User-based Security Model for version 3 of the
Simple Network Management Protocol (SNMPv3)
18 June 1997
U. Blumenthal
IBM T. J. Watson Research
uri@watson.ibm.com
B. Wijnen
IBM T. J. Watson Research
wijnen@vnet.ibm.com
<draft-ietf-snmpv3-usec-01.txt>
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as ``work in progress.''
To learn the current status of any Internet-Draft, please check the
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ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).
Abstract
This document describes the User-based Security Model (USEC) for SNMP
version 3 for use in the SNMP architecture [SNMP-ARCH]. This
document defines the Elements of Procedure for providing SNMP message
level security. This document also includes a MIB for remotely
monitoring/managing the configuration parameters for this Security
model.
0.1 Issues
- Do we indeed want to move all STATS counters to MPC, we
have assumed so for now.
- Do we need to do group mapping here and pass it back to MPC
we have assumed so for now... but other documents do not pass
groupName around.
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- Do we want to check reportableFlag to determine if caching
of securityData is needed or not.
0.2 Change Log
[version 1.2]
- changed (simplified) time sync in section 3 item 7.
- added usecUserMiId
- cleaned up text
- defined IV "salt" generation
- removed Statistics counters (now in MPC) and reportPDU
generation (now in MPC)
- Removed auth and des MIBs which are now merged into USEC MIB
- specified where cachedSecurityData needs to be discarded
- added abtract service interface definitions
- removed section on error reporting (is MPC responsibility)
- removed auth/priv protocol definitions, they are in ARCH now
- removed MIB definitions for snmpEngineID,Time,Boots. They
are in ARCH now.
[version 1.1]
- removed <securityCookie>.
- added <securityIdentity>, <securityCachedData>.
- added abstract function interface description of
inter-module communications.
- modified IV generation process to accomodate messages produced
faster than one-per-second (still open).
- always update the clock regardless of whether incoming message
was Report or not (if the message was properly authenticated
and its timestamp is ahead of our notion of their clock).
[version 1.0]
- first version posted to the v3editors mailing list.
- based on v2adv slides, v2adv items and issues list and on
RFC1910 and SNMPv2u and SNMPv2* documents.
- various iterations were done by the authors via private email.
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1. Introduction
The Architecture for describing Internet Management Frameworks
is composed of multiple subsystems:
1) a message processing and control subsystem,
2) a security subsystem,
3) an access control subsystem, and
4) orangelets.
It is important to understand the SNMP architecture and the
terminology of the architecture to understand where the model
described in this document fits into the architecture and interacts
with other subsystems within the architecture. The reader is
expected to have read and understood the description of the SNMP
architecture, as defined in [SNMP-ARCH].
This memo [SNMPv3-USEC] describes the User-Based Security model
as it is used within the SNMP Architecture. The main idea is that
we use the traditional concept of a user (identified by a userName)
to associate security information with.
This memo describes the use of Keyed-MD5 as the authentication
protocol and the use of CBC-DES as the privacy protocol.
The User-based Security model however allows for other such
protocols to be used instead of or concurrent with these protocols.
So the description of Keyed-MD5 and CBC-DES are in separate sections.
That way it shows that they are supposed to be self-contained
pieces that can be replaced or supplemented in the future.
1.1. Threats
Several of the classical threats to network protocols are applicable
to the network management problem and therefore would be applicable
to any SNMP security model. 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 this SNMPv3 security model
should provide protection are:
- Modification of Information
The modification threat is the danger that some unauthorized entity
may alter in-transit SNMPv3 messages generated on behalf of an
authorized user in such a way as to effect unauthorized management
operations, including falsifying the value of an object.
- Masquerade
The masquerade threat is the danger that management operations not
authorized for some user may be attempted by assuming the identity
of another user that has the appropriate authorizations.
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Two secondary threats are also identified. The security protocols
defined in this memo provide limited protection against:
- Disclosure
The disclosure threat is the danger of eavesdropping on the
exchanges between managed agents and a management station.
Protecting against this threat may be required as a matter of
local policy.
- Message Stream Modification
The SNMPv3 protocol is typically based upon a connection-less
transport service which may operate over any sub-network service.
The re-ordering, delay or replay of messages can and does occur
through the natural operation of many such sub-network 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
sub-network service, in order to effect unauthorized management
operations.
There are at least two threats that an SNMPv3 security protocol need
not protect against. The security protocols defined in this memo do
not provide protection against:
- Denial of Service
An SNMPv3 security protocol need not attempt to address the broad
range of attacks by which service on behalf of authorized users 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, an SNMPv3 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.
1.2. Goals and Constraints
Based on the foregoing account of threats in the SNMP network
management environment, the goals of this SNMPv3 security model are
as follows.
1) The protocol should provide for verification that each received
SNMPv3 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
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the user on whose behalf a received SNMPv3 message claims to have
been generated.
3) The protocol should provide for detection of received SNMPv3
messages, which request or contain management information, whose
time of generation was not recent.
4) The protocol should provide, when necessary, that the contents of
each received SNMPv3 message are protected from disclosure.
In addition to the principal goal of supporting secure network
management, the design of this SNMPv3 security model 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 design should
prefer the former.
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 key
management protocols).
3) A security mechanism should entail no changes to the basic SNMP
network management philosophy.
1.3. Security Services
The security services necessary to support the goals of an SNMPv3
security model 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 identity of the
user on whose behalf received data was originated is corroborated.
- Data Confidentiality
is the provision of the property that information is not made
available or disclosed to unauthorized individuals, entities, or
processes.
For the protocols specified in this memo, it is not possible to
assure the specific originator of a received SNMPv3 message; rather,
it is the user on whose behalf the message was originated that is
authenticated.
For these protocols, it not possible to obtain data integrity without
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data origin authentication, nor is it possible to obtain data origin
authentication without data integrity. Further, there is no
provision for data confidentiality without both data integrity and
data origin authentication.
The security protocols used in this memo are considered acceptably
secure at the time of writing. However, the procedures allow for new
authentication and privacy methods to be specified at a future time
if the need arises.
1.4. Implementation Organization
The security protocols defined in this memo are implemented in three
different modules and each have their specific responsibilities such
that together they realize the goals and security services described
above:
- The timeliness module must provide for:
- Protection against message delay or replay (to an extent greater
than can occur through normal operation)
- The authentication module must provide for:
- Data Integrity,
- Data Origin Authentication
- The privacy module must provide for
- Protection against disclosure of the message payload.
The timeliness module is fixed for this User-based Security model
while there is provision for multiple authentication and/or privacy
modules, each of which implements a specific authentication or
privacy protocol respectively.
1.4.1. Timeliness Module
Section 3 (Elements of procedure) uses the time values in an SNMPv3
message to do timeliness checking. The timeliness check is only
performed if authentication is applied to the message. Since the
complete message is checked for integrity, we can assume that the
time values in a message that passes the authentication module are
trustworthy.
1.4.2. Authentication Protocol
Section 6 describes the Keyed-MD5 authentication protocol which is
the first authentication protocol to be used with the User-based
Security model. In the future additional or replacement
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authentication protocols may be defined as new needs arise.
This User-based Security model prescribes that the complete message
is checked for integrity in the authentication module.
For a message to be authenticated, it needs to pass authentication
check by the authentication module and the timeliness check which
is a fixed part of this User-based Security model.
1.4.3. Privacy Protocol
Section 7 describes the CBC-DES Symmetric Encryption Protocol which
the first privacy protocol to be used with the User-based Security
model. In the future additional or replacement privacy protocols
may be defined as new needs arise.
This User-based Security model prescribes that the scopedPDU
is protected from disclosure when a message is sent with privacy.
This User-based Security model also prescribes that a message needs
to be authenticated if privacy is in use.
1.5 Protection against Message Replay, Delay and Redirection
1.5.1 Authoritative SNMP Engine
In order to protect against message replay, delay and redirection,
one of the SNMP engines involved in each communication is designated
to be the authoritative engine. For messages with a GET, GETNEXT,
GETBULK, SET or INFORM request as the payload, the receiver of such
messages is authoritative. For messages with a SNMPv2-TRAP,
RESPONSE or REPORT as the payload, the sender is authoritative.
1.5.2 The following mechanisms are used:
- To protect against the threat of message delay or replay (to an
extent greater than can occur through normal operation), a set of
time (at the authoritative source) indicators and a request-id are
included in each message generated. An SNMPv3 engine evaluates
the time indicators to determine if a received message is recent.
An SNMPv3 engine may evaluate the time indicators to ensure that
a received message is at least as recent as the last message it
received from the same source. A non-authoritative SNMPv3 engine
uses received authentic messages to advance its notion of time at
the remote authoritative source. An SNMPv3 engine also evaluates
the request-id in received Response messages and discards messages
which do not correspond to outstanding requests.
These mechanisms provide for the detection of messages whose time
of generation was not recent in all but one circumstance; this
circumstance is the delay or replay of a Report message (sent to a
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receiver) when the receiver has has not recently communicated with
the source of the Report message. In this circumstance, the
detection guarantees only that the Report message is more recent
than the last communication between source and destination of the
Report message. However, Report messages do not request or contain
management information, and thus, goal #3 in Section 1.2 above is
met; further, Report messages can at most cause the receiver to
advance its notion of time (at the source) by less than the proper
amount.
This protection against the threat of message delay or replay does
not imply nor provide any protection against unauthorized deletion
or suppression of messages. Also, an SNMPv3 engine may not be able
to detect message reordering if all the messages involved are sent
within the Time Window interval. Other mechanisms defined
independently of the security protocol can also be used to detect
the re-ordering replay, deletion, or suppression of messages
containing set operations (e.g., the MIB variable snmpSetSerialNo
[RFC1907]).
- verifying that a message sent to/from one SNMPv3 engine cannot be
replayed to/as-if-from another SNMPv3 engine.
Included in each message is an identifier unique to the SNMPv3
engine associated with the sender or intended recipient of the
message. Also, each message containing a Response PDU contains a
request-id which associates the message to a recently generated
request.
A Report message sent by one SNMPv3 engine to a second SNMPv3
engine can potentially be replayed to another SNMPv3 engine.
However, Report messages do not request or contain management
information, and thus, goal #3 in Section 1.2 above is met;
further, Report messages can at most cause the receiver to advance
its notion of time (at the authoritative source) by less than the
correct amount.
- detecting messages which were not recently generated.
A set of time indicators are included in the message, indicating
the time of generation. Messages (other than those containing
Report PDUs) without recent time indicators are not considered
authentic. In addition, messages containing Response PDUs have a
request-id; if the request-id does not match that of a recently
generated request, then the message is not considered to be
authentic.
A Report message sent by an SNMPv3 engine can potentially be
replayed at a later time to an SNMPv3 engine which has not
recently communicated with that source engine. However, Report
messages do not request or contain management information, and
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thus, goal #3 in Section 1.2 above is met; further, Report
messages can at most cause the receiver to advance its notion of
time (at the authoritative source) by less than the correct
amount.
This memo allows the same user to be defined on multiple SNMPv3
engines. Each SNMPv3 engine maintains a value, snmpEngineID,
which uniquely identifies the engine. This value is included in
each message sent to/from the engine that is authoritative (see
section 1.5.1). On receipt of a message, an authoritative engine
checks the value to ensure it is the intended recipient, and a
non-authoritative engine uses the value to ensure that the message
is processed using the correct state information.
Each SNMPv3 engine maintains two values, engineBoots and engineTime,
which taken together provide an indication of time at that engine.
Both of these values are included in an authenticated message sent
to/received from that engine. On receipt, the values are checked to
ensure that the indicated time is within a time window of the
current time. The time window represents an administrative upper
bound on acceptable delivery delay for protocol messages.
For an SNMPv3 engine to generate a message which an authoritative
engine will accept as authentic, and to verify that a message
received from that authoritative engine is authentic, such an engine
must first achieve time synchronization with the authoritative
engine.
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2. Elements of the Model
This section contains definitions required to realize the security
model defined by this memo.
2.1. SNMPv3 Users
Management operations using this security model make use of a defined
set of user identities. For any SNMPv3 user on whose behalf
management operations are authorized at a particular SNMPv3 engine,
that engine must have knowledge of that user. An SNMPv3 engine that
wishes to communicate with another SNMPv3 engine must also have
knowledge of a user known to that engine, including knowledge of the
applicable attributes of that user.
A user and its attributes are defined as follows:
<userName>
A string representing the name of the user.
<miId>
A human-readable string representing a (security) model
independent identity for this user.
<groupName>
A string representing the group that the user belongs to.
<authProtocol>
An indication of whether messages sent on behalf of this user can
be authenticated, and if so, the type of authentication protocol
which is used. One such protocol is defined in this memo: the
Digest Authentication Protocol.
<authKey>
If messages sent on behalf of this user can be authenticated, the
(private) authentication key for use with the authentication
protocol. Note that a user's authentication key will normally be
different at different authoritative engines. Not visible via
remote access.
<authKeyChange>
The only way to remotely update the authentication key. Does that
in a secure manner, so that the update can be completed without
the need to employ privacy protection.
<privProtocol>
An indication of whether messages sent on behalf of this user can
be protected from disclosure, and if so, the type of privacy
protocol which is used. One such protocol is defined in this memo:
the DES-based Encryption Protocol.
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<privKey>
If messages sent on behalf of this user can be en/decrypted, the
(private) privacy key for use with the privacy protocol. Note that
a user's privacy key will normally be different at different
authoritative engines. Not visible via remote access.
<privKeyChange>
The only way to remotely update the encryption key. Does that
in a secure manner, so that the update can be completed without
the need to employ privacy protection.
2.2. Replay Protection
Each SNMPv3 engine maintains three objects:
- snmpEngineID, which is an identifier unique among all SNMPv3
engines in (at least) an administrative domain;
- engineBoots, which is a count of the number of times the engine has
re-booted/re-initialized since snmpEngineID was last configured;
and,
- engineTime, which is the number of seconds since engineBoots was
last incremented.
Each SNMPv3 engine is always authoritative with respect to these
objects in its own engine. It is the responsibility of a non-
authoritative SNMPv3 engine to synchronize with the authoritative
engine, as appropriate.
An authoritative SNMPv3 engine is required to maintain the values of
its snmpEngineID and engineBoots in non-volatile storage.
2.2.1. snmpEngineID
The engineID value contained in an authenticated message is used to
defeat attacks in which messages from one engine to another engine
are replayed to a different engine.
When an authoritative engine is first installed, it sets its local
value of snmpEngineID according to a enterprise-specific algorithm
(see the definition of engineID in the SNMP Architecture document
[SNMP-ARCH]).
2.2.2. engineBoots and engineTime
The engineBoots and engineTime values contained in an authenticated
message are used to defeat attacks in which messages are replayed
when they are no longer valid. Through use of engineBoots and
engineTime, there is no requirement for an SNMPv3 engine to have a
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non-volatile clock which ticks (i.e., increases with the passage of
time) even when the engine is powered off. Rather, each time an
SNMPv3 engine re-boots, it retrieves, increments, and then stores
engineBoots in non-volatile storage, and resets engineTime to zero.
When an SNMPv3 engine is first installed, it sets its local values
of engineBoots and engineTime to zero. If engineTime ever
reaches its maximum value (2147483647), then engineBoots is
incremented as if the engine has re-booted and engineTime is reset to
zero and starts incrementing again.
Each time an authoritative SNMPv3 engine re-boots, any SNMPv3 engines
holding that authoritative engine's values of engineBoots and
engineTime need to re-synchronize prior to sending correctly
authenticated messages to that authoritative engine (see Section
2.3 for (re-)synchronization procedures). Note, however, that the
procedures do provide for a notification to be accepted as authentic
by a receiving engine, when sent by an authoritative engine which has
re-booted since the receiving engine last (re-)synchronized.
If an authoritative SNMPv3 engine is ever unable to determine its
latest engineBoots value, then it must set its engineBoots value to
0xffffffff.
Whenever the local value of engineBoots has the value 0xffffffff, it
latches at that value and an authenticated message always causes an
notInTimeWindow authentication failure.
In order to reset an engine whose engineBoots value has reached the
value 0xffffffff, manual intervention is required. The engine must
be physically visited and re-configured, either with a new
snmpEngineID value, or with new secret values for the authentication
and privacy protocols of all users known to that engine.
2.2.3. Time Window
The Time Window is a value that specifies the window of time in which
a message generated on behalf of any user is valid. This memo
specifies that the same value of the Time Window, 150 seconds, is
used for all users.
2.3. Time Synchronization
Time synchronization, required by a non-authoritative engine (see
section 5.1.1) in order to proceed with authentic communications,
has occurred when the non-authoritative engine has obtained local
values of engineBoots and engineTime from the authoritative engine
that are within the authoritative engine's time window. To remain
synchronized, the local values must remain within the authoritative
engine's time window and thus must be kept loosely synchronized
with the values stored at the authoritative engine.
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In addition to keeping a local version of engineBoots and engineTime,
a non-authoritative engine must also keep one other local variable,
latestReceivedEngineTime. This value records the highest value of
engineTime that was received by the non-authoritative engine from
the authoritative engine and is used to eliminate the possibility
of replaying messages that would prevent the non-authoritative
engine's notion of the engineTime from advancing.
Time synchronization occurs as part of the procedures of receiving
a message (Section 3.2, step 7b). As such, no explicit time
synchronization procedure is required by a non-authoritative engine.
Note, that whenever the local value of snmpEngineID is changed
(e.g., through discovery) or when secure communications are first
established with this engine, the local values of engineBoots and
latestReceivedEngineTime should be set to zero. This will cause
the time synchronization to occur when the next authentic message
is received.
2.4. SNMPv3 Messages Using this Model
The syntax of an SNMPv3 message using this security model adheres
to the message format defined in the SNMP Architecture document
[SNMP-ARCH]. The securityParameters in the message are
defined as an OCTET STRING. The format of that OCTET STRING for
the User-based Security model is as follows:
securityParameters ::=
SEQUENCE {
-- global parameters
engineID
OCTET STRING (SIZE(12)),
engineBoots
Unsigned32 (0..4294967295),
engineTime
Unsigned32 (0..2147483647),
userName
OCTET STRING (SIZE(1..16)),
authParameters
OCTET STRING,
privParameters
OCTET STRING,
}
END
The authParameters are defined by the authentication protocol in
use for the message (as defined by the authProtocol column in
the user's entry in the usecUserTable).
The privParameters are defined by the privacy protocol in
use for the message (as defined by the privProtocol column in
the user's entry in the usecUserTable).
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2.5 Input and Output of the User-based Security Module
This section describes the inputs and outputs that the User-based
Security module expects and produces when the Message Processing
and Control module (MPC) invokes the User-base Security module for
services.
2.5.1 Input and Output when generating an SNMPv3 Message
When the Message Processing and Control module (MPC) invokes the
User-based Security module to secure an outgoing SNMPv3 message,
there are two possibilities:
a) A new request is generated. The abstract service interface is:
generateRequestMsg(version, msgID, mms, msgFlags,
securityModel, securityParameters,
LoS, miId, engineID, scopedPDU)
b) A response is generated. The abtract service interface is:
generateResponseMsg(version, msgID, mms, msgFlags,
securityModel, securityParameters,
scopedPDU, cachedSecurityDataReference)
Where:
version
This is the version number for the SNMP message.
This data is not used by the USEC module.
It is part of the globalData of the message.
msgID
This is the msgID to be generated.
This data is not used by the USEC module.
It is part of the globalData of the message.
mms
This is the maximum message size.
This data is not used by the USEC module.
It is part of the globalData of the message.
msgFlags
This is the field containing the msgFlags.
This data is not used by the USEC module.
It is part of the globalData of the message.
It should be consistent with the LoS that is passed.
securityModel
This is the securityModel in use. Should be the USEC model.
This data is not used by the USEC module.
It is part of the globalData of the message.
securityParameters
These are the security parameters. They will be filled in
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by the User-based Security module.
LoS
The Level of Security (LoS) from which the User-based Security
module determines if the message needs to be protected from
disclosure and if the message needs to be authenticated.
scopedPDU
this is the message payload. The data is opaque as far as the
User-based Security module is concerned.
miId
this is the (security) model independent Identifier.
Together with the engineID it identifies a row in the
usecUserTable that is to be used for securing the message.
engineID
the engineID of the authoritative SNMP engine to which the
request is to be sent.
cachedSecurityDataReference
A handle/reference to cached security data to be used when
securing an outgoing response. This is the handle/reference
that was generated by the USEC module when the incoming
request was processed.
Upon completion of the process, the User-based Security module
returns either and error indication or the completed message
with privacy and authentication applied if such was requested
by the Level of Security (LoS) flags passed.
The abstract service interface is:
returnGeneratedMsg(wholeMsg, wholeMsgLen, statusCode)
Where:
wholeMsg
this is fully encoded and secured message ready to be sent on
the wire.
wholeMsgLen
this is the length of the encoded and secured message wholeMsg.
statusCode
this is the indicator of whether the encoding and securing of
the message was successful, and if not it is an indication of
the problem.
2.5.2 Input and Output when receiving an SNMPv3 Message
The Message Processing and Control module (MPC) invokes the
User-based Security module to verify proper security of an incoming
SNMPv3 message. The abstract service interface is:
processMsg(version, msgID, mms, msgFlags,
securityModel, securityParameters,
LoS, wholeMsg, wholeMsgLen)
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Where:
version
This is the version number for the SNMP message.
This data is not used by the USEC module.
It is part of the globalData of the message.
msgID
This is the msgID to be generated.
This data is not used by the USEC module.
It is part of the globalData of the message.
mms
This is the maximum message size.
This data is not used by the USEC module.
It is part of the globalData of the message.
msgFlags
This is the field containing the msgFlags.
This data is not used by the USEC module.
It is part of the globalData of the message.
It should be consistent with the LoS that is passed.
securityModel
This is the securityModel in use. Should be the USEC model.
This data is not used by the USEC module.
It is part of the globalData of the message.
securityParameters
These are the security parameters. They will be filled in
by the User-based Security module.
LoS
The Level of Security (LoS) from which the User-based Security
module determines if the message needs to be protected from
disclosure and if the message needs to be authenticated.
wholeMsg
this is the complete message as it was received by the Message
Processing and Control module (MPC).
wholeMsgLen
this is the length of the wholeMsg as received on the wire.
Upon completion of the process, the User-based Security module
returns a statusCode and in case of success authenticated and
decrypted data. The abstract service interface is:
returnMsg(miId, groupName, cachedSecurityDataReference,
scopedPDUmms, scopedPDU, statusCode)
Where:
miId
this is an Security Model-independent Identifier that identifies
an entry in the usecUserTable. It is to be used later when a
response message must be secured.
groupName
this is the group to which the user belongs. The User-based
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Security module retrieves this information from the usecUserTable.
cachedSecurityDataReference
cached security data to be used when securing a possible outgoing
response to this request. Will have to be released explicitly
by the MPC or the application.
scopedPDUmms
this is the maximum message size that a possible response PDU
may use. The User-based Security module calculates this size such
that there is always space available for any security parameters
that need to be added to the response message.
scopedPDU
this is the message payload. The data is opaque as far as the
User-based Security module is concerned. But if the data was
encrypted because privacy protection was in effect, then upon
return from the User-based Security module the data will have
been decrypted.
statusCode
this is an indicator of whether the message was parsed,
authenticated and possibly decrypted successfully. If
it was not - it indicates what the problem was.
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3. Elements of Procedure
This section describes the security related procedures followed by
an SNMPv3 engine when processing SNMPv3 messages according to the
User-based Security model.
3.1. Processing an Outgoing Message
This section describes the procedure followed by an SNMPv3 engine
whenever it generates a message containing a management operation
(either a request, a response, a notification, or a report) on
behalf of a user, with a particular Level of Security (LoS).
1) - If any cachedSecurityDataReference is passed, then
information concerning the user is extracted from the
cachedSecurityData. The cachedSecurityData can now be
discarded.
- Otherwise, based on the miId, information concerning the user
at the destination engineID is extracted from the Local
(security) Configuration Datastore (LCD, usecUserTable).
If information about the user is absent from the LCD,
then an error indication (unknownSecurityIdentity) is
returned to the calling module.
2) If the Level of Security (LoS) specifies that the message is to
be protected from disclosure, but the user does not support both
an authentication and a privacy protocol then the message cannot
be sent. An error indication (unsupportedLoS) is returned to
the calling module.
3) If the Level of Security (LoS) specifies that the message is to
be authenticated, but the user does not support an authentication
protocol, then the message cannot be sent. An error indication
(unsupportedLoS) is returned to the calling module.
4) If the Level of Security (LoS) specifies that the message is to
be protected from disclosure, then the octet sequence
representing the serialized scopedPDU is encrypted according to
the user's privacy protocol. To do so a call is made to the
privacy module that implements the user's privacy protocol.
The abstract service interface is:
encryptMsg(cryptKey, scopedPDU)
Where:
cryptKey
The user's usecUserPrivKey. This is the secret key
that can be used by the encryption algorithm.
scopedPDU
The data to be encrypted.
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Upon completion the privacy module returns:
returnEncryptedMsg(encryptedPDU, privParameters, statusCode)
encryptedPDU
The encrypted scopedPDU (encoded as an octet string).
privParameters
The privacy parameters (encoded as an octet string) that
need to be sent in the outgoing message.
statusCode
The indicator of whether the PDU was encrypted successfully
and if not, it indicates what went wrong.
If an error indication is returned by the privacy module then
the message cannot be sent and the error indication is returned
to the calling module.
If the privacy module returns success, then the privParameters
field is put into the securityParameters and the encryptedPDU
serves as the payload of the message being prepared.
5) If the Level of Security (LoS) specifies that the message is not
to be protected from disclosure, then the NULL string is encoded
as an octet string into the privParameters field of the
securityParameters and the scopedPDU serves as the payload of
the message being prepared.
6) The engineID is encoded as an octet string into the <engineID>
field of the securityParameters.
7) If the Level of Security (LoS) specifies that the message is to
be authenticated, then the current values of engineBoots, and
engineTime corresponding to engineID from the LCD are used.
Otherwise, a zero value is used for engineBoots and engineTime.
The values are encoded as Unsigned32 into the engineBoots and
engineTime fields of the securityParameters.
8) The userName is encoded as an octet string into the userName
field of the securityParameters.
9) If the Level of Security (LoS) specifies that the message is to
be authenticated, the message is authenticated according to the
user's authentication protocol. To do so, a call is made to the
authentication module that implements the user's authentication
protocol. The abstract service interface is:
authMsg(authKey, wholeMsg)
authKey
The user's usecUserAuthKey. This is the secret key
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that can be used by the authentication algorithm.
wholeMsg
the message to be authenticated.
Upon completion the authentication module returns:
returnAuthMsg(wholeMsg, statusCode)
wholeMsg
Same as in input, but with authParameters properly filled.
statusCode
The indicator of whether the message was successfully
processed by the authentication module.
If an error indication is returned by the authentication module,
then the message cannot be sent and the error indication is
returned to the calling module.
10) If the Level of Security (LoS) specifies that the message is not
to be authenticated then the NULL string is encoded as an octet
string into the authParameters field of the securityParameters.
11) The completed message is returned to the calling module with
the statusCode set to success.
3.2. Processing an Incoming Message
This section describes the procedure followed by an SNMPv3 engine
whenever it receives a message containing a management operation
on behalf of a user, with a particular Level of Security (LoS).
1) If the received securityParameters is not the serialization
(according to the conventions of [RFC1906]) of an OCTET STRING
formated according to the securityParameters defined in section
2.4, then the snmpInASNParseErrs counter [RFC1907] is
incremented, and an error indication (ASNParseError) is returned
to the calling module.
2) The values of the security parameter fields are extracted from
the securityParameters.
3) If the engineID field contained in the securityParameters is
unknown then:
- a manager that performs discovery may optionally create a
new entry in its Local (security) Configuration Database (LCD)
and continue processing; or
- an error indication (unknownEngineID) is returned to the
calling module.
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4) Information about the value of the userName and engineID
fields is extracted from the Local (security) Configuration
Database (LCD, usecUserTable). If no information is
available for this user, then an error indication
(unknownSecurityIdentity) is returned to the calling module.
5) If the information about the user indicates that it does not
support the Level of Security indicated by the LoS parameter,
then and an error indication (unsupportedLoS) is returned to
the calling module.
6) If the Level of Security (LoS) specifies that the message is to
be authenticated, then the message is authenticated according to
the user's authentication protocol. To do so, a call is made to
the authentication module that implements the user's
authentication protocol. The abstract service interface is:
authIncomingMsg(authKey, authParameters, wholeMsg)
authKey
The user's (secret) usecUserAuthKey
authParameters
the authParameters from the incoming message.
wholeMsg
the message to be authenticated.
The authentication module returns:
returnAuthIncomingMsg(wholeMsg, statusCode)
If the message is not authentic according to the authentication
protocol module (i.e. it returns an error indication), then the
error indication is returned to the calling module.
Otherwise, the authenticated wholeMsg is used for further
processing.
7) If the LoS field indicates an authenticated message, then
the local values of engineBoots and engineTime corresponding to
the value of the engineID field are extracted from the
Local (security) Configuration Database (LCD).
a) If the engineID value is the same as the snmpEngineID of
the processing SNMPv3 engine (meaning that this is the
authoritative engine), then if any of the following
conditions is true, then the message is considered to be
outside of the Time Window:
- the local value of engineBoots is 0xffffffff;
- the engineBoots field differs from the local value of
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engineBoots; or,
- the value of the engineTime field differs from the local
notion of engineTime by more than +/- 150 seconds.
If the message is considered to be outside of the Time Window
then an error indication (notInTimeWindow) is returned to
the calling module.
b) If the engineID value is not the same as the snmpEngineID of
the processing SNMPv3 engine (meaning that this engine is not
the authoritative engine), then:
- if at least one of the following conditions is true:
- the engineBoots field is greater than the local value
of engineBoots; or,
- the engineBoots field is equal to the local value of
engineBoots and the engineTime field is greater than
the value of latestReceivedEngineTime,
then the LCD entry corresponding to the value of the
engineID field is updated, by setting the local value of
engineBoots from the engineBoots field, the local value
latestReceivedEngineTime from the engineTime field, and
the local value of engineTime from the engineTime field.
- if any of the following conditions is true, then the message
is considered to be outside of the Time Window:
- the local value of engineBoots is 0xffffffff;
- the engineBoots field is less than the local value of
engineBoots; or,
- the engineBoots field is equal to the local value of
engineBoots and the engineTime field is more than 150
seconds less than the local notion of engineTime.
If the message is considered to be outside of the Time
Window then an error indication (notInTimeWindow) is
returned to the calling module;
however, time synchronization procedures may be invoked.
Note that this procedure allows for engineBoots in the
message to be greater than the local value of engineBoots
to allow for received messages to be accepted as authentic
when received from an authoritative SNMPv3 engine that
has re-booted since the receiving SNMPv3 engine last
(re-)synchronized.
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8) If the LoS field indicates that the message was protected from
disclosure, then the octet sequence representing the scopedPDU
is decrypted according to the user's privacy protocol to obtain
a serialized scopedPDUs value. Otherwise the data component is
assumed to directly contain the scopedPDUs value. To do the
decryption, a call is made to the privacy module that implements
the user's privacy protocol. The abstract service interface is:
decryptMsg(cryptKey, privParameters, encryptedPDU)
cryptKey
The user's secret usecUserPrivKey
privParameters
The privParameters field from the securityParameters from
the incoming message.
encryptedPDU
the data to be decrypted
The privacy module returns:
returnDecryptedMsg(scopedPDU, statusCode)
scopedPDU
The decrypted scopedPDU.
statusCode
The indicator whether the message was successfully decrypted.
If an error indication is returned by the privacy module, then
the error indication is returned to the calling module.
9) The scopedPDU-MMS is calculated.
10) The groupName is retrieved from the usecUserTable
11) The miId is retrieved from the usecUserTable
12) The securityData is cached, so that a possible response to
this message can use the same authentication and privacy
secrets. Information to be saved/cached is as follows:
usecUserName,
usecUserAuthProto, usecUserAuthKey,
usecUserPrivProto, usecUserPrivKey
-- Editor's note:
If we assume SNMPv3, then we could check the reportableFlag and if
it is not set, then we do not need to cache any security data
because then there is no response possible. Do we want to do that?
-- End Editor's note.
13) The statusCode is set to success and a return is made to the
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calling module according to this abstract service interface:
returnMsg(miId, groupName, cachedSecurityDataReference,
scopedPDUmms, scopedPDU, statusCode)
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4. Discovery
This security model requires that a discovery process obtains
sufficient information about other SNMP engines in order to
communicate with them. Discovery requires the SNMP manager to
learn the engine's snmpEngineID value before communication may
proceed. This may be accomplished by formulating a get-request
communication with the LoS set to noAuth/noPriv, the userName set
to "public", the snmpEngineID set to all zeros (binary), and the
varBindList left empty. The response to this message will be a
report PDU that contains the snmpEngineID within the
securityParameters field (and containing the snmpUnknownEngineIDs
counter in the varBindList).
If authenticated communication is required then the discovery
process may invoke the procedure described in Section 2.3 to
synchronize the timers.
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5. Definitions
SNMP-USEC-MIB DEFINITIONS ::= BEGIN
IMPORTS
MODULE-IDENTITY, OBJECT-TYPE, snmpModules FROM SNMPv2-SMI
TEXTUAL-CONVENTION, TestAndIncr,
RowStatus, StorageType FROM SNMPv2-TC
MODULE-COMPLIANCE, OBJECT-GROUP FROM SNMPv2-CONF,
SnmpAdminString, SnmpLoS, SnmpEngineID,
SnmpSecurityModel,
imfAuthMD5Protocol, imfNoPrivProtocol FROM IMF-MIB;
snmpUsecMIB MODULE-IDENTITY
LAST-UPDATED "9706180000Z" -- 18 June 1997, midnight
ORGANIZATION "SNMPv3 Working Group"
CONTACT-INFO "WG-email: snmpv3@tis.com
Subscribe: majordomo@tis.com
In msg body: subscribe snmpv3
Chair: Russ Mundy
Trusted Information Systems
postal: 3060 Washington Rd
Glenwood MD 21738
email: mundy@tis.com
phone: 301-854-6889
Co-editor Uri Blumenthal
IBM T. J. Watson Research
postal: 30 Saw Mill River Pkwy,
Hawthorne, NY 10532
USA
email: uri@watson.ibm.com
phone: +1.914.784.7964
Co-editor: Bert Wijnen
IBM T. J. Watson Research
postal: Schagen 33
3461 GL Linschoten
Netherlands
email: wijnen@vnet.ibm.com
phone: +31-348-432-794
"
DESCRIPTION "The management information definitions for the
SNMPv3 User-based Security model.
"
::= { snmpModules 99 } -- to be assigned
-- Administrative assignments ****************************************
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snmpUsecAdmin OBJECT IDENTIFIER ::= { snmpUsecMIB 1 }
snmpUsecMIBObjects OBJECT IDENTIFIER ::= { snmpUsecMIB 2 }
snmpUsecMIBConformance OBJECT IDENTIFIER ::= { snmpUsecMIB 3 }
-- Textual Conventions ***********************************************
UserName ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION "A string representing the name of a user for use in
accordance with the SNMP User-based Security model.
"
SYNTAX SnmpAdminString (SIZE(1..16))
-- Editor's note:
-- A real issue is whether the fact that MD5 is used in the following
-- TC is OK. It might be better to use 3DES for 3DES and IDEA for IDEA.
-- End Editor's note
KeyChange ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION
"Every definition of an object with this syntax must identify
a protocol, P, and a secret key, K. The object's value is a
manager-generated, partially-random value which, when
modified, causes the value of the secret key, K, to be
modified via a one-way function.
The value of an instance of this object is the concatenation
of two components: a 'random' component and a 'delta'
component. The lengths of the random and delta components are
given by the corresponding value of the protocol, P; if P
requires K to be a fixed length, the length of both the random
and delta components is that fixed length; if P allows the
length of K to be variable up to a particular maximum length,
the length of the random component is that maximum length and
the length of the delta component is any length less than or
equal to that maximum length. For example,
imfAuthMD5Protocol requires K to be a fixed length of 16
octets. Other protocols may define other sizes, as deemed
appropriate.
When an instance of this object is modified to have a new
value by the management protocol, the agent generates a new
value of K as follows:
- a temporary variable is initialized to the existing value
of K;
- if the length of the delta component is greater than 16
bytes, then:
- the random component is appended to the value of the
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temporary variable, and the result is input to the MD5
hash algorithm to produce a digest value, and the
temporary variable is set to this digest value;
- the value of the temporary variable is XOR-ed with the
first (next) 16-bytes of the delta component to produce
the first (next) 16-bytes of the new value of K.
- the above two steps are repeated until the unused
portion of the delta component is 16 bytes or less,
- the random component is appended to the value of the
temporary variable, and the result is input to the MD5
hash algorithm to produce a digest value;
- this digest value, truncated if necessary to be the same
length as the unused portion of the delta component, is
XOR-ed with the unused portion of the delta component to
produce the (final portion of the) new value of K.
i.e.,
iterations = (lenOfDelta - 1)/16; /* integer division */
temp = keyOld;
for (i = 0; i < iterations; i++) {
temp = MD5 (temp || random);
keyNew[i*16 .. (i*16)+15] =
temp XOR delta[i*16 .. (i*16)+15];
}
temp = MD5 (temp || random);
keyNew[i*16 .. lenOfDelta-1] =
temp XOR delta[i*16 .. lenOfDelta-1];
The value of an object with this syntax, whenever it is
retrieved by the management protocol, is always the zero-
length string."
SYNTAX OCTET STRING
-- *******************************************************************
-- The valid users for the User-based Security model ******************
usecUser OBJECT IDENTIFIER ::= { snmpUsecMIBObjects 1 }
usecUserTable OBJECT-TYPE
SYNTAX SEQUENCE OF UsecUserEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION "The table of users configured in the SNMP engine's
Local (security) Configuration Datastore (LCD)."
::= { usecUser 1 }
usecUserEntry OBJECT-TYPE
SYNTAX UsecUserEntry
MAX-ACCESS not-accessible
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STATUS current
DESCRIPTION "A user configured in the SNMP engine's Local
(security) Configuration Datastore (LCD) for
the User-based Security model.
"
INDEX { usecUserEngineID,
IMPLIED usecUserName
}
::= { usecUserTable 1 }
UsecUserEntry ::= SEQUENCE {
usecUserEngineID SnmpEngineID,
usecUserName UserName,
usecUserMiId SnmpAdminString,
usecUserGroupName SnmpAdminString,
usecUserCloneFrom RowPointer,
usecUserAuthProtocol OBJECT IDENTIFIER,
usecUserAuthKeyChange KeyChange,
-- usecUserAuthKey OCTET STRING, not visible
usecUserAuthPublic OCTET STRING,
usecUserPrivProtocol OBJECT IDENTIFIER,
usecUserPrivKeyChange KeyChange,
-- usecUserPrivKey OCTET STRING, not visible
usecUserPrivPublic OCTET STRING,
usecUserStorageType StorageType,
usecUserStatus RowStatus
}
usecUserEngineID OBJECT-TYPE
SYNTAX SnmpEngineID
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION "An SNMP engine's administratively-unique identifier.
In a simple agent, this value is always that agent's
own snmpEngineID value.
This value can also take the value of the snmpEngineID
of a remote SNMP engine with which this user can
communicate.
"
::= { usecUserEntry 1 }
usecUserName OBJECT-TYPE
SYNTAX UserName
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION "A string representing the name of the user. This is
the (User-based security) model dependent identity.
"
::= { usecUserEntry 2 }
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usecUserMiId OBJECT-TYPE
SYNTAX SnmpAdminString
MAX-ACCESS read-only
STATUS current
DESCRIPTION "A string representing the (security) model independent
identity for this user.
The default mapping for the User-based Security model
is that the miId is the same as the userName.
"
::= { usecUserEntry 3 }
usecUserGroupName OBJECT-TYPE
SYNTAX SnmpAdminString
MAX-ACCESS read-write
STATUS current
DESCRIPTION "A string representing the group to which the user
belongs. A group name of zero length indicates
that the user is not [perhaps yet] a member of any
group, possibly because the entry has not yet been
completely configured. Users which are not a part
of any group are effectively disabled to perform any
SNMP operations.
"
DEFVAL { ''H } -- the empty string
::= { usecUserEntry 4 }
usecUserCloneFrom OBJECT-TYPE
SYNTAX RowPointer
MAX-ACCESS read-create
STATUS current
DESCRIPTION "A pointer to another conceptual row in this
usecUserTable. The user in this other conceptual row
is called the clone-from user.
When a new user is created (i.e., a new conceptual row
is instantiated in this table), the authentication
parameters of the new user are cloned from its
clone-from user.
The first time an instance of this object is set by a
management operation (either at or after its
instantiation), the cloning process is invoked.
Subsequent writes are successful but invoke no action
to be taken by the agent.
The cloning process fails with an 'inconsistentName'
error if the conceptual row representing the
clone-from user is not in an active state when the
cloning process is invoked.
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Cloning also causes the initial values of the secret
authentication key and the secret encryption key of
the new user to be set to the same value as the
corresponding secret of the clone-from user.
When this object is read, the zero length string is
returned.
"
::= { usecUserEntry 5 }
usecUserAuthProtocol OBJECT-TYPE
SYNTAX OBJECT IDENTIFIER
MAX-ACCESS read-create
STATUS current
DESCRIPTION "An indication of whether messages sent on behalf of
this user to/from the SNMP engine identified by
usecUserEngineID, can be authenticated, and if so,
the type of authentication protocol which is used.
An instance of this object is created concurrently
with the creation of any other object instance for
the same user (i.e., as part of the processing of
the set operation which creates the first object
instance in the same conceptual row). Once created,
the value of an instance of this object can not be
changed.
"
DEFVAL { imfAuthMD5Protocol }
::= { usecUserEntry 6 }
usecUserAuthKeyChange OBJECT-TYPE
SYNTAX KeyChange -- typically (SIZE (0..32))
MAX-ACCESS read-create
STATUS current
DESCRIPTION "An object, which when modified, causes the secret
authentication key used for messages sent on behalf
of this user to/from the SNMP engine identified by
usecUserEngineID, to be modified via a one-way
function.
The associated protocol is the usecUserAuthProtocol.
The associated secret key is the user's secret
authentication key (usecUserAuthKey).
When creating a new user, it is an 'inconsistentName'
error for a set operation to refer to this object
unless it is previously or concurrently initialized
through a set operation on the corresponding value
of usecUserCloneFrom.
"
DEFVAL { ''H } -- the empty string
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::= { usecUserEntry 7 }
usecUserAuthPublic OBJECT-TYPE
SYNTAX OCTET STRING -- for MD5 (SIZE(0..32))
MAX-ACCESS read-create
STATUS current
DESCRIPTION "A publicly-readable value which is written as part
of the procedure for changing a user's secret key,
and later read to determine whether the change of
the secrets was effected.
"
DEFVAL { ''H } -- the empty string
::= { usecUserEntry 8 }
usecUserPrivProtocol OBJECT-TYPE
SYNTAX OBJECT IDENTIFIER
MAX-ACCESS read-create
STATUS current
DESCRIPTION "An indication of whether messages sent on behalf of
this user to/from the SNMP engine identified by
usecUserEngineID, can be protected from disclosure,
and if so, the type of privacy protocol which is used.
An instance of this object is created concurrently
with the creation of any other object instance for
the same user (i.e., as part of the processing of
the set operation which creates the first object
instance in the same conceptual row). Once created,
the value of an instance of this object can not be
changed.
"
DEFVAL { imfNoPrivProtocol }
::= { usecUserEntry 9 }
usecUserPrivKeyChange OBJECT-TYPE
SYNTAX KeyChange -- typically (SIZE (0..32))
MAX-ACCESS read-create
STATUS current
DESCRIPTION "An object, which when modified, causes the secret
encryption key used for messages sent on behalf
of this user to/from the SNMP engine identified by
usecUserEngineID, to be modified via a one-way
function.
The associated protocol is the usecUserPrivProtocol.
The associated secret key is the user's secret
encryption key (usecUserPrivKey).
When creating a new user, it is an 'inconsistentName'
error for a set operation to refer to this object
unless it is previously or concurrently initialized
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through a set operation on the corresponding value
of usecUserCloneFrom.
"
DEFVAL { ''H } -- the empty string
::= { usecUserEntry 10 }
usecUserPrivPublic OBJECT-TYPE
SYNTAX OCTET STRING -- for DES (SIZE(0..16))
MAX-ACCESS read-create
STATUS current
DESCRIPTION "A publicly-readable value which is written as part
of the procedure for changing a user's secret key,
and later read to determine whether the change of
the secrets was effected.
"
DEFVAL { ''H } -- the empty string
::= { usecUserEntry 11 }
usecUserStorageType OBJECT-TYPE
SYNTAX StorageType
MAX-ACCESS read-create
STATUS current
DESCRIPTION "The storage type for this conceptual row."
DEFVAL { nonVolatile }
::= { usecUserEntry 12 }
usecUserStatus OBJECT-TYPE
SYNTAX RowStatus
MAX-ACCESS read-create
STATUS current
DESCRIPTION "The status of this conceptual row. Until instances
of all corresponding columns are appropriately
configured, the value of the corresponding instance
of the usecUserStatus column is 'notReady'.
For those columnar objects which permit write-access,
their value in an existing conceptual row can be
changed irrespective of the value of usecUserStatus
for that row.
"
::= { usecUserEntry 13 }
usecUserSecretSpinLock OBJECT-TYPE
SYNTAX TestAndIncr
MAX-ACCESS read-write
STATUS current
DESCRIPTION "An advisory lock used to allow several cooperating
SNMPv3 engines, all acting in a manager role, to
coordinate their use of facilities to alter secrets
in the usecUserTable.
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"
::= { usecUser 2 }
--Editor's note
Is it enough to have just one spin-lock for such a table where
several secrets can be modified? Can the protocol ensure the
consistency? Should it?
--End editor's note
-- Conformance Information *******************************************
snmpUsecMIBCompliances
OBJECT IDENTIFIER ::= { snmpUsecMIBConformance 1 }
snmpUsecMIBGroups
OBJECT IDENTIFIER ::= { snmpUsecMIBConformance 2 }
-- Compliance statements
snmpUsecMIBCompliance MODULE-COMPLIANCE
STATUS current
DESCRIPTION "The compliance statement for SNMP engines which
implement the SNMP USEC MIB.
"
MODULE -- this module
MANDATORY-GROUPS { snmpUsecMIBBasicGroup }
OBJECT usecUserGroupName
MIN-ACCESS read-only
DESCRIPTION "Write access is not required."
OBJECT usecUserAuthProtocol
MIN-ACCESS read-only
DESCRIPTION "Write access is not required."
OBJECT usecUserPrivProtocol
MIN-ACCESS read-only
DESCRIPTION "Write access is not required."
::= { snmpUsecMIBCompliances 1 }
-- Units of compliance
snmpUsecMIBBasicGroup OBJECT-GROUP
OBJECTS {
usecUserMiId,
usecUserGroupName,
usecUserCloneFrom,
usecUserAuthProtocol,
usecUserAuthKeyChange,
usecUserAuthPublic,
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usecUserPrivProtocol,
usecUserPrivKeyChange,
usecUserPrivPublic,
usecUserStorageType,
usecUserStatus
}
STATUS current
DESCRIPTION "A collection of objects providing for configuration
of an SNMP engine which implements the SNMP
User-based Security model.
"
::= { snmpUsecMIBGroups 1 }
END
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6. MD5 Authentication Protocol
This section describes the Keyed-MD5 authentication protocol.
This protocol is the first authentication protocol defined for
the User-based Security model.
Over time, other authentication protocols may be defined either
as a replacement of this protocol or in addition to this protocol.
6.1 Mechanisms
- In support of data integrity, a message digest algorithm is
required. A digest is calculated over an appropriate portion
of an SNMPv3 message and included as part of the message sent
to the recipient.
- In support of data origin authentication and data integrity, a
secret value is both inserted into, and appended to, the SNMPv3
message prior to computing the digest; the inserted value is
overwritten prior to transmission, and the appended value is not
transmitted. The secret value is shared by all SNMPv3 engines
authorized to originate messages on behalf of the appropriate
user.
- In order to not expose the shared secrets (keys) at all SNMPv3
engines in case one of the engines is compromised, such secrets
(keys) are localized for each authoritative SNMPv3 engine, see
[Localized-Key].
6.1.1. Digest Authentication Protocol
The Digest Authentication Protocol defined in this memo provides for:
- verifying the integrity of a received message (i.e., the message
received is the message sent).
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.
- verifying the user on whose behalf the message was generated.
A secret value known only to SNMPv3 engines authorized to generate
messages on behalf of a user is both inserted into, and appended
to, the message prior to the digest computation. Thus, the
verification of the user is implicit with the verification of the
digest. (Note that the use of two copies of the secret, one near
the start and one at the end, is recommended by [KEYED-MD5].)
This protocol uses the MD5 [MD5] message digest algorithm. A 128-bit
digest is calculated over the designated portion of an SNMPv3 message
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and included as part of the message sent to the recipient. The size
of both the digest carried in a message and the private
authentication key (the secret) is 16 octets.
6.2 Elements of the Digest Authentication Protocol
This section contains definitions required to realize the
authentication module defined by this memo.
6.2.1. SNMPv3 Users
Authentication using this Digest Authentication protocol makes use
of a defined set of user identities. For any SNMPv3 user on whose
behalf a message must be authenticated at a particular SNMPv3 engine,
that engine must have knowledge of that user. An SNMPv3 engine that
wishes to communicate with another SNMPv3 engine must also have
knowledge of a user known to that engine, including knowledge of the
applicable attributes of that user.
A user and its attributes are defined as follows:
<userName>
A string representing the name of the user.
<authKey>
A user's secret key to be used when calculating a digest.
6.2.2. EngineID
The engineID value contained in an authenticated message specifies
the authoritative SNMPv3 engine for that particular message.
(see the definition of engineID in the SNMP Architecture document
[SNMP-ARCH]).
The user's (private) authentication key is normally different at
each authoritative SNMPv3 engine and so the snmpEngineID is used
to select the proper key for the authentication process.
6.2.3. SNMPv3 Messages Using this Authentication Protocol
Messages using this authentication protocol carry an authParameters
field as part of the securityParameters. For this protocol, the
authParameters field is the serialized octet string representing
the MD5 digest of the wholeMsg.
The digest is calculated over the wholeMsg so if a message is
authenticated, that also means that all the fields in the message
are intact and have not been tampered with.
6.2.4 Input and Output of the MD5 Authentication Module
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This section describes the inputs and outputs that the MD5
Authentication module expects and produces when the User-based
Security module invokes the MD5 Authentication module for
services.
6.2.4.1 Input and Output when generating an SNMPv3 Message
This MD5 authentication protocol assumes that the selection of the
authKey is done by the caller and that the caller passes
the secret key to be used. The abstract service interface is:
authMsg(authKey, wholeMsg)
Where:
authKey
The secret key to be used by the authentication algorithm.
wholeMsg
the message to be authenticated.
Upon completion the authentication module returns information.
The abstract service interface is:
returnAuthMsg(wholeMsg, statusCode)
Where:
wholeMsg
Same as in input, but with authParameters properly filled.
statusCode
The indicator of whether the message was successfully
processed or not.
Note, that <authParameters> is filled by the authentication module
and this field should be already present in the <wholeMsg> before
the MAC is generated.
6.2.4.2 Input and Output when receiving an SNMPv3 Message
This MD5 authentication protocol assumes that the selection of the
authKey is done by the caller and that the caller passes
the secret key to be used. The abstract service interface is:
authIncomingMsg(authKey, authParameters, wholeMsg)
Where:
authKey
The secret key to be used by the authentication algorithm.
authParameters
the authParameters from the incoming message.
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wholeMsg
the message to be authenticated.
Upon completion the authentication module returns information.
The abstract service interface is:
returnAuthIncomingMsg(wholeMsg, statusCode)
wholeMsg
Same as in input, data has been authenticated.
statusCode
The indicator of whether the message was successfully
processed or not.
6.3 Elements of Procedure
This section describes the procedures for the Keyed-MD5
authentication protocol.
6.3.1 Processing an Outgoing Message
This section describes the procedure followed by an SNMPv3 engine
whenever it must authenticate an outgoing message using the
imfAuthMD5Protocol.
1) The authParameters field is set to the serialization according
to the rules in [RFC1906] of an octet string representing the
secret (localized) authKey.
2) The secret (localized) authKey is then appended to the end of
the wholeMsg.
3) The MD5-Digest is calculated according to [MD5]. Then the
authParameters field is replaced with the calculated digest.
4) The wholeMsg (excluding the appended secret key) is then
returned to the caller together with a statusCode of success.
6.3.2 Processing an Incoming Message
This section describes the procedure followed by an SNMPv3 engine
whenever it must authenticate an incoming message using the
imfAuthMD5Protocol.
1) If the digest received in the authParameters field is not
16 octets long, then an error indication (authenticationError)
is returned to the calling module.
2) The digest received in the authParameters field is saved.
3) The digest in the authParameters field is replaced by the
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secret (localized) authKey.
4) The secret (localized) authKey is then appended to the end of
the wholeMsg.
5) The MD5-Digest is calculated according to [MD5].
The authParameters field is replaced with the digest value
that was saved in step 2).
6) Then the newly calculated digest is compared with the digest
saved in step 2). If the digests do not match, then an error
indication (authenticationError) is returned to the calling
module.
7) The wholeMsg (excluding the appended secret key) and a
statusCode of success are then returned to the caller.
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7. DES Privacy Protocol
This section describes the DES privacy protocol.
This protocol is the first privacy protocol defined for the
User-based Security model.
Over time, other privacy protocols may be defined either
as a replacement of this protocol or in addition to this protocol.
7.1 Mechanisms
- In support of data confidentiality, an encryption algorithm is
required. An appropriate portion of the message is encrypted
prior to being transmitted. The User-based Security model
specifies that the scopedPDU is the portion of the message
that needs to be encrypted.
- A secret value is in combination with a time value is used to
create the en/decryption key and the initialization vector.
The secret value is shared by all SNMPv3 engines authorized to
originate messages on behalf of the appropriate user.
- In order to not expose the shared secrets (keys) at all SNMPv3
engines in case one of the engines is compromised, such secrets
(keys) are localized for each authoritative SNMPv3 engine, see
[Localized-Key].
7.1.1. Symmetric Encryption Protocol
The Symmetric Encryption Protocol defined in this memo provides
support for data confidentiality. The designated portion of an
SNMPv3 message is encrypted and included as part of the message
sent to the recipient.
This memo requires that if data confidentiality is supported by
an SNMPv3 engine, this engine must implement at least the Data
Encryption Standard (DES) in the Cipher Block Chaining mode of
operation.
Two organizations have published specifications defining the DES: the
National Institute of Standards and Technology (NIST) [DES-NIST] and
the American National Standards Institute [DES-ANSI]. There is a
companion Modes of Operation specification for each definition
(see [DESO-NIST] and [DESO-ANSI], respectively).
The NIST has published three additional documents that implementors
may find useful.
- There is a document with guidelines for implementing and using the
DES, including functional specifications for the DES and its modes
of operation [DESG-NIST].
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- There is a specification of a validation test suite for the DES
[DEST-NIST]. The suite is designed to test all aspects of the DES
and is useful for pinpointing specific problems.
- There is a specification of a maintenance test for the DES
[DESM-NIST]. 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
re-boots.
7.1.1.1 DES key and Initialization Vector.
The first 8 bytes of the 16-byte secret (private privacy key) are
used as a DES key.
Since DES uses only 56 bits, the Least Significant Bit in each
byte is disregarded.
The Initialization Vector for encryption is obtained using the
following procedure.
The last 8 bytes of the 16-byte secret (private privacy key)
are used as pre-IV.
In order to ensure that IV for two different packets encrypted
by the same key, are not the same (i.e. IV does not repeat) we
need to "salt" the pre-IV with something unique per packet.
An 8-byte octet string is used as the "salt". The concatenation
of the generating engine's 32-bit snmpEngineBoots and a local
32-bit integer that the encryption engine maintains is input to
the "salt". The 32-bit integer is initialized to a random value
at boot time. The 32-bit snmpEngineBoots is converted to the first
4 bytes (Most Significant Byte first) of our "salt". The 32-bit
integer is then converted to the last 4 bytes (Most Significant
Byte first) of our "salt". The resulting "salt" is then XOR-ed
with the pre-IV. The 8-byte salt is then put into the privParameters
field as an octet-string. The "salt" integer is incremented by one
and wraps when it reaches the maximum value.
The "salt" must be placed in the privParameters field to enable the
receiving entity to compute the correct IV and to decrypt the
message.
How exactly the value of the "salt" (and thus of the IV) varies,
is an implementation issue, as long as the measures are taken to
avoid producing a duplicate IV.
7.1.1.2 Data Encryption.
The data to be encrypted is treated as sequence of octets. Its
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length should be an integral multiple of 8 - and if not, the
data is padded at the end as necessary. The actual pad value
is irrelevant.
The data is encrypted in Cipher Block Chaining mode.
The plaintext is divided into 64-bit blocks.
The plaintext for each block is XOR-ed with the ciphertext
of the previous block, the result is encrypted and the output
of the encryption is the ciphertext for the block.
This procedure is repeated until there are no more plaintext
blocks.
For the very first block, the Initialization Vector is used
instead of the ciphertext of the previous block.
7.1.1.3 Data Decryption
Before decryption, the encrypted data length is verified.
If the length of the octet sequence to be decrypted is not an
integral multiple of 8 octets, the processing of the octet sequence
is halted and an appropriate exception noted. When decrypting, the
padding is ignored.
The first ciphertext block is decrypted, the decryption output is
XOR-ed with the Initialization Vector, and the result is the first
plaintext block.
For each subsequent block, the ciphertext block is decrypted,
the decryption output is XOR-ed with the previous ciphertext
block and the result is the plaintext block.
7.2 Elements of the DES Privacy Protocol
This section contains definitions required to realize the privacy
module defined by this memo.
7.2.1. SNMPv3 Users
Data En/Decryption using this Symmetric Encryption Protocol makes use
of a defined set of user identities. For any SNMPv3 user on whose
behalf a message must be en/decrypted at a particular SNMPv3 engine,
that engine must have knowledge of that user. An SNMPv3 engine that
wishes to communicate with another SNMPv3 engine must also have
knowledge of a user known to that engine, including knowledge of the
applicable attributes of that user.
A user and its attributes are defined as follows:
<userName>
An octet string representing the name of the user.
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<privKey>
A user's secret key to be used as input for the DES key and IV.
7.2.2. EngineID
The engineID value contained in an authenticated message specifies
the authoritative SNMPv3 engine for that particular message.
(see the definition of engineID in the SNMP Architecture document
[SNMP-ARCH]).
The user's (private) privacy key is normally different at
each authoritative SNMPv3 engine and so the snmpEngineID is used
to select the proper key for the authentication process.
7.2.3. SNMPv3 Messages Using this Privacy Protocol
Messages using this privacy protocol carry a privParameters
field as part of the securityParameters. For this protocol, the
privParameters field is the serialized octet string representing
the "salt" that was used to create the IV.
7.2.4 Input and Output of the DES Privacy Module
This section describes the inputs and outputs that the DES Privacy
module expects and produces when the User-based Security module
invokes the DES Privacy module for services.
7.2.4.1 Input and Output when generating an SNMPv3 Message
This DES privacy protocol assumes that the selection of the
privKey is done by the caller and that the caller passes
the secret key to be used. The abstract service interface is:
encryptMsg(cryptKey, scopedPDU)
Where:
cryptKey
The secret key to be used by the encryption algorithm.
scopedPDU
The data to be encrypted.
Upon completion the privacy module returns information.
The abstract service interface is:
returnEncryptedMsg(encryptedPDU, privParameters, statusCode)
Where:
encryptedPDU
The encrypted scopedPDU (encoded as an octet string).
privParameters
The privacy parameters (encoded as an octet string) that
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need to be sent in the outgoing message.
statusCode
The indicator of whether the PDU was encrypted successfully
and if not, it indicates what went wrong.
7.2.4.2 Input and Output when receiving an SNMPv3 Message
This DES privacy protocol assumes that the selection of the
privKey is done by the caller and that the caller passes
the secret key to be used. The abstract service interface is:
decryptMsg(cryptKey, privParameters, encryptedPDU)
Where:
cryptKey
The secret key to be used by the decryption algorithm.
privParameters
The "salt" to be used to calculate the IV.
encryptedPDU
the data to be decrypted
Upon completion the privacy module returns information.
The abstract service interface is:
returnDecryptedMsg(scopedPDU, statusCode)
Where:
scopedPDU
The decrypted scopedPDU.
statusCode
The indicator whether the message was successfully decrypted.
7.3 Elements of Procedure.
This section describes the procedures for the DES privacy protocol.
7.3.1 Processing an Outgoing Message
This section describes the procedure followed by an SNMPv3 engine
whenever it must encrypt part of an outgoing message using the
imfPrivDESProtocol.
1) The secret (localized) cryptKey are used to construct the DES
encryption key, the "salt" and the DES pre-IV (as described in
7.1.1.1).
2) The authParameters field is set to the serialization according
to the rules in [RFC1906] of an octet string representing the
the "salt" string.
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2) The scopedPDU is encrypted (as described in 7.1.1.2) and the
encrypted data is serialized according to the rules in [RFC1906]
as an octet string.
3) The the serialized octet string representing the encrypted
scopedPDU together with the privParameters and a statusCode of
success is returned to the caller.
7.3.2 Processing an Incoming Message
This section describes the procedure followed by an SNMPv3 engine
whenever it must decrypt part of an incoming message using the
imfPrivDESProtocol.
1) If the privParameters field is not an 8-byte octet string,
then an error indication (privacyError) is returned to the
calling module.
2) The "salt" is extracted from the privParameters field.
3) The secret (localized) cryptKey and the "salt" are then used
to construct the DES decryption key and pre-IV
(as described in 7.1.1.1).
4) The encryptedPDU is decrypted (as described in 7.1.1.3).
5) If the encryptedPDU cannot be decrypted, then an error
indication (privacyError) is returned to the calling module.
6) The decrypted scopedPDU and a statusCode of success are returned
to the calling module.
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8. Editor's Addresses
Co-editor Uri Blumenthal
IBM T. J. Watson Research
postal: 30 Saw Mill River Pkwy,
Hawthorne, NY 10532
USA
email: uri@watson.ibm.com
phone: +1-914-784-7064
Co-editor: Bert Wijnen
IBM T. J. Watson Research
postal: Schagen 33
3461 GL Linschoten
Netherlands
email: wijnen@vnet.ibm.com
phone: +31-348-432-794
9. Acknowledgements
This document is based on the recommendations of the SNMP Security and
Administrative Framework Evolution team, comprised of
David Harrington (Cabletron Systems Inc.)
Jeff Johnson (Cisco)
David Levi (SNMP Research Inc.)
John Linn (Openvision)
Russ Mundy (Trusted Information Systems) chair
Shawn Routhier (Epilogue)
Glenn Waters (Nortel)
Bert Wijnen (IBM T. J. Watson Research)
Further a lot of "cut and paste" material comes from RFC1910 and from
earlier draft documents from the SNMPv2u and SNMPv2* series.
Further more a special thanks is due to the SNMPv3 WG, specifically:
....
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10. Security Considerations
10.1. Recommended Practices
This section describes practices that contribute to the secure,
effective operation of the mechanisms defined in this memo.
- A management station must discard SNMPv3 responses for which
neither the msgID nor the request-id component or 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, when using these security protocols it is
significant due to the possibility of message duplication
(malicious or otherwise).
- A management station must generate unpredictable msgIDs and
request-ids in authenticated messages in order to protect against
the possibility of message duplication (malicious or otherwise).
For example, start operations with msgID and/or request-id 0 is
not a good idea. Initializing them with a pseudorandom number
and then incrementing by one would be acceptable.
- A management station should perform time synchronization using
authenticated messages in order to protect against the possibility
of message duplication (malicious or otherwise).
- 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 SNMPv3. 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 of approximately 150 seconds 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 [RFC1907], 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.
- The frequency with which the secrets of an SNMPv3 user 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
less significant, and as such the changing of secrets may be less
frequent. However, when public data networks are the
communication paths, more caution is prudent.
10.2 Defining Users
The mechanisms defined in this document employ the notion of "users"
which map into "groups" and such "groups" have access rights.
How "users" are defined is subject to the security policy of the
network administration. For example, users could be individuals
(e.g., "joe" or "jane"), or a particular role (e.g., "operator" or
"administrator"), or a combination (e.g., "joe-operator",
"jane-operator" or "joe-admin"). Furthermore, a "user" may be a
logical entity, such as a manager station application or set
of manager station applications, acting on behalf of an individual
or role, or set of individuals, or set of roles, including
combinations.
Appendix A describes an algorithm for mapping a user "password" to a
16 octet value for use as either a user's authentication key or
privacy key (or both). Note however, that using the same password
(and therefore the same key) for both authentication and privacy is
very poor security practice and should be strongly discouraged.
Passwords are often generated, remembered, and input by a human.
Human-generated passwords may be less than the 16 octets required
by the authentication and privacy protocols, and brute force
attacks can be quite easy on a relatively short ASCII character set.
Therefore, the algorithm is Appendix A performs a transformation on
the password. If the Appendix A algorithm is used, SNMP
implementations (and SNMP configuration applications) must ensure
that passwords are at least 8 characters in length.
Because the Appendix A algorithm uses such passwords (nearly)
directly, it is very important that they not be easily guessed. It
is suggested that they be composed of mixed-case alphanumeric and
punctuation characters that don't form words or phrases that might
be found in a dictionary. Longer passwords improve the security of
the system. Users may wish to input multiword phrases to make their
password string longer while ensuring that it is memorable.
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Since it is infeasible for human users to maintain different
passwords for every engine, but security requirements strongly
discourage having the same key for more than one engine, SNMPv3
employs a compromise proposed in [Localized-key].
It derives the user keys for the SNMPv3 engines from user's password
in such a way that it is practically impossible to either determine
the user's password, or user's key for another SNMPv3 engine from
any combination of user's keys on SNMPv3 engines.
Note however, that if user's password is disclosed, key localization
will not help and network security may be compromised in this case.
10.3. Conformance
To be termed a "Secure SNMPv3 implementation" based on the User-base
Security model, an SNMPv3 implementation:
- must implement one or more Authentication Protocol(s). The MD5
Authentication Protocol defined in this memo is one such protocol.
- must, to the maximum extent possible, prohibit access to the
secret(s) of each user about which it maintains information in a
Local (security) Configuration Database (LCD) under all
circumstances except as required to generate and/or validate
SNMPv3 messages with respect to that user.
- must implement the SNMP USEC MIB.
In addition, an authoritative SNMPv3 engine must provide initial
configuration in accordance with Appendix A.1.
Implementation of a Privacy Protocol (the Symmetric Encryption
Protocol defined in this memo is one such protocol) is optional.
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11. References
[RFC1902] The SNMPv2 Working Group, Case, J., McCloghrie, K.,
Rose, M., and S., Waldbusser, "Structure of Management
Information for Version 2 of the Simple Network Management
Protocol (SNMPv2)", RFC 1905, January 1996.
[RFC1905] The SNMPv2 Working Group, Case, J., McCloghrie, K.,
Rose, M., and S., Waldbusser, "Protocol Operations for
Version 2 of the Simple Network Management Protocol (SNMPv2)",
RFC 1905, January 1996.
[RFC1906] The SNMPv2 Working Group, Case, J., McCloghrie, K.,
Rose, M., and S. Waldbusser, "Transport Mappings for
Version 2 of the Simple Network Management Protocol (SNMPv2)",
RFC 1906, January 1996.
[RFC1907] The SNMPv2 Working Group, Case, J., McCloghrie, K.,
Rose, M., and S. Waldbusser, "Management Information Base for
Version 2 of the Simple Network Management Protocol (SNMPv2)",
RFC 1907 January 1996.
[RFC1908] The SNMPv2 Working Group, Case, J., McCloghrie, K.,
Rose, M., and S. Waldbusser, "Coexistence between Version 1
and Version 2 of the Internet-standard Network Management
Framework", RFC 1908, January 1996.
[SNMP-ARCH] The SNMPv3 Working Group, Harrington, D., Wijnen, B.,
"An Architecture for describing Internet Management Frameworks",
draft-ietf-snmpv3-next-gen-arch-02.txt, June 1997.
[SNMPv3-MPC] The SNMPv3 Working Group, Wijnen, B., Harrington, D.,
"Message Processing and Control Model for version 3 of the Simple
Network Management Protocol (SNMPv3)",
draft-ietf-snmpv3-mpc-01.txt, June 1997.
[SNMPv3-ACM] The SNMPv3 Working Group, Wijnen, B., Harrington, D.,
"Access Control Model for Version 3 of the Simple Network
Management Protocol (SNMPv3)", draft-ietf-snmpv3-acm-00.txt,
June 1997.
[SNMPv3-USEC] The SNMPv3 Working Group, Blumenthal, U., Wijnen, B.
"User-Based Security Model for version 3 of the Simple Network
Management Protocol (SNMPv3)",
draft-ietf-snmpv3-usec-01.txt, June 1997.
[Localized-Key] U. Blumenthal, N. C. Hien, B. Wijnen
"Key Derivation for Network Management Applications"
IEEE Network Magazine, April/May issue, 1997.
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Draft User-based Security Model (USEC) for SNMPv3 June 1997
[KEYED-MD5] Krawczyk, H.,
"Keyed-MD5 for Message Authentication",
Work in Progress, IBM, June 1995.
[MD5] Rivest, R.
"Message Digest Algorithm MD5"
RFC 1321.
[DES-NIST] 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).
[DES-ANSI] Data Encryption Algorithm, American National Standards
Institute. ANSI X3.92-1981, (December, 1980).
[DESO-NIST] DES Modes of Operation, National Institute of Standards and
Technology. Federal Information Processing Standard (FIPS)
Publication 81, (December, 1980).
[DESO-ANSI] Data Encryption Algorithm - Modes of Operation, American
National Standards Institute. ANSI X3.106-1983, (May 1983).
[DESG-NIST] 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).
[DEST-NIST] Validating the Correctness of Hardware Implementations of
the NBS Data Encryption Standard, National Institute of Standards
and Technology. Special Publication 500-20.
[DESM-NIST] Maintenance Testing for the Data Encryption Standard,
National Institute of Standards and Technology.
Special Publication 500-61, (August, 1980).
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APPENDIX A - Installation
A.1. Engine Installation Parameters
During installation, an SNMPv3 engine acting in an authoritative role
is configured with several parameters. These include:
(1) one or more secrets
These are the authentication/privacy secrets for the first user
to be configured.
One way to accomplish this is to have the installer enter a
"password" for each required secret. The password is then
algorithmically converted into the required secret by:
- forming a string of length 1,048,576 octets by repeating the
value of the password as often as necessary, truncating
accordingly, and using the resulting string as the input to
the MD5 algorithm [MD5]. The resulting digest, termed "digest1",
is used in the next step.
- a second string of length 44 octets is formed by concatenating
digest1, the SNMPv3 engine's snmpEngineID value, and digest1.
This string is used as input to the MD5 algorithm [MD5].
The resulting digest is the required secret (see Appendix A.2).
With these configured parameters, the SNMPv3 engine instantiates
the following usecUserEntry in the usecUserTable:
no privacy support privacy support
------------------ ---------------
usecUserEngineID localEngineID localEngineID
usecUserName "public" "public"
usecUserMiId "public" "public"
usecUserGroupName "public" "public"
usecUserCloseFrom ZeroDotZero ZeroDotZero
usecUserAuthProtocol imfAuthMD5Protocol imfAuthMD5Protocol
usecUserAuthKeyChange "" ""
usecUserAuthPublic "" ""
usecUserPrivProtocol none imfPrivDESProtocol
usecUserPrivKeyChange "" ""
usecUserrivhPublic "" ""
usecUserStorageType permanent permanent
usecUserStatus active active
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A.2. Password to Key Algorithm
The following code fragment demonstrates the password to key
algorithm which can be used when mapping a password to an
authentication or privacy key. The calls to MD5 are as
documented in RFC1321 [RFC1321]
void password_to_key(
u_char *password, /* IN */
u_int passwordlen, /* IN */
u_char *engineID, /* IN - ptr to 12 octet long snmpEngineID */
u_char *key) /* OUT - caller's pointer to 16-byte buffer */
{
MD5_CTX MD;
u_char *cp, password_buf[64];
u_long password_index = 0;
u_long count = 0, i;
MD5Init (&MD); /* initialize MD5 */
/**********************************************/
/* Use while loop until we've done 1 Megabyte */
/**********************************************/
while (count < 1048576) {
cp = password_buf;
for (i = 0; i < 64; i++) {
/*************************************************/
/* Take the next byte of the password, wrapping */
/* to the beginning of the password as necessary.*/
/*************************************************/
*cp++ = password[password_index++ % passwordlen];
}
MDupdate (&MD, password_buf, 64);
count += 64;
}
MD5Final (key, &MD); /* tell MD5 we're done */
/*****************************************************/
/* Now localize the key with the engineID and pass */
/* through MD5 to produce final key */
/*****************************************************/
memcpy(password_buf, key, 16);
memcpy(password_buf+16, engineID, 12);
memcpy(password_buf+28, key, 16);
MD5Init(&MD);
MDupdate(&MD, password_buf, 44);
MD5Final(key, &MD);
return;
}
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A.3. Password to Key Sample
The following shows a sample output of the password to key algorithm.
With a password of "maplesyrup" the output of the password to key
algorithm before the key is localized with the engine's engineID is:
'9f af 32 83 88 4e 92 83 4e bc 98 47 d8 ed d9 63'H
After the intermediate key (shown above) is localized with the
snmpEngineID value of:
'00 00 00 00 00 00 00 00 00 00 00 02'H
the final output of the password to key algorithm is:
'52 6f 5e ed 9f cc e2 6f 89 64 c2 93 07 87 d8 2b'H
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Table of Contents
0.1 Issues 1
0.2 Change Log 2
1. Introduction 3
1.1. Threats 3
1.2. Goals and Constraints 4
1.3. Security Services 5
1.4. Implementation Organization 6
1.4.1. Timeliness Module 6
1.4.2. Authentication Protocol 6
1.4.3. Privacy Protocol 7
1.5 Protection against Message Replay, Delay and Redirection 7
1.5.1 Authoritative SNMP Engine 7
1.5.2 The following mechanisms are used: 7
2. Elements of the Model 10
2.1. SNMPv3 Users 10
2.2. Replay Protection 11
2.2.1. snmpEngineID 11
2.2.2. engineBoots and engineTime 11
2.3 for (re-)synchronization procedures). Note, however, that the 12
2.2.3. Time Window 12
2.3. Time Synchronization 12
2.4. SNMPv3 Messages Using this Model 13
2.5 Input and Output of the User-based Security Module 14
2.5.1 Input and Output when generating an SNMPv3 Message 14
2.5.2 Input and Output when receiving an SNMPv3 Message 15
3. Elements of Procedure 18
3.1. Processing an Outgoing Message 18
3.2. Processing an Incoming Message 20
2.4, then the snmpInASNParseErrs counter [RFC1907] is 20
4. Discovery 25
5. Definitions 26
6. MD5 Authentication Protocol 36
6.1 Mechanisms 36
6.1.1. Digest Authentication Protocol 36
6.2 Elements of the Digest Authentication Protocol 37
6.2.1. SNMPv3 Users 37
6.2.2. EngineID 37
6.2.3. SNMPv3 Messages Using this Authentication Protocol 37
6.2.4 Input and Output of the MD5 Authentication Module 37
6.2.4.1 Input and Output when generating an SNMPv3 Message 38
6.2.4.2 Input and Output when receiving an SNMPv3 Message 38
6.3 Elements of Procedure 39
6.3.1 Processing an Outgoing Message 39
6.3.2 Processing an Incoming Message 39
7. DES Privacy Protocol 41
7.1 Mechanisms 41
7.1.1. Symmetric Encryption Protocol 41
7.1.1.1 DES key and Initialization Vector. 42
7.1.1.2 Data Encryption. 42
7.1.1.3 Data Decryption 43
7.2 Elements of the DES Privacy Protocol 43
7.2.1. SNMPv3 Users 43
7.2.2. EngineID 44
7.2.3. SNMPv3 Messages Using this Privacy Protocol 44
7.2.4 Input and Output of the DES Privacy Module 44
7.2.4.1 Input and Output when generating an SNMPv3 Message 44
7.2.4.2 Input and Output when receiving an SNMPv3 Message 45
7.3 Elements of Procedure. 45
7.3.1 Processing an Outgoing Message 45
7.1.1.1). 45
7.3.2 Processing an Incoming Message 46
8. Editor's Addresses 47
9. Acknowledgements 47
A.1. Engine Installation Parameters 53
A.2. Password to Key Algorithm 54
A.3. Password to Key Sample 55
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