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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Secure Telnet Working Group Russell Housley (SPYRUS) 3 Todd Horting (SPYRUS) 4 Internet-Draft Peter Yee (SPYRUS) 5 Expire in six months July 1998 7 Telnet Authentication Using KEA and SKIPJACK 9 11 Status of this Memo 13 This document is an Internet-Draft. Internet-Drafts are working 14 documents of the Internet Engineering Task Force (IETF), its areas, 15 and its working groups. 17 Note that other groups may also distribute working documents as 18 Internet-Drafts. 20 Internet-Drafts are draft documents valid for a maximum of six months 21 and may be updated, replaced, or obsoleted by other documents at any 22 time. It is inappropriate to use Internet- Drafts as reference 23 material or to cite them other than as "work in progress." 25 To view the entire list of current Internet-Drafts, please check the 26 "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow 27 Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern 28 Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific 29 Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). 31 Distribution of this memo is unlimited. Please send comments to the 32 mailing list. 34 Abstract 36 This document defines a method to authenticate telnet using the Key 37 Exchanage Algorithm (KEA), and encryption of the telnet stream using 38 SKIPJACK. Two encryption modes are specified; one provides data 39 integrity and the other does not. It relies on the Telnet 40 Authentication Option [1]. 42 1 Introduction 44 The Telnet protocol provides no protocol security. Telnet servers may 45 require users to login. This is typically a host level login 46 consisting of a user name and a password, transmitted in the clear. 48 The mechanism specified in this document relies on the Telnet 49 Authentication Option [1]. 51 2 Telnet Security Extensions 53 Telnet, as a protocol, has no concept of security. Without 54 negotiated options, it merely passes characters back and forth 55 between the NVTs represented by the two Telnet processes. In its 56 most common usage as a protocol for remote terminal access (TCP port 57 23), Telnet normally connects to a server that requires user-level 58 authentication through a user name and password in the clear. The 59 server does not authenticate itself to the user. 61 The Telnet Authentication Option provides for: 63 * User authentication -- replacing or augmenting the normal host 64 password mechanism; 65 * Server authentication -- normally done in conjunction with user 66 authentication; 67 * Session parameter negotiation -- in particular, encryption key 68 and attributes; 69 * Session protection -- primarily encryption of the data and 70 embedded command stream, but the encryption algorithm may also 71 provide data integrity. 73 In order to support these security services, the two Telnet entities 74 must first negotiate their willingness to support the Telnet 75 Authentication Option and Data Encryption Options. Upon agreeing to 76 support these options, the parties are then able to perform 77 suboptions to determine the authentication protocol to be used, and 78 possibly the remote user name to be used for authorization checking. 79 Encryption is negotiated along with the type of the authentication. 81 Authentication and parameter negotiation occur within an unbounded 82 series of exchanges. The server proposes a preference-ordered list 83 of authentication types (mechanisms) which it supports. In addition 84 to listing the mechanisms it supports, the server qualifies each 85 mechanism with a modifier that specifies whether the authentication 86 is to be unilateral or mutual, and in which direction the 87 authentication is to be performed, and if encryption of data is 88 desired. The client selects one mechanism from the list and responds 89 to the server indicating its choice and the first set of 90 authentication data needed for the selected authentication type. The 91 client may ignore a request to encrypt data and so indicate, but the 92 server may also terminate the connection if the client refuses 93 encryption. The server and the client then proceed through whatever 94 number of iterations is required to arrive at the requested 95 authentication. 97 If encryption is requested, it is started immediately after the 98 Authentication options are completed. Afterwards either party may 99 use the Data Encryption Options to turn off encryption, but once this 100 has been disabled, there is no ability to re-enable encryption 101 without repeating the complete authentication phase. 103 3 Use of Key Exchange Algorithm (KEA) 105 This paper specifies the method in which KEA is used to achieve 106 Telnet Authentication. KEA (in conjunction with SKIPJACK) provides 107 authentication, integrity and confidentiality. Figure 1 illustrates 108 the authentication mechanism. 110 Telnet entities may use KEA to provide mutual authentication and 111 support for the setup of data encryption keys. A simple token format 112 and set of exchanges delivers these services. 114 The nonce used in this exchanged is a 64 bit unsigned integer. The 115 server generates one, and the client generates another. The nonce 116 value is selected randomly. The nonce is sent in a big endian form. 117 The encryption of the nonce will be done with the same mechanism that 118 the session will use, detailed in the next section. 120 In figure 1 the two-octet authentication type pair is denoted by 121 '0?06'. This will be filled in with either hexadecimal '0C06' for 122 KEA SKIPJACK without integrity mechanism or '0D06' for KEA SKIPJACK 123 with integrity mechanism. 125 --------------------------------------------------------------------- 126 Client (Party A) Server (Party B) 128 <-- IAC DO AUTHENTICATION 130 IAC WILL AUTHENTICATION --> 132 <-- IAC SB AUTHENTICATION SEND 133 134 IAC SE 136 IAC SB AUTHENTICATION 137 NAME --> 139 IAC SB AUTHENTICATION IS 140 '0?06' '1' CertA||Ra IAC SE --> 142 <-- IAC SB AUTHENTICATION REPLY 143 '0?06' '2' 144 CertB||Rb||WMEK||IV|| 145 Encrypt( NonceB ) 146 IAC SE 148 IAC SB AUTHENTICATION IS 149 '0?06' '3' 150 IV'||Encrypt( NonceB+1||NonceA ) 151 IAC SE --> 153 <-- IAC SB AUTHENTICATION REPLY 154 '0?06' '4' Encrypt( NonceA+1 ) 155 IAC SE 157 --------------------------------------------------------------------- 158 Figure 1 160 On completing these exchanges, the parties have a common SKIPJACK 161 key. Mutual authentication is provided by verification of the 162 certificates used to establish the SKIPJACK encryption key and 163 successful use of the derived SKIPJACK session key. To protect from 164 an active attacker, encryption will take place after successful 165 authentication. There will be no way to turn off encryption and 166 safely turn it back on; repeating the entire authentication is the 167 only safe way to restart it. If the user does not want to use 168 encryption, he will have to logoff and logon with the desired 169 security mechanism. 171 3.1 SKIPJACK Modes 173 There are two distinct modes for encrypting telnet streams; one 174 provides integrity and the other does not. Because telnet is 175 normally operated in a character-by-character mode, the KEA SKIPJACK 176 with stream integrity mechanism requires the transmission of 4 bytes 177 for every telnet data byte. However, a more simplified mode KEA 178 SKIPJACK without integrity mechanism will only require the 179 transmission of one byte for every telnet data byte. 181 The cryptographic mode for KEA SKIPJACK with stream integrity is 182 Cipher Feedback on 32 bits of data (CFB-32) and the mode of KEA 183 SKIPJACK is Cipher Feedback on 8 bits of data (CFB-8). 185 3.1.1 SKIPJACK without stream integrity 187 The first and least complicated mode is the SKIPJACK CFB-8. This 188 mode provides no stream integrity. 190 For SKIPJACK without stream integrity, the two-octet authentication 191 type pair (0?06 in Figure 1) is "KEA_SJ CLIENT_TO_SERVER MUTUAL 192 ENCRYPT_ON INI_CRED_FWD_OFF". This is encoded as two-octets: '0C06' 193 in hexadecimal. This indicates that the KEA SKIPJACK without 194 integrity mechanism will be used for mutual authentication and telnet 195 stream encryption. 197 3.1.2 SKIPJACK with stream integrity 199 SKIPJACK with stream integrity is more complicated. It uses a hash 200 function to bind integrity into the encryption stream as follows: 202 Set H0 to be the SHA-1 [3] hash of a zero length string. 203 Cn is the nth character in the stream. 204 Hn = SHA-1( Hn-1||Cn ), where Hn is the hash value 205 associated with the nth character in the stream. 206 ICVn is set to the three most significant bytes of Hn. 207 Transmit Encrypt( Cn||ICVn ). 209 The ciphertext that is transmitted is the SKIPJACK CFB-32 encryption 210 of ( Cn||ICVn ). The receiving end of the Telnet link reverses the 211 process, first decrypting the ciphertext, separating Cn and ICVn, 212 recalculating Hn, recalculating ICVn, and then comparing the received 213 ICVn with the recalculated ICVn. Integrity is indicated if the 214 comparison succeeds, and Cn can then be processed normally as part of 215 the Telnet stream. Failure of the comparison indicates some loss of 216 integrity, whether due to active manipulation or loss of 217 cryptographic synchronization. In either case, the only recourse is 218 to drop the telnet connection and start over. 220 For SKIPJACK with stream integrity, the two-octet authentication type 221 pair (XXXX in figure 1) is "KEA_SJ_INTEG CLIENT_TO_SERVER MUTUAL 222 ENCRYPT_ON INI_CRED_FWD_OFF". This is encoded as two-octets: '0D06' 223 in hexadecimal. This indicates that the KEA SKIPJACK with integrity 224 mechanism will be used for mutual authentication and telnet stream 225 encryption. 227 3.1.3 Telnet SYNCH Handling 229 Telnet supports a "Synch" mechanism to solve the problem of Telnet 230 control commands from being blocked by network flow control. 231 Basically, the sender of the Synch is telling the recipient to 232 discard all incoming data except Telnet commands until the DM (and 233 end of urgent) is reached. The Synch signal is sent via a TCP Urgent 234 notification, but does not arrive out of sequence, all data received 235 will be decrypted and only acted upon if it is a Telnet command. 236 Since sequence is preserved, no special cryptographic processing is 237 required. 239 4.0 Security Considerations 241 This entire memo is about security mechanisms. For KEA to provide 242 the authentication discussed, the implementation must protect the 243 private key from disclosure. Likewise, the SKIPJACK keys must be 244 protected form disclosure. 246 By linking the enabling of encryption as a side effect of successful 247 authentication, protection is provided against an active attacker. 248 If encryption were enabled as a separate negotiation, it would 249 provide a window of vulnerability from when the authentication 250 completes, up to and including the negotiation to turn on encryption. 251 The only safe way to restart encryption, if it is turned off, is to 252 repeat the entire authentication process. 254 5.0 Acknowledgements 256 We would like to thank William Nace for support during implementation 257 of this specification. 259 6.0 References 261 [1] - Borman, David A. "Telnet Authentication Option". 262 RFC 1416. February 1993. 264 [2] - T. Ts'o, "Telnet Data Encryption Option". 265 , February 1998. 267 [3] - Secure Hash Standard. FIPS Pub 180-1. April 17, 1995. 269 7.0 Author's Address 271 Russell Housley 272 SPYRUS 273 PO Box 1198 274 Herndon, VA 20172 275 USA 276 Phone: +1 703 435-7344 277 Email: housley@spyrus.com 279 Todd Horting 280 SPYRUS 281 PO Box 1198 282 Herndon, VA 20172 283 USA 284 Phone: +1 703 435-4711 285 Email: thorthing@spyrus.com 287 Peter Yee 288 SPYRUS 289 2460 N. First Street 290 Suite 100 291 San Jose, CA 95131-1023 292 USA 293 Phone: +1 408 432-8180 294 Email: yee@spyrus.com