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]. 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. 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 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 Blumenthal Experimental [Page 1] Internet Draft AES in USM SNMPv3 July 2001 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 Blumenthal Experimental [Page 2] Internet Draft AES in USM SNMPv3 July 2001 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). Blumenthal Experimental [Page 3] Internet Draft AES in USM SNMPv3 July 2001 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]; Blumenthal Experimental [Page 4] Internet Draft AES in USM SNMPv3 July 2001 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: Blumenthal Experimental [Page 5] Internet Draft AES in USM SNMPv3 July 2001 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 Blumenthal Experimental [Page 6] Internet Draft AES in USM SNMPv3 July 2001 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) ) Blumenthal Experimental [Page 7] Internet Draft AES in USM SNMPv3 July 2001 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. Blumenthal Experimental [Page 8] Internet Draft AES in USM SNMPv3 July 2001 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 Blumenthal Experimental [Page 9] Internet Draft AES in USM SNMPv3 July 2001 12.Full Copyright Statement "Copyright (C) The Internet Society (date). 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 Experimental [Page 10]