Network Working Group T. Ts'o, Editor Internet-Draft VA Linux Systems draft-tso-telnet-encryption-03.txt August 1999 Telnet Data Encryption 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 a the telnet encryption option as a generic method of providing data confidentiaility services of the telnet data stream. While this document summarizes currently utilized encrytion types and codesit does not define a specific encryption algorithm. Separate documents are to be published defining each encryption algo- rithms. 1. Command Names and Codes ENCRYPT 38 Encryption Commands IS 0 SUPPORT 1 REPLY 2 Expires February 19100 [Page 1] Internet-Draft Telnet Data Encryption Option August 1999 START 3 END 4 REQUEST-START 5 REQUEST-END 6 ENC_KEYID 7 DEC_KEYID 8 Encryption Types NULL 0 DES_CFB64 1 DES_OFB64 2 DES3_CFB64 3 DES3_OFB64 4 CAST5_40_CFB64 8 CAST5_40_OFB64 9 CAST128_CFB64 10 CAST128_OFB64 11 Following historical practice, future encryption type numbers will be assigned by the IANA under a First Come First Served pol- icy as outlined by RFC 2434 [3]. 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. The current list of encryption types is listed in http://www.isi.edu/in-notes/iana/assignments/telnet-options. 2. Command Meanings IAC WILL ENCRYPT The sender of this command is willing to send encrypted data. IAC WONT ENCRYPT The sender of this command refuses to send encrypted data. IAC DO ENCRYPT The sender of this command is willing to receive encrypted data. IAC DONT ENCRYPT The sender of this command refuses to accept encrypted data. Expires February 19100 [Page 2] Internet-Draft Telnet Data Encryption Option August 1999 IAC SB ENCRYPT SUPPORT encryption-type-list IAC SE The sender of this command is stating what types of encryption it will support. Only the side of the connection that is DO ENCRYPT may send the SUPPORT command. The current types of encryption are listed in the current version of the Assigned Numbers document[1]. IAC SB ENCRYPT IS encryption-type ... IAC SE The sender of this command is stating what type of encryption to use, and any initial data that is needed. Only the side of the connection that is WILL ENCRYPT may send the IS command. to ini- tialize the encryption-type scheme. IAC SB ENCRYPT REPLY encryption-type ... IAC SE The sender of this command is continuing the initial data exchange that is needed to initialize the encryption-type scheme. Only the side of the connection that is DO ENCRYPT may send the REPLY com- mand. IAC SB ENCRYPT START keyid IAC SE The sender of this command is stating that at this point in the data stream, all following data will be encrypted, via the previ- ously negotiated method of data encryption. Only the side of the connection that is WILL ENCRYPT may send the START command. The keyid is a variable length field. It is my be used by various encryption mechanisms to identify which encryption key is to be used, when multiple encryption keys might be known on either side of the connection. The keyid field is encoded with the most sig- nificant byte first, and a keyid value of zero is reserved to in- dicate the default encryption key (this would typically be an en- cryption key derived during authentication, with the AUTHENTICA- TION option). The keyid field must be at least one byte long. The only valid values for "keyid" will be those that have been re- ceived in a DEC_KEYID command. IAC SB ENCRYPT END IAC SE The sender of this command is stating that at this point in the data stream, all following data will no longer be encrypted. Only the side of the connection that is WILL ENCRYPT may send the END command. IAC SB ENCRYPT REQUEST-START keyid IAC SE The sender of this command requests that the remote side begin en- cryption of the telnet data stream. Only the side of the connec- Expires February 19100 [Page 3] Internet-Draft Telnet Data Encryption Option August 1999 tion that is DO ENCRYPT may send the REQUEST-START command. The keyid is only advisory, and my be omitted. IAC SB ENCRYPT REQUEST-END IAC SE The sender of this command requests that the remote side stop en- cryption of the telnet data stream. Only the side of the connec- tion that is DO ENCRYPT may send the REQUEST-END command. IAC SB ENCRYPT ENC_KEYID keyid IAC SE The sender of this requests that the remote side verify that "keyid" maps to a valid key on the remote side; or verifies that the "keyid" received in a DEC_KEYID command is valid. If keyid is omitted, it implies that there are no more known keyids, and that the attempt to find a common keyid has failed. Only the side of the connection that is WILL ENCRYPT may send the ENC_KEYID com- mand. IAC SB ENCRYPT DEC_KEYID keyid IAC SE The sender of this requests that the remote side verify that "keyid" maps to a valid key on the remote side; or verifies that the "keyid" received in a ENC_KEYID command is valid. If keyid is omitted, it implies that there are no more known keyids, and that the attempt to find a common keyid has failed. Only the side of the connection that is DO ENCRYPT may send the DEC_KEYID command. 3. Default Specification The default specification for this option is WONT ENCRYPT DONT ENCRYPT meaning there will not be any encryption of the Telnet data stream. 4. Motivation The Telnet protocol has no form of protection from some intervening gateway looking at IP packets as they travel through the network. This is especially dangerous when passwords are sent as clear text over the network. This option provides a method for encrypting the data stream. 5. Implementation Rules Once the Encryption option is in effect, all data in the negotiated direction, including TELNET options, is encrypted. Encryption begins Expires February 19100 [Page 4] Internet-Draft Telnet Data Encryption Option August 1999 with the octet of data immediately following the "IAC SB ENCRYPT START encryption-type IAC SE" command. Encryption ends after the "IAC SB ENCRYPT END IAC SE" command. WILL and DO are used only at the beginning of the connection to ob- tain and grant permission for future negotiations. The ENCRYPT op- tion must be negotiated in both directions. Once the two hosts have exchanged a WILL and a DO, the sender of the DO ENCRYPT must send a ENCRYPT SUPPORT command to let the remote side know what types of encryption it is willing to accept. In the re- quest, a list of supported encryption schemes is sent. Only the sender of the DO may send a list of supported encryption types (IAC SB ENCRYPT SUPPORT encryption-type-list IAC SE). Only the sender of the WILL may actually transmit encrypted data. This is initiated via the "IAC SB ENCRYPT START IAC SE" command, and terminated via the "IAC SB ENCRYPT END IAC SE" command. If a START is received, and then a second START is received before receiving an END, the second START is ignored. If the sender of the DO would like the remote side to begin sending encrypted data, it can send the "IAC SB ENCRYPT REQUEST-START IAC SE" command. If the sender of the DO would like the remote side to stop sending encrypted data, it can send the "IAC SB ENCRYPT REQUEST-STOP IAC SE" command. If the receiver of the SUPPORT command does not support any of the encryption types listed in the SUPPORT command, it should send an "IAC SB ENCRYPT IS NULL IAC SE" to indicate that there is not a com- mon encryption type. It may also send an IAC WONT ENCRYPT command to turn off the ENCRYPT option. The order of the encryption types in a SUPPORT command must be or- dered to indicate a preference for different encryption types, the first type being the most preferred, and the last type the least pre- ferred. If the ENCRYPT option has been enabled, and encrypted data is being received, the receipt of an "IAC WONT ENCRYPT" implies the receipt of an "IAC SB ENCRYPT END IAC SE", e.g., the Telnet data stream is no longer encrypted. The following is an example of use of the option: Host1 Host2 [ Host1 requests Host2 negotiate to encrypt data that it sends to Host1, and Host2 verifies that it will negotiate the encryption of data that it sends to Host1. ] DO ENCRYPT WILL ENCRYPT Expires February 19100 [Page 5] Internet-Draft Telnet Data Encryption Option August 1999 [ Host1 requests that Host2 enable encryption as soon as the initialization is completed, and informs Host2 that is supports DES_CFB64. ] IAC SB ENCRYPT REQUEST-START IAC SE IAC SB ENCRYPT SUPPORT DES_CFB64 IAC SE [ Host2 sends the initial feed to Host1, and Host1 acknowledges receipt of the IV. ] IAC SB ENCRYPT IS DES_CFB64 CFB64_IV 144 146 63 229 237 148 81 143 IAC SE IAC SB ENCRYPT REPLY DES_CFB64 CFB64_IV_OK 103 207 181 71 224 55 229 98 IAC SE [ Host2 is now free to start sending encrypted data, and since a REQUEST-START was received, it enables encryption. ] IAC SB ENCRYPT START IAC SE [ All data from Host2 to Host1 is now encrypted. ] IAC SB ENCRYPT END IAC SE [ All data from Host2 to Host1 is now in clear text again. ] It is expected that any implementation that supports the Telnet EN- CRYPT option will support all of this specification. 6. Security Considerations The ENCRYPT option used in isolation provides protection against pas- sive attacks, but not against active attacks. In other words, it will provide protection from someone who is just watching the IP packets as they pass through the network. However, an attacker who is able to modify packets in flight could prevent the ENCRYPT option from being negotiated. This flaw can be remedied by using the Telnet Authentication option alongside the ENCRYPT option. Specifically, setting ENCRYPT_US- ING_TELOPT in the authentication-type-pair can be used to force that Encryption be negotiated even in the face of active attacks. In addition, an active attacker can interfere with attempts to start or restart encryption. If encryption is requested by the user, and the client is unable to negotiate enabling or re-enabling encryption, the client must assume that it is being attacked, and MUST immediate- ly terminate the telnet connection. 6. TFhuetusrpeecdiifrieccattiioonnsdfeofrinTeeslnaetmeEtnhcordypftoironproviding data confidentiality to the telnet data stream. Unfortunately all of the encryption mech- anism provided under this option do not provide data integrity, be- cause of the complexity of specifying a protocol which provided in- tegrity services efficiently in a stream-oriented protocol. Expires February 19100 [Page 6] Internet-Draft Telnet Data Encryption Option August 1999 The TELNET_OVER_TLS specification provides a scheme which provides confidentiality, integrity, and compression, and future work for tel- net encryption should closely examine using this specification. One promising approach would use the anonymous Diffie-Hellman mode of TLS, followed by the telnet AUTHENTICATION option where the authenti- cation mechanism would include a signed hash of the Diffie-Hellman keying material negotiated by the TLS layer. 7. Acknowledgments This document was originally written by Dave Borman of Cray Research, with the assistance of Theodore Ts'o of MIT and the IETF Telnet Work- ing Group. 8. References [1] Reynolds, Joyce, and Postel, Jon, "Telnet Protocol Specifica- tion", RFC 854, May 1983. [2] Internet Engineering Task Force, "Telnet Authentication", draft- tso-telnet-auth-enc-03.txt, T. Ts'o, Editor, VA Linux Systems, August 1999. [3] 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@mit.edu [Page 7]