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rfc:rfc1446
        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
        Galvin & McCloghrie                                   [Page i]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        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
        Galvin & McCloghrie                                   [Page 1]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        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.
        Galvin & McCloghrie                                   [Page 2]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        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).
        Galvin & McCloghrie                                   [Page 3]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        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.
        Galvin & McCloghrie                                   [Page 4]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        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.
        Galvin & McCloghrie                                   [Page 5]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        (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.
        Galvin & McCloghrie                                   [Page 6]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        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
        Galvin & McCloghrie                                   [Page 7]
        RFC 1446        Security Protocols for SNMPv2       April 1993
             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.
        Galvin & McCloghrie                                   [Page 8]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        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.
        Galvin & McCloghrie                                   [Page 9]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        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.
        Galvin & McCloghrie                                  [Page 10]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        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.
        Galvin & McCloghrie                                  [Page 11]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        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.
        Galvin & McCloghrie                                  [Page 12]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        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.
        Galvin & McCloghrie                                  [Page 13]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        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.
        Galvin & McCloghrie                                  [Page 14]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        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
        Galvin & McCloghrie                                  [Page 15]
        RFC 1446        Security Protocols for SNMPv2       April 1993
             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
        Galvin & McCloghrie                                  [Page 16]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        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.
        Galvin & McCloghrie                                  [Page 17]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        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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        RFC 1446        Security Protocols for SNMPv2       April 1993
        (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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        RFC 1446        Security Protocols for SNMPv2       April 1993
             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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        RFC 1446        Security Protocols for SNMPv2       April 1993
        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
        Galvin & McCloghrie                                  [Page 35]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        when received by the recipient.
        Galvin & McCloghrie                                  [Page 36]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        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.
        Galvin & McCloghrie                                  [Page 37]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        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.
        Galvin & McCloghrie                                  [Page 38]
        RFC 1446        Security Protocols for SNMPv2       April 1993
             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:
        Galvin & McCloghrie                                  [Page 39]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        (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.
        Galvin & McCloghrie                                  [Page 40]
        RFC 1446        Security Protocols for SNMPv2       April 1993
                  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
        Galvin & McCloghrie                                  [Page 41]
        RFC 1446        Security Protocols for SNMPv2       April 1993
             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.
        Galvin & McCloghrie                                  [Page 42]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        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
        Galvin & McCloghrie                                  [Page 43]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        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.
        Galvin & McCloghrie                                  [Page 44]
        RFC 1446        Security Protocols for SNMPv2       April 1993
             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;
             }
        Galvin & McCloghrie                                  [Page 45]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        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
        Galvin & McCloghrie                                  [Page 46]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        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.
        Galvin & McCloghrie                                  [Page 47]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        7.  Acknowledgements
        This document is based, almost entirely, on RFC 1352.
        Galvin & McCloghrie                                  [Page 48]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        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.
        Galvin & McCloghrie                                  [Page 49]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        [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.
        Galvin & McCloghrie                                  [Page 50]
        RFC 1446        Security Protocols for SNMPv2       April 1993
        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
        Galvin & McCloghrie                                  [Page 51]
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