path: root/doc/rfc/rfc1446.txt

          Network Working Group                                J. Galvin
          Request for Comments: 1446         Trusted Information Systems
                                                           K. McCloghrie
                                                      Hughes LAN Systems
                                                              April 1993


                                Security Protocols
                               for version 2 of the
                   Simple Network Management Protocol (SNMPv2)


          Status of this Memo

          This RFC specifes an IAB standards track protocol for the
          Internet community, and requests discussion and suggestions
          for improvements.  Please refer to the current edition of the
          "IAB Official Protocol Standards" for the standardization
          state and status of this protocol.  Distribution of this memo
          is unlimited.


          Table of Contents


          1 Introduction ..........................................    2
          1.1 A Note on Terminology ...............................    3
          1.2 Threats .............................................    4
          1.3 Goals and Constraints ...............................    5
          1.4 Security Services ...................................    6
          1.5 Mechanisms ..........................................    7
          1.5.1 Message Digest Algorithm ..........................    8
          1.5.2 Symmetric Encryption Algorithm ....................    9
          2 SNMPv2 Party ..........................................   11
          3 Digest Authentication Protocol ........................   14
          3.1 Generating a Message ................................   16
          3.2 Receiving a Message .................................   18
          4 Symmetric Privacy Protocol ............................   21
          4.1 Generating a Message ................................   21
          4.2 Receiving a Message .................................   22
          5 Clock and Secret Distribution .........................   24
          5.1 Initial Configuration ...............................   25
          5.2 Clock Distribution ..................................   28
          5.3 Clock Synchronization ...............................   29
          5.4 Secret Distribution .................................   31
          5.5 Crash Recovery ......................................   34
          6 Security Considerations ...............................   37
          6.1 Recommended Practices ...............................   37
          6.2 Conformance .........................................   39
          6.3 Protocol Correctness ................................   42




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          6.3.1 Clock Monotonicity Mechanism ......................   43
          6.3.2 Data Integrity Mechanism ..........................   43
          6.3.3 Data Origin Authentication Mechanism ..............   44
          6.3.4 Restricted Administration Mechanism ...............   44
          6.3.5 Message Timeliness Mechanism ......................   45
          6.3.6 Selective Clock Acceleration Mechanism ............   46
          6.3.7 Confidentiality Mechanism .........................   47
          7 Acknowledgements ......................................   48
          8 References ............................................   49
          9 Authors' Addresses ....................................   51








































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          1.  Introduction

          A network 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 both authentication and
          authorization policies.

          Network management stations execute management applications
          which monitor and control network elements.  Network elements
          are devices such as hosts, routers, terminal servers, etc.,
          which are monitored and controlled through access to their
          management information.

          In the Administrative Model for SNMPv2 document [1], each
          SNMPv2 party is, by definition, associated with a single
          authentication protocol and a single privacy protocol.  It is
          the purpose of this document, Security Protocols for SNMPv2,
          to define one such authentication and one such privacy
          protocol.

          The authentication protocol provides a mechanism by which
          SNMPv2 management communications transmitted by the party may
          be reliably identified as having originated from that party.
          The authentication protocol defined in this memo also reliably
          determines that the message received is the message that was
          sent.

          The privacy protocol provides a mechanism by which SNMPv2
          management communications transmitted to said party are
          protected from disclosure.  The privacy protocol in this memo
          specifies that only authenticated messages may be protected
          from disclosure.

          These protocols are secure alternatives to the so-called
          "trivial" protocol defined in [2].

               USE OF THE TRIVIAL PROTOCOL ALONE DOES NOT CONSTITUTE
               SECURE NETWORK MANAGEMENT.  THEREFORE, A NETWORK
               MANAGEMENT SYSTEM THAT IMPLEMENTS ONLY THE TRIVIAL
               PROTOCOL IS NOT CONFORMANT TO THIS SPECIFICATION.






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          The Digest Authentication Protocol is described in Section 3.
          It provides a data integrity service by transmitting a message
          digest - computed by the originator and verified by the
          recipient - with each SNMPv2 message.  The data origin
          authentication service is provided by prefixing the message
          with a secret value known only to the originator and
          recipient, prior to computing the digest.  Thus, data
          integrity is supported explicitly while data origin
          authentication is supported implicitly in the verification of
          the digest.

          The Symmetric Privacy Protocol is described in Section 4.  It
          protects messages from disclosure by encrypting their contents
          according to a secret cryptographic key known only to the
          originator and recipient.  The additional functionality
          afforded by this protocol is assumed to justify its additional
          computational cost.

          The Digest Authentication Protocol depends on the existence of
          loosely synchronized clocks between the originator and
          recipient of a message.  The protocol specification makes no
          assumptions about the strategy by which such clocks are
          synchronized.  Section 5.3 presents one strategy that is
          particularly suited to the demands of SNMP network management.

          Both protocols described here require the sharing of secret
          information between the originator of a message and its
          recipient.  The protocol specifications assume the existence
          of the necessary secrets.  The selection of such secrets and
          their secure distribution to appropriate parties may be
          accomplished by a variety of strategies.  Section 5.4 presents
          one such strategy that is particularly suited to the demands
          of SNMP network management.


          1.1.  A Note on Terminology

          For the purpose of exposition, the original Internet-standard
          Network Management Framework, as described in RFCs 1155, 1157,
          and 1212, is termed the SNMP version 1 framework (SNMPv1).
          The current framework is termed the SNMP version 2 framework
          (SNMPv2).








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          1.2.  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 protocol.  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 any SNMPv2 security
          protocol should provide protection are:


          Modification of Information
               The SNMPv2 protocol provides the means for management
               stations to interrogate and to manipulate the value of
               objects in a managed agent.  The modification threat is
               the danger that some party may alter in-transit messages
               generated by an authorized party in such a way as to
               effect unauthorized management operations, including
               falsifying the value of an object.

          Masquerade
               The SNMPv2 administrative model includes an access
               control model.  Access control necessarily depends on
               knowledge of the origin of a message.  The masquerade
               threat is the danger that management operations not
               authorized for some party may be attempted by that party
               by assuming the identity of another party 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 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.





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          Disclosure
               The disclosure threat is the danger of eavesdropping on
               the exchanges between managed agents and a management
               station.  Protecting against this threat is mandatory
               when the SNMPv2 is used to create new SNMPv2 parties [1]
               on which subsequent secure operation might be based.
               Protecting against the disclosure threat may also be
               required as a matter of local policy.

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

          Denial of Service
               A SNMPv2 security protocol need not attempt to address
               the broad range of attacks by which service to authorized
               parties 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, a 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.


          1.3.  Goals and Constraints

          Based on the foregoing account of threats in the SNMP network
          management environment, the goals of a SNMPv2 security
          protocol are enumerated below.

          (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 originator of each received SNMPv2
               message.





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          (3)  The protocol should provide that the apparent time of
               generation for each received SNMPv2 message is 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 any SNMPv2 security protocol 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 former are preferred.

          (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 secret/key management protocols).

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


          1.4.  Security Services

          The security services necessary to support the goals of a
          SNMPv2 security protocol 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 origin
               of received data is corroborated.

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







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          The protocols specified in this memo require both data
          integrity and data origin authentication to be used at all
          times.  For these protocols, it is not possible to realize
          data integrity without data origin authentication, nor is it
          possible to realize data origin authentication without data
          integrity.

          Further, there is no provision for data confidentiality
          without both data integrity and data origin authentication.


          1.5.  Mechanisms

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

          o    In support of data integrity, a message digest algorithm
               is required.  A digest is calculated over an appropriate
               portion of a SNMPv2 message and included as part of the
               message sent to the recipient.

          o    In support of data origin authentication and data
               integrity, the portion of a SNMPv2 message that is
               digested is first prefixed with a secret value shared by
               the originator of that message and its intended
               recipient.

          o    To protect against the threat of message delay or replay,
               (to an extent greater than can occur through normal
               operation), a timestamp value is included in each message
               generated.  A recipient evaluates the timestamp to
               determine if the message is recent.  This protection
               against the threat of message delay or replay does not
               imply nor provide any protection against unauthorized
               deletion or suppression of messages.  Other mechanisms
               defined independently of the security protocol can also
               be used to detect message replay (e.g., the request-id
               [2]), or for set operations, the re-ordering, replay,
               deletion, or suppression of messages (e.g., the MIB
               variable snmpSetSerialNo [14]).

          o    In support of data confidentiality, a symmetric
               encryption algorithm is required.  An appropriate portion
               of the message is encrypted prior to being transmitted to





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               its recipient.

          The security protocols in this memo are defined independently
          of the particular choice of a message digest and encryption
          algorithm - owing principally to the lack of a suitable metric
          by which to evaluate the security of particular algorithm
          choices.  However, in the interests of completeness and in
          order to guarantee interoperability, Sections 1.5.1 and 1.5.2
          specify particular choices, which are considered acceptably
          secure as of this writing.  In the future, this memo may be
          updated by the publication of a memo specifying substitute or
          alternate choices of algorithms, i.e., a replacement for or
          addition to the sections below.


          1.5.1.  Message Digest Algorithm

          In support of data integrity, the use of the MD5 [3] message
          digest algorithm is chosen.  A 128-bit digest is calculated
          over the designated portion of a SNMPv2 message and included
          as part of the message sent to the recipient.

          An appendix of [3] contains a C Programming Language
          implementation of the algorithm.  This code was written with
          portability being the principal objective.  Implementors may
          wish to optimize the implementation with respect to the
          characteristics of their hardware and software platforms.

          The use of this algorithm in conjunction with the Digest
          Authentication Protocol (see Section 3) is identified by the
          ASN.1 object identifier value v2md5AuthProtocol, defined in
          [4].  (Note that this protocol is a modified version of the
          md5AuthProtocol protocol defined in RFC 1352.)

          For any SNMPv2 party for which the authentication protocol is
          v2md5AuthProtocol, the size of its private authentication key
          is 16 octets.

          Within an authenticated management communication generated by
          such a party, the size of the authDigest component of that
          communication (see Section 3) is 16 octets.









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          1.5.2.  Symmetric Encryption Algorithm

          In support of data confidentiality, the use of the Data
          Encryption Standard (DES) in the Cipher Block Chaining mode of
          operation is chosen.  The designated portion of a 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.

          o    There is a document with guidelines for implementing and
               using the DES, including functional specifications for
               the DES and its modes of operation [9].

          o    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.

          o    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.

          The use of this algorithm in conjunction with the Symmetric
          Privacy Protocol (see Section 4) is identified by the ASN.1
          object identifier value desPrivProtocol, defined in [4].

          For any SNMPv2 party for which the privacy protocol is
          desPrivProtocol, the size of the private 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.





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          The length of the octet sequence to be encrypted by the DES
          must be an integral multiple of 8.  When encrypting, the data
          should be padded at the end as necessary; the actual pad value
          is insignificant.

          If the length of the octet sequence to be decrypted is not an
          integral multiple of 8 octets, the processing of the octet
          sequence should be halted and an appropriate exception noted.
          Upon decrypting, the padding should be ignored.









































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          2.  SNMPv2 Party

          Recall from [1] that a SNMPv2 party is a conceptual, virtual
          execution context whose operation is restricted (for security
          or other purposes) to an administratively defined subset of
          all possible operations of a particular SNMPv2 entity.  A
          SNMPv2 entity is an actual process which performs network
          management operations by generating and/or responding to
          SNMPv2 protocol messages in the manner specified in [12].
          Architecturally, every SNMPv2 entity maintains a local
          database that represents all SNMPv2 parties known to it.







































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          A SNMPv2 party may be represented by an ASN.1 value with the
          following syntax:

               SnmpParty ::= SEQUENCE {
                 partyIdentity
                    OBJECT IDENTIFIER,
                 partyTDomain
                    OBJECT IDENTIFIER,
                 partyTAddress
                    OCTET STRING,
                 partyMaxMessageSize
                    INTEGER,
                 partyAuthProtocol
                    OBJECT IDENTIFIER,
                 partyAuthClock
                    INTEGER,
                 partyAuthPrivate
                    OCTET STRING,
                 partyAuthPublic
                    OCTET STRING,
                 partyAuthLifetime
                    INTEGER,
                 partyPrivProtocol
                    OBJECT IDENTIFIER,
                 partyPrivPrivate
                    OCTET STRING,
                 partyPrivPublic
                    OCTET STRING
               }

          For each SnmpParty value that represents a SNMPv2 party, the
          generic significance of each of its components is defined in
          [1].  For each SNMPv2 party that supports the generation of
          messages using the Digest Authentication Protocol, additional,
          special significance is attributed to certain components of
          that party's representation:

          o    Its partyAuthProtocol component is called the
               authentication protocol and identifies a combination of
               the Digest Authentication Protocol with a particular
               digest algorithm (such as that defined in Section 1.5.1).
               This combined mechanism is used to authenticate the
               origin and integrity of all messages generated by the
               party.






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          o    Its partyAuthClock component is called the authentication
               clock and represents a notion of the current time that is
               specific to the party.

          o    Its partyAuthPrivate component is called the private
               authentication key and represents any secret value needed
               to support the Digest Authentication Protocol and
               associated digest algorithm.

          o    Its partyAuthPublic component is called the public
               authentication key and represents any public value that
               may be needed to support the authentication protocol.
               This component is not significant except as suggested in
               Section 5.4.

          o    Its partyAuthLifetime component is called the lifetime
               and represents an administrative upper bound on
               acceptable delivery delay for protocol messages generated
               by the party.

          For each SNMPv2 party that supports the receipt of messages
          via the Symmetric Privacy Protocol, additional, special
          significance is attributed to certain components of that
          party's representation:

          o    Its partyPrivProtocol component is called the privacy
               protocol and identifies a combination of the Symmetric
               Privacy Protocol with a particular encryption algorithm
               (such as that defined in Section 1.5.2).  This combined
               mechanism is used to protect from disclosure all protocol
               messages received by the party.

          o    Its partyPrivPrivate component is called the private
               privacy key and represents any secret value needed to
               support the Symmetric Privacy Protocol and associated
               encryption algorithm.

          o    Its partyPrivPublic component is called the public
               privacy key and represents any public value that may be
               needed to support the privacy protocol.  This component
               is not significant except as suggested in Section 5.4.









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          3.  Digest Authentication Protocol

          This section describes the Digest Authentication Protocol.  It
          provides both for verifying the integrity of a received
          message (i.e., the message received is the message sent) and
          for verifying the origin of a message (i.e., the reliable
          identification of the originator).  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.

          A secret value known only to the originator and recipient of
          the message is prefixed to the message prior to the digest
          computation.  Thus, the origin of the message is known
          implicitly with the verification of the digest.

          A requirement on parties using this Digest Authentication
          Protocol is that they shall not originate messages for
          transmission to any destination party which does not also use
          this Digest Authentication Protocol.  This restriction
          excludes undesirable side effects of communication between a
          party which uses these security protocols and a party which
          does not.

          Recall from [1] that a SNMPv2 management communication is
          represented by an ASN.1 value with the following syntax:

               SnmpMgmtCom ::= [2] IMPLICIT SEQUENCE {
                 dstParty
                    OBJECT IDENTIFIER,
                 srcParty
                    OBJECT IDENTIFIER,
                 context
                    OBJECT IDENTIFIER,
                 pdu
                    PDUs
               }

          For each SnmpMgmtCom value that represents a SNMPv2 management
          communication, the following statements are true:

          o    Its dstParty component is called the destination and
               identifies the SNMPv2 party to which the communication is
               directed.





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          o    Its srcParty component is called the source and
               identifies the SNMPv2 party from which the communication
               is originated.

          o    Its context component identifies the SNMPv2 context
               containing the management information referenced by the
               communication.

          o    Its pdu component has the form and significance
               attributed to it in [12].

          Recall from [1] that a SNMPv2 authenticated management
          communication is represented by an ASN.1 value with the
          following syntax:

               SnmpAuthMsg ::= [1] IMPLICIT SEQUENCE {
                 authInfo
                    ANY, - defined by authentication protocol
                 authData
                    SnmpMgmtCom
               }

          For each SnmpAuthMsg value that represents a SNMPv2
          authenticated management communication, the following
          statements are true:

          o    Its authInfo component is called the authentication
               information and represents information required in
               support of the authentication protocol used by both the
               SNMPv2 party originating the message, and the SNMPv2
               party receiving the message.  The detailed significance
               of the authentication information is specific to the
               authentication protocol in use; it has no effect on the
               application semantics of the communication other than its
               use by the authentication protocol in determining whether
               the communication is authentic or not.

          o    Its authData component is called the authentication data












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               and represents a SNMPv2 management communication.

          In support of the Digest Authentication Protocol, an authInfo
          component is of type AuthInformation:

               AuthInformation ::= [2] IMPLICIT SEQUENCE {
                 authDigest
                    OCTET STRING,
                 authDstTimestamp
                    UInteger32,
                 authSrcTimestamp
                    UInteger32
               }

          For each AuthInformation value that represents authentication
          information, the following statements are true:

          o    Its authDigest component is called the authentication
               digest and represents the digest computed over an
               appropriate portion of the message, where the message is
               temporarily prefixed with a secret value for the purposes
               of computing the digest.

          o    Its authSrcTimestamp component is called the
               authentication timestamp and represents the time of the
               generation of the message according to the partyAuthClock
               of the SNMPv2 party that originated it.  Note that the
               granularity of the authentication timestamp is 1 second.

          o    Its authDstTimestamp component is called the
               authentication timestamp and represents the time of the
               generation of the message according to the partyAuthClock
               of the SNMPv2 party that is to receive it.  Note that the
               granularity of the authentication timestamp is 1 second.


          3.1.  Generating a Message

          This section describes the behavior of a SNMPv2 entity when it
          acts as a SNMPv2 party for which the authentication protocol
          is administratively specified as the Digest Authentication
          Protocol.  Insofar as the behavior of a SNMPv2 entity when
          transmitting protocol messages is defined generically in [1],
          only those aspects of that behavior that are specific to the
          Digest Authentication Protocol are described below.  In





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          particular, this section describes the encapsulation of a
          SNMPv2 management communication into a SNMPv2 authenticated
          management communication.

          According to Section 3.1 of [1], a SnmpAuthMsg value is
          constructed during Step 3 of generic processing.  In
          particular, it states the authInfo component is constructed
          according to the authentication protocol identified for the
          SNMPv2 party originating the message.  When the relevant
          authentication protocol is the Digest Authentication Protocol,
          the procedure performed by a SNMPv2 entity whenever a
          management communication is to be transmitted by a SNMPv2
          party is as follows.

          (1)  The local database is consulted to determine the
               authentication clock and private authentication key
               (extracted, for example, according to the conventions
               defined in Section 1.5.1) of the SNMPv2 party originating
               the message.  The local database is also consulted to
               determine the authentication clock of the receiving
               SNMPv2 party.

          (2)  The authSrcTimestamp component is set to the retrieved
               authentication clock value of the message's source.  The
               authDstTimestamp component is set to the retrieved
               authentication clock value of the message's intended
               recipient.

          (3)  The authentication digest is temporarily set to the
               private authentication key of the SNMPv2 party
               originating the message.  The SnmpAuthMsg value is
               serialized according to the conventions of [13] and [12].
               A digest is computed over the octet sequence representing
               that serialized value using, for example, the algorithm
               specified in Section 1.5.1.  The authDigest component is
               set to the computed digest value.

          As set forth in [1], the SnmpAuthMsg value is then
          encapsulated according to the appropriate privacy protocol
          into a SnmpPrivMsg value.  This latter value is then
          serialized and transmitted to the receiving SNMPv2 party.









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          3.2.  Receiving a Message

          This section describes the behavior of a SNMPv2 entity upon
          receipt of a protocol message from a SNMPv2 party for which
          the authentication protocol is administratively specified as
          the Digest Authentication Protocol.  Insofar as the behavior
          of a SNMPv2 entity when receiving protocol messages is defined
          generically in [1], only those aspects of that behavior that
          are specific to the Digest Authentication Protocol are
          described below.

          According to Section 3.2 of [1], a SnmpAuthMsg value is
          evaluated during Step 9 of generic processing.  In particular,
          it states the SnmpAuthMsg value is evaluated according to the
          authentication protocol identified for the SNMPv2 party that
          originated the message.  When the relevant authentication
          protocol is the Digest Authentication Protocol, the procedure
          performed by a SNMPv2 entity whenever a management
          communication is received by a SNMPv2 party is as follows.

          (1)  If the ASN.1 type of the authInfo component is not
               AuthInformation, the message is evaluated as unauthentic,
               and the snmpStatsBadAuths counter [14] is incremented.
               Otherwise, the authSrcTimestamp, authDstTimestamp, and
               authDigest components are extracted from the SnmpAuthMsg
               value.

          (2)  The local database is consulted to determine the
               authentication clock, private authentication key
               (extracted, for example, according to the conventions
               defined in Section 1.5.1), and lifetime of the SNMPv2
               party that originated the message.

          (3)  If the authSrcTimestamp component plus the lifetime is
               less than the authentication clock, the message is
               evaluated as unauthentic, and the snmpStatsNotInLifetimes
               counter [14] is incremented.

          (4)  The authDigest component is extracted and temporarily
               recorded.

          (5)  A new SnmpAuthMsg value is constructed such that its
               authDigest component is set to the private authentication
               key and its other components are set to the value of the
               corresponding components in the received SnmpAuthMsg





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               value.  This new SnmpAuthMsg value is serialized
               according to the conventions of [13] and [12].  A digest
               is computed over the octet sequence representing that
               serialized value using, for example, the algorithm
               specified in Section 1.5.1.

                                            NOTE
                    Because serialization rules are unambiguous but may
                    not be unique, great care must be taken in
                    reconstructing the serialized value prior to
                    computing the digest.  Implementations may find it
                    useful to keep a copy of the original serialized
                    value and then simply modify the octets which
                    directly correspond to the placement of the
                    authDigest component, rather than re-applying the
                    serialization algorithm to the new SnmpAuthMsg
                    value.

          (6)  If the computed digest value is not equal to the digest
               value temporarily recorded in step 4 above, the message
               is evaluated as unauthentic, and the
               snmpStatsWrongDigestValues counter [14] is incremented.

          (7)  The message is evaluated as authentic.

          (8)  The local database is consulted for access privileges
               permitted by the local access policy to the originating
               SNMPv2 party with respect to the receiving SNMPv2 party.
               If any level of access is permitted, then:

                 the authentication clock value locally recorded for the
                 originating SNMPv2 party is advanced to the
                 authSrcTimestamp value if this latter exceeds the
                 recorded value; and,

                 the authentication clock value locally recorded for the
                 receiving SNMPv2 party is advanced to the
                 authDstTimestamp value if this latter exceeds the
                 recorded value.

              (Note that this step is conceptually independent from
              Steps 15-17 of Section 3.2 in [1]).

          If the SnmpAuthMsg value is evaluated as unauthentic, an
          authentication failure is noted and the received message is





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          discarded without further processing.  Otherwise, processing
          of the received message continues as specified in [1].
















































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          4.  Symmetric Privacy Protocol

          This section describes the Symmetric Privacy Protocol.  It
          provides for protection from disclosure of a received message.
          An appropriate portion of the message is encrypted according
          to a secret key known only to the originator and recipient of
          the message.

          This protocol assumes the underlying mechanism is a symmetric
          encryption algorithm.  In addition, the message to be
          encrypted must be protected according to the conventions of
          the Digest Authentication Protocol.

          Recall from [1] that a SNMPv2 private management communication
          is represented by an ASN.1 value with the following syntax:

               SnmpPrivMsg ::= [1] IMPLICIT SEQUENCE {
                 privDst
                    OBJECT IDENTIFIER,
                 privData
                    [1] IMPLICIT OCTET STRING
               }

          For each SnmpPrivMsg value that represents a SNMPv2 private
          management communication, the following statements are true:

          o    Its privDst component is called the privacy destination
               and identifies the SNMPv2 party to which the
               communication is directed.

          o    Its privData component is called the privacy data and
               represents the (possibly encrypted) serialization
               (according to the conventions of [13] and [12]) of a
               SNMPv2 authenticated management communication.


          4.1.  Generating a Message

          This section describes the behavior of a SNMPv2 entity when it
          communicates with a SNMPv2 party for which the privacy
          protocol is administratively specified as the Symmetric
          Privacy Protocol.  Insofar as the behavior of a SNMPv2 entity
          when transmitting a protocol message is defined generically in
          [1], only those aspects of that behavior that are specific to
          the Symmetric Privacy Protocol are described below.  In





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          particular, this section describes the encapsulation of a
          SNMPv2 authenticated management communication into a SNMPv2
          private management communication.

          According to Section 3.1 of [1], a SnmpPrivMsg value is
          constructed during Step 5 of generic processing.  In
          particular, it states the privData component is constructed
          according to the privacy protocol identified for the SNMPv2
          party receiving the message.  When the relevant privacy
          protocol is the Symmetric Privacy Protocol, the procedure
          performed by a SNMPv2 entity whenever a management
          communication is to be transmitted by a SNMPv2 party is as
          follows.

          (1)  If the SnmpAuthMsg value is not authenticated according
               to the conventions of the Digest Authentication Protocol,
               the generation of the private management communication
               fails according to a local procedure, without further
               processing.

          (2)  The local database is consulted to determine the private
               privacy key of the SNMPv2 party receiving the message
               (represented, for example, according to the conventions
               defined in Section 1.5.2).

          (3)  The SnmpAuthMsg value is serialized according to the
               conventions of [13] and [12].

          (4)  The octet sequence representing the serialized
               SnmpAuthMsg value is encrypted using, for example, the
               algorithm specified in Section 1.5.2 and the extracted
               private privacy key.

          (5)  The privData component is set to the encrypted value.

          As set forth in [1], the SnmpPrivMsg value is then serialized
          and transmitted to the receiving SNMPv2 party.


          4.2.  Receiving a Message

          This section describes the behavior of a SNMPv2 entity when it
          acts as a SNMPv2 party for which the privacy protocol is
          administratively specified as the Symmetric Privacy Protocol.
          Insofar as the behavior of a SNMPv2 entity when receiving a





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          protocol message is defined generically in [1], only those
          aspects of that behavior that are specific to the Symmetric
          Privacy Protocol are described below.

          According to Section 3.2 of [1], the privData component of a
          received SnmpPrivMsg value is evaluated during Step 4 of
          generic processing.  In particular, it states the privData
          component is evaluated according to the privacy protocol
          identified for the SNMPv2 party receiving the message.  When
          the relevant privacy protocol is the Symmetric Privacy
          Protocol, the procedure performed by a SNMPv2 entity whenever
          a management communication is received by a SNMPv2 party is as
          follows.

          (1)  The local database is consulted to determine the private
               privacy key of the SNMPv2 party receiving the message
               (represented, for example, according to the conventions
               defined in Section 1.5.2).

          (2)  The contents octets of the privData component are
               decrypted using, for example, the algorithm specified in
               Section 1.5.2 and the extracted private privacy key.

          Processing of the received message continues as specified in
          [1].

























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          5.  Clock and Secret Distribution

          The protocols described in Sections 3 and 4 assume the
          existence of loosely synchronized clocks and shared secret
          values.  Three requirements constrain the strategy by which
          clock values and secrets are distributed.

          o    If the value of an authentication clock is decreased, the
               private authentication key must be changed concurrently.

               When the value of an authentication clock is decreased,
               messages that have been sent with a timestamp value
               between the value of the authentication clock and its new
               value may be replayed.  Changing the private
               authentication key obviates this threat.

          o    The private authentication key and private privacy key
               must be known only to the parties requiring knowledge of
               them.

               Protecting the secrets from disclosure is critical to the
               security of the protocols.  Knowledge of the secrets must
               be as restricted as possible within an implementation.
               In particular, although the secrets may be known to one
               or more persons during the initial configuration of a
               device, the secrets should be changed immediately after
               configuration such that their actual value is known only
               to the software.  A management station has the additional
               responsibility of recovering the state of all parties
               whenever it boots, and it may address this responsibility
               by recording the secrets on a long-term storage device.
               Access to information on this device must be as
               restricted as is practically possible.

          o    There must exist at least one SNMPv2 entity that assumes
               the role of a responsible management station.

               This management station is responsible for ensuring that
               all authentication clocks are synchronized and for
               changing the secret values when necessary.  Although more
               than one management station may share this
               responsibility, their coordination is essential to the
               secure management of the network.  The mechanism by which
               multiple management stations ensure that no more than one
               of them attempts to synchronize the clocks or update the





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               secrets at any one time is a local implementation issue.

               A responsible management station may either support clock
               synchronization and secret distribution as separate
               functions, or combine them into a single functional unit.

          The first section below specifies the procedures by which a
          SNMPv2 entity is initially configured.  The next two sections
          describe one strategy for distributing clock values and one
          for determining a synchronized clock value among SNMPv2
          parties supporting the Digest Authentication Protocol.  For
          SNMPv2 parties supporting the Symmetric Privacy Protocol, the
          next section describes a strategy for distributing secret
          values.  The last section specifies the procedures by which a
          SNMPv2 entity recovers from a "crash."


          5.1.  Initial Configuration

          This section describes the initial configuration of a SNMPv2
          entity that supports the Digest Authentication Protocol or
          both the Digest Authentication Protocol and the Symmetric
          Privacy Protocol.

          When a network device is first installed, its initial, secure
          configuration must be done manually, i.e., a person must
          physically visit the device and enter the initial secret
          values for at least its first secure SNMPv2 party.  This
          requirement suggests that the person will have knowledge of
          the initial secret values.

          In general, the security of a system is enhanced as the number
          of entities that know a secret is reduced.  Requiring a person
          to physically visit a device every time a SNMPv2 party is
          configured not only exposes the secrets unnecessarily but is
          administratively prohibitive.  In particular, when MD5 is
          used, the initial authentication secret is 128 bits long and
          when DES is used an additional 128 bits are needed - 64 bits
          each for the key and initialization vector.  Clearly, these
          values will need to be recorded on a medium in order to be
          transported between a responsible management station and a
          managed agent.  The recommended procedure is to configure a
          small set of initial SNMPv2 parties for each SNMPv2 entity,
          one pair of which may be used initially to configure all other
          SNMPv2 parties.





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          In fact, there is a minimal, useful set of SNMPv2 parties that
          could be configured between each responsible management
          station and managed agent.  This minimal set includes one of
          each of the following for both the responsible management
          station and the managed agent:

          o    a SNMPv2 party for which the authentication protocol and
               privacy protocol are the values noAuth and noPriv,
               respectively,

          o    a SNMPv2 party for which the authentication protocol
               identifies the mechanism defined in Section 1.5.1 and its
               privacy protocol is the value noPriv, and

          o    a SNMPv2 party for which the authentication protocol and
               privacy protocol identify the mechanisms defined in
               Section 1.5.1 and Section 1.5.2, respectively.

          The last of these SNMPv2 parties in both the responsible
          management station and the managed agent could be used to
          create all other SNMPv2 parties.

          Configuring one pair of SNMPv2 parties to be used to configure
          all other parties has the advantage of exposing only one pair
          of secrets - the secrets used to configure the minimal, useful
          set identified above.  To limit this exposure, the responsible
          management station should change these values as its first
          operation upon completion of the initial configuration.  In
          this way, secrets are known only to the peers requiring
          knowledge of them in order to communicate.

          The Management Information Base (MIB) document [4] supporting
          these security protocols specifies 6 initial party identities
          and initial values, which, by convention, are assigned to the
          parties and their associated parameters.

          These 6 initial parties are required to exist as part of the
          configuration of implementations when first installed, with
          the exception that implementations not providing support for a
          privacy protocol only need the 4 initial parties for which the
          privacy protocol is noPriv.  When installing a managed agent,
          these parties need to be configured with their initial
          secrets, etc., both in the responsible management station and
          in the new agent.






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          If the responsible management station is configured first, it
          can be used to generate the initial secrets and provide them
          to a person, on a suitable medium, for distribution to the
          managed agent.  The following sequence of steps describes the
          initial configuration of a managed agent and its responsible
          management station.

          (1)  Determine the initial values for each of the attributes
               of the SNMPv2 party to be configured.  Some of these
               values may be computed by the responsible management
               station, some may be specified in the MIB document, and
               some may be administratively determined.

          (2)  Configure the parties in the responsible management
               station, according to the set of initial values.  If the
               management station is computing some initial values to be
               entered into the agent, an appropriate medium must be
               present to record the values.

          (3)  Configure the parties in the managed agent, according to
               the set of initial values.

          (4)  The responsible management station must synchronize the
               authentication clock values for each party it shares with
               each managed agent.  Section 5.3 specifies one strategy
               by which this could be accomplished.

          (5)  The responsible management station should change the
               secret values manually configured to ensure the actual
               values are known only to the peers requiring knowledge of
               them in order to communicate.  To do this, the management
               station generates new secrets for each party to be
               reconfigured and distributes the updates using any
               strategy which protects the new values from disclosure;
               use of a SNMPv2 set operation acting on the managed
               objects defined in [4] is such a strategy.  Upon
               receiving positive acknowledgement that the new values
               have been distributed, the management station should
               update its local database with the new values.

          If the managed agent does not support a protocol that protects
          messages from disclosure, e.g., the Symmetric Privacy Protocol
          (see section 5.4), then the distribution of new secrets, after
          the compromise of existing secrets, is not possible.  In this
          case, the new secrets can only be distributed by a physical





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          visit to the device.

          If there are other SNMPv2 protocol entities requiring
          knowledge of the secrets, the responsible management station
          must distribute the information upon completion of the initial
          configuration.  The considerations, mentioned above,
          concerning the protection of secrets from disclosure, also
          apply in this case.


          5.2.  Clock Distribution

          A responsible management station must ensure that the
          authentication clock value for each SNMPv2 party for which it
          is responsible

          o    is loosely synchronized among all the local databases in
               which it appears,

          o    is reset, as indicated below, upon reaching its maximal
               value, and

          o    is non-decreasing, except as indicated below.

          The skew among the clock values must be accounted for in the
          lifetime value, in addition to the expected communication
          delivery delay.

          A skewed authentication clock may be detected by a number of
          strategies, including knowledge of the accuracy of the system
          clock, unauthenticated queries of the party database, and
          recognition of authentication failures originated by the
          party.

          Whenever clock skew is detected, and whenever the SNMPv2
          entities at both the responsible management station and the
          relevant managed agent support an appropriate privacy protocol
          (e.g., the Symmetric Privacy Protocol), a straightforward
          strategy for the correction of clock skew is simultaneous
          alteration of authentication clock and private key for the
          relevant SNMPv2 party.  If the request to alter the key and
          clock for a particular party originates from that same party,
          then, prior to transmitting that request, the local notion of
          the authentication clock is artificially advanced to assure
          acceptance of the request as authentic.





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          More generally, however, since an authentication clock value
          need not be protected from disclosure, it is not necessary
          that a managed agent support a privacy protocol in order for a
          responsible management station to correct skewed clock values.
          The procedure for correcting clock skew in the general case is
          presented in Section 5.3.

          In addition to correcting skewed notions of authentication
          clocks, every SNMPv2 entity must react correctly as an
          authentication clock approaches its maximal value.  If the
          authentication clock for a particular SNMPv2 party ever
          reaches the maximal time value, the clock must halt at that
          value.  (The value of interest may be the maximum less
          lifetime.  When authenticating a message, its authentication
          timestamp is added to lifetime and compared to the
          authentication clock.  A SNMPv2 entity must guarantee that the
          sum is never greater than the maximal time value.) In this
          state, the only authenticated request a management station
          should generate for this party is one that alters the value of
          at least its authentication clock and private authentication
          key.  In order to reset these values, the responsible
          management station may set the authentication timestamp in the
          message to the maximal time value.

          The value of the authentication clock for a particular SNMPv2
          party must never be altered such that its new value is less
          than its old value, unless its private authentication key is
          also altered at the same time.


          5.3.  Clock Synchronization

          Unless the secrets are changed at the same time, the correct
          way to synchronize clocks is to advance the slower clock to be
          equal to the faster clock.  Suppose that party agentParty is
          realized by the SNMPv2 entity in a managed agent; suppose that
          party mgrParty is realized by the SNMPv2 entity in the
          corresponding responsible management station.  For any pair of
          parties, there are four possible conditions of the
          authentication clocks that could require correction:

          (1)  The management station's notion of the value of the
               authentication clock for agentParty exceeds the agent's
               notion.






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          (2)  The management station's notion of the value of the
               authentication clock for mgrParty exceeds the agent's
               notion.

          (3)  The agent's notion of the value of the authentication
               clock for agentParty exceeds the management station's
               notion.

          (4)  The agent's notion of the value of the authentication
               clock for mgrParty exceeds the management station's
               notion.

          The selective clock acceleration mechanism intrinsic to the
          protocol corrects conditions 1, 2 and 3 as part of the normal
          processing of an authentic message.  Therefore, the clock
          adjustment procedure below does not provide for any
          adjustments in those cases.  Rather, the following sequence of
          steps specifies how the clocks may be synchronized when
          condition 4 is manifest.

          (1)  The responsible management station saves its existing
               notion of the authentication clock for the party
               mgrParty.

          (2)  The responsible management station retrieves the
               authentication clock value for mgrParty from the agent.
               This retrieval must be an unauthenticated request, since
               the management station does not know if the clocks are
               synchronized.  If the request fails, the clocks cannot be
               synchronized, and the clock adjustment procedure is
               aborted without further processing.

          (3)  If the notion of the authentication clock for mgrParty
               just retrieved from the agent exceeds the management
               station's notion, then condition 4 is manifest, and the
               responsible management station advances its notion of the
               authentication clock for mgrParty to match the agent's
               notion.

          (4)  The responsible management station retrieves the
               authentication clock value for mgrParty from the agent.
               This retrieval must be an authenticated request, in order
               that the management station may verify that the clock
               value is properly synchronized.  If this authenticated
               query fails, then the management station restores its





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               previously saved notion of the clock value, and the clock
               adjustment procedure is aborted without further
               processing.  Otherwise, clock synchronization has been
               successfully realized.

          Administrative advancement of a clock as described above does
          not introduce any new vulnerabilities, since the value of the
          clock is intended to increase with the passage of time.  A
          potential operational problem is the rejection of authentic
          management operations that were authenticated using a previous
          value of the relevant party clock.  This possibility may be
          avoided if a management station suppresses generation of
          management traffic between relevant parties while this clock
          adjustment procedure is in progress.


          5.4.  Secret Distribution

          This section describes one strategy by which a SNMPv2 entity
          that supports both the Digest Authentication Protocol and the
          Symmetric Privacy Protocol can change the secrets for a
          particular SNMPv2 party.

          The frequency with which the secrets of a SNMPv2 party should
          be changed is a local administrative issue.  However, the more
          frequently a secret is used, the more frequently it should be
          changed.  At a minimum, the secrets must be changed whenever
          the associated authentication clock approaches its maximal
          value (see Section 6).  Note that, owing to both
          administrative and automatic advances of the authentication
          clock described in this memo, the authentication clock for a
          SNMPv2 party may well approach its maximal value sooner than
          might otherwise be expected.

          The following sequence of steps specifies how a responsible
          management station alters a secret value (i.e., the private
          authentication key or the private privacy key) for a
          particular SNMPv2 party.  There are two cases.

          First, setting the initial secret for a new party:

          (1)  The responsible management station generates a new secret
               value.







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          (2)  The responsible management station encapsulates a SNMPv2
               setRequest in a SNMPv2 private management communication
               with at least the following properties.

                    Its source supports the Digest Authentication
                    Protocol and the Symmetric Privacy Protocol.

                    Its destination supports the Symmetric Privacy
                    Protocol and the Digest Authentication Protocol.

          (3)  The SNMPv2 private management communication is
               transmitted to its destination.

          (4)  Upon receiving the request, the recipient processes the
               message according to [12] and [1].

          (5)  The recipient encapsulates a SNMPv2 response in a SNMPv2
               private management communication with at least the
               following properties.

                    Its source supports the Digest Authentication
                    Protocol and the Symmetric Privacy Protocol.

                    Its destination supports the Symmetric Privacy
                    Protocol and the Digest Authentication Protocol.

          (6)  The SNMPv2 private management communication is
               transmitted to its destination.

          (7)  Upon receiving the response, the responsible management
               station updates its local database with the new value.

          Second, modifying the current secret of an existing party:

          (1)  The responsible management station generates a new secret
               value.

          (2)  The responsible management station encapsulates a SNMPv2
               setRequest in a SNMPv2 management communication with at
               least the following properties.

                    Its source and destination supports the Digest
                    Authentication Protocol.







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          RFC 1446        Security Protocols for SNMPv2       April 1993


          (3)  The SNMPv2 private management communication is
               transmitted to its destination.

          (4)  Upon receiving the request, the recipient processes the
               message according to [12] and [1].

          (5)  The recipient encapsulates a SNMPv2 response in a SNMPv2
               management communication with at least the following
               properties.

                    Its source and destination supports the Digest
                    Authentication Protocol.

          (6)  The SNMPv2 management communication is transmitted to its
               destination.

          (7)  Upon receiving the response, the responsible management
               station updates its local database with the new value.

          If the responsible management station does not receive a
          response to its request, there are two possible causes.

          o    The request may not have been delivered to the
               destination.

          o    The response may not have been delivered to the
               originator of the request.

          In order to distinguish the two possible error conditions, a
          responsible management station could check the destination to
          see if the change has occurred.  Unfortunately, since the
          secret values are unreadable, this is not directly possible.

          The recommended strategy for verifying key changes is to set
          the public value corresponding to the secret being changed to
          a recognizable, novel value: that is, alter the public
          authentication key value for the relevant party when changing
          its private authentication key, or alter its public privacy
          key value when changing its private privacy key.  In this way,
          the responsible management station may retrieve the public
          value when a response is not received, and verify whether or
          not the change has taken place.  (This strategy is available
          since the public values are not used by the protocols defined
          in this memo.  If this strategy is employed, then the public
          values are significant in this context.  Of course, protocols





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          using the public values may make use of this strategy
          directly.)

          One other scenario worthy of mention is using a SNMPv2 party
          to change its own secrets.  In this case, the destination will
          change its local database prior to generating a response.
          Thus, the response will be constructed according to the new
          value.  However, the responsible management station will not
          update its local database until after the response is
          received.  This suggests the responsible management station
          may receive a response which will be evaluated as unauthentic,
          unless the correct secret is used.  The responsible management
          station may either account for this scenario as a special
          case, or use an alteration of the relevant public values (as
          described above) to verify the key change.

          Note, during the period of time after the request has been
          sent and before the response is received, the management
          station must keep track of both the old and new secret values.
          Since the delay may be the result of a network failure, the
          management station must be prepared to retain both values for
          an extended period of time, including across reboots.


          5.5.  Crash Recovery

          This section describes the requirements for SNMPv2 protocol
          entities in connection with recovery from system crashes or
          other service interruptions.

          For each SNMPv2 party in the local database for a particular
          SNMPv2 entity, its identity, authentication clock, private
          authentication key, and private privacy key must enjoy non-
          volatile, incorruptible representations.  If possible,
          lifetime should also enjoy a non-volatile, incorruptible
          representation.  If said SNMPv2 entity supports other security
          protocols or algorithms in addition to the two defined in this
          memo, then the authentication protocol and the privacy
          protocol for each party also require non-volatile,
          incorruptible representation.

          The authentication clock of a SNMPv2 party is a critical
          component of the overall security of the protocols.  The
          inclusion of a reliable representation of a clock in a SNMPv2
          entity is required for overall security.  A reliable clock





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          representation ensures that a clock's value is monotonically
          increasing, even across a power loss or other system failure
          of the local SNMPv2 entity.  One example of a reliable clock
          representation is that provided by battery-powered clock-
          calendar devices incorporated into some contemporary systems.
          Another example is storing and updating a clock value in non-
          volatile storage at a frequency of once per U (e.g., 24)
          hours, and re-initialising that clock value on every reboot as
          the stored value plus U+1 hours.  It is assumed that
          management stations always support reliable clock
          representations, where clock adjustment by a human operator
          during crash recovery may contribute to that reliability.

          If a managed agent crashes and does not reboot in time for its
          responsible management station to prevent its authentication
          clock from reaching its maximal value, upon reboot the clock
          must be halted at its maximal value.  The procedures specified
          in Section 5.3 would then apply.

          Upon recovery, those attributes of each SNMPv2 party that do
          not enjoy non-volatile or reliable representation are
          initialized as follows.

          o    If the private authentication key is not the OCTET STRING
               of zero length, the authentication protocol is set to
               identify use of the Digest Authentication Protocol in
               conjunction with the algorithm specified in Section
               1.5.1.

          o    If the lifetime is not retained, it should be initialized
               to zero.

          o    If the private privacy key is not the OCTET STRING of
               zero length, the privacy protocol is set to identify use
               of the Symmetric Privacy Protocol in conjunction with the
               algorithm specified in Section 1.5.2.

          Upon detecting that a managed agent has rebooted, a
          responsible management station must reset all other party
          attributes, including the lifetime if it was not retained.  In
          order to reset the lifetime, the responsible management
          station should set the authentication timestamp in the message
          to the sum of the authentication clock and desired lifetime.
          This is an artificial advancement of the authentication
          timestamp in order to guarantee the message will be authentic





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          when received by the recipient.

















































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          6.  Security Considerations

          This section highlights security considerations relevant to
          the protocols and procedures defined in this memo.  Practices
          that contribute to secure, effective operation of the
          mechanisms defined here are described first.  Constraints on
          implementation behavior that are necessary to the security of
          the system are presented next.  Finally, an informal account
          of the contribution of each mechanism of the protocols to the
          required goals is presented.


          6.1.  Recommended Practices

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

          o    A management station should 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, in the context of these
               security protocols it is significant owing to the
               possibility of message duplication (malicious or
               otherwise).

          o    A management station should not interpret an agent's lack
               of response to an authenticated SNMPv2 management
               communication as a conclusive indication of agent or
               network failure.

               It is possible for authentication failure traps to be
               lost or suppressed as a result of authentication clock
               skew or inconsistent notions of shared secrets.  In order
               either to facilitate administration of such SNMPv2
               parties or to provide for continued management in times
               of network stress, a management station implementation
               may provide for arbitrary, artificial advancement of the
               timestamp or selection of shared secrets on locally
               generated messages.







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          o    The lifetime value for a SNMPv2 party should be chosen
               (by the local administration) to be as small as possible,
               given the accuracy of clock devices available, relevant
               round-trip communications delays, and the frequency with
               which a responsible management station will be able to
               verify all clock values.

               A large lifetime increases the vulnerability to malicious
               delays of SNMPv2 messages.  The implementation of a
               management station may accommodate changing network
               conditions during periods of network stress by
               effectively increasing the lifetimes of the source and
               destination parties.  The management station accomplishes
               this by artificially advancing its notion of the source
               party's clock on messages it sends, and by artificially
               increasing its notion of the source party`s lifetime on
               messages it receives.

          o    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 that does not exceed lifetime 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.

               However, a managed object, snmpSetSerialNo [14], 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.

          o    The frequency with which the secrets of a SNMPv2 party
               should be changed is indirectly related to the frequency
               of their use.





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               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 insignificant, and as such the changing
               of secrets may be less frequent.  However, when public
               data networks are the communication paths, more caution
               is prudent.

          o    In order to foster the greatest degree of security, a
               management station implementation must support
               constrained, pairwise sharing of secrets among SNMPv2
               entities as its default mode of operation.

               Owing to the use of symmetric cryptography in the
               protocols defined here, the secrets associated with a
               particular SNMPv2 party must be known to all other SNMPv2
               parties with which that party may wish to communicate.
               As the number of locations at which secrets are known and
               used increases, the likelihood of their disclosure also
               increases, as does the potential impact of that
               disclosure.  Moreover, if the set of SNMPv2 protocol
               entities with knowledge of a particular secret numbers
               more than two, data origin cannot be reliably
               authenticated because it is impossible to determine with
               any assurance which entity of that set may be the
               originator of a particular SNMPv2 message.  Thus, the
               greatest degree of security is afforded by configurations
               in which the secrets for each SNMPv2 party are known to
               at most two protocol entities.


          6.2.  Conformance

          A SNMPv2 entity implementation that claims conformance to this
          memo must satisfy the following requirements:






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          (1)  It must implement the noAuth and noPriv protocols whose
               object identifiers are defined in [4].

                    noAuth  This protocol signifies that messages
                    generated by a party using it are not protected as
                    to origin or integrity.  It is required to ensure
                    that a party's authentication clock is always
                    accessible.

                    noPriv  This protocol signifies that messages
                    received by a party using it are not protected from
                    disclosure.  It is required to ensure that a party's
                    authentication clock is always accessible.

          (2)  It must implement the Digest Authentication Protocol in
               conjunction with the algorithm defined in Section 1.5.1.

          (3)  It must include in its local database at least one SNMPv2
               party with the following parameters set as follows:

                    partyAuthProtocol is set to noAuth and

                    partyPrivProtocol is set to noPriv.

               This party must have a MIB view [1] specified that
               includes at least the authentication clock of all other
               parties.  Alternatively, the authentication clocks of the
               other parties may be partitioned among several similarly
               configured parties according to a local implementation
               convention.

          (4)  For each SNMPv2 party about which it maintains
               information in a local database, an implementation must
               satisfy the following requirements:

                    (a) It must not allow a party's parameters to be set
                    to a value inconsistent with its expected syntax.
                    In particular, Section 1.4 specifies constraints for
                    the chosen mechanisms.

                    (b) It must, to the maximal extent possible,
                    prohibit read-access to the private authentication
                    key and private encryption key under all
                    circumstances except as required to generate and/or
                    validate SNMPv2 messages with respect to that party.





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                    This prohibition includes prevention of read-access
                    by the entity's human operators.

                    (c) It must allow the party's authentication clock
                    to be publicly accessible.  The correct operation of
                    the Digest Authentication Protocol requires that it
                    be possible to determine this value at all times in
                    order to guarantee that skewed authentication clocks
                    can be resynchronized.

                    (d) It must prohibit alterations to its record of
                    the authentication clock for that party
                    independently of alterations to its record of the
                    private authentication key (unless the clock
                    alteration is an advancement).

                    (e) It must never allow its record of the
                    authentication clock for that party to be
                    incremented beyond the maximal time value and so
                    "roll-over" to zero.

                    (f) It must never increase its record of the
                    lifetime for that party except as may be explicitly
                    authorized (via imperative command or securely
                    represented configuration information) by the
                    responsible network administrator.

                    (g) In the event that the non-volatile,
                    incorruptible representations of a party's
                    parameters (in particular, either the private
                    authentication key or private encryption key) are
                    lost or destroyed, it must alter its record of these
                    quantities to random values so subsequent
                    interaction with that party requires manual
                    redistribution of new secrets and other parameters.

          (5)  If it selects new value(s) for a party's secret(s), it
               must avoid bad or obvious choices for said secret(s).
               Choices to be avoided are boundary values (such as all-
               zeros) and predictable values (such as the same value as
               previously or selecting from a predetermined set).

          (6)  It must ensure that a received message for which the
               originating party uses the Digest Authentication Protocol
               but the receiving party does not, is always declared to





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               be unauthentic.  This may be achieved explicitly via an
               additional step in the procedure for processing a
               received message, or implicitly by verifying that all
               local access control policies enforce this requirement.


          6.3.  Protocol Correctness

          The correctness of these SNMPv2 security protocols with
          respect to the stated goals depends on the following
          assumptions:

          (1)  The chosen message digest algorithm satisfies its design
               criteria.  In particular, it must be computationally
               infeasible to discover two messages that share the same
               digest value.

          (2)  It is computationally infeasible to determine the secret
               used in calculating a digest on the concatenation of the
               secret and a message when both the digest and the message
               are known.

          (3)  The chosen symmetric encryption algorithm satisfies its
               design criteria.  In particular, it must be
               computationally infeasible to determine the cleartext
               message from the ciphertext message without knowledge of
               the key used in the transformation.

          (4)  Local notions of a party's authentication clock while it
               is associated with a specific private key value are
               monotonically non-decreasing (i.e., they never run
               backwards) in the absence of administrative
               manipulations.

          (5)  The secrets for a particular SNMPv2 party are known only
               to authorized SNMPv2 protocol entities.

          (6)  Local notions of the authentication clock for a
               particular SNMPv2 party are never altered such that the
               authentication clock's new value is less than the current
               value without also altering the private authentication
               key.

          For each mechanism of the protocol, an informal account of its
          contribution to the required goals is presented below.





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          Pseudocode fragments are provided where appropriate to
          exemplify possible implementations; they are intended to be
          self-explanatory.


          6.3.1.  Clock Monotonicity Mechanism

          By pairing each sequence of a clock's values with a unique
          key, the protocols partially realize goal 3, and the
          conjunction of this property with assumption 6 above is
          sufficient for the claim that, with respect to a specific
          private key value, all local notions of a party's
          authentication clock are, in general, non-decreasing with
          time.


          6.3.2.  Data Integrity Mechanism

          The protocols require computation of a message digest computed
          over the SNMPv2 message prepended by the secret for the
          relevant party.  By virtue of this mechanism and assumptions 1
          and 2, the protocols realize goal 1.

          Normally, the inclusion of the message digest value with the
          digested message would not be sufficient to guarantee data
          integrity, since the digest value can be modified in addition
          to the message while it is enroute.  However, since not all of
          the digested message is included in the transmission to the
          destination, it is not possible to substitute both a message
          and a digest value while enroute to a destination.

          Strictly speaking, the specified strategy for data integrity
          does not detect a SNMPv2 message modification which appends
          extraneous material to the end of such messages.  However,
          owing to the representation of SNMPv2 messages as ASN.1
          values, such modifications cannot - consistent with goal 1 -
          result in unauthorized management operations.

          The data integrity mechanism specified in this memo protects
          only against unauthorized modification of individual SNMPv2
          messages.  A more general data integrity service that affords
          protection against the threat of message stream modification
          is not realized by this mechanism, although limited protection
          against reordering, delay, and duplication of messages within
          a message stream are provided by other mechanisms of the





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          protocol.


          6.3.3.  Data Origin Authentication Mechanism

          The data integrity mechanism requires the use of a secret
          value known only to communicating parties.  By virtue of this
          mechanism and assumptions 1 and 2, the protocols explicitly
          prevent unauthorized modification of messages.  Data origin
          authentication is implicit if the message digest value can be
          verified.  That is, the protocols realize goal 2.


          6.3.4.  Restricted Administration Mechanism

          This memo requires that implementations preclude
          administrative alterations of the authentication clock for a
          particular party independently from its private authentication
          key (unless that clock alteration is an advancement).  An
          example of an efficient implementation of this restriction is
          provided in a pseudocode fragment below.  This pseudocode
          fragment meets the requirements of assumption 6.  Observe that
          the requirement is not for simultaneous alteration but to
          preclude independent alteration.  This latter requirement is
          fairly easily realized in a way that is consistent with the
          defined semantics of the SNMPv2 set operation.
























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               Void partySetKey (party, newKeyValue)
               {
                   if (party->clockAltered) {
                      party->clockAltered = FALSE;
                      party->keyAltered = FALSE;
                      party->keyInUse = newKeyValue;
                      party->clockInUse = party->clockCache;
                   }
                   else {
                      party->keyAltered = TRUE;
                      party->keyCache = newKeyValue;
                   }
               }

               Void partySetClock (party, newClockValue)
               {
                   if (party->keyAltered) {
                      party->keyAltered = FALSE;
                      party->clockAltered = FALSE;
                      party->clockInUse = newClockValue;
                      party->keyInUse = party->keyCache;
                   }
                   else {
                      party->clockAltered = TRUE;
                      party->clockCache = newClockValue;
                   }
               }


          6.3.5.  Message Timeliness Mechanism

          The definition of the SNMPv2 security protocols requires that,
          if the authentication timestamp value on a received message -
          augmented by an administratively chosen lifetime value - is
          less than the local notion of the clock for the originating
          SNMPv2 party, the message is not delivered.


               if (timestampOfReceivedMsg +
                      party->administrativeLifetime <=
                      party->localNotionOfClock) {
                      msgIsValidated = FALSE;
               }







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          By virtue of this mechanism, the protocols realize goal 3.  In
          cases in which the local notions of a particular SNMPv2 party
          clock are moderately well-synchronized, the timeliness
          mechanism effectively limits the age of validly delivered
          messages.  Thus, if an attacker diverts all validated messages
          for replay much later, the delay introduced by this attack is
          limited to a period that is proportional to the skew among
          local notions of the party clock.


          6.3.6.  Selective Clock Acceleration Mechanism

          The definition of the SNMPv2 security protocols requires that,
          if either of the timestamp values for the originating or
          receiving parties on a received, validated message exceeds the
          corresponding local notion of the clock for that party, then
          the local notion of the clock for that party is adjusted
          forward to correspond to said timestamp value.  This mechanism
          is neither strictly necessary nor sufficient to the security
          of the protocol; rather, it fosters the clock synchronization
          on which valid message delivery depends - thereby enhancing
          the effectiveness of the protocol in a management context.


               if (msgIsValidated) {
                      if (timestampOfReceivedMsg >
                            party->localNotionOfClock) {
                            party->localNotionOfClock =
                                  timestampOfReceivedMsg;
                      }
               }


          The effect of this mechanism is to synchronize local notions
          of a party clock more closely in the case where a sender's
          notion is more advanced than a receiver's.  In the opposite
          case, this mechanism has no effect on local notions of a party
          clock and either the received message is validly delivered or
          not according to other mechanisms of the protocol.

          Operation of this mechanism does not, in general, improve the
          probability of validated delivery for messages generated by
          party participants whose local notion of the party clock is
          relatively less advanced.  In this case, queries from a
          management station may not be validly delivered and the





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          management station needs to react appropriately (e.g., by use
          of the strategy described in section 5.3).  In contrast, the
          delivery of SNMPv2 trap messages generated by an agent that
          suffers from a less advanced notion of a party clock is more
          problematic, for an agent may lack the capacity to recognize
          and react to security failures that prevent delivery of its
          messages.  Thus, the inherently unreliable character of trap
          messages is likely to be compounded by attempts to provide for
          their validated delivery.


          6.3.7.  Confidentiality Mechanism

          The protocols require the use of a symmetric encryption
          algorithm when the data confidentiality service is required.
          By virtue of this mechanism and assumption 3, the protocols
          realize goal 4.

































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          7.  Acknowledgements

          This document is based, almost entirely, on RFC 1352.















































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          8.  References

          [1]  Galvin, J., and McCloghrie, K., "Administrative Model for
               version 2 of the Simple Network Management Protocol
               (SNMPv2)", RFC 1445, Trusted Information Systems, Hughes
               LAN Systems, April 1993.

          [2]  Case, J., Fedor, M., Schoffstall, M., Davin, J., "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]  McCloghrie, K., and Galvin, J., "Party MIB for version 2
               of the Simple Network Management Protocol (SNMPv2)", RFC
               1447, Hughes LAN Systems, Trusted Information Systems,
               April 1993.

          [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).

          [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.






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

          [12] Case, J., McCloghrie, K., Rose, M., and Waldbusser, S.,
               "Protocol Operations for version 2 of the Simple Network
               Management Protocol (SNMPv2)", RFC 1448, SNMP Research,
               Inc., Hughes LAN Systems, Dover Beach Consulting, Inc.,
               Carnegie Mellon University, April 1993.

          [13] Case, J., McCloghrie, K., Rose, M., and Waldbusser, S.,
               "Transport Mappings for version 2 of the Simple Network
               Management Protocol (SNMPv2)", RFC 1449, SNMP Research,
               Inc., Hughes LAN Systems, Dover Beach Consulting, Inc.,
               Carnegie Mellon University, April 1993.

          [14] Case, J., McCloghrie, K., Rose, M., and Waldbusser, S.,
               "Management Information Base for version 2 of the Simple
               Network Management Protocol (SNMPv2)", RFC 1450, SNMP
               Research, Inc., Hughes LAN Systems, Dover Beach
               Consulting, Inc., Carnegie Mellon University, April 1993.





























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          9.  Authors' Addresses

               James M. Galvin
               Trusted Information Systems, Inc.
               3060 Washington Road, Route 97
               Glenwood, MD 21738

               Phone:  +1 301 854-6889
               EMail:  galvin@tis.com


               Keith McCloghrie
               Hughes LAN Systems
               1225 Charleston Road
               Mountain View, CA  94043
               US

               Phone: +1 415 966 7934
               Email: kzm@hls.com































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