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RFC 2574

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

Pages: 86
Obsoletes:  2274
Obsoleted by:  3414
Part 1 of 3 – Pages 1 to 30
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Network Working Group                                      U. Blumenthal
Request for Comments: 2574                     IBM T. J. Watson Research
Obsoletes: 2274                                                B. Wijnen
Category: Standards Track                      IBM T. J. Watson Research
                                                              April 1999


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

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

This document describes the User-based Security Model (USM) for SNMP version 3 for use in the SNMP architecture [RFC2571]. It defines the Elements of Procedure for providing SNMP message level security. This document also includes a MIB for remotely monitoring/managing the configuration parameters for this Security Model.

Table of Contents

1. Introduction 3 1.1. Threats 4 1.2. Goals and Constraints 5 1.3. Security Services 6 1.4. Module Organization 7 1.4.1. Timeliness Module 7 1.4.2. Authentication Protocol 8 1.4.3. Privacy Protocol 8 1.5. Protection against Message Replay, Delay and Redirection 8 1.5.1. Authoritative SNMP engine 8 1.5.2. Mechanisms 9 1.6. Abstract Service Interfaces 10 1.6.1. User-based Security Model Primitives for Authentication 11 1.6.2. User-based Security Model Primitives for Privacy 11 2. Elements of the Model 12 2.1. User-based Security Model Users 12
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   2.2.  Replay Protection                                           13
   2.2.1.  msgAuthoritativeEngineID                                  13
   2.2.2.  msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime 14
   2.2.3.  Time Window                                               15
   2.3.  Time Synchronization                                        15
   2.4.  SNMP Messages Using this Security Model                     16
   2.5.  Services provided by the User-based Security Model          17
   2.5.1.  Services for Generating an Outgoing SNMP Message          17
   2.5.2.  Services for Processing an Incoming SNMP Message          19
   2.6.  Key Localization Algorithm.                                 21
   3.  Elements of Procedure                                         21
   3.1.  Generating an Outgoing SNMP Message                         22
   3.2.  Processing an Incoming SNMP Message                         25
   4.  Discovery                                                     30
   5.  Definitions                                                   31
   6.  HMAC-MD5-96 Authentication Protocol                           50
   6.1.  Mechanisms                                                  50
   6.1.1.  Digest Authentication Mechanism                           50
   6.2.  Elements of the Digest Authentication Protocol              51
   6.2.1.  Users                                                     51
   6.2.2.  msgAuthoritativeEngineID                                  51
   6.2.3.  SNMP Messages Using this Authentication Protocol          51
   6.2.4.  Services provided by the HMAC-MD5-96 Authentication Module 52
   6.2.4.1.  Services for Generating an Outgoing SNMP Message        52
   6.2.4.2.  Services for Processing an Incoming SNMP Message        53
   6.3.  Elements of Procedure                                       53
   6.3.1.  Processing an Outgoing Message                            54
   6.3.2.  Processing an Incoming Message                            54
   7.  HMAC-SHA-96 Authentication Protocol                           55
   7.1.  Mechanisms                                                  55
   7.1.1.  Digest Authentication Mechanism                           56
   7.2.  Elements of the HMAC-SHA-96 Authentication Protocol         56
   7.2.1.  Users                                                     56
   7.2.2.  msgAuthoritativeEngineID                                  57
   7.2.3.  SNMP Messages Using this Authentication Protocol          57
   7.2.4.  Services provided by the HMAC-SHA-96 Authentication Module 57
   7.2.4.1.  Services for Generating an Outgoing SNMP Message        57
   7.2.4.2.  Services for Processing an Incoming SNMP Message        58
   7.3.  Elements of Procedure                                       59
   7.3.1.  Processing an Outgoing Message                            59
   7.3.2.  Processing an Incoming Message                            60
   8.  CBC-DES Symmetric Encryption Protocol                         61
   8.1.  Mechanisms                                                  61
   8.1.1.  Symmetric Encryption Protocol                             61
   8.1.1.1.  DES key and Initialization Vector.                      62
   8.1.1.2.  Data Encryption.                                        63
   8.1.1.3.  Data Decryption                                         63
   8.2.  Elements of the DES Privacy Protocol                        63
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   8.2.1.  Users                                                     63
   8.2.2.  msgAuthoritativeEngineID                                  64
   8.2.3.  SNMP Messages Using this Privacy Protocol                 64
   8.2.4.  Services provided by the DES Privacy Module               64
   8.2.4.1.  Services for Encrypting Outgoing Data                   64
   8.2.4.2.  Services for Decrypting Incoming Data                   65
   8.3.  Elements of Procedure.                                      66
   8.3.1.  Processing an Outgoing Message                            66
   8.3.2.  Processing an Incoming Message                            66
   9.  Intellectual Property                                         67
   10. Acknowledgements                                              67
   11. Security Considerations                                       69
   11.1. Recommended Practices                                       69
   11.2. Defining Users                                              71
   11.3. Conformance                                                 72
   11.4. Use of Reports                                              72
   11.5. Access to the SNMP-USER-BASED-SM-MIB                        72
   12. References                                                    73
   13. Editors' Addresses                                            75
   A.1.  SNMP engine Installation Parameters                         76
   A.2.  Password to Key Algorithm                                   78
   A.2.1.  Password to Key Sample Code for MD5                       79
   A.2.2.  Password to Key Sample Code for SHA                       80
   A.3.  Password to Key Sample Results                              81
   A.3.1.  Password to Key Sample Results using MD5                  81
   A.3.2.  Password to Key Sample Results using SHA                  81
   A.4.  Sample encoding of msgSecurityParameters                    82
   A.5.  Sample keyChange Results                                    83
   A.5.1.  Sample keyChange Results using MD5                        83
   A.5.2.  Sample keyChange Results using SHA                        84
   B.  Change Log                                                    85
   C.  Full Copyright Statement                                      86

1. Introduction

The Architecture for describing Internet Management Frameworks [RFC2571] 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
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   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 [RFC2571].

   This memo [RFC2274] describes the User-based Security Model as it is
   used within the SNMP Architecture.  The main idea is that we use the
   traditional concept of a user (identified by a userName) with which
   to associate security information.

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

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

1.1. Threats

Several of the classical threats to network protocols are applicable to the network management problem and therefore would be applicable to any SNMP Security Model. Other threats are not applicable to the network management problem. This section discusses principal threats, secondary threats, and threats which are of lesser importance. The principal threats against which this SNMP Security Model should provide protection are: - Modification of Information The modification threat is the danger that some unauthorized entity may alter in-transit SNMP messages generated on behalf of an authorized principal in such a way as to effect unauthorized management operations, including falsifying the value of an object. - Masquerade The masquerade threat is the danger that management operations not authorized for some user may be attempted by assuming the identity of another user that has the appropriate authorizations. Two secondary threats are also identified. The Security Model defined in this memo provides limited protection against:
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   - 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 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.
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   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. Note that message reordering is not dealt with and can occur in normal conditions too.
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   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 the
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   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 HMAC-MD5-96 authentication protocol which is the first authentication protocol that MUST be supported with the User-based Security Model. Section 7 describes the HMAC-SHA-96 authentication protocol which is another authentication protocol that SHOULD be supported with the User-based Security Model. In the future additional or replacement authentication protocols may be defined as new needs arise. The User-based Security Model prescribes that, if authentication is used, then the complete message is checked for integrity in the authentication module. For a message to be authenticated, it needs to pass authentication check by the authentication module and the timeliness check which is a fixed part of this User-based Security model.

1.4.3. Privacy Protocol

Section 8 describes the CBC-DES Symmetric Encryption Protocol which is the first privacy protocol to be used with the User-based Security Model. In the future additional or replacement privacy protocols may be defined as new needs arise. The User-based Security Model prescribes that the scopedPDU is protected from disclosure when a message is sent with privacy. The User-based Security Model also prescribes that a message needs to be authenticated if privacy is in use.

1.5. Protection against Message Replay, Delay and Redirection

1.5.1. Authoritative SNMP engine

In order to protect against message replay, delay and redirection, one of the SNMP engines involved in each communication is designated to be the authoritative SNMP engine. When an SNMP message contains a payload which expects a response (those messages that contain a Confirmed Class PDU [RFC2571]), then the receiver of such messages is authoritative. When an SNMP message contains a payload which does not expect a response (those messages that contain an Unconfirmed Class PDU [RFC2571]), then the sender of such a message is authoritative.
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1.5.2. Mechanisms

The following mechanisms are used: 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 message containing an Unconfirmed Class PDU 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
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      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 engine, which theoretically can happen without the
      knowledge of the second SNMP engine anyway. Anyway, the second
      SNMP engine may not expect to receive this Trap, but is allowed to
      see the management information contained in it.

   3) Detection of messages which were not recently generated.

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

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

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

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

1.6. Abstract Service Interfaces

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

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

1.6.1. User-based Security Model Primitives for Authentication

The User-based Security Model provides the following internal primitives to pass data back and forth between the Security Model itself and the authentication service: statusInformation = authenticateOutgoingMsg( IN authKey -- secret key for authentication IN wholeMsg -- unauthenticated complete message OUT authenticatedWholeMsg -- complete authenticated message ) statusInformation = authenticateIncomingMsg( IN authKey -- secret key for authentication IN authParameters -- as received on the wire IN wholeMsg -- as received on the wire OUT authenticatedWholeMsg -- complete authenticated message )

1.6.2. User-based Security Model Primitives for Privacy

The User-based Security Model provides the following internal primitives to pass data back and forth between the Security Model itself and the privacy service: statusInformation = encryptData( IN encryptKey -- secret key for encryption IN dataToEncrypt -- data to encrypt (scopedPDU) OUT encryptedData -- encrypted data (encryptedPDU) OUT privParameters -- filled in by service provider ) statusInformation = decryptData( IN decryptKey -- secret key for decrypting IN privParameters -- as received on the wire
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     IN    encryptedData            -- encrypted data (encryptedPDU)
     OUT   decryptedData            -- decrypted data (scopedPDU)
              )

2. Elements of the Model

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

2.1. User-based Security Model Users

Management operations using this Security Model make use of a defined set of user identities. For any user on whose behalf management operations are authorized at a particular SNMP engine, that SNMP engine must have knowledge of that user. An SNMP engine that wishes to communicate with another SNMP engine must also have knowledge of a user known to that engine, including knowledge of the applicable attributes of that user. A user and its attributes are defined as follows: userName A string representing the name of the user. securityName A human-readable string representing the user in a format that is Security Model independent. authProtocol An indication of whether messages sent on behalf of this user can be authenticated, and if so, the type of authentication protocol which is used. Two such protocols are defined in this memo: - the HMAC-MD5-96 authentication protocol. - the HMAC-SHA-96 authentication protocol. authKey If messages sent on behalf of this user can be authenticated, the (private) authentication key for use with the authentication protocol. Note that a user's authentication key will normally be different at different authoritative SNMP engines. The authKey is not accessible via SNMP. The length requirements of the authKey are defined by the authProtocol in use. authKeyChange and authOwnKeyChange The only way to remotely update the authentication key. Does that in a secure manner, so that the update can be completed without the need to employ privacy protection.
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   privProtocol
     An indication of whether messages sent on behalf of this user
     can be protected from disclosure, and if so, the type of privacy
     protocol which is used.  One such protocol is defined in this
     memo: the CBC-DES Symmetric Encryption Protocol.

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

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

2.2. Replay Protection

Each SNMP engine maintains three objects: - snmpEngineID, which (at least within an administrative domain) uniquely and unambiguously identifies an SNMP engine. - snmpEngineBoots, which is a count of the number of times the SNMP engine has re-booted/re-initialized since snmpEngineID was last configured; and, - snmpEngineTime, which is the number of seconds since the snmpEngineBoots counter was last incremented. Each SNMP engine is always authoritative with respect to these objects in its own SNMP entity. It is the responsibility of a non-authoritative SNMP engine to synchronize with the authoritative SNMP engine, as appropriate. An authoritative SNMP engine is required to maintain the values of its snmpEngineID and snmpEngineBoots in non-volatile storage.

2.2.1. msgAuthoritativeEngineID

The msgAuthoritativeEngineID value contained in an authenticated message is used to defeat attacks in which messages from one SNMP engine to another SNMP engine are replayed to a different SNMP engine. It represents the snmpEngineID at the authoritative SNMP engine involved in the exchange of the message.
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   When an authoritative SNMP engine is first installed, it sets its
   local value of snmpEngineID according to a enterprise-specific
   algorithm (see the definition of the Textual Convention for
   SnmpEngineID in the SNMP Architecture document [RFC2571]).

2.2.2. msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime

The msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime values contained in an authenticated message are used to defeat attacks in which messages are replayed when they are no longer valid. They represent the snmpEngineBoots and snmpEngineTime values at the authoritative SNMP engine involved in the exchange of the message. Through use of snmpEngineBoots and snmpEngineTime, there is no requirement for an SNMP engine to have a non-volatile clock which ticks (i.e., increases with the passage of time) even when the SNMP engine is powered off. Rather, each time an SNMP engine re-boots, it retrieves, increments, and then stores snmpEngineBoots in non-volatile storage, and resets snmpEngineTime to zero. When an SNMP engine is first installed, it sets its local values of snmpEngineBoots and snmpEngineTime to zero. If snmpEngineTime ever reaches its maximum value (2147483647), then snmpEngineBoots is incremented as if the SNMP engine has re-booted and snmpEngineTime is reset to zero and starts incrementing again. Each time an authoritative SNMP engine re-boots, any SNMP engines holding that authoritative SNMP engine's values of snmpEngineBoots and snmpEngineTime need to re-synchronize prior to sending correctly authenticated messages to that authoritative SNMP engine (see Section 2.3 for (re-)synchronization procedures). Note, however, that the procedures do provide for a notification to be accepted as authentic by a receiving SNMP engine, when sent by an authoritative SNMP engine which has re-booted since the receiving SNMP engine last (re-)synchronized. If an authoritative SNMP engine is ever unable to determine its latest snmpEngineBoots value, then it must set its snmpEngineBoots value to 2147483647. Whenever the local value of snmpEngineBoots has the value 2147483647 it latches at that value and an authenticated message always causes an notInTimeWindow authentication failure. In order to reset an SNMP engine whose snmpEngineBoots value has reached the value 2147483647, manual intervention is required. The engine must be physically visited and re-configured, either
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   with a new snmpEngineID value, or with new secret values for the
   authentication and privacy protocols of all users known to that
   SNMP engine. Note that even if an SNMP engine re-boots once a second
   that it would still take approximately 68 years before the max value
   of 2147483647 would be reached.

2.2.3. Time Window

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

2.3. Time Synchronization

Time synchronization, required by a non-authoritative SNMP engine in order to proceed with authentic communications, has occurred when the non-authoritative SNMP engine has obtained a local notion of the authoritative SNMP engine's values of snmpEngineBoots and snmpEngineTime from the authoritative SNMP engine. These values must be (and remain) within the authoritative SNMP engine's Time Window. So the local notion of the authoritative SNMP engine's values must be kept loosely synchronized with the values stored at the authoritative SNMP engine. In addition to keeping a local copy of snmpEngineBoots and snmpEngineTime from the authoritative SNMP engine, a non-authoritative SNMP engine must also keep one local variable, latestReceivedEngineTime. This value records the highest value of snmpEngineTime that was received by the non-authoritative SNMP engine from the authoritative SNMP engine and is used to eliminate the possibility of replaying messages that would prevent the non-authoritative SNMP engine's notion of the snmpEngineTime from advancing. A non-authoritative SNMP engine must keep local notions of these values (snmpEngineBoots, snmpEngineTime and latestReceivedEngineTime) for each authoritative SNMP engine with which it wishes to communicate. Since each authoritative SNMP engine is uniquely and unambiguously identified by its value of snmpEngineID, the non-authoritative SNMP engine may use this value as a key in order to cache its local notions of these values. Time synchronization occurs as part of the procedures of receiving an SNMP message (Section 3.2, step 7b). As such, no explicit time synchronization procedure is required by a non-authoritative SNMP engine. Note, that whenever the local value of snmpEngineID is changed (e.g., through discovery) or when secure communications are first established with an authoritative SNMP engine, the local
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   values of snmpEngineBoots and latestReceivedEngineTime should be
   set to zero.  This will cause the time synchronization to occur
   when the next authentic message is received.

2.4. SNMP Messages Using this Security Model

The syntax of an SNMP message using this Security Model adheres to the message format defined in the version-specific Message Processing Model document (for example [RFC2572]). The field msgSecurityParameters in SNMPv3 messages has a data type of OCTET STRING. Its value is the BER serialization of the following ASN.1 sequence: USMSecurityParametersSyntax DEFINITIONS IMPLICIT TAGS ::= BEGIN UsmSecurityParameters ::= SEQUENCE { -- global User-based security parameters msgAuthoritativeEngineID OCTET STRING, msgAuthoritativeEngineBoots INTEGER (0..2147483647), msgAuthoritativeEngineTime INTEGER (0..2147483647), msgUserName OCTET STRING (SIZE(0..32)), -- authentication protocol specific parameters msgAuthenticationParameters OCTET STRING, -- privacy protocol specific parameters msgPrivacyParameters OCTET STRING } END The fields of this sequence are: - The msgAuthoritativeEngineID specifies the snmpEngineID of the authoritative SNMP engine involved in the exchange of the message. - The msgAuthoritativeEngineBoots specifies the snmpEngineBoots value at the authoritative SNMP engine involved in the exchange of the message. - The msgAuthoritativeEngineTime specifies the snmpEngineTime value at the authoritative SNMP engine involved in the exchange of the message. - The msgUserName specifies the user (principal) on whose behalf the message is being exchanged. Note that a zero-length userName will not match any user, but it can be used for snmpEngineID discovery.
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   - The msgAuthenticationParameters are defined by the authentication
     protocol in use for the message, as defined by the
     usmUserAuthProtocol column in the user's entry in the usmUserTable.

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

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

2.5. Services provided by the User-based Security Model

This section describes the services provided by the User-based Security Model with their inputs and outputs. The services are described as primitives of an abstract service interface and the inputs and outputs are described as abstract data elements as they are passed in these abstract service primitives.

2.5.1. Services for Generating an Outgoing SNMP Message

When the Message Processing (MP) Subsystem invokes the User-based Security module to secure an outgoing SNMP message, it must use the appropriate service as provided by the Security module. These two services are provided: 1) A service to generate a Request message. The abstract service primitive is: statusInformation = -- success or errorIndication generateRequestMsg( IN messageProcessingModel -- typically, SNMP version IN globalData -- message header, admin data IN maxMessageSize -- of the sending SNMP entity IN securityModel -- for the outgoing message IN securityEngineID -- authoritative SNMP entity IN securityName -- on behalf of this principal IN securityLevel -- Level of Security requested IN scopedPDU -- message (plaintext) payload OUT securityParameters -- filled in by Security Module OUT wholeMsg -- complete generated message OUT wholeMsgLength -- length of generated message ) 2) A service to generate a Response message. The abstract service primitive is:
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      statusInformation =            -- success or errorIndication
        generateResponseMsg(
        IN   messageProcessingModel  -- typically, SNMP version
        IN   globalData              -- message header, admin data
        IN   maxMessageSize          -- of the sending SNMP entity
        IN   securityModel           -- for the outgoing message
        IN   securityEngineID        -- authoritative SNMP entity
        IN   securityName            -- on behalf of this principal
        IN   securityLevel           -- Level of Security requested
        IN   scopedPDU               -- message (plaintext) payload
        IN   securityStateReference  -- reference to security state
                                     -- information from original
                                     -- request
        OUT  securityParameters      -- filled in by Security Module
        OUT  wholeMsg                -- complete generated message
        OUT  wholeMsgLength          -- length of generated message
             )

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

    statusInformation
      An indication of whether the encoding and securing of the message
      was successful.  If not it is an indication of the problem.
    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 cache is used.
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    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.
    securityEngineID
      The snmpEngineID of the authoritative SNMP engine to which a
      Request message is to be sent. In case of a response it is implied
      to be the processing SNMP engine's snmpEngineID and so if it is
      specified, then it is ignored.
    scopedPDU
      The message payload.  The data is opaque as far as the User-based
      Security Model is concerned.
    securityStateReference
      A handle/reference to cachedSecurityData to be used when securing
      an outgoing Response message.  This is the exact same
      handle/reference as it was generated by the User-based Security
      module when processing the incoming Request message to which this
      is the Response message.
    wholeMsg
      The fully encoded and secured message ready for sending on the
      wire.
    wholeMsgLength
      The length of the encoded and secured message (wholeMsg).

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

2.5.2. Services for Processing an Incoming SNMP Message

When the Message Processing (MP) Subsystem invokes the User-based Security module to verify proper security of an incoming message, it must use the service provided for an incoming message. The abstract service primitive is: statusInformation = -- errorIndication or success -- error counter OID/value if error processIncomingMsg( IN messageProcessingModel -- typically, SNMP version IN maxMessageSize -- of the sending SNMP entity IN securityParameters -- for the received message IN securityModel -- for the received message IN securityLevel -- Level of Security IN wholeMsg -- as received on the wire IN wholeMsgLength -- length as received on the wire OUT securityEngineID -- authoritative SNMP entity
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     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.
    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.
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    maxSizeResponseScopedPDU
      The maximum size of a scopedPDU to be included in a possible
      Response message.  The User-based Security module calculates this
      size based on the msgMaxSize (as received in the message) and the
      space required for the message header (including the
      securityParameters) for such a Response message.
    securityStateReference
      A handle/reference to cachedSecurityData to be used when securing
      an outgoing Response message.  When the Message Processing
      Subsystem calls the User-based Security module to generate a
      response to this incoming message it must pass this
      handle/reference.

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

2.6. Key Localization Algorithm.

A localized key is a secret key shared between a user U and one authoritative SNMP engine E. Even though a user may have only one password and therefore one key for the whole network, the actual secrets shared between the user and each authoritative SNMP engine will be different. This is achieved by key localization [Localized- key]. First, if a user uses a password, then the user's password is converted into a key Ku using one of the two algorithms described in Appendices A.2.1 and A.2.2. To convert key Ku into a localized key Kul of user U at the authoritative SNMP engine E, one appends the snmpEngineID of the authoritative SNMP engine to the key Ku and then appends the key Ku to the result, thus enveloping the snmpEngineID within the two copies of user's key Ku. Then one runs a secure hash function (which one depends on the authentication protocol defined for this user U at authoritative SNMP engine E; this document defines two authentication protocols with their associated algorithms based on MD5 and SHA). The output of the hash-function is the localized key Kul for user U at the authoritative SNMP engine E.

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.
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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 or Report message), then information concerning the user is extracted from the cachedSecurityData. The cachedSecurityData can now be discarded. The securityEngineID is set to the local snmpEngineID. The securityLevel is set to the value specified by the calling module. 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 localized privKey IN dataToEncrypt -- serialized scopedPDU OUT encryptedData -- serialized encryptedPDU OUT privParameters -- serialized privacy parameters )
ToP   noToC   RFC2574 - Page 23
          statusInformation
            indicates if the encryption process was successful or not.
          encryptKey
            the user's localized private privKey is the secret key that
            can be used by the encryption algorithm.
          dataToEncrypt
            the serialized scopedPDU is the data 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 (encryptionError) 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
          be protected from disclosure, then a zero-length OCTET STRING
          is encoded into the msgPrivacyParameters field of the
          securityParameters and the plaintext scopedPDU serves as the
          payload of the message being prepared.

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

          Otherwise,

       b) If this is a Response or Report message, then the current
          value of snmpEngineBoots and snmpEngineTime corresponding to
          the local snmpEngineID from the LCD are used.
ToP   noToC   RFC2574 - Page 24
          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 INTEGER respectively 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 localized authKey
            IN  wholeMsg              -- unauthenticated message
            OUT authenticatedWholeMsg -- authenticated complete message
                )

          statusInformation
            indicates if authentication was successful or not.
          authKey
            the user's localized 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.
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          Otherwise,

       b) If the securityLevel specifies that the message is not to be
          authenticated then a zero-length OCTET STRING is encoded 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, an error indication can return an OID and value for an incremented counter and optionally a value for securityLevel, and values for contextEngineID or contextName for the counter. In addition, the securityStateReference data is returned if any such information is available at the point where the error is detected. 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 3) If the value of the msgAuthoritativeEngineID field in the securityParameters is unknown then:
ToP   noToC   RFC2574 - Page 26
       a) a non-authoritative SNMP engine that performs discovery may
          optionally create a new entry in its Local Configuration
          Datastore (LCD) and continue processing;

          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.

       Note in the event that a zero-length, or other illegally
       sized msgAuthoritativeEngineID is received, b) should be
       chosen to facilitate engineID discovery.
       Otherwise the choice between a) and b) is an implementation
       issue.

   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 localized authKey
         IN   authParameters        -- as received on the wire
         IN   wholeMsg              -- as received on the wire
         OUT  authenticatedWholeMsg -- checked for authentication
                 )
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       statusInformation
         indicates if authentication was successful or not.
       authKey
         the user's localized 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
       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, snmpEngineTime
       and latestReceivedEngineTime
       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 2147483647;

           - 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,
          the value of the incremented counter, and an indication that
          the error must be reported with a securityLevel of authNoPriv,
          is returned to the calling module
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       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, and the extracted value of
               msgAuthoritativeEngineTime field is greater than the
               value of latestReceivedEngineTime,

             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
               2147483647;

             - 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 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.
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             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.

   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 localized 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 localized 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 (decryptionError) together
          with the OID and value of the incremented counter is returned
          to the calling module.
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          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
       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.  Information to be
       saved/cached is as follows:

          msgUserName,
          usmUserAuthProtocol, usmUserAuthKey
          usmUserPrivProtocol, usmUserPrivKey

   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.



(page 30 continued on part 2)

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