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RFC 2274

 
 
 

User-based Security Model (USM) for version 3 of the Simple Network Management Protocol (SNMPv3)

Part 3 of 3, p. 51 to 76
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7.  HMAC-SHA-96 Authentication Protocol

   This section describes the HMAC-SHA-96 authentication protocol.  This
   protocol uses the SHA hash-function which is described in [SHA-NIST],
   in HMAC mode described in [RFC2104], truncating the output to 96
   bits.

   This protocol is identified by usmHMACSHAAuthProtocol.

   Over time, other authentication protocols may be defined either as a
   replacement of this protocol or in addition to this protocol.

7.1.  Mechanisms

   - In support of data integrity, a message digest algorithm is
     required.  A digest is calculated over an appropriate portion of an
     SNMP message and included as part of the message sent to the
     recipient.

   - In support of data origin authentication and data integrity,
     a secret value is prepended to the SNMP message prior to computing
     the digest; the calculated digest is then partially inserted into
     the message prior to transmission. The prepended secret is not
     transmitted.  The secret value is shared by all SNMP engines
     authorized to originate messages on behalf of the appropriate user.

7.1.1.  Digest Authentication Mechanism

   The Digest Authentication Mechanism defined in this memo provides
   for:

   - verification of the integrity of a received message, i.e., the
     the message received is the message sent.

     The integrity of the message is protected by computing a digest
     over an appropriate portion of the message.  The digest is computed
     by the originator of the message, transmitted with the message, and
     verified by the recipient of the message.

   - verification of the user on whose behalf the message was generated.

     A secret value known only to SNMP engines authorized to generate
     messages on behalf of a user is used in HMAC mode (see [RFC2104]).
     It also recommends the hash-function output used as Message
     Authentication Code, to be truncated.

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   This mechanism uses the SHA [SHA-NIST] message digest algorithm.  A
   160-bit SHA digest is calculated in a special (HMAC) way over the
   designated portion of an SNMP message and the first 96 bits of this
   digest is included as part of the message sent to the recipient. The
   size of the digest carried in a message is 12 octets. The size of the
   private authentication key (the secret) is 20 octets. For the details
   see section 7.3.

7.2.  Elements of the HMAC-SHA-96 Authentication Protocol

   This section contains definitions required to realize the
   authentication module defined in this section of this memo.

7.2.1.  Users

   Authentication using this authentication protocol makes use of a
   defined set of userNames.  For any user on whose behalf a message
   must be authenticated at a particular SNMP engine, that SNMP engine
   must have knowledge of that user.  An SNMP engine that wishes to
   communicate with another SNMP engine must also have knowledge of a
   user known to that engine, including knowledge of the applicable
   attributes of that user.

   A user and its attributes are defined as follows:

   <userName>
     A string representing the name of the user.
   <authKey>
     A user's secret key to be used when calculating a digest.
     It MUST be 20 octets long for SHA.

7.2.2.  msgAuthoritativeEngineID

   The msgAuthoritativeEngineID value contained in an authenticated
   message specifies the authoritative SNMP engine for that particular
   message (see the definition of SnmpEngineID in the SNMP Architecture
   document [RFC2271]).

   The user's (private) authentication key is normally different at each
   authoritative SNMP engine and so the snmpEngineID is used to select
   the proper key for the authentication process.

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7.2.3.  SNMP Messages Using this Authentication Protocol

   Messages using this authentication protocol carry a
   msgAuthenticationParameters field as part of the
   msgSecurityParameters. For this protocol, the
   msgAuthenticationParameters field is the serialized OCTET STRING
   representing the first 12 octets of HMAC-SHA-96 output done over the
   wholeMsg.

   The digest is calculated over the wholeMsg so if a message is
   authenticated, that also means that all the fields in the message are
   intact and have not been tampered with.

7.2.4.  Services provided by the HMAC-SHA-96 Authentication Module

   This section describes the inputs and outputs that the HMAC-SHA-96
   Authentication module expects and produces when the User-based
   Security module calls the HMAC-SHA-96 Authentication module for
   services.

7.2.4.1.  Services for Generating an Outgoing SNMP Message

   HMAC-SHA-96 authentication protocol assumes that the selection of the
   authKey is done by the caller and that the caller passes the secret
   key to be used.

   Upon completion the authentication module returns statusInformation
   and, if the message digest was correctly calculated, the wholeMsg
   with the digest inserted at the proper place. The abstract service
   primitive is:

   statusInformation =              -- success or failure
     authenticateOutgoingMsg(
     IN   authKey                   -- secret key for authentication
     IN   wholeMsg                  -- unauthenticated complete message
     OUT  authenticatedWholeMsg     -- complete authenticated message
          )

   The abstract data elements are:

     statusInformation
       An indication of whether the authentication process was
       successful.  If not it is an indication of the problem.
     authKey
       The secret key to be used by the authentication algorithm.
       The length of this key MUST be 20 octets.
     wholeMsg
       The message to be authenticated.

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     authenticatedWholeMsg
       The authenticated message (including inserted digest) on output.

   Note, that authParameters field is filled by the authentication
   module and this field should be already present in the wholeMsg
   before the Message Authentication Code (MAC) is generated.

7.2.4.2.  Services for Processing an Incoming SNMP Message

   HMAC-SHA-96 authentication protocol assumes that the selection of the
   authKey is done by the caller and that the caller passes the secret
   key to be used.

   Upon completion the authentication module returns statusInformation
   and, if the message digest was correctly calculated, the wholeMsg as
   it was processed. The abstract service primitive is:

   statusInformation =              -- success or failure
     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
       )

   The abstract data elements are:

     statusInformation
       An indication of whether the authentication process was
       successful.  If not it is an indication of the problem.
     authKey
       The secret key to be used by the authentication algorithm.
       The length of this key MUST be 20 octets.
     authParameters
       The authParameters from the incoming message.
     wholeMsg
       The message to be authenticated on input and the authenticated
       message on output.
     authenticatedWholeMsg
       The whole message after the authentication check is complete.

7.3.  Elements of Procedure

   This section describes the procedures for the HMAC-SHA-96
   authentication protocol.

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7.3.1.  Processing an Outgoing Message

   This section describes the procedure followed by an SNMP engine
   whenever it must authenticate an outgoing message using the
   usmHMACSHAAuthProtocol.

   1) The msgAuthenticationParameters field is set to the
      serialization, according to the rules in [RFC1906], of an OCTET
      STRING containing 12 zero octets.

   2) From the secret authKey, two keys K1 and K2 are derived:

         a) extend the authKey to 64 octets by appending 44 zero
            octets; save it as extendedAuthKey
         b) obtain IPAD by replicating the octet 0x36 64 times;
         c) obtain K1 by XORing extendedAuthKey with IPAD;
         d) obtain OPAD by replicating the octet 0x5C 64 times;
         e) obtain K2 by XORing extendedAuthKey with OPAD.

   3) Prepend K1 to the wholeMsg and calculate the SHA digest over it
      according to [SHA-NIST].

   4) Prepend K2 to the result of the step 4 and calculate SHA digest
      over it according to [SHA-NIST]. Take the first 12 octets of the
      final digest - this is Message Authentication Code (MAC).

   5) Replace the msgAuthenticationParameters field with MAC obtained
      in the step 5.

   6) The authenticatedWholeMsg is then returned to the caller
      together with statusInformation indicating success.

7.3.2.  Processing an Incoming Message

   This section describes the procedure followed by an SNMP engine
   whenever it must authenticate an incoming message using the
   usmHMACSHAAuthProtocol.

   1)  If the digest received in the msgAuthenticationParameters field
       is not 12 octets long, then an failure and an errorIndication
       (authenticationError) is returned to the calling module.

   2)  The MAC received in the msgAuthenticationParameters field
       is saved.

   3)  The digest in the msgAuthenticationParameters field is
       replaced by the 12 zero octets.

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   4)  From the secret authKey, two keys K1 and K2 are derived:

         a) extend the authKey to 64 octets by appending 44 zero
            octets; save it as extendedAuthKey
         b) obtain IPAD by replicating the octet 0x36 64 times;
         c) obtain K1 by XORing extendedAuthKey with IPAD;
         d) obtain OPAD by replicating the octet 0x5C 64 times;
         e) obtain K2 by XORing extendedAuthKey with OPAD.

   5)  The MAC is calculated over the wholeMsg:

         a) prepend K1 to the wholeMsg and calculate the SHA digest
            over it;
         b) prepend K2 to the result of step 5.a and calculate the
            SHA digest over it;
         c) first 12 octets of the result of step 5.b is the MAC.

       The msgAuthenticationParameters field is replaced with the MAC
       value that was saved in step 2.

   6)  The the newly calculated MAC is compared with the MAC saved in
       step 2. If they do not match, then a failure and an
       errorIndication (authenticationFailure) are returned to the
       calling module.

   7)  The authenticatedWholeMsg and statusInformation indicating
       success are then returned to the caller.

8.  CBC-DES Symmetric Encryption Protocol

   This section describes the CBC-DES Symmetric Encryption Protocol.
   This protocol is the first privacy protocol defined for the User-
   based Security Model.

   This protocol is identified by usmDESPrivProtocol.

   Over time, other privacy protocols may be defined either as a
   replacement of this protocol or in addition to this protocol.

8.1.  Mechanisms

   - In support of data confidentiality, an encryption algorithm is
     required.  An appropriate portion of the message is encrypted prior
     to being transmitted. The User-based Security Model specifies that
     the scopedPDU is the portion of the message that needs to be
     encrypted.

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   - A secret value in combination with a timeliness value is used
     to create the en/decryption key and the initialization vector.  The
     secret value is shared by all SNMP engines authorized to originate
     messages on behalf of the appropriate user.

8.1.1.  Symmetric Encryption Protocol

   The Symmetric Encryption Protocol defined in this memo provides
   support for data confidentiality.  The designated portion of an SNMP
   message is encrypted and included as part of the message sent to the
   recipient.

   Two organizations have published specifications defining the DES: the
   National Institute of Standards and Technology (NIST) [DES-NIST] and
   the American National Standards Institute [DES-ANSI].  There is a
   companion Modes of Operation specification for each definition
   ([DESO-NIST] and [DESO-ANSI], respectively).

   The NIST has published three additional documents that implementors
   may find useful.

   - There is a document with guidelines for implementing and using
     the DES, including functional specifications for the DES and its
     modes of operation [DESG-NIST].

   - There is a specification of a validation test suite for the DES
     [DEST-NIST].  The suite is designed to test all aspects of the DES
     and is useful for pinpointing specific problems.

   - There is a specification of a maintenance test for the DES
     [DESM-NIST].  The test utilizes a minimal amount of data and
     processing to test all components of the DES.  It provides a simple
     yes-or-no indication of correct operation and is useful to run as
     part of an initialization step, e.g., when a computer re-boots.

8.1.1.1.  DES key and Initialization Vector.

   The first 8 octets of the 16-octet secret (private privacy key) are
   used as a DES key.  Since DES uses only 56 bits, the Least
   Significant Bit in each octet is disregarded.

   The Initialization Vector for encryption is obtained using the
   following procedure.

   The last 8 octets of the 16-octet secret (private privacy key) are
   used as pre-IV.

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   In order to ensure that the IV for two different packets encrypted by
   the same key, are not the same (i.e., the IV does not repeat) we need
   to "salt" the pre-IV with something unique per packet.  An 8-octet
   string is used as the "salt".  The concatenation of the generating
   SNMP engine's 32-bit snmpEngineBoots and a local 32-bit integer, that
   the encryption engine maintains, is input to the "salt".  The 32-bit
   integer is initialized to an arbitrary value at boot time.

   The 32-bit snmpEngineBoots is converted to the first 4 octets (Most
   Significant Byte first) of our "salt".  The 32-bit integer is then
   converted to the last 4 octet (Most Significant Byte first) of our
   "salt".  The resulting "salt" is then XOR-ed with the pre-IV. The 8-
   octet "salt" is then put into the privParameters field encoded as an
   OCTET STRING.  The "salt" integer is then modified.  We recommend
   that it be incremented by one and wrap when it reaches the maximum
   value.

   How exactly the value of the "salt" (and thus of the IV) varies, is
   an implementation issue, as long as the measures are taken to avoid
   producing a duplicate IV.

   The "salt" must be placed in the privParameters field to enable the
   receiving entity to compute the correct IV and to decrypt the
   message.

8.1.1.2.  Data Encryption.

   The data to be encrypted is treated as sequence of octets. Its length
   should be an integral multiple of 8 - and if it is not, the data is
   padded at the end as necessary.  The actual pad value is irrelevant.

   The data is encrypted in Cipher Block Chaining mode.

   The plaintext is divided into 64-bit blocks.

   The plaintext for each block is XOR-ed with the ciphertext of the
   previous block, the result is encrypted and the output of the
   encryption is the ciphertext for the block.  This procedure is
   repeated until there are no more plaintext blocks.

   For the very first block, the Initialization Vector is used instead
   of the ciphertext of the previous block.

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8.1.1.3.  Data Decryption

   Before decryption, the encrypted data length is verified.  If the
   length of the OCTET STRING to be decrypted is not an integral
   multiple of 8 octets, the decryption process is halted and an
   appropriate exception noted.  When decrypting, the padding is
   ignored.

   The first ciphertext block is decrypted, the decryption output is
   XOR-ed with the Initialization Vector, and the result is the first
   plaintext block.

   For each subsequent block, the ciphertext block is decrypted, the
   decryption output is XOR-ed with the previous ciphertext block and
   the result is the plaintext block.

8.2.  Elements of the DES Privacy Protocol

   This section contains definitions required to realize the privacy
   module defined by this memo.

8.2.1.  Users

   Data en/decryption using this Symmetric Encryption Protocol makes use
   of a defined set of userNames.  For any user on whose behalf a
   message must be en/decrypted at a particular SNMP engine, that SNMP
   engine must have knowledge of that user.  An SNMP engine that wishes
   to communicate with another SNMP engine must also have knowledge of a
   user known to that SNMP engine, including knowledge of the applicable
   attributes of that user.

   A user and its attributes are defined as follows:

   <userName>
     An octet string representing the name of the user.
   <privKey>
     A user's secret key to be used as input for the DES key and IV.
     The length of this key MUST be 16 octets.

8.2.2.  msgAuthoritativeEngineID

   The msgAuthoritativeEngineID value contained in an authenticated
   message specifies the authoritative SNMP engine for that particular
   message (see the definition of SnmpEngineID in the SNMP Architecture
   document [RFC2271]).

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   The user's (private) privacy key is normally different at each
   authoritative SNMP engine and so the snmpEngineID is used to select
   the proper key for the en/decryption process.

8.2.3.  SNMP Messages Using this Privacy Protocol

   Messages using this privacy protocol carry a msgPrivacyParameters
   field as part of the msgSecurityParameters. For this protocol, the
   msgPrivacyParameters field is the serialized OCTET STRING
   representing the "salt" that was used to create the IV.

8.2.4.  Services provided by the DES Privacy Module

   This section describes the inputs and outputs that the DES Privacy
   module expects and produces when the User-based Security module
   invokes the DES Privacy module for services.

8.2.4.1.  Services for Encrypting Outgoing Data

   This DES privacy protocol assumes that the selection of the privKey
   is done by the caller and that the caller passes the secret key to be
   used.

   Upon completion the privacy module returns statusInformation and, if
   the encryption process was successful, the encryptedPDU and the
   msgPrivacyParameters encoded as an OCTET STRING.  The abstract
   service primitive is:

   statusInformation =              -- success of failure
     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
           )

   The abstract data elements are:

     statusInformation
       An indication of the success or failure of the encryption
       process.  In case of failure, it is an indication of the error.
     encryptKey
       The secret key to be used by the encryption algorithm.
       The length of this key MUST be 16 octets.
     dataToEncrypt
       The data that must be encrypted.
     encryptedData
       The encrypted data upon successful completion.

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     privParameters
       The privParameters encoded as an OCTET STRING.

8.2.4.2.  Services for Decrypting Incoming Data

   This DES privacy protocol assumes that the selection of the privKey
   is done by the caller and that the caller passes the secret key to be
   used.

   Upon completion the privacy module returns statusInformation and, if
   the decryption process was successful, the scopedPDU in plain text.
   The abstract service primitive is:

   statusInformation =
     decryptData(
     IN    decryptKey               -- secret key for decryption
     IN    privParameters           -- as received on the wire
     IN    encryptedData            -- encrypted data (encryptedPDU)
     OUT   decryptedData            -- decrypted data (scopedPDU)
           )

   The abstract data elements are:

     statusInformation
       An indication whether the data was successfully decrypted
       and if not an indication of the error.
     decryptKey
       The secret key to be used by the decryption algorithm.
       The length of this key MUST be 16 octets.
     privParameters
       The "salt" to be used to calculate the IV.
     encryptedData
       The data to be decrypted.
     decryptedData
       The decrypted data.

8.3.  Elements of Procedure.

   This section describes the procedures for the DES privacy protocol.

8.3.1.  Processing an Outgoing Message

   This section describes the procedure followed by an SNMP engine
   whenever it must encrypt part of an outgoing message using the
   usmDESPrivProtocol.

   1)  The secret cryptKey is used to construct the DES encryption key,
       the "salt" and the DES pre-IV (as described in section 8.1.1.1).

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   2)  The privParameters field is set to the serialization according
       to the rules in [RFC1906] of an OCTET STRING representing the the
       "salt" string.

   3)  The scopedPDU is encrypted (as described in section 8.1.1.2)
       and the encrypted data is serialized according to the rules in
       [RFC1906] as an OCTET STRING.

   4)  The serialized OCTET STRING representing the encrypted
       scopedPDU together with the privParameters and statusInformation
       indicating success is returned to the calling module.

8.3.2.  Processing an Incoming Message

   This section describes the procedure followed by an SNMP engine
   whenever it must decrypt part of an incoming message using the
   usmDESPrivProtocol.

   1)  If the privParameters field is not an 8-octet OCTET STRING,
       then an error indication (decryptionError) is returned to the
       calling module.

   2)  The "salt" is extracted from the privParameters field.

   3)  The secret cryptKey and the "salt" are then used to construct the
       DES decryption key and pre-IV (as described in section 8.1.1.1).

   4)  The encryptedPDU is then decrypted (as described in
       section 8.1.1.3).

   5)  If the encryptedPDU cannot be decrypted, then an error
       indication (decryptionError) is returned to the calling module.

   6)  The decrypted scopedPDU and statusInformation indicating
       success are returned to the calling module.

9.  Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11.  Copies of
   claims of rights made available for publication and any assurances of
   licenses to be made available, or the result of an attempt made to

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   obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive
   Director.

10.  Acknowledgements

   This document is the result of the efforts of the SNMPv3 Working
   Group.  Some special thanks are in order to the following SNMPv3 WG
   members:

      Dave Battle (SNMP Research, Inc.)
      Uri Blumenthal (IBM T.J. Watson Research Center)
      Jeff Case (SNMP Research, Inc.)
      John Curran (BBN)
      T. Max Devlin (Hi-TECH Connections)
      John Flick (Hewlett Packard)
      David Harrington (Cabletron Systems Inc.)
      N.C. Hien (IBM T.J. Watson Research Center)
      Dave Levi (SNMP Research, Inc.)
      Louis A Mamakos (UUNET Technologies Inc.)
      Paul Meyer (Secure Computing Corporation)
      Keith McCloghrie (Cisco Systems)
      Russ Mundy (Trusted Information Systems, Inc.)
      Bob Natale (ACE*COMM Corporation)
      Mike O'Dell (UUNET Technologies Inc.)
      Dave Perkins (DeskTalk)
      Peter Polkinghorne (Brunel University)
      Randy Presuhn (BMC Software, Inc.)
      David Reid (SNMP Research, Inc.)
      Shawn Routhier (Epilogue)
      Juergen Schoenwaelder (TU Braunschweig)
      Bob Stewart (Cisco Systems)
      Bert Wijnen (IBM T.J. Watson Research Center)

   The document is based on recommendations of the IETF Security and
   Administrative Framework Evolution for SNMP Advisory Team.  Members
   of that Advisory Team were:

      David Harrington (Cabletron Systems Inc.)
      Jeff Johnson (Cisco Systems)
      David Levi (SNMP Research Inc.)
      John Linn (Openvision)

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      Russ Mundy (Trusted Information Systems) chair
      Shawn Routhier (Epilogue)
      Glenn Waters (Nortel)
      Bert Wijnen (IBM T. J. Watson Research Center)

   As recommended by the Advisory Team and the SNMPv3 Working Group
   Charter, the design incorporates as much as practical from previous
   RFCs and drafts. As a result, special thanks are due to the authors
   of previous designs known as SNMPv2u and SNMPv2*:

      Jeff Case (SNMP Research, Inc.)
      David Harrington (Cabletron Systems Inc.)
      David Levi (SNMP Research, Inc.)
      Keith McCloghrie (Cisco Systems)
      Brian O'Keefe (Hewlett Packard)
      Marshall T. Rose (Dover Beach Consulting)
      Jon Saperia (BGS Systems Inc.)
      Steve Waldbusser (International Network Services)
      Glenn W. Waters (Bell-Northern Research Ltd.)

11.  Security Considerations

11.1.  Recommended Practices

   This section describes practices that contribute to the secure,
   effective operation of the mechanisms defined in this memo.

   - An SNMP engine must discard SNMP Response messages that do not
     correspond to any currently outstanding Request message. It is the
     responsibility of the Message Processing module to take care of
     this. For example it can use a msgID for that.

     An SNMP Command Generator Application must discard any Response PDU
     for which there is no currently outstanding Request PDU; for
     example for SNMPv2 [RFC1905] PDUs, the request-id component in the
     PDU can be used to correlate Responses to outstanding Requests.

     Although it would be typical for an SNMP engine and an SNMP Command
     Generator Application to do this as a matter of course, when using
     these security protocols it is significant due to the possibility
     of message duplication (malicious or otherwise).

   - If an SNMP engine uses a msgID for correlating Response messages
     to outstanding Request messages, then it MUST use different msgIDs
     in all such Request messages that it sends out during a Time Window
     (150 seconds) period.

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     A Command Generator or Notification Originator Application MUST use
     different request-ids in all Request PDUs that it sends out during
     a TimeWindow (150 seconds) period.

     This must be done to protect against the possibility of message
     duplication (malicious or otherwise).

     For example, starting operations with a msgID and/or request-id
     value of zero is not a good idea.  Initializing them with an
     unpredictable number (so they do not start out the same after each
     reboot) and then incrementing by one would be acceptable.

   - An SNMP engine should perform time synchronization using
     authenticated messages in order to protect against the possibility
     of message duplication (malicious or otherwise).

   - When sending state altering messages to a managed authoritative
     SNMP engine, a Command Generator Application should delay sending
     successive messages to that managed SNMP engine until a positive
     acknowledgement is received for the previous message or until the
     previous message expires.

     No message ordering is imposed by the SNMP. Messages may be
     received in any order relative to their time of generation and each
     will be processed in the ordered received.  Note that when an
     authenticated message is sent to a managed SNMP engine, it will be
     valid for a period of time of approximately 150 seconds under
     normal circumstances, and is subject to replay during this period.
     Indeed, an SNMP engine and SNMP Command Generator Applications must
     cope with the loss and re-ordering of messages resulting from
     anomalies in the network as a matter of course.

     However, a managed object, snmpSetSerialNo [RFC1907], is
     specifically defined for use with SNMP Set operations in order to
     provide a mechanism to ensure that the processing of SNMP messages
     occurs in a specific order.

   - The frequency with which the secrets of a User-based Security
     Model user should be changed is indirectly related to the frequency
     of their use.

     Protecting the secrets from disclosure is critical to the overall
     security of the protocols.  Frequent use of a secret provides a
     continued source of data that may be useful to a cryptanalyst in
     exploiting known or perceived weaknesses in an algorithm.  Frequent
     changes to the secret avoid this vulnerability.

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     Changing a secret after each use is generally regarded as the most
     secure practice, but a significant amount of overhead may be
     associated with that approach.

     Note, too, in a local environment the threat of disclosure may be
     less significant, and as such the changing of secrets may be less
     frequent.  However, when public data networks are used as the
     communication paths, more caution is prudent.

11.2  Defining Users

   The mechanisms defined in this document employ the notion of users on
   whose behalf messages are sent.  How "users" are defined is subject
   to the security policy of the network administration.  For example,
   users could be individuals (e.g., "joe" or "jane"), or a particular
   role (e.g., "operator" or "administrator"), or a combination (e.g.,
   "joe-operator", "jane-operator" or "joe-admin").  Furthermore, a user
   may be a logical entity, such as an SNMP Application or a set of SNMP
   Applications, acting on behalf of an individual or role, or set of
   individuals, or set of roles, including combinations.

   Appendix A describes an algorithm for mapping a user "password" to a
   16 octet value for use as either a user's authentication key or
   privacy key (or both).  Note however, that using the same password
   (and therefore the same key) for both authentication and privacy is
   very poor security practice and should be strongly discouraged.
   Passwords are often generated, remembered, and input by a human.
   Human-generated passwords may be less than the 16 octets required by
   the authentication and privacy protocols, and brute force attacks can
   be quite easy on a relatively short ASCII character set.  Therefore,
   the algorithm is Appendix A performs a transformation on the
   password.  If the Appendix A algorithm is used, SNMP implementations
   (and SNMP configuration applications) must ensure that passwords are
   at least 8 characters in length.

   Because the Appendix A algorithm uses such passwords (nearly)
   directly, it is very important that they not be easily guessed.  It
   is suggested that they be composed of mixed-case alphanumeric and
   punctuation characters that don't form words or phrases that might be
   found in a dictionary.  Longer passwords improve the security of the
   system.  Users may wish to input multiword phrases to make their
   password string longer while ensuring that it is memorable.

   Since it is infeasible for human users to maintain different
   passwords for every SNMP engine, but security requirements strongly
   discourage having the same key for more than one SNMP engine, the
   User-based Security Model employs a compromise proposed in
   [Localized-key].  It derives the user keys for the SNMP engines from

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   user's password in such a way that it is practically impossible to
   either determine the user's password, or user's key for another SNMP
   engine from any combination of user's keys on SNMP engines.

   Note however, that if user's password is disclosed, then key
   localization will not help and network security may be compromised in
   this case. Therefore a user's password or non-localized key MUST NOT
   be stored on a managed device/node. Instead the localized key SHALL
   be stored (if at all) , so that, in case a device does get
   compromised, no other managed or managing devices get compromised.

11.3.  Conformance

   To be termed a "Secure SNMP implementation" based on the User-based
   Security Model, an SNMP implementation MUST:

   - implement one or more Authentication Protocol(s). The HMAC-MD5-96
     and HMAC-SHA-96 Authentication Protocols defined in this memo are
     examples of such protocols.

   - to the maximum extent possible, prohibit access to the secret(s)
     of each user about which it maintains information in a Local
     Configuration Datastore (LCD) under all circumstances except as
     required to generate and/or validate SNMP messages with respect to
     that user.

   - implement the key-localization mechanism.

   - implement the SNMP-USER-BASED-SM-MIB.

   In addition, an authoritative SNMP engine SHOULD provide initial
   configuration in accordance with Appendix A.1.

   Implementation of a Privacy Protocol (the DES Symmetric Encryption
   Protocol defined in this memo is one such protocol) is optional.

12.  References

   [RFC1321] Rivest, R.,  "Message Digest Algorithm MD5",
      RFC 1321, April 1992.

   [RFC1903] Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
      "Textual Conventions for Version 2 of the Simple Network
      Management Protocol (SNMPv2)", RFC 1903, January 1996.

   [RFC1905] Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
      "Protocol Operations for Version 2 of the Simple Network
      Management Protocol (SNMPv2)", RFC 1905, January 1996.

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   [RFC1906] Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
      "Transport Mappings for Version 2 of the Simple Network Management
      Protocol (SNMPv2)", RFC 1906, January 1996.

   [RFC1907] Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
      "Management Information Base for Version 2 of the Simple Network
      Management Protocol (SNMPv2)", RFC 1907 January 1996.

   [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
      Keyed-Hashing  for Message Authentication", RFC 2104, February
      1997.

   [RFC2028] Hovey, R., and S. Bradner, "The Organizations Involved in
      the IETF Standards Process", BCP 11, RFC 2028, October 1996.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
      Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2271] Harrington, D., Presuhn, R., and B. Wijnen, "An
      Architecture for describing SNMP Management Frameworks", RFC 2271,
      January 1998.

   [RFC2272] Case, J., Harrington, D., Presuhn, R., and B. Wijnen,
      "Message Processing and Dispatching for the Simple Network
      Management Protocol (SNMP)", RFC 2272, January 1998.

   [Localized-Key] U. Blumenthal, N. C. Hien, B. Wijnen
      "Key Derivation for Network Management Applications" IEEE Network
      Magazine, April/May issue, 1997.

   [DES-NIST] Data Encryption Standard, National Institute of Standards
      and Technology.  Federal Information Processing Standard (FIPS)
      Publication 46-1.  Supersedes FIPS Publication 46, (January, 1977;
      reaffirmed January, 1988).

   [DES-ANSI] Data Encryption Algorithm, American National Standards
      Institute.  ANSI X3.92-1981, (December, 1980).

   [DESO-NIST] DES Modes of Operation, National Institute of Standards
      and Technology.  Federal Information Processing Standard (FIPS)
      Publication 81, (December, 1980).

   [DESO-ANSI] Data Encryption Algorithm - Modes of Operation, American
      National Standards Institute.  ANSI X3.106-1983, (May 1983).

   [DESG-NIST] Guidelines for Implementing and Using the NBS Data
      Encryption Standard, National Institute of Standards and
      Technology.  Federal Information Processing Standard (FIPS)

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      Publication 74, (April, 1981).

   [DEST-NIST] Validating the Correctness of Hardware Implementations of
      the NBS Data Encryption Standard, National Institute of Standards
      and Technology.  Special Publication 500-20.

   [DESM-NIST] Maintenance Testing for the Data Encryption Standard,
      National Institute of Standards and Technology.  Special
      Publication 500-61, (August, 1980).

   [SHA-NIST] Secure Hash Algorithm. NIST FIPS 180-1, (April, 1995)
      http://csrc.nist.gov/fips/fip180-1.txt (ASCII)
      http://csrc.nist.gov/fips/fip180-1.ps  (Postscript)

13.  Editors' Addresses

   Uri Blumenthal
   IBM T. J. Watson Research
   30 Saw Mill River Pkwy,
   Hawthorne, NY 10532
   USA

   EMail:      uri@watson.ibm.com
   Phone:      +1-914-784-7064


   Bert Wijnen
   IBM T. J. Watson Research
   Schagen 33
   3461 GL Linschoten
   Netherlands

   EMail:      wijnen@vnet.ibm.com
   Phone:      +31-348-432-794

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APPENDIX A - Installation

A.1.  SNMP engine Installation Parameters

   During installation, an authoritative SNMP engine SHOULD (in the
   meaning as defined in [RFC2119]) be configured with several initial
   parameters.  These include:

   1) A security posture

      The choice of security posture determines if initial configuration
      is implemented and if so how.  One of three possible choices is
      selected:

            minimum-secure,
            semi-secure,
            very-secure (i.e., no-initial-configuration)

      In the case of a very-secure posture, there is no initial
      configuration, and so the following steps are irrelevant.

2) one or more secrets

   These are the authentication/privacy secrets for the first user to be
   configured.

   One way to accomplish this is to have the installer enter a
   "password" for each required secret. The password is then
   algorithmically converted into the required secret by:

   - forming a string of length 1,048,576 octets by repeating the
     value of the password as often as necessary, truncating
     accordingly, and using the resulting string as the input to the MD5
     algorithm [MD5].  The resulting digest, termed "digest1", is used
     in the next step.

   - a second string is formed by concatenating digest1, the SNMP
     engine's snmpEngineID value, and digest1.  This string is used as
     input to the MD5 algorithm [MD5].

     The resulting digest is the required secret (see Appendix A.2).

   With these configured parameters, the SNMP engine instantiates the
   following usmUserEntry in the usmUserTable:

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                           no privacy support     privacy support
                           ------------------     ---------------
   usmUserEngineID         localEngineID          localEngineID
   usmUserName             "initial"              "initial"
   usmUserSecurityName     "initial"              "initial"
   usmUserCloneFrom        ZeroDotZero            ZeroDotZero
   usmUserAuthProtocol     usmHMACMD5AuthProtocol usmHMACMD5AuthProtocol
   usmUserAuthKeyChange    ""                     ""
   usmUserOwnAuthKeyChange ""                     ""
   usmUserPrivProtocol     none                   usmDESPrivProtocol
   usmUserPrivKeyChange    ""                     ""
   usmUserOwnPrivKeyChange ""                     ""
   usmUserPublic           ""                     ""
   usmUserStorageType      anyValidStorageType    anyValidStorageType
   usmUserStatus           active                 active

A.2.  Password to Key Algorithm

   A sample code fragment (section A.2.1) demonstrates the password to
   key algorithm which can be used when mapping a password to an
   authentication or privacy key using MD5. The reference source code of
   MD5 is available in [RFC1321].

   Another sample code fragment (section A.2.2) demonstrates the
   password to key algorithm which can be used when mapping a password
   to an authentication or privacy key using SHA (documented in SHA-
   NIST).

   An example of the results of a correct implementation is provided
   (section A.3) which an implementor can use to check if his
   implementation produces the same result.

A.2.1.  Password to Key Sample Code for MD5

void password_to_key_md5(
   u_char *password,    /* IN */
   u_int   passwordlen, /* IN */
   u_char *engineID,    /* IN  - pointer to snmpEngineID  */
   u_int   engineLength /* IN  - length of snmpEngineID */
   u_char *key)         /* OUT - pointer to caller 16-octet buffer */
{
   MD5_CTX     MD;
   u_char     *cp, password_buf[64];
   u_long      password_index = 0;
   u_long      count = 0, i;

   MD5Init (&MD);   /* initialize MD5 */

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   /**********************************************/
   /* Use while loop until we've done 1 Megabyte */
   /**********************************************/
   while (count < 1048576) {
      cp = password_buf;
      for (i = 0; i < 64; i++) {
          /*************************************************/
          /* Take the next octet of the password, wrapping */
          /* to the beginning of the password as necessary.*/
          /*************************************************/
          *cp++ = password[password_index++ % passwordlen];
      }
      MD5Update (&MD, password_buf, 64);
      count += 64;
   }
   MD5Final (key, &MD);          /* tell MD5 we're done */

   /*****************************************************/
   /* Now localize the key with the engineID and pass   */
   /* through MD5 to produce final key                  */
   /* May want to ensure that engineLength <= 32,       */
   /* otherwise need to use a buffer larger than 64     */
   /*****************************************************/
   memcpy(password_buf, key, 16);
   memcpy(password_buf+16, engineID, engineLength);
   memcpy(password_buf+engineLength, key, 16);

   MD5Init(&MD);
   MD5Update(&MD, password_buf, 32+engineLength);
   MD5Final(key, &MD);

   return;
}

A.2.2.  Password to Key Sample Code for SHA

void password_to_key_sha(
   u_char *password,    /* IN */
   u_int   passwordlen, /* IN */
   u_char *engineID,    /* IN  - pointer to snmpEngineID  */
   u_int   engineLength /* IN  - length of snmpEngineID */
   u_char *key)         /* OUT - pointer to caller 20-octet buffer */
{
   SHA_CTX     SH;
   u_char     *cp, password_buf[72];
   u_long      password_index = 0;
   u_long      count = 0, i;

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   SHAInit (&SH);   /* initialize SHA */

   /**********************************************/
   /* Use while loop until we've done 1 Megabyte */
   /**********************************************/
   while (count < 1048576) {
      cp = password_buf;
      for (i = 0; i < 64; i++) {
          /*************************************************/
          /* Take the next octet of the password, wrapping */
          /* to the beginning of the password as necessary.*/
          /*************************************************/
          *cp++ = password[password_index++ % passwordlen];
      }
      SHAUpdate (&SH, password_buf, 64);
      count += 64;
   }
   SHAFinal (key, &SH);          /* tell SHA we're done */

   /*****************************************************/
   /* Now localize the key with the engineID and pass   */
   /* through SHA to produce final key                  */
   /* May want to ensure that engineLength <= 32,       */
   /* otherwise need to use a buffer larger than 72     */
   /*****************************************************/
   memcpy(password_buf, key, 20);
   memcpy(password_buf+20, engineID, engineLength);
   memcpy(password_buf+engineLength, key, 20);

   SHAInit(&SH);
   SHAUpdate(&SH, password_buf, 40+engineLength);
   SHAFinal(key, &SH);

   return;
}

A.3.  Password to Key Sample Results

A.3.1.  Password to Key Sample Results using MD5

   The following shows a sample output of the password to key algorithm
   for a 16-octet key using MD5.

   With a password of "maplesyrup" the output of the password to key
   algorithm before the key is localized with the SNMP engine's
   snmpEngineID is:

      '9f af 32 83 88 4e 92 83 4e bc 98 47 d8 ed d9 63'H

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   After the intermediate key (shown above) is localized with the
   snmpEngineID value of:

      '00 00 00 00 00 00 00 00 00 00 00 02'H

   the final output of the password to key algorithm is:

      '52 6f 5e ed 9f cc e2 6f 89 64 c2 93 07 87 d8 2b'H

A.3.2.  Password to Key Sample Results using SHA

      The following shows a sample output of the password to key
      algorithm for a 20-octet key using SHA.

      With a password of "maplesyrup" the output of the password to key
      algorithm before the key is localized with the SNMP engine's
      snmpEngineID is:

      'f1 be a9 ae 66 7f 4f b6 34 1e 51 af 06 80 7e 91 e4 3b 01 ac'H

   After the intermediate key (shown above) is localized with the
   snmpEngineID value of:

      '00 00 00 00 00 00 00 00 00 00 00 02'H

   the final output of the password to key algorithm is:

      '8a a3 d9 9e 3e 30 56 f2 bf e3 a9 ee f3 45 d5 39 54 91 12 be'H

A.4.  Sample encoding of msgSecurityParameters

   The msgSecurityParameters in an SNMP message are represented as an
   OCTET STRING. This OCTET STRING should be considered opaque outside a
   specific Security Model.

   The User-based Security Model defines the contents of the OCTET
   STRING as a SEQUENCE (see section 2.4).

   Given these two properties, the following is an example of the
   msgSecurityParameters for the User-based Security Model, encoded as
   an OCTET STRING:

     04 <length>
     30 <length>
     04 <length> <msgAuthoritativeEngineID>
     02 <length> <msgAuthoritativeEngineBoots>
     02 <length> <msgAuthoritativeEngineTime>
     04 <length> <msgUserName>

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     04 0c       <HMAC-MD5-96-digest>
     04 08       <salt>

   Here is the example once more, but now with real values (except for
   the digest in msgAuthenticationParameters and the salt in
   msgPrivacyParameters, which depend on variable data that we have not
   defined here):

     Hex Data                         Description
     --------------  -----------------------------------------------
     04 39           OCTET STRING,                  length 57
     30 37           SEQUENCE,                      length 55
     04 0c 80000002  msgAuthoritativeEngineID:      IBM
           01                                       IPv4 address
           09840301                                 9.132.3.1
     02 01 01        msgAuthoritativeEngineBoots:   1
     02 02 0101      msgAuthoritativeEngineTime:    257
     04 04 62657274  msgUserName:                   bert
     04 0c 01234567  msgAuthenticationParameters:   sample value
           89abcdef
           fedcba98
     04 08 01234567  msgPrivacyParameters:          sample value
           89abcdef

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B.  Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.