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rfc:rfc1352

Network Working Group J. Galvin Request for Comments: 1352 Trusted Information Systems, Inc.

                                                        K. McCloghrie
                                             Hughes LAN Systems, Inc.
                                                             J. Davin
                                  MIT Laboratory for Computer Science
                                                            July 1992
                      SNMP Security Protocols

Status of this Memo

 This document specifies 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.    Abstract . . . . . . . . . . . . . . . . . . . . . . . . . .   2
 2.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .   2
 2.1   Threats  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
 2.2   Goals and Constraints  . . . . . . . . . . . . . . . . . . .   5
 2.3   Security Services  . . . . . . . . . . . . . . . . . . . . .   6
 2.4   Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . .   6
 2.4.1   Message Digest Algorithm . . . . . . . . . . . . . . . . .   7
 2.4.2   Symmetric Encryption Algorithm . . . . . . . . . . . . . .   8
 3.    SNMP Party   . . . . . . . . . . . . . . . . . . . . . . . .   9
 4.    Digest Authentication Protocol . . . . . . . . . . . . . . .  11
 4.1   Generating a Message   . . . . . . . . . . . . . . . . . . .  14
 4.2   Receiving a Message  . . . . . . . . . . . . . . . . . . . .  15
 5.    Symmetric Privacy Protocol . . . . . . . . . . . . . . . . .  16
 5.1   Generating a Message   . . . . . . . . . . . . . . . . . . .  17
 5.2   Receiving a Message  . . . . . . . . . . . . . . . . . . . .  18
 6.    Clock and Secret Distribution  . . . . . . . . . . . . . . .  19
 6.1   Initial Configuration    . . . . . . . . . . . . . . . . . .  20
 6.2   Clock Distribution   . . . . . . . . . . . . . . . . . . . .  22
 6.3   Clock Synchronization  . . . . . . . . . . . . . . . . . . .  24
 6.4   Secret Distribution  . . . . . . . . . . . . . . . . . . . .  26
 6.5   Crash Recovery   . . . . . . . . . . . . . . . . . . . . . .  28
 7.    Security Considerations  . . . . . . . . . . . . . . . . . .  30
 7.1   Recommended Practices  . . . . . . . . . . . . . . . . . . .  30
 7.2   Conformance    . . . . . . . . . . . . . . . . . . . . . . .  33
 7.3   Protocol Correctness . . . . . . . . . . . . . . . . . . . .  34
 7.3.1   Clock Monotonicity Mechanism . . . . . . . . . . . . . . .  35
 7.3.2   Data Integrity Mechanism . . . . . . . . . . . . . . . . .  36

Galvin, McCloghrie, & Davin [Page 1] RFC 1352 SNMP Security Protocols July 1992

 7.3.3   Data Origin Authentication Mechanism . . . . . . . . . . .  36
 7.3.4   Restricted Administration Mechanism  . . . . . . . . . . .  36
 7.3.5   Ordered Delivery Mechanism   . . . . . . . . . . . . . . .  37
 7.3.6   Message Timeliness Mechanism . . . . . . . . . . . . . . .  38
 7.3.7   Selective Clock Acceleration Mechanism . . . . . . . . . .  38
 7.3.8   Confidentiality Mechanism  . . . . . . . . . . . . . . . .  39
 8.    Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  39
 9.    References . . . . . . . . . . . . . . . . . . . . . . . . .  40
 10.   Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  41

1. Abstract

 The Simple Network Management Protocol (SNMP) specification [1]
 allows for the protection of network management operations by a
 variety of security protocols.  The SNMP administrative model
 described in [2] provides a framework for securing SNMP network
 management. In the context of that framework, this memo defines
 protocols to support the following three security services:
   o data integrity,
   o data origin authentication, and
   o data confidentiality.
 Please send comments to the SNMP Security Developers mailing list
 (snmp-sec-dev@tis.com).

2. Introduction

 In the model described in [2], each SNMP party is, by definition,
 associated with a single authentication protocol.  The authentication
 protocol provides a mechanism by which SNMP 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.
 Similarly, each SNMP party is, by definition, associated with a
 single privacy protocol. The privacy protocol provides a mechanism by
 which SNMP 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 [1].

Galvin, McCloghrie, & Davin [Page 2] RFC 1352 SNMP Security Protocols July 1992

    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.
 The Digest Authentication Protocol is described in Section 4.  It
 provides a data integrity service by transmitting a message digest --
 computed by the originator and verified by the recipient -- with each
 SNMP 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 5. 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 6.3 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 6.4 presents one such strategy that is
 particularly suited to the demands of SNMP network management.

2.1 Threats

 Several of the classical threats to network protocols are applicable
 to the network management problem and therefore would be applicable
 to any SNMP security 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 SNMP security protocol should
 provide protection are:

Galvin, McCloghrie, & Davin [Page 3] RFC 1352 SNMP Security Protocols July 1992

 Modification of Information.
    The SNMP 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 SNMP 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 SNMP protocol is based upon connectionless transport services.
    The message stream modification threat is the danger that messages
    may be arbitrarily re-ordered, delayed or replayed to effect
    unauthorized management operations.  This threat may arise either
    by the work of a malicious attacker or by the natural operation of
    a subnetwork service.
 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 SNMP is used
    to administer private parameters on which its security is based.
    Protecting against the disclosure threat may also be required as a
    matter of local policy.
 There are at least two threats that a SNMP security protocol need not
 protect against. The security protocols defined in this memo do not
 provide protection against:
 Denial of Service.
    A SNMP 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.

Galvin, McCloghrie, & Davin [Page 4] RFC 1352 SNMP Security Protocols July 1992

 Traffic Analysis.
    In addition, a SNMP 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.

2.2 Goals and Constraints

 Based on the foregoing account of threats in the SNMP network
 management environment, the goals of a SNMP security protocol are
 enumerated below.
  1. The protocol should provide for verification that each
     received SNMP 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 SNMP
     message.
  3. The protocol should provide that the apparent time of
     generation for each received SNMP message is recent.
  4. The protocol should provide that the apparent time of
     generation for each received SNMP message is
     subsequent to that for all previously delivered messages
     of similar origin.
  5. The protocol should provide, when necessary, that the
     contents of each received SNMP message are protected
     from disclosure.
 In addition to the principal goal of supporting secure network
 management, the design of any SNMP 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).

Galvin, McCloghrie, & Davin [Page 5] RFC 1352 SNMP Security Protocols July 1992

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

2.3 Security Services

 The security services necessary to support the goals of a SNMP
 security protocol are as follows.
 Data Integrity   is the provision of the property that data
     and data sequences have not been altered or destroyed
     in an unauthorized manner.
 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.
    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.

2.4 Mechanisms

    The security protocols defined in this memo employ several
    types of mechanisms in order to realize the goals and security
    services described above:
   o In support of data integrity, a message digest algorithm
     is required. A digest is calculated over an appropriate
     portion of a SNMP 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 SNMP 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 reordering, a
     timestamp value is included in each message generated.
     A recipient evaluates the timestamp to determine if the

Galvin, McCloghrie, & Davin [Page 6] RFC 1352 SNMP Security Protocols July 1992

     message is recent and it uses the timestamp to determine
     if the message is ordered relative to other messages it
     has received. In conjunction with other readily available
     information (e.g., the request-id), the timestamp also
     indicates whether or not the message is a replay of a
     previous message. This protection against the threat of
     message reordering implies no protection against
     unauthorized deletion or suppression of messages.
   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 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 2.4.1 and 2.4.2 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.

2.4.1 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 SNMP 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 4) is identified by the ASN.1
 object identifier value md5AuthProtocol, defined in [4].
 For any SNMP party for which the authentication protocol is
 md5AuthProtocol, 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

Galvin, McCloghrie, & Davin [Page 7] RFC 1352 SNMP Security Protocols July 1992

 (see Section 4) is 16 octets.

2.4.2 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 SNMP 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 5) is identified by the ASN.1 object identifier
 value desPrivProtocol, defined in [4].
 For any SNMP 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.
 The length of the octet sequence to be encrypted by the DES must be

Galvin, McCloghrie, & Davin [Page 8] RFC 1352 SNMP Security Protocols July 1992

 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.

3. SNMP Party

 Recall from [2] that a SNMP 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 SNMP protocol entity. A SNMP protocol
 entity is an actual process which performs network management
 operations by generating and/or responding to SNMP protocol messages
 in the manner specified in [1]. Architecturally, every SNMP protocol
 entity maintains a local database that represents all SNMP parties
 known to it.
 A SNMP party may be represented by an ASN.1 value with the following
 syntax.
    SnmpParty ::= SEQUENCE {
      partyIdentity
         OBJECT IDENTIFIER,
      partyTDomain
         OBJECT IDENTIFIER,
      partyTAddr
         OCTET STRING,
      partyProxyFor
         OBJECT IDENTIFIER,
      partyMaxMessageSize
         INTEGER,
      partyAuthProtocol
         OBJECT IDENTIFIER,
      partyAuthClock
         INTEGER,
      partyAuthLastMsg
         INTEGER,
      partyAuthNonce
         INTEGER,
      partyAuthPrivate
         OCTET STRING,
      partyAuthPublic
         OCTET STRING,
      partyAuthLifetime

Galvin, McCloghrie, & Davin [Page 9] RFC 1352 SNMP Security Protocols July 1992

         INTEGER,
      partyPrivProtocol
         OBJECT IDENTIFIER,
      partyPrivPrivate
         OCTET STRING,
      partyPrivPublic
         OCTET STRING
    }
 For each SnmpParty value that represents a SNMP party, the generic
 significance of each of its components is defined in [2]. For each
 SNMP 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 2.4.1).
     This combined mechanism is used to authenticate the
     origin and integrity of all messages generated by the
     party.
   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 partyAuthLastMsg component is called the
     last-timestamp and represents a notion of time
     associated with the most recent, authentic protocol
     message generated by the party.
   o Its partyAuthNonce component is called the nonce
     and represents a monotonically increasing integer
     associated with the most recent, authentic protocol
     message generated by the party. The nonce associated
     with a particular message distinguishes it among all
     others transmitted in the same unit time interval.
   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.

Galvin, McCloghrie, & Davin [Page 10] RFC 1352 SNMP Security Protocols July 1992

     This component is not significant except as suggested in
     Section 6.4.
   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 SNMP 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 2.4.2). 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 6.4.

4. 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.
 Recall from [2] that a SNMP management communication is represented
 by an ASN.1 value with the following syntax.

Galvin, McCloghrie, & Davin [Page 11] RFC 1352 SNMP Security Protocols July 1992

    SnmpMgmtCom ::= [1] IMPLICIT SEQUENCE {
      dstParty
         OBJECT IDENTIFIER,
      srcParty
         OBJECT IDENTIFIER,
      pdu   PDUs
    }
 For each SnmpMgmtCom value that represents a SNMP management
 communication, the following statements are true:
   o Its dstParty component is called the destination and
     identifies the SNMP party to which the communication
     is directed.
   o Its srcParty component is called the source and
     identifies the SNMP party from which the
     communication is originated.
   o Its pdu component has the form and significance
     attributed to it in [1].
 Recall from [2] that a SNMP 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 SNMP 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 the
     SNMP party originating 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.

Galvin, McCloghrie, & Davin [Page 12] RFC 1352 SNMP Security Protocols July 1992

   o Its authData component is called the authentication
     data and represents a SNMP management
     communication.
 In support of the Digest Authentication Protocol, an authInfo
 component is of type AuthInformation:
    AuthInformation ::= [1] IMPLICIT SEQUENCE {
      authTimestamp
         INTEGER (0..2147483647),
      authNonce
         INTEGER (0..2147483647),
      authDigest
         OCTET STRING
    }
 For each AuthInformation value that represents authentication
 information, the following statements are true:
   o Its authTimestamp component is called the
     authentication timestamp and represents the time of the
     generation of the message according to the
     partyAuthClock of the SNMP party that originated
     it. Note that the granularity of the authentication
     timestamp is 1 second.
   o Its authNonce component is called the authentication
     nonce and represents a non-negative integer value
     evaluated according to the authTimestamp value. In
     order not to limit transmission frequency of management
     communications to the granularity of the authentication
     timestamp, the authentication nonce is provided to
     differentiate between multiple messages sent with the
     same value of authTimestamp. The authentication
     nonce is a monotonically increasing sequence number,
     that is reset for each new authentication timestamp
     value.
   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.

Galvin, McCloghrie, & Davin [Page 13] RFC 1352 SNMP Security Protocols July 1992

4.1 Generating a Message

 This section describes the behavior of a SNMP protocol entity when it
 acts as a SNMP party for which the authentication protocol is
 administratively specified as the Digest Authentication Protocol.
 Insofar as the behavior of a SNMP protocol entity when transmitting
 protocol messages is defined generically in [2], only those aspects
 of that behavior that are specific to the Digest Authentication
 Protocol are described below. In particular, this section describes
 the encapsulation of a SNMP management communication into a SNMP
 authenticated management communication.
 According to [2], 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 SNMP party originating the message. When the relevant
 authentication protocol is the Digest Authentication Protocol, the
 procedure performed by a SNMP protocol entity whenever a management
 communication is to be transmitted by a SNMP party is as follows.
  1. The local database is consulted to determine the
     authentication clock, last-timestamp, nonce, and private
     authentication key (extracted, for example, according to
     the conventions defined in Section 2.4.1) of the SNMP
     party originating the message.
  2. The authTimestamp component is set to the retrieved
     authentication clock value.
  3. If the last-timestamp is equal to the authentication
     clock, the nonce is incremented. Otherwise the nonce is
     set to zero. The authNonce component is set to the
     nonce value. In the local database, the originating
     SNMP party's nonce and last-timestamp are set to the
     nonce value and the authentication clock, respectively.
  4. The authentication digest is temporarily set to the
     private authentication key. The SnmpAuthMsg value
     is serialized according to the conventions of [12] and [1].
     A digest is computed over the octet sequence
     representing that serialized value using, for example, the
     algorithm specified in Section 2.4.1. The authDigest
     component is set to the computed digest value.
 As set forth in [2], 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 SNMP party.

Galvin, McCloghrie, & Davin [Page 14] RFC 1352 SNMP Security Protocols July 1992

4.2 Receiving a Message

 This section describes the behavior of a SNMP protocol entity upon
 receipt of a protocol message from a SNMP party for which the
 authentication protocol is administratively specified as the Digest
 Authentication Protocol. Insofar as the behavior of a SNMP protocol
 entity when receiving protocol messages is defined generically in
 [2], only those aspects of that behavior that are specific to the
 Digest Authentication Protocol are described below.
 According to [2], 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
 SNMP party that originated the message. When the relevant
 authentication protocol is the Digest Authentication Protocol, the
 procedure performed by a SNMP protocol entity whenever a management
 communication is received by a SNMP party is as follows.
  1. If the ASN.1 type of the authInfo component is not
     AuthInformation, the message is evaluated as
     unauthentic. Otherwise, the authTimestamp,
     authNonce, and authDigest components are
     extracted from the SnmpAuthMsg value.
  2. The local database is consulted to determine the
     authentication clock, last-timestamp, nonce, private
     authentication key (extracted, for example, according to
     the conventions defined in Section 2.4.1), and lifetime of
     the SNMP party that originated the message.
  3. If the authTimestamp component plus the lifetime is
     less than the authentication clock, the message is
     evaluated as unauthentic.
  4. If the authTimestamp component is less than the
     last-timestamp recorded for the originating party in the
     local database, the message is evaluated as unauthentic.
  5. If the authTimestamp component is equal to the
     last-timestamp and if the authNonce component is less
     than or equal to the nonce, the message is evaluated as
     unauthentic.
  6. The authDigest component is extracted and
     temporarily recorded.
  7. A new SnmpAuthMsg value is constructed such that
     its authDigest component is set to the private

Galvin, McCloghrie, & Davin [Page 15] RFC 1352 SNMP Security Protocols July 1992

     authentication key and its other components are set to
     the value of the corresponding components in the
     received SnmpAuthMsg value. This new
     SnmpAuthMsg value is serialized according to the
     conventions of [12] and [1]. A digest is computed over
     the octet sequence representing that serialized value
     using, for example, the algorithm specified in
     Section 2.4.1.
  8. If the computed digest value is not equal to the
     previously recorded digest value, the message is
     evaluated as unauthentic.
  9. The message is evaluated as authentic.
 10. The last-timestamp and nonce values locally recorded
     for the originating SNMP party are set to the
     authTimestamp value and the authNonce value,
     respectively.
 11. The authentication clock value locally recorded for the
     originating SNMP party is advanced to the
     authTimestamp value if this latter exceeds the
     recorded value.
 If the SnmpAuthMsg value is evaluated as unauthentic, an
 authentication failure is noted and the received message is discarded
 without further processing. Otherwise, processing of the received
 message continues as specified in [2].

5. 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 [2] that a SNMP private management communication is
 represented by an ASN.1 value with the following syntax.

Galvin, McCloghrie, & Davin [Page 16] RFC 1352 SNMP Security Protocols July 1992

    SnmpPrivMsg ::= [1] IMPLICIT SEQUENCE {
      privDst
         OBJECT IDENTIFIER,
      privData
         [1] IMPLICIT OCTET STRING
    }
 For each SnmpPrivMsg value that represents a SNMP private management
 communication, the following statements are true:
   o Its privDst component is called the privacy destination
     and identifies the SNMP 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 [12] and [1]) of a SNMP
     authenticated management communication.

5.1 Generating a Message

 This section describes the behavior of a SNMP protocol entity when it
 communicates with a SNMP party for which the privacy protocol is
 administratively specified as the Symmetric Privacy Protocol. Insofar
 as the behavior of a SNMP protocol entity when transmitting a
 protocol message is defined generically in [2], only those aspects of
 that behavior that are specific to the Symmetric Privacy Protocol are
 described below. In particular, this section describes the
 encapsulation of a SNMP authenticated management communication into a
 SNMP private management communication.
 According to [2], 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
 SNMP party receiving the message.  When the relevant privacy protocol
 is the Symmetric Privacy Protocol, the procedure performed by a SNMP
 protocol entity whenever a management communication is to be
 transmitted by a SNMP 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 SNMP party receiving the message

Galvin, McCloghrie, & Davin [Page 17] RFC 1352 SNMP Security Protocols July 1992

     (represented, for example, according to the conventions
     defined in Section 2.4.2).
  3. The SnmpAuthMsg value is serialized according to the
     conventions of [12] and [1].
  4. The octet sequence representing the serialized
     SnmpAuthMsg value is encrypted using, for example,
     the algorithm specified in Section 2.4.2 and the
     extracted private privacy key.
  5. The privData component is set to the encrypted value.
    As set forth in [2], the SnmpPrivMsg value is then serialized
    and transmitted to the receiving SNMP party.

5.2 Receiving a Message

 This section describes the behavior of a SNMP protocol entity when it
 acts as a SNMP party for which the privacy protocol is
 administratively specified as the Symmetric Privacy Protocol. Insofar
 as the behavior of a SNMP protocol entity when receiving a protocol
 message is defined generically in [2], only those aspects of that
 behavior that are specific to the Symmetric Privacy Protocol are
 described below.
 According to [2], 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 SNMP party receiving the
 message. When the relevant privacy protocol is the Symmetric Privacy
 Protocol, the procedure performed by a SNMP protocol entity whenever
 a management communication is received by a SNMP party is as follows.
  1. The local database is consulted to determine the private
     privacy key of the SNMP party receiving the message
     (represented, for example, according to the conventions
     defined in Section 2.4.2).
  2. The contents octets of the privData component are
     decrypted using, for example, the algorithm specified in
     Section 2.4.2 and the extracted private privacy key.
    Processing of the received message continues as specified in [2].

Galvin, McCloghrie, & Davin [Page 18] RFC 1352 SNMP Security Protocols July 1992

6. Clock and Secret Distribution

 The protocols described in Sections 4 and 5 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
     last-timestamp and 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. However,
     changing the authentication clock and the private
     authentication key is not sufficient to ensure proper
     operation. If the last-timestamp is not reduced similarly
     to the authentication clock, no message will be
     considered authentic until the value of the authentication
     clock exceeds the value of the last-timestamp.
   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. In particular, if the secrets are
     distributed via a network, the secrets must be protected
     with a protocol that supports confidentiality, e.g., the
     Symmetric Privacy Protocol. Further, 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 SNMP protocol entity that
     assumes the role of a responsible management station.
     This management station is responsible for ensuring that

Galvin, McCloghrie, & Davin [Page 19] RFC 1352 SNMP Security Protocols July 1992

     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 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 SNMP
 protocol entity is initially configured. The next two sections
 describe one strategy for distributing clock values and one for
 determining a synchronized clock value among SNMP parties supporting
 the Digest Authentication Protocol. For SNMP 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 SNMP protocol entity recovers from a "crash."

6.1 Initial Configuration

 This section describes the initial configuration of a SNMP protocol
 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 SNMP 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 SNMP 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 SNMP parties for each SNMP
 protocol entity, one pair of which may be used initially to configure
 all other SNMP parties.

Galvin, McCloghrie, & Davin [Page 20] RFC 1352 SNMP Security Protocols July 1992

 In fact, there is a minimal, useful set of SNMP 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 SNMP party for which the authentication protocol and
     privacy protocol are the values noAuth and noPriv,
     respectively,
   o a SNMP party for which the authentication protocol
     identifies the mechanism defined in Section 2.4.1 and its
     privacy protocol is the value noPriv, and
   o a SNMP party for which the authentication protocol and
     privacy protocol identify the mechanisms defined in
     Section 2.4.1 and Section 2.4.2, respectively.
 The last of these SNMP parties in both the responsible management
 station and the managed agent could be used to configure all other
 SNMP parties. It is the only suitable party for this purpose because
 it is the only party that supports data confidentiality, which is
 necessary in order to protect the distributed secrets from disclosure
 to unauthorized entities.
 Configuring one pair of SNMP 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.
 All 6 parties should be configured in each new managed agent and its
 responsible management station. The responsible management station
 should be configured first, since the management station 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 SNMP party to be configured. Some of these values
     may be computed by the responsible management

Galvin, McCloghrie, & Davin [Page 21] RFC 1352 SNMP Security Protocols July 1992

     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 6.3 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 those secrets with a
     strategy that uses a protocol that protects them from
     disclosure, e.g., Symmetric Privacy Protocol (see
     Section 6.4). 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, then automatic maintenance and
 configuration of parties is not possible, i.e., the last step above
 is not possible. The secrets can only be changed by a physical visit
 to the device.
 If there are other SNMP protocol entities requiring knowledge of the
 secrets, the responsible management station must distribute the
 information upon completion of the initial configuration. The
 mechanism used must protect the secrets from disclosure to
 unauthorized entities. The Symmetric Privacy Protocol, for example,
 is an acceptable mechanism.

6.2 Clock Distribution

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

Galvin, McCloghrie, & Davin [Page 22] RFC 1352 SNMP Security Protocols July 1992

   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 SNMP 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 SNMP 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.
 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
 6.3.
 In addition to correcting skewed notions of authentication clocks,
 every SNMP entity must react correctly as an authentication clock
 approaches its maximal value. If the authentication clock for a
 particular SNMP 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 SNMP protocol 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. In this case, the

Galvin, McCloghrie, & Davin [Page 23] RFC 1352 SNMP Security Protocols July 1992

 nonce value may be used to distinguish multiple messages.
 The value of the authentication clock for a particular SNMP party
 must never be altered such that its new value is less than its old
 value, unless its last-timestamp and private authentication key are
 also altered at the same time.

6.3 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 SNMP
 entity in a managed agent; suppose that party mgrParty is realized by
 the SNMP 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.
  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 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 1, condition 4, or both of those
 conditions are manifest.
  1. The responsible management station saves its existing
     notions of the authentication clocks for the two parties
     agentParty and mgrParty.
  2. The responsible management station retrieves the
     authentication clock values for both agentParty and
     mgrParty from the agent. This retrieval must be an

Galvin, McCloghrie, & Davin [Page 24] RFC 1352 SNMP Security Protocols July 1992

     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 management station's notion of the authentication
     clock for agentParty exceeds the notion just retrieved
     from the agent by more than the amount of the
     communications delay between the two protocol entities,
     then condition 1 is manifest. The recommended estimate
     of communication delay in this context is one half of the
     lifetime value recorded for agentParty.
  4. 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.
  5. If condition 1 is manifest, then the responsible
     management station sends an authenticated
     management operation to the agent that advances the
     agent's notion of the authentication clock for
     agentParty to be equal to the management station's
     notion. If this management operation fails, then the
     management station restores its previously saved notions
     of the clock values, and the clock adjustment procedure
     is aborted without further processing.
  6. The responsible management station retrieves the
     authentication clock values for both agentParty and
     mgrParty from the agent. This retrieval must be an
     authenticated request, in order that the management
     station may verify that the clock values are properly
     synchronized. If this authenticated query fails, then the
     management station restores its previously saved notions
     of the clock values, and the clock adjustment procedure
     is aborted without further processing. Otherwise, clock
     synchronization has been successfully realized.
 It is important to note step 4 above must be completed before
 attempting step 5. Otherwise, the agent may evaluate the request in
 step 5 as unauthentic. Similarly, step 5 above must be completed
 before attempting step 6. Otherwise, the management station may
 evaluate the query response in step 6 as unauthentic.

Galvin, McCloghrie, & Davin [Page 25] RFC 1352 SNMP Security Protocols July 1992

 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 management operations that
 are 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.

6.4 Secret Distribution

 This section describes one strategy by which a SNMP protocol entity
 that supports both the Digest Authentication Protocol and the
 Symmetric Privacy Protocol can change the secrets for a particular
 SNMP party.
 The frequency with which the secrets of a SNMP 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 7).
 Note that, owing to both administrative and automatic advances of the
 authentication clock described in this memo, the authentication clock
 for a SNMP 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 SNMP
 party.
  1. The responsible management station generates a new
     secret value.
  2. The responsible management station encapsulates a
     SNMP Set request in a SNMP private management
     communication with at least the following properties.
      o Its source supports the Digest Authentication
        Protocol and the Symmetric Privacy Protocol.
      o Its destination supports the Symmetric Privacy
        Protocol and the Digest Authentication Protocol.
  3. The SNMP private management communication is
     transmitted to its destination.
  4. Upon receiving the request, the recipient processes the

Galvin, McCloghrie, & Davin [Page 26] RFC 1352 SNMP Security Protocols July 1992

     message according to [1] and [2].
  5. The recipient encapsulates a SNMP Set response in a
     SNMP private management communication with at least
     the following properties.
      o Its source supports the Digest Authentication
        Protocol and the Symmetric Privacy Protocol.
      o Its destination supports the Symmetric Privacy
        Protocol and the Digest Authentication Protocol.
  6. The SNMP 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.
 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 using the public values may make use of this strategy
 directly.)
 One other scenario worthy of mention is using a SNMP party to change

Galvin, McCloghrie, & Davin [Page 27] RFC 1352 SNMP Security Protocols July 1992

 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.

6.5 Crash Recovery

 This section describes the requirements for SNMP protocol entities in
 connection with recovery from system crashes or other service
 interruptions.
 For each SNMP party in the local database for a particular SNMP
 protocol 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 protocol
 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 SNMP party is a critical component of
 the overall security of the protocols. The inclusion of a reliable
 representation of a clock in a SNMP protocol entity enhances overall
 security. A reliable clock representation continues to increase
 according to the passage of time, even when the local SNMP protocol
 entity -- due to power loss or other system failure -- may not be
 operating.  An example of a reliable clock representation is that
 provided by battery-powered clock-calendar devices incorporated into
 some contemporary systems. 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

Galvin, McCloghrie, & Davin [Page 28] RFC 1352 SNMP Security Protocols July 1992

 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 6.3 would
 then apply.
 If a managed network element supports a reliable clock
 representation, recovering from a crash requires few special actions.
 Upon recovery, those attributes of each SNMP 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 2.4.1.
   o The last-timestamp is initialized to the value of the
     authentication clock.
   o The nonce is initialized to zero.
   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 2.4.2.
 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 when received by the recipient.
 If, alternatively, a managed network element does not support a
 reliable clock representation, then those attributes of each SNMP
 party that do not enjoy non-volatile 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 2.4.1.

Galvin, McCloghrie, & Davin [Page 29] RFC 1352 SNMP Security Protocols July 1992

   o The authentication clock is initialized to the maximal
     time value.
   o The last-timestamp is initialized to the maximal time
     value.
   o The nonce is initialized to zero.
   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 2.4.2.
 The only authenticated request a management station should generate
 for a party in this initial state is one that alters the value of at
 least its authentication clock, private authentication key, and
 lifetime (if that was not retained). In order to reset these values,
 the responsible management station must set the authentication
 timestamp in the message to the maximal time value. The nonce value
 may be used to distinguish multiple messages.

7. 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.

7.1 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 SNMP 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).

Galvin, McCloghrie, & Davin [Page 30] RFC 1352 SNMP Security Protocols July 1992

   o A management station should not interpret an agent's
     lack of response to an authenticated SNMP 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 SNMP 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.
   o The lifetime value for a SNMP 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 SNMP messages. The implementation of a
     management station may, when explicitly authorized,
     provide for dynamic adjustment of the lifetime in order
     to accommodate changing network conditions.
   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.
     When using the noAuth protocol, no message ordering
     is imposed by the SNMP. Messages may be received in
     any order relative to their time of generation and each
     will be processed in the ordered received. In contrast,
     the security protocols guarantee that received messages
     are ordered insofar as each received message must have
     been sent subsequent to the sending of a previously
     received message.
     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. During the
     period of time this message is valid, if the management
     station sends another authenticated message to the

Galvin, McCloghrie, & Davin [Page 31] RFC 1352 SNMP Security Protocols July 1992

     managed agent that is received and processed prior to
     the first message, the first message will be considered
     unauthentic when it is received by the managed agent.
     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. A management
     station implementation may choose to prevent the loss
     of messages resulting from re-ordering when using the
     security protocols defined in this memo by delaying
     sending successive messages.
   o The frequency with which the secrets of a SNMP party
     should be changed is indirectly related to the frequency
     of their use.
     Protecting the secrets from disclosure is critical to the
     overall security of the protocols. Frequent use of a secret
     provides a continued source of data that may be useful
     to a cryptanalyst in exploiting known or perceived
     weaknesses in an algorithm. Frequent changes to the
     secret avoid this vulnerability.
     Changing a secret after each use is 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 SNMP
     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 SNMP party must be known to all other
     SNMP 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 SNMP
     protocol entities with knowledge of a particular secret
     numbers more than two, data origin cannot be reliably

Galvin, McCloghrie, & Davin [Page 32] RFC 1352 SNMP Security Protocols July 1992

     authenticated because it is impossible to determine with
     any assurance which entity of that set may be the
     originator of a particular SNMP message. Thus, the
     greatest degree of security is afforded by configurations
     in which the secrets for each SNMP party are known to
     at most two protocol entities.

7.2 Conformance

 A SNMP protocol entity implementation that claims conformance to this
 memo must satisfy the following requirements:
  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 2.4.1.
  3. It must include in its local database at least one SNMP
     party with the following parameters set as follows:
      o partyAuthProtocol is set to noAuth and
      o partyPrivProtocol is set to noPriv.
     This party must have a MIB view [2] 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 SNMP 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 2.4 specifies constraints for the
        chosen mechanisms.

Galvin, McCloghrie, & Davin [Page 33] RFC 1352 SNMP Security Protocols July 1992

    (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
        SNMP messages with respect to that party. 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).

7.3 Protocol Correctness

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

Galvin, McCloghrie, & Davin [Page 34] RFC 1352 SNMP Security Protocols July 1992

  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 SNMP party are known only
     to authorized SNMP protocol entities.
  6. Local notions of the authentication clock for a particular
     SNMP 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.  Pseudocode
 fragments are provided where appropriate to exemplify possible
 implementations; they are intended to be self-explanatory.

7.3.1 Clock Monotonicity Mechanism

 By pairing each sequence of a clock's values with a unique key, the
 protocols partially realize goals 3 and 4, 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.

Galvin, McCloghrie, & Davin [Page 35] RFC 1352 SNMP Security Protocols July 1992

7.3.2 Data Integrity Mechanism

 The protocols require computation of a message digest computed over
 the SNMP 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 SNMP message modification which appends extraneous material
 to the end of such messages. However, owing to the representation of
 SNMP 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 SNMP 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 protocol.

7.3.3 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.

7.3.4 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.

Galvin, McCloghrie, & Davin [Page 36] RFC 1352 SNMP Security Protocols July 1992

 Pseudocode Fragment. 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 SNMP Set operation.
 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;
     }
 }

7.3.5 Ordered Delivery Mechanism

 The definition of the Digest Authentication Protocol requires that,
 if the timestamp value on a received message does not exceed the
 timestamp of the most recent validated message locally delivered from
 the originating party, then that message is not delivered. Otherwise,
 the record of the timestamp for the most recent locally delivered
 validated message is updated.
 if (msgIsValidated) {
     if (timestampOfReceivedMsg >
        party->timestampOfLastDeliveredMsg) {

Galvin, McCloghrie, & Davin [Page 37] RFC 1352 SNMP Security Protocols July 1992

        party->timestampOfLastDeliveredMsg =
           timestampOfReceivedMsg;
     }
     else {
        msgIsValidated = FALSE;
     }
 }
 Although not explicitly represented in the pseudocode above, in the
 Digest Authentication Protocol, the ordered delivery mechanism must
 ensure that, when the authentication timestamp of the received
 message is equal to the last-timestamp, received messages continue to
 be delivered as long as their nonce values are monotonically
 increasing. By virtue of this mechanism, the protocols realize goal
 4.

7.3.6 Message Timeliness Mechanism

 The definition of the SNMP 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 SNMP party, the message is
 not delivered.
 if (timestampOfReceivedMsg +
        party->administrativeLifetime <=
        party->localNotionOfClock) {
        msgIsValidated = FALSE;
 }
 By virtue of this mechanism, the protocols realize goal 3. In cases
 in which the local notions of a particular SNMP 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.

7.3.7 Selective Clock Acceleration Mechanism

 The definition of the SNMP security protocols requires that, if the
 timestamp value on a received, validated message exceeds the local
 notion of the clock for the originating party, then that notion is
 adjusted forward to correspond to said timestamp value. This
 mechanism is neither strictly necessary nor sufficient to the

Galvin, McCloghrie, & Davin [Page 38] RFC 1352 SNMP Security Protocols July 1992

 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 the
 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 the 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 management station needs to react
 appropriately (e.g., by administratively resynchronizing local
 notions of the clock in conjunction with a key change). In contrast,
 the delivery of SNMP 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.

7.3.8 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 5.

8. Acknowledgements

 The authors would like to thank the members of the SNMP Security
 Working Group of the IETF for their patience and comments. Special
 thanks go to Jeff Case who provided the first implementation of the
 protocols. Dave Balenson, John Linn, Dan Nessett, and all the members
 of the Privacy and Security Research Group provided many valuable and

Galvin, McCloghrie, & Davin [Page 39] RFC 1352 SNMP Security Protocols July 1992

 detailed comments.

9. References

 [1] Case, J., M. Fedor, M. Schoffstall, and J. Davin, The Simple
     Network Management Protocol", RFC 1157, University of Tennessee
     at Knoxville, Performance Systems International, Performance
     Systems International, and the MIT Laboratory for Computer
     Science, May 1990.  (Obsoletes RFC 1098.)
 [2] Davin, J., Galvin, J., and K. McCloghrie, "SNMP Administrative
     Model", RFC 1351, MIT Laboratory for Computer Science, Trusted
     Information Systems, Inc., Hughes LAN Systems, Inc., July 1992.
 [3] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, MIT
     Laboratory for Computer Science, April 1992.
 [4] McCloghrie, K., Davin, J., and J. Galvin, "Definitions of Managed
     Objects for Administration of SNMP Parties", RFC 1353, Hughes LAN
     Systems, Inc., MIT Laboratory for Computer Science, Trusted
     Information Systems, Inc., July 1992.
 [5] FIPS Publication 46-1, "Data Encryption Standard", National
     Institute of Standards and Technology, Federal Information
     Processing Standard (FIPS); Supersedes FIPS Publication 46,
     January 15, 1977; Reaffirmed January 22, 1988.
 [6] ANSI X3.92-1981, "Data Encryption Algorithm", American National
     Standards Institute, December 30, 1980.
 [7] FIPS Publication 81, "DES Modes of Operation", National Institute
     of Standards and Technology, December 2, 1980, Federal
     Information Processing Standard (FIPS).
 [8] ANSI X3.106-1983, "Data Encryption Algorithm - Modes of
     Operation", American National Standards Institute, May 16, 1983.
 [9] FIPS Publication 74, "Guidelines for Implementing and Using the
     NBS Data Encryption Standard", National Institute of Standards
     and Technology, April 1, 1981.  Federal Information Processing
     Standard (FIPS).
[10] Special Publication 500-20, "Validating the Correctness of
     Hardware Implementations of the NBS Data Encryption Standard",
     National Institute of Standards and Technology.
[11] Special Publication 500-61, "Maintenance Testing for the Data
     Encryption Standard", National Institute of Standards and

Galvin, McCloghrie, & Davin [Page 40] RFC 1352 SNMP Security Protocols July 1992

     Technology, August 1980.
[12] Information Processing -- Open Systems Interconnection --
     Specification of Basic Encoding Rules for Abstract Syntax
     Notation One (ASN.1), International Organization for
     Standardization/International Electrotechnical Institute, 1987,
     International Standard 8825.

10. Authors' Addresses

     James M. Galvin
     Trusted Information Systems, Inc.
     3060 Washington Road, Route 97
     Glenwood, MD 21738
     Phone:  (301) 854-6889
     EMail:  galvin@tis.com
     Keith McCloghrie
     Hughes LAN Systems, Inc.
     1225 Charleston Road
     Mountain View, CA 94043
     Phone:  (415) 966-7934
     EMail:  kzm@hls.com
     James R. Davin
     MIT Laboratory for Computer Science
     545 Technology Square
     Cambridge, MA 02139
     Phone:  (617) 253-6020
     EMail:  jrd@ptt.lcs.mit.edu

Galvin, McCloghrie, & Davin [Page 41]

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