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:

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
       )

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-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.

2.1. User-based Security Model Users​

2.2. Replay Protection​

2.2.1. msgAuthoritativeEngineID​

2.2.2. msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime​

2.2.3. Time Window​

2.3. Time Synchronization​

2.4. SNMP Messages Using this Security Model​

2.5. Services provided by the User-based Security Model​

2.5.1. Services for Generating an Outgoing SNMP Message​

2.5.2. Services for Processing an Incoming SNMP Message​

2.6. Key Localization Algorithm.​