User-based Security Model (USM) for version 3 of the
Simple Network Management Protocol (SNMPv3)
14 July 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-usm-00.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
and may be updated, replaced, or obsoleted by other documents at any
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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 (USM) for SNMP
version 3 for use in the SNMP architecture [SNMP-ARCH]. It 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.
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0. Issues and Change Log
0.1. Current Open Issues
- Is it OK to use MD5 for KeyChange Algorithm ??
- Improve acknowledgements and sync it up with other documents
- Should the USM define checking such that a received Response
messages used the same or better LoS than the Request message
that this is a response to.
In section 3.1 step 9, we return a completed outgoing message
to the calling module (Message Processing). We believe it is
the Message Processing Subsystem that should cache information
about outgoing messages regarding msgID and such so that a
possible Response Message can be mapped to an outstanding
request. At the same time that piece of code can then ensure
that the same securityModel and the same (or better??) LoS
has been used for the Response Message. So in step 9 we do
not save any cachedSecurityData for outgoing messages.
- Can you all please review section 3.1 steps 7a and 7b to
ensure that we have the timeliness checking and the
automagic timeliness sync up correct? Quite some text changed
in this writeup compared to what we used to see in SNMPv2u
and SNMPv2*. I think the current text is much better and
makes things simpler. But we need to make sure we cover
everything.
0.2. Change Log
[version 1.8]
- Add reference to RFC2119 about use of SHOULD and MUST
- paginate and generate table of contents
- posted as I-D <draft-ietf-snmpv3-usm-00.txt> on 15 July 1997
[version 1.7]
- Changed the KeyChange description so it allows for other
hash algorithms instead of MD5. If in the future the MD5 gets
replaced by another Authentication -- algorithm, then it seems
we also need to use that new algorithm to -- calculate the
digest during KeyChange.
- Updated the password to key code fragment to cater for the
variable length of the snmpEngineID.
- Added issue on cacheing of data on outgoing messages and one
on required review of timeliness handling.
[version 1.4 - version 1.6]
- Editorial changes because of internal review by authors
- Adapt to latest list of Primitive names and parameters
- Change USEC to USM
- Changes based on comments from Jeff Case.
- Checked MIB with SMICng
[version 1.3]
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- Too many changes have taken place, so marking it was skipped
The most important changes are listed here.
However, changes that just split text on different lines
and changes like different capitalization of words/terms
has not been listed. Also changes to fit new terms and such
have not been listed.
- Split/Join some lines to ensure we stay within 72 columns
as required by RFC guidelines.
- Addressed Dave Perkins comments:
1) Section 1.3, last para's, timeliness was left off. -done
2) Section 1.5.1, the operations need to be made general, since
additional one may be added later. - done
3) Section 1.5.2, the field "request-id" is used throughout
this section when it should be field "msgID" - done
4) The document must allow the value of engineID in the
security to be a zero length string. There are several
places that are affected by this change. An actual value is
never needed, since secrets are never the same on different
agents (see your paper). - done
5) Last sentence of description for object usmUserCloneFrom is
not correct, since the object has a OID data type - done
- Removed groupName from usmUserTable.
Now done in Access Control as agreed at 2nd interim
- Stats counters back in this document as agreed at 2nd interim
- Use AutonomousType for usmUserPrivProtocol and
usmUserAuthProtocol. Also use OBJECT-IDENTITY for the
protocol OIDs (John Flick).
- Changed "SNMPv3 engine" to "SNMP engine" at various places
- added appendix with sample encoding of securityParameters
- cleanup elements of procedure to use consistent terms
- fix up some problems in elements of procedure
- Do not use IMPLIED on usmUserTable as agreed at 2nd interim.
For one thing, SNMPv1 cannot handle it.
- cleanup section 2.3 and 3.3 step 7b based on comments by
Dave Levi.
[version 1.2]
- changed (simplified) time sync in section 3 item 7.
- added usmUserMiId
- cleaned up text
- defined IV "salt" generation
- removed Statistics counters (now in MPC) and Report PDU
generation (now in MPC)
- Removed auth and DES MIBs which are now merged into
User-based Security MIB
- specified where cachedSecurityData needs to be discarded
- added abstract 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.
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[version 1.1]
- removed <securityCookie>.
- added <securityIdentity>, <securityCachedData>.
- added abstract function interface description of
inter-module communications.
- modified IV generation process to accommodate 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 time-stamp is ahead of our notion of their clock).
[version 1.0]
- first version posted to the SNMPv3 editor's 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
[SNMP-ARCH] is composed of multiple subsystems:
1) a Message Processing Subsystem,
2) a Security Subsystem,
3) an Access Control Subsystem,
Applications make use of the services of these subsystems.
It is important to understand the SNMP architecture and the
terminology of the architecture to understand where the Security
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 [SNMP-USM] 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.
Therefore, the description of Keyed-MD5 and CBC-DES are in separate
sections to reflect their self-contained nature and to indicate
that they 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 SNMP Security Model
should provide protection are:
- Modification of Information
The modification threat is the danger that some unauthorized
entity may alter in-transit SNMP 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
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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.
Two secondary threats are also identified. The Security Model
defined in this memo provides 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 SNMP 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 SNMP Security Model need
not protect against. The security protocols defined in this memo
do not provide protection against:
- Denial of Service
This SNMP Security Model does 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
This SNMP Security Model does not attempt to address traffic
analysis attacks. Indeed, many traffic patterns are predictable
- devices may be managed on a regular basis by a relatively small
number of management applications - 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 SNMP Security Model
are as follows.
1) Provide for verification that each received SNMP message has
not been modified during its transmission through the network.
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2) Provide for verification of the identity of the user on whose
behalf a received SNMP message claims to have been generated.
3) Provide for detection of received SNMP messages, which request
or contain management information, whose time of generation was
not recent.
4) Provide, when necessary, that the contents of each received
SNMP message are protected from disclosure.
In addition to the principal goal of supporting secure network
management, the design of this SNMP 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 this SNMP
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.
- Message timeliness and limited replay protection
is the provision of the property that a message whose generation
time is outside of a specified time window is not accepted.
Note that message reordering is not dealt with and can occur in
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normal conditions too.
For the protocols specified in this memo, it is not possible to
assure the specific originator of a received SNMP 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 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. Module Organization
The security protocols defined in this memo are split in three
different modules and each has its specific responsibilities such
that together they realize the goals and security services
described above:
- The authentication module MUST provide for:
- Data Integrity,
- Data Origin Authentication
- The timeliness module MUST provide for:
- Protection against message delay or replay (to an extent
greater than can occur through normal operation)
- The privacy module MUST provide for
- Protection against disclosure of the message payload.
The timeliness module is fixed for the 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 timeliness values in an
SNMP 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
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the timeliness 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 authentication protocols may be defined as new
needs arise.
The User-based Security Model prescribes that, if authentication
is used, then 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 is 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.
The User-based Security Model prescribes that the scopedPDU
is protected from disclosure when a message is sent with privacy.
The 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 SNMP engine. When an SNMP
message contains a payload which expects a response (for example
a Get, GetNext, GetBulk, Set or Inform PDU), then the receiver of
such messages is authoritative. When an SNMP message contains a
payload which does not expect a response (for example an
SNMPv2-Trap, Response or Report PDU), then the sender of such a
message is authoritative.
1.5.2. Mechanisms
The following mechanisms are used:
- To protect against the threat of message delay or replay (to an
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extent greater than can occur through normal operation), a set
of timeliness (at the authoritative source) indicators and a
msgID are included in each message generated. An SNMP engine
evaluates the timeliness indicators to determine if a received
message is recent. An SNMP engine may evaluate the timeliness
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 SNMP engine uses received authentic messages
to advance its notion of the timeliness indicators at the remote
authoritative source. An SNMP engine also evaluates the msgID in
received Response messages and discards those Response messages
which do not correspond to an outstanding Request message.
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 receiver) when the receiver 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 sensitive
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 the timeliness indicators (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 SNMP 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 SNMP engine cannot
be replayed to/as-if-from another SNMP engine.
Included in each message is an identifier unique to the SNMP
engine associated with the sender or intended recipient of the
message. Also, each message containing a Response PDU contains
a msgID which associates the message with a recently generated
Request message.
A Report message sent by one SNMP engine to a second SNMP
engine can potentially be replayed to another SNMP engine but
that is not considered a threat (see above);
- detecting messages which were not recently generated.
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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 msgID; if the msgID does not match that of a recently
generated Request message, then the message is not considered
to be authentic.
A Report message sent by an SNMP engine can potentially be
replayed at a later time to an SNMP engine which has not
recently communicated with that source engine, which is
not a threat (see above).
This memo allows the same user to be defined on multiple SNMP
engines. Each SNMP engine maintains a value, snmpEngineID,
which uniquely identifies the SNMP engine. This value is included
in each message sent to/from the SNMP engine that is authoritative
(see section 1.5.1). On receipt of a message, an authoritative
SNMP engine checks the value to ensure that it is the intended
recipient, and a non-authoritative SNMP engine uses the value to
ensure that the message is processed using the correct state
information.
Each SNMP engine maintains two values, snmpEngineBoots and
snmpEngineTime, which taken together provide an indication of
time at that SNMP engine. Both of these values are included in
an authenticated message sent to/received from that SNMP engine.
On receipt, the values are checked to ensure that the indicated
timeliness value 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 SNMP engine to generate a message which an authoritative
SNMP engine will accept as authentic, and to verify that a message
received from that authoritative SNMP engine is authentic, such an
SNMP engine must first achieve timeliness synchronization with the
authoritative SNMP engine. See section 2.3.
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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. User-based Security Model Users
Management operations using this Security Model make use of a
defined set of user identities. For any user on whose behalf
management operations are authorized at a particular SNMP engine,
that SNMP engine must have knowledge of that user. An SNMP engine
that wishes to communicate with another SNMP 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.
securityName
A human-readable string representing the user in a format that
is Security Model independent.
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 SNMP engines.
The authKey is not accessible via SNMP.
authKeyChange and authOwnKeyChange
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.
privKey
If messages sent on behalf of this user can be en/decrypted,
the (private) privacy key for use with the privacy protocol.
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Note that a user's privacy key will normally be different at
different authoritative SNMP engines. The privKey is not
accessible via SNMP.
privKeyChange and privOwnKeyChange
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 SNMP engine maintains three objects:
- snmpEngineID, which (at least within an administrative domain)
uniquely and unambiguously identifies an SNMP engine.
- snmpEngineBoots, which is a count of the number of times the
SNMP engine has re-booted/re-initialized since snmpEngineID
was last configured; and,
- snmpEngineTime, which is the number of seconds since the
snmpEngineBoots counter was last incremented.
Each SNMP engine is always authoritative with respect to these
objects in its own SNMP entity. It is the responsibility of a
non-authoritative SNMP engine to synchronize with the
authoritative SNMP engine, as appropriate.
An authoritative SNMP engine is required to maintain the values of
its snmpEngineID and snmpEngineBoots in non-volatile storage.
2.2.1. authEngineID
The authEngineID value contained in an authenticated message is
used to defeat attacks in which messages from one SNMP engine to
another SNMP engine are replayed to a different SNMP engine.
It represents the snmpEngineID at the authoritative SNMP engine
involved in the exchange of the message.
When an authoritative SNMP engine is first installed, it sets its
local value of snmpEngineID according to a enterprise-specific
algorithm (see the definition of the Textual Convention for
SnmpEngineID in the SNMP Architecture document [SNMP-ARCH]).
2.2.2. authEngineBoots and authEngineTime
The authEngineBoots and authEngineTime values contained in an
authenticated message are used to defeat attacks in which messages
are replayed when they are no longer valid. They represent the
snmpEngineBoots and snmpEngineTime values at the authoritative
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SNMP engine involved in the exchange of the message.
Through use of snmpEngineBoots and snmpEngineTime, there is no
requirement for an SNMP engine to have a non-volatile clock which
ticks (i.e., increases with the passage of time) even when the
SNMP engine is powered off. Rather, each time an SNMP engine
re-boots, it retrieves, increments, and then stores snmpEngineBoots
in non-volatile storage, and resets snmpEngineTime to zero.
When an SNMP engine is first installed, it sets its local values
of snmpEngineBoots and snmpEngineTime to zero. If snmpEngineTime
ever reaches its maximum value (2147483647), then snmpEngineBoots
is incremented as if the SNMP engine has re-booted and
snmpEngineTime is reset to zero and starts incrementing again.
Each time an authoritative SNMP engine re-boots, any SNMP engines
holding that authoritative SNMP engine's values of snmpEngineBoots
and snmpEngineTime need to re-synchronize prior to sending
correctly authenticated messages to that authoritative SNMP 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 SNMP engine, when sent by an
authoritative SNMP engine which has re-booted since the receiving
SNMP engine last (re-)synchronized.
If an authoritative SNMP engine is ever unable to determine its
latest snmpEngineBoots value, then it must set its snmpEngineBoots
value to 0xffffffff.
Whenever the local value of snmpEngineBoots has the value
0xffffffff, it latches at that value and an authenticated message
always causes an notInTimeWindow authentication failure.
In order to reset an SNMP engine whose snmpEngineBoots 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
SNMP 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 SNMP engine
in order to proceed with authentic communications, has occurred
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when the non-authoritative SNMP engine has obtained a local notion
of the authoritative SNMP engine's values of snmpEngineBoots and
snmpEngineTime from the authoritative SNMP engine. These values
must be (and remain) within the authoritative SNMP engine's Time
Window. So the local notion of the authoritative SNMP engine's
values must be kept loosely synchronized with the values stored
at the authoritative SNMP engine. In addition to keeping a local
copy of snmpEngineBoots and snmpEngineTime from the authoritative
SNMP engine, a non-authoritative SNMP engine must also keep one
local variable, latestReceivedEngineTime. This value records the
highest value of snmpEngineTime that was received by the
non-authoritative SNMP engine from the authoritative SNMP engine
and is used to eliminate the possibility of replaying messages
that would prevent the non-authoritative SNMP engine's notion of
the snmpEngineTime from advancing.
A non-authoritative SNMP engine must keep local notions of these
values for each authoritative SNMP engine with which it wishes to
communicate. Since each authoritative SNMP engine is uniquely
and unambiguously identified by its value of snmpEngineID, the
non-authoritative SNMP engine may use this value as a key in
order to cache its local notions of these values.
Time synchronization occurs as part of the procedures of receiving
an SNMP message (Section 3.2, step 7b). As such, no explicit time
synchronization procedure is required by a non-authoritative SNMP
engine. Note, that whenever the local value of snmpEngineID is
changed (e.g., through discovery) or when secure communications
are first established with an authoritative SNMP engine, the local
values of snmpEngineBoots and latestReceivedEngineTime should be
set to zero. This will cause the time synchronization to occur
when the next authentic message is received.
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2.4. SNMP Messages Using this Security Model
The syntax of an SNMP message using this Security Model adheres
to the message format defined in the version-specific Message
Processing Model document (for example [SNMP-MP]).
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 User-based security parameters
authEngineID
OCTET STRING (SIZE(12)),
authEngineBoots
Unsigned32 (0..4294967295),
authEngineTime
Unsigned32 (0..2147483647),
userName
OCTET STRING (SIZE(1..16)),
-- authentication protocol specific parameters
authParameters
OCTET STRING,
-- privacy protocol specific parameters
privParameters
OCTET STRING,
}
END
The authEngineID is the snmpEngineID of the authoritative SNMP
engine involved in the exchange of the message.
The authEngineBoots is the snmpEngineBoots value at the
authoritative SNMP engine involved in the exchange of the message.
The authEngineTime is the snmpEngineTime value at the
authoritative SNMP engine involved in the exchange of the message.
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 usmUserTable).
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 usmUserTable).
See appendix A.4 for en example of the encoding.
2.5. Services provided by the User-based Security Model
This section describes the services provided by the User-based
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Security Model with their inputs and outputs.
The services are described as primitives of an abstract service
interface and the inputs and outputs are described as abstract
data elements as they are passed in these abstract service
interface primitives.
2.5.1. Services for Generating an Outgoing SNMP Message
When the Message Processing (MP) Subsystem invokes the User-based
Security module to secure an outgoing SNMP message, it must use
the appropriate service as provided by the Security module. These
two services are provided:
1) A service to generate a Request message.
2) A service to generate a Response message.
Upon completion of the process, the User-based Security module
returns statusInformation and, if the process was successful,
the completed message with privacy and authentication applied
if such was requested by the specified Level of Security (LoS).
The abstract service interface primitives are:
generateRequestMsg(
messageProcessingModel -- typically, SNMP version
msgID -- for the outgoing message
mms -- of the sending SNMP entity
msgFlags -- for the outgoing message
securityParameters -- filled in by Security Module
securityModel -- for the outgoing message
securityName -- on behalf of this principal
LoS -- Level of Security requested
snmpEngineID -- authoritative SNMP entity
scopedPDU -- message (plaintext) payload
)
generateResponseMsg(
messageProcessingModel -- typically, SNMP version
msgID -- for the outgoing message
mms -- of the sending SNMP entity
msgFlags -- for the outgoing message
securityParameters -- filled in by Security Module
securityModel -- for the outgoing message
scopedPDU -- message (plaintext) payload
securityStateReference -- reference to security state
-- information, as received in
) -- processPdu primitive
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returnGeneratedMsg(
wholeMsg -- complete generated message
wholeMsgLength -- length of the generated message
statusInformation -- errorIndication or success
)
Where:
messageProcessingModel
The SNMP version number for the message to be generated.
This data is not used by the User-based Security module.
It is part of the globalData of the message.
msgID
The msgID for the message to be generated.
This data is not used by the User-based Security module.
It is part of the globalData of the message.
mms
The maximum message size to be included as mms in the message.
This data is not used by the User-based Security module.
It is part of the globalData of the message.
msgFlags
The msgFlags to be included in the message.
This data is not used by the User-based Security module.
It is part of the globalData of the message.
It should be consistent with the LoS that is passed.
securityParameters
These are the security parameters. They will be filled in
by the User-based Security module.
securityModel
The securityModel in use.
Should be the User-based Security Model.
This data is not used by the User-based Security module.
It is part of the globalData of the message.
securityName
Together with the snmpEngineID it identifies a row in the
usmUserTable that is to be used for securing the message.
The securityName has a format that is independent of the
Security Model.
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.
snmpEngineID
The snmpEngineID of the authoritative SNMP engine to which the
Request message is to be sent or from which the Response
message originates. In case of a response the snmpEngineID
is implied to be the processing SNMP engine's snmpEngineID.
scopedPDU
The message payload. The data is opaque as far as the
User-based Security Model is concerned.
securityStateReference
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A handle/reference to cached security data to be used when
securing an outgoing Response message. This is the exact same
handle/reference as it was generated by the User-based Security
module when processing the incoming Request message to which
this is the Response message.
wholeMsg
The fully encoded and secured message ready for sending on
the wire.
wholeMsgLength
The length of the encoded and secured message (wholeMsg).
statusInformation
An indication of whether the encoding and securing of the
message was successful. If not it is an indication of the
problem.
2.5.2. Services for Processing an Incoming SNMP Message
When the Message Processing (MP) Subsystem invokes the User-based
Security module to verify proper security of an incoming message,
it must use the service provided for an incoming message.
Upon completion of the process, the User-based Security module
returns statusInformation and, if the process was successful,
the additional data elements for further processing of the message.
The abstract service interface primitives are:
processMsg(
messageProcessingModel -- typically, SNMP version
msgID -- of the received message
mms -- of the sending SNMP entity
msgFlags -- for the received message
securityParameters -- for the received message
securityModel -- for the received message
LoS -- Level of Security
wholeMsg -- as received on the wire
wholeMsgLength -- length as received on the wire
)
returnProcessedMsg(
securityName -- identification of the principal
scopedPDU, -- message (plaintext) payload
maxSizeResponseScopedPDU -- maximum size of the Response PDU
securityStateReference -- reference to security state
-- information, needed for response
statusInformation -- errorIndication or success
) -- error counter OID/value if error
Where:
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messageProcessingModel
The SNMP version number as received in the message.
This data is not used by the User-based Security module.
It is part of the globalData of the message.
msgID
The msgID as received in the message.
This data is not used by the User-based Security module.
It is part of the globalData of the message.
mms
The maximum message size as received in the message.
It is part of the globalData of the message.
The USM module uses this information to calculate the
maxSizeResponseScopedPDU that it returns upon completion.
msgFlags
The msgFlags as received in the message.
This data is not used by the User-based Security module.
It is part of the globalData of the message.
It should be consistent with the LoS that is passed.
securityParameters
These are the security parameters as received in the message.
securityModel
The securityModel in use.
Should be the User-based Security Model.
This data is not used by the User-based Security module.
It is part of the globalData of the message.
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
The whole message as it was received.
wholeMsgLength
The length of the message as it was received (wholeMsg).
securityName
The security name representing the user on whose behalf the
message was received. The securityName has a format that is
independent of the Security Model.
scopedPDU
The message payload. The data is opaque as far as the
User-based Security Model is concerned.
maxSizeResponseScopedPDU
The maximum size of a scopedPDU to be included in a possible
Response message. The User-base Security module calculates
this size based on the mms (as received in the message) and
the space required for the message header (including the
securityParameters) for such a Response message.
securityStateReference
A handle/reference to cached security data to be used when
securing an outgoing Response message. When the Message
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Processing Subsystem calls the User-based Security module to
generate a response to this incoming message it must pass this
handle/reference.
statusInformation
An indication of whether the process was successful or not.
If not, then the statusInformation includes the OID and the
value of the error counter that was incremented.
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3. Elements of Procedure
This section describes the security related procedures followed by
an SNMP engine when processing SNMP messages according to the
User-based Security Model.
3.1. Generating an Outgoing SNMP Message
This section describes the procedure followed by an SNMP engine
whenever it generates a message containing a management operation
(like a request, a response, a notification, or a report) on
behalf of a user, with a particular Level of Security (LoS).
1) a) If any securityStateReference is passed (Response message),
then information concerning the user is extracted from the
cachedSecurityData. The snmpEngineID and the Level of
Security (LoS) are extracted from the cachedSecurityData.
The cachedSecurityData can now be discarded.
Otherwise,
b) based on the securityName, information concerning the
user at the destination snmpEngineID is extracted from
the Local Configuration Datastore (LCD, usmUserTable).
If information about the user is absent from the LCD,
then an error indication (unknownSecurityName) 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) a) 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 according to
the abstract service interface primitive:
encryptData(
cryptKey -- user's privKey
dataToEncrypt) -- serialized scopedPDU
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The user's private privKey is the secret key that can
be used by the encryption algorithm. The serialized
scopedPDU is the data that must be encrypted.
Upon completion the privacy module returns the result
according to the abstract service interface primitive:
returnEncryptedData(
encryptedData -- serialized encryptedPDU
privParameters -- serialized privParameters
statusInformation) -- success or failure
The encryptedPDU represents the encrypted scopedPDU,
encoded as an OCTET STRING.
The privParameters represents the privacy parameters,
encoded as an OCTET STRING.
The statusInformation indicates if the scopedPDU was
encrypted successfully or not.
If the privacy module returns failure, then the message
cannot be sent and an error indication (encryptionFailure)
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.
Otherwise,
b) 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 and put into the
privParameters field of the securityParameters and the
plaintext scopedPDU serves as the payload of the message
being prepared.
5) The snmpEngineID is encoded as an OCTET STRING into the
authEngineID field of the securityParameters.
6) a) If the Level of Security (LoS) specifies that the message
is to be authenticated, then the current values of
snmpEngineBoots and snmpEngineTime corresponding to the
snmpEngineID from the LCD are used.
Otherwise,
b) If this is a Response message, then the current value of
snmpEngineBoots and snmpEngineTime corresponding to the
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local snmpEngineID from the LCD are used.
Otherwise,
c) If this is a Request message, then a zero value is used
for both snmpEngineBoots and snmpEngineTime.
The values are encoded as Unsigned32 into the authEngineBoots
and authEngineTime fields of the securityParameters.
7) The userName is encoded as an OCTET STRING into the userName
field of the securityParameters.
8) a) 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 according to
the abstract service interface primitive:
authenticateOutgoingMsg(
authKey -- the user's authKey
wholeMsg) -- the complete serialized message
The user's private authKey is the secret key that can
be used by the authentication algorithm.
The wholeMsg is the complete serialized message that
must be authenticated.
Upon completion the authentication module returns the result
according to the abstract service interface primitive:
returnAuthenticatedOutgoingMsg(
wholeMsg -- secured serialized message
statusInformation) -- success or failure
The wholeMsg is the same as the input given to the
authenticateOutgoingMsg service, but with authParameters
properly filled in.
The statusInformation indicates if the message was
successfully processed by the authentication module or not.
If the authentication module returns failure, then the
message cannot be sent and an error indication
(authenticationFailure) is returned to the calling module.
If the authentication module returns success, then the
authParameters field is put into the securityParameters
and the wholeMsg represents the serialization of the
authenticated message being prepared.
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Otherwise,
b) 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. The wholeMsg is now serialized and
then represents the unauthenticated message being prepared.
9) The completed message with its length is returned to the
calling module with the statusInformation set to success.
This is done according to the following abstract service
interface primitive:
returnGeneratedMsg(
wholeMsg -- LoS secured serialized message
wholeMsgLength -- length of message
statusInformation) -- success
3.2. Processing an Incoming SNMP Message
This section describes the procedure followed by an SNMP 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
formatted according to the securityParameters defined in
section 2.4, then the snmpInASNParseErrs counter [RFC1907] is
incremented, and an error indication (parseError) together
with the OID and value of the incremented counter is returned
to the calling module.
2) The values of the security parameter fields are extracted from
the securityParameters.
3) If the value of the authEngineID contained in the
securityParameters is unknown then:
a) a manager that performs discovery may optionally create a
new entry in its Local Configuration Datastore (LCD)
and continue processing;
or
b) the usmStatsUnknownEngineIDs counter is incremented, and
an error indication (unknownEngineID) together with the
OID and value of the incremented counter is returned to
the calling module.
4) Information about the value of the userName and authEngineID
fields is extracted from the Local Configuration Datastore
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(LCD, usmUserTable). If no information is available for
the user, then the usmStatsUnknownUserNames counter is
incremented and an error indication (unknownSecurityName)
together with the OID and value of the incremented counter
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 the usmStatsUnsupportedLoS counter is incremented and
an error indication (unsupportedLoS) together with the OID
and value of the incremented counter 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 according to
the abstract service interface primitive:
authenticateIncomingMsg(
authKey -- the user's authKey
authParameters -- as received on the wire
wholeMsg) -- as received on the wire
The user's private authKey is the secret key that can
be used by the authentication algorithm.
The authParameters and the wholeMsg are passed as received
on the wire.
Upon completion the authentication module returns the result
according to the abstract service interface primitive:
returnAuthenticatedIncomingMsg(
wholeMsg -- authenticated serialized message
statusInformation) -- success or failure
The wholeMsg is the same as the input given to the
authenticateIncomingMsg service.
The statusInformation indicates if the message was successfully
authenticated by the authentication module or not.
If the authentication module returns failure, then the message
cannot trusted, so the usmStatsWrongDigests counter is
incremented and an error indication (authenticationFailure)
together with the OID and value of the incremented counter is
returned to the calling module.
If the authentication module returns success, then the message
is authentic and can be trusted so processing continues.
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7) If the Level of Security (LoS) indicates an authenticated
message, then the local values of snmpEngineBoots and
snmpEngineTime corresponding to the value of the authEngineID
field are extracted from the Local Configuration Datastore.
a) If the extracted value of authEngineID is the same as
the value of SnmpEngineID of the processing SNMP engine
(meaning this is the authoritative SNMP 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 snmpEngineBoots is 0xffffffff;
- the value of the authEngineBoots field differs from
the local value of snmpEngineBoots; or,
- the value of the authEngineTime field differs from
the local notion of snmpEngineTime by more than
+/- 150 seconds.
If the message is considered to be outside of the Time
Window then the usmStatsNotInTimeWindows counter is
incremented and an error indication (notInTimeWindow)
together with the OID and value of the incremented counter
is returned to the calling module.
b) If the extracted value of authEngineID is not the same as
the value snmpEngineID of the processing SNMP engine
(meaning this is not the authoritative SNMP engine), then:
1) if at least one of the following conditions is true:
- the extracted value of the authEngineBoots field is
greater than the local notion of the value of
snmpEngineBoots; or,
- the extracted value of the authEngineBoots field is
equal to the local notion of the value of
snmpEngineBoots and the extracted value of
authEngineTime field is greater than the value of
latestReceivedEngineTime,
then the LCD entry corresponding to the extracted value
of the authEngineID field is updated, by setting:
- the local notion of the value of snmpEngineBoots
to the value of the authEngineBoots field,
- the local notion of the value of snmpEngineTime
to the value of the authEngineTime field, and
- the latestReceivedEngineTime to the value of the
authEngineTime field.
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2) if any of the following conditions is true, then the
message is considered to be outside of the Time Window:
- the local notion of the value of snmpEngineBoots is
0xffffffff;
- the value of the authEngineBoots field is less than
the local notion of the value of snmpEngineBoots; or,
- the value of the authEngineBoots field is equal to
the local notion of the value of snmpEngineBoots
and the value of the authEngineTime field is more
than 150 seconds less than the local notion of
of the value of snmpEngineTime.
If the message is considered to be outside of the Time
Window then an error indication (notInTimeWindow) is
returned to the calling module;
Note that this means that a too old (possibly replayed)
message has been detected and is deemed unauthentic.
Note that this procedure allows for the value of
authEngineBoots in the message to be greater than the
local notion of the value of snmpEngineBoots to allow
for received messages to be accepted as authentic when
received from an authoritative SNMP engine that has
re-booted since the receiving SNMP engine last
(re-)synchronized.
8) a) If the Level of Security (LoS) indicates that the message
was protected from disclosure, then the OCTET STRING
representing the encryptedPDU is decrypted according to
the user's privacy protocol to obtain an unencrypted
serialized scopedPDU value.
To do so a call is made to the privacy module that
implements the user's privacy protocol according to
the abstract service interface primitive:
decryptData(
decryptKey -- user's privKey
privParameters -- as received on the wire
encryptedData) -- encryptedPDU received on wire
The user's private privKey is the secret key that can
be used by the decryption algorithm. The serialized
encryptedPDU is the data that must be decrypted.
Upon completion the privacy module returns the result
according to the abstract service interface primitive:
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returnDecryptedData(
decryptedData -- serialized decrypted scopedPDU
statusInformation) -- success or failure
The statusInformation indicates if the scopedPDU was
decrypted successfully or not.
If the privacy module returns failure, then the message can
not be processed, so the usmStatsDecryptionErrors counter
is incremented and an error indication (encryptionFailure)
together with the OID and value of the incremented counter
is returned to the calling module.
If the privacy module returns success, then the decrypted
scopedPDU is the message payload to be returned to the
calling module.
Otherwise,
b) The scopedPDU component is assumed to be in plain text
and is the message payload to be returned to the calling
module.
9) The maxSizeResponseScopedPDU is calculated. This is the
maximum size allowed for a scopedPDU for a possible Response
message. Provision is made for a message header that allows
the same Level of Security as the received Request.
10) The securityName for the user is retrieved from the
usmUserTable.
11) The security data is cached as cachedSecurityData, so that a
possible response to this message can and will use the same
authentication and privacy secrets, the same Level of Security
and the same authEngineID. Information to be saved/cached is
as follows:
usmUserName, LoS
usmUserAuthProtocol, usmUserAuthKey
usmUserPrivProtocol, usmUserPrivKey
authEngineID
12) The statusInformation is set to success and a return is made
to the calling module according to this abstract service interface primitive:
returnProcessedMsg(
securityName -- identification of the principal
scopedPDU, -- message (plaintext) payload
maxSizeResponseScopedPDU -- maximum size of the Response PDU
securityStateReference -- reference to security state
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-- information, needed for response
statusInformation -- errorIndication or success
) -- error counter OID/value if error
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4. Discovery
The User-based Security Model requires that a discovery process
obtains sufficient information about other SNMP engines in order
to communicate with them. Discovery requires an non-authoritative
SNMP engine to learn the authoritative SNMP engine's snmpEngineID
value before communication may proceed. This may be accomplished
by generating a Request message with a Level of Security (LoS) of
noAuthNoPriv, a userName of "initial", an authEngineID value of
zero length or all zeroes (binary), and the varBindList left empty.
The response to this message will be a Report message containing
the snmpEngineID of the authoritative SNMP engine as the value of
the authEngineID field within the securityParameters field. It
also contains a Report PDU with the usmStatsUnknownEngineIDs
counter in the varBindList.
If authenticated communication is required, then the discovery
process should also establish time synchronization with the
authoritative SNMP engine. This may be accomplished by sending an
authenticated Request message with the value of authEngineID set
to the newly learned snmpEngineID and with the values of
authEngineBoots and authEngineTime set to zero.
The response to this authenticated message will be a Report message
containing the up to date values of the authoritative SNMP engine's
snmpEngineBoots and snmpEngineTime as the value of the
authEngineBoots and authEngineTime fields respectively. It also
contains the usmStatsNotInTimeWindows counter in the varBindList
of the Report PDU. The time synchronization then happens
automatically as part of the procedures in section 3.2 step 7b.
See also section 2.3.
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5. Definitions
SNMP-USER-BASED-SM-MIB DEFINITIONS ::= BEGIN
IMPORTS
MODULE-IDENTITY, OBJECT-TYPE,
OBJECT-IDENTITY,
snmpModules, Counter32 FROM SNMPv2-SMI
TEXTUAL-CONVENTION, TestAndIncr,
RowStatus, RowPointer,
StorageType, AutonomousType FROM SNMPv2-TC
MODULE-COMPLIANCE, OBJECT-GROUP FROM SNMPv2-CONF
SnmpAdminString, SnmpLoS,
SnmpEngineID, SnmpSecurityModel,
snmpAuthProtocols, snmpPrivProtocols FROM SNMP-FRAMEWORK-MIB;
snmpUsmMIB MODULE-IDENTITY
LAST-UPDATED "9707140000Z" -- 14 July 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
USA
email: mundy@tis.com
phone: +1-301-854-6889
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
SNMP User-based Security Model.
"
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::= { snmpModules 9 } -- to be verified with IANA
-- Administrative assignments ****************************************
usmAdmin OBJECT IDENTIFIER ::= { snmpUsmMIB 1 }
usmMIBObjects OBJECT IDENTIFIER ::= { snmpUsmMIB 2 }
usmMIBConformance OBJECT IDENTIFIER ::= { snmpUsmMIB 3 }
-- Identification of Authentication and Privacy Protocols ************
usmNoAuthProtocol OBJECT-IDENTITY
STATUS current
DESCRIPTION "No Authentication Protocol."
::= { snmpAuthProtocols 1 }
usmMD5AuthProtocol OBJECT-IDENTITY
STATUS current
DESCRIPTION "The Keyed MD5 Digest Authentication Protocol."
REFERENCE "Rivest, R., Message Digest Algorithm MD5, RFC1321."
::= { snmpAuthProtocols 2 }
usmNoPrivProtocol OBJECT-IDENTITY
STATUS current
DESCRIPTION "No Privacy Protocol."
::= { snmpPrivProtocols 1 }
usmDESPrivProtocol OBJECT-IDENTITY
STATUS current
DESCRIPTION "The CBC-DES Symmetric Encryption Protocol."
REFERENCE "- 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).
- Data Encryption Algorithm, American National
Standards Institute. ANSI X3.92-1981,
(December, 1980).
- DES Modes of Operation, National Institute of
Standards and Technology. Federal Information
Processing Standard (FIPS) Publication 81,
(December, 1980).
- Data Encryption Algorithm - Modes of Operation,
American National Standards Institute.
ANSI X3.106-1983, (May 1983).
"
::= { snmpPrivProtocols 2 }
-- Textual Conventions ***********************************************
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-- Editor's note:
-- If in the future the MD5 gets replaced by another Authentication
-- algorithm, then it seems we also need to use that new algorithm to
-- calculate the digest during KeyChange. So this TC has been changed
-- End Editor's note
KeyChange ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION
"Every definition of an object with this syntax must identify
a protocol, P, a secret key, K, and a hash algorithm, H.
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, usmMD5AuthProtocol 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
temporary variable, and the result is input to the
the hash algorithm H 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
hash algorithm H to produce a digest value;
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- 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.
for example, using MD5 as the hash algorithm H:
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
-- Statistics for the User-based Security Model **********************
usmStats OBJECT IDENTIFIER ::= { usmMIBObjects 1 }
usmStatsUnsupportedLoS OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The total number of packets received by the SNMP
engine which were dropped because they requested
a Level of Security (LoS) that was unknown to the
SNMP engine or otherwise unavailable.
"
::= { usmStats 1 }
usmStatsNotInTimeWindows OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The total number of packets received by the SNMP
engine which were dropped because they appeared
outside of the authoritative SNMP engine's window.
"
::= { usmStats 2 }
usmStatsUnknownUserNames OBJECT-TYPE
SYNTAX Counter32
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MAX-ACCESS read-only
STATUS current
DESCRIPTION "The total number of packets received by the SNMP
engine which were dropped because they referenced a
user that was not known to the SNMP engine.
"
::= { usmStats 3 }
usmStatsUnknownEngineIDs OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The total number of packets received by the SNMP
engine which were dropped because they referenced an
snmpEngineID that was not known to the SNMP engine.
"
::= { usmStats 4 }
usmStatsWrongDigests OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The total number of packets received by the SNMP
engine which were dropped because they didn't
contain the expected digest value.
"
::= { usmStats 5 }
usmStatsDecryptionErrors OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The total number of packets received by the SNMP
engine which were dropped because they could not be
decrypted.
"
::= { usmStats 6 }
-- The usmUser Group ************************************************
usmUser OBJECT IDENTIFIER ::= { usmMIBObjects 2 }
usmUserSpinLock OBJECT-TYPE
SYNTAX TestAndIncr
MAX-ACCESS read-write
STATUS current
DESCRIPTION "An advisory lock used to allow several cooperating
Command Generator Applications to coordinate their
use of facilities to alter secrets in the
usmUserTable.
"
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::= { usmUser 1 }
-- The table of valid users for the User-based Security Model ********
usmUserTable OBJECT-TYPE
SYNTAX SEQUENCE OF UsmUserEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION "The table of users configured in the SNMP engine's
Local Configuration Datastore (LCD)."
::= { usmUser 2 }
usmUserEntry OBJECT-TYPE
SYNTAX UsmUserEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION "A user configured in the SNMP engine's Local
Configuration Datastore (LCD) for the User-based
Security Model.
"
INDEX { usmUserEngineID,
usmUserName
}
::= { usmUserTable 1 }
UsmUserEntry ::= SEQUENCE
{
usmUserEngineID SnmpEngineID,
usmUserName SnmpAdminString,
usmUserSecurityName SnmpAdminString,
usmUserCloneFrom RowPointer,
usmUserAuthProtocol AutonomousType,
usmUserAuthKeyChange KeyChange,
usmUserOwnAuthKeyChange KeyChange,
usmUserPrivProtocol AutonomousType,
usmUserPrivKeyChange KeyChange,
usmUserOwnPrivKeyChange KeyChange,
usmUserPublic OCTET STRING,
usmUserStorageType StorageType,
usmUserStatus RowStatus
}
usmUserEngineID 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.
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The value can also take the value of the snmpEngineID
of a remote SNMP engine with which this user can
communicate.
"
::= { usmUserEntry 1 }
usmUserName OBJECT-TYPE
SYNTAX SnmpAdminString (SIZE(1..16))
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION "A human readable string representing the name of
the user.
This is the (User-based Security) Model dependent
security ID.
"
::= { usmUserEntry 2 }
usmUserSecurityName OBJECT-TYPE
SYNTAX SnmpAdminString
MAX-ACCESS read-only
STATUS current
DESCRIPTION "A human readable string representing the user in
Security Model independent format.
The default transformation of the User-based Security
Model dependent security ID to the securityName and
vice versa is the identity function so that the
securityName is the same as the userName.
"
::= { usmUserEntry 3 }
usmUserCloneFrom OBJECT-TYPE
SYNTAX RowPointer
MAX-ACCESS read-create
STATUS current
DESCRIPTION "A pointer to another conceptual row in this
usmUserTable. 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 privacy and
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 receiver.
The cloning process fails with an 'inconsistentName'
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error if the conceptual row representing the
clone-from user is not in an active state when the
cloning process is invoked.
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 ZeroDotZero OID
is returned.
"
::= { usmUserEntry 4 }
usmUserAuthProtocol OBJECT-TYPE
SYNTAX AutonomousType
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
usmUserEngineID, 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 { usmMD5AuthProtocol }
::= { usmUserEntry 5 }
usmUserAuthKeyChange 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
usmUserEngineID, to be modified via a one-way
function.
The associated protocol is the usmUserAuthProtocol.
The associated secret key is the user's secret
authentication key (authKey). The associated hash
algorithm is the algorithm used by the user's
usmUserAuthProtocol.
When creating a new user, it is an 'inconsistentName'
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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 usmUserCloneFrom.
"
DEFVAL { ''H } -- the empty string
::= { usmUserEntry 6 }
usmUserOwnAuthKeyChange OBJECT-TYPE
SYNTAX KeyChange -- typically (SIZE (0..32))
MAX-ACCESS read-create
STATUS current
DESCRIPTION "Behaves exactly as usmUserAuthKeyChange, with one
notable difference: in order for the Set operation
to succeed, the usmUserName of the operation
requester must match the usmUserName that
indexes the row which is targeted by this
operation.
The idea here is that access to this column can be
public, since it will only allow a user to change
his own secret authentication key (authKey).
"
DEFVAL { ''H } -- the empty string
::= { usmUserEntry 7 }
usmUserPrivProtocol OBJECT-TYPE
SYNTAX AutonomousType
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
usmUserEngineID, 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 { usmNoPrivProtocol }
::= { usmUserEntry 8 }
usmUserPrivKeyChange OBJECT-TYPE
SYNTAX KeyChange -- typically (SIZE (0..32))
MAX-ACCESS read-create
STATUS current
DESCRIPTION "An object, which when modified, causes the secret
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encryption key used for messages sent on behalf
of this user to/from the SNMP engine identified by
usmUserEngineID, to be modified via a one-way
function.
The associated protocol is the usmUserPrivProtocol.
The associated secret key is the user's secret
privacy key (privKey). The associated hash
algorithm is the algorithm used by the user's
usmUserAuthProtocol.
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 usmUserCloneFrom.
"
DEFVAL { ''H } -- the empty string
::= { usmUserEntry 9 }
usmUserOwnPrivKeyChange OBJECT-TYPE
SYNTAX KeyChange -- typically (SIZE (0..32))
MAX-ACCESS read-create
STATUS current
DESCRIPTION "Behaves exactly as usmUserPrivKeyChange, with one
notable difference: in order for the Set operation
to succeed, the usmUserName of the operation
requester must match the usmUserName that indexes
the row which is targeted by this operation.
The idea here is that access to this column can be
public, since it will only allow a user to change
his own secret privacy key (privKey).
"
DEFVAL { ''H } -- the empty string
::= { usmUserEntry 10 }
usmUserPublic OBJECT-TYPE
SYNTAX OCTET STRING (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
authentication and/or privacy key, and later read to
determine whether the change of the secret was
effected.
"
DEFVAL { ''H } -- the empty string
::= { usmUserEntry 11 }
usmUserStorageType OBJECT-TYPE
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SYNTAX StorageType
MAX-ACCESS read-create
STATUS current
DESCRIPTION "The storage type for this conceptual row.
Conceptual rows having the value 'permanent'
must allow write-access at a minimum to:
- usmUserAuthKeyChange, usmUserOwnAuthKeyChange
and usmUserPublic for a user who employs
authentication, and
- usmUserPrivKeyChange, usmUserOwnPrivKeyChange
and usmUserPublic for a user who employs
privacy.
Note that any user who employs authentication or
privacy must allow its secret(s) to be updated and
thus cannot be 'readOnly'.
"
DEFVAL { nonVolatile }
::= { usmUserEntry 12 }
usmUserStatus 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 usmUserStatus column
is 'notReady'.
In particular, a newly created row cannot be made
active until the corresponding usmUserCloneFrom,
usmUserAuthKeyChange, usmUserOwnAuthKeyChange,
usmUserPrivKeyChange and usmUserOwnPrivKeyChange
have all been set.
The value of this object has no effect on whether
other objects in this conceptual row can be modified.
"
::= { usmUserEntry 13 }
-- Conformance Information *******************************************
usmMIBCompliances OBJECT IDENTIFIER ::= { usmMIBConformance 1 }
usmMIBGroups OBJECT IDENTIFIER ::= { usmMIBConformance 2 }
-- Compliance statements
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usmMIBCompliance MODULE-COMPLIANCE
STATUS current
DESCRIPTION "The compliance statement for SNMP engines which
implement the SNMP-USER-BASED-SM-MIB.
"
MODULE -- this module
MANDATORY-GROUPS { usmMIBBasicGroup }
OBJECT usmUserAuthProtocol
MIN-ACCESS read-only
DESCRIPTION "Write access is not required."
OBJECT usmUserPrivProtocol
MIN-ACCESS read-only
DESCRIPTION "Write access is not required."
::= { usmMIBCompliances 1 }
-- Units of compliance
usmMIBBasicGroup OBJECT-GROUP
OBJECTS {
usmStatsUnsupportedLoS,
usmStatsNotInTimeWindows,
usmStatsUnknownUserNames,
usmStatsUnknownEngineIDs,
usmStatsWrongDigests,
usmStatsDecryptionErrors,
usmUserSpinLock,
usmUserSecurityName,
usmUserCloneFrom,
usmUserAuthProtocol,
usmUserAuthKeyChange,
usmUserOwnAuthKeyChange,
usmUserPrivProtocol,
usmUserPrivKeyChange,
usmUserOwnPrivKeyChange,
usmUserPublic,
usmUserStorageType,
usmUserStatus
}
STATUS current
DESCRIPTION "A collection of objects providing for configuration
of an SNMP engine which implements the SNMP
User-based Security Model.
"
::= { usmMIBGroups 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.
This protocol is identified by usmMD5AuthProtocol.
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 SNMP 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
SNMP 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 SNMP
engines authorized to originate messages on behalf of the
appropriate user.
- In order to not expose the shared secrets (keys) at all SNMP
engines in case one of the SNMP engines is compromised, such
secrets (keys) are localized for each authoritative SNMP
engine, see [Localized-Key].
6.1.1. Digest Authentication Protocol
The Digest Authentication Protocol defined in this memo provides
for:
- verification of 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 the message. The digest is
computed by the originator of the message, transmitted with the
message, and verified by the recipient of the message.
- verification of the user on whose behalf the message was
generated.
A secret value known only to SNMP 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
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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
SNMP message 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. Users
Authentication using this Digest Authentication protocol makes use
of a defined set of userNames. For any user on whose behalf a
message must be authenticated at a particular SNMP engine, that
SNMP engine must have knowledge of that user. An SNMP engine that
wishes to communicate with another SNMP 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. authEngineID
The authEngineID value contained in an authenticated message
specifies the authoritative SNMP engine for that particular
message (see the definition of SnmpEngineID in the SNMP
Architecture document [SNMP-ARCH]).
The user's (private) authentication key is normally different at
each authoritative SNMP engine and so the snmpEngineID is used
to select the proper key for the authentication process.
6.2.3. SNMP 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
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are intact and have not been tampered with.
6.2.4. Services provided by the MD5 Authentication Module
This section describes the inputs and outputs that the MD5
Authentication module expects and produces when the User-based
Security module calls the MD5 Authentication module for services.
6.2.4.1. Services for Generating an Outgoing SNMP 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.
Upon completion the authentication module returns statusInformation
and, if the message digest was correctly calculated, the wholeMsg
with the digest inserted at the proper place.
The abstract service interfaces are:
authenticateOutgoingMsg(
authKey -- secret key for authentication
wholeMsg -- complete message
)
returnAuthenticatedOutgoingMsg(
wholeMsg -- complete authenticated message
statusInformation -- success or errorIndication
)
Where:
authKey
The secret key to be used by the authentication algorithm.
wholeMsg
The message to be authenticated on input or the authenticated
message (including inserted digest) on output.
statusInformation
An indication of whether the authentication process was
successful. If not it is an indication of the problem.
Note, that authParameters field is filled by the authentication
module and this field should be already present in the wholeMsg
before the Message Authentication Code (MAC) is generated.
6.2.4.2. Services for Processing an Incoming SNMP 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.
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Upon completion the authentication module returns statusInformation
and, if the message digest was correctly calculated, the wholeMsg
as it was processed.
The abstract service interfaces are:
authenticateIncomingMsg(
authKey -- secret key for authentication
authParameters -- filled in by service provider
wholeMsg -- as received on the wire
)
returnAuthenticatedIncomingMsg(
wholeMsg -- complete authenticated message
statusInformation -- success or errorIndication
)
Where:
authKey
The secret key to be used by the authentication algorithm.
authParameters
The authParameters from the incoming message.
wholeMsg
The message to be authenticated on input and the authenticated
message on output.
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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 SNMP engine
whenever it must authenticate an outgoing message using the
usmMD5AuthProtocol.
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 statusInformation
indicating success.
6.3.2. Processing an Incoming Message
This section describes the procedure followed by an SNMP engine
whenever it must authenticate an incoming message using the
usmMD5AuthProtocol.
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
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 (authenticationFailure) is returned to the calling
module.
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7) The wholeMsg (excluding the appended secret key) and
statusInformation indicating 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.
This protocol is identified by usmDESPrivProtocol.
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 in combination with a timeliness value is used
to create the en/decryption key and the initialization vector.
The secret value is shared by all SNMP engines authorized to
originate messages on behalf of the appropriate user.
- In order to not expose the shared secrets (keys) at all SNMP
engines in case one of the SNMP engines is compromised, such
secrets (keys) are localized for each authoritative SNMP 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
SNMP message is encrypted and included as part of the message
sent to the recipient.
Two organizations have published specifications defining the DES:
the National Institute of Standards and Technology (NIST)
[DES-NIST] and the American National Standards Institute
[DES-ANSI]. There is a companion Modes of Operation specification
for each definition ([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].
- There is a specification of a validation test suite for the DES
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[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 the IV for two different packets encrypted
by the same key, are not the same (i.e. the 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 SNMP 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 an arbitrary
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 encoded as an OCTET STRING. The "salt" integer is then
modified. We recommend that it be incremented by one and wrap
when it reaches the maximum value.
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.
The "salt" must be placed in the privParameters field to enable the
receiving entity to compute the correct IV and to decrypt the
message.
7.1.1.2. Data Encryption.
The data to be encrypted is treated as sequence of octets. Its
length should be an integral multiple of 8 - and if t is not, the
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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 STRING to be decrypted is not an
integral multiple of 8 octets, the decryption process 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. Users
Data en/decryption using this Symmetric Encryption Protocol makes
use of a defined set of userNames. For any user on whose behalf
a message must be en/decrypted at a particular SNMP engine, that
SNMP engine must have knowledge of that user. An SNMP engine that
wishes to communicate with another SNMP engine must also have
knowledge of a user known to that SNMP 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.
<privKey>
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A user's secret key to be used as input for the DES key and IV.
7.2.2. authEngineID
The authEngineID value contained in an authenticated message
specifies the authoritative SNMP engine for that particular
message (see the definition of SnmpEngineID in the SNMP
Architecture document [SNMP-ARCH]).
The user's (private) privacy key is normally different at
each authoritative SNMP engine and so the snmpEngineID is used
to select the proper key for the en/decryption process.
7.2.3. SNMP 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. Services provided by 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. Services for Encrypting Outgoing Data
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.
Upon completion the privacy module returns statusInformation
and, if the encryption process was successful, the encryptedPDU
and the privParameters encoded as an OCTET STRING.
The abstract service interface primitives are:
encryptData(
encryptKey -- secret key for encryption
dataToEncrypt -- data to encrypt (scopedPDU)
)
returnEncryptedData(
encryptedData -- encrypted data (encryptedPDU)
privParameters -- filled in by service provider
statusInformation -- success or errorIndication
)
Where:
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encryptKey
The secret key to be used by the encryption algorithm.
dataToEncrypt
The data that must be encrypted.
encryptedData
The encrypted data upon successful completion.
privParameters
The privParameters encoded as an OCTET STRING.
statusInformation
An indication of the success or failure of the encryption
process. In case of failure, it is an indication of the
error.
7.2.4.2. Services for Decrypting Incoming Data
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.
Upon completion the privacy module returns statusInformation
and, if the decryption process was successful, the scopedPDU
in plain text.
The abstract service interface primitives are:
decryptData(
decryptKey -- secret key for decryption
privParameters -- as received on the wire
encryptedData -- encrypted data (encryptedPDU)
returnDecryptedData(
decryptedData -- decrypted data (scopedPDU)
statusInformation -- success or errorIndication
)
Where:
decryptKey
The secret key to be used by the decryption algorithm.
privParameters
The "salt" to be used to calculate the IV.
encryptedData
The data to be decrypted.
decryptedData
The decrypted data.
statusInformation
An indication whether the data was successfully decrypted
and if not an indication of the error.
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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 SNMP engine
whenever it must encrypt part of an outgoing message using the
usmDESPrivProtocol.
1) The secret (localized) cryptKey is used to construct the DES
encryption key, the "salt" and the DES pre-IV (as described
in section 7.1.1.1).
2) The privParameters field is set to the serialization according
to the rules in [RFC1906] of an OCTET STRING representing the
the "salt" string.
2) The scopedPDU is encrypted (as described in section 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 statusInformation
indicating success is returned to the calling module.
7.3.2. Processing an Incoming Message
This section describes the procedure followed by an SNMP engine
whenever it must decrypt part of an incoming message using the
usmDESPrivProtocol.
1) If the privParameters field is not an 8-byte OCTET STRING,
then an error indication (decryptionError) 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 section 7.1.1.1).
4) The encryptedPDU is then decrypted (as described in
section 7.1.1.3).
5) If the encryptedPDU cannot be decrypted, then an error
indication (decryptionError) is returned to the calling module.
6) The decrypted scopedPDU and statusInformation indicating
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.
- An SNMP engine must discard SNMP Response messages for which
the msgID component does not correspond to any currently
outstanding Request message.
An SNMP Command Generator Application must discard any Response
PDU for which the request-id component or the represented
management information does not correspond to any currently
outstanding Request PDU.
Although it would be typical for an SNMP engine and an SNMP
Command Generator Application 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).
- An SNMP engine must generate unpredictable msgIDs and an SNMP
Command Generator or Notification Originator Application must
generate unpredictable request-ids in authenticated messages in
order to protect against the possibility of message duplication
(malicious or otherwise).
For example, starting operations with a msgID and/or request-id
value of zero is not a good idea. Initializing them with an
unpredictable number (so they do not start out the same after
each reboot) and then incrementing by one would be acceptable.
- An SNMP engine 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 authoritative
SNMP engine, a Command Generator Application should delay sending
successive messages to that managed SNMP engine until a positive
acknowledgement is received for the previous message or until
the previous message expires.
No message ordering is imposed by the SNMP. Messages may be
received in any order relative to their time of generation and
each will be processed in the ordered received. Note that when
an authenticated message is sent to a managed SNMP engine, 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, an SNMP engine and SNMP Command Generator
Applications 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
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specifically defined for use with SNMP Set operations in order
to provide a mechanism to ensure that the processing of SNMP
messages occurs in a specific order.
- The frequency with which the secrets of a User-based Security
Model user should be changed is indirectly related to the
frequency of their use.
Protecting the secrets from disclosure is critical to the overall
security of the protocols. Frequent use of a secret provides a
continued source of data that may be useful to a cryptanalyst in
exploiting known or perceived weaknesses in an algorithm.
Frequent changes to the secret avoid this vulnerability.
Changing a secret after each use is 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 used as the
communication paths, more caution is prudent.
10.2 Defining Users
The mechanisms defined in this document employ the notion of users
on whose behalf messages are sent. 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 an SNMP
Application or a set of SNMP 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)
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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.
Since it is infeasible for human users to maintain different
passwords for every SNMP engine, but security requirements
strongly discourage having the same key for more than one SNMP
engine, the User-based Security Model employs a compromise
proposed in [Localized-key]. It derives the user keys for the
SNMP 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 SNMP engine from any combination of
user's keys on SNMP 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 SNMP implementation" based on the
User-based Security Model, an SNMP implementation MUST:
- implement one or more Authentication Protocol(s). The MD5
Authentication Protocol defined in this memo is one such
protocol.
- to the maximum extent possible, prohibit access to the
secret(s) of each user about which it maintains information
in a Local Configuration Datastore (LCD) under all
circumstances except as required to generate and/or
validate SNMP messages with respect to that user.
- implement the SNMP-USER-BASE-SM-MIB.
In addition, an authoritative SNMP engine SHOULD [RFC2119] provide
initial configuration in accordance with Appendix A.1.
Implementation of a Privacy Protocol (the DES 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.
[RFC2119] Network Working Group, Bradner, S., "Key words for use in
RFCs to Indicate Requirement Levels", RFC 2119, March 1997.
[SNMP-ARCH] The SNMPv3 Working Group, Harrington, D., Wijnen, B.,
"An Architecture for describing Internet Management Frameworks",
draft-ietf-snmpv3-next-gen-arch-03.txt, July 1997.
[SNMP-v3MP] The SNMPv3 Working Group, Wijnen, B., Harrington, D.,
Case, J., "Message Processing Model for version 3 of the Simple
Network Management Protocol (SNMPv3)",
draft-ietf-snmpv3-mpc-03.txt, July 1997.
[SNMP-ACM] The SNMPv3 Working Group, Wijnen, B., Harrington, D.,
"View-based Access Control Model for the Simple Network
Management Protocol (SNMP)",
draft-ietf-snmpv3-acm-01.txt, July 1997.
[Localized-Key] U. Blumenthal, N. C. Hien, B. Wijnen
"Key Derivation for Network Management Applications"
IEEE Network Magazine, April/May issue, 1997.
[KEYED-MD5] Krawczyk, H.,
"Keyed-MD5 for Message Authentication",
Work in Progress, IBM, June 1995.
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[MD5] Rivest, R., "Message Digest Algorithm MD5",
RFC 1321, April 1992.
[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. SNMP engine Installation Parameters
During installation, an authoritative SNMP engine SHOULD (in the
meaning as defined in [RFC2119]) be configured with several initial
parameters. These include:
(1) A security posture
The choice of security posture determines if initial configuration
is implemented and if so how. One of three possible choices
is selected:
minimum-secure,
semi-secure,
very-secure (i.e. no-initial-configuration)
In the case of a very-secure posture, there is no initial
configuration, and so the following steps are irrelevant.
(2) 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).
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With these configured parameters, the SNMP engine instantiates
the following usmUserEntry in the usmUserTable:
no privacy support privacy support
------------------ ---------------
usmUserEngineID localEngineID localEngineID
usmUserName "initial" "initial"
usmUserSecurityName "initial" "initial"
usmUserCloneFrom ZeroDotZero ZeroDotZero
usmUserAuthProtocol usmMD5AuthProtocol usmMD5AuthProtocol
usmUserAuthKeyChange "" ""
usmUserOwnAuthKeyChange "" ""
usmUserPrivProtocol none usmDESPrivProtocol
usmUserPrivKeyChange "" ""
usmUserOwnPrivKeyChange "" ""
usmUserPublic "" ""
usmUserStorageType anyValidStorageType anyValidStorageType
usmUserStatus active active
A.2. Password to Key Algorithm
The following code fragment (section A.2.1) 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].
An example of the results of a correct implementation is provided
(section A.2.2) whihc an implementer can use to check if his
implementation produces the same result.
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A.2.1. Password to Key Sample Code
void password_to_key(
u_char *password, /* IN */
u_int passwordlen, /* IN */
u_char *engineID, /* IN - pointer to snmpEngineID */
u_int engineLength /* IN - length of snmpEngineID */
u_char *key) /* OUT - pointer to caller 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];
}
MD5Update (&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 */
/* May want to ensure that engineLength <= 32, */
/* otherwise need to use a buffer larger than 64 */
/*****************************************************/
memcpy(password_buf, key, 16);
memcpy(password_buf+16, engineID, engineLength);
memcpy(password_buf+engineLength, key, 16);
MD5Init(&MD);
MD5Update(&MD, password_buf, 32+engineLength);
MD5Final(key, &MD);
return;
}
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A.3. Password to Key Sample Results
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 SNMP engine's
snmpEngineID 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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A.4. Sample encoding of securityParameters
The securityParameters in an SNMP message are represented as
an OCTET STRING. This OCTET STRING should be considered opaque
outside a specific Security Model.
The User-based Security Model defines the contents of the OCTET
STRING as a SEQUENCE (see section 2.4).
Given these two properties, the following is an example of the
securityParameters for the User-based Security Model, encoded
as an OCTET STRING:
04 <length>
30 <length>
04 <length> <authEngineID>
02 <length> <authEngineBoots>
02 <length> <authEngineTime>
04 <length> <userName>
04 10 <MD5-digest>
04 08 <salt>
Here is the example once more. but now with real values (except
for the digest in authParameters and the salt in privParameters,
which depend on variable data that we have not defined here):
Hex Data Description
-------------------------- --------------------------------
04 39 OCTET STRING, length 57
30 37 SEQUENCE, length 55
04 0c 00000002 00000000 authEngineID: IBM, IP, 9.132.3.1
09840301
02 01 01 authEngineBoots: 1
02 02 0101 authEngineTime: 257
04 04 62657274 userName: bert
04 10 01234567 89abcdef authParameters: sample value
fedcba98 76543210
04 08 01234567 89abcdef privParameters: sample value
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Table of Contents
0. Issues and Change Log 2
0.1. Current Open Issues 2
0.2. Change Log 2
1. Introduction 5
1.1. Threats 5
1.2. Goals and Constraints 6
1.3. Security Services 7
1.4. Module Organization 8
1.4.1. Timeliness Module 8
1.4.2. Authentication Protocol 9
1.4.3. Privacy Protocol 9
1.5. Protection against Message Replay, Delay and Redirection 9
1.5.1. Authoritative SNMP engine 9
1.5.2. Mechanisms 9
2. Elements of the Model 12
2.1. User-based Security Model Users 12
2.2. Replay Protection 13
2.2.1. authEngineID 13
2.2.2. authEngineBoots and authEngineTime 13
2.2.3. Time Window 14
2.3. Time Synchronization 14
2.4. SNMP Messages Using this Security Model 16
2.5. Services provided by the User-based Security Model 16
2.5.1. Services for Generating an Outgoing SNMP Message 17
2.5.2. Services for Processing an Incoming SNMP Message 19
3. Elements of Procedure 22
3.1. Generating an Outgoing SNMP Message 22
3.2. Processing an Incoming SNMP Message 25
4. Discovery 31
5. Definitions 32
6. MD5 Authentication Protocol 44
6.1. Mechanisms 44
6.1.1. Digest Authentication Protocol 44
6.2. Elements of the Digest Authentication Protocol 45
6.2.1. Users 45
6.2.2. authEngineID 45
6.2.3. SNMP Messages Using this Authentication Protocol 45
6.2.4. Services provided by the MD5 Authentication Module 46
6.2.4.1. Services for Generating an Outgoing SNMP Message 46
6.2.4.2. Services for Processing an Incoming SNMP Message 46
6.3. Elements of Procedure 48
6.3.1. Processing an Outgoing Message 48
6.3.2. Processing an Incoming Message 48
7. DES Privacy Protocol 50
7.1. Mechanisms 50
7.1.1. Symmetric Encryption Protocol 50
7.1.1.1. DES key and Initialization Vector. 51
7.1.1.2. Data Encryption. 51
7.1.1.3. Data Decryption 52
7.2. Elements of the DES Privacy Protocol 52
7.2.1. Users 52
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7.2.2. authEngineID 53
7.2.3. SNMP Messages Using this Privacy Protocol 53
7.2.4. Services provided by the DES Privacy Module 53
7.2.4.1. Services for Encrypting Outgoing Data 53
7.2.4.2. Services for Decrypting Incoming Data 54
7.3. Elements of Procedure. 55
7.3.1. Processing an Outgoing Message 55
7.3.2. Processing an Incoming Message 55
8. Editor's Addresses 56
9. Acknowledgements 56
A.1. SNMP engine Installation Parameters 62
A.2. Password to Key Algorithm 63
A.2.1. Password to Key Sample Code 64
A.3. Password to Key Sample Results 65
A.4. Sample encoding of securityParameters 66
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