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RFC 1446

          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)


          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
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



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