RFC 2574
User-based Security Model (USM) for version 3 of the Simple Network Management Protocol (SNMPv3)
Network Working Group U. Blumenthal Request for Comments: 2574 IBM T. J. Watson Research Obsoletes: 2274 B. Wijnen Category: Standards Track IBM T. J. Watson Research April 1999 User-based Security Model (USM) for version 3 of the Simple Network Management Protocol (SNMPv3) Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (1999). All Rights Reserved.Abstract
This document describes the User-based Security Model (USM) for SNMP version 3 for use in the SNMP architecture [RFC2571]. 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.Table of Contents
1. Introduction 3 1.1. Threats 4 1.2. Goals and Constraints 5 1.3. Security Services 6 1.4. Module Organization 7 1.4.1. Timeliness Module 7 1.4.2. Authentication Protocol 8 1.4.3. Privacy Protocol 8 1.5. Protection against Message Replay, Delay and Redirection 8 1.5.1. Authoritative SNMP engine 8 1.5.2. Mechanisms 9 1.6. Abstract Service Interfaces 10 1.6.1. User-based Security Model Primitives for Authentication 11 1.6.2. User-based Security Model Primitives for Privacy 11 2. Elements of the Model 12 2.1. User-based Security Model Users 12
2.2. Replay Protection 13 2.2.1. msgAuthoritativeEngineID 13 2.2.2. msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime 14 2.2.3. Time Window 15 2.3. Time Synchronization 15 2.4. SNMP Messages Using this Security Model 16 2.5. Services provided by the User-based Security Model 17 2.5.1. Services for Generating an Outgoing SNMP Message 17 2.5.2. Services for Processing an Incoming SNMP Message 19 2.6. Key Localization Algorithm. 21 3. Elements of Procedure 21 3.1. Generating an Outgoing SNMP Message 22 3.2. Processing an Incoming SNMP Message 25 4. Discovery 30 5. Definitions 31 6. HMAC-MD5-96 Authentication Protocol 50 6.1. Mechanisms 50 6.1.1. Digest Authentication Mechanism 50 6.2. Elements of the Digest Authentication Protocol 51 6.2.1. Users 51 6.2.2. msgAuthoritativeEngineID 51 6.2.3. SNMP Messages Using this Authentication Protocol 51 6.2.4. Services provided by the HMAC-MD5-96 Authentication Module 52 6.2.4.1. Services for Generating an Outgoing SNMP Message 52 6.2.4.2. Services for Processing an Incoming SNMP Message 53 6.3. Elements of Procedure 53 6.3.1. Processing an Outgoing Message 54 6.3.2. Processing an Incoming Message 54 7. HMAC-SHA-96 Authentication Protocol 55 7.1. Mechanisms 55 7.1.1. Digest Authentication Mechanism 56 7.2. Elements of the HMAC-SHA-96 Authentication Protocol 56 7.2.1. Users 56 7.2.2. msgAuthoritativeEngineID 57 7.2.3. SNMP Messages Using this Authentication Protocol 57 7.2.4. Services provided by the HMAC-SHA-96 Authentication Module 57 7.2.4.1. Services for Generating an Outgoing SNMP Message 57 7.2.4.2. Services for Processing an Incoming SNMP Message 58 7.3. Elements of Procedure 59 7.3.1. Processing an Outgoing Message 59 7.3.2. Processing an Incoming Message 60 8. CBC-DES Symmetric Encryption Protocol 61 8.1. Mechanisms 61 8.1.1. Symmetric Encryption Protocol 61 8.1.1.1. DES key and Initialization Vector. 62 8.1.1.2. Data Encryption. 63 8.1.1.3. Data Decryption 63 8.2. Elements of the DES Privacy Protocol 63
8.2.1. Users 63 8.2.2. msgAuthoritativeEngineID 64 8.2.3. SNMP Messages Using this Privacy Protocol 64 8.2.4. Services provided by the DES Privacy Module 64 8.2.4.1. Services for Encrypting Outgoing Data 64 8.2.4.2. Services for Decrypting Incoming Data 65 8.3. Elements of Procedure. 66 8.3.1. Processing an Outgoing Message 66 8.3.2. Processing an Incoming Message 66 9. Intellectual Property 67 10. Acknowledgements 67 11. Security Considerations 69 11.1. Recommended Practices 69 11.2. Defining Users 71 11.3. Conformance 72 11.4. Use of Reports 72 11.5. Access to the SNMP-USER-BASED-SM-MIB 72 12. References 73 13. Editors' Addresses 75 A.1. SNMP engine Installation Parameters 76 A.2. Password to Key Algorithm 78 A.2.1. Password to Key Sample Code for MD5 79 A.2.2. Password to Key Sample Code for SHA 80 A.3. Password to Key Sample Results 81 A.3.1. Password to Key Sample Results using MD5 81 A.3.2. Password to Key Sample Results using SHA 81 A.4. Sample encoding of msgSecurityParameters 82 A.5. Sample keyChange Results 83 A.5.1. Sample keyChange Results using MD5 83 A.5.2. Sample keyChange Results using SHA 84 B. Change Log 85 C. Full Copyright Statement 861. Introduction
The Architecture for describing Internet Management Frameworks [RFC2571] describes that an SNMP engine is composed of: 1) a Dispatcher 2) a Message Processing Subsystem, 3) a Security Subsystem, and 4) 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 [RFC2571]. This memo [RFC2274] 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) with which to associate security information. This memo describes the use of HMAC-MD5-96 and HMAC-SHA-96 as the authentication protocols 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 HMAC-MD5-96, HMAC-SHA-96 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. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].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 principal in such a way as to effect unauthorized management operations, including falsifying the value of an object. - Masquerade The masquerade threat is the danger that management operations not authorized for some user may be attempted by assuming the identity of another user that has the appropriate authorizations. 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.
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 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 the timeliness values in a message that passes the authentication module are trustworthy.1.4.2. Authentication Protocol
Section 6 describes the HMAC-MD5-96 authentication protocol which is the first authentication protocol that MUST be supported with the User-based Security Model. Section 7 describes the HMAC-SHA-96 authentication protocol which is another authentication protocol that SHOULD be supported 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 8 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 (those messages that contain a Confirmed Class PDU [RFC2571]), then the receiver of such messages is authoritative. When an SNMP message contains a payload which does not expect a response (those messages that contain an Unconfirmed Class PDU [RFC2571]), then the sender of such a message is authoritative.
1.5.2. Mechanisms
The following mechanisms are used: 1) To protect against the threat of message delay or replay (to an extent greater than can occur through normal operation), a set of timeliness indicators (for the authoritative SNMP engine) 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 MUST also use a mechanism to match incoming Responses to outstanding Requests and it MUST drop any Responses that do not match an outstanding request. For example, a msgID can be inserted in every message to cater for this functionality. These mechanisms provide for the detection of authenticated messages whose time of generation was not recent. This protection against the threat of message delay or replay does not imply nor provide any protection against unauthorized deletion or suppression of messages. 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]). 2) Verification that a message sent to/from one authoritative SNMP engine cannot be replayed to/as-if-from another authoritative SNMP engine. Included in each message is an identifier unique to the authoritative SNMP engine associated with the sender or intended recipient of the message. A message containing an Unconfirmed Class PDU sent by an authoritative SNMP engine to one non-authoritative SNMP engine can potentially be replayed to another non-authoritative SNMP engine. The latter non-authoritative SNMP engine might (if it knows about the same userName with the same secrets at the authoritative SNMP engine) as a result update its notion of timeliness indicators of the authoritative SNMP engine, but that is not considered a
threat. In this case, A Report or Response message will be
discarded by the Message Processing Model, because there should
not be an outstanding Request message. A Trap will possibly be
accepted. Again, that is not considered a threat, because the
communication was authenticated and timely. It is as if the
authoritative SNMP engine was configured to start sending Traps to
the second SNMP engine, which theoretically can happen without the
knowledge of the second SNMP engine anyway. Anyway, the second
SNMP engine may not expect to receive this Trap, but is allowed to
see the management information contained in it.
3) Detection of messages which were not recently generated.
A set of time indicators are included in the message, indicating
the time of generation. Messages without recent time indicators
are not considered authentic. In addition, an SNMP engine MUST
drop any Responses that do not match an outstanding request. This
however is the responsibility of the Message Processing Model.
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.
1.6. Abstract Service Interfaces
Abstract service interfaces have been defined to describe the
conceptual interfaces between the various subsystems within an SNMP
entity. Similarly a set of abstract service interfaces have been
defined within the User-based Security Model (USM) to describe the conceptual interfaces between the generic USM services and the self- contained authentication and privacy services. These abstract service interfaces are defined by a set of primitives that define the services provided and the abstract data elements that must be passed when the services are invoked. This section lists the primitives that have been defined for the User-based Security Model.1.6.1. User-based Security Model Primitives for Authentication
The User-based Security Model provides the following internal primitives to pass data back and forth between the Security Model itself and the authentication service: statusInformation = authenticateOutgoingMsg( IN authKey -- secret key for authentication IN wholeMsg -- unauthenticated complete message OUT authenticatedWholeMsg -- complete authenticated message ) statusInformation = authenticateIncomingMsg( IN authKey -- secret key for authentication IN authParameters -- as received on the wire IN wholeMsg -- as received on the wire OUT authenticatedWholeMsg -- complete authenticated message )1.6.2. User-based Security Model Primitives for Privacy
The User-based Security Model provides the following internal primitives to pass data back and forth between the Security Model itself and the privacy service: statusInformation = encryptData( IN encryptKey -- secret key for encryption IN dataToEncrypt -- data to encrypt (scopedPDU) OUT encryptedData -- encrypted data (encryptedPDU) OUT privParameters -- filled in by service provider ) statusInformation = decryptData( IN decryptKey -- secret key for decrypting IN privParameters -- as received on the wire
IN encryptedData -- encrypted data (encryptedPDU)
OUT decryptedData -- decrypted data (scopedPDU)
)
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. 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 [RFC2571]).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 [RFC2572]). 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
usmUserTable that 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
Request 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.
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 [RFC1906]) of an OCTET STRING
formatted according to the UsmSecurityParameters defined in
section 2.4, then the snmpInASNParseErrs counter [RFC1907] 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.
(page 30 continued on part 2)
