rfc2264.txt

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Network Working Group                                      U. Blumenthal
Request for Comments: 2264                     IBM T. J. Watson Research
Category: Standards Track                                      B. Wijnen
                                               IBM T. J. Watson Research
                                                            January 1998


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

Status of this Memo

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

Copyright Notice

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

Abstract

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

Table of Contents

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



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



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

1.  Introduction

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

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

   Applications make use of the services of these subsystems.

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






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

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

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

1.1.  Threats

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

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

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

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

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

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



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

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

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

1.2.  Goals and Constraints

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

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

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

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

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




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

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