outside of the Time Window:

           - the local value of snmpEngineBoots is 0xffffffff;

           - the value of the msgAuthoritativeEngineBoots field differs
             from the local value of snmpEngineBoots; or,

           - the value of the msgAuthoritativeEngineTime field differs
             from the local notion of snmpEngineTime by more than
             +/- 150 seconds.

          If the message is considered to be outside of the Time Window
          then the usmStatsNotInTimeWindows counter is incremented and
          an error indication (notInTimeWindow) together with the OID
          and value of the incremented counter is returned to the
          calling module.

       b) If the extracted value of msgAuthoritativeEngineID is not the
          same as the value snmpEngineID of the processing SNMP engine
          (meaning this is not the authoritative SNMP engine), then:

          1) if at least one of the following conditions is true:

             - the extracted value of the msgAuthoritativeEngineBoots
               field is greater than the local notion of the value of
               snmpEngineBoots; or,

             - the extracted value of the msgAuthoritativeEngineBoots
               field is equal to the local notion of the value of
               snmpEngineBoots, the extracted value of
               msgAuthoritativeEngineTime field is greater than the
               value of latestReceivedEngineTime, and the extracted
               value of msgAuthoritativeEngineTime field is not more
               than 150 seconds less than the local notion of the value
               of snmpEngineBoots,



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             then the LCD entry corresponding to the extracted value
             of the msgAuthoritativeEngineID field is updated, by
             setting:

                - the local notion of the value of snmpEngineBoots to
                  the value of the msgAuthoritativeEngineBoots field,
                - the local notion of the value of snmpEngineTime to
                  the value of the msgAuthoritativeEngineTime field,
                  and
                - the latestReceivedEngineTime to the value of the
                  value of the msgAuthoritativeEngineTime field.

          2) if any of the following conditions is true, then the
             message is considered to be outside of the Time Window:

             - the local notion of the value of snmpEngineBoots is
               0xffffffff;

             - the value of the msgAuthoritativeEngineBoots field is
               less than the local notion of the value of
               snmpEngineBoots; or,

             - the value of the msgAuthoritativeEngineBoots field is
               equal to the local notion of the value of
               snmpEngineBoots and the value of the
               msgAuthoritativeEngineTime field is more than 150
               seconds less than the local notion of of the value of
               snmpEngineTime.

             If the message is considered to be outside of the Time
             Window then an error indication (notInTimeWindow) is
             returned to the calling module;

             Note that this means that a too old (possibly replayed)
             message has been detected and is deemed unauthentic.

             Note that this procedure allows for the value of
             msgAuthoritativeEngineBoots in the message to be greater
             than the local notion of the value of snmpEngineBoots to
             allow for received messages to be accepted as authentic
             when received from an authoritative SNMP engine that has
             re-booted since the receiving SNMP engine last
             (re-)synchronized.

             Note that this procedure does not allow for automatic
             time synchronization if the non-authoritative SNMP engine
             has a real out-of-sync situation whereby the authoritative
             SNMP engine is more than 150 seconds behind the
             non-authoritative SNMP engine.




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   8)  a) If the securityLevel indicates that the message was protected
          from disclosure, then the OCTET STRING representing the
          encryptedPDU is decrypted according to the user's privacy
          protocol to obtain an unencrypted serialized scopedPDU value.
          To do so a call is made to the privacy module that implements
          the user's privacy protocol according to the abstract
          primitive:

          statusInformation =       -- success or failure
            decryptData(
            IN    decryptKey        -- the user's privKey
            IN    privParameters    -- as received on the wire
            IN    encryptedData     -- encryptedPDU as received
            OUT   decryptedData     -- serialized decrypted scopedPDU
                  )

          statusInformation
            indicates if the decryption process was successful or not.
          decryptKey
            the user's private privKey is the secret key that can be
            used by the decryption algorithm.
          privParameters
            the msgPrivacyParameters, encoded as an OCTET STRING.
          encryptedData
            the encryptedPDU represents the encrypted scopedPDU,
            encoded as an OCTET STRING.
          decryptedData
            the serialized scopedPDU if decryption is successful.

          If the privacy module returns failure, then the message can
          not be processed, so the usmStatsDecryptionErrors counter
          is incremented and an error indication (decryptionFailure)
          together with the OID and value of the incremented counter
          is returned to the calling module.

          If the privacy module returns success, then the decrypted
          scopedPDU is the message payload to be returned to the
          calling module.

          Otherwise,

       b) The scopedPDU component is assumed to be in plain text
          and is the message payload to be returned to the calling
          module.

   9)  The maxSizeResponseScopedPDU is calculated.  This is the
       maximum size allowed for a scopedPDU for a possible Response
       message.  Provision is made for a message header that allows
       the same securityLevel as the received Request.

   10) The securityName for the user is retrieved from the



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       usmUserTable.

   11) The security data is cached as cachedSecurityData, so that a
       possible response to this message can and will use the same
       authentication and privacy secrets, the same securityLevel and
       the same value for msgAuthoritativeEngineID.  Information to be
       saved/cached is as follows:

          usmUserName,
          usmUserAuthProtocol, usmUserAuthKey
          usmUserPrivProtocol, usmUserPrivKey
          securityEngineID, securityLevel

   12) The statusInformation is set to success and a return is made to
       the calling module passing back the OUT parameters as specified
       in the processIncomingMsg primitive.






































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4.  Discovery

   The User-based Security Model requires that a discovery process
   obtains sufficient information about other SNMP engines in order
   to communicate with them.  Discovery requires an non-authoritative
   SNMP engine to learn the authoritative SNMP engine's snmpEngineID
   value before communication may proceed.  This may be accomplished by
   generating a Request message with a securityLevel of noAuthNoPriv,
   a msgUserName of "initial", a msgAuthoritativeEngineID value of zero
   length, and the varBindList left empty.
   The response to this message will be a Report message containing
   the snmpEngineID of the authoritative SNMP engine as the value of
   the msgAuthoritativeEngineID field within the msgSecurityParameters
   field.  It contains a Report PDU with the usmStatsUnknownEngineIDs
   counter in the varBindList.

   If authenticated communication is required, then the discovery
   process should also establish time synchronization with the
   authoritative SNMP engine.  This may be accomplished by sending an
   authenticated Request message with the value of
   msgAuthoritativeEngineID set to the newly learned snmpEngineID and
   with the values of msgAuthoritativeEngineBoots and
   msgAuthoritativeEngineTime set to zero.
   The response to this authenticated message will be a Report message
   containing the up to date values of the authoritative SNMP engine's
   snmpEngineBoots and snmpEngineTime as the value of the
   msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime fields
   respectively.  It also contains the usmStatsNotInTimeWindows counter
   in the varBindList of the Report PDU.  The time synchronization then
   happens automatically as part of the procedures in section 3.2
   step 7b. See also section 2.3.























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5.  Definitions

SNMP-USER-BASED-SM-MIB DEFINITIONS ::= BEGIN

IMPORTS
    MODULE-IDENTITY, OBJECT-TYPE,
    OBJECT-IDENTITY,
    snmpModules, Counter32                FROM SNMPv2-SMI
    TEXTUAL-CONVENTION, TestAndIncr,
    RowStatus, RowPointer,
    StorageType, AutonomousType           FROM SNMPv2-TC
    MODULE-COMPLIANCE, OBJECT-GROUP       FROM SNMPv2-CONF
    SnmpAdminString, SnmpEngineID,
    snmpAuthProtocols, snmpPrivProtocols  FROM SNMP-FRAMEWORK-MIB;

snmpUsmMIB MODULE-IDENTITY
    LAST-UPDATED "9707290000Z"            -- 29 July 1997, midnight
    ORGANIZATION "SNMPv3 Working Group"
    CONTACT-INFO "WG-email:   snmpv3@tis.com
                  Subscribe:  majordomo@tis.com
                              In msg body:  subscribe snmpv3

                  Chair:      Russ Mundy
                              Trusted Information Systems
                  postal:     3060 Washington Rd
                              Glenwood MD 21738
                              USA
                  email:      mundy@tis.com
                  phone:      +1-301-854-6889

                  Co-editor   Uri Blumenthal
                              IBM T. J. Watson Research
                  postal:     30 Saw Mill River Pkwy,
                              Hawthorne, NY 10532
                              USA
                  email:      uri@watson.ibm.com
                  phone:      +1-914-784-7964

                  Co-editor:  Bert Wijnen
                              IBM T. J. Watson Research
                  postal:     Schagen 33
                              3461 GL Linschoten
                              Netherlands
                  email:      wijnen@vnet.ibm.com
                  phone:      +31-348-432-794
                 "

    DESCRIPTION  "The management information definitions for the
                  SNMP User-based Security Model.
                 "
    ::= { snmpModules 9 }     -- to be verified with IANA



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-- Administrative assignments ****************************************

usmAdmin          OBJECT IDENTIFIER ::= { snmpUsmMIB 1 }
usmMIBObjects     OBJECT IDENTIFIER ::= { snmpUsmMIB 2 }
usmMIBConformance OBJECT IDENTIFIER ::= { snmpUsmMIB 3 }

-- Identification of Authentication and Privacy Protocols ************

usmNoAuthProtocol OBJECT-IDENTITY
    STATUS        current
    DESCRIPTION  "No Authentication Protocol."
    ::= { snmpAuthProtocols 1 }

usmMD5AuthProtocol OBJECT-IDENTITY
    STATUS        current
    DESCRIPTION  "The Keyed MD5 Digest Authentication Protocol."
    REFERENCE    "Rivest, R., Message Digest Algorithm MD5, RFC1321."
    ::= { snmpAuthProtocols 2 }

usmNoPrivProtocol OBJECT-IDENTITY
    STATUS        current
    DESCRIPTION  "No Privacy Protocol."
    ::= { snmpPrivProtocols 1 }

usmDESPrivProtocol OBJECT-IDENTITY
    STATUS        current
    DESCRIPTION  "The CBC-DES Symmetric Encryption Protocol."
    REFERENCE    "- Data Encryption Standard, National Institute of
                    Standards and Technology.  Federal Information
                    Processing Standard (FIPS) Publication 46-1.
                    Supersedes FIPS Publication 46,
                    (January, 1977; reaffirmed January, 1988).

                  - Data Encryption Algorithm, American National
                    Standards Institute.  ANSI X3.92-1981,
                    (December, 1980).

                  - DES Modes of Operation, National Institute of
                    Standards and Technology.  Federal Information
                    Processing Standard (FIPS) Publication 81,
                    (December, 1980).

                  - Data Encryption Algorithm - Modes of Operation,
                    American National Standards Institute.
                    ANSI X3.106-1983, (May 1983).
                 "
    ::= { snmpPrivProtocols 2 }

-- Textual Conventions ***********************************************




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-- Editor's note:
-- If in the future the MD5 gets replaced by another Authentication
-- algorithm, then it seems we also need to use that new algorithm to
-- calculate the digest during KeyChange. So this TC has been defined
-- to cater for that.
-- End Editor's note

KeyChange ::=     TEXTUAL-CONVENTION
   STATUS         current
   DESCRIPTION
         "Every definition of an object with this syntax must identify
          a protocol, P, a secret key, K, and a hash algorithm, H.
          The object's value is a manager-generated, partially-random
          value which, when modified, causes the value of the secret
          key, K, to be modified via a one-way function.

          The value of an instance of this object is the concatenation
          of two components: a 'random' component and a 'delta'
          component.  The lengths of the random and delta components
          are given by the corresponding value of the protocol, P;
          if P requires K to be a fixed length, the length of both the
          random and delta components is that fixed length; if P
          allows the length of K to be variable up to a particular
          maximum length, the length of the random component is that
          maximum length and the length of the delta component is any
          length less than or equal to that maximum length.
          For example, usmMD5AuthProtocol requires K to be a fixed
          length of 16 octets.  Other protocols may define other
          sizes, as deemed appropriate.

          When an instance of this object is modified to have a new
          value by the management protocol, the agent generates a new
          value of K as follows:

           - a temporary variable is initialized to the existing value
             of K;
           - if the length of the delta component is greater than 16
             bytes, then:
              - the random component is appended to the value of the
                temporary variable, and the result is input to the
                the hash algorithm H to produce a digest value, and
                the temporary variable is set to this digest value;
              - the value of the temporary variable is XOR-ed with
                the first (next) 16-bytes of the delta component to
                produce the first (next) 16-bytes of the new value
                of K.
              - the above two steps are repeated until the unused
                portion of the delta component is 16 bytes or less,
           - the random component is appended to the value of the
             temporary variable, and the result is input to the
             hash algorithm H to produce a digest value;



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           - this digest value, truncated if necessary to be the same
             length as the unused portion of the delta component, is
             XOR-ed with the unused portion of the delta component to
             produce the (final portion of the) new value of K.

             for example, using MD5 as the hash algorithm H:

              iterations = (lenOfDelta - 1)/16; /* integer division */
              temp = keyOld;
              for (i = 0; i < iterations; i++) {
                  temp = MD5 (temp || random);
                  keyNew[i*16 .. (i*16)+15] =
                         temp XOR delta[i*16 .. (i*16)+15];
              }
              temp = MD5 (temp || random);
              keyNew[i*16 .. lenOfDelta-1] =
                     temp XOR delta[i*16 .. lenOfDelta-1];

          The value of an object with this syntax, whenever it is
          retrieved by the management protocol, is always the zero
          length string.
         "
    SYNTAX       OCTET STRING

-- Statistics for the User-based Security Model **********************

usmStats         OBJECT IDENTIFIER ::= { usmMIBObjects 1 }

usmStatsUnsupportedSecLevels OBJECT-TYPE
    SYNTAX       Counter32
    MAX-ACCESS   read-only
    STATUS       current
    DESCRIPTION "The total number of packets received by the SNMP
                 engine which were dropped because they requested a
                 securityLevel that was unknown to the SNMP engine
                 or otherwise unavailable.
                "
    ::= { usmStats 1 }

usmStatsNotInTimeWindows OBJECT-TYPE
    SYNTAX       Counter32
    MAX-ACCESS   read-only
    STATUS       current
    DESCRIPTION "The total number of packets received by the SNMP
                 engine which were dropped because they appeared
                 outside of the authoritative SNMP engine's window.
                "
    ::= { usmStats 2 }

usmStatsUnknownUserNames OBJECT-TYPE
    SYNTAX       Counter32



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    MAX-ACCESS   read-only
    STATUS       current
    DESCRIPTION "The total number of packets received by the SNMP
                 engine which were dropped because they referenced a
                 user that was not known to the SNMP engine.
                "
    ::= { usmStats 3 }

usmStatsUnknownEngineIDs OBJECT-TYPE
    SYNTAX       Counter32
    MAX-ACCESS   read-only
    STATUS       current
    DESCRIPTION "The total number of packets received by the SNMP
                 engine which were dropped because they referenced an
                 snmpEngineID that was not known to the SNMP engine.
                "
    ::= { usmStats 4 }

usmStatsWrongDigests OBJECT-TYPE
    SYNTAX       Counter32
    MAX-ACCESS   read-only
    STATUS       current
    DESCRIPTION "The total number of packets received by the SNMP
                 engine which were dropped because they didn't
                 contain the expected digest value.
                "
    ::= { usmStats 5 }

usmStatsDecryptionErrors OBJECT-TYPE
    SYNTAX       Counter32
    MAX-ACCESS   read-only
    STATUS       current
    DESCRIPTION "The total number of packets received by the SNMP
                 engine which were dropped because they could not be
                 decrypted.
                "
    ::= { usmStats 6 }

-- The usmUser Group ************************************************

usmUser          OBJECT IDENTIFIER ::= { usmMIBObjects 2 }

usmUserSpinLock  OBJECT-TYPE
    SYNTAX       TestAndIncr
    MAX-ACCESS   read-write
    STATUS       current
    DESCRIPTION "An advisory lock used to allow several cooperating
                 Command Generator Applications to coordinate their
                 use of facilities to alter secrets in the
                 usmUserTable.
                "



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    ::= { usmUser 1 }

-- The table of valid users for the User-based Security Model ********

usmUserTable     OBJECT-TYPE
    SYNTAX       SEQUENCE OF UsmUserEntry
    MAX-ACCESS   not-accessible
    STATUS       current
    DESCRIPTION "The table of users configured in the SNMP engine's
                 Local Configuration Datastore (LCD)."
    ::= { usmUser 2 }

usmUserEntry     OBJECT-TYPE
    SYNTAX       UsmUserEntry
    MAX-ACCESS   not-accessible
    STATUS       current
    DESCRIPTION "A user configured in the SNMP engine's Local
                 Configuration Datastore (LCD) for the User-based
                 Security Model.
                "
    INDEX       { usmUserEngineID,
                  usmUserName
                }
    ::= { usmUserTable 1 }

UsmUserEntry ::= SEQUENCE
    {
        usmUserEngineID         SnmpEngineID,
        usmUserName             SnmpAdminString,
        usmUserSecurityName     SnmpAdminString,
        usmUserCloneFrom        RowPointer,
        usmUserAuthProtocol     AutonomousType,
        usmUserAuthKeyChange    KeyChange,
        usmUserOwnAuthKeyChange KeyChange,
        usmUserPrivProtocol     AutonomousType,
        usmUserPrivKeyChange    KeyChange,
        usmUserOwnPrivKeyChange KeyChange,
        usmUserPublic           OCTET STRING,
        usmUserStorageType      StorageType,
        usmUserStatus           RowStatus
    }

usmUserEngineID  OBJECT-TYPE
    SYNTAX       SnmpEngineID
    MAX-ACCESS   not-accessible
    STATUS       current
    DESCRIPTION "An SNMP engine's administratively-unique identifier.

                 In a simple agent, this value is always that agent's
                 own snmpEngineID value.




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                 The value can also take the value of the snmpEngineID
                 of a remote SNMP engine with which this user can
                 communicate.
                "
    ::= { usmUserEntry 1 }

usmUserName      OBJECT-TYPE
    SYNTAX       SnmpAdminString (SIZE(1..32))
    MAX-ACCESS   not-accessible
    STATUS       current
    DESCRIPTION "A human readable string representing the name of
                 the user.

                 This is the (User-based Security) Model dependent
                 security ID.
                "
    ::= { usmUserEntry 2 }

usmUserSecurityName OBJECT-TYPE
    SYNTAX       SnmpAdminString
    MAX-ACCESS   read-only
    STATUS       current
    DESCRIPTION "A human readable string representing the user in
                 Security Model independent format.

                 The default transformation of the User-based Security
                 Model dependent security ID to the securityName and
                 vice versa is the identity function so that the
                 securityName is the same as the userName.
                "
    ::= { usmUserEntry 3 }

usmUserCloneFrom OBJECT-TYPE
    SYNTAX       RowPointer
    MAX-ACCESS   read-create
    STATUS       current
    DESCRIPTION "A pointer to another conceptual row in this
                 usmUserTable.  The user in this other conceptual
                 row is called the clone-from user.

                 When a new user is created (i.e., a new conceptual
                 row is instantiated in this table), the privacy and
                 authentication parameters of the new user are cloned
                 from its clone-from user.

                 The first time an instance of this object is set by
                 a management operation (either at or after its
                 instantiation), the cloning process is invoked.
                 Subsequent writes are successful but invoke no
                 action to be taken by the receiver.
                 The cloning process fails with an 'inconsistentName'



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                 error if the conceptual row representing the
                 clone-from user is not in an active state when the
                 cloning process is invoked.

                 Cloning also causes the initial values of the secret
                 authentication key and the secret encryption key of
                 the new user to be set to the same value as the
                 corresponding secret of the clone-from user.

                 When this object is read, the ZeroDotZero OID
                 is returned.
                "
    ::= { usmUserEntry 4 }

usmUserAuthProtocol OBJECT-TYPE
    SYNTAX       AutonomousType
    MAX-ACCESS   read-create
    STATUS       current
    DESCRIPTION "An indication of whether messages sent on behalf of
                 this user to/from the SNMP engine identified by
                 usmUserEngineID, can be authenticated, and if so,
                 the type of authentication protocol which is used.

                 An instance of this object is created concurrently
                 with the creation of any other object instance for
                 the same user (i.e., as part of the processing of
                 the set operation which creates the first object
                 instance in the same conceptual row).  Once created,
                 the value of an instance of this object can not be
                 changed.
                "
    DEFVAL      { usmMD5AuthProtocol }
    ::= { usmUserEntry 5 }

usmUserAuthKeyChange OBJECT-TYPE
    SYNTAX       KeyChange   -- typically (SIZE (0..32))
    MAX-ACCESS   read-create
    STATUS       current
    DESCRIPTION "An object, which when modified, causes the secret
                 authentication key used for messages sent on behalf
                 of this user to/from the SNMP engine identified by
                 usmUserEngineID, to be modified via a one-way
                 function.

                 The associated protocol is the usmUserAuthProtocol.
                 The associated secret key is the user's secret
                 authentication key (authKey). The associated hash
                 algorithm is the algorithm used by the user's
                 usmUserAuthProtocol.

                 When creating a new user, it is an 'inconsistentName'



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                 error for a Set operation to refer to this object
                 unless it is previously or concurrently initialized
                 through a set operation on the corresponding value
                 of usmUserCloneFrom.
                "
    DEFVAL      { ''H }    -- the empty string
    ::= { usmUserEntry 6 }

usmUserOwnAuthKeyChange OBJECT-TYPE
    SYNTAX       KeyChange  -- typically (SIZE (0..32))
    MAX-ACCESS   read-create
    STATUS       current
    DESCRIPTION "Behaves exactly as usmUserAuthKeyChange, with one
                 notable difference: in order for the Set operation
                 to succeed, the usmUserName of the operation
                 requester must match the usmUserName that
                 indexes the row which is targeted by this
                 operation.

                 The idea here is that access to this column can be
                 public, since it will only allow a user to change
                 his own secret authentication key (authKey).
                "
    DEFVAL      { ''H }    -- the empty string
    ::= { usmUserEntry 7 }

usmUserPrivProtocol OBJECT-TYPE
    SYNTAX       AutonomousType
    MAX-ACCESS   read-create
    STATUS       current
    DESCRIPTION "An indication of whether messages sent on behalf of
                 this user to/from the SNMP engine identified by
                 usmUserEngineID, can be protected from disclosure,
                 and if so, the type of privacy protocol which is used.

                 An instance of this object is created concurrently
                 with the creation of any other object instance for
                 the same user (i.e., as part of the processing of
                 the set operation which creates the first object
                 instance in the same conceptual row).  Once created,
                 the value of an instance of this object can not be
                 changed.
                "
    DEFVAL      { usmNoPrivProtocol }
    ::= { usmUserEntry 8 }

usmUserPrivKeyChange OBJECT-TYPE
    SYNTAX       KeyChange  -- typically (SIZE (0..32))
    MAX-ACCESS   read-create
    STATUS       current
    DESCRIPTION "An object, which when modified, causes the secret



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                 encryption key used for messages sent on behalf
                 of this user to/from the SNMP engine identified by
                 usmUserEngineID, to be modified via a one-way
                 function.

                 The associated protocol is the usmUserPrivProtocol.
                 The associated secret key is the user's secret
                 privacy key (privKey). The associated hash
                 algorithm is the algorithm used by the user's
                 usmUserAuthProtocol.

                 When creating a new user, it is an 'inconsistentName'
                 error for a set operation to refer to this object
                 unless it is previously or concurrently initialized
                 through a set operation on the corresponding value
                 of usmUserCloneFrom.
                "
    DEFVAL      { ''H }    -- the empty string
    ::= { usmUserEntry 9 }

usmUserOwnPrivKeyChange OBJECT-TYPE
    SYNTAX       KeyChange  -- typically (SIZE (0..32))
    MAX-ACCESS   read-create
    STATUS       current
    DESCRIPTION "Behaves exactly as usmUserPrivKeyChange, with one
                 notable difference: in order for the Set operation
                 to succeed, the usmUserName of the operation
                 requester must match the usmUserName that indexes
                 the row which is targeted by this operation.

                 The idea here is that access to this column can be
                 public, since it will only allow a user to change
                 his own secret privacy key (privKey).
                "
    DEFVAL      { ''H }    -- the empty string
    ::= { usmUserEntry 10 }

usmUserPublic    OBJECT-TYPE
    SYNTAX       OCTET STRING (SIZE(0..32))
    MAX-ACCESS   read-create
    STATUS       current
    DESCRIPTION "A publicly-readable value which is written as part
                 of the procedure for changing a user's secret
                 authentication and/or privacy key, and later read to
                 determine whether the change of the secret was
                 effected.
                "
    DEFVAL      { ''H }  -- the empty string
    ::= { usmUserEntry 11 }

usmUserStorageType OBJECT-TYPE



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    SYNTAX       StorageType
    MAX-ACCESS   read-create
    STATUS       current
    DESCRIPTION "The storage type for this conceptual row.

                 Conceptual rows having the value 'permanent'
                 must allow write-access at a minimum to:

                 - usmUserAuthKeyChange, usmUserOwnAuthKeyChange
                   and usmUserPublic for a user who employs
                   authentication, and
                 - usmUserPrivKeyChange, usmUserOwnPrivKeyChange
                   and usmUserPublic for a user who employs
                   privacy.

                 Note that any user who employs authentication or
                 privacy must allow its secret(s) to be updated and
                 thus cannot be 'readOnly'.
                "
    DEFVAL      { nonVolatile }
    ::= { usmUserEntry 12 }

usmUserStatus    OBJECT-TYPE
    SYNTAX       RowStatus
    MAX-ACCESS   read-create
    STATUS       current
    DESCRIPTION "The status of this conceptual row.

                 Until instances of all corresponding columns are
                 appropriately configured, the value of the
                 corresponding instance of the usmUserStatus column
                 is 'notReady'.

                 In particular, a newly created row cannot be made
                 active until the corresponding usmUserCloneFrom,
                 usmUserAuthKeyChange, usmUserOwnAuthKeyChange,
                 usmUserPrivKeyChange and usmUserOwnPrivKeyChange
                 have all been set.

                 The  RowStatus TC [RFC1903] requires that this
                 DESCRIPTION clause states under which circumstances
                 other objects in this row can be modified:

                 The value of this object has no effect on whether
                 other objects in this conceptual row can be modified.
                "
    ::= { usmUserEntry 13 }

-- Conformance Information *******************************************

usmMIBCompliances OBJECT IDENTIFIER ::= { usmMIBConformance 1 }



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usmMIBGroups      OBJECT IDENTIFIER ::= { usmMIBConformance 2 }

-- Compliance statements

usmMIBCompliance MODULE-COMPLIANCE
    STATUS       current
    DESCRIPTION "The compliance statement for SNMP engines which
                 implement the SNMP-USER-BASED-SM-MIB.
                "

    MODULE       -- this module
        MANDATORY-GROUPS { usmMIBBasicGroup }

        OBJECT           usmUserAuthProtocol
        MIN-ACCESS       read-only
        DESCRIPTION     "Write access is not required."

        OBJECT           usmUserPrivProtocol
        MIN-ACCESS       read-only
        DESCRIPTION     "Write access is not required."

    ::= { usmMIBCompliances 1 }

-- Units of compliance

usmMIBBasicGroup OBJECT-GROUP
    OBJECTS     {
                  usmStatsUnsupportedSecLevels,
                  usmStatsNotInTimeWindows,
                  usmStatsUnknownUserNames,
                  usmStatsUnknownEngineIDs,
                  usmStatsWrongDigests,
                  usmStatsDecryptionErrors,
                  usmUserSpinLock,
                  usmUserSecurityName,
                  usmUserCloneFrom,
                  usmUserAuthProtocol,
                  usmUserAuthKeyChange,
                  usmUserOwnAuthKeyChange,
                  usmUserPrivProtocol,
                  usmUserPrivKeyChange,
                  usmUserOwnPrivKeyChange,
                  usmUserPublic,
                  usmUserStorageType,
                  usmUserStatus
                }
    STATUS       current
    DESCRIPTION "A collection of objects providing for configuration
                 of an SNMP engine which implements the SNMP
                 User-based Security Model.
                "



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    ::= { usmMIBGroups 1 }

END



















































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6.  MD5 Authentication Protocol

   This section describes the Keyed-MD5 authentication protocol.
   This protocol is the first authentication protocol defined for
   the User-based Security Model.

   This protocol is identified by usmMD5AuthProtocol.

   Over time, other authentication protocols may be defined either
   as a replacement of this protocol or in addition to this protocol.

6.1.  Mechanisms

   - In support of data integrity, a message digest algorithm is
     required.  A digest is calculated over an appropriate portion
     of an SNMP message and included as part of the message sent
     to the recipient.

   - In support of data origin authentication and data integrity,
     a secret value is both inserted into, and appended to, the
     SNMP message prior to computing the digest; the inserted value
     is overwritten prior to transmission, and the appended value
     is not transmitted.  The secret value is shared by all SNMP
     engines authorized to originate messages on behalf of the
     appropriate user.

   - In order to not expose the shared secrets (keys) at all SNMP
     engines in case one of the SNMP engines is compromised, such
     secrets (keys) are localized for each authoritative SNMP
     engine, see [Localized-Key].

6.1.1.  Digest Authentication Protocol

   The Digest Authentication Protocol defined in this memo provides
   for:

   - verification of the integrity of a received message
     (i.e., the message received is the message sent).

     The integrity of the message is protected by computing a digest
     over an appropriate portion of the message.  The digest is
     computed by the originator of the message, transmitted with the
     message, and verified by the recipient of the message.

   - verification of the user on whose behalf the message was
     generated.

     A secret value known only to SNMP engines authorized to generate
     messages on behalf of a user is both inserted into, and appended
     to, the message prior to the digest computation.  Thus, the
     verification of the user is implicit with the verification of the



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     digest.  Note that the use of two copies of the secret, one near
     the start and one at the end, is recommended by [KEYED-MD5].

   This protocol uses the MD5 [MD5] message digest algorithm.
   A 128-bit digest is calculated over the designated portion of an
   SNMP message and included as part of the message sent to the
   recipient.  The size of both the digest carried in a message and
   the private authentication key (the secret) is 16 octets.

6.2.  Elements of the Digest Authentication Protocol

   This section contains definitions required to realize the
   authentication module defined by this memo.

6.2.1.  Users

   Authentication using this Digest Authentication protocol makes use
   of a defined set of userNames.  For any user on whose behalf a
   message must be authenticated 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.
   <authKey>
     A user's secret key to be used when calculating a digest.

6.2.2.  msgAuthoritativeEngineID

   The msgAuthoritativeEngineID value contained in an authenticated
   message specifies the authoritative SNMP engine for that particular
   message (see the definition of SnmpEngineID in the SNMP
   Architecture document [SNMP-ARCH]).

   The user's (private) authentication key is normally different at
   each authoritative SNMP engine and so the snmpEngineID is used
   to select the proper key for the authentication process.

6.2.3.  SNMP Messages Using this Authentication Protocol

   Messages using this authentication protocol carry a
   msgAuthenticationParameters field as part of the
   msgSecurityParameters.  For this protocol, the
   msgAuthenticationParameters field is the serialized OCTET STRING
   representing the MD5 digest of the wholeMsg.

   The digest is calculated over the wholeMsg so if a message is



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   authenticated, that also means that all the fields in the message
   are intact and have not been tampered with.

6.2.4.  Services provided by the MD5 Authentication Module

   This section describes the inputs and outputs that the MD5
   Authentication module expects and produces when the User-based
   Security module calls the MD5 Authentication module for services.

6.2.4.1.  Services for Generating an Outgoing SNMP Message

   This MD5 authentication protocol assumes that the selection of the
   authKey is done by the caller and that the caller passes the
   secret key to be used.

   Upon completion the authentication module returns statusInformation
   and, if the message digest was correctly calculated, the wholeMsg
   with the digest inserted at the proper place. The abstract service
   primitive is:

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

   The abstract data elements are:

     statusInformation
       An indication of whether the authentication process was
       successful.  If not it is an indication of the problem.
     authKey
       The secret key to be used by the authentication algorithm.
     wholeMsg
       The message to be authenticated.
     authenticatedWholeMsg
       The authenticated message (including inserted digest) on output.

   Note, that authParameters field is filled by the authentication
   module and this field should be already present in the wholeMsg
   before the Message Authentication Code (MAC) is generated.

6.2.4.2.  Services for Processing an Incoming SNMP Message

   This MD5 authentication protocol assumes that the selection of the
   authKey is done by the caller and that the caller passes
   the secret key to be used.

   Upon completion the authentication module returns statusInformation
   and, if the message digest was correctly calculated, the wholeMsg



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   as it was processed. The abstract service primitive is:

   statusInformation =              -- success or failure
     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
       )

   The abstract data elements are:

     statusInformation
       An indication of whether the authentication process was
       successful.  If not it is an indication of the problem.
     authKey
       The secret key to be used by the authentication algorithm.
     authParameters
       The authParameters from the incoming message.
     wholeMsg
       The message to be authenticated on input and the authenticated
       message on output.
     authenticatedWholeMsg
       The whole message after the authentication check is complete.






























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6.3.  Elements of Procedure

   This section describes the procedures for the Keyed-MD5
   authentication protocol.

6.3.1.  Processing an Outgoing Message

   This section describes the procedure followed by an SNMP engine
   whenever it must authenticate an outgoing message using the
   usmMD5AuthProtocol.

   1)  The msgAuthenticationParameters field is set to the
       serialization, according to the rules in [RFC1906], of an
       OCTET STRING representing the secret (localized) authKey.

   2)  The secret (localized) authKey is then appended to the end of
       the wholeMsg.

   3)  The MD5-Digest is calculated according to [MD5]. Then the
       msgAuthenticationParameters field is replaced with the
       calculated digest.

   4)  The authenticatedWholeMsg (excluding the appended secret key)
       is then returned to the caller together with statusInformation
       indicating success.

6.3.2.  Processing an Incoming Message

   This section describes the procedure followed by an SNMP engine
   whenever it must authenticate an incoming message using the
   usmMD5AuthProtocol.

   1)  If the digest received in the msgAuthenticationParameters field
       is not 16 octets long, then an failure and an errorIndication
       (authenticationError) is returned to the calling module.

   2)  The digest received in the msgAuthenticationParameters field
       is saved.

   3)  The digest in the msgAuthenticationParameters field is replaced
       by the secret (localized) authKey.

   4)  The secret (localized) authKey is then appended to the end of
       the wholeMsg.

   5)  The MD5-Digest is calculated according to [MD5].
       The msgAuthenticationParameters field is replaced with the
       digest value that was saved in step 2).

   6)  Then the newly calculated digest is compared with the digest
       saved in step 2). If the digests do not match, then an failure



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       and an errorIndication (authenticationFailure) is returned to
       the calling module.

   7)  The authenticatedWholeMsg (excluding the appended secret key)
       and statusInformation indicating success are then returned to
       the caller.
















































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7.  DES Privacy Protocol

   This section describes the DES privacy protocol.
   This protocol is the first privacy protocol defined for the
   User-based Security Model.

   This protocol is identified by usmDESPrivProtocol.

   Over time, other privacy protocols may be defined either
   as a replacement of this protocol or in addition to this protocol.


7.1.  Mechanisms

   - In support of data confidentiality, an encryption algorithm is
     required.  An appropriate portion of the message is encrypted
     prior to being transmitted. The User-based Security Model
     specifies that the scopedPDU is the portion of the message
     that needs to be encrypted.

   - A secret value in combination with a timeliness value is used
     to create the en/decryption key and the initialization vector.
     The secret value is shared by all SNMP engines authorized to
     originate messages on behalf of the appropriate user.

   - In order to not expose the shared secrets (keys) at all SNMP
     engines in case one of the SNMP engines is compromised, such
     secrets (keys) are localized for each authoritative SNMP engine,
     see [Localized-Key].

7.1.1.  Symmetric Encryption Protocol

   The Symmetric Encryption Protocol defined in this memo provides
   support for data confidentiality.  The designated portion of an
   SNMP message is encrypted and included as part of the message
   sent to the recipient.

   Two organizations have published specifications defining the DES:
   the National Institute of Standards and Technology (NIST)
   [DES-NIST] and the American National Standards Institute
   [DES-ANSI].  There is a companion Modes of Operation specification
   for each definition ([DESO-NIST] and [DESO-ANSI], respectively).

   The NIST has published three additional documents that implementers
   may find useful.

   - There is a document with guidelines for implementing and using
     the DES, including functional specifications for the DES and
     its modes of operation [DESG-NIST].

   - There is a specification of a validation test suite for the DES



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     [DEST-NIST].  The suite is designed to test all aspects of the
     DES and is useful for pinpointing specific problems.

   - There is a specification of a maintenance test for the DES
     [DESM-NIST].  The test utilizes a minimal amount of data and
     processing to test all components of the DES.  It provides a
     simple yes-or-no indication of correct operation and is useful
     to run as part of an initialization step, e.g., when a computer
     re-boots.

7.1.1.1.  DES key and Initialization Vector.

   The first 8 bytes of the 16-byte secret (private privacy key) are
   used as a DES key.  Since DES uses only 56 bits, the Least
   Significant Bit in each byte is disregarded.

   The Initialization Vector for encryption is obtained using the
   following procedure.

   The last 8 bytes of the 16-byte secret (private privacy key)
   are used as pre-IV.

   In order to ensure that the IV for two different packets encrypted
   by the same key, are not the same (i.e. the IV does not repeat) we
   need to "salt" the pre-IV with something unique per packet.
   An 8-byte octet string is used as the "salt".  The concatenation
   of the generating SNMP engine's 32-bit snmpEngineBoots and a local
   32-bit integer, that the encryption engine maintains, is input to
   the "salt".  The 32-bit integer is initialized to an arbitrary
   value at boot time.
   The 32-bit snmpEngineBoots is converted to the first 4 bytes
   (Most Significant Byte first) of our "salt".  The 32-bit integer
   is then converted to the last 4 bytes (Most Significant Byte first)
   of our "salt".  The resulting "salt" is then XOR-ed with the
   pre-IV. The 8-byte "salt" is then put into the privParameters
   field encoded as an OCTET STRING.  The "salt" integer is then
   modified.  We recommend that it be incremented by one and wrap
   when it reaches the maximum value.

   How exactly the value of the "salt" (and thus of the IV) varies,
   is an implementation issue, as long as the measures are taken to
   avoid producing a duplicate IV.

   The "salt" must be placed in the privParameters field to enable the
   receiving entity to compute the correct IV and to decrypt the
   message.

7.1.1.2.  Data Encryption.

   The data to be encrypted is treated as sequence of octets. Its
   length should be an integral multiple of 8 - and if it is not, the



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   data is padded at the end as necessary.  The actual pad value
   is irrelevant.

   The data is encrypted in Cipher Block Chaining mode.
   The plaintext is divided into 64-bit blocks.

   The plaintext for each block is XOR-ed with the ciphertext
   of the previous block, the result is encrypted and the output
   of the encryption is the ciphertext for the block.
   This procedure is repeated until there are no more plaintext
   blocks.

   For the very first block, the Initialization Vector is used
   instead of the ciphertext of the previous block.

7.1.1.3.  Data Decryption

   Before decryption, the encrypted data length is verified.
   If the length of the OCTET STRING to be decrypted is not an
   integral multiple of 8 octets, the decryption process is halted
   and an appropriate exception noted.  When decrypting, the padding
   is ignored.

   The first ciphertext block is decrypted, the decryption output is
   XOR-ed with the Initialization Vector, and the result is the first
   plaintext block.

   For each subsequent block, the ciphertext block is decrypted,
   the decryption output is XOR-ed with the previous ciphertext
   block and the result is the plaintext block.

7.2.  Elements of the DES Privacy Protocol

   This section contains definitions required to realize the privacy
   module defined by this memo.

7.2.1.  Users

   Data en/decryption using this Symmetric Encryption Protocol makes
   use of a defined set of userNames.  For any user on whose behalf
   a message must be en/decrypted 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 SNMP engine, including knowledge
   of the applicable attributes of that user.

   A user and its attributes are defined as follows:

   <userName>
     An octet string representing the name of the user.
   <privKey>



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     A user's secret key to be used as input for the DES key and IV.

7.2.2.  msgAuthoritativeEngineID

   The msgAuthoritativeEngineID value contained in an authenticated
   message specifies the authoritative SNMP engine for that particular
   message (see the definition of SnmpEngineID in the SNMP
   Architecture document [SNMP-ARCH]).

   The user's (private) privacy key is normally different at each
   authoritative SNMP engine and so the snmpEngineID is used to select
   the proper key for the en/decryption process.

7.2.3.  SNMP Messages Using this Privacy Protocol

   Messages using this privacy protocol carry a msgPrivacyParameters
   field as part of the msgSecurityParameters. For this protocol, the
   msgPrivacyParameters field is the serialized OCTET STRING
   representing the "salt" that was used to create the IV.

7.2.4.  Services provided by the DES Privacy Module

   This section describes the inputs and outputs that the DES Privacy
   module expects and produces when the User-based Security module
   invokes the DES Privacy module for services.

7.2.4.1.  Services for Encrypting Outgoing Data

   This DES privacy protocol assumes that the selection of the
   privKey is done by the caller and that the caller passes
   the secret key to be used.

   Upon completion the privacy module returns statusInformation
   and, if the encryption process was successful, the encryptedPDU
   and the msgPrivacyParameters encoded as an OCTET STRING.
   The abstract service primitive is:

   statusInformation =              -- success of failure
     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
           )

   The abstract data elements are:

     statusInformation
       An indication of the success or failure of the encryption
       process.  In case of failure, it is an indication of the error.
     encryptKey



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       The secret key to be used by the encryption algorithm.
     dataToEncrypt
       The data that must be encrypted.
     encryptedData
       The encrypted data upon successful completion.
     privParameters
       The privParameters encoded as an OCTET STRING.

7.2.4.2.  Services for Decrypting Incoming Data

   This DES privacy protocol assumes that the selection of the
   privKey is done by the caller and that the caller passes
   the secret key to be used.

   Upon completion the privacy module returns statusInformation
   and, if the decryption process was successful, the scopedPDU
   in plain text. The abstract service primitive is:

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

   The abstract data elements are:

     statusInformation
       An indication whether the data was successfully decrypted
       and if not an indication of the error.
     decryptKey
       The secret key to be used by the decryption algorithm.
     privParameters
       The "salt" to be used to calculate the IV.
     encryptedData
       The data to be decrypted.
     decryptedData
       The decrypted data.















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7.3.  Elements of Procedure.

   This section describes the procedures for the DES privacy protocol.

7.3.1.  Processing an Outgoing Message

   This section describes the procedure followed by an SNMP engine
   whenever it must encrypt part of an outgoing message using the
   usmDESPrivProtocol.

   1)  The secret (localized) cryptKey is used to construct the DES
       encryption key, the "salt" and the DES pre-IV (as described
       in section 7.1.1.1).

   2)  The privParameters field is set to the serialization according
       to the rules in [RFC1906] of an OCTET STRING representing the
       the "salt" string.

   3)  The scopedPDU is encrypted (as described in section 7.1.1.2)
       and the encrypted data is serialized according to the rules
       in [RFC1906] as an OCTET STRING.

   4)  The serialized OCTET STRING representing the encrypted
       scopedPDU together with the privParameters and statusInformation
       indicating success is returned to the calling module.

7.3.2.  Processing an Incoming Message

   This section describes the procedure followed by an SNMP engine
   whenever it must decrypt part of an incoming message using the
   usmDESPrivProtocol.

   1)  If the privParameters field is not an 8-byte OCTET STRING,
       then an error indication (decryptionError) is returned to
       the calling module.

   2)  The "salt" is extracted from the privParameters field.

   3)  The secret (localized) cryptKey and the "salt" are then used
       to construct the DES decryption key and pre-IV (as described
       in section 7.1.1.1).

   4)  The encryptedPDU is then decrypted (as described in
       section 7.1.1.3).

   5)  If the encryptedPDU cannot be decrypted, then an error
       indication (decryptionError) is returned to the calling module.

   6)  The decrypted scopedPDU and statusInformation indicating
       success are returned to the calling module.




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8.  Editors' Addresses

   Co-editor   Uri Blumenthal
               IBM T. J. Watson Research
   postal:     30 Saw Mill River Pkwy,
               Hawthorne, NY 10532
               USA
   email:      uri@watson.ibm.com
   phone:      +1-914-784-7064

   Co-editor:  Bert Wijnen
               IBM T. J. Watson Research
   postal:     Schagen 33
               3461 GL Linschoten
               Netherlands
   email:      wijnen@vnet.ibm.com
   phone:      +31-348-432-794





































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9.  Acknowledgements

This document is the result of the efforts of the SNMPv3 Working Group.
Some special thanks are in order to the following SNMPv3 WG members:

    Dave Battle (SNMP Research, Inc.)
    Uri Blumenthal (IBM T.J. Watson Research Center)
    Jeff Case (SNMP Research, Inc.)
    John Curran (BBN)
    T. Max Devlin (Hi-TECH Connections)
    John Flick (Hewlett Packard)
    David Harrington (Cabletron Systems Inc.)
    N.C. Hien (IBM T.J. Watson Research Center)
    Dave Levi (SNMP Research, Inc.)
    Louis A Mamakos (UUNET Technologies Inc.)
    Paul Meyer (Secure Computing Corporation)
    Keith McCloghrie (Cisco Systems)
    Russ Mundy (Trusted Information Systems, Inc.)
    Bob Natale (ACE*COMM Corporation)
    Mike O'Dell (UUNET Technologies Inc.)
    Dave Perkins (DeskTalk)
    Peter Polkinghorne (Brunel University)
    Randy Presuhn (BMC Software, Inc.)
    David Reid (SNMP Research, Inc.)
    Shawn Routhier (Epilogue)
    Juergen Schoenwaelder (TU Braunschweig)
    Bob Stewart (Cisco Systems)
    Bert Wijnen (IBM T.J. Watson Research Center)

The document is based on recommendations of the IETF Security and
Administrative Framework Evolution for SNMP Advisory Team.
Members of that Advisory Team were:

    David Harrington (Cabletron Systems Inc.)
    Jeff Johnson (Cisco Systems)
    David Levi (SNMP Research Inc.)
    John Linn (Openvision)
    Russ Mundy (Trusted Information Systems) chair
    Shawn Routhier (Epilogue)
    Glenn Waters (Nortel)
    Bert Wijnen (IBM T. J. Watson Research Center)

As recommended by the Advisory Team and the SNMPv3 Working Group
Charter, the design incorporates as much as practical from previous
RFCs and drafts. As a result, special thanks are due to the authors
of previous designs known as SNMPv2u and SNMPv2*:

    Jeff Case (SNMP Research, Inc.)
    David Harrington (Cabletron Systems Inc.)
    David Levi (SNMP Research, Inc.)
    Keith McCloghrie (Cisco Systems)



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    Brian O'Keefe (Hewlett Packard)
    Marshall T. Rose (Dover Beach Consulting)
    Jon Saperia (BGS Systems Inc.)
    Steve Waldbusser (International Network Services)
    Glenn W. Waters (Bell-Northern Research Ltd.)

















































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10.  Security Considerations

10.1.  Recommended Practices

   This section describes practices that contribute to the secure,
   effective operation of the mechanisms defined in this memo.

   - An SNMP engine must discard SNMP Response messages that do not
     correspond to any currently outstanding Request message. It is
     the responsibility of the Message Processing module to take care
     of this. For example it can use a msgID for that.

     An SNMP Command Generator Application must discard any Response
     PDU for which there is no currently outstanding Request PDU;
     for example for SNMPv2 [RFC1905] PDUs, the request-id component
     in the PDU can be used to correlate Responses to outstanding
     Requests.

     Although it would be typical for an SNMP engine and an SNMP
     Command Generator Application to do this as a matter of course,
     when using these security protocols it is significant due to
     the possibility of message duplication (malicious or otherwise).

   - If an SNMP engine uses a msgID for correlating Response messages
     to outstanding Request messages, then it MUST use different
     msgIDs in all such Request messages that it sends out during a
     Time Window (150 seconds) period.

     A Command Generator or Notification Originator Application MUST
     use different request-ids in all Request PDUs that it sends out
     during a TimeWindow (150 seconds) period.

     This must be done to protect against the possibility of message
     duplication (malicious or otherwise).

     For example, starting operations with a msgID and/or request-id
     value of zero is not a good idea.  Initializing them with an
     unpredictable number (so they do not start out the same after
     each reboot) and then incrementing by one would be acceptable.

   - An SNMP engine should perform time synchronization using
     authenticated messages in order to protect against the
     possibility of message duplication (malicious or otherwise).

   - When sending state altering messages to a managed authoritative
     SNMP engine, a Command Generator Application should delay sending
     successive messages to that managed SNMP engine until a positive
     acknowledgement is received for the previous message or until
     the previous message expires.

     No message ordering is imposed by the SNMP. Messages may be



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     received in any order relative to their time of generation and
     each will be processed in the ordered received.  Note that when
     an authenticated message is sent to a managed SNMP engine, it
     will be valid for a period of time of approximately 150 seconds
     under normal circumstances, and is subject to replay during this
     period.  Indeed, an SNMP engine and SNMP Command Generator
     Applications must cope with the loss and re-ordering of messages
     resulting from anomalies in the network as a matter of course.

     However, a managed object, snmpSetSerialNo [RFC1907], is
     specifically defined for use with SNMP Set operations in order
     to provide a mechanism to ensure that the processing of SNMP
     messages occurs in a specific order.

   - The frequency with which the secrets of a User-based Security
     Model user should be changed is indirectly related to the
     frequency of their use.

     Protecting the secrets from disclosure is critical to the overall
     security of the protocols.  Frequent use of a secret provides a
     continued source of data that may be useful to a cryptanalyst in
     exploiting known or perceived weaknesses in an algorithm.
     Frequent changes to the secret avoid this vulnerability.

     Changing a secret after each use is generally regarded as the
     most secure practice, but a significant amount of overhead may
     be associated with that approach.

     Note, too, in a local environment the threat of disclosure may be
     less significant, and as such the changing of secrets may be less
     frequent.  However, when public data networks are used as the
     communication paths, more caution is prudent.

10.2.  Defining Users

   The mechanisms defined in this document employ the notion of users
   on whose behalf messages are sent.  How "users" are defined is
   subject to the security policy of the network administration.
   For example, users could be individuals (e.g., "joe" or "jane"),
   or a particular role (e.g., "operator" or "administrator"), or a
   combination (e.g., "joe-operator", "jane-operator" or "joe-admin").
   Furthermore, a user may be a logical entity, such as an SNMP
   Application or a set of SNMP Applications, acting on behalf of an
   individual or role, or set of individuals, or set of roles,
   including combinations.

   Appendix A describes an algorithm for mapping a user "password" to
   a 16 octet value for use as either a user's authentication key or
   privacy key (or both).  Note however, that using the same password
   (and therefore the same key) for both authentication and privacy
   is very poor security practice and should be strongly discouraged.



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   Passwords are often generated, remembered, and input by a human.
   Human-generated passwords may be less than the 16 octets required
   by the authentication and privacy protocols, and brute force
   attacks can be quite easy on a relatively short ASCII character
   set.  Therefore, the algorithm is Appendix A performs a
   transformation on the password.  If the Appendix A algorithm is
   used, SNMP implementations (and SNMP configuration applications)
   must ensure that passwords are at least 8 characters in length.

   Because the Appendix A algorithm uses such passwords (nearly)
   directly, it is very important that they not be easily guessed.
   It is suggested that they be composed of mixed-case alphanumeric
   and punctuation characters that don't form words or phrases that
   might be found in a dictionary.  Longer passwords improve the
   security of the system.  Users may wish to input multiword
   phrases to make their password string longer while ensuring that
   it is memorable.

   Since it is infeasible for human users to maintain different
   passwords for every SNMP engine, but security requirements
   strongly discourage having the same key for more than one SNMP
   engine, the User-based Security Model employs a compromise
   proposed in [Localized-key].  It derives the user keys for the
   SNMP engines from user's password in such a way that it is
   practically impossible to either determine the user's password,
   or user's key for another SNMP engine from any combination of
   user's keys on SNMP engines.

   Note however, that if user's password is disclosed, then key
   localization will not help and network security may be
   compromised in this case.

10.3.  Conformance

   To be termed a "Secure SNMP implementation" based on the
   User-based Security Model, an SNMP implementation MUST:

   - implement one or more Authentication Protocol(s). The MD5
     Authentication Protocol defined in this memo is one such
     protocol.

   - to the maximum extent possible, prohibit access to the
     secret(s) of each user about which it maintains information
     in a Local Configuration Datastore (LCD) under all
     circumstances except as required to generate and/or
     validate SNMP messages with respect to that user.

   - implement the key-localization mechanism

   - implement the SNMP-USER-BASE-SM-MIB.




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   In addition, an authoritative SNMP engine SHOULD [RFC2119] provide
   initial configuration in accordance with Appendix A.1.

   Implementation of a Privacy Protocol (the DES Symmetric Encryption
   Protocol defined in this memo is one such protocol) is optional.

















































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11.  References

[RFC1902] The SNMPv2 Working Group, Case, J., McCloghrie, K.,
     Rose, M., and S., Waldbusser, "Structure of Management
     Information for Version  2 of the Simple Network Management
     Protocol (SNMPv2)", RFC 1902, January 1996.

[RFC1905] The SNMPv2 Working Group, Case, J., McCloghrie, K.,
     Rose, M., and S., Waldbusser, "Protocol Operations for
     Version 2 of the Simple Network Management Protocol (SNMPv2)",
     RFC 1905, January 1996.

[RFC1906] The SNMPv2 Working Group, Case, J., McCloghrie, K.,
     Rose, M., and S. Waldbusser, "Transport Mappings for
     Version 2 of the Simple Network Management Protocol (SNMPv2)",
     RFC 1906, January 1996.

[RFC1907] The SNMPv2 Working Group, Case, J., McCloghrie, K.,
     Rose, M., and S. Waldbusser, "Management Information Base for
     Version 2 of the Simple Network Management Protocol (SNMPv2)",
     RFC 1907 January 1996.

[RFC1908] The SNMPv2 Working Group, Case, J., McCloghrie, K.,
     Rose, M., and S. Waldbusser, "Coexistence between Version 1
     and Version 2 of the Internet-standard Network Management
     Framework", RFC 1908, January 1996.

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

[SNMP-ARCH] The SNMPv3 Working Group, Harrington, D., Wijnen, B.,
     "An Architecture for describing SNMP Management Frameworks",
     draft-ietf-snmpv3-next-gen-arch-04.txt, August 1997.

[SNMP-MPD] The SNMPv3 Working Group, Case, J., Harrington, D.,
     Wijnen, B., "Message Processing and Dispatching for the Simple
     Network Management Protocol (SNMP)",
     draft-ietf-snmpv3-mpc-03.txt, August 1997

[SNMP-USM] The SNMPv3 Working Group, Blumenthal, U., Wijnen, B.,
     "The User-Based Security Model for Version 3 of the Simple
     Network Management Protocol (SNMPv3)",
     draft-ietf-snmpv3-usm-01.txt, August 1997.

[SNMP-ACM] The SNMPv3 Working Group, Wijnen, B., Presuhn, R.,
     McCloghrie, K., "View-based Access Control Model for the Simple
     Network Management Protocol (SNMP)",
     draft-ietf-snmpv3-acm-02.txt, August 1997.

[SNMP-APPL] The SNMPv3 Working Group, Levi, D. B., Meyer, P.,
     Stewart, B., "SNMPv3 Applications",



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     <draft-ietf-snmpv3-appl-01.txt>, August 1997

[Localized-Key] U. Blumenthal, N. C. Hien, B. Wijnen
     "Key Derivation for Network Management Applications"
     IEEE Network Magazine, April/May issue, 1997.

[KEYED-MD5] Krawczyk, H.,
     "Keyed-MD5 for Message Authentication",
     Work in Progress, IBM, June 1995.

[MD5] Rivest, R.,  "Message Digest Algorithm MD5",
     RFC 1321, April 1992.

[DES-NIST] Data Encryption Standard, National Institute of Standards
     and Technology.  Federal Information Processing Standard (FIPS)
     Publication 46-1.  Supersedes FIPS Publication 46, (January, 1977;
     reaffirmed January, 1988).

[DES-ANSI] Data Encryption Algorithm, American National Standards
     Institute.  ANSI X3.92-1981, (December, 1980).

[DESO-NIST] DES Modes of Operation, National Institute of Standards and
     Technology.  Federal Information Processing Standard (FIPS)
     Publication 81, (December, 1980).

[DESO-ANSI] Data Encryption Algorithm - Modes of Operation, American
     National Standards Institute.  ANSI X3.106-1983, (May 1983).

[DESG-NIST] Guidelines for Implementing and Using the NBS Data
     Encryption Standard, National Institute of Standards and
     Technology.  Federal Information Processing Standard (FIPS)
     Publication 74, (April, 1981).

[DEST-NIST] Validating the Correctness of Hardware Implementations of
     the NBS Data Encryption Standard, National Institute of Standards
     and Technology.  Special Publication 500-20.

[DESM-NIST] Maintenance Testing for the Data Encryption Standard,
     National Institute of Standards and Technology.
     Special Publication 500-61, (August, 1980).














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APPENDIX A - Installation

A.1.  SNMP engine Installation Parameters

During installation, an authoritative SNMP engine SHOULD (in the
meaning as defined in [RFC2119]) be configured with several initial
parameters.  These include:

(1) A security posture

    The choice of security posture determines if initial configuration
    is implemented and if so how.  One of three possible choices
    is selected:

          minimum-secure,
          semi-secure,
          very-secure (i.e. no-initial-configuration)

    In the case of a very-secure posture, there is no initial
    configuration, and so the following steps are irrelevant.

(2) one or more secrets

    These are the authentication/privacy secrets for the first user
    to be configured.

    One way to accomplish this is to have the installer enter a
    "password" for each required secret. The password is then
    algorithmically converted into the required secret by:

    - forming a string of length 1,048,576 octets by repeating the
      value of the password as often as necessary, truncating
      accordingly, and using the resulting string as the input to
      the MD5 algorithm [MD5].  The resulting digest, termed
      "digest1", is used in the next step.

    - a second string is formed by concatenating digest1, the SNMP
      engine's snmpEngineID value, and digest1.
      This string is used as input to the MD5 algorithm [MD5].

      The resulting digest is the required secret (see Appendix A.2).













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    With these configured parameters, the SNMP engine instantiates
    the following usmUserEntry in the usmUserTable:

                              no privacy support  privacy support
                              ------------------  ---------------
     usmUserEngineID          localEngineID       localEngineID
     usmUserName              "initial"           "initial"
     usmUserSecurityName      "initial"           "initial"
     usmUserCloneFrom         ZeroDotZero         ZeroDotZero
     usmUserAuthProtocol      usmMD5AuthProtocol  usmMD5AuthProtocol
     usmUserAuthKeyChange     ""                  ""
     usmUserOwnAuthKeyChange  ""                  ""
     usmUserPrivProtocol      none                usmDESPrivProtocol
     usmUserPrivKeyChange     ""                  ""
     usmUserOwnPrivKeyChange  ""                  ""
     usmUserPublic            ""                  ""
     usmUserStorageType       anyValidStorageType anyValidStorageType
     usmUserStatus            active              active


A.2.  Password to Key Algorithm

   The following code fragment (section A.2.1) demonstrates the
   password to key algorithm which can be used when mapping a password
   to an authentication or privacy key. The calls to MD5 are as
   documented in [RFC1321].

   An example of the results of a correct implementation is provided
   (section A.2.2) which an implementer can use to check if his
   implementation produces the same result.
























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A.2.1.  Password to Key Sample Code

void password_to_key(
   u_char *password,    /* IN */
   u_int   passwordlen, /* IN */
   u_char *engineID,    /* IN  - pointer to snmpEngineID  */
   u_int   engineLength /* IN  - length of snmpEngineID */
   u_char *key)         /* OUT - pointer to caller 16-byte buffer */
{
   MD5_CTX     MD;
   u_char     *cp, password_buf[64];
   u_long      password_index = 0;
   u_long      count = 0, i;

   MD5Init (&MD);   /* initialize MD5 */

   /**********************************************/
   /* Use while loop until we've done 1 Megabyte */
   /**********************************************/
   while (count < 1048576) {
      cp = password_buf;
      for (i = 0; i < 64; i++) {
          /*************************************************/
          /* Take the next byte of the password, wrapping  */
          /* to the beginning of the password as necessary.*/
          /*************************************************/
          *cp++ = password[password_index++ % passwordlen];
      }
      MD5Update (&MD, password_buf, 64);
      count += 64;
   }
   MD5Final (key, &MD);          /* tell MD5 we're done */

   /*****************************************************/
   /* Now localize the key with the engineID and pass   */
   /* through MD5 to produce final key                  */
   /* May want to ensure that engineLength <= 32,       */
   /* otherwise need to use a buffer larger than 64     */
   /*****************************************************/
   memcpy(password_buf, key, 16);
   memcpy(password_buf+16, engineID, engineLength);
   memcpy(password_buf+engineLength, key, 16);

   MD5Init(&MD);
   MD5Update(&MD, password_buf, 32+engineLength);
   MD5Final(key, &MD);

   return;
}





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A.3.  Password to Key Sample Results

   The following shows a sample output of the password to key
   algorithm.

   With a password of "maplesyrup" the output of the password to key
   algorithm before the key is localized with the SNMP engine's
   snmpEngineID is:

      '9f af 32 83 88 4e 92 83 4e bc 98 47 d8 ed d9 63'H

   After the intermediate key (shown above) is localized with the
   snmpEngineID value of:

      '00 00 00 00 00 00 00 00 00 00 00 02'H

   the final output of the password to key algorithm is:

      '52 6f 5e ed 9f cc e2 6f 89 64 c2 93 07 87 d8 2b'H



































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A.4.  Sample encoding of msgSecurityParameters

   The msgSecurityParameters in an SNMP message are represented as
   an OCTET STRING. This OCTET STRING should be considered opaque
   outside a specific Security Model.

   The User-based Security Model defines the contents of the OCTET
   STRING as a SEQUENCE (see section 2.4).

   Given these two properties, the following is an example of the
   msgSecurityParameters for the User-based Security Model, encoded
   as an OCTET STRING:

     04 <length>
     30 <length>
     04 <length> <msgAuthoritativeEngineID>
     02 <length> <msgAuthoritativeEngineBoots>
     02 <length> <msgAuthoritativeEngineTime>
     04 <length> <msgUserName>
     04 10       <MD5-digest>
     04 08       <salt>

   Here is the example once more, but now with real values (except
   for the digest in msgAuthenticationParameters and the salt in
   msgPrivacyParameters, which depend on variable data that we have
   not defined here):

     Hex Data                         Description
     --------------  -----------------------------------------------
     04 39           OCTET STRING,                  length 57
     30 37           SEQUENCE,                      length 55
     04 0c 00000002  msgAuthoritativeEngineID:      IBM
           00000000                                 IPv4 address
           09840301                                 9.132.3.1
     02 01 01        msgAuthoritativeEngineBoots:   1
     02 02 0101      msgAuthoritativeEngineTime:    257
     04 04 62657274  msgUserName:                   bert
     04 10 01234567  msgAuthenticationParameters:   sample value
           89abcdef
           fedcba98
           76543210
     04 08 01234567  msgPrivacyParameters:          sample value
           89abcdef




















































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Table of Contents

0.  Issues and Change Log                                             2
0.1.  Resolved Issues                                                 2
0.2.  Change Log                                                      2
1.  Introduction                                                      6
1.1.  Threats                                                         6
1.2.  Goals and Constraints                                           7
1.3.  Security Services                                               8
1.4.  Module Organization                                             9
1.4.1.  Timeliness Module                                             9
1.4.2.  Authentication Protocol                                      10
1.4.3.  Privacy Protocol                                             10
1.5.  Protection against Message Replay, Delay and Redirection       10
1.5.1.  Authoritative SNMP engine                                    10
1.5.2.  Mechanisms                                                   10
2.  Elements of the Model                                            13
2.1.  User-based Security Model Users                                13
2.2.  Replay Protection                                              14
2.2.1.  msgAuthoritativeEngineID                                     14
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                        17
2.5.  Services provided by the User-based Security Model             18
2.5.1.  Services for Generating an Outgoing SNMP Message             18
2.5.2.  Services for Processing an Incoming SNMP Message             20
3.  Elements of Procedure                                            22
3.1.  Generating an Outgoing SNMP Message                            22
3.2.  Processing an Incoming SNMP Message                            25
4.  Discovery                                                        31
5.  Definitions                                                      32
6.  MD5 Authentication Protocol                                      45
6.1.  Mechanisms                                                     45
6.1.1.  Digest Authentication Protocol                               45
6.2.  Elements of the Digest Authentication Protocol                 46
6.2.1.  Users                                                        46
6.2.2.  msgAuthoritativeEngineID                                     46
6.2.3.  SNMP Messages Using this Authentication Protocol             46
6.2.4.  Services provided by the MD5 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           47
6.3.  Elements of Procedure                                          49
6.3.1.  Processing an Outgoing Message                               49
6.3.2.  Processing an Incoming Message                               49
7.  DES Privacy Protocol                                             51
7.1.  Mechanisms                                                     51
7.1.1.  Symmetric Encryption Protocol                                51
7.1.1.1.  DES key and Initialization Vector.                         52
7.1.1.2.  Data Encryption.                                           52
7.1.1.3.  Data Decryption                                            53
7.2.  Elements of the DES Privacy Protocol                           53
7.2.1.  Users                                                        53



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7.2.2.  msgAuthoritativeEngineID                                     54
7.2.3.  SNMP Messages Using this Privacy Protocol                    54
7.2.4.  Services provided by the DES Privacy Module                  54
7.2.4.1.  Services for Encrypting Outgoing Data                      54
7.2.4.2.  Services for Decrypting Incoming Data                      55
7.3.  Elements of Procedure.                                         56
7.3.1.  Processing an Outgoing Message                               56
7.3.2.  Processing an Incoming Message                               56
8.  Editors' Addresses                                               57
9.  Acknowledgements                                                 58
10.  Security Considerations                                         60
10.1.  Recommended Practices                                         60
10.2.  Defining Users                                                61
10.3.  Conformance                                                   62
11.  References                                                      64
A.1.  SNMP engine Installation Parameters                            66
A.2.  Password to Key Algorithm                                      67
A.2.1.  Password to Key Sample Code                                  68
A.3.  Password to Key Sample Results                                 69
A.4.  Sample encoding of msgSecurityParameters                       70


































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