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RFC 3414

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

Pages: 88
Internet Standard: 62
Errata
STD 62 is also:  3411341234133415341634173418
Obsoletes:  2574
Updated by:  5590
Part 4 of 4 – Pages 63 to 88
First   Prev   None

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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. - 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.
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   - 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. 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 to obtain the 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.
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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.
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
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   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 [RFC3411]). 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:
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   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.

   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) )
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   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 (from which the IV is computed as described in section 8.1.1.1). 2) The privParameters field is set to the serialization according to the rules in [RFC3417] of an OCTET STRING representing 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 [RFC3417] 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.
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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 (from which the IV is computed 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 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.
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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: Harald Tveit Alvestrand (Maxware) Dave Battle (SNMP Research, Inc.) Alan Beard (Disney Worldwide Services) Paul Berrevoets (SWI Systemware/Halcyon Inc.) Martin Bjorklund (Ericsson) Uri Blumenthal (IBM T.J. Watson Research Center) Jeff Case (SNMP Research, Inc.) John Curran (BBN) Mike Daniele (Compaq Computer Corporation)) T. Max Devlin (Eltrax Systems) John Flick (Hewlett Packard) Rob Frye (MCI) Wes Hardaker (U.C.Davis, Information Technology - D.C.A.S.) David Harrington (Cabletron Systems Inc.) Lauren Heintz (BMC Software, Inc.) N.C. Hien (IBM T.J. Watson Research Center) Michael Kirkham (InterWorking Labs, Inc.) Dave Levi (SNMP Research, Inc.) Louis A Mamakos (UUNET Technologies Inc.) Joe Marzot (Nortel Networks) Paul Meyer (Secure Computing Corporation) Keith McCloghrie (Cisco Systems) Bob Moore (IBM) Russ Mundy (TIS Labs at Network Associates) Bob Natale (ACE*COMM Corporation) Mike O'Dell (UUNET Technologies Inc.) Dave Perkins (DeskTalk) Peter Polkinghorne (Brunel University) Randy Presuhn (BMC Software, Inc.) David Reeder (TIS Labs at Network Associates) David Reid (SNMP Research, Inc.) Aleksey Romanov (Quality Quorum) Shawn Routhier (Epilogue) Juergen Schoenwaelder (TU Braunschweig) Bob Stewart (Cisco Systems) Mike Thatcher (Independent Consultant) Bert Wijnen (IBM T.J. Watson Research Center)
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   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)
      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 Class PDU for which there is no currently outstanding Confirmed Class PDU; for example for SNMPv2 [RFC3416] PDUs, the request-id component in the PDU can be used to correlate Responses to outstanding Requests.
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     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.

     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 [RFC3418], 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.
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   - The frequency with which the secrets of a User-based Security Model
     user should be changed is indirectly related to the frequency of
     their use.

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

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

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

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 16/20 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 16/20 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. Please note that longer passwords with repetitive strings may result in exactly the same key. For example, a password 'bertbert' will result in exactly the same key as password 'bertbertbert'.
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   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
   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.
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11.4. Use of Reports

The use of unsecure reports (i.e., sending them with a securityLevel of noAuthNoPriv) potentially exposes a non-authoritative SNMP engine to some form of attacks. Some people consider these denial of service attacks, others don't. An installation should evaluate the risk involved before deploying unsecure Report PDUs.

11.5 Access to the SNMP-USER-BASED-SM-MIB

The objects in this MIB may be considered sensitive in many environments. Specifically the objects in the usmUserTable contain information about users and their authentication and privacy protocols. It is important to closely control (both read and write) access to these MIB objects by using appropriately configured Access Control models (for example the View-based Access Control Model as specified in [RFC3415]).

12. References

12.1 Normative References

[RFC1321] Rivest, R., "Message Digest Algorithm MD5", RFC 1321, April 1992. [RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2578] McCloghrie, K., Perkins, D., Schoenwaelder, J., Case, J., Rose, M. and S. Waldbusser, "Structure of Management Information Version 2 (SMIv2)", STD 58, RFC 2578, April 1999. [RFC2579] McCloghrie, K., Perkins, D., Schoenwaelder, J., Case, J., Rose, M. and S. Waldbusser, "Textual Conventions for SMIv2", STD 58, RFC 2579, April 1999. [RFC2580] McCloghrie, K., Perkins, D., Schoenwaelder, J., Case, J., Rose, M. and S. Waldbusser, "Conformance Statements for SMIv2", STD 58, RFC 2580, April 1999.
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   [RFC3411]       Harrington, D., Presuhn, R. and B. Wijnen, "An
                   Architecture for Describing Simple Network Management
                   Protocol (SNMP) Management Frameworks", STD 62, RFC
                   3411, December 2002.

   [RFC3412]       Case, J., Harrington, D., Presuhn, R. and B. Wijnen,
                   "Message Processing and Dispatching for the Simple
                   Network Management Protocol (SNMP)", STD 62, RFC
                   3412, December 2002.

   [RFC3415]       Wijnen, B., Presuhn, R. and K. McCloghrie, "View-
                   based Access Control Model (VACM) for the Simple
                   Network Management Protocol (SNMP)", STD 62, RFC
                   3415, December 2002.

   [RFC3416]       Presuhn, R., Case, J., McCloghrie, K., Rose, M. and
                   S. Waldbusser, "Version 2 of the Protocol Operations
                   for the Simple Network Management Protocol (SNMP)",
                   STD 62, RFC 3416, December 2002.

   [RFC3417]       Presuhn, R., Case, J., McCloghrie, K., Rose, M. and
                   S.  Waldbusser, "Transport Mappings for the Simple
                   Network Management Protocol (SNMP)", STD 62, RFC
                   3417, December 2002.

   [RFC3418]       Presuhn, R., Case, J., McCloghrie, K., Rose, M. and
                   S. Waldbusser, "Management Information Base (MIB) for
                   the Simple Network Management Protocol (SNMP)", STD
                   62, RFC 3418, December 2002.

   [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).

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

[Localized-Key] U. Blumenthal, N. C. Hien, B. Wijnen "Key Derivation for Network Management Applications" IEEE Network Magazine, April/May issue, 1997. [DES-ANSI] Data Encryption Algorithm, American National Standards Institute. ANSI X3.92-1981, (December, 1980). [DESO-ANSI] Data Encryption Algorithm - Modes of Operation, American National Standards Institute. ANSI X3.106- 1983, (May 1983). [DESG-NIST] Guidelines for Implementing and Using the NBS Data Encryption Standard, National Institute of Standards and Technology. Federal Information Processing Standard (FIPS) Publication 74, (April, 1981). [DEST-NIST] Validating the Correctness of Hardware Implementations of the NBS Data Encryption Standard, National Institute of Standards and Technology. Special Publication 500-20. [DESM-NIST] Maintenance Testing for the Data Encryption Standard, National Institute of Standards and Technology. Special Publication 500-61, (August, 1980). [RFC3174] Eastlake, D. 3rd and P. Jones, "US Secure Hash Algorithm 1 (SHA1)", RFC 3174, September 2001.
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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 [RFC1321]. 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 [RFC1321]. The resulting digest is the required secret (see Appendix A.2).
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      With these configured parameters, the SNMP engine instantiates the
      following usmUserEntry in the usmUserTable:

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

      It is recommended to also instantiate a set of template
      usmUserEntries which can be used as clone-from users for newly
      created usmUserEntries.  These are the two suggested entries:

                           no privacy support     privacy support
                           ------------------     ---------------
   usmUserEngineID         localEngineID          localEngineID
   usmUserName             "templateMD5"          "templateMD5"
   usmUserSecurityName     "templateMD5"          "templateMD5"
   usmUserCloneFrom        ZeroDotZero            ZeroDotZero
   usmUserAuthProtocol     usmHMACMD5AuthProtocol usmHMACMD5AuthProtocol
   usmUserAuthKeyChange    ""                     ""
   usmUserOwnAuthKeyChange ""                     ""
   usmUserPrivProtocol     none                   usmDESPrivProtocol
   usmUserPrivKeyChange    ""                     ""
   usmUserOwnPrivKeyChange ""                     ""
   usmUserPublic           ""                     ""
   usmUserStorageType      permanent              permanent
   usmUserStatus           active                 active
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                           no privacy support     privacy support
                           ------------------     ---------------
   usmUserEngineID         localEngineID          localEngineID
   usmUserName             "templateSHA"          "templateSHA"
   usmUserSecurityName     "templateSHA"          "templateSHA"
   usmUserCloneFrom        ZeroDotZero            ZeroDotZero
   usmUserAuthProtocol     usmHMACSHAAuthProtocol usmHMACSHAAuthProtocol
   usmUserAuthKeyChange    ""                     ""
   usmUserOwnAuthKeyChange ""                     ""
   usmUserPrivProtocol     none                   usmDESPrivProtocol
   usmUserPrivKeyChange    ""                     ""
   usmUserOwnPrivKeyChange ""                     ""
   usmUserPublic           ""                     ""
   usmUserStorageType      permanent              permanent
   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.
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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 */ /**********************************************/ /* 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+16+engineLength, key, 16); MD5Init(&MD); MD5Update(&MD, password_buf, 32+engineLength); MD5Final(key, &MD); return; }
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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; 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+20+engineLength, key, 20); SHAInit(&SH); SHAUpdate(&SH, password_buf, 40+engineLength); SHAFinal(key, &SH); return; }
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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 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: '9f b5 cc 03 81 49 7b 37 93 52 89 39 ff 78 8d 5d 79 14 52 11'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: '66 95 fe bc 92 88 e3 62 82 23 5f c7 15 1f 12 84 97 b3 8f 3f'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.
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   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 they
   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>
      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

A.5. Sample keyChange Results

A.5.1. Sample keyChange Results using MD5

Let us assume that a user has a current password of "maplesyrup" as in section A.3.1. and let us also assume the snmpEngineID of 12 octets: '00 00 00 00 00 00 00 00 00 00 00 02'H
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   If we now want to change the password to "newsyrup", then we first
   calculate the key for the new password.  It is as follows:

      '01 ad d2 73 10 7c 4e 59 6b 4b 00 f8 2b 1d 42 a7'H

   If we localize it for the above snmpEngineID, then the localized new
   key becomes:

      '87 02 1d 7b d9 d1 01 ba 05 ea 6e 3b f9 d9 bd 4a'H

   If we then use a (not so good, but easy to test) random value of:

      '00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00'H

   Then the value we must send for keyChange is:

      '00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
       88 05 61 51 41 67 6c c9 19 61 74 e7 42 a3 25 51'H

   If this were for the privacy key, then it would be exactly the same.

A.5.2. Sample keyChange Results using SHA

Let us assume that a user has a current password of "maplesyrup" as in section A.3.2. and let us also assume the snmpEngineID of 12 octets: '00 00 00 00 00 00 00 00 00 00 00 02'H If we now want to change the password to "newsyrup", then we first calculate the key for the new password. It is as follows: '3a 51 a6 d7 36 aa 34 7b 83 dc 4a 87 e3 e5 5e e4 d6 98 ac 71'H If we localize it for the above snmpEngineID, then the localized new key becomes: '78 e2 dc ce 79 d5 94 03 b5 8c 1b ba a5 bf f4 63 91 f1 cd 25'H If we then use a (not so good, but easy to test) random value of: '00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00'H Then the value we must send for keyChange is: '00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 9c 10 17 f4 fd 48 3d 2d e8 d5 fa db f8 43 92 cb 06 45 70 51'
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   For the key used for privacy, the new nonlocalized key would be:

      '3a 51 a6 d7 36 aa 34 7b 83 dc 4a 87 e3 e5 5e e4 d6 98 ac 71'H

   For the key used for privacy, the new localized key would be (note
   that they localized key gets truncated to 16 octets for DES):

      '78 e2 dc ce 79 d5 94 03 b5 8c 1b ba a5 bf f4 63'H

   If we then use a (not so good, but easy to test) random value of:

      '00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00'H

   Then the value we must send for keyChange for the privacy key is:

      '00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
      '7e f8 d8 a4 c9 cd b2 6b 47 59 1c d8 52 ff 88 b5'H

B. Change Log

Changes made since RFC2574: - Updated references - Updated contact info - Clarifications - to first constraint item 1) on page 6. - to usmUserCloneFrom DESCRIPTION clause - to securityName in section 2.1 - Fixed "command responder" into "command generator" in last para of DESCRIPTION clause of usmUserTable. Changes made since RFC2274: - Fixed msgUserName to allow size of zero and explain that this can be used for snmpEngineID discovery. - Clarified section 3.1 steps 4.b, 5, 6 and 8.b. - Clarified section 3.2 paragraph 2. - Clarified section 3.2 step 7.a last paragraph, step 7.b.1 second bullet and step 7.b.2 third bullet. - Clarified section 4 to indicate that discovery can use a userName of zero length in unAuthenticated messages, whereas a valid userName must be used in authenticated messages. - Added REVISION clauses to MODULE-IDENTITY - Clarified KeyChange TC by adding a note that localized keys must be used when calculating a KeyChange value. - Added clarifying text to the DESCRIPTION clause of usmUserTable. Added text describes a recommended procedure for adding a new user. - Clarified the use of usmUserCloneFrom object.
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   - Clarified how and under which conditions the usmUserAuthProtocol
     and usmUserPrivProtocol can be initialized and/or changed.
   - Added comment on typical sizes for usmUserAuthKeyChange and
     usmUserPrivKeyChange.  Also for usmUserOwnAuthKeyChange and
     usmUserOwnPrivKeyChange.
   - Added clarifications to the DESCRIPTION clauses of
     usmUserAuthKeyChange, usmUserOwnAuthKeychange, usmUserPrivKeyChange
     and usmUserOwnPrivKeychange.
   - Added clarification to DESCRIPTION clause of usmUserStorageType.
   - Added clarification to DESCRIPTION clause of usmUserStatus.
   - Clarified IV generation procedure in section 8.1.1.1 and in
     addition clarified section 8.3.1 step 1 and section 8.3.2. step 3.
   - Clarified section 11.2 and added a warning that different size
     passwords with repetitive strings may result in same key.
   - Added template users to appendix A for cloning process.
   - Fixed C-code examples in Appendix A.
   - Fixed examples of generated keys in Appendix A.
   - Added examples of KeyChange values to Appendix A.
   - Used PDU Classes instead of RFC1905 PDU types.
   - Added text in the security section about Reports and Access Control
     to the MIB.
   - Removed a incorrect note at the end of section 3.2 step 7.
   - Added a note in section 3.2 step 3.
   - Corrected various spelling errors and typos.
   - Corrected procedure for 3.2 step 2.a)
   - various clarifications.
   - Fixed references to new/revised documents
   - Change to no longer cache data that is not used

Editors' Addresses

Uri Blumenthal Lucent Technologies 67 Whippany Rd. Whippany, NJ 07981 USA Phone: +1-973-386-2163 EMail: uri@lucent.com Bert Wijnen Lucent Technologies Schagen 33 3461 GL Linschoten Netherlands Phone: +31-348-480-685 EMail: bwijnen@lucent.com
Top   ToC   RFC3414 - Page 88
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