Network Working Group U. Blumenthal
Internet Draft Lucent Technologies
Document: 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].
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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
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.
IV - 32-bit Initialization Vector for the AES engine
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i - 32-bit counter (initialized to one).
E(K,P) - encrypting P in ECB mode under key K.
P[i] - i-th block of the plaintext(all but last: 128-bit).
C[i] - i-th block of the 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.
XOR - bitwise operation eXclusive OR.
A * B - A multiplied by 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]. The
underlying cipher and protocol differ from RFC 2574 as
follows:
.Rijndael uses longer keys (AES permits 128-, 192- and
256-bit long keys, with USM we recommend 128-bit
key for most applications);
.Rijndael block size is 128 bits (instead of 64 bits
in DES), which may affect the resulting message
size, depending on what encryption mode is used;
.Recommended encryption mode is GCFB, for the purpose
of maximizing performance and preserving the
message size;
.Explicit Initialization Vector (IV) is truncated to
32 bits, and the rest of the IV is 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 fast, does not increase the size of the ciphertext,
has property of error propagation (due to 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. 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.
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]).
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 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 (non-repeating from one packet to another);
.Varying "rapidly" (considerable amount of bits change
from one IV to another).
It is preferable but not required, that IV is
unpredictable.
4.3. Message encryption
The data to be encrypted is treated as sequence of octets.
The data is encrypted in Generalized Counter Feedback
(GCFB) mode.
The plaintext is divided into a sequence of n 128-bit
blocks P[1], P[2], P[3], à , P[i], à , P[n]. Possibly the
last block P[n] is shorter than 128 bits.
Let 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 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 SNMPv3 USM Message header.
for (i=1; i <= n; i++) do:
1.S[i][1] = i;
2.Obtain O by encrypting S using key K:
O[i] = E(K,S[i]);
3.Ciphertext C is XOR of plaintext P and O (result of
encryption at step 1): C[i] = P[i] XOR 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.Fill the first 32 bits of S[i] with the
value i;
2.Rijndael-encrypt the value of S[i] with
secret key, obtaining O[i];
3.Take the plaintext block P[i] and XOR it
with O[i], obtaining C[i];
4.Take the rightmost 32 bits of 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 C[i].
If the last block P[n] has length L that is shorter than
128 bits, only the leftmost L bits of O[n] are used at
step 3 to obtain C[n].
4.4. Message decryption
The data to be decrypted is treated as sequence of octets.
The data is decrypted in Generalized Counter Feedback
(GCFB) mode.
The ciphertext is divided into a sequence of n 128-bit
blocks C[1], C[2], C[3], à , C[i], à , C[n]. Possibly the
last block C[n] is shorter than 128 bits.
Form S[i] (i=1) the following way:
1. Copy the 32-bit value of IV retrieved from the
privParameters to 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:
1.Complete S[i]: S[i][1] = i;
2.Encrypt S[i], obtaining O[i]: O[i] = E(K,S[i]);
3.Obtain i-th block of plaintext: P[i] = C[i] XOR
O[i];
4.Update S[i] to S[i+1]: S[i+1][2] = 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. Fill the first 32 bits of S[i] with i (value of
the counter);
2. Rijndael-encrypt the value S[i] using secret key
K, obtaining O[i];
3. XOR O[i] with C[i], obtaining plaintext block
P[i];
4. Take rightmost 32 bits of C[i] and replace with
them the current value of second word of S[i],
obtaining S[i+1];
5. Output P[i] as i-th block of plaintext.
If the last block C[n] has length L that is shorter than
128 bits, only the leftmost L bits of O[n] are used at
step 3 to obtain P[n].
5. MIB Definitions
usmAESPrivProtocol OBJECT-IDENTITY
STATUS current
DESCRIPTION "The Rijndael Symmetric Encryption
Protocol"
REFERENCE "Advanced Encryption Standard - NIST.
http://www.nist.gov/aes"
::= { snmpPrivProtocols 4 }
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.
6.1. Services for encrypting outgoing data
This Rijndael privacy protocol assumes that the caller
does the selection of the privKey and that the caller
passes the secret key to be used.
To encrypt the payload (scopedPDU - see [6]) the 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
)
6.2. Services for decrypting incoming data
This Rijndael privacy protocol assumes that the caller
does the selection of the privKey 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)
)
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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.
7.1. Processing an Outgoing Message
1.IV is computed.
2.privParameters field is set to the serialization
according to the rules in [RFC1906] of the OCTET
STRING representing the 4-octet-long IV.
3.The scopedPDU is encrypted (as described above in 4.3)
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.
7.2. Processing an Incoming Message
1.If the privParameters field is not a 4-octet OCTET
STRING, then an error indication (decryptionError)
is returned to the calling module.
2.IV is extracted from privParameters.
3.The encryptedPDU is decrypted, as described above
in 4.4.
4.The decrypted scopedPDU and the statusInformation
are returned to the caller.
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 (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 GCFB mode is used. Therefore it is vital to
adhere to USM requirement given in RFC 2574 and always use
authentication with encryption.
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9. References
1.S. Bradner ôThe Internet Standard Process û Revision 3ö,
RFC 2026. Oct 1996.
2.S. Bradner ôKey words to use in the RFCsö, RFC 2119. Mar
1997.
3.U. Blumenthal, B. Wijnen ôUser-based Security Model
(USM) for version 3 of the Simple Network Management
Protocol (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.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, especially of Dr. Ganesh Sundaram, SNMPv3 WG
and Security Area Directorate is gratefully acknowledged.
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
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