Network Working Group J. Galvin Request for Comments: 1446 Trusted Information Systems K. McCloghrie Hughes LAN Systems April 1993
Security Protocols for version 2 of the Simple Network Management Protocol (SNMPv2)
Status of this Memo
This RFC specifes an IAB standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "IAB Official Protocol Standards" for the standardization state and status of this protocol. Distribution of this memo is unlimited.
A network management system contains: several (potentially many) nodes, each with a processing entity, termed an agent, which has access to management instrumentation; at least one management station; and, a management protocol, used to convey management information between the agents and management stations. Operations of the protocol are carried out under an administrative framework which defines both authentication and authorization policies.
Network management stations execute management applications which monitor and control network elements. Network elements are devices such as hosts, routers, terminal servers, etc., which are monitored and controlled through access to their management information.
In the Administrative Model for SNMPv2 document [1], each SNMPv2 party is, by definition, associated with a single authentication protocol and a single privacy protocol. It is the purpose of this document, Security Protocols for SNMPv2, to define one such authentication and one such privacy protocol.
The authentication protocol provides a mechanism by which SNMPv2 management communications transmitted by the party may be reliably identified as having originated from that party. The authentication protocol defined in this memo also reliably determines that the message received is the message that was sent.
The privacy protocol provides a mechanism by which SNMPv2 management communications transmitted to said party are protected from disclosure. The privacy protocol in this memo specifies that only authenticated messages may be protected from disclosure.
These protocols are secure alternatives to the so-called "trivial" protocol defined in [2].
USE OF THE TRIVIAL PROTOCOL ALONE DOES NOT CONSTITUTE SECURE NETWORK MANAGEMENT. THEREFORE, A NETWORK MANAGEMENT SYSTEM THAT IMPLEMENTS ONLY THE TRIVIAL PROTOCOL IS NOT CONFORMANT TO THIS SPECIFICATION.
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The Digest Authentication Protocol is described in Section 3. It provides a data integrity service by transmitting a message digest - computed by the originator and verified by the recipient - with each SNMPv2 message. The data origin authentication service is provided by prefixing the message with a secret value known only to the originator and recipient, prior to computing the digest. Thus, data integrity is supported explicitly while data origin authentication is supported implicitly in the verification of the digest.
The Symmetric Privacy Protocol is described in Section 4. It protects messages from disclosure by encrypting their contents according to a secret cryptographic key known only to the originator and recipient. The additional functionality afforded by this protocol is assumed to justify its additional computational cost.
The Digest Authentication Protocol depends on the existence of loosely synchronized clocks between the originator and recipient of a message. The protocol specification makes no assumptions about the strategy by which such clocks are synchronized. Section 5.3 presents one strategy that is particularly suited to the demands of SNMP network management.
Both protocols described here require the sharing of secret information between the originator of a message and its recipient. The protocol specifications assume the existence of the necessary secrets. The selection of such secrets and their secure distribution to appropriate parties may be accomplished by a variety of strategies. Section 5.4 presents one such strategy that is particularly suited to the demands of SNMP network management.
1.1. A Note on Terminology
For the purpose of exposition, the original Internet-standard Network Management Framework, as described in RFCs 1155, 1157, and 1212, is termed the SNMP version 1 framework (SNMPv1). The current framework is termed the SNMP version 2 framework (SNMPv2).
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1.2. Threats
Several of the classical threats to network protocols are applicable to the network management problem and therefore would be applicable to any SNMPv2 security protocol. Other threats are not applicable to the network management problem. This section discusses principal threats, secondary threats, and threats which are of lesser importance.
The principal threats against which any SNMPv2 security protocol should provide protection are:
Modification of Information The SNMPv2 protocol provides the means for management stations to interrogate and to manipulate the value of objects in a managed agent. The modification threat is the danger that some party may alter in-transit messages generated by an authorized party in such a way as to effect unauthorized management operations, including falsifying the value of an object.
Masquerade The SNMPv2 administrative model includes an access control model. Access control necessarily depends on knowledge of the origin of a message. The masquerade threat is the danger that management operations not authorized for some party may be attempted by that party by assuming the identity of another party that has the appropriate authorizations.
Two secondary threats are also identified. The security protocols defined in this memo do provide protection against:
Message Stream Modification The SNMPv2 protocol is based upon a connectionless transport service which may operate over any subnetwork service. The re-ordering, delay or replay of messages can and does occur through the natural operation of many such subnetwork services. The message stream modification threat is the danger that messages may be maliciously re-ordered, delayed or replayed to an extent which is greater than can occur through the natural operation of a subnetwork service, in order to effect unauthorized management operations.
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Disclosure The disclosure threat is the danger of eavesdropping on the exchanges between managed agents and a management station. Protecting against this threat is mandatory when the SNMPv2 is used to create new SNMPv2 parties [1] on which subsequent secure operation might be based. Protecting against the disclosure threat may also be required as a matter of local policy.
There are at least two threats that a SNMPv2 security protocol need not protect against. The security protocols defined in this memo do not provide protection against:
Denial of Service A SNMPv2 security protocol need not attempt to address the broad range of attacks by which service to authorized parties is denied. Indeed, such denial-of-service attacks are in many cases indistinguishable from the type of network failures with which any viable network management protocol must cope as a matter of course.
Traffic Analysis In addition, a SNMPv2 security protocol need not attempt to address traffic analysis attacks. Indeed, many traffic patterns are predictable - agents may be managed on a regular basis by a relatively small number of management stations - and therefore there is no significant advantage afforded by protecting against traffic analysis.
1.3. Goals and Constraints
Based on the foregoing account of threats in the SNMP network management environment, the goals of a SNMPv2 security protocol are enumerated below.
(1) The protocol should provide for verification that each received SNMPv2 message has not been modified during its transmission through the network in such a way that an unauthorized management operation might result.
(2) The protocol should provide for verification of the identity of the originator of each received SNMPv2 message.
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(3) The protocol should provide that the apparent time of generation for each received SNMPv2 message is recent.
(4) The protocol should provide, when necessary, that the contents of each received SNMPv2 message are protected from disclosure.
In addition to the principal goal of supporting secure network management, the design of any SNMPv2 security protocol is also influenced by the following constraints:
(1) When the requirements of effective management in times of network stress are inconsistent with those of security, the former are preferred.
(2) Neither the security protocol nor its underlying security mechanisms should depend upon the ready availability of other network services (e.g., Network Time Protocol (NTP) or secret/key management protocols).
(3) A security mechanism should entail no changes to the basic SNMP network management philosophy.
1.4. Security Services
The security services necessary to support the goals of a SNMPv2 security protocol are as follows.
Data Integrity is the provision of the property that data has not been altered or destroyed in an unauthorized manner, nor have data sequences been altered to an extent greater than can occur non-maliciously.
Data Origin Authentication is the provision of the property that the claimed origin of received data is corroborated.
Data Confidentiality is the provision of the property that information is not made available or disclosed to unauthorized individuals, entities, or processes.
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The protocols specified in this memo require both data integrity and data origin authentication to be used at all times. For these protocols, it is not possible to realize data integrity without data origin authentication, nor is it possible to realize data origin authentication without data integrity.
Further, there is no provision for data confidentiality without both data integrity and data origin authentication.
1.5. Mechanisms
The security protocols defined in this memo employ several types of mechanisms in order to realize the goals and security services described above:
o In support of data integrity, a message digest algorithm is required. A digest is calculated over an appropriate portion of a SNMPv2 message and included as part of the message sent to the recipient.
o In support of data origin authentication and data integrity, the portion of a SNMPv2 message that is digested is first prefixed with a secret value shared by the originator of that message and its intended recipient.
o To protect against the threat of message delay or replay, (to an extent greater than can occur through normal operation), a timestamp value is included in each message generated. A recipient evaluates the timestamp to determine if the message is recent. This protection against the threat of message delay or replay does not imply nor provide any protection against unauthorized deletion or suppression of messages. Other mechanisms defined independently of the security protocol can also be used to detect message replay (e.g., the request-id [2]), or for set operations, the re-ordering, replay, deletion, or suppression of messages (e.g., the MIB variable snmpSetSerialNo [14]).
o In support of data confidentiality, a symmetric encryption algorithm is required. An appropriate portion of the message is encrypted prior to being transmitted to
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its recipient.
The security protocols in this memo are defined independently of the particular choice of a message digest and encryption algorithm - owing principally to the lack of a suitable metric by which to evaluate the security of particular algorithm choices. However, in the interests of completeness and in order to guarantee interoperability, Sections 1.5.1 and 1.5.2 specify particular choices, which are considered acceptably secure as of this writing. In the future, this memo may be updated by the publication of a memo specifying substitute or alternate choices of algorithms, i.e., a replacement for or addition to the sections below.
1.5.1. Message Digest Algorithm
In support of data integrity, the use of the MD5 [3] message digest algorithm is chosen. A 128-bit digest is calculated over the designated portion of a SNMPv2 message and included as part of the message sent to the recipient.
An appendix of [3] contains a C Programming Language implementation of the algorithm. This code was written with portability being the principal objective. Implementors may wish to optimize the implementation with respect to the characteristics of their hardware and software platforms.
The use of this algorithm in conjunction with the Digest Authentication Protocol (see Section 3) is identified by the ASN.1 object identifier value v2md5AuthProtocol, defined in [4]. (Note that this protocol is a modified version of the md5AuthProtocol protocol defined in RFC 1352.)
For any SNMPv2 party for which the authentication protocol is v2md5AuthProtocol, the size of its private authentication key is 16 octets.
Within an authenticated management communication generated by such a party, the size of the authDigest component of that communication (see Section 3) is 16 octets.
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1.5.2. Symmetric Encryption Algorithm
In support of data confidentiality, the use of the Data Encryption Standard (DES) in the Cipher Block Chaining mode of operation is chosen. The designated portion of a SNMPv2 message is encrypted and included as part of the message sent to the recipient.
Two organizations have published specifications defining the DES: the National Institute of Standards and Technology (NIST) [5] and the American National Standards Institute [6]. There is a companion Modes of Operation specification for each definition (see [7] and [8], respectively).
The NIST has published three additional documents that implementors may find useful.
o There is a document with guidelines for implementing and using the DES, including functional specifications for the DES and its modes of operation [9].
o There is a specification of a validation test suite for the DES [10]. The suite is designed to test all aspects of the DES and is useful for pinpointing specific problems.
o There is a specification of a maintenance test for the DES [11]. The test utilizes a minimal amount of data and processing to test all components of the DES. It provides a simple yes-or-no indication of correct operation and is useful to run as part of an initialization step, e.g., when a computer reboots.
The use of this algorithm in conjunction with the Symmetric Privacy Protocol (see Section 4) is identified by the ASN.1 object identifier value desPrivProtocol, defined in [4].
For any SNMPv2 party for which the privacy protocol is desPrivProtocol, the size of the private privacy key is 16 octets, of which the first 8 octets are a DES key and the second 8 octets are a DES Initialization Vector. The 64-bit DES key in the first 8 octets of the private key is a 56 bit quantity used directly by the algorithm plus 8 parity bits - arranged so that one parity bit is the least significant bit of each octet. The setting of the parity bits is ignored.
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The length of the octet sequence to be encrypted by the DES must be an integral multiple of 8. When encrypting, the data should be padded at the end as necessary; the actual pad value is insignificant.
If the length of the octet sequence to be decrypted is not an integral multiple of 8 octets, the processing of the octet sequence should be halted and an appropriate exception noted. Upon decrypting, the padding should be ignored.
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2. SNMPv2 Party
Recall from [1] that a SNMPv2 party is a conceptual, virtual execution context whose operation is restricted (for security or other purposes) to an administratively defined subset of all possible operations of a particular SNMPv2 entity. A SNMPv2 entity is an actual process which performs network management operations by generating and/or responding to SNMPv2 protocol messages in the manner specified in [12]. Architecturally, every SNMPv2 entity maintains a local database that represents all SNMPv2 parties known to it.
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A SNMPv2 party may be represented by an ASN.1 value with the following syntax:
For each SnmpParty value that represents a SNMPv2 party, the generic significance of each of its components is defined in [1]. For each SNMPv2 party that supports the generation of messages using the Digest Authentication Protocol, additional, special significance is attributed to certain components of that party's representation:
o Its partyAuthProtocol component is called the authentication protocol and identifies a combination of the Digest Authentication Protocol with a particular digest algorithm (such as that defined in Section 1.5.1). This combined mechanism is used to authenticate the origin and integrity of all messages generated by the party.
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o Its partyAuthClock component is called the authentication clock and represents a notion of the current time that is specific to the party.
o Its partyAuthPrivate component is called the private authentication key and represents any secret value needed to support the Digest Authentication Protocol and associated digest algorithm.
o Its partyAuthPublic component is called the public authentication key and represents any public value that may be needed to support the authentication protocol. This component is not significant except as suggested in Section 5.4.
o Its partyAuthLifetime component is called the lifetime and represents an administrative upper bound on acceptable delivery delay for protocol messages generated by the party.
For each SNMPv2 party that supports the receipt of messages via the Symmetric Privacy Protocol, additional, special significance is attributed to certain components of that party's representation:
o Its partyPrivProtocol component is called the privacy protocol and identifies a combination of the Symmetric Privacy Protocol with a particular encryption algorithm (such as that defined in Section 1.5.2). This combined mechanism is used to protect from disclosure all protocol messages received by the party.
o Its partyPrivPrivate component is called the private privacy key and represents any secret value needed to support the Symmetric Privacy Protocol and associated encryption algorithm.
o Its partyPrivPublic component is called the public privacy key and represents any public value that may be needed to support the privacy protocol. This component is not significant except as suggested in Section 5.4.
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3. Digest Authentication Protocol
This section describes the Digest Authentication Protocol. It provides both for verifying the integrity of a received message (i.e., the message received is the message sent) and for verifying the origin of a message (i.e., the reliable identification of the originator). The integrity of the message is protected by computing a digest over an appropriate portion of a message. The digest is computed by the originator of the message, transmitted with the message, and verified by the recipient of the message.
A secret value known only to the originator and recipient of the message is prefixed to the message prior to the digest computation. Thus, the origin of the message is known implicitly with the verification of the digest.
A requirement on parties using this Digest Authentication Protocol is that they shall not originate messages for transmission to any destination party which does not also use this Digest Authentication Protocol. This restriction excludes undesirable side effects of communication between a party which uses these security protocols and a party which does not.
Recall from [1] that a SNMPv2 management communication is represented by an ASN.1 value with the following syntax:
For each SnmpMgmtCom value that represents a SNMPv2 management communication, the following statements are true:
o Its dstParty component is called the destination and identifies the SNMPv2 party to which the communication is directed.
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o Its srcParty component is called the source and identifies the SNMPv2 party from which the communication is originated.
o Its context component identifies the SNMPv2 context containing the management information referenced by the communication.
o Its pdu component has the form and significance attributed to it in [12].
Recall from [1] that a SNMPv2 authenticated management communication is represented by an ASN.1 value with the following syntax:
SnmpAuthMsg ::= [1] IMPLICIT SEQUENCE { authInfo ANY, - defined by authentication protocol authData SnmpMgmtCom }
For each SnmpAuthMsg value that represents a SNMPv2 authenticated management communication, the following statements are true:
o Its authInfo component is called the authentication information and represents information required in support of the authentication protocol used by both the SNMPv2 party originating the message, and the SNMPv2 party receiving the message. The detailed significance of the authentication information is specific to the authentication protocol in use; it has no effect on the application semantics of the communication other than its use by the authentication protocol in determining whether the communication is authentic or not.
o Its authData component is called the authentication data
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and represents a SNMPv2 management communication.
In support of the Digest Authentication Protocol, an authInfo component is of type AuthInformation:
For each AuthInformation value that represents authentication information, the following statements are true:
o Its authDigest component is called the authentication digest and represents the digest computed over an appropriate portion of the message, where the message is temporarily prefixed with a secret value for the purposes of computing the digest.
o Its authSrcTimestamp component is called the authentication timestamp and represents the time of the generation of the message according to the partyAuthClock of the SNMPv2 party that originated it. Note that the granularity of the authentication timestamp is 1 second.
o Its authDstTimestamp component is called the authentication timestamp and represents the time of the generation of the message according to the partyAuthClock of the SNMPv2 party that is to receive it. Note that the granularity of the authentication timestamp is 1 second.
3.1. Generating a Message
This section describes the behavior of a SNMPv2 entity when it acts as a SNMPv2 party for which the authentication protocol is administratively specified as the Digest Authentication Protocol. Insofar as the behavior of a SNMPv2 entity when transmitting protocol messages is defined generically in [1], only those aspects of that behavior that are specific to the Digest Authentication Protocol are described below. In
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particular, this section describes the encapsulation of a SNMPv2 management communication into a SNMPv2 authenticated management communication.
According to Section 3.1 of [1], a SnmpAuthMsg value is constructed during Step 3 of generic processing. In particular, it states the authInfo component is constructed according to the authentication protocol identified for the SNMPv2 party originating the message. When the relevant authentication protocol is the Digest Authentication Protocol, the procedure performed by a SNMPv2 entity whenever a management communication is to be transmitted by a SNMPv2 party is as follows.
(1) The local database is consulted to determine the authentication clock and private authentication key (extracted, for example, according to the conventions defined in Section 1.5.1) of the SNMPv2 party originating the message. The local database is also consulted to determine the authentication clock of the receiving SNMPv2 party.
(2) The authSrcTimestamp component is set to the retrieved authentication clock value of the message's source. The authDstTimestamp component is set to the retrieved authentication clock value of the message's intended recipient.
(3) The authentication digest is temporarily set to the private authentication key of the SNMPv2 party originating the message. The SnmpAuthMsg value is serialized according to the conventions of [13] and [12]. A digest is computed over the octet sequence representing that serialized value using, for example, the algorithm specified in Section 1.5.1. The authDigest component is set to the computed digest value.
As set forth in [1], the SnmpAuthMsg value is then encapsulated according to the appropriate privacy protocol into a SnmpPrivMsg value. This latter value is then serialized and transmitted to the receiving SNMPv2 party.
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3.2. Receiving a Message
This section describes the behavior of a SNMPv2 entity upon receipt of a protocol message from a SNMPv2 party for which the authentication protocol is administratively specified as the Digest Authentication Protocol. Insofar as the behavior of a SNMPv2 entity when receiving protocol messages is defined generically in [1], only those aspects of that behavior that are specific to the Digest Authentication Protocol are described below.
According to Section 3.2 of [1], a SnmpAuthMsg value is evaluated during Step 9 of generic processing. In particular, it states the SnmpAuthMsg value is evaluated according to the authentication protocol identified for the SNMPv2 party that originated the message. When the relevant authentication protocol is the Digest Authentication Protocol, the procedure performed by a SNMPv2 entity whenever a management communication is received by a SNMPv2 party is as follows.
(1) If the ASN.1 type of the authInfo component is not AuthInformation, the message is evaluated as unauthentic, and the snmpStatsBadAuths counter [14] is incremented. Otherwise, the authSrcTimestamp, authDstTimestamp, and authDigest components are extracted from the SnmpAuthMsg value.
(2) The local database is consulted to determine the authentication clock, private authentication key (extracted, for example, according to the conventions defined in Section 1.5.1), and lifetime of the SNMPv2 party that originated the message.
(3) If the authSrcTimestamp component plus the lifetime is less than the authentication clock, the message is evaluated as unauthentic, and the snmpStatsNotInLifetimes counter [14] is incremented.
(4) The authDigest component is extracted and temporarily recorded.
(5) A new SnmpAuthMsg value is constructed such that its authDigest component is set to the private authentication key and its other components are set to the value of the corresponding components in the received SnmpAuthMsg
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value. This new SnmpAuthMsg value is serialized according to the conventions of [13] and [12]. A digest is computed over the octet sequence representing that serialized value using, for example, the algorithm specified in Section 1.5.1.
NOTE Because serialization rules are unambiguous but may not be unique, great care must be taken in reconstructing the serialized value prior to computing the digest. Implementations may find it useful to keep a copy of the original serialized value and then simply modify the octets which directly correspond to the placement of the authDigest component, rather than re-applying the serialization algorithm to the new SnmpAuthMsg value.
(6) If the computed digest value is not equal to the digest value temporarily recorded in step 4 above, the message is evaluated as unauthentic, and the snmpStatsWrongDigestValues counter [14] is incremented.
(7) The message is evaluated as authentic.
(8) The local database is consulted for access privileges permitted by the local access policy to the originating SNMPv2 party with respect to the receiving SNMPv2 party. If any level of access is permitted, then:
the authentication clock value locally recorded for the originating SNMPv2 party is advanced to the authSrcTimestamp value if this latter exceeds the recorded value; and,
the authentication clock value locally recorded for the receiving SNMPv2 party is advanced to the authDstTimestamp value if this latter exceeds the recorded value.
(Note that this step is conceptually independent from Steps 15-17 of Section 3.2 in [1]).
If the SnmpAuthMsg value is evaluated as unauthentic, an authentication failure is noted and the received message is
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discarded without further processing. Otherwise, processing of the received message continues as specified in [1].
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4. Symmetric Privacy Protocol
This section describes the Symmetric Privacy Protocol. It provides for protection from disclosure of a received message. An appropriate portion of the message is encrypted according to a secret key known only to the originator and recipient of the message.
This protocol assumes the underlying mechanism is a symmetric encryption algorithm. In addition, the message to be encrypted must be protected according to the conventions of the Digest Authentication Protocol.
Recall from [1] that a SNMPv2 private management communication is represented by an ASN.1 value with the following syntax:
For each SnmpPrivMsg value that represents a SNMPv2 private management communication, the following statements are true:
o Its privDst component is called the privacy destination and identifies the SNMPv2 party to which the communication is directed.
o Its privData component is called the privacy data and represents the (possibly encrypted) serialization (according to the conventions of [13] and [12]) of a SNMPv2 authenticated management communication.
4.1. Generating a Message
This section describes the behavior of a SNMPv2 entity when it communicates with a SNMPv2 party for which the privacy protocol is administratively specified as the Symmetric Privacy Protocol. Insofar as the behavior of a SNMPv2 entity when transmitting a protocol message is defined generically in [1], only those aspects of that behavior that are specific to the Symmetric Privacy Protocol are described below. In
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particular, this section describes the encapsulation of a SNMPv2 authenticated management communication into a SNMPv2 private management communication.
According to Section 3.1 of [1], a SnmpPrivMsg value is constructed during Step 5 of generic processing. In particular, it states the privData component is constructed according to the privacy protocol identified for the SNMPv2 party receiving the message. When the relevant privacy protocol is the Symmetric Privacy Protocol, the procedure performed by a SNMPv2 entity whenever a management communication is to be transmitted by a SNMPv2 party is as follows.
(1) If the SnmpAuthMsg value is not authenticated according to the conventions of the Digest Authentication Protocol, the generation of the private management communication fails according to a local procedure, without further processing.
(2) The local database is consulted to determine the private privacy key of the SNMPv2 party receiving the message (represented, for example, according to the conventions defined in Section 1.5.2).
(3) The SnmpAuthMsg value is serialized according to the conventions of [13] and [12].
(4) The octet sequence representing the serialized SnmpAuthMsg value is encrypted using, for example, the algorithm specified in Section 1.5.2 and the extracted private privacy key.
(5) The privData component is set to the encrypted value.
As set forth in [1], the SnmpPrivMsg value is then serialized and transmitted to the receiving SNMPv2 party.
4.2. Receiving a Message
This section describes the behavior of a SNMPv2 entity when it acts as a SNMPv2 party for which the privacy protocol is administratively specified as the Symmetric Privacy Protocol. Insofar as the behavior of a SNMPv2 entity when receiving a
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protocol message is defined generically in [1], only those aspects of that behavior that are specific to the Symmetric Privacy Protocol are described below.
According to Section 3.2 of [1], the privData component of a received SnmpPrivMsg value is evaluated during Step 4 of generic processing. In particular, it states the privData component is evaluated according to the privacy protocol identified for the SNMPv2 party receiving the message. When the relevant privacy protocol is the Symmetric Privacy Protocol, the procedure performed by a SNMPv2 entity whenever a management communication is received by a SNMPv2 party is as follows.
(1) The local database is consulted to determine the private privacy key of the SNMPv2 party receiving the message (represented, for example, according to the conventions defined in Section 1.5.2).
(2) The contents octets of the privData component are decrypted using, for example, the algorithm specified in Section 1.5.2 and the extracted private privacy key.
Processing of the received message continues as specified in [1].
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5. Clock and Secret Distribution
The protocols described in Sections 3 and 4 assume the existence of loosely synchronized clocks and shared secret values. Three requirements constrain the strategy by which clock values and secrets are distributed.
o If the value of an authentication clock is decreased, the private authentication key must be changed concurrently.
When the value of an authentication clock is decreased, messages that have been sent with a timestamp value between the value of the authentication clock and its new value may be replayed. Changing the private authentication key obviates this threat.
o The private authentication key and private privacy key must be known only to the parties requiring knowledge of them.
Protecting the secrets from disclosure is critical to the security of the protocols. Knowledge of the secrets must be as restricted as possible within an implementation. In particular, although the secrets may be known to one or more persons during the initial configuration of a device, the secrets should be changed immediately after configuration such that their actual value is known only to the software. A management station has the additional responsibility of recovering the state of all parties whenever it boots, and it may address this responsibility by recording the secrets on a long-term storage device. Access to information on this device must be as restricted as is practically possible.
o There must exist at least one SNMPv2 entity that assumes the role of a responsible management station.
This management station is responsible for ensuring that all authentication clocks are synchronized and for changing the secret values when necessary. Although more than one management station may share this responsibility, their coordination is essential to the secure management of the network. The mechanism by which multiple management stations ensure that no more than one of them attempts to synchronize the clocks or update the
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secrets at any one time is a local implementation issue.
A responsible management station may either support clock synchronization and secret distribution as separate functions, or combine them into a single functional unit.
The first section below specifies the procedures by which a SNMPv2 entity is initially configured. The next two sections describe one strategy for distributing clock values and one for determining a synchronized clock value among SNMPv2 parties supporting the Digest Authentication Protocol. For SNMPv2 parties supporting the Symmetric Privacy Protocol, the next section describes a strategy for distributing secret values. The last section specifies the procedures by which a SNMPv2 entity recovers from a "crash."
5.1. Initial Configuration
This section describes the initial configuration of a SNMPv2 entity that supports the Digest Authentication Protocol or both the Digest Authentication Protocol and the Symmetric Privacy Protocol.
When a network device is first installed, its initial, secure configuration must be done manually, i.e., a person must physically visit the device and enter the initial secret values for at least its first secure SNMPv2 party. This requirement suggests that the person will have knowledge of the initial secret values.
In general, the security of a system is enhanced as the number of entities that know a secret is reduced. Requiring a person to physically visit a device every time a SNMPv2 party is configured not only exposes the secrets unnecessarily but is administratively prohibitive. In particular, when MD5 is used, the initial authentication secret is 128 bits long and when DES is used an additional 128 bits are needed - 64 bits each for the key and initialization vector. Clearly, these values will need to be recorded on a medium in order to be transported between a responsible management station and a managed agent. The recommended procedure is to configure a small set of initial SNMPv2 parties for each SNMPv2 entity, one pair of which may be used initially to configure all other SNMPv2 parties.
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In fact, there is a minimal, useful set of SNMPv2 parties that could be configured between each responsible management station and managed agent. This minimal set includes one of each of the following for both the responsible management station and the managed agent:
o a SNMPv2 party for which the authentication protocol and privacy protocol are the values noAuth and noPriv, respectively,
o a SNMPv2 party for which the authentication protocol identifies the mechanism defined in Section 1.5.1 and its privacy protocol is the value noPriv, and
o a SNMPv2 party for which the authentication protocol and privacy protocol identify the mechanisms defined in Section 1.5.1 and Section 1.5.2, respectively.
The last of these SNMPv2 parties in both the responsible management station and the managed agent could be used to create all other SNMPv2 parties.
Configuring one pair of SNMPv2 parties to be used to configure all other parties has the advantage of exposing only one pair of secrets - the secrets used to configure the minimal, useful set identified above. To limit this exposure, the responsible management station should change these values as its first operation upon completion of the initial configuration. In this way, secrets are known only to the peers requiring knowledge of them in order to communicate.
The Management Information Base (MIB) document [4] supporting these security protocols specifies 6 initial party identities and initial values, which, by convention, are assigned to the parties and their associated parameters.
These 6 initial parties are required to exist as part of the configuration of implementations when first installed, with the exception that implementations not providing support for a privacy protocol only need the 4 initial parties for which the privacy protocol is noPriv. When installing a managed agent, these parties need to be configured with their initial secrets, etc., both in the responsible management station and in the new agent.
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If the responsible management station is configured first, it can be used to generate the initial secrets and provide them to a person, on a suitable medium, for distribution to the managed agent. The following sequence of steps describes the initial configuration of a managed agent and its responsible management station.
(1) Determine the initial values for each of the attributes of the SNMPv2 party to be configured. Some of these values may be computed by the responsible management station, some may be specified in the MIB document, and some may be administratively determined.
(2) Configure the parties in the responsible management station, according to the set of initial values. If the management station is computing some initial values to be entered into the agent, an appropriate medium must be present to record the values.
(3) Configure the parties in the managed agent, according to the set of initial values.
(4) The responsible management station must synchronize the authentication clock values for each party it shares with each managed agent. Section 5.3 specifies one strategy by which this could be accomplished.
(5) The responsible management station should change the secret values manually configured to ensure the actual values are known only to the peers requiring knowledge of them in order to communicate. To do this, the management station generates new secrets for each party to be reconfigured and distributes the updates using any strategy which protects the new values from disclosure; use of a SNMPv2 set operation acting on the managed objects defined in [4] is such a strategy. Upon receiving positive acknowledgement that the new values have been distributed, the management station should update its local database with the new values.
If the managed agent does not support a protocol that protects messages from disclosure, e.g., the Symmetric Privacy Protocol (see section 5.4), then the distribution of new secrets, after the compromise of existing secrets, is not possible. In this case, the new secrets can only be distributed by a physical
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visit to the device.
If there are other SNMPv2 protocol entities requiring knowledge of the secrets, the responsible management station must distribute the information upon completion of the initial configuration. The considerations, mentioned above, concerning the protection of secrets from disclosure, also apply in this case.
5.2. Clock Distribution
A responsible management station must ensure that the authentication clock value for each SNMPv2 party for which it is responsible
o is loosely synchronized among all the local databases in which it appears,
o is reset, as indicated below, upon reaching its maximal value, and
o is non-decreasing, except as indicated below.
The skew among the clock values must be accounted for in the lifetime value, in addition to the expected communication delivery delay.
A skewed authentication clock may be detected by a number of strategies, including knowledge of the accuracy of the system clock, unauthenticated queries of the party database, and recognition of authentication failures originated by the party.
Whenever clock skew is detected, and whenever the SNMPv2 entities at both the responsible management station and the relevant managed agent support an appropriate privacy protocol (e.g., the Symmetric Privacy Protocol), a straightforward strategy for the correction of clock skew is simultaneous alteration of authentication clock and private key for the relevant SNMPv2 party. If the request to alter the key and clock for a particular party originates from that same party, then, prior to transmitting that request, the local notion of the authentication clock is artificially advanced to assure acceptance of the request as authentic.
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More generally, however, since an authentication clock value need not be protected from disclosure, it is not necessary that a managed agent support a privacy protocol in order for a responsible management station to correct skewed clock values. The procedure for correcting clock skew in the general case is presented in Section 5.3.
In addition to correcting skewed notions of authentication clocks, every SNMPv2 entity must react correctly as an authentication clock approaches its maximal value. If the authentication clock for a particular SNMPv2 party ever reaches the maximal time value, the clock must halt at that value. (The value of interest may be the maximum less lifetime. When authenticating a message, its authentication timestamp is added to lifetime and compared to the authentication clock. A SNMPv2 entity must guarantee that the sum is never greater than the maximal time value.) In this state, the only authenticated request a management station should generate for this party is one that alters the value of at least its authentication clock and private authentication key. In order to reset these values, the responsible management station may set the authentication timestamp in the message to the maximal time value.
The value of the authentication clock for a particular SNMPv2 party must never be altered such that its new value is less than its old value, unless its private authentication key is also altered at the same time.
5.3. Clock Synchronization
Unless the secrets are changed at the same time, the correct way to synchronize clocks is to advance the slower clock to be equal to the faster clock. Suppose that party agentParty is realized by the SNMPv2 entity in a managed agent; suppose that party mgrParty is realized by the SNMPv2 entity in the corresponding responsible management station. For any pair of parties, there are four possible conditions of the authentication clocks that could require correction:
(1) The management station's notion of the value of the authentication clock for agentParty exceeds the agent's notion.
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(2) The management station's notion of the value of the authentication clock for mgrParty exceeds the agent's notion.
(3) The agent's notion of the value of the authentication clock for agentParty exceeds the management station's notion.
(4) The agent's notion of the value of the authentication clock for mgrParty exceeds the management station's notion.
The selective clock acceleration mechanism intrinsic to the protocol corrects conditions 1, 2 and 3 as part of the normal processing of an authentic message. Therefore, the clock adjustment procedure below does not provide for any adjustments in those cases. Rather, the following sequence of steps specifies how the clocks may be synchronized when condition 4 is manifest.
(1) The responsible management station saves its existing notion of the authentication clock for the party mgrParty.
(2) The responsible management station retrieves the authentication clock value for mgrParty from the agent. This retrieval must be an unauthenticated request, since the management station does not know if the clocks are synchronized. If the request fails, the clocks cannot be synchronized, and the clock adjustment procedure is aborted without further processing.
(3) If the notion of the authentication clock for mgrParty just retrieved from the agent exceeds the management station's notion, then condition 4 is manifest, and the responsible management station advances its notion of the authentication clock for mgrParty to match the agent's notion.
(4) The responsible management station retrieves the authentication clock value for mgrParty from the agent. This retrieval must be an authenticated request, in order that the management station may verify that the clock value is properly synchronized. If this authenticated query fails, then the management station restores its
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previously saved notion of the clock value, and the clock adjustment procedure is aborted without further processing. Otherwise, clock synchronization has been successfully realized.
Administrative advancement of a clock as described above does not introduce any new vulnerabilities, since the value of the clock is intended to increase with the passage of time. A potential operational problem is the rejection of authentic management operations that were authenticated using a previous value of the relevant party clock. This possibility may be avoided if a management station suppresses generation of management traffic between relevant parties while this clock adjustment procedure is in progress.
5.4. Secret Distribution
This section describes one strategy by which a SNMPv2 entity that supports both the Digest Authentication Protocol and the Symmetric Privacy Protocol can change the secrets for a particular SNMPv2 party.
The frequency with which the secrets of a SNMPv2 party should be changed is a local administrative issue. However, the more frequently a secret is used, the more frequently it should be changed. At a minimum, the secrets must be changed whenever the associated authentication clock approaches its maximal value (see Section 6). Note that, owing to both administrative and automatic advances of the authentication clock described in this memo, the authentication clock for a SNMPv2 party may well approach its maximal value sooner than might otherwise be expected.
The following sequence of steps specifies how a responsible management station alters a secret value (i.e., the private authentication key or the private privacy key) for a particular SNMPv2 party. There are two cases.
First, setting the initial secret for a new party:
(1) The responsible management station generates a new secret value.
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(2) The responsible management station encapsulates a SNMPv2 setRequest in a SNMPv2 private management communication with at least the following properties.
Its source supports the Digest Authentication Protocol and the Symmetric Privacy Protocol.
Its destination supports the Symmetric Privacy Protocol and the Digest Authentication Protocol.
(3) The SNMPv2 private management communication is transmitted to its destination.
(4) Upon receiving the request, the recipient processes the message according to [12] and [1].
(5) The recipient encapsulates a SNMPv2 response in a SNMPv2 private management communication with at least the following properties.
Its source supports the Digest Authentication Protocol and the Symmetric Privacy Protocol.
Its destination supports the Symmetric Privacy Protocol and the Digest Authentication Protocol.
(6) The SNMPv2 private management communication is transmitted to its destination.
(7) Upon receiving the response, the responsible management station updates its local database with the new value.
Second, modifying the current secret of an existing party:
(1) The responsible management station generates a new secret value.
(2) The responsible management station encapsulates a SNMPv2 setRequest in a SNMPv2 management communication with at least the following properties.
Its source and destination supports the Digest Authentication Protocol.
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(3) The SNMPv2 private management communication is transmitted to its destination.
(4) Upon receiving the request, the recipient processes the message according to [12] and [1].
(5) The recipient encapsulates a SNMPv2 response in a SNMPv2 management communication with at least the following properties.
Its source and destination supports the Digest Authentication Protocol.
(6) The SNMPv2 management communication is transmitted to its destination.
(7) Upon receiving the response, the responsible management station updates its local database with the new value.
If the responsible management station does not receive a response to its request, there are two possible causes.
o The request may not have been delivered to the destination.
o The response may not have been delivered to the originator of the request.
In order to distinguish the two possible error conditions, a responsible management station could check the destination to see if the change has occurred. Unfortunately, since the secret values are unreadable, this is not directly possible.
The recommended strategy for verifying key changes is to set the public value corresponding to the secret being changed to a recognizable, novel value: that is, alter the public authentication key value for the relevant party when changing its private authentication key, or alter its public privacy key value when changing its private privacy key. In this way, the responsible management station may retrieve the public value when a response is not received, and verify whether or not the change has taken place. (This strategy is available since the public values are not used by the protocols defined in this memo. If this strategy is employed, then the public values are significant in this context. Of course, protocols
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using the public values may make use of this strategy directly.)
One other scenario worthy of mention is using a SNMPv2 party to change its own secrets. In this case, the destination will change its local database prior to generating a response. Thus, the response will be constructed according to the new value. However, the responsible management station will not update its local database until after the response is received. This suggests the responsible management station may receive a response which will be evaluated as unauthentic, unless the correct secret is used. The responsible management station may either account for this scenario as a special case, or use an alteration of the relevant public values (as described above) to verify the key change.
Note, during the period of time after the request has been sent and before the response is received, the management station must keep track of both the old and new secret values. Since the delay may be the result of a network failure, the management station must be prepared to retain both values for an extended period of time, including across reboots.
5.5. Crash Recovery
This section describes the requirements for SNMPv2 protocol entities in connection with recovery from system crashes or other service interruptions.
For each SNMPv2 party in the local database for a particular SNMPv2 entity, its identity, authentication clock, private authentication key, and private privacy key must enjoy non- volatile, incorruptible representations. If possible, lifetime should also enjoy a non-volatile, incorruptible representation. If said SNMPv2 entity supports other security protocols or algorithms in addition to the two defined in this memo, then the authentication protocol and the privacy protocol for each party also require non-volatile, incorruptible representation.
The authentication clock of a SNMPv2 party is a critical component of the overall security of the protocols. The inclusion of a reliable representation of a clock in a SNMPv2 entity is required for overall security. A reliable clock
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representation ensures that a clock's value is monotonically increasing, even across a power loss or other system failure of the local SNMPv2 entity. One example of a reliable clock representation is that provided by battery-powered clock- calendar devices incorporated into some contemporary systems. Another example is storing and updating a clock value in non- volatile storage at a frequency of once per U (e.g., 24) hours, and re-initialising that clock value on every reboot as the stored value plus U+1 hours. It is assumed that management stations always support reliable clock representations, where clock adjustment by a human operator during crash recovery may contribute to that reliability.
If a managed agent crashes and does not reboot in time for its responsible management station to prevent its authentication clock from reaching its maximal value, upon reboot the clock must be halted at its maximal value. The procedures specified in Section 5.3 would then apply.
Upon recovery, those attributes of each SNMPv2 party that do not enjoy non-volatile or reliable representation are initialized as follows.
o If the private authentication key is not the OCTET STRING of zero length, the authentication protocol is set to identify use of the Digest Authentication Protocol in conjunction with the algorithm specified in Section 1.5.1.
o If the lifetime is not retained, it should be initialized to zero.
o If the private privacy key is not the OCTET STRING of zero length, the privacy protocol is set to identify use of the Symmetric Privacy Protocol in conjunction with the algorithm specified in Section 1.5.2.
Upon detecting that a managed agent has rebooted, a responsible management station must reset all other party attributes, including the lifetime if it was not retained. In order to reset the lifetime, the responsible management station should set the authentication timestamp in the message to the sum of the authentication clock and desired lifetime. This is an artificial advancement of the authentication timestamp in order to guarantee the message will be authentic
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when received by the recipient.
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6. Security Considerations
This section highlights security considerations relevant to the protocols and procedures defined in this memo. Practices that contribute to secure, effective operation of the mechanisms defined here are described first. Constraints on implementation behavior that are necessary to the security of the system are presented next. Finally, an informal account of the contribution of each mechanism of the protocols to the required goals is presented.
6.1. Recommended Practices
This section describes practices that contribute to the secure, effective operation of the mechanisms defined in this memo.
o A management station should discard SNMPv2 responses for which neither the request-id component nor the represented management information corresponds to any currently outstanding request.
Although it would be typical for a management station to do this as a matter of course, in the context of these security protocols it is significant owing to the possibility of message duplication (malicious or otherwise).
o A management station should not interpret an agent's lack of response to an authenticated SNMPv2 management communication as a conclusive indication of agent or network failure.
It is possible for authentication failure traps to be lost or suppressed as a result of authentication clock skew or inconsistent notions of shared secrets. In order either to facilitate administration of such SNMPv2 parties or to provide for continued management in times of network stress, a management station implementation may provide for arbitrary, artificial advancement of the timestamp or selection of shared secrets on locally generated messages.
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o The lifetime value for a SNMPv2 party should be chosen (by the local administration) to be as small as possible, given the accuracy of clock devices available, relevant round-trip communications delays, and the frequency with which a responsible management station will be able to verify all clock values.
A large lifetime increases the vulnerability to malicious delays of SNMPv2 messages. The implementation of a management station may accommodate changing network conditions during periods of network stress by effectively increasing the lifetimes of the source and destination parties. The management station accomplishes this by artificially advancing its notion of the source party's clock on messages it sends, and by artificially increasing its notion of the source party`s lifetime on messages it receives.
o When sending state altering messages to a managed agent, a management station should delay sending successive messages to the managed agent until a positive acknowledgement is received for the previous message or until the previous message expires.
No message ordering is imposed by the SNMPv2. Messages may be received in any order relative to their time of generation and each will be processed in the ordered received. Note that when an authenticated message is sent to a managed agent, it will be valid for a period of time that does not exceed lifetime under normal circumstances, and is subject to replay during this period.
Indeed, a management station must cope with the loss and re-ordering of messages resulting from anomalies in the network as a matter of course.
However, a managed object, snmpSetSerialNo [14], is specifically defined for use with SNMPv2 set operations in order to provide a mechanism to ensure the processing of SNMPv2 messages occurs in a specific order.
o The frequency with which the secrets of a SNMPv2 party should be changed is indirectly related to the frequency of their use.
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Protecting the secrets from disclosure is critical to the overall security of the protocols. Frequent use of a secret provides a continued source of data that may be useful to a cryptanalyst in exploiting known or perceived weaknesses in an algorithm. Frequent changes to the secret avoid this vulnerability.
Changing a secret after each use is generally regarded as the most secure practice, but a significant amount of overhead may be associated with that approach.
Note, too, in a local environment the threat of disclosure may be insignificant, and as such the changing of secrets may be less frequent. However, when public data networks are the communication paths, more caution is prudent.
o In order to foster the greatest degree of security, a management station implementation must support constrained, pairwise sharing of secrets among SNMPv2 entities as its default mode of operation.
Owing to the use of symmetric cryptography in the protocols defined here, the secrets associated with a particular SNMPv2 party must be known to all other SNMPv2 parties with which that party may wish to communicate. As the number of locations at which secrets are known and used increases, the likelihood of their disclosure also increases, as does the potential impact of that disclosure. Moreover, if the set of SNMPv2 protocol entities with knowledge of a particular secret numbers more than two, data origin cannot be reliably authenticated because it is impossible to determine with any assurance which entity of that set may be the originator of a particular SNMPv2 message. Thus, the greatest degree of security is afforded by configurations in which the secrets for each SNMPv2 party are known to at most two protocol entities.
6.2. Conformance
A SNMPv2 entity implementation that claims conformance to this memo must satisfy the following requirements:
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(1) It must implement the noAuth and noPriv protocols whose object identifiers are defined in [4].
noAuth This protocol signifies that messages generated by a party using it are not protected as to origin or integrity. It is required to ensure that a party's authentication clock is always accessible.
noPriv This protocol signifies that messages received by a party using it are not protected from disclosure. It is required to ensure that a party's authentication clock is always accessible.
(2) It must implement the Digest Authentication Protocol in conjunction with the algorithm defined in Section 1.5.1.
(3) It must include in its local database at least one SNMPv2 party with the following parameters set as follows:
partyAuthProtocol is set to noAuth and
partyPrivProtocol is set to noPriv.
This party must have a MIB view [1] specified that includes at least the authentication clock of all other parties. Alternatively, the authentication clocks of the other parties may be partitioned among several similarly configured parties according to a local implementation convention.
(4) For each SNMPv2 party about which it maintains information in a local database, an implementation must satisfy the following requirements:
(a) It must not allow a party's parameters to be set to a value inconsistent with its expected syntax. In particular, Section 1.4 specifies constraints for the chosen mechanisms.
(b) It must, to the maximal extent possible, prohibit read-access to the private authentication key and private encryption key under all circumstances except as required to generate and/or validate SNMPv2 messages with respect to that party.
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This prohibition includes prevention of read-access by the entity's human operators.
(c) It must allow the party's authentication clock to be publicly accessible. The correct operation of the Digest Authentication Protocol requires that it be possible to determine this value at all times in order to guarantee that skewed authentication clocks can be resynchronized.
(d) It must prohibit alterations to its record of the authentication clock for that party independently of alterations to its record of the private authentication key (unless the clock alteration is an advancement).
(e) It must never allow its record of the authentication clock for that party to be incremented beyond the maximal time value and so "roll-over" to zero.
(f) It must never increase its record of the lifetime for that party except as may be explicitly authorized (via imperative command or securely represented configuration information) by the responsible network administrator.
(g) In the event that the non-volatile, incorruptible representations of a party's parameters (in particular, either the private authentication key or private encryption key) are lost or destroyed, it must alter its record of these quantities to random values so subsequent interaction with that party requires manual redistribution of new secrets and other parameters.
(5) If it selects new value(s) for a party's secret(s), it must avoid bad or obvious choices for said secret(s). Choices to be avoided are boundary values (such as all- zeros) and predictable values (such as the same value as previously or selecting from a predetermined set).
(6) It must ensure that a received message for which the originating party uses the Digest Authentication Protocol but the receiving party does not, is always declared to
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be unauthentic. This may be achieved explicitly via an additional step in the procedure for processing a received message, or implicitly by verifying that all local access control policies enforce this requirement.
6.3. Protocol Correctness
The correctness of these SNMPv2 security protocols with respect to the stated goals depends on the following assumptions:
(1) The chosen message digest algorithm satisfies its design criteria. In particular, it must be computationally infeasible to discover two messages that share the same digest value.
(2) It is computationally infeasible to determine the secret used in calculating a digest on the concatenation of the secret and a message when both the digest and the message are known.
(3) The chosen symmetric encryption algorithm satisfies its design criteria. In particular, it must be computationally infeasible to determine the cleartext message from the ciphertext message without knowledge of the key used in the transformation.
(4) Local notions of a party's authentication clock while it is associated with a specific private key value are monotonically non-decreasing (i.e., they never run backwards) in the absence of administrative manipulations.
(5) The secrets for a particular SNMPv2 party are known only to authorized SNMPv2 protocol entities.
(6) Local notions of the authentication clock for a particular SNMPv2 party are never altered such that the authentication clock's new value is less than the current value without also altering the private authentication key.
For each mechanism of the protocol, an informal account of its contribution to the required goals is presented below.
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Pseudocode fragments are provided where appropriate to exemplify possible implementations; they are intended to be self-explanatory.
6.3.1. Clock Monotonicity Mechanism
By pairing each sequence of a clock's values with a unique key, the protocols partially realize goal 3, and the conjunction of this property with assumption 6 above is sufficient for the claim that, with respect to a specific private key value, all local notions of a party's authentication clock are, in general, non-decreasing with time.
6.3.2. Data Integrity Mechanism
The protocols require computation of a message digest computed over the SNMPv2 message prepended by the secret for the relevant party. By virtue of this mechanism and assumptions 1 and 2, the protocols realize goal 1.
Normally, the inclusion of the message digest value with the digested message would not be sufficient to guarantee data integrity, since the digest value can be modified in addition to the message while it is enroute. However, since not all of the digested message is included in the transmission to the destination, it is not possible to substitute both a message and a digest value while enroute to a destination.
Strictly speaking, the specified strategy for data integrity does not detect a SNMPv2 message modification which appends extraneous material to the end of such messages. However, owing to the representation of SNMPv2 messages as ASN.1 values, such modifications cannot - consistent with goal 1 - result in unauthorized management operations.
The data integrity mechanism specified in this memo protects only against unauthorized modification of individual SNMPv2 messages. A more general data integrity service that affords protection against the threat of message stream modification is not realized by this mechanism, although limited protection against reordering, delay, and duplication of messages within a message stream are provided by other mechanisms of the
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protocol.
6.3.3. Data Origin Authentication Mechanism
The data integrity mechanism requires the use of a secret value known only to communicating parties. By virtue of this mechanism and assumptions 1 and 2, the protocols explicitly prevent unauthorized modification of messages. Data origin authentication is implicit if the message digest value can be verified. That is, the protocols realize goal 2.
6.3.4. Restricted Administration Mechanism
This memo requires that implementations preclude administrative alterations of the authentication clock for a particular party independently from its private authentication key (unless that clock alteration is an advancement). An example of an efficient implementation of this restriction is provided in a pseudocode fragment below. This pseudocode fragment meets the requirements of assumption 6. Observe that the requirement is not for simultaneous alteration but to preclude independent alteration. This latter requirement is fairly easily realized in a way that is consistent with the defined semantics of the SNMPv2 set operation.
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.
By virtue of this mechanism, the protocols realize goal 3. In cases in which the local notions of a particular SNMPv2 party clock are moderately well-synchronized, the timeliness mechanism effectively limits the age of validly delivered messages. Thus, if an attacker diverts all validated messages for replay much later, the delay introduced by this attack is limited to a period that is proportional to the skew among local notions of the party clock.
6.3.6. Selective Clock Acceleration Mechanism
The definition of the SNMPv2 security protocols requires that, if either of the timestamp values for the originating or receiving parties on a received, validated message exceeds the corresponding local notion of the clock for that party, then the local notion of the clock for that party is adjusted forward to correspond to said timestamp value. This mechanism is neither strictly necessary nor sufficient to the security of the protocol; rather, it fosters the clock synchronization on which valid message delivery depends - thereby enhancing the effectiveness of the protocol in a management context.
if (msgIsValidated) { if (timestampOfReceivedMsg > party->localNotionOfClock) { party->localNotionOfClock = timestampOfReceivedMsg; } }
The effect of this mechanism is to synchronize local notions of a party clock more closely in the case where a sender's notion is more advanced than a receiver's. In the opposite case, this mechanism has no effect on local notions of a party clock and either the received message is validly delivered or not according to other mechanisms of the protocol.
Operation of this mechanism does not, in general, improve the probability of validated delivery for messages generated by party participants whose local notion of the party clock is relatively less advanced. In this case, queries from a management station may not be validly delivered and the
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management station needs to react appropriately (e.g., by use of the strategy described in section 5.3). In contrast, the delivery of SNMPv2 trap messages generated by an agent that suffers from a less advanced notion of a party clock is more problematic, for an agent may lack the capacity to recognize and react to security failures that prevent delivery of its messages. Thus, the inherently unreliable character of trap messages is likely to be compounded by attempts to provide for their validated delivery.
6.3.7. Confidentiality Mechanism
The protocols require the use of a symmetric encryption algorithm when the data confidentiality service is required. By virtue of this mechanism and assumption 3, the protocols realize goal 4.
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7. Acknowledgements
This document is based, almost entirely, on RFC 1352.
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8. References
[1] Galvin, J., and McCloghrie, K., "Administrative Model for version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1445, Trusted Information Systems, Hughes LAN Systems, April 1993.
[2] Case, J., Fedor, M., Schoffstall, M., Davin, J., "Simple Network Management Protocol", STD 15, RFC 1157, SNMP Research, Performance Systems International, MIT Laboratory for Computer Science, May 1990.
[3] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, MIT Laboratory for Computer Science, April 1992.
[4] McCloghrie, K., and Galvin, J., "Party MIB for version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1447, Hughes LAN Systems, Trusted Information Systems, April 1993.
[5] Data Encryption Standard, National Institute of Standards and Technology. Federal Information Processing Standard (FIPS) Publication 46-1. Supersedes FIPS Publication 46, (January, 1977; reaffirmed January, 1988).
[6] Data Encryption Algorithm, American National Standards Institute. ANSI X3.92-1981, (December, 1980).
[7] DES Modes of Operation, National Institute of Standards and Technology. Federal Information Processing Standard (FIPS) Publication 81, (December, 1980).
[8] Data Encryption Algorithm - Modes of Operation, American National Standards Institute. ANSI X3.106-1983, (May 1983).
[9] Guidelines for Implementing and Using the NBS Data Encryption Standard, National Institute of Standards and Technology. Federal Information Processing Standard (FIPS) Publication 74, (April, 1981).
[10] Validating the Correctness of Hardware Implementations of the NBS Data Encryption Standard, National Institute of Standards and Technology. Special Publication 500-20.
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[11] Maintenance Testing for the Data Encryption Standard, National Institute of Standards and Technology. Special Publication 500-61, (August, 1980).
[12] Case, J., McCloghrie, K., Rose, M., and Waldbusser, S., "Protocol Operations for version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1448, SNMP Research, Inc., Hughes LAN Systems, Dover Beach Consulting, Inc., Carnegie Mellon University, April 1993.
[13] Case, J., McCloghrie, K., Rose, M., and Waldbusser, S., "Transport Mappings for version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1449, SNMP Research, Inc., Hughes LAN Systems, Dover Beach Consulting, Inc., Carnegie Mellon University, April 1993.
[14] Case, J., McCloghrie, K., Rose, M., and Waldbusser, S., "Management Information Base for version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1450, SNMP Research, Inc., Hughes LAN Systems, Dover Beach Consulting, Inc., Carnegie Mellon University, April 1993.
Galvin & McCloghrie [Page 50]
RFC 1446 Security Protocols for SNMPv2 April 1993
9. Authors' Addresses
James M. Galvin Trusted Information Systems, Inc. 3060 Washington Road, Route 97 Glenwood, MD 21738
Phone: +1 301 854-6889 EMail: galvin@tis.com
Keith McCloghrie Hughes LAN Systems 1225 Charleston Road Mountain View, CA 94043 US
