Network Working Group T. Ts'o, Editor Internet-Draft Massachusetts Institute of Technology draft-tso-telnet-auth-enc-00.txt February 1998 Telnet Authentication Option Status of this Memo This document is an Internet-Draft. 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." To view the entire list of current Internet-Drafts, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). 1. Command Names and Codes AUTHENTICATION 37 Authentication Commands IS 0 SEND 1 REPLY 2 NAME 3 Authentication Types NULL 0 KERBEROS_V4 1 KERBEROS_V5 2 SPX 3 RSA 6 LOKI 10 DSS 11 KEA_SJ 12 KEA_SJ_INTEG 13 Modifiers AUTH_WHO_MASK 1 Expires August 1998 [Page 1]
Internet-Draft Telnet Authentication Option February 1998 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 4 ENCRYPT_OFF 0 ENCRYPT_ON 4 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 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 sends this command if it receives a DO AUTHENTICATION 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 sends this command if it receives a WILL AUTHENTICA- TION 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 Expires August 1998 [Page 2]
Internet-Draft Telnet Authentication Option February 1998 types listed in "authentication-type-pair-list". The "authentication-type-pair-list" is an ordered list of "authentication-type" pairs. Only the server side (DO AUTHENTICA- TION) 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 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. There are currently three one bit fields defined in the modifier. Two of these 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 negotiation is successful, the client will know that it is con- nected to the server that it wants to be connected to. Expires August 1998 [Page 3]
Internet-Draft Telnet Authentication Option February 1998 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- 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 bit field in the modifier is the ENCRYPT_MASK bit. This bit is either set to ENCRYPT_ON or ENCRYPT_OFF. Setting this bit to ENCRYPT_ON implies that once authentication completes, the tel- net ENCRYPT option must be negotiated. If the negotiation for the ENCRYPT option fails, the data connection must be terminated. 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. Setting this bit to INI_CRED_FWD_ON implies that once authentica- tion completes, the client will immediately forward authentication credentials to the server. This bit is set by the client to ad- vise the server to expect forwarded credentials from the client. 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 mechan- ism does not support credentials forwarding. 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 Expires August 1998 [Page 4]
Internet-Draft Telnet Authentication Option February 1998 One of the deficiences 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 passwords 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. 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 suc- ceeded, the data stream can be encrypted to provide protection against active attacks. It is intended that the AUTHENTICATION option be general enough that 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. By linking the enabling of encryption as a side effect of successful authentication, protection is provided against an active attacker. An active attack is one where the underlying TCP stream can be modi- fied or taken over by the active attacker. If encryption were en- abled as a separate negotiation, it would provide a window of vulner- ability from when the authentication completes, up to and including the negotiation to turn on encryption. It is because of this that, while there is a command to end encryption, there is not a command to restart encryption. The only safe way to restart encryption once it has been turned off, is to repeat the entire authentication processes. 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 directions; the server must send the "DO", and the client must send the "WILL". This res- Expires August 1998 [Page 5]
Internet-Draft Telnet Authentication Option February 1998 triction is due to the nature of authentication; there are three pos- sible 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 replys ("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. 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. 7. User Interface Normally protocol specifications do not address user interface specifications. However, due to the fact that the user will probably want to be able to specify the things about authentication and en- cryption and also know whether or not things succeeded, some guidance needs to be given to implementors to provide some minimum level of user control. 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. Expires August 1998 [Page 6]
Internet-Draft Telnet Authentication Option February 1998 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 switchs should be WARN. Both of these switchs may be implemented as a single switch, though having them seperate 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 33 134 148 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 REPLY 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 Expires August 1998 [Page 7]
Internet-Draft Telnet Authentication Option February 1998 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: Client Server IAC DO AUTHENTICATION IAC WILL AUTHENTICATION [ The server is now free to request authentication information. ] IAC SB AUTHENTICATION SEND KERBEROS_V4 ENCRYPT_ON|CLIENT|MUTUAL KERBEROS_V4 ENCRYPT_ON|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_ON|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 33 134 148 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_ON ACCEPT IAC SE [ Next, the client sends across a CHALLENGE to verify that it is really talking to the right server. ] IAC SB AUTHENTICATION REPLY KERBEROS_V4 CLIENT|MUTUAL CHALLENGE xx xx xx xx xx xx xx xx IAC SE [ At this point, the client begins to encrypt the outgoing data Expires August 1998 [Page 8]
Internet-Draft Telnet Authentication Option February 1998 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 KERBEROS_V4 CLIENT|MUTUAL RESPONSE yy yy yy yy yy yy yy yy 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. References [1] Reynolds, Joyce, and Postel, Jon, "Assigned Numbers", RFC 1060, ISI, March 1990 Author's Address Theodore Ts'o, Editor Massachusetts Institute of Technology MIT Room E40-343 77 Massachusetts Aveneue Cambridge, MA 02139 Phone: (617) 253-8091 Mailing List: telnet-ietf@CRAY.COM EMail: tytso@mit.edu Expires August 1998 [Page 9]