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Network Working Group U. Blumenthal
Internet Draft Lucent Technologies
Document: draft-blumenthal-aes-usm-00.txt December 2000
Rijndael draft-blumenthal-aes-usm-01.doc July 2001
Category: Experimental
AES (Rijndael) Encryption Protocol with SNMPv3 USM
Status of this Memo
This document is an Internet-Draft and is in full
conformance with all provisions of Section 10 of RFC2026
[1].
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. Internet-Drafts are draft
documents valid for a maximum of six months and may be
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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.
For potential updates to the above required-text see:
http://www.ietf.org/ietf/1id-guidelines.txt
1. Abstract
This document describes the use of Rijndael encryption
protocol with User-based Security Model (USM) for SNMP
version 3. This protocol provides data confidentiality.
This document augments and should be used with RFC 2574
[1].
2. Conventions used in this document
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
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 RFC-2119 [2].
K _ - secret key for the AES encryption engine engine.
IV _ - 32-bit Initialization Vector for the encryption AES engine
Si _ the input value
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Oi _ the output value computed via USM SNMPv3 July 2001
i - 32-bit counter (initialized to one).
E(K,P) - encrypting the shift register
Ti _ 64 leftmost bits of Oi
Pi _ P in ECB mode under key K.
P[i] - i-th 64-bit block of the plaintext
Ci _ plaintext(all but last: 128-bit).
C[i] - i-th 64-bit block of the ciphertext (last block may be shorter)
Ek(P) _ encrypting P in ECB mode under key K ciphertext(size - same as above).
C[i][j] û j-th 4-byte word of O[i] (1 <= j <= 4).
S[i] - the encryptor input value for i-th step.
S[i][j] û j-th 4-byte word of S[i] (1 <= j <= 4).
O[i] û encryptor output value O[i]=E(K,S[i]).
A^b - A raised in power b b.
XOR - bitwise operation eXclusive OR OR.
A * B _ - A multiplied by B B.
When an integer value (i, snmpEngineTime, snmpEngineBoots)
is placed in the octet string such as S[i], it is
converted to Network Byte Order if necessary (Big-Endian),
and then copied byte by byte from left to right.
3. Overview
At the time of writing of this document, Rijndael [4] has
been declared the proposed AES (Advanced Encryption
Standard) [5] by NIST. This, together with the fact that
practical attacks on DES became feasible, makes it
necessary to define new privacy protocols for USM.
Rijndael is the natural candidate to base them on.
The protocol is very similar to CBC-DES Symmetric
Encryption Protocol described in RFC 2574 [3], with some exceptions:
. Rijndael [3]. The
underlying cipher and protocol differ from RFC 2574 as
follows:
.Rijndael uses longer keys (USM (AES permits 128-, 192- and
256-bit long keys, and recommends with USM we recommend 128-bit
key for most applications);
. Rijndael
.Rijndael block size is 128 bits (instead of 64 bits
in DES), which affects may affect the resulting message size;
. Recommended
size, depending on what encryption mode is used;
.Recommended encryption mode is CFB, GCFB, for the purpose
of non-
increasing maximizing performance and preserving the
message size;
.
.Explicit Initialization Vector (IV) is twice as long (128 bits) truncated to
32 bits, and the rest of the IV is
generated differently (the procedure filled according
to the algorithm described below;
.Encryption and decryption processes are the same,
thus the crypto engine must implement only
encryption and does not have to implement
decryption procedure.
3.1. Generalized Counter Feedback Mode
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GCFB is a stream cipher mode. It combines the advantages
of CTR-mode (Counter) and of CFB (Cipher Feedback) mode.
It is outlined below).
Due fast, does not increase the size of the ciphertext,
has property of error propagation (due to increased block length, the feedback).
The cipher engine is used only in encryption mode (AES
decryption feature is not needed). It produces a
pseudorandom stream that is XOR-ed with the plaintext. To
create pseudorandom stream, a 128-bit input string is
encrypted. Like the CTR-mode, part of that string
comprises of a counter that increments by one with each
encryption iteration. Like CFB-mode, part of the resulting
ciphertext is fed back to the 128-bit string, affecting
the next 128-bit of pseudorandom stream.
4. Rijndael AES (Rijndael) Symmetric Encryption Protocol
Rijndael is a modern 128-bit block cipher developed by
Joan Deamen and Vincent Rijmen [4], declared by NIST a
proposed AES (successor to DES). Its description, modes of
operation, validation test suite and reference
implementation code are available on the AES NIST Web site
[5].
Rijndael takes 128-, 192- and 256-bit long keys. For USM
it is believed that 128-bit keys are sufficient. However
neither USM [3] nor the Rijndael protocol as specified
here, mandate any particular key length - thus all the
three key length options are acceptable.
Rijndael encryption algorithm is used to encrypt the
designated portion of an SNMP message, which along with
Rijndael Initialization Vector is included as a part of
the message sent to the recipient.
<Title> <month> <year>
4.1. Rijndael Key
Rijndael key is an octet string of 16, 24, or 32 bytes.
The recommended length is 16 bytes, which is deemed enough
for most applications.
The key is (implicitly) stored in the USM User table and
can be manipulated using SNMPv3 protocol via access to USM
User Table [3].
The whole length of the octet string representing the
secret privacy key is used as a Rijndael key (see
usmUserPrivKeyChange and usmUserOwnPrivKeyChange in [3]).
If a password or other variable-length user input needs to be
converted to
KeyChange Textual Convention governs the process, for the
keys of 128-, 192- and 256-bit length. It is strongly
recommended that only SHA-1 is used, and not MD5 (SHA-256
and SHA-512 are good choices to replace SHA-1).
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If a password or other variable-length user input needs to
be converted to a Rijndael key, follow the algorithm given
in RFC 2574.
Throughout this document it is assumed that the Rijndael
key is localized, as described in RFC 2574.
4.2. Rijndael Initialization Vector
It is up to the entity in question how to obtain/compute
the
Initialization Vector (IV). 32-bit IV. On Unix operating systems one can use
reasonably secure random number sources such as
/dev/random.
IV should satisfy the following requirements:
. Unique
.Unique (non-repeating from one packet to another);
. Varying _rapidly_
.Varying "rapidly" (considerable amount of bits change
from one IV to another).
It is preferable but not required, that IV is
unpredictable.
If a good source of randomness is unavailable, one can generate IV's
running Rijndael in FCB (filtered counter) mode, following the below
procedure.
When the SNMPv3 entity is activated, it obtains one 128-bit random
number to use it as the enciphering key for the Rijndael encryption
engine. Then it sets an 8-byte octet string A to the concatenation
of 32-bit octet strings agentBoots and agentTime. Then the octet
string A is padded with zeroes to 16-byte length. Octet string A is
used as input for Rijndael encryption engine. This completes
initialization of Rijndael-based Pseudo-Random Number Generator.
From now on, when a message needs to be encrypted, the following
steps are performed:
1.Rijndael engine performs one iteration.
2.IV is set to the 128-bit result of step 1.
3.Treating octet string A as a 128-bit integer number,
increment it by one (to prepare for the next request for IV).
4.3. Message encryption
The data to be encrypted is treated as sequence of octets.
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The data is encrypted in Cipher feedback (CFB) Generalized Counter Feedback
(GCFB) mode.
The plaintext is divided into a sequence of n 64-bit 128-bit
blocks P1, P2,
P3, _, Pi, _, Pn. P[1], P[2], P[3], à , P[i], à , P[n]. Possibly the
last block Pn P[n] is shorter than 64 128 bits.
Let Si (i=1) i be 32-bit counter, initialized to 1.
After 32-bit IV is selected (se 4.2), 128-bit S[i] for i=1
is constructed in the input value of the shift register. Assign following way:
1. First 32 bits are filled with 32-bit counter i.
2. Second 32 bits are filled with 32-bit IV.
3. Third 32 bits are filled with snmpEngineBoots.
4. Fourth 32 bits are filled with snmpEngineTime.
SnmpEngineBoots and snmpEngineTime must match those that
will be inserted in the
value of IV to S1. SNMPv3 USM Message header.
for (i=1; i <= n; i++) do:
1.
Pi
1.S[i][1] = next 64-bit block i;
2.Obtain O by encrypting S using key K:
O[i] = E(K,S[i]);
3.Ciphertext C is XOR of the plaintext (of the message)
2.
Oi = Ek(Ii)
3.
Ti = 64 leftmost bits P and O (result of Oi
4.
Ci
encryption at step 1): C[i] = Pi P[i] XOR Ti
5.
S[i+1] = ((2^64 * Ii) + Ci ) mod 2^128
Algorithmically it means:
for O[i];
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4.Copy the last 32 bits of C[i] to the second word
(second 32 bits of S: S[i+1][2] = C[i][4];
5.Output C[i] as encryption of P[i].
Algorithmically it means:
for as long as there are input plaintext blocks
1.
Rijndael-encrypt
1.Fill the value first 32 bits of the register Si S[i] with
secret key;
2.
Take the next plaintext block Pi;
3.
r is the length in bits of
value i;
2.Rijndael-encrypt the plaintext block Pi
being currently processed;
4.
Assign 64 leftmost bits value of the result to Oi;
5.
XOR them S[i] with
secret key, obtaining O[i];
3.Take the plaintext block (Ci = Pi P[i] and XOR Oi),
if the plaintext block is only r-bits long (r < 64)
use it
with O[i], obtaining C[i];
4.Take the leftmost r rightmost 32 bits of Oi;
6.
Output C[i] and
replace with them second 32-bit word) of
S[i], obtaining S[i+1] (counter will also
be updated: here it is shown at step 1);
5.Output the result of the step 3, as the
next ciphertext block Ci, and if r C[i].
If the last block P[n] has length L that is less shorter than 64, stop.
7.
Shift the shift-register 64 bits to the left,
discarding
128 bits, only the leftmost 64 bits;
8.
Fill the rightmost 64 L bits of the shift register with
the ciphertext block obtained O[n] are used at the
step 3. This
prepares the Rijndael CFB engine 3 to process the next
plaintext block. obtain C[n].
4.4. Message decryption
The data to be decrypted is treated as sequence of octets.
The data is decrypted in Cipher feedback (CFB) Generalized Counter Feedback
(GCFB) mode.
The ciphertext is divided into a sequence of n 64-bit 128-bit
blocks C1, C2,
C3, _, Ci,_, Cn. C[1], C[2], C[3], à , C[i], à , C[n]. Possibly the
last block Cn C[n] is shorter than 64 128 bits.
Let Si
Form S[i] (i=1) be the input value of the shift register. Assign following way:
1. Copy the 32-bit value of IV retrieved from the
privParameters to S1. second 32-bit word of S[1].
2. Copy the 32-bit msgSnmpEngineBoots value to the
third 32-bit word of S[1].
3. Copy the 32-bit msgSnmpEngineTime value to the
fourth 32-bit word of S[1].
for (i=1; i <= n; i++) do:
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1.
Ci = next 64-bit block of the plaintext (of the message)
2.
Oi
1.Complete S[i]: S[i][1] = Ek(Ii)
3.
Ti i;
2.Encrypt S[i], obtaining O[i]: O[i] = 64 leftmost bits E(K,S[i]);
3.Obtain i-th block of Oi
4.
Pi plaintext: P[i] = Ci C[i] XOR Ti
5.
S[i+1]
O[i];
4.Update S[i] to S[i+1]: S[i+1][2] = ((2^64 * Ii) + Ci ) mod 2^128 C[i][4];
5.Output P[i] as i-th block of plaintext.
Algorithmically it means:
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for as long as there are input ciphertext blocks
1.
Rijndael-encrypt Fill the value first 32 bits of the register Si S[i] with
secret key; i (value of
the counter);
2.
Take Rijndael-encrypt the next ciphertext block Ci; value S[i] using secret key
K, obtaining O[i];
3.
r is the length in bits of the ciphertext XOR O[i] with C[i], obtaining plaintext block Ci
being currently processed;
P[i];
4. Take 64 leftmost rightmost 32 bits of the result (Oi)
5.
XOR them with the ciphertext block (Pi = Ci XOR Oi),
if the ciphertext block is only r-bits long C[i] and r<64,
use replace with
them the leftmost r bits current value of Oi;
6.
Output the result second word of the step 3, S[i],
obtaining S[i+1];
5. Output P[i] as i-th block of plaintext.
If the next
plaintext last block Pi, and if r C[n] has length L that is less shorter than 64, stop;
7.
Shift the shift-register 64 bits to the left,
discarding
128 bits, only the leftmost 64 bits;
8.
Fill the rightmost 64 L bits of the shift-register with
the ciphertext block obtained O[n] are used at the
step 3. This
prepares the Rijndael CFB engine 3 to process the next
ciphertext block.
1. obtain P[n].
5. MIB Definitions
usmAESPrivProtocol OBJECT-IDENTITY
STATUS current
DESCRIPTION _The "The Rijndael Symmetric Encryption Protocol_
Protocol"
REFERENCE _Advanced "Advanced Encryption Standard _ - NIST.
http://www.nist.gov/aes_
http://www.nist.gov/aes"
::= { snmpPrivProtocols 4 }
5.
6. Rijndael Encryption Services
Here we describe the Rijndael-based privacy services,
which are called upon by User-based Security Model (USM)
to encrypt and decrypt SNMPv3 message payload.
These are the same as described in RFC 2574.
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 IV.
<Title> <month> <year>
5.1.
6.1. Services for encrypting outgoing data
This Rijndael privacy protocol assumes that the caller
does the selection of the privKey is done by the caller and that the caller
passes the secret key to be used.
To encrypt the payload (scopedPDU _ - see [6]) the User-based User-
based Security Model (USM) will pass the payload and the
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encryption key to the privacy service which implements
Rijndael protocol, receiving back the encryptedPDU (see
[6]) and the privParameters containing IV (see [3]).
Upon completion, the privacy service 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 =
encryptData(
IN encryptKey -- secret key for encryption
IN dataToEncrypt -- data to encrypt (scopedPDU)
OUT encryptedData -- encrypted data (encryptedPDU)
OUT privParamets -- filled in by service provider
)
5.2.
6.2. Services for decrypting incoming data
This Rijndael privacy protocol assumes that the caller
does the selection of the privKey is done by the caller and that the caller
passes the secret key to be used.
To decrypt the payload (encryptedPDU - see[4]) the USM
will pass the encryptedPDU, secret key and privParameters
to the privacy service, receiving back the decrypted
plaintext scopedPDU.
statusInformation indicates whether the decryption was
successful.
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 decrypting
IN privParameters -- as received on the wire
IN encryptedData -- encrypted data (encryptedPDU)
OUT decryptedData -- decrypted data (scopedPDU)
)
<Title> <month> <year>
2.
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7. Elements of the procedure
This section describes the procedure followed by an SNMP
engine whenever it must encrypt part of an outgoing
message using the usmAESPrivProtocol.
3.
7.1. Processing an Outgoing Message
1.
IV
1.IV is computed.
2.
privParameters
2.privParameters field is set to the serialization
according to the rules in [RFC1906] of the OCTET
STRING representing the 16-
octet-long 4-octet-long IV.
3.
The
3.The scopedPDU is encrypted (as described above) above in 4.3)
and the encrypted data is serialized according to the
rules in [RFC1906] as an OCTET STRING.
4.
The
4.The serialized OCTET STRING representing the encrypted
scopedPDU together with the privParameters and
statusInformation indicating success is returned to
the calling module.
3.
7.2. Processing an Incoming Message
1.
If
1.If the privParameters field is not an 16-octet a 4-octet OCTET
STRING, then an error indication (decryptionError)
is returned to the calling module.
2.
IV
2.IV is extracted from privParameters.
3.
The
3.The encryptedPDU is decrypted decrypted, as described above.
4.
The above
in 4.4.
4.The decrypted scopedPDU and the statusInformation
are returned to the caller.
6.
8. Security Considerations
The strength of this protocol depends on the cryptographic
strength of SHA-1 hash-function (properties of the
generated key) and of Rijndael block cipher. cipher (security of
the encryption). It will be better to use SHA-256 or SHA-
512 for AES key generation, but we want to give more time
to their studying by the world cryptographic community.
An adversary can predictably change the plaintext bits by
modifying the corresponding ciphertext bits when
encryption in CFB GCFB mode is used. Therefore it is vital to
adhere to USM requirement given in RFC 2574 and always use
authentication with encryption.
7.
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9. References
1 Bradner, S., "The
1.S. Bradner ôThe Internet Standards Standard Process -- û Revision 3", BCP
9, 3ö,
RFC 2026, October 2026. Oct 1996.
2 Bradner, S., "Key
2.S. Bradner ôKey words for to use in RFCs to Indicate Requirement
Levels", BCP 14, the RFCsö, RFC 2119, March 1997
<Title> <month> <year>
1.RFC 2026.
2.RFC 2119. Mar
1997.
3.U. Blumenthal, B. Wijnen "User-based ôUser-based Security Model
(USM) for version 3 of the Simple Network Management
Protocol (SNMPv3).
RFC2574, (SNMPv3)ö, RFC 2574, April 1999.
4.J. Daemen, V. Rijmen "The Block Cipher Rijndael"
http://www.esat.kuleuven.ac.be/~rijmen/rijndael/
5.Rijndael: NIST's Selection for the AES
http://csrc.nist.gov/encryption/aes/rijndael/
6.RFC
6.D. Harrington, R. Presuhn, B. Wijnen ôAn Architecture
for Describing SNMP Management Frameworkö, RFC 2571.
April 1999.
10. Acknowledgments
Help of the members of Wireless Security Group at Lucent
Technologies, SAGE group, especially of Dr. Ganesh Sundaram, SNMPv3 WG
and Security Area Directorate is gratefully acknowledged.
4.
Special thanks go to Wes Hardaker and Randy Presuhn for
detailed review and helpful comments.
11. Author's Addresses
Uri Blumenthal
Lucent Technologies / Bell Labs
14D-318
67 Whippany Rd
Whippany, NY 07981
USA
Phone: +1.973.386.2163
Email: uri@lucent.com
12. Appendix 1. Creating IV-generating Key
The procedure described here helps obtaining an IV-generating key
IVK when no good source of randomness is available.
1.
Obtain a 128-bit random number R of poor quality.
2.
Encrypt is using the user's secret key.
3.
XOR the result with R itself and assign it to W = R XOR Ek(R)
4.
Concatenate agentBoots with agentTime and snmpEngineId of this
entity (if it exists) and pad the results with 0xBE to 16 bytes.
Assign the result to RND.
5.
XOR the result of the previous step with W.
6.
Encrypt the result of the previous step using secret key K.
7.
XOR the result of the previous step with W.
8.
Cyclic-rotate the result of the previous step to the left by 53
bits.
9.
Encrypt the result of the previous step using secret key K.
10. XOR the result of the previous step with W.
<Title> <month> <year>
11. Output the result of the previous step as IVK.
<Title> <month> <year>
5.
Full
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12.Full Copyright Statement
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Reserved. This document and translations of it may be
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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
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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
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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
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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.
<Title> <month> <year>
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