📄 rfc2228.txt
字号:
hexadecimal C5 in an EBCDIC-based system, the local significance of the two representations is equivalent). A 64-character subset of International Alphabet IA5 is used, enabling 6 bits to be represented per printable character. (The proposed subset of characters is represented identically in IA5 and ASCII.) The character "=" signifies a special processing function used for padding within the printable encoding procedure. The encoding process represents 24-bit groups of input bits as output strings of 4 encoded characters. Proceeding from left to right across a 24-bit input group output from the security mechanism specific message protection procedure, each 6-bit group is used as an index into an array of 64 printable characters, namely "[A-Z][a- z][0-9]+/". The character referenced by the index is placed in the output string. These characters are selected so as to be universally representable, and the set excludes characters with particular significance to Telnet (e.g., "<CR>", "<LF>", IAC). Special processing is performed if fewer than 24 bits are available in an input group at the end of a message. A full encoding quantum is always completed at the end of a message. When fewer than 24 input bits are available in an input group, zero bits are added (on the right) to form an integral number of 6-bit groups. Output character positions which are not required to represent actual input data are set to the character "=". Since all canonically encoded output is an integral number of octets, only the following cases can arise: (1) the final quantum of encoding input is an integral multiple of 24 bits; here, the final unit of encoded output will be an integral multiple of 4 characters with no "=" padding, (2) the final quantum of encoding input is exactly 8 bits; here, the final unit of encoded output will be two characters followed by two "=" padding characters, or (3) the final quantum of encoding input is exactly 16 bits; here, the final unit of encoded output will be three characters followed by one "=" padding character.Horowitz & Lunt Standards Track [Page 21] RFC 2228 FTP Security Extensions October 1997 Implementors must keep in mind that the base 64 encodings in ADAT, MIC, CONF, and ENC commands, and in 63z replies may be arbitrarily long. Thus, the entire line must be read before it can be processed. Several successive reads on the control channel may be necessary. It is not appropriate to for a server to reject a command containing a base 64 encoding simply because it is too long (assuming that the decoding is otherwise well formed in the context in which it was sent). Case must not be ignored when reading commands and replies containing base 64 encodings.11. Security Considerations This entire document deals with security considerations related to the File Transfer Protocol. Third party file transfers cannot be secured using these extensions, since a security context cannot be established between two servers using these facilities (no control connection exists between servers over which to pass ADAT tokens). Further work in this area is deferred.12. Acknowledgements I would like to thank the members of the CAT WG, as well as all participants in discussions on the "cat-ietf@mit.edu" mailing list, for their contributions to this document. I would especially like to thank Sam Sjogren, John Linn, Ted Ts'o, Jordan Brown, Michael Kogut, Derrick Brashear, John Gardiner Myers, Denis Pinkas, and Karri Balk for their contributions to this work. Of course, without Steve Lunt, the author of the first six revisions of this document, it would not exist at all.13. References [TELNET-SEC] Borman, D., "Telnet Authentication and Encryption Option", Work in Progress. [RFC-1123] Braden, R., "Requirements for Internet Hosts -- Application and Support", STD 3, RFC 1123, October 1989. [RFC-1421] Linn, J., "Privacy Enhancement for Internet Electronic Mail: Part I: Message Encryption and Authentication Procedures", RFC 1421, February 1993.Horowitz & Lunt Standards Track [Page 22] RFC 2228 FTP Security Extensions October 199714. Author's Address Marc Horowitz Cygnus Solutions 955 Massachusetts Avenue Cambridge, MA 02139 Phone: +1 617 354 7688 EMail: marc@cygnus.comHorowitz & Lunt Standards Track [Page 23] RFC 2228 FTP Security Extensions October 1997Appendix I: Specification under the GSSAPI In order to maximise the utility of new security mechanisms, it is desirable that new mechanisms be implemented as GSSAPI mechanisms rather than as FTP security mechanisms. This will enable existing ftp implementations to support the new mechanisms more easily, since little or no code will need to be changed. In addition, the mechanism will be usable by other protocols, such as IMAP, which are built on top of the GSSAPI, with no additional specification or implementation work needed by the mechanism designers. The security mechanism name (for the AUTH command) associated with all mechanisms employing the GSSAPI is GSSAPI. If the server supports a security mechanism employing the GSSAPI, it must respond with a 334 reply code indicating that an ADAT command is expected next. The client must begin the authentication exchange by calling GSS_Init_Sec_Context, passing in 0 for input_context_handle (initially), and a targ_name equal to output_name from GSS_Import_Name called with input_name_type of Host-Based Service and input_name_string of "ftp@hostname" where "hostname" is the fully qualified host name of the server with all letters in lower case. (Failing this, the client may try again using input_name_string of "host@hostname".) The output_token must then be base 64 encoded and sent to the server as the argument to an ADAT command. If GSS_Init_Sec_Context returns GSS_S_CONTINUE_NEEDED, then the client must expect a token to be returned in the reply to the ADAT command. This token must subsequently be passed to another call to GSS_Init_Sec_Context. In this case, if GSS_Init_Sec_Context returns no output_token, then the reply code from the server for the previous ADAT command must have been 235. If GSS_Init_Sec_Context returns GSS_S_COMPLETE, then no further tokens are expected from the server, and the client must consider the server authenticated. The server must base 64 decode the argument to the ADAT command and pass the resultant token to GSS_Accept_Sec_Context as input_token, setting acceptor_cred_handle to NULL (for "use default credentials"), and 0 for input_context_handle (initially). If an output_token is returned, it must be base 64 encoded and returned to the client by including "ADAT=base64string" in the text of the reply. If GSS_Accept_Sec_Context returns GSS_S_COMPLETE, the reply code must be 235, and the server must consider the client authenticated. If GSS_Accept_Sec_Context returns GSS_S_CONTINUE_NEEDED, the reply code must be 335. Otherwise, the reply code should be 535, and the text of the reply should contain a descriptive error message.Horowitz & Lunt Standards Track [Page 24] RFC 2228 FTP Security Extensions October 1997 The chan_bindings input to GSS_Init_Sec_Context and GSS_Accept_Sec_Context should use the client internet address and server internet address as the initiator and acceptor addresses, respectively. The address type for both should be GSS_C_AF_INET. No application data should be specified. Since GSSAPI supports anonymous peers to security contexts, it is possible that the client's authentication of the server does not actually establish an identity. The procedure associated with MIC commands, 631 replies, and Safe file transfers is: GSS_Wrap for the sender, with conf_flag == FALSE GSS_Unwrap for the receiver The procedure associated with ENC commands, 632 replies, and Private file transfers is: GSS_Wrap for the sender, with conf_flag == TRUE GSS_Unwrap for the receiver CONF commands and 633 replies are not supported. Both the client and server should inspect the value of conf_avail to determine whether the peer supports confidentiality services. When the security state is reset (when AUTH is received a second time, or when REIN is received), this should be done by calling the GSS_Delete_sec_context function.Appendix II: Specification under Kerberos version 4 The security mechanism name (for the AUTH command) associated with Kerberos Version 4 is KERBEROS_V4. If the server supports KERBEROS_V4, it must respond with a 334 reply code indicating that an ADAT command is expected next. The client must retrieve a ticket for the Kerberos principal "ftp.hostname@realm" by calling krb_mk_req(3) with a principal name of "ftp", an instance equal to the first part of the canonical host name of the server with all letters in lower case (as returned by krb_get_phost(3)), the server's realm name (as returned by krb_realmofhost(3)), and an arbitrary checksum. The ticket must then be base 64 encoded and sent as the argument to an ADAT command.Horowitz & Lunt Standards Track [Page 25] RFC 2228 FTP Security Extensions October 1997 If the "ftp" principal name is not a registered principal in the Kerberos database, then the client may fall back on the "rcmd" principal name (same instance and realm). However, servers must accept only one or the other of these principal names, and must not be willing to accept either. Generally, if the server has a key for the "ftp" principal in its srvtab, then that principal only must be used, otherwise the "rcmd" principal only must be used. The server must base 64 decode the argument to the ADAT command and pass the result to krb_rd_req(3). The server must add one to the checksum from the authenticator, convert the result to network byte order (most significant byte first), and sign it using krb_mk_safe(3), and base 64 encode the result. Upon success, the server must reply to the client with a 235 code and include "ADAT=base64string" in the text of the reply. Upon failure, the server should reply 535. Upon receipt of the 235 reply from the server, the client must parse the text of the reply for the base 64 encoded data, decode it, convert it from network byte order, and pass the result to krb_rd_safe(3). The client must consider the server authenticated if the resultant checksum is equal to one plus the value previously sent. The procedure associated with MIC commands, 631 replies, and Safe file transfers is: krb_mk_safe(3) for the sender krb_rd_safe(3) for the receiver The procedure associated with ENC commands, 632 replies, and Private file transfers is: krb_mk_priv(3) for the sender krb_rd_priv(3) for the receiver CONF commands and 633 replies are not supported. Note that this specification for KERBEROS_V4 contains no provision for negotiating alternate means for integrity and confidentiality routines. Note also that the ADAT exchange does not convey whether the peer supports confidentiality services. In order to stay within the allowed PBSZ, implementors must take note that a cleartext buffer will grow by 31 bytes when processed by krb_mk_safe(3) and will grow by 26 bytes when processed by krb_mk_priv(3).Horowitz & Lunt Standards Track [Page 26] RFC 2228 FTP Security Extensions October 1997Full Copyright Statement Copyright (C) The Internet Society (1997). 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 implmentation may be prepared, copied, published andand 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.Horowitz & Lunt Standards Track [Page 27] ⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -