Internet Draft U. Blumenthal
draft-blumenthal-aes-usm-04.txt Lucent Technologies
Expires: April 2003 F. Maino
Andiamo Systems, Inc.
K. McCloghrie
Cisco Systems, Inc.
October 2002
The AES Cipher Algorithm in the SNMP's 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 [RFC2026].
Internet-Drafts are working documents of the Internet Engineering
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Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document describes a set of symmetric encryption protocols that
supplement the protocols described in the User-based Security Model
(USM) [RFC2574], which is a Security Subsystem for version 3 of the
Simple Network Management Protocol for use in the SNMP Architecture
[RFC2571]. The symmetric encryption protocols described in this
document are based on the AES cipher algorithm [FIPS-AES], used in
Cipher FeedBack Mode (CFB), with key size of 128 (mandated), 192,
and 256 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
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2. Definitions.....................................................3
3. CFB128-AES-128/192/256 Symmetric Encryption Protocols...........5
3.1. Mechanisms.................................................5
3.1.1. The AES-based Symmetric Encryption Protocols..........6
3.1.2. Localized Key, AES Encryption Key and Initialization
Vector.......................................................7
3.1.3. Data Encryption.......................................8
3.1.4. Data Decryption.......................................8
3.2. Elements of the AES Privacy Protocols......................9
3.2.1. Users.................................................9
3.2.2. msgAuthoritativeEngineID..............................9
3.2.3. SNMP Messages Using this Privacy Protocol.............9
3.2.4. Services provided by the AES Privacy Modules..........9
3.3. Elements of Procedure.....................................11
3.3.1. Processing an Outgoing Message.......................11
3.3.2. Processing an Incoming Message.......................11
4. Security Considerations........................................12
5. Intellectual Property Rights Statement.........................12
6. Acknowledgements...............................................13
7. References.....................................................13
8. Authors Addresses..............................................13
Appendix A........................................................14
A.1.Sample Results of Extension of Localized Keys..............14
1.Introduction
Within the Architecture for describing Internet Management
Frameworks [RFC2571], the User-based Security Model (USM) [RFC2574]
for SNMPv3 is defined as a Security Subsystem within an SNMP engine.
[RFC2574] 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-128/192/256 as three
alternative privacy protocols for the User-based Security Model.
This memo describes also the Key Localization Algorithm for use with
the new authentication protocol.
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 goals of this memo are as follows.
1)Provide a set of new privacy protocols for USM based on the
Advanced Encryption Standard.
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2)Provide a key localization mechanism that generates an adequate
amount of key material for the privacy protocols.
The major constraint is to maintain a complete interchangeability of
the new protocols defined on this memo with existing authentication
and privacy protocols already defined in USM.
For a given user, the AES-based privacy protocols MAY be used with
the authentication protocols described in [RFC2574].
1.2.Key Localization
As defined in [RFC2574] 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 [RFC2574], the key localization is performed according to
the two steps process described in section 2.6 of [RFC2574].
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.
Password SHOULD be changed at least every 90 days.
It worth to remember that, as specified in [RFC2574], 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
SNMP-USM-AES-MIB DEFINITIONS ::= BEGIN
IMPORTS
MODULE-IDENTITY, OBJECT-IDENTITY FROM SNMPv2-SMI
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xxx FROM XXX-MIB;
snmpUsmAesMIB MODULE-IDENTITY
LAST-UPDATED "200206300000Z"
ORGANIZATION "???"
CONTACT-INFO "Uri Blumenthal
Lucent Technologies / Bell Labs
67 Whippany Rd.
14D-318
Whippany, NJ 07981, USA
973-386-2163
uri@bell-labs.com
Fabio Maino
Andiamo Systems, Inc.
375 East Tasman Drive
San Jose, CA 95134, USA
408-853-7530
fmaino@andiamo.com
Keith McCloghrie
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134-1706, USA
408-526-5260
kzm@cisco.com"
DESCRIPTION "Definitions of Object Identities needed for
the use of AES by SNMP's User-based Security
Model."
REVISION "200110120000Z"
DESCRIPTION "Initial version, published as RFCnnnn"
::= { xxx nn } -- to be assigned by TBD
snmpUsmAesProtocols OBJECT IDENTIFIER ::= { snmpUsmAesMIB 1 }
-- Identification of Privacy Protocols
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).
- Dworkin, M., NIST Recommendation for Block
Cipher Modes of Operation, Methods and
Techniques (DRAFT).
NIST Special Publication 800-38A
(December 2001).
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"
::= { snmpUsmAesProtocols 2 }
usmAesCfb192Protocol OBJECT-IDENTITY
STATUS current
DESCRIPTION "The CFB128-AES-192 Privacy Protocol."
REFERENCE "- Specification for the ADVANCED ENCRYPTION
STANDARD (DRAFT). Federal Information Processing
Standard (FIPS) Publication 197.
(November 2001).
- Dworkin, M., NIST Recommendation for Block
Cipher Modes of Operation, Methods and
Techniques (DRAFT).
NIST Special Publication 800-38A
(December 2001).
"
::= { snmpUsmAesProtocols 3 }
usmAesCfb256Protocol OBJECT-IDENTITY
STATUS current
DESCRIPTION "The CFB128-AES-256 Privacy Protocol."
REFERENCE "- Specification for the ADVANCED ENCRYPTION
STANDARD (DRAFT). Federal Information Processing
Standard (FIPS) Publication 197
(November 2001).
- Dworkin, M., NIST Recommendation for Block
Cipher Modes of Operation, Methods and
Techniques (DRAFT).
NIST Special Publication 800-38A
(December 2001).
"
::= { snmpUsmAesProtocols 4 }
END
3.CFB128-AES-128/192/256 Symmetric Encryption Protocols
This section describes three Symmetric Encryption Protocols 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,
192, and 256 bits.
These protocols are identified by:
-usmAesCfb128PrivProtocol;
-usmAesCfb192PrivProtocol;
-usmAesCfb256PrivProtocol;
These protocols are alternatives to the privacy protocol defined in
[RFC2574].
3.1.Mechanisms
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- 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 Protocols
The Symmetric Encryption Protocols defined in this memo provide
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
protocols described in this memo.
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 protocols 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 a 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 protocols described by this memo AES is used with
key sizes of 128, 192, and 256 bits.
3.1.1.3.Block Size and Padding
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The block size of the AES cipher algorithms used in the encryption
protocols described by this memo is 128 bits.
3.1.1.4.Rounds
This parameter determines how many times a block is encrypted. The
encryption protocols described on this memo use:
-10 rounds for AES-128;
-12 rounds for AES-192;
-14 rounds for AES-256
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
[RFC2574], depends on the authentication protocol defined for that
user U at the authoritative SNMP engine E.
3.1.2.1.Short Localized Keys
The encryption protocols defined on this memo SHOULD be used with an
authentication protocol that generates a localized key with enough
key material to derive a 128/192/256 bits encryption key. At the
time of this writing an authentication protocol with such
characteristics has not been defined within the USM model for the
SNMPv3 architecture.
However, if the size of the localized key is not large enough to
generate an encryption key the following algorithm is applied to
extend the localized key:
1)Let Hnnn() the hash function of the authentication protocol for
the user U on the SNMP authoritative engine E. nnn being the size
of the output of the hash function (e.g. nnn=128 bits for MD5, or
nnn=160 bits for SHA1).
2)Set c = ceil ( 256 / nnn )
3)For i = 1, 2, ..., c
a.Set Kul = Kul || Hnnn(Kul); Where Hnnn() is the hash
function of the authentication protocol defined for that user
As an example if the user authentication protocol is HMAC-SHA1-96,
the hash function Hnnn is SHA1 with nnn=160 bits. The algorithm will
generate a localized key 480-bit long:
Kul' = Kul || SHA1(Kul) || SHA1(Kul||SHA1(Kul))
3.1.2.2.AES Encryption Key and IV
The first 128/192/256 bits of the localized key Kul are used as the
AES encryption key, according to the AES cipher algorithm key size
of the encryption protocol used.
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.
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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).
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 be
incremented by one and wrap when it reaches the maximum value.
How exactly the value of the IV varies is an implementation issue,
as long as measures are taken to avoid producing a duplicate IV.
The 64-bit integer must be placed in the msgPrivacyParameters field
to enable the receiving entity to compute the correct IV and to
decrypt the message.
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.
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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.
3.2.Elements of the AES Privacy Protocols
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) for AES-128
- 192 bits (24 octets) for AES-192
- 254 bits (32 octets) for AES-256
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 [RFC2571]).
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
msgPrivacyParameters 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.
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3.2.4.1.Services for Encrypting Outgoing Data
The AES privacy protocols assume 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 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 16/24/32 octets for AES
128/192/256.
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)
)
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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 16/24/32 octets for AES
128/192/256.
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 protocols.
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
usmAesCfbxxxPrivProtocol (where xxx can be any of 128, 192, or 256).
1)The secret cryptKey is used to construct the AES encryption key,
as described in section .
2)The privParameters field is set to the serialization according to
the rules in [RFC1906] of an OCTET STRING representing the 64-
bit integer that will be used in the IV as described in
3)The scopedPDU is encrypted (as described in section ) 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.
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
usmAesCfbxxxPrivProtocol (where xxx can be any of 128, 192, or 256).
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.
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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.
4)The encryptedPDU is then decrypted (as described in section ).
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
Implementations are encouraged to use the largest key sizes they can
when taking into account performance considerations for their
particular hardware and software configuration. However, a key size
of 128 bits is considered secure for the foreseeable future.
At the recommendation of cryptographic experts, we will recommend
that the IESG include usmAesCfb128PrivProtocol within the default
and mandatory-to-implement authentication and privacy algorithms for
USM.
For more information regarding the necessary use of random IV
values, see [CRYPTO-B].
For further security considerations, the reader is encouraged to
read the documents that describe the actual cipher algorithms.
5.Intellectual Property Rights Statement
Pursuant to the provisions of [RFC2026], the authors represent that
they have disclosed the existence of any proprietary or intellectual
property rights in the contribution that are reasonably and
personally known to the authors. The authors do not represent that
they personally know of all potentially pertinent proprietary and
intellectual property rights owned or claimed by the organizations
they represent or third parties.
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.
Blumenthal/Maino/McCloghrie Expires April 2003 [Page 12]
6.Acknowledgements
Portions of this text, as well as its general structure, were
unabashedly lifted from [RFC2574].
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.
[PKCS-12] "PKCS 12 v1.0: personal Information Exchange Syntax",
RSA Laboratories, June 1999.
[RFC1906] Case, J., McCloghrie, K., Rose, M., Waldbusser, S.,
"Transport Mappings for Version 2 of the Simple Network
Management Protocol (SNMPv2)", RFC1906, January 1996.
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", RFC2026, October 1996.
[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", RFC2119, March 1997.
[RFC2574] Blumenthal, U., Wijnen, B., "User-based Security Model
(USM) for version 3 of the Simple Network Management
Protocol (SNMPv3)",.RFC2574, April 1999.
[RFC2571] Wijnen, B., Harrington, D., Presuhn, R., "An
Architecture for Describing SNMP Management Frameworks",
RFC2571, April 1999.
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.
8.Authors Addresses
Uri Blumenthal
Lucent Technologies / Bell Labs
67 Whippany Rd. Phone: +1-973-386-2163
Blumenthal/Maino/McCloghrie Expires April 2003 [Page 13]
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
Appendix A
A.1.Sample Results of Extension of Localized Keys
The following shows a sample output of the algorithm that would be
used to extend a 160-bit localized key generated with SHA, to a 256-
bit localized key (e.g. to have enough key material to generate a
256-bit privKey for the usmAesCfb256PrivProtocol.
Let's assume that the user U has a password of "maplesyrup" and that
the key has been localized using SHA for the SNMP engine whose
snmpEngineID is:
'00000000 00000000 00000002'H
The localized key will be the 160 bit long hex number:
'6695febc 9288e362 82235fc7 151f1284 97b38f3f'H
The 256-bit extended localized key will be generating applying the
mechanism described in 1.2, using the SHA algorithm. The resulting
extended localized key is:
Kul = '6695febc 9288e362 82235fc7 151f1284 97b38f3f 505e07eb
9af25568 fa1f5dbe'H
Note that the last 64 bits of the result of the extended key
algorithm have been truncated to obtain a Kul that is exactly 256-
bit long.
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Full Copyright Statement
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lumenthal/Maino/McCloghrie Expires April 2003 [Page 15]
