Internet Draft U. Blumenthal draft-blumenthal-aes-usm-05.txt Lucent Technologies Expires: August 2003 F. Maino Andiamo Systems, Inc. K. McCloghrie Cisco Systems, Inc. February 2003 The AES Cipher Algorithm in the SNMP User-based Security Model Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract This document describes a symmetric encryption protocol that supplement the protocols described in the User-based Security Model (USM), which is a Security Subsystem for version 3 of the Simple Network Management Protocol for use in the SNMP Architecture. The symmetric encryption protocol described in this document is based on the AES cipher algorithm, used in Cipher FeedBack Mode (CFB), with key size of 128 bits. Table of Contents 1. Introduction....................................................2 1.1. Goals and Constraints......................................2 1.2. Key Localization...........................................3 1.3. Password Entropy and Storage...............................3 2. Definitions.....................................................3 Blumenthal/Maino/McCloghrie Expires August 2003 [Page 1]
^L Internet Draft AES in the SNMP's USM February 2003 3. CFB128-AES-128 Symmetric Encryption Protocol....................4 3.1. Mechanisms.................................................4 3.1.1. The AES-based Symmetric Encryption Protocol...........4 3.1.2. Localized Key, AES Encryption Key and Initialization Vector.......................................................5 3.1.3. Data Encryption.......................................6 3.1.4. Data Decryption.......................................6 3.2. Elements of the AES Privacy Protocol.......................7 3.2.1. Users.................................................7 3.2.2. msgAuthoritativeEngineID..............................7 3.2.3. SNMP Messages Using this Privacy Protocol.............7 3.2.4. Services provided by the AES Privacy Modules..........7 3.3. Elements of Procedure......................................9 3.3.1. Processing an Outgoing Message........................9 3.3.2. Processing an Incoming Message........................9 4. Security Considerations........................................10 5. Intellectual Property Rights Statement.........................10 6. Acknowledgements...............................................10 7. References.....................................................10 8. Authors' Addresses.............................................11 1. Introduction Within the Architecture for describing Internet Management Frameworks [RFC3411], the User-based Security Model (USM) [RFC3414] for SNMPv3 is defined as a Security Subsystem within an SNMP engine. RFC 3414 describes the use of HMAC-MD5-96 and HMAC-SHA-96 as the (initial) authentication protocols and the use of CBC-DES as the (initial) privacy protocol. The User-based Security Model however allows for other such protocols to be used instead of or concurrently with these protocols. This memo describes the use of CFB128-AES-128 as an alternative privacy protocol for the User-based Security Model. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 1.1 Goals and Constraints The main goal of this memo is to provide a new privacy protocol for USM based on the Advanced Encryption Standard. The major constraint is to maintain a complete interchangeability of the new protocol defined on this memo with existing authentication and privacy protocols already defined in USM. For a given user, the AES-based privacy protocol MAY be used with the authentication protocols described in [RFC3414]. Blumenthal/Maino/McCloghrie Expires August 2003 [Page 2]
^L Internet Draft AES in the SNMP's USM February 2003 1.2 Key Localization As defined in [RFC3414], a localized key is a secret key shared between a user U and one authoritative SNMP engine E. Even though a user may have only one pair of authentication and privacy passwords (and consequently only one pair of keys) for the whole network, the actual secrets shared between the user and each authoritative SNMP engine will be different. This is achieved by key localization. If the authentication protocol defined for a user U at the authoritative SNMP engine E is one of the authentication protocols defined on [RFC3414], the key localization is performed according to the two steps process described in section 2.6 of [RFC3414]. 1.3 Password Entropy and Storage The security of various cryptographic functions lies both in the strength of the functions themselves against various forms of attack, and also, perhaps more importantly, in the keying material that is used with them. While theoretical attacks against the cryptographic functions specified by this document are possible, it is vastly more probable that key guessing is the main threat. The following can be suggested with regard to the user password: - Passwords lengths SHOULD be at least 12 bytes. - Password sharing SHOULD be limited so that passwords aren't shared among multiple SNMP users. It is worth to remember that, as specified in [RFC3414], 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. 2. Definitions An OID for the privacy algorithm is added, to be included in the snmpUsmMIB. usmAesCfb128Protocol OBJECT-IDENTITY STATUS current DESCRIPTION "The CFB128-AES-128 Privacy Protocol." REFERENCE "- Specification for the ADVANCED ENCRYPTION STANDARD (DRAFT). Federal Information Processing Standard (FIPS) Publication 197. (November 2001). Blumenthal/Maino/McCloghrie Expires August 2003 [Page 3]
^L Internet Draft AES in the SNMP's USM February 2003 - Dworkin, M., NIST Recommendation for Block Cipher Modes of Operation, Methods and Techniques (DRAFT). NIST Special Publication 800-38A (December 2001). " ::= { snmpPrivProtocols n } -- to be assigned by IANA 3. CFB128-AES-128 Symmetric Encryption Protocol This section describes a Symmetric Encryption Protocol based on the AES Cipher Algorithm [FIPS-AES], used in Cipher Feedback Mode as described in [AES-MODE], using encryption keys with a size of 128 bits. This protocol is identified by usmAesCfb128PrivProtocol. The protocol usmAesCfb128PrivProtocol is an alternative to the privacy protocol defined in [RFC3414]. 3.1 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 and a 64-bit integer 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. 3.1.1 The AES-based 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. The AES (Advanced Encryption Standard) is the symmetric cipher algorithm that the NIST (National Institute of Standards and Technology) has selected in a four-year competitive process. The AES homepage, http://www.nist.gov/aes, contains a wealth of information on AES including the Federal Information Processing Standard [FIPS-AES] that will finally specify the Advanced Encryption Standard. The following subsections contain description of the relevant characteristics of the AES ciphers used in the symmetric encryption protocol described in this memo. Blumenthal/Maino/McCloghrie Expires August 2003 [Page 4]
^L Internet Draft AES in the SNMP's USM February 2003 3.1.1.1 Mode of operation The NIST Special Publication 800-38A [AES-MODE] recommends five confidentiality modes of operation for use with AES: Electronic Codebook (ECB), Cipher Block Chaining (CBC), Cipher Feedback (CFB), Output Feedback (OFB), and Counter (CTR). The symmetric encryption protocol described in this memo use AES in CFB mode with the parameter s set to 128 according to the definition of CFB mode given in [AES-MODE]. This mode requires an Initialization Vector (IV) that is the same size as the block size of the cipher algorithm. 3.1.1.2 Key Size In the encryption protocol described by this memo AES is used with key size of 128 bits. 3.1.1.3 Block Size and Padding The block size of the AES cipher algorithms used in the encryption protocol described by this memo is 128 bits. 3.1.1.4 Rounds This parameter determines how many times a block is encrypted. The encryption protocol described on this memo uses 10 rounds. 3.1.2 Localized Key, AES Encryption Key and Initialization Vector The size of the Localized Key (Kul) of an SNMP user, as described in [RFC3414], depends on the authentication protocol defined for that user U at the authoritative SNMP engine E. The encryption protocol defined on this memo MUST be used with an authentication protocol that generates a localized key with of at least 128 bits. The authentication protocols described in [RFC3414] satisfy the requirement above. 3.1.2.1 AES Encryption Key and IV The first 128 bits of the localized key Kul are used as the AES encryption key. The 128-bit IV is obtained as the concatenation of the generating SNMP engine's 32-bit snmpEngineBoots, the SNMP engine's 32-bit snmpEngineTime, and a local 64-bit integer. The 64-bit integer is initialized to a pseudo-random value at boot time. The IV is concatenated as follows: the 32-bit snmpEngineBoots is converted to the first 4 octets (Most Significant Byte first), the 32-bit snmpEngineTime is converted to the subsequent 4 octets (Most Significant Byte first), and the 64-bit integer is then converted to the last 8 octets (Most Significant Byte first). Blumenthal/Maino/McCloghrie Expires August 2003 [Page 5]
^L Internet Draft AES in the SNMP's USM February 2003 The 64-bit integer is then put into the msgPrivacyParameters field encoded as an OCTET STRING of length 8 octets. The integer is then modified for the subsequent message. We recommend that it is incremented by one until it reaches its maximum value at which time it is wrapped. An implementation can use any method to vary the value of the IV providing the chosen method never generates duplicate IV for the same key. The 64-bit integer must be placed in the privParameters field to enable the receiving entity to compute the correct IV and to decrypt the message. This 64-bit value is called "salt" in this document. See RFC 3414. 3.1.3 Data Encryption. The data to be encrypted is treated as sequence of octets. The data is encrypted in Cipher Feedback mode with the parameter s set to 128 according to the definition of CFB mode given in [AES- MODE]. The plaintext is divided into 128-bit blocks. The last block may have less than 128 bits, and no padding is required. The first input block is the IV, and the forward cipher operation is applied to the IV to produce the first output block. The first ciphertext block is produced by exclusive-ORing the first plaintext block with the first output block. The ciphertext block is also used as the input block for the subsequent forward cipher operation. The process is repeated with the successive input blocks until a ciphertext segment is produced from every plaintext segment. The last ciphertext block is produced by exclusive-ORing the last plaintext segment of r bits (r is less or equal to 128) with the segment of the r most significant bits of the last output block. 3.1.4 Data Decryption In CFB decryption, the IV is the first input block, the first ciphertext is used for the second input block, the second ciphertext is used for the third input block, etc. The forward cipher function is applied to each input block to produce the output blocks. The output blocks are exclusive-ORed with the corresponding ciphertext blocks to recover the plaintext blocks. The last ciphertext block (whose size r is less or equal to 128) is exclusive-ORed with the segment of the r most significant bits of the last output block to recover the last plaintext block of r bits. Blumenthal/Maino/McCloghrie Expires August 2003 [Page 6]
^L Internet Draft AES in the SNMP's USM February 2003 3.2 Elements of the AES Privacy Protocol This section contains definitions required to realize the privacy modules defined by this memo. 3.2.1 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 the AES key. The length of this key MUST be 128 bits (16 octets). 3.2.2 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 [RFC3414]). 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. 3.2.3 SNMP Messages Using this Privacy Protocol Messages using this privacy protocol carry a msgPrivacyParameters field as part of the msgSecurityParameters. For this protocol, the privParameters field is the serialized OCTET STRING representing the "salt" that was used to create the IV. 3.2.4 Services provided by the AES Privacy Modules This section describes the inputs and outputs that the AES Privacy modules expects and produces when the User-based Security module invokes one of the AES Privacy modules for services. 3.2.4.1 Services for Encrypting Outgoing Data The AES 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. Blumenthal/Maino/McCloghrie Expires August 2003 [Page 7]
^L Internet Draft AES in the SNMP's USM February 2003 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 or 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. 3.2.4.2 Services for Decrypting Incoming Data This AES 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. Blumenthal/Maino/McCloghrie Expires August 2003 [Page 8]
^L Internet Draft AES in the SNMP's USM February 2003 decryptKey The secret key to be used by the decryption algorithm. The length of this key MUST be 16 octets. privParameters The 64-bit integer to be used to calculate the IV. encryptedData The data to be decrypted. decryptedData The decrypted data. 3.3 Elements of Procedure. This section describes the procedures for the AES privacy protocol. 3.3.1 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 usmAesCfb128PrivProtocol. 1)The secret cryptKey is used to construct the AES encryption key, as described in section 3.1.2.1. 2)The privParameters field is set to the serialization according to the rules in [RFC3417] of an OCTET STRING representing the 64- bit integer that will be used in the IV as described in [RFC3414]. 3)The scopedPDU is encrypted (as described in section 3.1.3) 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. 3.3.2 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 usmAesCfb128PrivProtocol. 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 64-bit integer is extracted from the privParameters field. 3)The secret cryptKey and the 64-bit integer are then used to construct the AES decryption key and the IV that is computed as described in section 3.1.2.2. Blumenthal/Maino/McCloghrie Expires August 2003 [Page 9]
^L Internet Draft AES in the SNMP's USM February 2003 4)The encryptedPDU is then decrypted (as described in section 3.1.4). 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. 4. Security Considerations For information regarding the necessary use of random IV values, see [CRYPTO-B]. This algorithm must be used with an authentication and/or integrity protection algorithm (including but not limited to those defined in RFC 3414), because CFB encryption mode does not detect ciphertext modifications. For further security considerations, the reader is encouraged to read the documents that describe the actual cipher algorithms. 5. Intellectual Property Rights Statement The authors made no IPR claims on the contents of this document or the algorithms defined in it. 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 implementers 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. 6. Acknowledgements Portions of this text, as well as its general structure, were unabashedly lifted from [RFC3414]. Blumenthal/Maino/McCloghrie Expires August 2003 [Page 10]
^L Internet Draft AES in the SNMP's USM February 2003 7. References Normative References [AES-MODE] Dworkin, M., "NIST Recommendation for Block Cipher Modes of Operation, Methods and Techniques", NIST Special Publication 800-38A, December 2001. [FIPS-AES] "Specification for the ADAVANCED ENCRYPTION STANDARD (AES)", Federal Information Processing Standard (FIPS) Publication 197, November 2001. [RFC2104] Bellare, M., Canetti, R., Krawczyk, H., "HMAC: Keyed- Hashing for Message Authentication", RFC2104, 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. [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. [RFC3414] Blumenthal, U. and B. Wijnen, "User-based Security Model(USM) for version 3 of the Simple Network Management Protocol (SNMPv3)", STD 62, RFC 3414, 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. Informative References [CRYPTO-B] Bellovin, S., "Probable Plaintext Cryptanalysis of the IP Security Protocols", Proceedings of the Symposium on Network and Distributed System Security, San Diego, CA, pp. 155-160, February 1997. Blumenthal/Maino/McCloghrie Expires August 2003 [Page 11]
^L Internet Draft AES in the SNMP's USM February 2003 8. Authors' Addresses Uri Blumenthal Lucent Technologies / Bell Labs 67 Whippany Rd. Phone: +1-973-386-2163 14D-318 Email: uri@bell-labs.com Whippany, NJ 07981, USA Fabio Maino Andiamo Systems, Inc. 375 East Tasman Drive Phone: +1-408-853-7530 San Jose, CA. 95134 USA Email: fmaino@andiamo.com Keith McCloghrie Cisco Systems, Inc. 170 East Tasman Drive Phone: +1-408-526-5260 San Jose, CA. 95134-1706 USA Email: kzm@cisco.com Full Copyright Statement Copyright (C) The Internet Society (2003). 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. Blumenthal/Maino/McCloghrie Expires August 2003 [Page 12]
