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Keywords: [--------|h], SNMP, Management, Framework
Network Working Group J. Galvin
Request for Comments: 1446 Trusted Information Systems
K. McCloghrie
Hughes LAN Systems
April 1993
Security Protocols
for version 2 of the
Simple Network Management Protocol (SNMPv2)
Status of this Memo
This RFC specifes an IAB standards track protocol for the
Internet community, and requests discussion and suggestions
for improvements. Please refer to the current edition of the
"IAB Official Protocol Standards" for the standardization
state and status of this protocol. Distribution of this memo
is unlimited.
Table of Contents
1 Introduction .......................................... 2
1.1 A Note on Terminology ............................... 3
1.2 Threats ............................................. 4
1.3 Goals and Constraints ............................... 5
1.4 Security Services ................................... 6
1.5 Mechanisms .......................................... 7
1.5.1 Message Digest Algorithm .......................... 8
1.5.2 Symmetric Encryption Algorithm .................... 9
2 SNMPv2 Party .......................................... 11
3 Digest Authentication Protocol ........................ 14
3.1 Generating a Message ................................ 16
3.2 Receiving a Message ................................. 18
4 Symmetric Privacy Protocol ............................ 21
4.1 Generating a Message ................................ 21
4.2 Receiving a Message ................................. 22
5 Clock and Secret Distribution ......................... 24
5.1 Initial Configuration ............................... 25
5.2 Clock Distribution .................................. 28
5.3 Clock Synchronization ............................... 29
5.4 Secret Distribution ................................. 31
5.5 Crash Recovery ...................................... 34
6 Security Considerations ............................... 37
6.1 Recommended Practices ............................... 37
6.2 Conformance ......................................... 39
6.3 Protocol Correctness ................................ 42
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6.3.1 Clock Monotonicity Mechanism ...................... 43
6.3.2 Data Integrity Mechanism .......................... 43
6.3.3 Data Origin Authentication Mechanism .............. 44
6.3.4 Restricted Administration Mechanism ............... 44
6.3.5 Message Timeliness Mechanism ...................... 45
6.3.6 Selective Clock Acceleration Mechanism ............ 46
6.3.7 Confidentiality Mechanism ......................... 47
7 Acknowledgements ...................................... 48
8 References ............................................ 49
9 Authors' Addresses .................................... 51
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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.
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The Digest Authentication Protocol is described in Section 3.
It provides a data integrity service by transmitting a message
digest - computed by the originator and verified by the
recipient - with each SNMPv2 message. The data origin
authentication service is provided by prefixing the message
with a secret value known only to the originator and
recipient, prior to computing the digest. Thus, data
integrity is supported explicitly while data origin
authentication is supported implicitly in the verification of
the digest.
The Symmetric Privacy Protocol is described in Section 4. It
protects messages from disclosure by encrypting their contents
according to a secret cryptographic key known only to the
originator and recipient. The additional functionality
afforded by this protocol is assumed to justify its additional
computational cost.
The Digest Authentication Protocol depends on the existence of
loosely synchronized clocks between the originator and
recipient of a message. The protocol specification makes no
assumptions about the strategy by which such clocks are
synchronized. Section 5.3 presents one strategy that is
particularly suited to the demands of SNMP network management.
Both protocols described here require the sharing of secret
information between the originator of a message and its
recipient. The protocol specifications assume the existence
of the necessary secrets. The selection of such secrets and
their secure distribution to appropriate parties may be
accomplished by a variety of strategies. Section 5.4 presents
one such strategy that is particularly suited to the demands
of SNMP network management.
1.1. A Note on Terminology
For the purpose of exposition, the original Internet-standard
Network Management Framework, as described in RFCs 1155, 1157,
and 1212, is termed the SNMP version 1 framework (SNMPv1).
The current framework is termed the SNMP version 2 framework
(SNMPv2).
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1.2. Threats
Several of the classical threats to network protocols are
applicable to the network management problem and therefore
would be applicable to any SNMPv2 security protocol. Other
threats are not applicable to the network management problem.
This section discusses principal threats, secondary threats,
and threats which are of lesser importance.
The principal threats against which any SNMPv2 security
protocol should provide protection are:
Modification of Information
The SNMPv2 protocol provides the means for management
stations to interrogate and to manipulate the value of
objects in a managed agent. The modification threat is
the danger that some party may alter in-transit messages
generated by an authorized party in such a way as to
effect unauthorized management operations, including
falsifying the value of an object.
Masquerade
The SNMPv2 administrative model includes an access
control model. Access control necessarily depends on
knowledge of the origin of a message. The masquerade
threat is the danger that management operations not
authorized for some party may be attempted by that party
by assuming the identity of another party that has the
appropriate authorizations.
Two secondary threats are also identified. The security
protocols defined in this memo do provide protection against:
Message Stream Modification
The SNMPv2 protocol is based upon a connectionless
transport service which may operate over any subnetwork
service. The re-ordering, delay or replay of messages
can and does occur through the natural operation of many
such subnetwork services. The message stream
modification threat is the danger that messages may be
maliciously re-ordered, delayed or replayed to an extent
which is greater than can occur through the natural
operation of a subnetwork service, in order to effect
unauthorized management operations.
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Disclosure
The disclosure threat is the danger of eavesdropping on
the exchanges between managed agents and a management
station. Protecting against this threat is mandatory
when the SNMPv2 is used to create new SNMPv2 parties [1]
on which subsequent secure operation might be based.
Protecting against the disclosure threat may also be
required as a matter of local policy.
There are at least two threats that a SNMPv2 security protocol
need not protect against. The security protocols defined in
this memo do not provide protection against:
Denial of Service
A SNMPv2 security protocol need not attempt to address
the broad range of attacks by which service to authorized
parties is denied. Indeed, such denial-of-service
attacks are in many cases indistinguishable from the type
of network failures with which any viable network
management protocol must cope as a matter of course.
Traffic Analysis
In addition, a SNMPv2 security protocol need not attempt
to address traffic analysis attacks. Indeed, many
traffic patterns are predictable - agents may be managed
on a regular basis by a relatively small number of
management stations - and therefore there is no
significant advantage afforded by protecting against
traffic analysis.
1.3. Goals and Constraints
Based on the foregoing account of threats in the SNMP network
management environment, the goals of a SNMPv2 security
protocol are enumerated below.
(1) The protocol should provide for verification that each
received SNMPv2 message has not been modified during its
transmission through the network in such a way that an
unauthorized management operation might result.
(2) The protocol should provide for verification of the
identity of the originator of each received SNMPv2
message.
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(3) The protocol should provide that the apparent time of
generation for each received SNMPv2 message is recent.
(4) The protocol should provide, when necessary, that the
contents of each received SNMPv2 message are protected
from disclosure.
In addition to the principal goal of supporting secure network
management, the design of any SNMPv2 security protocol is also
influenced by the following constraints:
(1) When the requirements of effective management in times of
network stress are inconsistent with those of security,
the former are preferred.
(2) Neither the security protocol nor its underlying security
mechanisms should depend upon the ready availability of
other network services (e.g., Network Time Protocol (NTP)
or secret/key management protocols).
(3) A security mechanism should entail no changes to the
basic SNMP network management philosophy.
1.4. Security Services
The security services necessary to support the goals of a
SNMPv2 security protocol are as follows.
Data Integrity
is the provision of the property that data has not been
altered or destroyed in an unauthorized manner, nor have
data sequences been altered to an extent greater than can
occur non-maliciously.
Data Origin Authentication
is the provision of the property that the claimed origin
of received data is corroborated.
Data Confidentiality
is the provision of the property that information is not
made available or disclosed to unauthorized individuals,
entities, or processes.
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The protocols specified in this memo require both data
integrity and data origin authentication to be used at all
times. For these protocols, it is not possible to realize
data integrity without data origin authentication, nor is it
possible to realize data origin authentication without data
integrity.
Further, there is no provision for data confidentiality
without both data integrity and data origin authentication.
1.5. Mechanisms
The security protocols defined in this memo employ several
types of mechanisms in order to realize the goals and security
services described above:
o In support of data integrity, a message digest algorithm
is required. A digest is calculated over an appropriate
portion of a SNMPv2 message and included as part of the
message sent to the recipient.
o In support of data origin authentication and data
integrity, the portion of a SNMPv2 message that is
digested is first prefixed with a secret value shared by
the originator of that message and its intended
recipient.
o To protect against the threat of message delay or replay,
(to an extent greater than can occur through normal
operation), a timestamp value is included in each message
generated. A recipient evaluates the timestamp to
determine if the message is recent. This protection
against the threat of message delay or replay does not
imply nor provide any protection against unauthorized
deletion or suppression of messages. Other mechanisms
defined independently of the security protocol can also
be used to detect message replay (e.g., the request-id
[2]), or for set operations, the re-ordering, replay,
deletion, or suppression of messages (e.g., the MIB
variable snmpSetSerialNo [14]).
o In support of data confidentiality, a symmetric
encryption algorithm is required. An appropriate portion
of the message is encrypted prior to being transmitted to
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its recipient.
The security protocols in this memo are defined independently
of the particular choice of a message digest and encryption
algorithm - owing principally to the lack of a suitable metric
by which to evaluate the security of particular algorithm
choices. However, in the interests of completeness and in
order to guarantee interoperability, Sections 1.5.1 and 1.5.2
specify particular choices, which are considered acceptably
secure as of this writing. In the future, this memo may be
updated by the publication of a memo specifying substitute or
alternate choices of algorithms, i.e., a replacement for or
addition to the sections below.
1.5.1. Message Digest Algorithm
In support of data integrity, the use of the MD5 [3] message
digest algorithm is chosen. A 128-bit digest is calculated
over the designated portion of a SNMPv2 message and included
as part of the message sent to the recipient.
An appendix of [3] contains a C Programming Language
implementation of the algorithm. This code was written with
portability being the principal objective. Implementors may
wish to optimize the implementation with respect to the
characteristics of their hardware and software platforms.
The use of this algorithm in conjunction with the Digest
Authentication Protocol (see Section 3) is identified by the
ASN.1 object identifier value v2md5AuthProtocol, defined in
[4]. (Note that this protocol is a modified version of the
md5AuthProtocol protocol defined in RFC 1352.)
For any SNMPv2 party for which the authentication protocol is
v2md5AuthProtocol, the size of its private authentication key
is 16 octets.
Within an authenticated management communication generated by
such a party, the size of the authDigest component of that
communication (see Section 3) is 16 octets.
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1.5.2. Symmetric Encryption Algorithm
In support of data confidentiality, the use of the Data
Encryption Standard (DES) in the Cipher Block Chaining mode of
operation is chosen. The designated portion of a SNMPv2
message is encrypted and included as part of the message sent
to the recipient.
Two organizations have published specifications defining the
DES: the National Institute of Standards and Technology (NIST)
[5] and the American National Standards Institute [6]. There
is a companion Modes of Operation specification for each
definition (see [7] and [8], respectively).
The NIST has published three additional documents that
implementors may find useful.
o There is a document with guidelines for implementing and
using the DES, including functional specifications for
the DES and its modes of operation [9].
o There is a specification of a validation test suite for
the DES [10]. The suite is designed to test all aspects
of the DES and is useful for pinpointing specific
problems.
o There is a specification of a maintenance test for the
DES [11]. The test utilizes a minimal amount of data and
processing to test all components of the DES. It
provides a simple yes-or-no indication of correct
operation and is useful to run as part of an
initialization step, e.g., when a computer reboots.
The use of this algorithm in conjunction with the Symmetric
Privacy Protocol (see Section 4) is identified by the ASN.1
object identifier value desPrivProtocol, defined in [4].
For any SNMPv2 party for which the privacy protocol is
desPrivProtocol, the size of the private privacy key is 16
octets, of which the first 8 octets are a DES key and the
second 8 octets are a DES Initialization Vector. The 64-bit
DES key in the first 8 octets of the private key is a 56 bit
quantity used directly by the algorithm plus 8 parity bits -
arranged so that one parity bit is the least significant bit
of each octet. The setting of the parity bits is ignored.
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The length of the octet sequence to be encrypted by the DES
must be an integral multiple of 8. When encrypting, the data
should be padded at the end as necessary; the actual pad value
is insignificant.
If the length of the octet sequence to be decrypted is not an
integral multiple of 8 octets, the processing of the octet
sequence should be halted and an appropriate exception noted.
Upon decrypting, the padding should be ignored.
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2. SNMPv2 Party
Recall from [1] that a SNMPv2 party is a conceptual, virtual
execution context whose operation is restricted (for security
or other purposes) to an administratively defined subset of
all possible operations of a particular SNMPv2 entity. A
SNMPv2 entity is an actual process which performs network
management operations by generating and/or responding to
SNMPv2 protocol messages in the manner specified in [12].
Architecturally, every SNMPv2 entity maintains a local
database that represents all SNMPv2 parties known to it.
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A SNMPv2 party may be represented by an ASN.1 value with the
following syntax:
SnmpParty ::= SEQUENCE {
partyIdentity
OBJECT IDENTIFIER,
partyTDomain
OBJECT IDENTIFIER,
partyTAddress
OCTET STRING,
partyMaxMessageSize
INTEGER,
partyAuthProtocol
OBJECT IDENTIFIER,
partyAuthClock
INTEGER,
partyAuthPrivate
OCTET STRING,
partyAuthPublic
OCTET STRING,
partyAuthLifetime
INTEGER,
partyPrivProtocol
OBJECT IDENTIFIER,
partyPrivPrivate
OCTET STRING,
partyPrivPublic
OCTET STRING
}
For each SnmpParty value that represents a SNMPv2 party, the
generic significance of each of its components is defined in
[1]. For each SNMPv2 party that supports the generation of
messages using the Digest Authentication Protocol, additional,
special significance is attributed to certain components of
that party's representation:
o Its partyAuthProtocol component is called the
authentication protocol and identifies a combination of
the Digest Authentication Protocol with a particular
digest algorithm (such as that defined in Section 1.5.1).
This combined mechanism is used to authenticate the
origin and integrity of all messages generated by the
party.
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o Its partyAuthClock component is called the authentication
clock and represents a notion of the current time that is
specific to the party.
o Its partyAuthPrivate component is called the private
authentication key and represents any secret value needed
to support the Digest Authentication Protocol and
associated digest algorithm.
o Its partyAuthPublic component is called the public
authentication key and represents any public value that
may be needed to support the authentication protocol.
This component is not significant except as suggested in
Section 5.4.
o Its partyAuthLifetime component is called the lifetime
and represents an administrative upper bound on
acceptable delivery delay for protocol messages generated
by the party.
For each SNMPv2 party that supports the receipt of messages
via the Symmetric Privacy Protocol, additional, special
significance is attributed to certain components of that
party's representation:
o Its partyPrivProtocol component is called the privacy
protocol and identifies a combination of the Symmetric
Privacy Protocol with a particular encryption algorithm
(such as that defined in Section 1.5.2). This combined
mechanism is used to protect from disclosure all protocol
messages received by the party.
o Its partyPrivPrivate component is called the private
privacy key and represents any secret value needed to
support the Symmetric Privacy Protocol and associated
encryption algorithm.
o Its partyPrivPublic component is called the public
privacy key and represents any public value that may be
needed to support the privacy protocol. This component
is not significant except as suggested in Section 5.4.
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3. Digest Authentication Protocol
This section describes the Digest Authentication Protocol. It
provides both for verifying the integrity of a received
message (i.e., the message received is the message sent) and
for verifying the origin of a message (i.e., the reliable
identification of the originator). The integrity of the
message is protected by computing a digest over an appropriate
portion of a message. The digest is computed by the
originator of the message, transmitted with the message, and
verified by the recipient of the message.
A secret value known only to the originator and recipient of
the message is prefixed to the message prior to the digest
computation. Thus, the origin of the message is known
implicitly with the verification of the digest.
A requirement on parties using this Digest Authentication
Protocol is that they shall not originate messages for
transmission to any destination party which does not also use
this Digest Authentication Protocol. This restriction
excludes undesirable side effects of communication between a
party which uses these security protocols and a party which
does not.
Recall from [1] that a SNMPv2 management communication is
represented by an ASN.1 value with the following syntax:
SnmpMgmtCom ::= [2] IMPLICIT SEQUENCE {
dstParty
OBJECT IDENTIFIER,
srcParty
OBJECT IDENTIFIER,
context
OBJECT IDENTIFIER,
pdu
PDUs
}
For each SnmpMgmtCom value that represents a SNMPv2 management
communication, the following statements are true:
o Its dstParty component is called the destination and
identifies the SNMPv2 party to which the communication is
directed.
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o Its srcParty component is called the source and
identifies the SNMPv2 party from which the communication
is originated.
o Its context component identifies the SNMPv2 context
containing the management information referenced by the
communication.
o Its pdu component has the form and significance
attributed to it in [12].
Recall from [1] that a SNMPv2 authenticated management
communication is represented by an ASN.1 value with the
following syntax:
SnmpAuthMsg ::= [1] IMPLICIT SEQUENCE {
authInfo
ANY, - defined by authentication protocol
authData
SnmpMgmtCom
}
For each SnmpAuthMsg value that represents a SNMPv2
authenticated management communication, the following
statements are true:
o Its authInfo component is called the authentication
information and represents information required in
support of the authentication protocol used by both the
SNMPv2 party originating the message, and the SNMPv2
party receiving the message. The detailed significance
of the authentication information is specific to the
authentication protocol in use; it has no effect on the
application semantics of the communication other than its
use by the authentication protocol in determining whether
the communication is authentic or not.
o Its authData component is called the authentication data
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and represents a SNMPv2 management communication.
In support of the Digest Authentication Protocol, an authInfo
component is of type AuthInformation:
AuthInformation ::= [2] IMPLICIT SEQUENCE {
authDigest
OCTET STRING,
authDstTimestamp
UInteger32,
authSrcTimestamp
UInteger32
}
For each AuthInformation value that represents authentication
information, the following statements are true:
o Its authDigest component is called the authentication
digest and represents the digest computed over an
appropriate portion of the message, where the message is
temporarily prefixed with a secret value for the purposes
of computing the digest.
o Its authSrcTimestamp component is called the
authentication timestamp and represents the time of the
generation of the message according to the partyAuthClock
of the SNMPv2 party that originated it. Note that the
granularity of the authentication timestamp is 1 second.
o Its authDstTimestamp component is called the
authentication timestamp and represents the time of the
generation of the message according to the partyAuthClock
of the SNMPv2 party that is to receive it. Note that the
granularity of the authentication timestamp is 1 second.
3.1. Generating a Message
This section describes the behavior of a SNMPv2 entity when it
acts as a SNMPv2 party for which the authentication protocol
is administratively specified as the Digest Authentication
Protocol. Insofar as the behavior of a SNMPv2 entity when
transmitting protocol messages is defined generically in [1],
only those aspects of that behavior that are specific to the
Digest Authentication Protocol are described below. In
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particular, this section describes the encapsulation of a
SNMPv2 management communication into a SNMPv2 authenticated
management communication.
According to Section 3.1 of [1], a SnmpAuthMsg value is
constructed during Step 3 of generic processing. In
particular, it states the authInfo component is constructed
according to the authentication protocol identified for the
SNMPv2 party originating the message. When the relevant
authentication protocol is the Digest Authentication Protocol,
the procedure performed by a SNMPv2 entity whenever a
management communication is to be transmitted by a SNMPv2
party is as follows.
(1) The local database is consulted to determine the
authentication clock and private authentication key
(extracted, for example, according to the conventions
defined in Section 1.5.1) of the SNMPv2 party originating
the message. The local database is also consulted to
determine the authentication clock of the receiving
SNMPv2 party.
(2) The authSrcTimestamp component is set to the retrieved
authentication clock value of the message's source. The
authDstTimestamp component is set to the retrieved
authentication clock value of the message's intended
recipient.
(3) The authentication digest is temporarily set to the
private authentication key of the SNMPv2 party
originating the message. The SnmpAuthMsg value is
serialized according to the conventions of [13] and [12].
A digest is computed over the octet sequence representing
that serialized value using, for example, the algorithm
specified in Section 1.5.1. The authDigest component is
set to the computed digest value.
As set forth in [1], the SnmpAuthMsg value is then
encapsulated according to the appropriate privacy protocol
into a SnmpPrivMsg value. This latter value is then
serialized and transmitted to the receiving SNMPv2 party.
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3.2. Receiving a Message
This section describes the behavior of a SNMPv2 entity upon
receipt of a protocol message from a SNMPv2 party for which
the authentication protocol is administratively specified as
the Digest Authentication Protocol. Insofar as the behavior
of a SNMPv2 entity when receiving protocol messages is defined
generically in [1], only those aspects of that behavior that
are specific to the Digest Authentication Protocol are
described below.
According to Section 3.2 of [1], a SnmpAuthMsg value is
evaluated during Step 9 of generic processing. In particular,
it states the SnmpAuthMsg value is evaluated according to the
authentication protocol identified for the SNMPv2 party that
originated the message. When the relevant authentication
protocol is the Digest Authentication Protocol, the procedure
performed by a SNMPv2 entity whenever a management
communication is received by a SNMPv2 party is as follows.
(1) If the ASN.1 type of the authInfo component is not
AuthInformation, the message is evaluated as unauthentic,
and the snmpStatsBadAuths counter [14] is incremented.
Otherwise, the authSrcTimestamp, authDstTimestamp, and
authDigest components are extracted from the SnmpAuthMsg
value.
(2) The local database is consulted to determine the
authentication clock, private authentication key
(extracted, for example, according to the conventions
defined in Section 1.5.1), and lifetime of the SNMPv2
party that originated the message.
(3) If the authSrcTimestamp component plus the lifetime is
less than the authentication clock, the message is
evaluated as unauthentic, and the snmpStatsNotInLifetimes
counter [14] is incremented.
(4) The authDigest component is extracted and temporarily
recorded.
(5) A new SnmpAuthMsg value is constructed such that its
authDigest component is set to the private authentication
key and its other components are set to the value of the
corresponding components in the received SnmpAuthMsg
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value. This new SnmpAuthMsg value is serialized
according to the conventions of [13] and [12]. A digest
is computed over the octet sequence representing that
serialized value using, for example, the algorithm
specified in Section 1.5.1.
NOTE
Because serialization rules are unambiguous but may
not be unique, great care must be taken in
reconstructing the serialized value prior to
computing the digest. Implementations may find it
useful to keep a copy of the original serialized
value and then simply modify the octets which
directly correspond to the placement of the
authDigest component, rather than re-applying the
serialization algorithm to the new SnmpAuthMsg
value.
(6) If the computed digest value is not equal to the digest
value temporarily recorded in step 4 above, the message
is evaluated as unauthentic, and the
snmpStatsWrongDigestValues counter [14] is incremented.
(7) The message is evaluated as authentic.
(8) The local database is consulted for access privileges
permitted by the local access policy to the originating
SNMPv2 party with respect to the receiving SNMPv2 party.
If any level of access is permitted, then:
the authentication clock value locally recorded for the
originating SNMPv2 party is advanced to the
authSrcTimestamp value if this latter exceeds the
recorded value; and,
the authentication clock value locally recorded for the
receiving SNMPv2 party is advanced to the
authDstTimestamp value if this latter exceeds the
recorded value.
(Note that this step is conceptually independent from
Steps 15-17 of Section 3.2 in [1]).
If the SnmpAuthMsg value is evaluated as unauthentic, an
authentication failure is noted and the received message is
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discarded without further processing. Otherwise, processing
of the received message continues as specified in [1].
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4. Symmetric Privacy Protocol
This section describes the Symmetric Privacy Protocol. It
provides for protection from disclosure of a received message.
An appropriate portion of the message is encrypted according
to a secret key known only to the originator and recipient of
the message.
This protocol assumes the underlying mechanism is a symmetric
encryption algorithm. In addition, the message to be
encrypted must be protected according to the conventions of
the Digest Authentication Protocol.
Recall from [1] that a SNMPv2 private management communication
is represented by an ASN.1 value with the following syntax:
SnmpPrivMsg ::= [1] IMPLICIT SEQUENCE {
privDst
OBJECT IDENTIFIER,
privData
[1] IMPLICIT OCTET STRING
}
For each SnmpPrivMsg value that represents a SNMPv2 private
management communication, the following statements are true:
o Its privDst component is called the privacy destination
and identifies the SNMPv2 party to which the
communication is directed.
o Its privData component is called the privacy data and
represents the (possibly encrypted) serialization
(according to the conventions of [13] and [12]) of a
SNMPv2 authenticated management communication.
4.1. Generating a Message
This section describes the behavior of a SNMPv2 entity when it
communicates with a SNMPv2 party for which the privacy
protocol is administratively specified as the Symmetric
Privacy Protocol. Insofar as the behavior of a SNMPv2 entity
when transmitting a protocol message is defined generically in
[1], only those aspects of that behavior that are specific to
the Symmetric Privacy Protocol are described below. In
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particular, this section describes the encapsulation of a
SNMPv2 authenticated management communication into a SNMPv2
private management communication.
According to Section 3.1 of [1], a SnmpPrivMsg value is
constructed during Step 5 of generic processing. In
particular, it states the privData component is constructed
according to the privacy protocol identified for the SNMPv2
party receiving the message. When the relevant privacy
protocol is the Symmetric Privacy Protocol, the procedure
performed by a SNMPv2 entity whenever a management
communication is to be transmitted by a SNMPv2 party is as
follows.
(1) If the SnmpAuthMsg value is not authenticated according
to the conventions of the Digest Authentication Protocol,
the generation of the private management communication
fails according to a local procedure, without further
processing.
(2) The local database is consulted to determine the private
privacy key of the SNMPv2 party receiving the message
(represented, for example, according to the conventions
defined in Section 1.5.2).
(3) The SnmpAuthMsg value is serialized according to the
conventions of [13] and [12].
(4) The octet sequence representing the serialized
SnmpAuthMsg value is encrypted using, for example, the
algorithm specified in Section 1.5.2 and the extracted
private privacy key.
(5) The privData component is set to the encrypted value.
As set forth in [1], the SnmpPrivMsg value is then serialized
and transmitted to the receiving SNMPv2 party.
4.2. Receiving a Message
This section describes the behavior of a SNMPv2 entity when it
acts as a SNMPv2 party for which the privacy protocol is
administratively specified as the Symmetric Privacy Protocol.
Insofar as the behavior of a SNMPv2 entity when receiving a
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protocol message is defined generically in [1], only those
aspects of that behavior that are specific to the Symmetric
Privacy Protocol are described below.
According to Section 3.2 of [1], the privData component of a
received SnmpPrivMsg value is evaluated during Step 4 of
generic processing. In particular, it states the privData
component is evaluated according to the privacy protocol
identified for the SNMPv2 party receiving the message. When
the relevant privacy protocol is the Symmetric Privacy
Protocol, the procedure performed by a SNMPv2 entity whenever
a management communication is received by a SNMPv2 party is as
follows.
(1) The local database is consulted to determine the private
privacy key of the SNMPv2 party receiving the message
(represented, for example, according to the conventions
defined in Section 1.5.2).
(2) The contents octets of the privData component are
decrypted using, for example, the algorithm specified in
Section 1.5.2 and the extracted private privacy key.
Processing of the received message continues as specified in
[1].
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5. Clock and Secret Distribution
The protocols described in Sections 3 and 4 assume the
existence of loosely synchronized clocks and shared secret
values. Three requirements constrain the strategy by which
clock values and secrets are distributed.
o If the value of an authentication clock is decreased, the
private authentication key must be changed concurrently.
When the value of an authentication clock is decreased,
messages that have been sent with a timestamp value
between the value of the authentication clock and its new
value may be replayed. Changing the private
authentication key obviates this threat.
o The private authentication key and private privacy key
must be known only to the parties requiring knowledge of
them.
Protecting the secrets from disclosure is critical to the
security of the protocols. Knowledge of the secrets must
be as restricted as possible within an implementation.
In particular, although the secrets may be known to one
or more persons during the initial configuration of a
device, the secrets should be changed immediately after
configuration such that their actual value is known only
to the software. A management station has the additional
responsibility of recovering the state of all parties
whenever it boots, and it may address this responsibility
by recording the secrets on a long-term storage device.
Access to information on this device must be as
restricted as is practically possible.
o There must exist at least one SNMPv2 entity that assumes
the role of a responsible management station.
This management station is responsible for ensuring that
all authentication clocks are synchronized and for
changing the secret values when necessary. Although more
than one management station may share this
responsibility, their coordination is essential to the
secure management of the network. The mechanism by which
multiple management stations ensure that no more than one
of them attempts to synchronize the clocks or update the
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secrets at any one time is a local implementation issue.
A responsible management station may either support clock
synchronization and secret distribution as separate
functions, or combine them into a single functional unit.
The first section below specifies the procedures by which a
SNMPv2 entity is initially configured. The next two sections
describe one strategy for distributing clock values and one
for determining a synchronized clock value among SNMPv2
parties supporting the Digest Authentication Protocol. For
SNMPv2 parties supporting the Symmetric Privacy Protocol, the
next section describes a strategy for distributing secret
values. The last section specifies the procedures by which a
SNMPv2 entity recovers from a "crash."
5.1. Initial Configuration
This section describes the initial configuration of a SNMPv2
entity that supports the Digest Authentication Protocol or
both the Digest Authentication Protocol and the Symmetric
Privacy Protocol.
When a network device is first installed, its initial, secure
configuration must be done manually, i.e., a person must
physically visit the device and enter the initial secret
values for at least its first secure SNMPv2 party. This
requirement suggests that the person will have knowledge of
the initial secret values.
In general, the security of a system is enhanced as the number
of entities that know a secret is reduced. Requiring a person
to physically visit a device every time a SNMPv2 party is
configured not only exposes the secrets unnecessarily but is
administratively prohibitive. In particular, when MD5 is
used, the initial authentication secret is 128 bits long and
when DES is used an additional 128 bits are needed - 64 bits
each for the key and initialization vector. Clearly, these
values will need to be recorded on a medium in order to be
transported between a responsible management station and a
managed agent. The recommended procedure is to configure a
small set of initial SNMPv2 parties for each SNMPv2 entity,
one pair of which may be used initially to configure all other
SNMPv2 parties.
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In fact, there is a minimal, useful set of SNMPv2 parties that
could be configured between each responsible management
station and managed agent. This minimal set includes one of
each of the following for both the responsible management
station and the managed agent:
o a SNMPv2 party for which the authentication protocol and
privacy protocol are the values noAuth and noPriv,
respectively,
o a SNMPv2 party for which the authentication protocol
identifies the mechanism defined in Section 1.5.1 and its
privacy protocol is the value noPriv, and
o a SNMPv2 party for which the authentication protocol and
privacy protocol identify the mechanisms defined in
Section 1.5.1 and Section 1.5.2, respectively.
The last of these SNMPv2 parties in both the responsible
management station and the managed agent could be used to
create all other SNMPv2 parties.
Configuring one pair of SNMPv2 parties to be used to configure
all other parties has the advantage of exposing only one pair
of secrets - the secrets used to configure the minimal, useful
set identified above. To limit this exposure, the responsible
management station should change these values as its first
operation upon completion of the initial configuration. In
this way, secrets are known only to the peers requiring
knowledge of them in order to communicate.
The Management Information Base (MIB) document [4] supporting
these security protocols specifies 6 initial party identities
and initial values, which, by convention, are assigned to the
parties and their associated parameters.
These 6 initial parties are required to exist as part of the
configuration of implementations when first installed, with
the exception that implementations not providing support for a
privacy protocol only need the 4 initial parties for which the
privacy protocol is noPriv. When installing a managed agent,
these parties need to be configured with their initial
secrets, etc., both in the responsible management station and
in the new agent.
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If the responsible management station is configured first, it
can be used to generate the initial secrets and provide them
to a person, on a suitable medium, for distribution to the
managed agent. The following sequence of steps describes the
initial configuration of a managed agent and its responsible
management station.
(1) Determine the initial values for each of the attributes
of the SNMPv2 party to be configured. Some of these
values may be computed by the responsible management
station, some may be specified in the MIB document, and
some may be administratively determined.
(2) Configure the parties in the responsible management
station, according to the set of initial values. If the
management station is computing some initial values to be
entered into the agent, an appropriate medium must be
present to record the values.
(3) Configure the parties in the managed agent, according to
the set of initial values.
(4) The responsible management station must synchronize the
authentication clock values for each party it shares with
each managed agent. Section 5.3 specifies one strategy
by which this could be accomplished.
(5) The responsible management station should change the
secret values manually configured to ensure the actual
values are known only to the peers requiring knowledge of
them in order to communicate. To do this, the management
station generates new secrets for each party to be
reconfigured and distributes the updates using any
strategy which protects the new values from disclosure;
use of a SNMPv2 set operation acting on the managed
objects defined in [4] is such a strategy. Upon
receiving positive acknowledgement that the new values
have been distributed, the management station should
update its local database with the new values.
If the managed agent does not support a protocol that protects
messages from disclosure, e.g., the Symmetric Privacy Protocol
(see section 5.4), then the distribution of new secrets, after
the compromise of existing secrets, is not possible. In this
case, the new secrets can only be distributed by a physical
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visit to the device.
If there are other SNMPv2 protocol entities requiring
knowledge of the secrets, the responsible management station
must distribute the information upon completion of the initial
configuration. The considerations, mentioned above,
concerning the protection of secrets from disclosure, also
apply in this case.
5.2. Clock Distribution
A responsible management station must ensure that the
authentication clock value for each SNMPv2 party for which it
is responsible
o is loosely synchronized among all the local databases in
which it appears,
o is reset, as indicated below, upon reaching its maximal
value, and
o is non-decreasing, except as indicated below.
The skew among the clock values must be accounted for in the
lifetime value, in addition to the expected communication
delivery delay.
A skewed authentication clock may be detected by a number of
strategies, including knowledge of the accuracy of the system
clock, unauthenticated queries of the party database, and
recognition of authentication failures originated by the
party.
Whenever clock skew is detected, and whenever the SNMPv2
entities at both the responsible management station and the
relevant managed agent support an appropriate privacy protocol
(e.g., the Symmetric Privacy Protocol), a straightforward
strategy for the correction of clock skew is simultaneous
alteration of authentication clock and private key for the
relevant SNMPv2 party. If the request to alter the key and
clock for a particular party originates from that same party,
then, prior to transmitting that request, the local notion of
the authentication clock is artificially advanced to assure
acceptance of the request as authentic.
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More generally, however, since an authentication clock value
need not be protected from disclosure, it is not necessary
that a managed agent support a privacy protocol in order for a
responsible management station to correct skewed clock values.
The procedure for correcting clock skew in the general case is
presented in Section 5.3.
In addition to correcting skewed notions of authentication
clocks, every SNMPv2 entity must react correctly as an
authentication clock approaches its maximal value. If the
authentication clock for a particular SNMPv2 party ever
reaches the maximal time value, the clock must halt at that
value. (The value of interest may be the maximum less
lifetime. When authenticating a message, its authentication
timestamp is added to lifetime and compared to the
authentication clock. A SNMPv2 entity must guarantee that the
sum is never greater than the maximal time value.) In this
state, the only authenticated request a management station
should generate for this party is one that alters the value of
at least its authentication clock and private authentication
key. In order to reset these values, the responsible
management station may set the authentication timestamp in the
message to the maximal time value.
The value of the authentication clock for a particular SNMPv2
party must never be altered such that its new value is less
than its old value, unless its private authentication key is
also altered at the same time.
5.3. Clock Synchronization
Unless the secrets are changed at the same time, the correct
way to synchronize clocks is to advance the slower clock to be
equal to the faster clock. Suppose that party agentParty is
realized by the SNMPv2 entity in a managed agent; suppose that
party mgrParty is realized by the SNMPv2 entity in the
corresponding responsible management station. For any pair of
parties, there are four possible conditions of the
authentication clocks that could require correction:
(1) The management station's notion of the value of the
authentication clock for agentParty exceeds the agent's
notion.
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(2) The management station's notion of the value of the
authentication clock for mgrParty exceeds the agent's
notion.
(3) The agent's notion of the value of the authentication
clock for agentParty exceeds the management station's
notion.
(4) The agent's notion of the value of the authentication
clock for mgrParty exceeds the management station's
notion.
The selective clock acceleration mechanism intrinsic to the
protocol corrects conditions 1, 2 and 3 as part of the normal
processing of an authentic message. Therefore, the clock
adjustment procedure below does not provide for any
adjustments in those cases. Rather, the following sequence of
steps specifies how the clocks may be synchronized when
condition 4 is manifest.
(1) The responsible management station saves its existing
notion of the authentication clock for the party
mgrParty.
(2) The responsible management station retrieves the
authentication clock value for mgrParty from the agent.
This retrieval must be an unauthenticated request, since
the management station does not know if the clocks are
synchronized. If the request fails, the clocks cannot be
synchronized, and the clock adjustment procedure is
aborted without further processing.
(3) If the notion of the authentication clock for mgrParty
just retrieved from the agent exceeds the management
station's notion, then condition 4 is manifest, and the
responsible management station advances its notion of the
authentication clock for mgrParty to match the agent's
notion.
(4) The responsible management station retrieves the
authentication clock value for mgrParty from the agent.
This retrieval must be an authenticated request, in order
that the management station may verify that the clock
value is properly synchronized. If this authenticated
query fails, then the management station restores its
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previously saved notion of the clock value, and the clock
adjustment procedure is aborted without further
processing. Otherwise, clock synchronization has been
successfully realized.
Administrative advancement of a clock as described above does
not introduce any new vulnerabilities, since the value of the
clock is intended to increase with the passage of time. A
potential operational problem is the rejection of authentic
management operations that were authenticated using a previous
value of the relevant party clock. This possibility may be
avoided if a management station suppresses generation of
management traffic between relevant parties while this clock
adjustment procedure is in progress.
5.4. Secret Distribution
This section describes one strategy by which a SNMPv2 entity
that supports both the Digest Authentication Protocol and the
Symmetric Privacy Protocol can change the secrets for a
particular SNMPv2 party.
The frequency with which the secrets of a SNMPv2 party should
be changed is a local administrative issue. However, the more
frequently a secret is used, the more frequently it should be
changed. At a minimum, the secrets must be changed whenever
the associated authentication clock approaches its maximal
value (see Section 6). Note that, owing to both
administrative and automatic advances of the authentication
clock described in this memo, the authentication clock for a
SNMPv2 party may well approach its maximal value sooner than
might otherwise be expected.
The following sequence of steps specifies how a responsible
management station alters a secret value (i.e., the private
authentication key or the private privacy key) for a
particular SNMPv2 party. There are two cases.
First, setting the initial secret for a new party:
(1) The responsible management station generates a new secret
value.
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(2) The responsible management station encapsulates a SNMPv2
setRequest in a SNMPv2 private management communication
with at least the following properties.
Its source supports the Digest Authentication
Protocol and the Symmetric Privacy Protocol.
Its destination supports the Symmetric Privacy
Protocol and the Digest Authentication Protocol.
(3) The SNMPv2 private management communication is
transmitted to its destination.
(4) Upon receiving the request, the recipient processes the
message according to [12] and [1].
(5) The recipient encapsulates a SNMPv2 response in a SNMPv2
private management communication with at least the
following properties.
Its source supports the Digest Authentication
Protocol and the Symmetric Privacy Protocol.
Its destination supports the Symmetric Privacy
Protocol and the Digest Authentication Protocol.
(6) The SNMPv2 private management communication is
transmitted to its destination.
(7) Upon receiving the response, the responsible management
station updates its local database with the new value.
Second, modifying the current secret of an existing party:
(1) The responsible management station generates a new secret
value.
(2) The responsible management station encapsulates a SNMPv2
setRequest in a SNMPv2 management communication with at
least the following properties.
Its source and destination supports the Digest
Authentication Protocol.
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(3) The SNMPv2 private management communication is
transmitted to its destination.
(4) Upon receiving the request, the recipient processes the
message according to [12] and [1].
(5) The recipient encapsulates a SNMPv2 response in a SNMPv2
management communication with at least the following
properties.
Its source and destination supports the Digest
Authentication Protocol.
(6) The SNMPv2 management communication is transmitted to its
destination.
(7) Upon receiving the response, the responsible management
station updates its local database with the new value.
If the responsible management station does not receive a
response to its request, there are two possible causes.
o The request may not have been delivered to the
destination.
o The response may not have been delivered to the
originator of the request.
In order to distinguish the two possible error conditions, a
responsible management station could check the destination to
see if the change has occurred. Unfortunately, since the
secret values are unreadable, this is not directly possible.
The recommended strategy for verifying key changes is to set
the public value corresponding to the secret being changed to
a recognizable, novel value: that is, alter the public
authentication key value for the relevant party when changing
its private authentication key, or alter its public privacy
key value when changing its private privacy key. In this way,
the responsible management station may retrieve the public
value when a response is not received, and verify whether or
not the change has taken place. (This strategy is available
since the public values are not used by the protocols defined
in this memo. If this strategy is employed, then the public
values are significant in this context. Of course, protocols
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using the public values may make use of this strategy
directly.)
One other scenario worthy of mention is using a SNMPv2 party
to change its own secrets. In this case, the destination will
change its local database prior to generating a response.
Thus, the response will be constructed according to the new
value. However, the responsible management station will not
update its local database until after the response is
received. This suggests the responsible management station
may receive a response which will be evaluated as unauthentic,
unless the correct secret is used. The responsible management
station may either account for this scenario as a special
case, or use an alteration of the relevant public values (as
described above) to verify the key change.
Note, during the period of time after the request has been
sent and before the response is received, the management
station must keep track of both the old and new secret values.
Since the delay may be the result of a network failure, the
management station must be prepared to retain both values for
an extended period of time, including across reboots.
5.5. Crash Recovery
This section describes the requirements for SNMPv2 protocol
entities in connection with recovery from system crashes or
other service interruptions.
For each SNMPv2 party in the local database for a particular
SNMPv2 entity, its identity, authentication clock, private
authentication key, and private privacy key must enjoy non-
volatile, incorruptible representations. If possible,
lifetime should also enjoy a non-volatile, incorruptible
representation. If said SNMPv2 entity supports other security
protocols or algorithms in addition to the two defined in this
memo, then the authentication protocol and the privacy
protocol for each party also require non-volatile,
incorruptible representation.
The authentication clock of a SNMPv2 party is a critical
component of the overall security of the protocols. The
inclusion of a reliable representation of a clock in a SNMPv2
entity is required for overall security. A reliable clock
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representation ensures that a clock's value is monotonically
increasing, even across a power loss or other system failure
of the local SNMPv2 entity. One example of a reliable clock
representation is that provided by battery-powered clock-
calendar devices incorporated into some contemporary systems.
Another example is storing and updating a clock value in non-
volatile storage at a frequency of once per U (e.g., 24)
hours, and re-initialising that clock value on every reboot as
the stored value plus U+1 hours. It is assumed that
management stations always support reliable clock
representations, where clock adjustment by a human operator
during crash recovery may contribute to that reliability.
If a managed agent crashes and does not reboot in time for its
responsible management station to prevent its authentication
clock from reaching its maximal value, upon reboot the clock
must be halted at its maximal value. The procedures specified
in Section 5.3 would then apply.
Upon recovery, those attributes of each SNMPv2 party that do
not enjoy non-volatile or reliable representation are
initialized as follows.
o If the private authentication key is not the OCTET STRING
of zero length, the authentication protocol is set to
identify use of the Digest Authentication Protocol in
conjunction with the algorithm specified in Section
1.5.1.
o If the lifetime is not retained, it should be initialized
to zero.
o If the private privacy key is not the OCTET STRING of
zero length, the privacy protocol is set to identify use
of the Symmetric Privacy Protocol in conjunction with the
algorithm specified in Section 1.5.2.
Upon detecting that a managed agent has rebooted, a
responsible management station must reset all other party
attributes, including the lifetime if it was not retained. In
order to reset the lifetime, the responsible management
station should set the authentication timestamp in the message
to the sum of the authentication clock and desired lifetime.
This is an artificial advancement of the authentication
timestamp in order to guarantee the message will be authentic
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when received by the recipient.
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6. Security Considerations
This section highlights security considerations relevant to
the protocols and procedures defined in this memo. Practices
that contribute to secure, effective operation of the
mechanisms defined here are described first. Constraints on
implementation behavior that are necessary to the security of
the system are presented next. Finally, an informal account
of the contribution of each mechanism of the protocols to the
required goals is presented.
6.1. Recommended Practices
This section describes practices that contribute to the
secure, effective operation of the mechanisms defined in this
memo.
o A management station should discard SNMPv2 responses for
which neither the request-id component nor the
represented management information corresponds to any
currently outstanding request.
Although it would be typical for a management station to
do this as a matter of course, in the context of these
security protocols it is significant owing to the
possibility of message duplication (malicious or
otherwise).
o A management station should not interpret an agent's lack
of response to an authenticated SNMPv2 management
communication as a conclusive indication of agent or
network failure.
It is possible for authentication failure traps to be
lost or suppressed as a result of authentication clock
skew or inconsistent notions of shared secrets. In order
either to facilitate administration of such SNMPv2
parties or to provide for continued management in times
of network stress, a management station implementation
may provide for arbitrary, artificial advancement of the
timestamp or selection of shared secrets on locally
generated messages.
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o The lifetime value for a SNMPv2 party should be chosen
(by the local administration) to be as small as possible,
given the accuracy of clock devices available, relevant
round-trip communications delays, and the frequency with
which a responsible management station will be able to
verify all clock values.
A large lifetime increases the vulnerability to malicious
delays of SNMPv2 messages. The implementation of a
management station may accommodate changing network
conditions during periods of network stress by
effectively increasing the lifetimes of the source and
destination parties. The management station accomplishes
this by artificially advancing its notion of the source
party's clock on messages it sends, and by artificially
increasing its notion of the source party`s lifetime on
messages it receives.
o When sending state altering messages to a managed agent,
a management station should delay sending successive
messages to the managed agent until a positive
acknowledgement is received for the previous message or
until the previous message expires.
No message ordering is imposed by the SNMPv2. Messages
may be received in any order relative to their time of
generation and each will be processed in the ordered
received. Note that when an authenticated message is
sent to a managed agent, it will be valid for a period of
time that does not exceed lifetime under normal
circumstances, and is subject to replay during this
period.
Indeed, a management station must cope with the loss and
re-ordering of messages resulting from anomalies in the
network as a matter of course.
However, a managed object, snmpSetSerialNo [14], is
specifically defined for use with SNMPv2 set operations
in order to provide a mechanism to ensure the processing
of SNMPv2 messages occurs in a specific order.
o The frequency with which the secrets of a SNMPv2 party
should be changed is indirectly related to the frequency
of their use.
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Protecting the secrets from disclosure is critical to the
overall security of the protocols. Frequent use of a
secret provides a continued source of data that may be
useful to a cryptanalyst in exploiting known or perceived
weaknesses in an algorithm. Frequent changes to the
secret avoid this vulnerability.
Changing a secret after each use is generally regarded as
the most secure practice, but a significant amount of
overhead may be associated with that approach.
Note, too, in a local environment the threat of
disclosure may be insignificant, and as such the changing
of secrets may be less frequent. However, when public
data networks are the communication paths, more caution
is prudent.
o In order to foster the greatest degree of security, a
management station implementation must support
constrained, pairwise sharing of secrets among SNMPv2
entities as its default mode of operation.
Owing to the use of symmetric cryptography in the
protocols defined here, the secrets associated with a
particular SNMPv2 party must be known to all other SNMPv2
parties with which that party may wish to communicate.
As the number of locations at which secrets are known and
used increases, the likelihood of their disclosure also
increases, as does the potential impact of that
disclosure. Moreover, if the set of SNMPv2 protocol
entities with knowledge of a particular secret numbers
more than two, data origin cannot be reliably
authenticated because it is impossible to determine with
any assurance which entity of that set may be the
originator of a particular SNMPv2 message. Thus, the
greatest degree of security is afforded by configurations
in which the secrets for each SNMPv2 party are known to
at most two protocol entities.
6.2. Conformance
A SNMPv2 entity implementation that claims conformance to this
memo must satisfy the following requirements:
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(1) It must implement the noAuth and noPriv protocols whose
object identifiers are defined in [4].
noAuth This protocol signifies that messages
generated by a party using it are not protected as
to origin or integrity. It is required to ensure
that a party's authentication clock is always
accessible.
noPriv This protocol signifies that messages
received by a party using it are not protected from
disclosure. It is required to ensure that a party's
authentication clock is always accessible.
(2) It must implement the Digest Authentication Protocol in
conjunction with the algorithm defined in Section 1.5.1.
(3) It must include in its local database at least one SNMPv2
party with the following parameters set as follows:
partyAuthProtocol is set to noAuth and
partyPrivProtocol is set to noPriv.
This party must have a MIB view [1] specified that
includes at least the authentication clock of all other
parties. Alternatively, the authentication clocks of the
other parties may be partitioned among several similarly
configured parties according to a local implementation
convention.
(4) For each SNMPv2 party about which it maintains
information in a local database, an implementation must
satisfy the following requirements:
(a) It must not allow a party's parameters to be set
to a value inconsistent with its expected syntax.
In particular, Section 1.4 specifies constraints for
the chosen mechanisms.
(b) It must, to the maximal extent possible,
prohibit read-access to the private authentication
key and private encryption key under all
circumstances except as required to generate and/or
validate SNMPv2 messages with respect to that party.
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This prohibition includes prevention of read-access
by the entity's human operators.
(c) It must allow the party's authentication clock
to be publicly accessible. The correct operation of
the Digest Authentication Protocol requires that it
be possible to determine this value at all times in
order to guarantee that skewed authentication clocks
can be resynchronized.
(d) It must prohibit alterations to its record of
the authentication clock for that party
independently of alterations to its record of the
private authentication key (unless the clock
alteration is an advancement).
(e) It must never allow its record of the
authentication clock for that party to be
incremented beyond the maximal time value and so
"roll-over" to zero.
(f) It must never increase its record of the
lifetime for that party except as may be explicitly
authorized (via imperative command or securely
represented configuration information) by the
responsible network administrator.
(g) In the event that the non-volatile,
incorruptible representations of a party's
parameters (in particular, either the private
authentication key or private encryption key) are
lost or destroyed, it must alter its record of these
quantities to random values so subsequent
interaction with that party requires manual
redistribution of new secrets and other parameters.
(5) If it selects new value(s) for a party's secret(s), it
must avoid bad or obvious choices for said secret(s).
Choices to be avoided are boundary values (such as all-
zeros) and predictable values (such as the same value as
previously or selecting from a predetermined set).
(6) It must ensure that a received message for which the
originating party uses the Digest Authentication Protocol
but the receiving party does not, is always declared to
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be unauthentic. This may be achieved explicitly via an
additional step in the procedure for processing a
received message, or implicitly by verifying that all
local access control policies enforce this requirement.
6.3. Protocol Correctness
The correctness of these SNMPv2 security protocols with
respect to the stated goals depends on the following
assumptions:
(1) The chosen message digest algorithm satisfies its design
criteria. In particular, it must be computationally
infeasible to discover two messages that share the same
digest value.
(2) It is computationally infeasible to determine the secret
used in calculating a digest on the concatenation of the
secret and a message when both the digest and the message
are known.
(3) The chosen symmetric encryption algorithm satisfies its
design criteria. In particular, it must be
computationally infeasible to determine the cleartext
message from the ciphertext message without knowledge of
the key used in the transformation.
(4) Local notions of a party's authentication clock while it
is associated with a specific private key value are
monotonically non-decreasing (i.e., they never run
backwards) in the absence of administrative
manipulations.
(5) The secrets for a particular SNMPv2 party are known only
to authorized SNMPv2 protocol entities.
(6) Local notions of the authentication clock for a
particular SNMPv2 party are never altered such that the
authentication clock's new value is less than the current
value without also altering the private authentication
key.
For each mechanism of the protocol, an informal account of its
contribution to the required goals is presented below.
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Pseudocode fragments are provided where appropriate to
exemplify possible implementations; they are intended to be
self-explanatory.
6.3.1. Clock Monotonicity Mechanism
By pairing each sequence of a clock's values with a unique
key, the protocols partially realize goal 3, and the
conjunction of this property with assumption 6 above is
sufficient for the claim that, with respect to a specific
private key value, all local notions of a party's
authentication clock are, in general, non-decreasing with
time.
6.3.2. Data Integrity Mechanism
The protocols require computation of a message digest computed
over the SNMPv2 message prepended by the secret for the
relevant party. By virtue of this mechanism and assumptions 1
and 2, the protocols realize goal 1.
Normally, the inclusion of the message digest value with the
digested message would not be sufficient to guarantee data
integrity, since the digest value can be modified in addition
to the message while it is enroute. However, since not all of
the digested message is included in the transmission to the
destination, it is not possible to substitute both a message
and a digest value while enroute to a destination.
Strictly speaking, the specified strategy for data integrity
does not detect a SNMPv2 message modification which appends
extraneous material to the end of such messages. However,
owing to the representation of SNMPv2 messages as ASN.1
values, such modifications cannot - consistent with goal 1 -
result in unauthorized management operations.
The data integrity mechanism specified in this memo protects
only against unauthorized modification of individual SNMPv2
messages. A more general data integrity service that affords
protection against the threat of message stream modification
is not realized by this mechanism, although limited protection
against reordering, delay, and duplication of messages within
a message stream are provided by other mechanisms of the
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protocol.
6.3.3. Data Origin Authentication Mechanism
The data integrity mechanism requires the use of a secret
value known only to communicating parties. By virtue of this
mechanism and assumptions 1 and 2, the protocols explicitly
prevent unauthorized modification of messages. Data origin
authentication is implicit if the message digest value can be
verified. That is, the protocols realize goal 2.
6.3.4. Restricted Administration Mechanism
This memo requires that implementations preclude
administrative alterations of the authentication clock for a
particular party independently from its private authentication
key (unless that clock alteration is an advancement). An
example of an efficient implementation of this restriction is
provided in a pseudocode fragment below. This pseudocode
fragment meets the requirements of assumption 6. Observe that
the requirement is not for simultaneous alteration but to
preclude independent alteration. This latter requirement is
fairly easily realized in a way that is consistent with the
defined semantics of the SNMPv2 set operation.
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Void partySetKey (party, newKeyValue)
{
if (party->clockAltered) {
party->clockAltered = FALSE;
party->keyAltered = FALSE;
party->keyInUse = newKeyValue;
party->clockInUse = party->clockCache;
}
else {
party->keyAltered = TRUE;
party->keyCache = newKeyValue;
}
}
Void partySetClock (party, newClockValue)
{
if (party->keyAltered) {
party->keyAltered = FALSE;
party->clockAltered = FALSE;
party->clockInUse = newClockValue;
party->keyInUse = party->keyCache;
}
else {
party->clockAltered = TRUE;
party->clockCache = newClockValue;
}
}
6.3.5. Message Timeliness Mechanism
The definition of the SNMPv2 security protocols requires that,
if the authentication timestamp value on a received message -
augmented by an administratively chosen lifetime value - is
less than the local notion of the clock for the originating
SNMPv2 party, the message is not delivered.
if (timestampOfReceivedMsg +
party->administrativeLifetime <=
party->localNotionOfClock) {
msgIsValidated = FALSE;
}
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By virtue of this mechanism, the protocols realize goal 3. In
cases in which the local notions of a particular SNMPv2 party
clock are moderately well-synchronized, the timeliness
mechanism effectively limits the age of validly delivered
messages. Thus, if an attacker diverts all validated messages
for replay much later, the delay introduced by this attack is
limited to a period that is proportional to the skew among
local notions of the party clock.
6.3.6. Selective Clock Acceleration Mechanism
The definition of the SNMPv2 security protocols requires that,
if either of the timestamp values for the originating or
receiving parties on a received, validated message exceeds the
corresponding local notion of the clock for that party, then
the local notion of the clock for that party is adjusted
forward to correspond to said timestamp value. This mechanism
is neither strictly necessary nor sufficient to the security
of the protocol; rather, it fosters the clock synchronization
on which valid message delivery depends - thereby enhancing
the effectiveness of the protocol in a management context.
if (msgIsValidated) {
if (timestampOfReceivedMsg >
party->localNotionOfClock) {
party->localNotionOfClock =
timestampOfReceivedMsg;
}
}
The effect of this mechanism is to synchronize local notions
of a party clock more closely in the case where a sender's
notion is more advanced than a receiver's. In the opposite
case, this mechanism has no effect on local notions of a party
clock and either the received message is validly delivered or
not according to other mechanisms of the protocol.
Operation of this mechanism does not, in general, improve the
probability of validated delivery for messages generated by
party participants whose local notion of the party clock is
relatively less advanced. In this case, queries from a
management station may not be validly delivered and the
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management station needs to react appropriately (e.g., by use
of the strategy described in section 5.3). In contrast, the
delivery of SNMPv2 trap messages generated by an agent that
suffers from a less advanced notion of a party clock is more
problematic, for an agent may lack the capacity to recognize
and react to security failures that prevent delivery of its
messages. Thus, the inherently unreliable character of trap
messages is likely to be compounded by attempts to provide for
their validated delivery.
6.3.7. Confidentiality Mechanism
The protocols require the use of a symmetric encryption
algorithm when the data confidentiality service is required.
By virtue of this mechanism and assumption 3, the protocols
realize goal 4.
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7. Acknowledgements
This document is based, almost entirely, on RFC 1352.
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8. References
[1] Galvin, J., and McCloghrie, K., "Administrative Model for
version 2 of the Simple Network Management Protocol
(SNMPv2)", RFC 1445, Trusted Information Systems, Hughes
LAN Systems, April 1993.
[2] Case, J., Fedor, M., Schoffstall, M., Davin, J., "Simple
Network Management Protocol", STD 15, RFC 1157, SNMP
Research, Performance Systems International, MIT
Laboratory for Computer Science, May 1990.
[3] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
MIT Laboratory for Computer Science, April 1992.
[4] McCloghrie, K., and Galvin, J., "Party MIB for version 2
of the Simple Network Management Protocol (SNMPv2)", RFC
1447, Hughes LAN Systems, Trusted Information Systems,
April 1993.
[5] Data Encryption Standard, National Institute of Standards
and Technology. Federal Information Processing Standard
(FIPS) Publication 46-1. Supersedes FIPS Publication 46,
(January, 1977; reaffirmed January, 1988).
[6] Data Encryption Algorithm, American National Standards
Institute. ANSI X3.92-1981, (December, 1980).
[7] DES Modes of Operation, National Institute of Standards
and Technology. Federal Information Processing Standard
(FIPS) Publication 81, (December, 1980).
[8] Data Encryption Algorithm - Modes of Operation, American
National Standards Institute. ANSI X3.106-1983, (May
1983).
[9] Guidelines for Implementing and Using the NBS Data
Encryption Standard, National Institute of Standards and
Technology. Federal Information Processing Standard
(FIPS) Publication 74, (April, 1981).
[10] Validating the Correctness of Hardware Implementations of
the NBS Data Encryption Standard, National Institute of
Standards and Technology. Special Publication 500-20.
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[11] Maintenance Testing for the Data Encryption Standard,
National Institute of Standards and Technology. Special
Publication 500-61, (August, 1980).
[12] Case, J., McCloghrie, K., Rose, M., and Waldbusser, S.,
"Protocol Operations for version 2 of the Simple Network
Management Protocol (SNMPv2)", RFC 1448, SNMP Research,
Inc., Hughes LAN Systems, Dover Beach Consulting, Inc.,
Carnegie Mellon University, April 1993.
[13] Case, J., McCloghrie, K., Rose, M., and Waldbusser, S.,
"Transport Mappings for version 2 of the Simple Network
Management Protocol (SNMPv2)", RFC 1449, SNMP Research,
Inc., Hughes LAN Systems, Dover Beach Consulting, Inc.,
Carnegie Mellon University, April 1993.
[14] Case, J., McCloghrie, K., Rose, M., and Waldbusser, S.,
"Management Information Base for version 2 of the Simple
Network Management Protocol (SNMPv2)", RFC 1450, SNMP
Research, Inc., Hughes LAN Systems, Dover Beach
Consulting, Inc., Carnegie Mellon University, April 1993.
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9. Authors' Addresses
James M. Galvin
Trusted Information Systems, Inc.
3060 Washington Road, Route 97
Glenwood, MD 21738
Phone: +1 301 854-6889
EMail: galvin@tis.com
Keith McCloghrie
Hughes LAN Systems
1225 Charleston Road
Mountain View, CA 94043
US
Phone: +1 415 966 7934
Email: kzm@hls.com
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