Network Working Group T. Ts'o, Editor Internet-Draft VA Linux Systems draft-tso-telnet-auth-enc-04.txt September 1999 Telnet Authentication Option 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 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. 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. 0. Abstract This document describes the authentication option to the telnet[1] protocol as a generic method for negotiating an authentication type and mode including whether encryption should be used and if creden- tials should be for- warded. While this document summarizes current- ly utilized commands and types it does not define a specific authen- tication type. Separate documents are to be published defining each authentication type. This document updates a previous specification of the telnet authen- tication option, RFC 1409 [2], so that it can be used to its use can be used to securely enable the telnet encryption option[3]. 1. Command Names and Codes AUTHENTICATION 37 Expires March 2000 [Page 1]
Internet-Draft Telnet Authentication Option September 1999 Authentication Commands IS 0 SEND 1 REPLY 2 NAME 3 Authentication Types NULL 0 KERBEROS_V4 1 KERBEROS_V5 2 SRP 5 KEA_SJ 12 KEA_SJ_INTEG 13 DSS 14 Following historical practice, future authentication type numbers will be assigned by the IANA under a First Come First Served pol- icy as outlined by RFC 2434 [4]. Despite the fact that authenti- cation type numbers are allocated out of an 8-bit number space (as are most values in the telnet specification) it is not antic- ipated that the number space is or will become in danger of being exhausted. However, if this should become an issue, when over 50% of the number space becomes allocated, the IANA shall refer allocation requests to either the IESG or a designated expert for approval. Modifiers AUTH_WHO_MASK 1 AUTH_CLIENT_TO_SERVER 0 AUTH_SERVER_TO_CLIENT 1 AUTH_HOW_MASK 2 AUTH_HOW_ONE_WAY 0 AUTH_HOW_MUTUAL 2 ENCRYPT_MASK 20 ENCRYPT_OFF 0 ENCRYPT_USING_TELOPT 4 ENCRYPT_AFTER_EXCHANGE 16 ENCRYPT_RESERVED 20 INI_CRED_FWD_MASK 8 INI_CRED_FWD_OFF 0 INI_CRED_FWD_ON 8 2. Command Meanings This document makes reference to a "server" and a "client". For the purposes of this document, the "server" is the side of the connection that did the passive TCP open (TCP LISTEN state), and the "client" is Expires March 2000 [Page 2]
Internet-Draft Telnet Authentication Option September 1999 the side of the connection that did the active open. IAC WILL AUTHENTICATION The client side of the connection sends this command to indicate that it is willing to send and receive authentication information. IAC DO AUTHENTICATION The servers side of the connection sends this command to indicate that it is willing to send and receive authentication information. IAC WONT AUTHENTICATION The client side of the connection sends this command to indicate that it refuses to send or receive authentication information; the server side must send this command if it receives a DO AUTHENTICA- TION command. IAC DONT AUTHENTICATION The server side of the connection sends this command to indicate that it refuses to send or receive authentication information; the client side must send this command if it receives a WILL AUTHENTI- CATION command. IAC SB AUTHENTICATION SEND authentication-type-pair-list IAC SE The sender of this command (the server) requests that the remote side send authentication information for one of the authentication types listed in "authentication-type-pair-list". The "authentica- tion-type-pair-list" is an ordered list of "authentication-type" pairs. Only the server side (DO AUTHENTICATION) is allowed to send this. IAC SB AUTHENTICATION IS authentication-type-pair <auth data> IAC SE The sender of this command (the client) is sending the authentica- tion information for authentication type "authentication-type- pair". Only the client side (WILL AUTHENTICATION) is allowed to send this. IAC SB AUTHENTICATION REPLY authentication-type-pair <auth data> IAC SE The sender of this command (the server) is sending a reply to the the authentication information received in a previous IS command. Only the server side (DO AUTHENTICATION) is allowed to send this. IAC SB AUTHENTICATION NAME remote-user IAC SE Expires March 2000 [Page 3]
Internet-Draft Telnet Authentication Option September 1999 This optional command is sent to specify the account name on the remote host that the user wishes to be authorized to use. Note that authentication may succeed, and the authorization to use a particular account may still fail. Some authentication mechanisms may ignore this command. The "authentication-type-pair" is two octets, the first is the au- thentication type, and the second is a modifier to the type. The au- thentication type may or may not include built-in encryption. For instance, when the Kerberos 4 authentication type is negotiated en- cryption must be negotiated with the telnet ENCRYPT option. However, the SSL and KEA_SJ authentication types provide an encrypted channel as part of a successful telnet AUTH option negotiation. There are currently five one bit fields defined in the modifier. The first two of these bits are processed as a pair, the AUTH_WHO_MASK bit and the AUTH_HOW_MASK bit. There are four possible combinations of these two bits: AUTH_CLIENT_TO_SERVER AUTH_HOW_ONE_WAY The client will send authentication information about the local user to the server. If the negotiation is successful, the server will have authenticated the user on the client side of the connection. AUTH_SERVER_TO_CLIENT AUTH_HOW_ONE_WAY The server will authenticate itself to the client. If the ne- gotiation is successful, the client will know that it is con- nected to the server that it wants to be connected to. AUTH_CLIENT_TO_SERVER AUTH_HOW_MUTUAL The client will send authentication information about the local user to the server, and then the server will authenticate it- self to the client. If the negotiation is successful, the server will have authenticated the user on the client side of the connection, and the client will know that it is connected to the server that it wants to be connected to. AUTH_SERVER_TO_CLIENT AUTH_HOW_MUTUAL The server will authenticate itself to the client, and then the client will authenticate itself to the server. If the negotia- Expires March 2000 [Page 4]
Internet-Draft Telnet Authentication Option September 1999 tion is successful, the client will know that it is connected to the server that it wants to be connected to, and the server will know that the client is who it claims to be. The third and fifth bits in the modifier are the ENCRYPT_MASK bits. These bits are used to determine if and how encryption should be enabled. Of the four possible combinations only three are currently defined: ENCRYPT_OFF Encryption will not be used for this session. TELOPT EN- CRYPT SHOULD NOT be negotiated. This mode MUST be used with all AUTH types that do not provide a shared secret to be used as a session key. ENCRYPT_USING_TELOPT Encryption will be negotiated via the use of TELOPT ENCRYPT. Immediately after authentication has completed TELOPT EN- CRYPT MUST be negotiated in both directions. This is re- quired to occur before credentials forwarding; other telnet options are negotiated; or any user data is transmitted. A failure to successfully negotiate TELOPT ENCRYPT in either direction MUST result in immediate session termination. ENCRYPT_AFTER_EXCHANGE Encryption will be activated in both directions immediately after the successful exchange of the shared secret to be used as the session key. The encryption algorithm to be used MUST be implied by the AUTH type. The fourth bit field in the modifier is the INI_CRED_FWD_MASK bit. This bit is either set to INI_CRED_FWD_ON or INI_CRED_FWD_OFF. This bit is set by the client to advise the server to expect for- warded credentials from the client. INI_CRED_FWD_OFF The client will not be forwarding credentials to the server. This mode must be used if the selected authentication method does not support credentials forwarding. INI_CRED_FWD_ON Once authentication, and perhaps encryption, completes, the client will immediately forward authentication credentials to the server. Expires March 2000 [Page 5]
Internet-Draft Telnet Authentication Option September 1999 The motivation for this advisory bit is that the server may wish to wait until the forwarded credentials have been sent before starting any operating system specific login procedures which may depend on these credentials. Note that credentials forwarding may not be supported by all authentication mechanisms. It is a proto- col error to set this bit if the underlying authentication mecha- nism does not support credentials forwarding. Credentials forwarding MUST NOT be performed if AUTH_CLIENT_TO_SERVER|AUTH_HOW_ONE_WAY was used since the identity of the server can not be assured. Credentials SHOULD NOT be for- warded if the telnet connection is not protected using some en- cryption or integrity protection services. Note that older implementations of the telnet authentication op- tion will not understand the ENCRYPT_MASK and INI_CRED_FWD_MASK bits. Hence an implementation wishing to offer these bits should offer authentication type pairs with these bits both set and not set if backwards compatibility is required. 3. Default Specification The default specification for this option is WONT AUTHENTICATION DONT AUTHENTICATION meaning there will not be any exchange of authentication information. 4. Motivation One of the deficiencies of the Telnet protocol is that in order to log into remote systems, users have to type their passwords, which are passed in clear text through the network. If the connections goes through untrusted networks, there is the possibility that pass- words will be compromised by someone watching the packets as they go by. The purpose of the AUTHENTICATION option is to provide a framework for the passing of authentication information through the TELNET ses- sion, and a mechanism to enable encryption of the data stream as a side effect of successful authentication or via subsequent use of the telnet ENCRYPT option. This means that: 1) the users password will not be sent in clear text across the network, 2) if the front end telnet process has the appropriate authentication information, it can automatically send the information, and the user will not have to type any password. 3) once authentication has succeeded, the data stream can be encrypted to provide protection against active attacks. It is intended that the AUTHENTICATION option be general enough that Expires March 2000 [Page 6]
Internet-Draft Telnet Authentication Option September 1999 it can be used to pass information for any authentication and encryp- tion system. 5. Security Implications The ability to negotiate a common authentication mechanism between client and server is a feature of the authentication option that should be used with caution. When the negotiation is performed, no authentication has yet occurred. Therefore each system has no way of knowing whether or not it is talking to the system it intends. An in- truder could attempt to negotiate the use of an authentication system which is either weak, or already compromised by the intruder. If the authentication type requires that encryption be enabled as a separate optional negotiation (the ENCRYPT option), it will provide a window of vulnerability from when the authentication completes, up to and including the negotiation to turn on encryption by an active at- tacker. An active attack is one where the underlying TCP stream can be modified or taken over by the active attacker. If the server only offers authentication type pairs that include the ENCRYPT_US- ING_TELOPT set in the ENCRYPT_MASK field, this will avoid the window of vulnerability, since both parties will agree that telnet ENCRYPT option must be successfully negotiated immediately following the suc- cessful completion of telnet AUTH. Other authentication types link the enabling of encryption as a side effect of successful authentication. This will also provide protec- tion against the active attacker. The ENCRYPT_AFTER_EXCHANGE bit al- lows these authentication types to negotiate encryption so that it can be made optional. Another opportunity for active attacks is presented when encryption may be turned on and off without re-authentication. Once encryption is disabled, an attacker may hijack the telnet stream, and interfere with attempts to restart encryption. Therefore, a client SHOULD NOT support the ability to turn off encryption. Once encryption is dis- abled, if an attempt to re-enable encryption fails, the client MUST terminate the telnet connection. It is important that in both cases the authentication type pair be integrity protected at the end of the authentication exchange. This must be specified for each authentication type to ensure that the re- sult of the telnet authentication option negotiation is agreed to by both the client and the server. Some authentication type suboptions may wish to include all of the telnet authentication negotiation ex- changes in the integrity checksum, to fully protect the entire ex- change. Each side MUST verify the consistency of the auth-type-pairs in each message received. Any variation in the auth-type-pair MUST be treat- Expires March 2000 [Page 7]
Internet-Draft Telnet Authentication Option September 1999 ed as a fatal protocol error. 6. Implementation Rules WILL and DO are used only at the beginning of the connection to ob- tain and grant permission for future negotiations. The authentication is only negotiated in one direction; the server must send the "DO", and the client must send the "WILL". This re- striction is due to the nature of authentication; there are three possible cases; server authenticates client, client authenticates server, and server and client authenticate each other. By only nego- tiating the option in one direction, and then determining which of the three cases is being used via the suboption, potential ambiguity is removed. If the server receives a "DO", it must respond with a "WONT". If the client receives a "WILL", it must respond with a "DONT". Once the two hosts have exchanged a DO and a WILL, the server is free to request authentication information. In the request, a list of supported authentication types is sent. Only the server may send re- quests ("IAC SB AUTHENTICATION SEND authentication-type-pair-list IAC SE"). Only the client may transmit authentication information via the "IAC SB AUTHENTICATION IS authentication-type ... IAC SE" com- mand. Only the server may send replies ("IAC SB AUTHENTICATION REPLY authentication-type ... IAC SE"). As many IS and REPLY suboptions may be exchanged as are needed for the particular authentication scheme chosen. If the client does not support any of the authentication types listed in the authentication-type-pair-list, a type of NULL should be used to indicate this in the IS reply. Note that if the client responds with a type of NULL, the server may choose to close the connection. When the server has concluded that authentication cannot be nego- tiated with the client it should send IAC DONT AUTH to the client. The order of the authentication types MUST be ordered to indicate a preference for different authentication types, the first type being the most preferred, and the last type the least preferred. As long as the server is WILL AUTH it may request authentication in- formation at any time. This is done by sending a new list of sup- ported authentication types. Requesting authentication information may be done as a way of verifying the validity of the client's cre- dentials after an extended period of time or to negotiate a new ses- sion key for use during encryption. 7. User Interface Normally protocol specifications do not address user interface speci- fications. However, due to the fact that the user will probably want Expires March 2000 [Page 8]
Internet-Draft Telnet Authentication Option September 1999 to be able to specify the things about authentication and encryption and also know whether or not things succeeded, some guidance needs to be given to implementors to provide some minimum level of user con- trol. The user MUST be able to specify whether or not authentication is to be used, and whether or not encryption is to used if the authentica- tion succeeds. There SHOULD be at least four settings, REQUIRE, PROMPT, WARN and DISABLE. Setting the authentication switch to RE- QUIRE means that if the authentication fails, then an appropriate er- ror message must be displayed and the TELNET connection must be ter- minated. Setting the authentication switch to PROMPT means that if the authentication fails, then an appropriate error message must be displayed and the user must be prompted for confirmation before con- tinuing the TELNET session. Setting the authentication switch to WARN means that if the authentication fails, then an appropriate er- ror message must be displayed before continuing the TELNET session. Setting the authentication switch to DISABLE means that authentica- tion will not be attempted. The encryption switch SHOULD have the same settings as the authentication switch; however its settings are only used when authentication succeeds. The default setting for both switches should be WARN. Both of these switches may be implemented as a single switch, though having them separate gives more control to the user. 8. Example The following is an example of use of the option: Client Server IAC DO AUTHENTICATION IAC WILL AUTHENTICATION [ The server is now free to request authentication information. ] IAC SB AUTHENTICATION SEND KERBEROS_V4 CLIENT|MUTUAL KERBEROS_V4 CLIENT|ONE_WAY IAC SE [ The server has requested mutual Kerberos authentication, but is willing to do just one-way Kerberos authentication. The client will now respond with the name of the user that it wants to log in as, and the Kerberos ticket. ] IAC SB AUTHENTICATION NAME "joe" IAC SE IAC SB AUTHENTICATION IS KERBEROS_V4 CLIENT|MUTUAL AUTH 4 7 1 67 82 65 89 46 67 7 9 77 0 48 24 49 244 109 240 50 208 43 35 25 116 104 44 167 21 201 224 229 145 20 2 244 213 220 33 134 Expires March 2000 [Page 9]
Internet-Draft Telnet Authentication Option September 1999 148 4 251 249 233 229 152 77 2 109 130 231 33 146 190 248 1 9 31 95 94 15 120 224 0 225 76 205 70 136 245 190 199 147 155 13 IAC SE [ The server responds with an ACCEPT command to state that the authentication was successful. ] IAC SB AUTHENTICATION REPLY KERBEROS_V4 CLIENT|MUTUAL ACCEPT IAC SE [ Next, the client sends across a CHALLENGE to verify that it is really talking to the right server. ] IAC SB AUTHENTICATION IS KERBEROS_V4 CLIENT|MUTUAL CHALLENGE xx xx xx xx xx xx xx xx IAC SE [ Lastly, the server sends across a RESPONSE to prove that it really is the right server. IAC SB AUTHENTICATION REPLY KERBEROS_V4 CLIENT|MUTUAL RESPONSE yy yy yy yy yy yy yy yy IAC SE The following is an example of use of the option with encryption ne- gotiated via telnet ENCRYPT: Client Server IAC DO AUTHENTICATION IAC WILL AUTHENTICATION [ The server is now free to request authentication information. ] IAC SB AUTHENTICATION SEND KERBEROS_V4 ENCRYPT_USING_TELOPT|CLIENT|MUTUAL KERBEROS_V4 CLIENT|ONE_WAY IAC SE [ The server has requested mutual Kerberos authentication, but is willing to do just one-way Kerberos authentication. In both cases it is willing to encrypt the data stream. The client will now respond with the name of the user that it wants to log in as, and the Kerberos ticket. ] IAC SB AUTHENTICATION NAME "joe" IAC SE IAC SB AUTHENTICATION IS KERBEROS_V4 ENCRYPT_USING_TELOPT|CLIENT|MUTUAL AUTH 4 7 1 67 82 65 89 46 67 7 9 77 0 48 24 49 244 109 240 50 208 43 35 25 116 104 44 167 21 201 224 229 145 20 2 244 213 220 33 Expires March 2000 [Page 10]
Internet-Draft Telnet Authentication Option September 1999 134 148 4 251 249 233 229 152 77 2 109 130 231 33 146 190 248 1 9 31 95 94 15 120 224 0 225 76 205 70 136 245 190 199 147 155 13 IAC SE [ The server responds with an ACCEPT command to state that the authentication was successful. ] IAC SB AUTHENTICATION REPLY KERBEROS_V4 CLIENT|MUTUAL|ENCRYPT_USING_TELOPT ACCEPT IAC SE [ Next, the client sends across a CHALLENGE to verify that it is really talking to the right server. ] IAC SB AUTHENTICATION IS KERBEROS_V4 CLIENT|MUTUAL|ENCRYPT_USING_TELOPT CHALLENGE xx xx xx xx xx xx xx xx IAC SE [ The server sends across a RESPONSE to prove that it really is the right server. ] IAC SB AUTHENTICATION REPLY KERBEROS_V4 CLIENT|MUTUAL|ENCRYPT_USING_TELOPT RESPONSE yy yy yy yy yy yy yy yy IAC SE [ At this point, the client and server begin to negotiate the telnet ENCRYPT option in each direction for a secure channel. If the option fails in either direction for any reason the connection must be immediately terminated. ] The following is an example of use of the option with integrated en- cryption: Client Server IAC DO AUTHENTICATION IAC WILL AUTHENTICATION [ The server is now free to request authentication information. ] IAC SB AUTHENTICATION SEND KEA_SJ CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE IAC SE [ The server has requested mutual KEA authentication with SKIPJACK encryption. The client will now respond with the name of the user that it wants to log in as and the KEA cert. ] IAC SB AUTHENTICATION NAME "joe" IAC SE IAC SB AUTHENTICATION IS KEA_SJ CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE '1' CertA||Ra IAC SE Expires March 2000 [Page 11]
Internet-Draft Telnet Authentication Option September 1999 [ The server responds with its KEA Cert. ] IAC SB AUTHENTICATION REPLY KEA_SJ CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE '2' CertB||Rb||IVb||Encrypt(NonceB) IAC SE [ Next, the client sends across a CHALLENGE to verify that it is really talking to the right server. ] IAC SB AUTHENTICATION IS KEA_SJ CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE '3' IVa||Encrypt( NonceB xor 0x0C18 || NonceA ) IAC SE [ At this point, the client begins to encrypt the outgoing data stream, and the server, after receiving this command, begins to decrypt the incoming data stream. Lastly, the server sends across a RESPONSE to prove that it really is the right server. ] IAC SB AUTHENTICATION REPLY KEA_SJ CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE '4' Encrypt( NonceA xor 0x0C18 ) IAC SE [ At this point, the server begins to encrypt its outgoing data stream, and the client, after receiving this command, begins to decrypt its incoming data stream. ] It is expected that any implementation that supports the Telnet AU- THENTICATION option will support all of this specification. 9. Security Considerations This memo describes a general framework for adding authentication and encryption to the telnet protocol. The actual authentication mecha- nism is described in the authentication suboption specifications, and the security of the authentication option is dependent on the strengths and weaknesses of the authentication suboption. It should also be noted that the negotiation of the authentication type pair is not protected, thus allowing an attacker to force the result of the authentication to the weakest mutually acceptable method. (For example, even if both sides of the negotiation can ac- cept a "strong" mechanism and a "40-bit" mechanism, an attacker could force selection of the "40-bit" mechanism.) An implementation should therefore only accept an authentication mechanism to be negotiated if it is willing to trust it as being secure. 11. Acknowledgements Many people have worked on this document over the span of many years. Expires March 2000 [Page 12]
Internet-Draft Telnet Authentication Option September 1999 Dave Borman was a document editor and author of much of the original text. Other folks who have contributed ideas and suggestions to this text include: David Carrel, Jeff Schiller, and Richard Basch. 10. References [1] Reynolds, Joyce, and Postel, Jon, "Telnet Protocol Specifica- tion", RFC 854, May 1983. [2] D. Borman, "Telnet Authentication Option", RFC 1409, January 1993. [3] Internet Engineering Task Force, "Telnet Data Encryption Option", draft-tso-telnet-encryption-04.txt, T. Ts'o, Editor, VA Linux Systems, September 1999. [4] Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. Author's Address Theodore Ts'o, Editor VA Linux Systems 43 Pleasant St. Medford, MA 02155 Phone: (781) 391-3464 EMail: tytso@valinux.com Jeffrey Altman Columbia University Watson Hall Room 716 612 West 115th Street New York NY 10025 Phone: +1 (212) 854-1344 EMail: jaltman@columbia.edu Mailing List: telnet-wg@BSDI.COM Expires March 2000 [Page 13]