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Network Working Group G. Waters, Editor Request for Comments: 1910 Bell-Northern Research Ltd. Category: Experimental February 1996

                User-based Security Model for SNMPv2

Status of this Memo

 This memo defines an Experimental Protocol for the Internet
 community.  This memo does not specify an Internet standard of any
 kind.  Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

Table of Contents

 1. Introduction ................................................    2
 1.1 Threats ....................................................    3
 1.2 Goals and Constraints ......................................    4
 1.3 Security Services ..........................................    5
 1.4 Mechanisms .................................................    5
 1.4.1 Digest Authentication Protocol ...........................    7
 1.4.2 Symmetric Encryption Protocol ............................    8
 2. Elements of the Model .......................................   10
 2.1 SNMPv2 Users ...............................................   10
 2.2 Contexts and Context Selectors .............................   11
 2.3 Quality of Service (qoS) ...................................   13
 2.4 Access Policy ..............................................   13
 2.5 Replay Protection ..........................................   13
 2.5.1 agentID ..................................................   14
 2.5.2 agentBoots and agentTime .................................   14
 2.5.3 Time Window ..............................................   15
 2.6 Error Reporting ............................................   15
 2.7 Time Synchronization .......................................   16
 2.8 Proxy Error Propagation ....................................   16
 2.9 SNMPv2 Messages Using this Model ...........................   16
 2.10 Local Configuration Datastore (LCD) .......................   18
 3. Elements of Procedure .......................................   19
 3.1 Generating a Request or Notification .......................   19
 3.2 Processing a Received Communication ........................   20
 3.2.1 Additional Details .......................................   28
 3.2.1.1 ASN.1 Parsing Errors ...................................   28
 3.2.1.2 Incorrectly Encoded Parameters .........................   29
 3.2.1.3 Generation of a Report PDU .............................   29
 3.2.1.4 Cache Timeout ..........................................   29
 3.3 Generating a Response ......................................   30
 4. Discovery ...................................................   30
 5. Definitions .................................................   31

Waters Experimental [Page 1] RFC 1910 User-based Security Model for SNMPv2 February 1996

 4.1 The USEC Basic Group .......................................   32
 4.2 Conformance Information ....................................   35
 4.2.1 Compliance Statements ....................................   35
 4.2.2 Units of Conformance .....................................   35
 6. Security Considerations .....................................   36
 6.1 Recommended Practices ......................................   36
 6.2 Defining Users .............................................   37
 6.3 Conformance ................................................   38
 7. Editor's Address ............................................   38
 8. Acknowledgements ............................................   39
 9. References ..................................................   39
 Appendix A Installation ........................................   41
 Appendix A.1 Agent Installation Parameters .....................   41
 Appendix A.2 Password to Key Algorithm .........................   43
 Appendix A.3 Password to Key Sample ............................   44

1. Introduction

 A management system contains:  several (potentially many) nodes, each
 with a processing entity, termed an agent, which has access to
 management instrumentation; at least one management station; and, a
 management protocol, used to convey management information between
 the agents and management stations.  Operations of the protocol are
 carried out under an administrative framework which defines
 authentication, authorization, access control, and privacy policies.
 Management stations execute management applications which monitor and
 control managed elements.  Managed elements are devices such as
 hosts, routers, terminal servers, etc., which are monitored and
 controlled via access to their management information.
 The Administrative Infrastructure for SNMPv2 document [1] defines an
 administrative framework which realizes effective management in a
 variety of configurations and environments.
 In this administrative framework, a security model defines the
 mechanisms used to achieve an administratively-defined level of
 security for protocol interactions.  Although many such security
 models might be defined, it is the purpose of this document, User-
 based Security Model for SNMPv2, to define the first, and, as of this
 writing, only, security model for this administrative framework.
 This administrative framework includes the provision of an access
 control model.  The enforcement of access rights requires the means
 to identify the entity on whose behalf a request is generated.  This
 SNMPv2 security model identifies an entity on whose behalf an SNMPv2
 message is generated as a "user".

Waters Experimental [Page 2] RFC 1910 User-based Security Model for SNMPv2 February 1996

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 SNMPv2 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 SNMPv2 security model should
 provide protection are:

Modification of Information

   The modification threat is the danger that some unauthorized entity
   may alter in-transit SNMPv2 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 protocols
 defined in this memo do provide protection against:

Message Stream Modification

   The SNMPv2 protocol is typically based upon a connectionless
   transport service which may operate over any subnetwork service.
   The re-ordering, delay or replay of messages can and does occur
   through the natural operation of many such subnetwork 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
   subnetwork service, in order to effect unauthorized management
   operations.

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.
 There are at least two threats that an SNMPv2 security protocol need
 not protect against.  The security protocols defined in this memo do
 not provide protection against:

Waters Experimental [Page 3] RFC 1910 User-based Security Model for SNMPv2 February 1996

Denial of Service

   An SNMPv2 security protocol need 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

   In addition, an SNMPv2 security protocol need not attempt to
   address traffic analysis attacks.  Indeed, many traffic patterns
   are predictable - agents may be managed on a regular basis by a
   relatively small number of management stations - 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 SNMPv2 security model are
 as follows.

(1) The protocol should provide for verification that each received

   SNMPv2 message has not been modified during its transmission
   through the network in such a way that an unauthorized management
   operation might result.

(2) The protocol should provide for verification of the identity of the

   user on whose behalf a received SNMPv2 message claims to have been
   generated.

(3) The protocol should provide for detection of received SNMPv2

   messages, which request or contain management information, whose
   time of generation was not recent.

(4) The protocol should provide, when necessary, that the contents of

   each received SNMPv2 message are protected from disclosure.
 In addition to the principal goal of supporting secure network
 management, the design of this SNMPv2 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

Waters Experimental [Page 4] RFC 1910 User-based Security Model for SNMPv2 February 1996

   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 an SNMPv2
 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.
 For the protocols specified in this memo, it is not possible to
 assure the specific originator of a received SNMPv2 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. Mechanisms

 The security protocols defined in this memo employ several types of
 mechanisms in order to realize the goals and security services
 described above:

Waters Experimental [Page 5] RFC 1910 User-based Security Model for SNMPv2 February 1996

  1. In support of data integrity, a message digest algorithm is

required. A digest is calculated over an appropriate portion of an

   SNMPv2 message and included as part of the message sent to the
   recipient.
  1. In support of data origin authentication and data integrity, a

secret value is both inserted into, and appended to, the SNMPv2

   message prior to computing the digest; the inserted value
   overwritten prior to transmission, and the appended value is not
   transmitted.  The secret value is shared by all SNMPv2 entities
   authorized to originate messages on behalf of the appropriate user.
  1. To protect against the threat of message delay or replay (to an

extent greater than can occur through normal operation), a set of

   time (at the agent) indicators and a request-id are included in
   each message generated.  An SNMPv2 agent evaluates the time
   indicators to determine if a received message is recent.  An SNMPv2
   manager evaluates the time indicators to ensure that a received
   message is at least as recent as the last message it received from
   the same source.  An SNMPv2 manager uses received authentic
   messages to advance its notion of time (at the agent).  An  SNMPv2
   manager also evaluates the request-id in received Response messages
   and discards messages which do not correspond to outstanding
   requests.
   These mechanisms provide for the detection of messages whose time
   of generation was not recent in all but one circumstance; this
   circumstance is the delay or replay of a Report  message (sent to a
   manager) when the manager has has not recently communicated with
   the source of the Report message.  In this circumstance, the
   detection guarantees only that the Report message is more recent
   than the last communication between source and destination of the
   Report message.  However, Report messages do not request or contain
   management information, and thus, goal #3 in Section 1.2 above is
   met; further, Report messages can at most cause the manager to
   advance its notion of time (at the agent) by less than the proper
   amount.
   This protection against the threat of message delay or replay does
   not imply nor provide any protection against unauthorized deletion
   or suppression of messages.  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 [15]).
  1. In support of data confidentiality, an encryption algorithm is

required. An appropriate portion of the message is encrypted prior

   to being transmitted.

Waters Experimental [Page 6] RFC 1910 User-based Security Model for SNMPv2 February 1996

1.4.1. Digest Authentication Protocol

 The Digest Authentication Protocol defined in this memo provides for:
  1. verifying 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 a message.  The digest is computed
   by the originator of the message, transmitted with the message, and
   verified by the recipient of the message.
  1. verifying the user on whose behalf the message was generated.
   A secret value known only to SNMPv2 entities authorized to generate
   messages on behalf of this 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
   digest.  (Note that the use of two copies of the secret, one near
   the start and one at the end, is recommended by [14].)
  1. verifying that a message sent to/from one SNMPv2 entity cannot be

replayed to/as-if-from another SNMPv2 entity.

   Included in each message is an identifier unique to the SNMPv2
   agent associated with the sender or intended recipient of the
   message.  Also, each message containing a Response PDU contains a
   request-id which associates the message to a recently generated
   request.
   A Report message sent by one SNMPv2 agent to one SNMPv2 manager can
   potentially be replayed to another SNMPv2 manager.  However, Report
   messages do not request or contain management information, and
   thus, goal #3 in Section 1.2 above is met; further, Report messages
   can at most cause the manager to advance its notion of time (at the
   agent) by less than the correct amount.
  1. detecting messages which were not recently generated.
   A set of time indicators are included in the message, indicating
   the time of generation.  Messages (other than those containing
   Report PDUs) without recent time indicators are not considered
   authentic.  In addition, messages containing Response PDUs have a
   request-id; if the request-id does not match that of a recently
   generated request, then the message is not considered to be
   authentic.

Waters Experimental [Page 7] RFC 1910 User-based Security Model for SNMPv2 February 1996

   A Report message sent by an SNMPv2 agent can potentially be
   replayed at a later time to an SNMPv2 manager which has not
   recently communicated with that agent.  However, Report messages do
   not request or contain management information, and thus, goal #3 in
   Section 1.2 above is met; further, Report messages can at most
   cause the manager to advance its notion of time (at the agent) by
   less than the correct amount.
 This protocol uses the MD5 [3] message digest algorithm.  A 128-bit
 digest is calculated over the designated portion of an SNMPv2 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 is 16 octets.
 This memo allows the same user to be defined on multiple SNMPv2
 agents and managers.  Each SNMPv2 agent maintains a value, agentID,
 which uniquely identifies the agent. This value is included in each
 message sent to/from that agent.  Messages sent from a SNMPv2 dual-
 role entity [1] to a SNMPv2 manager include the agentID value
 maintained by the dual-role entity's agent.  On receipt of a message,
 an agent checks the value to ensure it is the intended recipient, and
 a manager uses the value to ensure that the message is processed
 using the correct state information.
 Each SNMPv2 agent maintains two values, agentBoots and agentTime,
 which taken together provide an indication of time at that agent.
 Both of these values are included in an authenticated message sent
 to/received from that agent.  Authenticated messages sent from a
 SNMPv2 dual-role entity to a SNMPv2 manager include the agentBoots
 and agentTime values maintained by the dual-role entity's agent.  On
 receipt, the values are checked to ensure that the indicated time 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 SNMPv2 manager to generate a message which an agent will
 accept as authentic, and to verify that a message received from that
 agent is authentic, that manager must first achieve time
 synchronization with that agent.  Similarly, for a manger to verify
 that a message received from an SNMPv2 dual-role entity is authentic,
 that manager must first achieve time synchronization with the dual-
 role entity's agent.

1.4.2. Symmetric Encryption Protocol

 The Symmetric Encryption Protocol defined in this memo provides
 support for data confidentiality through the use of the Data
 Encryption Standard (DES) in the Cipher Block Chaining mode of

Waters Experimental [Page 8] RFC 1910 User-based Security Model for SNMPv2 February 1996

 operation.  The designated portion of an SNMPv2 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) [5] and the
 American National Standards Institute [6].  There is a companion
 Modes of Operation specification for each definition (see [7] and
 [8], respectively).
 The NIST has published three additional documents that implementors
 may find useful.
  1. There is a document with guidelines for implementing and using the

DES, including functional specifications for the DES and its modes

   of operation [9].
  1. There is a specification of a validation test suite for the DES

[10]. The suite is designed to test all aspects of the DES and is

   useful for pinpointing specific problems.
  1. There is a specification of a maintenance test for the DES [11].

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 reboots.
 This Symmetric Encryption Protocol specifies that the size of the
 privacy key is 16 octets, of which the first 8 octets are a DES key
 and the second 8 octets are a DES Initialization Vector.  The 64-bit
 DES key in the first 8 octets of the private key is a 56 bit quantity
 used directly by the algorithm plus 8 parity bits - arranged so that
 one parity bit is the least significant bit of each octet.  The
 setting of the parity bits is ignored by this protocol.
 The length of an octet sequence to be encrypted by the DES must be an
 integral multiple of 8.  When encrypting, the data is padded at the
 end as necessary; the actual pad value is irrelevant.
 If the length of the octet sequence to be decrypted is not an
 integral multiple of 8 octets, the processing of the octet sequence
 is halted and an appropriate exception noted.  When decrypting, the
 padding is ignored.

Waters Experimental [Page 9] RFC 1910 User-based Security Model for SNMPv2 February 1996

2. Elements of the Model

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

2.1. SNMPv2 Users

 Management operations using this security model make use of a defined
 set of user identities.  For any SNMPv2 user on whose behalf
 management operations are authorized at a particular SNMPv2 agent,
 that agent must have knowledge of that user.  A SNMPv2 manager that
 wishes to communicate with a particular agent must also have
 knowledge of a user known to that agent, including knowledge of the
 applicable attributes of that user.  Similarly, a SNMPv2 manager that
 wishes to receive messages from a SNMPv2 dual-role entity must have
 knowledge of the user on whose behalf the dual-role entity sends the
 message.
 A user and its attributes are defined as follows:

<userName>

   An octet string representing the name of the user.

<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.

<authPrivateKey>

   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 agents.

<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 Symmetric Encryption Protocol.

<privPrivateKey>

   If messages sent on behalf of this user can be protected from
   disclosure, the (private) privacy key for use with the privacy
   protocol.  Note that a user's privacy key will normally be
   different at different agents.

Waters Experimental [Page 10] RFC 1910 User-based Security Model for SNMPv2 February 1996

2.2. Contexts and Context Selectors

 An SNMPv2 context is a collection of management information
 accessible (locally or via proxy) by an SNMPv2 agent.  An item of
 management information may exist in more than one context.  An SNMPv2
 agent potentially has access to many contexts.  Each SNMPv2 message
 contains a context selector which unambiguously identifies an SNMPv2
 context accessible by the SNMPv2 agent to which the message is
 directed or by the SNMPv2 agent associated with the sender of the
 message.
 For a local SNMPv2 context which is realized by an SNMPv2 entity,
 that SNMPv2 entity uses locally-defined mechanisms to access the
 management information identified by the SNMPv2 context.
 For a proxy SNMPv2 context, the SNMPv2 entity acts as a proxy SNMPv2
 agent to access the management information identified by the SNMPv2
 context.
 The term remote SNMPv2 context is used at an SNMPv2 manager to
 indicate a SNMPv2 context (either local or proxy) which is not
 realized by the local SNMPv2 entity (i.e., the local SNMPv2 entity
 uses neither locally-defined mechanisms, nor acts as a proxy SNMPv2
 agent to access the management information identified by the SNMPv2
 context).
 Proxy SNMPv2 contexts are further categorized as either local-proxy
 contexts or remote-proxy contexts.  A proxy SNMPv2 agent receives
 Get/GetNext/GetBulk/Set operations for a local-proxy context, and
 forwards them with a remote-proxy context; it receives SNMPv2-Trap
 and Inform operations for a remote-proxy context, and forwards them
 with a local-proxy context; for Response operations, a proxy SNMPv2
 agent receives them with either a local-proxy or remote-proxy
 context, and forwards them with a remote-proxy or local-proxy
 context, respectively.

Waters Experimental [Page 11] RFC 1910 User-based Security Model for SNMPv2 February 1996

   For the non-proxy situation:
                    context-A
       Manager <----------------> Agent
   the type of context is:
                         +-----------------+
                         |   context-A     |
       +-----------------+-----------------+
       | Manager         |    remote       |
       +-----------------+-----------------+
       | Agent           |    local        |
       +-----------------+-----------------+
       | agentID         |   of Agent      |
       +-----------------+-----------------+
       | contextSelector | locally unique  |
       +-----------------+-----------------+
   For proxy:
                    context-B               context-C
       Manager <----------------> Proxy <----------------> Agent
                                  Agent
   the type and identity of the contexts are:
                         +-----------------+-----------------+
                         |   context-B     |    context-C    |
       +-----------------+-----------------+-----------------+
       | Manager         |    remote       |       --        |
       +-----------------+-----------------+-----------------+
       | Proxy-Agent     |  local-proxy    |   remote-proxy  |
       +-----------------+-----------------+-----------------+
       | Agent           |      --         |      local      |
       +-----------------+-----------------+-----------------+
       | agentID         | of Proxy agent  |     of Agent    |
       +-----------------+-----------------+-----------------+
       | contextSelector | locally unique  |  locally unique |
       +-----------------+-----------------+-----------------+
 The combination of an agentID value and a context selector provides a
 globally-unique identification of a context.  When a context is
 accessible by multiple agents (e.g., including by proxy SNMPv2
 agents), it has multiple such globally-unique identifications, one
 associated with each agent which can access it. In the example above,
 "context-B" and "context-C" are different names for the same context.

Waters Experimental [Page 12] RFC 1910 User-based Security Model for SNMPv2 February 1996

2.3. Quality of Service (qoS)

 Messages are generated with a particular Quality of Service (qoS),
 either:
  1. without authentication and privacy,
  1. with authentication but not privacy,
  1. with authentication and privacy.
 All users are capable of having messages without authentication and
 privacy generated on their behalf.  Users having an authentication
 protocol and an authentication key can have messages with
 authentication but not privacy generated on their behalf. Users
 having an authentication protocol, an authentication key, a privacy
 protocol and a privacy key can have messages with authentication and
 privacy generated on their behalf.
 In addition to its indications of authentication and privacy, the qoS
 may also indicate that the message contains an operation that may
 result in a report PDU being generated (see Section 2.6 below).

2.4. Access Policy

 An administration's access policy determines the access rights of
 users.  For a particular SNMPv2 context to which a user has access
 using a particular qoS, that user's access rights are given by a list
 of authorized operations, and for a local context, a read-view and a
 write-view.  The read-view is the set of object instances authorized
 for the user when reading objects.  Reading objects occurs when
 processing a retrieval (get, get-next, get-bulk) operation and when
 sending a notification.  The write-view is the set of object
 instances authorized for the user when writing objects.  Writing
 objects occurs when processing a set operation.  A user's access
 rights may be different at different agents.

2.5. Replay Protection

 Each SNMPv2 agent (or dual-role entity) maintains three objects:
  1. agentID, which is an identifier unique among all agents in (at

least) an administrative domain;

  1. agentBoots, which is a count of the number of times the agent has

rebooted/re-initialized since agentID was last configured; and,

Waters Experimental [Page 13] RFC 1910 User-based Security Model for SNMPv2 February 1996

  1. agentTime, which is the number of seconds since agentBoots was last

incremented.

 An SNMPv2 agent is always authoritative with respect to these
 variables.  It is the responsibility of an SNMPv2 manager to
 synchronize with the agent, as appropriate.  In the case of an SNMPv2
 dual-role entity sending an Inform-Request, it is that entity acting
 in an agent role which is authoritative with respect to these
 variables for the Inform-Request.
 An agent is required to maintain the values of agentID and agentBoots
 in non-volatile storage.

2.5.1. agentID

 The agentID value contained in an authenticated message is used to
 defeat attacks in which messages from a manager are replayed to a
 different agent and/or messages from one agent (or dual-role entity)
 are replayed as if from a different agent (or dual-role entity).
 When an agent (or dual-role entity) is first installed, it sets its
 local value of agentID according to a enterprise-specific algorithm
 (see the definition of agentID in Section 4.1).

2.5.2. agentBoots and agentTime

 The agentBoots and agentTime values contained in an authenticated
 message are used to defeat attacks in which messages are replayed
 when they are no longer valid.  Through use of agentBoots and
 agentTime, there is no requirement for an SNMPv2 agent to have a
 non-volatile clock which ticks (i.e., increases with the passage of
 time) even when the agent is powered off.  Rather, each time an
 SNMPv2 agent reboots, it retrieves, increments, and then stores
 agentBoots in non-volatile storage, and resets agentTime to zero.
 When an agent (or dual-role entity) is first installed, it sets its
 local values of agentBoots and agentTime to zero.  If agentTime ever
 reaches its maximum value (2147483647), then agentBoots is
 incremented as if the agent has rebooted and agentTime is reset to
 zero and starts incrementing again.
 Each time an agent (or dual-role entity) reboots, any SNMPv2 managers
 holding that agent's values of agentBoots and agentTime need to re-
 synchronize prior to sending correctly authenticated messages to that
 agent (see Section 2.7 for re-synchronization procedures).  Note,
 however, that the procedures do provide for a notification to be
 accepted as authentic by a manager, when sent by an agent which has
 rebooted since the manager last re-synchronized.

Waters Experimental [Page 14] RFC 1910 User-based Security Model for SNMPv2 February 1996

 If an agent (or dual-role entity) is ever unable to determine its
 latest agentBoots value, then it must set its agentBoots value to
 0xffffffff.
 Whenever the local value of agentBoots has the value 0xffffffff, it
 latches at that value and an authenticated message always causes an
 usecStatsNotInWindows authentication failure.
 In order to reset an agent whose agentBoots value has reached the
 value 0xffffffff, manual intervention is required.  The agent must be
 physically visited and re-configured, either with a new agentID
 value, or with new secret values for the authentication and privacy
 keys of all users known to that agent.

2.5.3. 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.

2.6. Error Reporting

 While processing a received communication, an SNMPv2 entity may
 determine that the message is unacceptable (see Section 3.2).  In
 this case, the appropriate counter from the snmpGroup [15] or
 usecStatsGroup object groups is incremented and the received message
 is discarded without further processing.
 If an SNMPv2 entity acting in the agent role makes such a
 determination and the qoS indicates that a report may be generated,
 then after incrementing the appropriate counter, it is required to
 generate a message containing a report PDU, with the same user and
 context as the received message, and to send it to the transport
 address which originated the received message.  For all report PDUs,
 except those generated due to incrementing the usecStatsNotInWindows
 counter, the report PDU is unauthenticated.  For those generated due
 to incrementing usecStatsNotInWindows, the report PDU is
 authenticated only if the received message was authenticated.
 The report flag in the qoS may only be set if the message contains a
 Get, GetNext, GetBulk, Set operation.  The report flag should never
 be set for a message that contains a Response, Inform, SNMPv2-Trap or
 Report operation.  Furthermore, a report PDU is never sent by an
 SNMPv2 entity acting in a manager role.

Waters Experimental [Page 15] RFC 1910 User-based Security Model for SNMPv2 February 1996

2.7. Time Synchronization

 Time synchronization, required by a management entity in order to
 proceed with authentic communications, has occurred when the
 management entity has obtained local values of agentBoots and
 agentTime from the agent that are within the agent's time window.  To
 remain synchronized, the local values must remain within the agent's
 time window and thus must be kept loosely synchronized with the
 values stored at the agent.  In addition to keeping a local version
 of agentBoots and agentTime, a manager must also keep one other local
 variable, latestReceivedAgentTime.  This value records the highest
 value of agentTime that was received by the manager from the agent
 and is used to eliminate the possibility of replaying messages that
 would prevent the manager's notion of the agentTime from advancing.
 Time synchronization occurs as part of the procedures of receiving a
 message (Section 3.2, step 9d). As such, no explicit time
 synchronization procedure is required by a management entity.  Note,
 that whenever the local value of agentID is changed (e.g., through
 discovery) or when a new secret is configured, the local values of
 agentBoots and latestReceivedAgentTime should be set to zero. This
 will cause the time synchronization to occur when the next authentic
 message is received.

2.8. Proxy Error Propagation

 When a proxy SNMPv2 agent receives a report PDU from a proxied agent
 and it is determined that a proxy-forwarded request cannot be
 delivered to the proxied agent, then the snmpProxyDrops counter [15]
 is incremented and a report PDU is generated and transmitted to the
 transport address from which the original request was received.
 (Note that the receipt of a report PDU containing snmpProxyDrops as a
 VarBind, is included among the reasons why a proxy-forwarded request
 cannot be delivered.)

2.9. SNMPv2 Messages Using this Model

 The syntax of an SNMPv2 message using this security model differs
 from that of an SNMPv1 [2] message as follows:
  1. The version component is changed to 2.
  1. The data component contains either a PDU or an OCTET STRING

containing an encrypted PDU.

 The SNMPv1 community string is now termed the "parameters" component
 and contains a set of administrative information for the message.

Waters Experimental [Page 16] RFC 1910 User-based Security Model for SNMPv2 February 1996

 Only the PDU is protected from disclosure by the privacy protocol.
 This exposes the administrative information to eavesdroppers.
 However, malicious use of this information is considered to be a
 Traffic Analysis attack against which protection is not provided.
 For an authenticated SNMPv2 message, the message digest is applied to
 the entire message given to the transport service.  As such, message
 generation first privatizes the PDU, then adds the message wrapper,
 and then authenticates the message.
 An SNMPv2 message is an ASN.1 value with the following syntax:
   Message ::=
       SEQUENCE {
           version
               INTEGER { v2 (2) },
           parameters
               OCTET STRING,
           -- <model=1>
           --      <qoS><agentID><agentBoots><agentTime><maxSize>
           --      <userLen><userName><authLen><authDigest>
           --      <contextSelector>
           data
               CHOICE {
                   plaintext
                       PDUs,
                   encrypted
                       OCTET STRING
               }
       }

where:

parameters
   a concatenation of the following values in network-byte order.  If
   the first octet (<model>) is one, then
   <qoS>    = 8-bits of quality-of-service
            bitnumber
            7654 3210     meaning
            ---- ----     --------------------------------
            .... ..00     no authentication nor privacy
            .... ..01     authentication, no privacy
            .... ..1.     authentication and privacy
            .... .1..     generation of report PDU allowed

Waters Experimental [Page 17] RFC 1910 User-based Security Model for SNMPv2 February 1996

            where bit 7 is the most significant bit.
   <agentID>    = 12 octets
        a unique identifier for the agent (or dual-role entity).
   <agentBoots> = 32-bits
        an unsigned quantity (0..4294967295) in network-byte order.
   <agentTime>  = 32-bits
        an unsigned quantity (0..2147483647) in network-byte order.
   <maxSize>    = 16-bits
        an unsigned quantity (484..65507) in network-byte order, which
        identifies the maximum message size which the sender of this
        message can receive using the same transport domain as used
        for this message.
   <userLen>    = 1 octet
        the length of following <userName> field.
   <userName>   = 1..16 arbitrary octets
        the user on whose behalf this message is sent.
   <authLen>    = 1 octet
        the length of following <authDigest> field.
   <authDigest> = 0..255 octets
        for authenticated messages, the authentication digest.
        Otherwise, the value has zero-length on transmission and is
        ignored on receipt.
   <contextSelector> = 0..40 arbitrary octets
        the context selector which in combination with agentID
        identifies the SNMPv2 context containing the management
        information referenced by the SNMPv2 message.
plaintext
   an SNMPv2 PDU as defined in [12].
encrypted
   the encrypted form of an SNMPv2 PDU.

2.10. Local Configuration Datastore (LCD)

 Each SNMPv2 entity maintains a local conceptually database, called
 the Local Configuration Datastore (LCD), which holds its known set of
 information about SNMPv2 users and other associated (e.g., access
 control) information.  An LCD may potentially be required to hold

Waters Experimental [Page 18] RFC 1910 User-based Security Model for SNMPv2 February 1996

 information about multiple SNMPv2 agent entities. As such, the
 <agentID> should be used to identify a particular agent entity in the
 LCD.
 It is a local implementation issue as to whether information in the
 LCD is stored information or whether it is obtained dynamically
 (e.g., as a part of an SNMPv2 manager's API) on an as-needed basis.

3. Elements of Procedure

 This section describes the procedures followed by an SNMPv2 entity in
 processing SNMPv2 messages.

3.1. Generating a Request or Notification

 This section describes the procedure followed by an SNMPv2 entity
 whenever it generates a message containing a management operation
 (either a request or a notification) on behalf of a user, for a
 particular context and with a particular qoS value.

(1) Information concerning the user is extracted from the LCD. The

   transport domain and transport address to which the operation is to
   be sent is determined.  The context is resolved into an agentID
   value and a contextSelector value.

(2) If the qoS specifies that the message is to be protected from

   disclosure, but the user does not support both an authentication
   and a privacy protocol, or does not have configured authentication
   and privacy keys, then the operation cannot be sent.

(3) If the qoS specifies that the message is to be authenticated, but

   the user does not support an authentication protocol, or does not
   have a configured authentication key, then the operation cannot be
   sent.

(4) The operation is serialized (i.e., encoded) according to the

   conventions of [13] and [12] into a PDUs value.

(5) If the operation is a Get, GetNext, GetBulk, or Set then the report

   flag in the qoS is set to the value 1.

(6) An SNMPv2 message is constructed using the ASN.1 Message syntax:

  1. the version component is set to the value 2.
  1. if the qoS specifies that the message is to be protected from

disclosure, then the octet sequence representing the serialized

     PDUs value is encrypted according to the user's privacy protocol

Waters Experimental [Page 19] RFC 1910 User-based Security Model for SNMPv2 February 1996

     and privacy key, and the encrypted data is encoded as an octet
     string and is used as the data component of the message.
  1. if the qoS specifies that the message is not to be protected from

disclosure, then the serialized PDUs value is used directly as

     the value of the data component.
  1. the parameters component is constructed using:
  1. the requested qoS, userName, agentID and context selector,
  1. if the qoS specifies that the message is to be authenticated or

the management operation is a notification, then the current

       values of agentBoots, and agentTime corresponding to agentID
       from the LCD are used.  Otherwise, the <agentBoots> and
       <agentTime> fields are set to zero-filled octets.
  1. the <maxSize> field is set to the maximum message size which

the local SNMPv2 entity can receive using the transport domain

       which will be used to send this message.
  1. if the qoS specifies that the message is to be authenticated,

then the <authDigest> field is temporarily set to the user's

       authentication key.  Otherwise, the <authDigest> field is set
       to the zero-length string.

(7) The constructed Message value is serialized (i.e., encoded)

   according to the conventions of [13] and [12].

(8) If the qoS specifies that the message is to be authenticated, then

   an MD5 digest value is computed over the octet sequence
   representing the concatenation of the serialized Message value and
   the user's authentication key.  The <authDigest> field is then set
   to the computed digest value.

(9) The serialized Message value is transmitted to the determined

   transport address.

3.2. Processing a Received Communication

 This section describes the procedure followed by an SNMPv2 entity
 whenever it receives an SNMPv2 message.  This procedure is
 independent of the transport service address at which the message was
 received.  For clarity, some of the details of this procedure are
 left out and are described in Section 3.2.1 and its sub-sections.

(1) The snmpInPkts counter [15] is incremented. If the received

   message is not the serialization (according to the conventions of

Waters Experimental [Page 20] RFC 1910 User-based Security Model for SNMPv2 February 1996

   [13]) of a Message value, then the snmpInASNParseErrs counter [15]
   is incremented, and the message is discarded without further
   processing.

(2) If the value of the version component has a value other than 2,

   then the message is either processed according to some other
   version of this protocol, or the snmpInBadVersions counter [15] is
   incremented, and the message is discarded without further
   processing.

(3) The value of the <model> field is extracted from the parameters

   component of the Message value.  If the value of the <model> field
   is not 1, then either the message is processed according to some
   other security model, or the usecStatsBadParameters counter is
   incremented, and the message is discarded without further
   processing.

(4) The values of the rest of the fields are extracted from the

   parameters component of the Message value.

(5) If the <agentID> field contained in the parameters is unknown then:

  1. a manager that performs discovery may optionally create a new LCD

entry and continue processing; or

  1. the usecStatsUnknownContexts counter is incremented, a report PDU

is generated, and the received message is discarded without

     further processing.

(6) The LCD is consulted for information about the SNMPv2 context

   identified by the combination of the <agentID> and
   <contextSelector> fields.  If information about this SNMPv2 context
   is absent from the LCD, then the usecStatsUnknownContexts counter
   is incremented, a report PDU is generated, and the received message
   is discarded without further processing.

(7) Information about the value of the <userName> field is extracted

   from the LCD.  If no information is available, then the
   usecStatsUnknownUserNames counter is incremented, a report PDU [1]
   is generated, and the received message is discarded without further
   processing.

(8) If the information about the user indicates that it does not

   support the quality of service indicated by the <qoS> field, then
   the usecStatsUnsupportedQoS counter is incremented, a report PDU is
   generated, and the received message is discarded without further
   processing.

Waters Experimental [Page 21] RFC 1910 User-based Security Model for SNMPv2 February 1996

(9) If the <qoS> field indicates an authenticated message and the

   user's authentication protocol is the Digest Authentication
   Protocol described in this memo, then:
   a) the local values of agentBoots and agentTime corresponding to
      the value of the <agentID> field are extracted from the LCD.
   b) the value of <authDigest> field is temporarily saved.  A new
      serialized Message is constructed which differs from that
      received in exactly one respect: that the <authDigest> field
      within it has the value of the user's authentication key.  An
      MD5 digest value is computed over the octet sequence
      representing the concatenation of the new serialized Message and
      the user's authentication key.
   c) if the LCD information indicates the SNMPv2 context is of type
      local (i.e., an agent), then:
  1. if the computed digest differs from the saved authDigest

value, then the usecStatsWrongDigestValues counter is

        incremented, a report PDU is generated, and the received
        message is discarded without further processing. However, if
        the snmpEnableAuthenTraps object [15] is enabled, then the
        SNMPv2 entity sends authenticationFailure traps [15] according
        to its configuration.
  1. if any of the following conditions is true, then the message

is considered to be outside of the Time Window:

  1. the local value of agentBoots is 0xffffffff;
  1. the <agentBoots> field differs from the local value of

agentBoots; or,

  1. the value of the <agentTime> field differs from the local

notion of agentTime by more than +/- 150 seconds.

  1. if the message is considered to be outside of the Time Window

then the usecStatsNotInWindows counter is incremented, an

        authenticated report PDU is generated (see section 2.7), and
        the received message is discarded without further processing.
   d) if the LCD information indicates the SNMPv2 context is not
      realized by the local SNMPv2 entity (i.e., a manager), then:
  1. if the computed digest differs from the saved authDigest

value, then the usecStatsWrongDigestValues counter is

        incremented and the received message is discarded without

Waters Experimental [Page 22] RFC 1910 User-based Security Model for SNMPv2 February 1996

        further processing.
  1. if all of the following conditions are true:
  1. if the <qoS> field indicates that privacy is not in use;
  1. the SNMPv2 operation type determined from the ASN.1 tag

value associated with the PDU's component is a Report;

  1. the Report was generated due to a usecStatsNotInWindows

error condition; and,

  1. the <agentBoots> field is greater than the local value of

agentBoots, or the <agentBoots> field is equal to the

             local value of agentBoots and the <agentTime> field is
             greater than the value of latestReceivedAgentTime,
        then the LCD entry corresponding to the value of the <agentID>
        field is updated, by setting the local value of agentBoots
        from the <agentBoots> field, the value latestReceivedAgentTime
        from the <agentTime> field, and the local value of agentTime
        from the <agentTime> field.
  1. if any of the following conditions is true, then the message

is considered to be outside of the Time Window:

  1. the local value of agentBoots is 0xffffffff;
  1. the <agentBoots> field is less than the local value of

agentBoots; or,

  1. the <agentBoots> field is equal to the local value of

agentBoots and the <agentTime> field is more than 150

          seconds less than the local notion of agentTime.
  1. if the message is considered to be outside of the Time Window

then the usecStatsNotInWindows counter is incremented, and the

        received message is discarded without further processing;
        however, time synchronization procedures may be invoked.  Note
        that this procedure allows for <agentBoots> to be greater than
        the local value of agentBoots to allow for received messages
        to be accepted as authentic when received from an agent that
        has rebooted since the manager last re-synchronized.
  1. if at least one of the following conditions is true:
  1. the <agentBoots> field is greater than the local value of

agentBoots; or,

Waters Experimental [Page 23] RFC 1910 User-based Security Model for SNMPv2 February 1996

  1. the <agentBoots> field is equal to the local value of

agentBoots and the <agentTime> field is greater than the

             value of latestReceivedAgentTime,
        then the LCD entry corresponding to the value of the <agentID>
        field is updated, by setting the local value of agentBoots
        from the <agentBoots> field, the local value
        latestReceivedAgentTime from the <agentTime> field, and the
        local value of agentTime from the <agentTime> field.

(10) If the <qoS> field indicates use of a privacy protocol, then the

   octet sequence representing the data component is decrypted
   according to the user's privacy protocol to obtain a serialized
   PDUs value.  Otherwise the data component is assumed to directly
   contain the PDUs value.

(11) The SNMPv2 operation type is determined from the ASN.1 tag value

   associated with the PDUs component.

(12) If the SNMPv2 operation type is a Report, then the request-id in

   the PDU is correlated to an outstanding request, and if the
   correlation is successful, the appropriate action is taken (e.g.,
   time synchronization, proxy error propagation, etc.); in
   particular, if the report PDU indicates a usecStatsNotInWindows
   condition, then the outstanding request may be retransmitted (since
   the procedure in Step 9d above should have resulted in time
   synchronization).

(13) If the SNMPv2 operation type is either a Get, GetNext, GetBulk, or

   Set operation, then:
   a) if the LCD information indicates that the SNMPv2 context is of
      type remote or remote-proxy, then the
      usecStatsUnauthorizedOperations counter is incremented, a report
      PDU is generated, and the received message is discarded without
      further processing.
   b) the LCD is consulted for access rights authorized for
      communications using the indicated qoS, on behalf of the
      indicated user, and concerning management information in the
      indicated SNMPv2 context for the particular SNMPv2 operation
      type.
   c) if the SNMPv2 operation type is not among the authorized access
      rights, then the usecStatsUnauthorizedOperations counter is
      incremented, a report PDU is generated, and the received message
      is discarded without further processing.

Waters Experimental [Page 24] RFC 1910 User-based Security Model for SNMPv2 February 1996

   d) The information extracted from the LCD concerning the user and
      the SNMPv2 context, together with the sending transport address
      of the received message is cached for later use in generating a
      response message.
   e) if the LCD information indicates the SNMPv2 context is of type
      local, then the management operation represented by the PDUs
      value is performed by the receiving SNMPv2 entity with respect
      to the relevant MIB view within the SNMPv2 context according to
      the procedures set forth in [12], where the relevant MIB view is
      determined according to the user, the agentID, the
      contextSelector, the qoS values and the type of operation
      requested.
   f) if the LCD information indicates the SNMPv2 context is of type
      local-proxy, then:
      i. the user, qoS, agentID, contextSelector and transport address
         to be used to forward the request are extracted from the LCD.
         If insufficient information concerning the user is currently
         available, then snmpProxyDrops counter [15] is incremented, a
         report PDU is generated, and the received message is
         discarded.
      ii. if an administrative flag in the LCD indicates that the
         message is to be forwarded using the SNMPv1 administrative
         framework, then the procedures described in [4] are invoked.
         Otherwise, a new SNMPv2 message is constructed: its PDUs
         component is copied from that in the received message except
         that the contained request-id is replaced by a unique value
         (this value will enable a subsequent response message to be
         correlated with this request); the <userName>, <qoS>,
         <agentID> and <contextSelector> fields are set to the values
         extracted from the LCD; the <maxSize> field is set to the
         minimum of the value in the received message and the local
         system's maximum message size for the transport domain which
         will be used to forward the message; and finally, the message
         is authenticated and/or protected from disclosure according
         to the qoS value.
      iii. the information cached in Step 13d above is augmented with
         the request-id of the received message as well as the
         request-id, agentID and contextSelector of the constructed
         message.
      iv. the constructed message is forwarded to the extracted
         transport address.

Waters Experimental [Page 25] RFC 1910 User-based Security Model for SNMPv2 February 1996

(14) If the SNMPv2 operation type is an Inform, then:

   a) if the LCD information indicates the SNMPv2 context is of type
      local or local-proxy then the usecStatsUnauthorizedOperations
      counter is incremented, a report PDU is generated, and the
      received message is discarded without further processing.
   b) if the LCD information indicates the SNMPv2 context is of type
      remote, then the Inform operation represented by the PDUs value
      is performed by the receiving SNMPv2 entity according to the
      procedures set forth in [12].
   c) if the LCD information indicates the SNMPv2 context is of type
      remote-proxy, then:
      i. a single unique request-id is selected for use by all
         forwarded copies of this request.  This value will enable the
         first response message to be correlated with this request;
         other responses are not required and should be discarded when
         received, since the agent that originated the Inform only
         requires one response to its Inform.
      ii. information is extracted from the LCD concerning all
         combinations of userName, qoS, agentID, contextSelector and
         transport address with which the received message is to be
         forwarded.
      iii. for each such combination whose access rights permit Inform
         operations to be forwarded, a new SNMPv2 message is
         constructed, as follows: its PDUs component is copied from
         that in the received message except that the contained
         request-id is replaced by the value selected in Step i above;
         its <userName>, <qoS>, <agentID> and <contextSelector> fields
         are set to the values extracted in Step ii above; and its
         <maxSize> field is set to the minimum of the value in the
         received message and the local system's maximum message size
         for the transport domain which will be used to forward this
         message.
      iv. for each constructed SNMPv2 message, information concerning
         the <userName>, <qoS>, <agentID>, <contextSelector>,
         request-id and sending transport address of the received
         message, as well as the request- id, agentID and
         contextSelector of the constructed message, is cached for
         later use in generating a response message.
      v. each constructed message is forwarded to the appropriate
         transport address extracted from the LCD in step ii above.

Waters Experimental [Page 26] RFC 1910 User-based Security Model for SNMPv2 February 1996

(15) If the SNMPv2 operation type is a Response, then:

   a) if the LCD information indicates the SNMPv2 context is of type
      local, then the usecStatsUnauthorizedOperations counter is
      incremented, a report PDU is generated, and the received message
      is discarded without further processing.
   b) if the LCD information indicates the SNMPv2 context is of type
      remote, then the Response operation represented by the PDUs
      value is performed by the receiving SNMPv2 entity according to
      the procedures set forth in [12].
   c) if the LCD information indicates the SNMPv2 context is of type
      local-proxy or remote-proxy, then:
      i. the request-id is extracted from the PDUs component of the
         received message.  The context's agentID and contextSelector
         values together with the extracted request-id are used to
         correlate this response message to the corresponding values
         for a previously forwarded request by inspecting the cache of
         information as augmented in Substep iii of Step 13f above or
         in Substep iv of 14c above.  If no such correlated
         information is found, then the received message is discarded
         without further processing.
      ii. a new SNMPv2 message is constructed: its PDUs component is
         copied from that in the received message except that the
         contained request-id is replaced by the value saved in the
         correlated information from the original request; its
         <userName>, <qoS>, <agentID> and <contextSelector> fields are
         set to the values saved from the received message. The
         <maxSize> field is set to the minimum of the value in the
         received message and the local system's maximum message size
         for the transport domain which will be used to forward the
         message. The message is authenticated and/or protected from
         disclosure according to the saved qoS value.
      iii. the constructed message is forwarded to the transport
         address saved in the correlated information as the sending
         transport address of the original request.
      iv. the correlated information is deleted from the cache of
         information.

(16) If the SNMPv2 operation type is a SNMPv2-Trap, then:

   a) if the LCD information indicates the SNMPv2 context is of type
      local or local-proxy, then the usecStatsUnauthorizedOperations

Waters Experimental [Page 27] RFC 1910 User-based Security Model for SNMPv2 February 1996

      counter is incremented, a report PDU is generated, and the
      received message is discarded without further processing.
   b) if the LCD information indicates the SNMPv2 context is of type
      remote, then the SNMPv2-Trap operation represented by the PDUs
      value is performed by the receiving SNMPv2 entity according to
      the procedures set forth in [12].
   c) if the LCD information indicates the SNMPv2 context is of type
      remote-proxy, then:
      i. a unique request-id is selected for use in forwarding the
         message.
      ii. information is extracted from the LCD concerning all
         combinations of userName, qoS, agentID, contextSelector and
         transport address with which the received message is to be
         forwarded.
      iii. for each such combination whose access rights permit
         SNMPv2-Trap operations to be forwarded, a new SNMPv2 message
         is constructed, as follows: its PDUs component is copied from
         that in the received message except that the contained
         request-id is replaced by the value selected in Step i above;
         its <userName>, <qoS>, <agentID> and <contextSelector> fields
         are set to the values extracted in Step ii above.
      iv. each constructed message is forwarded to the appropriate
         transport address extracted from the LCD in step ii above.

3.2.1. Additional Details

 For the sake of clarity and to prevent the above procedure from being
 even longer, the following details were omitted from the above
 procedure.

3.2.1.1. ASN.1 Parsing Errors

 For ASN.1 parsing errors, the snmpInASNParseErrs counter [15] is
 incremented and a report PDU is generated whenever such an ASN.1
 parsing error is discovered.  However, if the parsing error causes
 the information able to be extracted from the message to be
 insufficient for generating a report PDU, then the report PDU is not
 sent.

Waters Experimental [Page 28] RFC 1910 User-based Security Model for SNMPv2 February 1996

3.2.1.2. Incorrectly Encoded Parameters

 For an incorrectly encoded parameters component of the Message value
 (e.g., incorrect or inconsistent value of the <userLen> or <authLen>
 fields), the usecStatsBadParameters counter is incremented. Since the
 encoded parameters are in error, the report flag in the qoS cannot be
 reliably determined. Thus, no report PDU is generated for the
 incorrectly encoded parameters error condition.

3.2.1.3. Generation of a Report PDU

 Some steps specify that the received message is discarded without
 further processing whenever a report PDU is generated.  However:
  1. An SNMPv2 manager never generates a report PDU.
  1. If the operation type can reliably be determined and it is

determined to be a Report, SNMPv2-Trap, Inform, or a Response then

   a report PDU is not generated.
  1. A report PDU is only generated when the report flag in the qoS is

set to the value 1.

 A generated report PDU must always use the current values of agentID,
 agentBoots, and agentTime from the LCD.  In addition, a generated
 report PDU must whenever possible contain the same request-id value
 as in the PDU contained in the received message.  Meeting this
 constraint normally requires the message to be further processed just
 enough so as to extract its request-id. There are two situations in
 which the SNMPv2 request-id cannot be determined. The first situation
 occurs when the userName is unknown and the qoS indicates that the
 message is encrypted.  The other situation is when there is an ASN.1
 parsing error.  In cases where the the request-id cannot be
 determined, the default request-id value 2147483647 is used.

3.2.1.4. Cache Timeout

 Some steps specify that information is cached so that a Response
 operation may be correlated to the appropriate Request operation.
 However, a number of situations could cause the cache to grow without
 bound. One such situation is when the Response operation does not
 arrive or arrives "late" at the entity. In order to ensure that the
 cache does not grow without bound, it is recommended that cache
 entries be deleted when they are determined to be no longer valid. It
 is an implementation dependent decision as to how long cache entries
 remain valid, however, caching entries more than 150 seconds is not
 useful since any use of the cache entry after that time would
 generate a usecStatsNotInWindows error condition.

Waters Experimental [Page 29] RFC 1910 User-based Security Model for SNMPv2 February 1996

3.3. Generating a Response

 The procedure for generating a response to an SNMPv2 management
 request is identical to the procedure for transmitting a request (see
 Section 3.1), with these exceptions:
  1. The response is sent on behalf of the same user and with the same

value of the agentID and contextSelector as the request.

  1. The PDUs value of the responding Message value is the response

which results from performing the operation specified in the

   original PDUs value.
  1. The authentication protocol and other relevant information for the

user is obtained, not from the LCD, but rather from information

   cached (in Step 13d) when processing the original message.
  1. The serialized Message value is transmitted using any transport

address belonging to the agent for the transport domain from which

   the corresponding request originated - even if that is different
   from any transport information obtained from the LCD.
  1. If the qoS specifies that the message is to be authenticated or the

response is being generated by a SNMPv2 entity acting in an agent

   role, then the current values of agentBoots and agentTime from the
   LCD are used.  Otherwise, the <agentBoots> and <agentTime> fields
   are set to zero-filled octets.
  1. The report flag in the qoS is set to the value 0.

4. Discovery

 This security model requires that a discovery process obtain
 sufficient information about an SNMPv2 entity's agent in order to
 communicate with it.  Discovery requires the SNMPv2 manager to learn
 the agent's agentID value before communication may proceed.  This may
 be accomplished by formulating a get-request communication with the
 qoS set to noAuth/noPriv, the userName set to "public", the agentID
 set to all zeros (binary), the contextSelector set to "", and the
 VarBindList left empty.  The response to this message will be an
 reportPDU that contains the agentID within the <parameters> field
 (and containing the usecStatsUnknownContexts counter in the
 VarBindList). If authenticated communication is required then the
 discovery process may invoke the procedure described in Section 2.7
 to synchronize the clocks.

Waters Experimental [Page 30] RFC 1910 User-based Security Model for SNMPv2 February 1996

5. Definitions

SNMPv2-USEC-MIB DEFINITIONS ::= BEGIN

IMPORTS

  MODULE-IDENTITY, OBJECT-TYPE, Counter32, Unsigned32,
  snmpModules
      FROM SNMPv2-SMI
  TEXTUAL-CONVENTION
      FROM SNMPv2-TC
  MODULE-COMPLIANCE, OBJECT-GROUP
      FROM SNMPv2-CONF;

usecMIB MODULE-IDENTITY

  LAST-UPDATED "9601120000Z"
  ORGANIZATION "IETF SNMPv2 Working Group"
  CONTACT-INFO
          "        Glenn W. Waters
           Postal: Bell-Northern Research, Ltd.
                   P.O. Box 3511, Station C
                   Ottawa, ON, K1Y 4H7
                   Canada
              Tel: +1 613 763 3933
           E-mail: gwaters@bnr.ca"
  DESCRIPTION
          "The MIB module for SNMPv2 entities implementing the user-
          based security model."
  ::= { snmpModules 6 }

usecMIBObjects OBJECT IDENTIFIER ::= { usecMIB 1 }

– Textual Conventions

AgentID ::= TEXTUAL-CONVENTION

  STATUS       current
  DESCRIPTION
          "An agent's administratively-unique identifier.
          The value for this object may not be all zeros or all 'ff'H.
          The initial value for this object may be configured via an
          operator console entry or via an algorithmic function.  In

Waters Experimental [Page 31] RFC 1910 User-based Security Model for SNMPv2 February 1996

          the later case, the following guidelines are recommended:
            1) The first four octets are set to the binary equivalent
               of the agent's SNMP network management private
               enterprise number as assigned by the Internet Assigned
               Numbers Authority (IANA).  For example, if Acme
               Networks has been assigned { enterprises 696 }, the
               first four octets would be assigned '000002b8'H.
            2) The remaining eight octets are the cookie whose
               contents are determined via one or more enterprise-
               specific methods.  Such methods must be designed so as
               to maximize the possibility that the value of this
               object will be unique in the agent's administrative
               domain.  For example, the cookie may be the IP address
               of the agent, or the MAC address of one of the
               interfaces, with each address suitably padded with
               random octets.  If multiple methods are defined, then
               it is recommended that the cookie be further divided
               into one octet that indicates the method being used and
               seven octets which are a function of the method."
  SYNTAX     OCTET STRING (SIZE (12))

– the USEC Basic group – – a collection of objects providing basic instrumentation of – the SNMPv2 entity implementing the user-based security model

usecAgent OBJECT IDENTIFIER ::= { usecMIBObjects 1 }

agentID OBJECT-TYPE

  SYNTAX     AgentID
  MAX-ACCESS read-only
  STATUS     current
  DESCRIPTION
          "The agent's administratively-unique identifier."
  ::= { usecAgent 1 }

agentBoots OBJECT-TYPE

  SYNTAX     Unsigned32
  MAX-ACCESS read-only
  STATUS     current
  DESCRIPTION
          "The number of times that the agent has re-initialized
          itself since its initial configuration."
  ::= { usecAgent 2 }

Waters Experimental [Page 32] RFC 1910 User-based Security Model for SNMPv2 February 1996

agentTime OBJECT-TYPE

  SYNTAX     Unsigned32 (0..2147483647)
  UNITS      "seconds"
  MAX-ACCESS read-only
  STATUS     current
  DESCRIPTION
          "The number of seconds since the agent last incremented the
          agentBoots object."
  ::= { usecAgent 3 }

agentSize OBJECT-TYPE

  SYNTAX     INTEGER (484..65507)
  MAX-ACCESS read-only
  STATUS     current
  DESCRIPTION
          "The maximum length in octets of an SNMPv2 message which
          this agent will accept using any transport mapping."
  ::= { usecAgent 4 }

– USEC statistics – – a collection of objects providing basic instrumentation of – the SNMPv2 entity implementing the user-based security model

usecStats OBJECT IDENTIFIER ::= { usecMIBObjects 2 }

usecStatsUnsupportedQoS OBJECT-TYPE

  SYNTAX     Counter32
  MAX-ACCESS read-only
  STATUS     current
  DESCRIPTION
          "The total number of packets received by the SNMPv2 entity
          which were dropped because they requested a quality-of-
          service that was unknown to the agent or otherwise
          unavailable."
  ::= { usecStats 1 }

usecStatsNotInWindows OBJECT-TYPE

  SYNTAX     Counter32
  MAX-ACCESS read-only
  STATUS     current
  DESCRIPTION
          "The total number of packets received by the SNMPv2 entity
          which were dropped because they appeared outside of the
          agent's window."
  ::= { usecStats 2 }

Waters Experimental [Page 33] RFC 1910 User-based Security Model for SNMPv2 February 1996

usecStatsUnknownUserNames OBJECT-TYPE

  SYNTAX     Counter32
  MAX-ACCESS read-only
  STATUS     current
  DESCRIPTION
          "The total number of packets received by the SNMPv2 entity
          which were dropped because they referenced a user that was
          not known to the agent."
  ::= { usecStats 3 }

usecStatsWrongDigestValues OBJECT-TYPE

  SYNTAX     Counter32
  MAX-ACCESS read-only
  STATUS     current
  DESCRIPTION
          "The total number of packets received by the SNMPv2 entity
          which were dropped because they didn't contain the expected
          digest value."
  ::= { usecStats 4 }

usecStatsUnknownContexts OBJECT-TYPE

  SYNTAX     Counter32
  MAX-ACCESS read-only
  STATUS     current
  DESCRIPTION
          "The total number of packets received by the SNMPv2 entity
          which were dropped because they referenced a context that
          was not known to the agent."
  ::= { usecStats 5 }

usecStatsBadParameters OBJECT-TYPE

  SYNTAX     Counter32
  MAX-ACCESS read-only
  STATUS     current
  DESCRIPTION
          "The total number of packets received by the SNMPv2 entity
          which were dropped because the <parameters> field was
          improperly encoded or had invalid syntax."
  ::= { usecStats 6 }

usecStatsUnauthorizedOperations OBJECT-TYPE

  SYNTAX     Counter32
  MAX-ACCESS read-only
  STATUS     current
  DESCRIPTION
          "The total number of packets received by the SNMPv2 entity
          which were dropped because the PDU type referred to an
          operation that is invalid or not authorized."

Waters Experimental [Page 34] RFC 1910 User-based Security Model for SNMPv2 February 1996

  ::= { usecStats 7 }

– conformance information

usecMIBConformance

             OBJECT IDENTIFIER ::= { usecMIB 2 }

usecMIBCompliances

             OBJECT IDENTIFIER ::= { usecMIBConformance 1 }

usecMIBGroups OBJECT IDENTIFIER ::= { usecMIBConformance 2 }

– compliance statements

usecMIBCompliance MODULE-COMPLIANCE

  STATUS  current
  DESCRIPTION
          "The compliance statement for SNMPv2 entities which
          implement the SNMPv2 USEC model."
  MODULE  -- this module
      MANDATORY-GROUPS { usecBasicGroup,
                        usecStatsGroup }
  ::= { usecMIBCompliances 1 }

– units of conformance

usecBasicGroup OBJECT-GROUP

  OBJECTS { agentID,
            agentBoots,
            agentTime,
            agentSize }
  STATUS  current
  DESCRIPTION
          "A collection of objects providing identification, clocks,
          and capabilities of an SNMPv2 entity which implements the
          SNMPv2 USEC model."
  ::= { usecMIBGroups 1 }

usecStatsGroup OBJECT-GROUP

  OBJECTS { usecStatsUnsupportedQoS,
            usecStatsNotInWindows,
            usecStatsUnknownUserNames,
            usecStatsWrongDigestValues,
            usecStatsUnknownContexts,
            usecStatsBadParameters,
            usecStatsUnauthorizedOperations }

Waters Experimental [Page 35] RFC 1910 User-based Security Model for SNMPv2 February 1996

  STATUS  current
  DESCRIPTION
          "A collection of objects providing basic error statistics of
          an SNMPv2 entity which implements the SNMPv2 USEC model."
  ::= { usecMIBGroups 2 }

END

6. Security Considerations

6.1. Recommended Practices

 This section describes practices that contribute to the secure,
 effective operation of the mechanisms defined in this memo.
  1. A management station must discard SNMPv2 responses for which

neither the request-id component nor the represented management

   information corresponds to any currently outstanding request.
   Although it would be typical for a management station 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).
  1. A management station must generate unpredictable request-ids in

authenticated messages in order to protect against the possibility

   of message duplication (malicious or otherwise).
  1. A management station should perform time synchronization using

authenticated messages in order to protect against the possibility

   of message duplication (malicious or otherwise).
  1. When sending state altering messages to a managed agent, a

management station should delay sending successive messages to the

   managed agent until a positive acknowledgement is received for the
   previous message or until the previous message expires.
   No message ordering is imposed by the SNMPv2. Messages may be
   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 agent, 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, a management station must cope with the loss and re-
   ordering of messages resulting from anomalies in the network as a
   matter of course.

Waters Experimental [Page 36] RFC 1910 User-based Security Model for SNMPv2 February 1996

   However, a managed object, snmpSetSerialNo [15], is specifically
   defined for use with SNMPv2 set operations in order to provide a
   mechanism to ensure the processing of SNMPv2 messages occurs in a
   specific order.
  1. The frequency with which the secrets of an SNMPv2 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 the communication
   paths, more caution is prudent.

6.2. Defining Users

 The mechanisms defined in this document employ the notion of "users"
 having access rights.  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 a manager station application or set
 of manager station applications, acting on behalf of a 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).  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,
 agent implementations (and agent 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

Waters Experimental [Page 37] RFC 1910 User-based Security Model for SNMPv2 February 1996

 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.
 Note that there is security risk in configuring the same "user" on
 multiple systems where the same password is used on each system,
 since the compromise of that user's secrets on one system results in
 the compromise of that user on all other systems having the same
 password.
 The algorithm in Appendix A avoids this problem by including the
 agent's agentID value as well as the user's password in the
 calculation of a user's secrets; this results in the user's secrets
 being different at different agents; however, if the password is
 compromised the algorithm in Appendix A is not effective.

6.3. Conformance

 To be termed a "Secure SNMPv2 implementation", an SNMPv2
 implementation:

- must implement the Digest Authentication Protocol.

- must, to the maximal extent possible, prohibit access to the

 secret(s) of each user about which it maintains information in a LCD,
 under all circumstances except as required to generate and/or
 validate SNMPv2 messages with respect to that user.

- must implement the SNMPv2 USEC MIB.

 In addition, an SNMPv2 agent must provide initial configuration in
 accordance with Appendix A.1.
 Implementation of the Symmetric Encryption Protocol is optional.

7. Editor's Address

 Glenn W. Waters
 Bell-Northern Research Ltd.
 P.O. Box 3511, Station C
 Ottawa, Ontario  K1Y 4H7
 CA
 Phone: +1 613 763 3933
 EMail: gwaters@bnr.ca

Waters Experimental [Page 38] RFC 1910 User-based Security Model for SNMPv2 February 1996

8. Acknowledgements

 This document is the result of significant work by three major
 contributors:
   Keith McCloghrie (Cisco Systems, kzm@cisco.com)
   Marshall T. Rose (Dover Beach Consulting, mrose@dbc.mtview.ca.us)
   Glenn W. Waters (Bell-Northern Research Ltd., gwaters@bnr.ca)
 The authors wish to acknowledge James M. Galvin of Trusted
 Information Systems who contributed significantly to earlier work on
 which this memo is based, and the general contributions of members of
 the SNMPv2 Working Group, and, in particular, Aleksey Y. Romanov and
 Steven L. Waldbusser.
 A special thanks is extended for the contributions of:
   Uri Blumenthal (IBM)
   Shawn Routhier (Epilogue)
   Barry Sheehan (IBM)
   Bert Wijnen (IBM)

9. References

[1] McCloghrie, K., Editor, "An Administrative Infrastructure for

   SNMPv2", RFC 1909, Cisco Systems, January 1996.

[2] Case, J., Fedor, M., Schoffstall, M., and J. Davin, "Simple

   Network Management Protocol", STD 15, RFC 1157, SNMP Research,
   Performance Systems International, MIT Laboratory for Computer
   Science, May 1990.

[3] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, MIT

   Laboratory for Computer Science, April 1992.

[4] 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.

[5] 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).

[6] Data Encryption Algorithm, American National Standards Institute.

   ANSI X3.92-1981, (December, 1980).

Waters Experimental [Page 39] RFC 1910 User-based Security Model for SNMPv2 February 1996

[7] DES Modes of Operation, National Institute of Standards and

   Technology.  Federal Information Processing Standard (FIPS)
   Publication 81, (December, 1980).

[8] Data Encryption Algorithm - Modes of Operation, American National

   Standards Institute.  ANSI X3.106-1983, (May 1983).

[9] 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).

[10] Validating the Correctness of Hardware Implementations of the NBS

   Data Encryption Standard, National Institute of Standards and
   Technology.  Special Publication 500-20.

[11] Maintenance Testing for the Data Encryption Standard, National

   Institute of Standards and Technology.  Special Publication 500-61,
   (August, 1980).

[12] 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.

[13] 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.

[14] Krawczyk, H., "Keyed-MD5 for Message Authentication", Work in

   Progress, IBM, June 1995.

[15] 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.

Waters Experimental [Page 40] RFC 1910 User-based Security Model for SNMPv2 February 1996

APPENDIX A - Installation

A.1. Agent Installation Parameters

During installation, an agent is configured with several parameters. These include:

(1) a security posture

   The choice of security posture determines the extent of the view
   configured for unauthenticated access.  One of three possible
   choices is selected:
        minimum-secure,
        semi-secure, or
        very-secure.

(2) one or more transport service addresses

   These parameters may be specified explicitly, or they may be
   specified implicitly as the same set of network-layer addresses
   configured for other uses by the device together with the well-
   known transport-layer "port" information for the appropriate
   transport domain [13].  The agent listens on each of these
   transport service addresses for messages sent on behalf of any user
   it knows about.

(3) 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:
  1. 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. The resulting digest, termed "digest1", is used in
     the next step.
  1. a second string of length 44 octets is formed by concatenating

digest1, the agent's agentID value, and digest1. This string is

     used as input to the MD5 algorithm. The resulting digest is the
     required secret (see Appendix A.2).

Waters Experimental [Page 41] RFC 1910 User-based Security Model for SNMPv2 February 1996

 With these configured parameters, the agent instantiates the
 following user, context, views and access rights.  This configuration
 information should be readOnly (persistent).
  1. One user:
                       privacy not supported   privacy supported
                       ---------------------   -----------------
     <userName>        "public"                "public"
     <authProtocol>    Digest Auth. Protocol   Digest Auth. Protocol
     <authPrivateKey>  authentication key      authentication key
     <privProtocol>    none                    Symmetric Privacy Protocol
     <privPrivateKey>  --                      privacy key
  1. One local context with its <contextSelector> as the empty-string.
  1. One view for authenticated access:
  1. the <all> MIB view is the "internet" subtree.
  1. A second view for unauthenticated access. This view is configured

according to the selected security posture. For the "very-secure"

   posture:
  1. the <restricted> MIB view is the union of the "snmp" [15],

"usecAgent" and "usecStats" subtrees.

   For the "semi-secure" posture:
  1. the <restricted> MIB view is the union of the "snmp" [15],

"usecAgent", "usecStats" and "system" subtrees.

   For the "minimum-secure" posture:
  1. the <restricted> MIB view is the "internet" subtree.
  1. Access rights to allow:
  1. read-only access for unauthenticated messages on behalf of the

user "public" to the <restricted> MIB view of contextSelector

        "".
  1. read-write access for authenticated but not private messages

on behalf of the user "public" to the <all> MIB view of

        contextSelector "".
  1. if privacy is supported, read-write access for authenticated

and private messages on behalf of the user "public" to the

Waters Experimental [Page 42] RFC 1910 User-based Security Model for SNMPv2 February 1996

        <all> MIB view of contextSelector "".

A.2. Password to Key Algorithm

 The following code fragment 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
 RFC 1321.)

void password_to_key(password, passwordlen, agentID, key)

  u_char *password;       /* IN */
  u_int   passwordlen;    /* IN */
  u_char *agentID;        /* IN - pointer to 12 octet long agentID */
  u_char *key;            /* OUT - caller supplies pointer to 16
                             octet buffer */ {
  MD5_CTX     MD;
  u_char      *cp, password_buf[64];
  u_long      password_index = 0;
  u_long      count = 0, i;
  MD5Init (&MD);   /* initialize MD5 */
  /* loop until we've done 1 Megabyte */
  while (count < 1048576) {
      cp = password_buf;
      for(i = 0; i < 64; i++) {
          *cp++ = password[ password_index++ % passwordlen ];
          /*
           * Take the next byte of the password, wrapping to the
           * beginning of the password as necessary.
           */
      }
      MDupdate (&MD, password_buf, 64);
      count += 64;
  }
  MD5Final (key, &MD);              /* tell MD5 we're done */
  /* localize the key with the agentID and pass through MD5
    to produce final key */
  memcpy (password_buf, key, 16);
  memcpy (password_buf+16, agentID, 12);
  memcpy (password_buf+28, key, 16);
  MD5Init (&MD);
  MDupdate (&MD, password_buf, 44);
  MD5Final (key, &MD);
  return; }

Waters Experimental [Page 43] RFC 1910 User-based Security Model for SNMPv2 February 1996

A.3. Password to Key Sample

 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 agent's agentID 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
 agentID 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

Waters Experimental [Page 44]

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