RFC 3414
User-based Security Model (USM) for version 3 of the Simple Network Management Protocol (SNMPv3)
Pages: 88
Internet Standard: 62
→ Errata
STD 62 is also: 3411 3412 3413 3415 3416 3417 3418
Obsoletes: 2574
Updated by: 5590
Internet Standard: 62
→ Errata
STD 62 is also: 3411 3412 3413 3415 3416 3417 3418
Obsoletes: 2574
Updated by: 5590
Part 2 of 4 – Pages 12 to 32
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. There is a one-to-one relationship between userName and securityName.
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. Two such protocols are defined in this memo:
- the HMAC-MD5-96 authentication protocol.
- the HMAC-SHA-96 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. The length requirements of the authKey
are defined by the authProtocol in use.
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 CBC-DES Symmetric 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. Note
that a user's privacy key will normally be different at different
authoritative SNMP engines. The privKey is not accessible via
SNMP. The length requirements of the privKey are defined by the
privProtocol in use.
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. msgAuthoritativeEngineID
The msgAuthoritativeEngineID 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 [RFC3411]).
2.2.2. msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime
The msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime 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 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 2147483647. Whenever the local value of snmpEngineBoots has the value 2147483647 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 2147483647, 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. Note that even if an SNMP engine re-boots once a second that it would still take approximately 68 years before the max value of 2147483647 would be reached.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 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 (snmpEngineBoots, snmpEngineTime and latestReceivedEngineTime) 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.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 [RFC3412]). The field msgSecurityParameters in SNMPv3 messages has a data type of OCTET STRING. Its value is the BER serialization of the following ASN.1 sequence: USMSecurityParametersSyntax DEFINITIONS IMPLICIT TAGS ::= BEGIN UsmSecurityParameters ::= SEQUENCE { -- global User-based security parameters msgAuthoritativeEngineID OCTET STRING, msgAuthoritativeEngineBoots INTEGER (0..2147483647), msgAuthoritativeEngineTime INTEGER (0..2147483647), msgUserName OCTET STRING (SIZE(0..32)), -- authentication protocol specific parameters msgAuthenticationParameters OCTET STRING, -- privacy protocol specific parameters msgPrivacyParameters OCTET STRING
}
END
The fields of this sequence are:
- The msgAuthoritativeEngineID specifies the snmpEngineID of the
authoritative SNMP engine involved in the exchange of the message.
- The msgAuthoritativeEngineBoots specifies the snmpEngineBoots value
at the authoritative SNMP engine involved in the exchange of the
message.
- The msgAuthoritativeEngineTime specifies the snmpEngineTime value
at the authoritative SNMP engine involved in the exchange of the
message.
- The msgUserName specifies the user (principal) on whose behalf the
message is being exchanged. Note that a zero-length userName will
not match any user, but it can be used for snmpEngineID discovery.
- The msgAuthenticationParameters are defined by the authentication
protocol in use for the message, as defined by the
usmUserAuthProtocol column in the user's entry in the usmUserTable.
- The msgPrivacyParameters are defined by the privacy protocol in use
for the message, as defined by the usmUserPrivProtocol column in
the user's entry in the usmUserTable).
See appendix A.4 for an example of the BER encoding of field
msgSecurityParameters.
2.5. Services provided by the User-based Security Model
This section describes the services provided by the User-based
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 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. The abstract service
primitive is:
statusInformation = -- success or errorIndication
generateRequestMsg(
IN messageProcessingModel -- typically, SNMP version
IN globalData -- message header, admin data
IN maxMessageSize -- of the sending SNMP entity
IN securityModel -- for the outgoing message
IN securityEngineID -- authoritative SNMP entity
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security requested
IN scopedPDU -- message (plaintext) payload
OUT securityParameters -- filled in by Security Module
OUT wholeMsg -- complete generated message
OUT wholeMsgLength -- length of generated message
)
2) A service to generate a Response message. The abstract service
primitive is:
statusInformation = -- success or errorIndication
generateResponseMsg(
IN messageProcessingModel -- typically, SNMP version
IN globalData -- message header, admin data
IN maxMessageSize -- of the sending SNMP entity
IN securityModel -- for the outgoing message
IN securityEngineID -- authoritative SNMP entity
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security requested
IN scopedPDU -- message (plaintext) payload
IN securityStateReference -- reference to security state
-- information from original
-- request
OUT securityParameters -- filled in by Security Module
OUT wholeMsg -- complete generated message
OUT wholeMsgLength -- length of generated message
)
The abstract data elements passed as parameters in the abstract
service primitives are as follows:
statusInformation
An indication of whether the encoding and securing of the message
was successful. If not it is an indication of the problem.
messageProcessingModel
The SNMP version number for the message to be generated. This
data is not used by the User-based Security module.
globalData
The message header (i.e., its administrative information). This
data is not used by the User-based Security module.
maxMessageSize
The maximum message size as included in the message. This data is
not used by the User-based Security module.
securityParameters
These are the security parameters. They will be filled in by the
User-based Security module.
securityModel
The securityModel in use. Should be User-based Security Model.
This data is not used by the User-based Security module.
securityName
Together with the snmpEngineID it identifies a row in the
usmUserTablethat is to be used for securing the message. The
securityName has a format that is independent of the Security
Model. In case of a response this parameter is ignored and the
value from the cache is used.
securityLevel
The Level of Security 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.
securityEngineID
The snmpEngineID of the authoritative SNMP engine to which a
dateRequest message is to be sent. In case of a response it is
implied to be the processing SNMP engine's snmpEngineID and so if
it is specified, then it is ignored.
scopedPDU
The message payload. The data is opaque as far as the User-based
Security Model is concerned.
securityStateReference
A handle/reference to cachedSecurityData 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).
Upon completion of the process, the User-based Security module
returns statusInformation. If the process was successful, the
completed message with privacy and authentication applied if such was
requested by the specified securityLevel is returned. If the process
was not successful, then an errorIndication is returned.
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. The abstract
service primitive is:
statusInformation = -- errorIndication or success
-- error counter OID/value if error
processIncomingMsg(
IN messageProcessingModel -- typically, SNMP version
IN maxMessageSize -- of the sending SNMP entity
IN securityParameters -- for the received message
IN securityModel -- for the received message
IN securityLevel -- Level of Security
IN wholeMsg -- as received on the wire
IN wholeMsgLength -- length as received on the wire
OUT securityEngineID -- authoritative SNMP entity
OUT securityName -- identification of the principal
OUT scopedPDU, -- message (plaintext) payload
OUT maxSizeResponseScopedPDU -- maximum size of the Response PDU
OUT securityStateReference -- reference to security state
) -- information, needed for response
The abstract data elements passed as parameters in the abstract
service primitives are as follows:
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.
messageProcessingModel
The SNMP version number as received in the message. This data is
not used by the User-based Security module.
maxMessageSize
The maximum message size as included in the message. The User-bas
User-based Security module uses this value to calculate the
maxSizeResponseScopedPDU.
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.
securityLevel
The Level of Security 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).
securityEngineID
The snmpEngineID that was extracted from the field
msgAuthoritativeEngineID and that was used to lookup the secrets
in the usmUserTable.
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-based Security module calculates this
size based on the msgMaxSize (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 cachedSecurityData to be used when securing
an outgoing Response message. When the Message Processing
Subsystem calls the User-based Security module to generate a
response to this incoming message it must pass this
handle/reference.
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. If
the process was not successful, then an errorIndication, possibly
with a OID and value pair of an error counter that was incremented.
2.6. Key Localization Algorithm.
A localized key is a secret key shared between a user U and one
authoritative SNMP engine E. Even though a user may have only one
password and therefore one key for the whole network, the actual
secrets shared between the user and each authoritative SNMP engine
will be different. This is achieved by key localization [Localized-
key].
First, if a user uses a password, then the user's password is
converted into a key Ku using one of the two algorithms described in
Appendices A.2.1 and A.2.2.
To convert key Ku into a localized key Kul of user U at the
authoritative SNMP engine E, one appends the snmpEngineID of the
authoritative SNMP engine to the key Ku and then appends the key Ku
to the result, thus enveloping the snmpEngineID within the two copies
of user's key Ku. Then one runs a secure hash function (which one
depends on the authentication protocol defined for this user U at
authoritative SNMP engine E; this document defines two authentication
protocols with their associated algorithms based on MD5 and SHA).
The output of the hash-function is the localized key Kul for user U
at the authoritative SNMP engine E.
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 securityLevel.
1) a) If any securityStateReference is passed (Response or Report
message), then information concerning the user is extracted
from the cachedSecurityData. The cachedSecurityData can now be
discarded. The securityEngineID is set to the local
snmpEngineID. The securityLevel is set to the value specified
by the calling module.
Otherwise,
b) based on the securityName, information concerning the user at
the destination snmpEngineID, specified by the
securityEngineID, 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 securityLevel 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 (unsupportedSecurityLevel) is returned
to the calling module.
3) If the securityLevel 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
(unsupportedSecurityLevel) is returned to the calling module.
4) a) If the securityLevel 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 primitive:
statusInformation = -- success or failure
encryptData(
IN encryptKey -- user's localized privKey
IN dataToEncrypt -- serialized scopedPDU
OUT encryptedData -- serialized encryptedPDU
OUT privParameters -- serialized privacy parameters
)
statusInformation
indicates if the encryption process was successful or not.
encryptKey
the user's localized private privKey is the secret key that
can be used by the encryption algorithm.
dataToEncrypt
the serialized scopedPDU is the data to be encrypted.
encryptedData
the encryptedPDU represents the encrypted scopedPDU, encoded
as an OCTET STRING.
privParameters
the privacy parameters, encoded as an OCTET STRING.
If the privacy module returns failure, then the message cannot
be sent and an error indication (encryptionError) is returned
to the calling module.
If the privacy module returns success, then the returned
privParameters are put into the msgPrivacyParameters field of
the securityParameters and the encryptedPDU serves as the
payload of the message being prepared.
Otherwise,
b) If the securityLevel specifies that the message is not to be be
protected from disclosure, then a zero-length OCTET STRING is
encoded into the msgPrivacyParameters field of the
securityParameters and the plaintext scopedPDU serves as the
payload of the message being prepared.
5) The securityEngineID is encoded as an OCTET STRING into the
msgAuthoritativeEngineID field of the securityParameters. Note
that an empty (zero length) securityEngineID is OK for a Request
message, because that will cause the remote (authoritative) SNMP
engine to return a Report PDU with the proper securityEngineID
included in the msgAuthoritativeEngineID in the securityParameters
of that returned Report PDU.
6) a) If the securityLevel specifies that the message is to be
authenticated, then the current values of snmpEngineBoots and
snmpEngineTime corresponding to the securityEngineID from the
LCD are used.
Otherwise,
b) If this is a Response or Report message, then the current value
of snmpEngineBoots and snmpEngineTime corresponding to the
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. This zero value gets
used if snmpEngineID is empty.
The values are encoded as INTEGER respectively into the
msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime
fields of the securityParameters.
7) The userName is encoded as an OCTET STRING into the msgUserName
field of the securityParameters.
8) a) If the securityLevel 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 primitive:
statusInformation =
authenticateOutgoingMsg(
IN authKey -- the user's localized authKey
IN wholeMsg -- unauthenticated message
OUT authenticatedWholeMsg -- authenticated complete message
)
statusInformation
indicates if authentication was successful or not.
authKey
the user's localized private authKey is the secret key that
can be used by the authentication algorithm.
wholeMsg
the complete serialized message to be authenticated.
authenticatedWholeMsg
the same as the input given to the authenticateOutgoingMsg
service, but with msgAuthenticationParameters properly
filled in.
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
msgAuthenticationParameters field is put into the
securityParameters and the authenticatedWholeMsg represents the
serialization of the authenticated message being prepared.
Otherwise,
b) If the securityLevel specifies that the message is not to be
authenticated then a zero-length OCTET STRING is encoded into
the msgAuthenticationParameters 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.
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 securityLevel.
To simplify the elements of procedure, the release of state
information is not always explicitly specified. As a general rule,
if state information is available when a message gets discarded, the
state information should also be released. Also, an error indication
can return an OID and value for an incremented counter and optionally
a value for securityLevel, and values for contextEngineID or
contextName for the counter. In addition, the securityStateReference
data is returned if any such information is available at the point
where the error is detected.
1) If the received securityParameters is not the serialization
(according to the conventions of [RFC3417]) of an OCTET STRING
formatted according to the UsmSecurityParameters defined in
section 2.4, then the snmpInASNParseErrs counter [RFC3418] is
incremented, and an error indication (parseError) is returned to
the calling module. Note that we return without the OID and
value of the incremented counter, because in this case there is
not enough information to generate a Report PDU.
2) The values of the security parameter fields are extracted from
the securityParameters. The securityEngineID to be returned to
the caller is the value of the msgAuthoritativeEngineID field.
The cachedSecurityData is prepared and a securityStateReference
is prepared to reference this data. Values to be cached are:
msgUserName
3) If the value of the msgAuthoritativeEngineID field in the
securityParameters is unknown then:
a) a non-authoritative SNMP engine 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.
Note in the event that a zero-length, or other illegally sized
msgAuthoritativeEngineID is received, b) should be chosen to
facilitate engineID discovery. Otherwise the choice between a)
and b) is an implementation issue.
4) Information about the value of the msgUserName and
msgAuthoritativeEngineID fields is extracted from the Local
Configuration Datastore (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 securityLevel requested by the caller, then the
usmStatsUnsupportedSecLevels counter is incremented and an error
indication (unsupportedSecurityLevel) together with the OID and
value of the incremented counter is returned to the calling
module.
6) If the securityLevel 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 primitive:
statusInformation = -- success or failure
authenticateIncomingMsg(
IN authKey -- the user's localized authKey
IN authParameters -- as received on the wire
IN wholeMsg -- as received on the wire
OUT authenticatedWholeMsg -- checked for authentication
)
statusInformation
indicates if authentication was successful or not.
authKey
the user's localized private authKey is the secret key that
can be used by the authentication algorithm.
wholeMsg
the complete serialized message to be authenticated.
authenticatedWholeMsg
the same as the input given to the authenticateIncomingMsg
service, but after authentication has been checked.
If the authentication module returns failure, then the message
cannot be 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.
7) If the securityLevel indicates an authenticated message, then the
local values of snmpEngineBoots, snmpEngineTime and
latestReceivedEngineTime corresponding to the value of the
msgAuthoritativeEngineID field are extracted from the Local
Configuration Datastore.
a) If the extracted value of msgAuthoritativeEngineID 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 2147483647;
- the value of the msgAuthoritativeEngineBoots field differs
from the local value of snmpEngineBoots; or,
- the value of the msgAuthoritativeEngineTime 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,
the value of the incremented counter, and an indication that
the error must be reported with a securityLevel of authNoPriv,
is returned to the calling module
b) If the extracted value of msgAuthoritativeEngineID 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 msgAuthoritativeEngineBoots
field is greater than the local notion of the value of
snmpEngineBoots; or,
- the extracted value of the msgAuthoritativeEngineBoots
field is equal to the local notion of the value of
snmpEngineBoots, and the extracted value of
msgAuthoritativeEngineTime field is greater than the
value of latestReceivedEngineTime,
then the LCD entry corresponding to the extracted value of
the msgAuthoritativeEngineID field is updated, by setting:
- the local notion of the value of snmpEngineBoots to the
value of the msgAuthoritativeEngineBoots field,
- the local notion of the value of snmpEngineTime to the
value of the msgAuthoritativeEngineTime field, and
- the latestReceivedEngineTime to the value of the value of
the msgAuthoritativeEngineTime field.
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
2147483647;
- the value of the msgAuthoritativeEngineBoots field is
less than the local notion of the value of
snmpEngineBoots; or,
- the value of the msgAuthoritativeEngineBoots field is
equal to the local notion of the value of snmpEngineBoots
and the value of the msgAuthoritativeEngineTime field is
more than 150 seconds less than the local notion 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
msgAuthoritativeEngineBoots 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 securityLevel 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
primitive:
statusInformation = -- success or failure
decryptData(
IN decryptKey -- the user's localized privKey
IN privParameters -- as received on the wire
IN encryptedData -- encryptedPDU as received
OUT decryptedData -- serialized decrypted scopedPDU
)
statusInformation
indicates if the decryption process was successful or not.
decryptKey
the user's localized private privKey is the secret key that
can be used by the decryption algorithm.
privParameters
the msgPrivacyParameters, encoded as an OCTET STRING.
encryptedData
the encryptedPDU represents the encrypted scopedPDU,
encoded as an OCTET STRING.
decryptedData
the serialized scopedPDU if decryption is successful.
If the privacy module returns failure, then the message can
not be processed, so the usmStatsDecryptionErrors counter is
incremented and an error indication (decryptionError) 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
securityLevel 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. Information to be
saved/cached is as follows:
msgUserName,
usmUserAuthProtocol, usmUserAuthKey
usmUserPrivProtocol, usmUserPrivKey
12) The statusInformation is set to success and a return is made to
the calling module passing back the OUT parameters as specified
in the processIncomingMsg primitive.
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 securityLevel of noAuthNoPriv, a
msgUserName of zero-length, a msgAuthoritativeEngineID value of zero
length, 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
msgAuthoritativeEngineID field within the msgSecurityParameters
field. It 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 msgAuthoritativeEngineID set to the newly learned snmpEngineID and with the values of msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime set to zero. For an authenticated Request message, a valid userName must be used in the msgUserName field. 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 msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime 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.
(page 32 continued on part 3)