Network Working Group M. Horowitz Openvision Technologies Updates: RFC 959 S. J. Lunt Internet-Draft Bellcore March, 1995 FTP Security Extensions 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 learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the Internet- Drafts Shadow Directories on ds.internic.net (US East Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). Distribution of this memo is unlimited. Please send comments to the mailing list. Abstract This document defines extensions to the FTP specification RFC 959, "FILE TRANSFER PROTOCOL (FTP)" (October 1985). These extensions provide strong authentication, integrity, and confidentiality on both the control and data channels with the introduction of new optional commands, replies, and file transfer encodings. The following new optional commands are introduced in this specification: AUTH (Authentication Mechanism), ADAT (Authentication Data), PROT (Data Channel Protection Level), PBSZ (Protection Buffer Size), MIC (Integrity Protected Command), CONF (Confidentiality Protected Command), and ENC (Privacy Protected Command). Expires: September 30, 1995 [Page 1] draft-ietf-cat-ftpsec-06.txtFTP Security Extensions March, 1995 A new class of reply types (6yz) is also introduced for protected replies. None of the above commands are required to be implemented, but interdependencies exist. Note that this specification is compatible with RFC 959. 1. Introduction The File Transfer Protocol (FTP) currently defined in RFC 959 and in place on the Internet uses usernames and passwords passed in clear- text to authenticate clients to servers (via the USER and PASS com- mands). Except for services such as 'anonymous' FTP archives, this represents a security risk whereby passwords can be stolen through monitoring of local and wide-area networks. This either aids poten- tial attackers through password exposure and/or limits accessibility of files by FTP servers who cannot or will not accept the inherent security risks. Aside from the problem of authenticating users in a secure manner, there is also the problem of protecting sensitive data and/or verify- ing its integrity. An attacker may be able to access valuable or sensitive data merely by monitoring a network, or through active means may be able to delete or modify the data being transferred so as to corrupt its integrity. An active attacker may also initiate spurious file transfers to and from a site of the attacker's choice, and may invoke other commands on the server. FTP does not currently have any provision for the encryption or verification of the authen- ticity of commands, replies, or transferred data. Note that these security services have value even to anonymous file access. Current practice for sending files securely is generally either: 1. via FTP of files pre-encrypted under keys which are manually distributed, 2. via electronic mail containing an encoding of a file encrypted under keys which are manually distributed, 3. via a PEM message, or 4. via the rcp command enhanced to use Kerberos. None of these means could be considered even a de facto standard, and none are truly interactive. A need exists to securely transfer files using FTP in a secure manner which is supported within the FTP proto- col in a consistent manner and which takes advantage of existing Expires: September 30, 1995 [Page 2] draft-ietf-cat-ftpsec-06.txtFTP Security Extensions March, 1995 security infrastructure and technology. Extensions are necessary to the FTP specification if these security services are to be introduced into the protocol in an interoperable way. Although the FTP control connection follows the Telnet protocol, and Telnet has defined an authentication and encryption option [TELNET- SEC], [RFC-1123] explicitly forbids the use of Telnet option negotia- tion over the control connection (other than Synch and IP). Also, the Telnet authentication and encryption option does not provide for integrity protection only (without confidentiality), and does not address the protection of the data channel. 2. FTP Security Overview At the highest level, the FTP security extensions seek to provide an abstract mechanism for authenticating and/or authorizing connections, and integrity and/or confidentiality protecting commands, replies, and data transfers. In the context of FTP security, authentication is the establishment of a client's identity in a secure way, usually using cryptographic techniques. The basic FTP protocol does not have a concept of authentication. Authorization is the process of validating a user for login. The basic authorization process involves the USER, PASS, and ACCT com- mands. With the FTP security extensions, authentication established using the authentication mechanism may also be used to make the authorization decision. Without the security extensions, FTP authentication is established by assertion (with the USER command), and authorization is validated by a password on the network in the clear. An FTP security interaction begins with a client telling the server what authentication mechanism it wants to use with the AUTH command. The server will either accept this mechanism, reject this mechanism, or, in the case of a server which does not implement the security extensions, reject the command completely. The client may try multi- ple authentication mechanisms until it requests one which the server accepts. This allows a rudimentary form of negotiation to take place. The server's reply will will indicate if the client should respond with additional data for the authentication mechanism to interpret. If none is needed, this will usually mean that the mechanism is one where the password (specified by the PASS command) is to be interpreted differently, such as with a token or one-time password system. Expires: September 30, 1995 [Page 3] draft-ietf-cat-ftpsec-06.txtFTP Security Extensions March, 1995 If the server requires additional authentication information, then the client and server will enter into an authorization data exchange. The client will send an ADAT command containing the first block of authentication data. The server's reply will indicate if the data exchange is complete, if there was an error, or if more data is needed. The server's reply can optionally contain authentication data for the client to interpret. If more data is needed, the client will send another ADAT command containing the next block of data, and await the server's reply. This exchange can continue as many times as necessary. Once this exchange completes, the client and server have established authentication, which may be one-way or mutual, depending on the mechanism in use. Once authentication is established, this authentication may be used instead of or in addition to the standard username/password exchange for authorizing a user to connect to the server. In addition, the client and server may begin integrity and/or confidentiality protect- ing commands with the MIC, CONF, and ENC commands, and replies with the 63z reply codes. Once the client and server have negotiated with the PBSZ command an acceptable buffer size for encapsulating protected data over the data channel, the authentication mechanism may also be used to protect data channel transfers. Policy is not specified by this document. In particular, client and server implementations may choose to implement restrictions on what operations can be performed depending on the authentication which exists. For example, a server might require that a client authorize via an authentication mechanism rather than using a password, require at least integrity protection on the command channel, or require that certain files only be transmitted encrypted. An anonymous ftp client might refuse to do file transfers without integrity protection in order to insure the validity of files downloaded. No particular set of functionality is required, except as dependen- cies described in the next section. This means that none of authori- zation, integrity, or confidentiality are required of an implementa- tion, although a mechanism which does none is not of much use. For example, it is acceptable for a mechanism to implement only integrity protection, one-way authentication and/or encryption, encryption without any authentication or integrity protection, or any other sub- set of functionality if policy or technical considerations make this desirable. Of course, a peer might require stronger protection as a matter of policy, preventing perfect interoperability. Expires: September 30, 1995 [Page 4] draft-ietf-cat-ftpsec-06.txtFTP Security Extensions March, 1995 3. New FTP Commands The following commands are optional, but dependent on each other. They are extensions to the FTP Access Control Commands. AUTHENTICATION TYPE (AUTH) The argument field is a Telnet string identifying a supported mechanism. This string is case-insensitive. Values must be registered with the IANA, except that values beginning with "X-" are reserved for local use. If the server does not implement the security extensions, it should respond with reply code 500. If the server does not understand the named authentication mechan- ism, it should respond with reply code 504. If the server is not willing to accept the named authentication mechanism, it should respond with reply code 534. If the server is not able to accept the named authentication mechanism, such as if a required resource is unavailable, it should respond with reply code 431. If the server is willing to accept the named authentication mechanism, but requires authentication data, it should respond with reply code 334. If the server is willing to accept the named authentication mechanism, and does not require any authentication data, it should respond with reply code 234. Some servers will allow the AUTH command to be reissued in order to establish new authentication. The AUTH command, if accepted, removes any state associated with prior FTP Security commands. The server may also choose to require that the user reauthorize in this case (see section 2). AUTHENTICATION DATA (ADAT) The argument field is a Telnet string representing base 64 encoded authentication data (see Section xx, "Base 64 Encoding"). If a reply code indicating success is returned, the server may also use a string of the form "ADAT=base64data" as the text part of the reply if it wishes to convey authentication data back to the client. Expires: September 30, 1995 [Page 5] draft-ietf-cat-ftpsec-06.txtFTP Security Extensions March, 1995 The data in both cases is specific to the authentication mechanism specified by the previous AUTH command. The ADAT command, and the associated replies, allow the client and server to conduct an arbitrary authentication protocol. The authentication data exchange should include enough information for both peers to be aware of which optional features are available. For example, if the client does not support authentication, the server should be aware of this, so it will know not to send encrypted command chan- nel replies. The authentication mechanism should provide sequenc- ing on the command channel, to insure that commands are not deleted, reordered, or replayed. The ADAT command must be preceded by a successful AUTH command, and cannot be issued once an authentication data exchange com- pletes (successfully or unsuccessfully), unless it is preceded by an AUTH command to reset the authentication state. If the server has not yet received an AUTH command, or if a prior authentication data exchange completed, but the authentication state has not been reset with an AUTH command, it should reply with reply code 503. If the server cannot base 64 decode the argument, it should respond with reply code 501. If the server rejects the authentication data (if a checksum fails, for instance), it should respond with reply code 535. If the server accepts the authentication data, and requires addi- tional data, it should respond with reply code 336. If the server accepts the authentication data, but does not require any additional data (i.e., the authentication data exchange has completed successfully), it should respond with reply code 235. If the server is responding with a 234 or 334 reply code, then it may include authentication data in the text part of the reply as specified above. If the authentication mechanism exchange returns an error, the authentication state of the client is reset. If the client becomes unsynchronized with the server (for example, the server sends a 234 reply code to an AUTH command, but the client has more data to transmit), then the client should reset the server's authentication state. PROTECTION BUFFER SIZE (PBSZ) Expires: September 30, 1995 [Page 6] draft-ietf-cat-ftpsec-06.txtFTP Security Extensions March, 1995 The argument is a decimal integer representing the maximum size, in bytes, of the encoded data blocks to be sent or received during file transfer. This number shall be no greater than can be represented in a 32-bit unsigned integer. This command allows the FTP client and server to negotiate a max- imum protected buffer size for the connection. There is no default size; the client must issue a PBSZ command before it can issue the first PROT command. The PBSZ command must be preceded by a successful authentication data exchange. If the server cannot parse the argument, or if it will not fit in 32 bits, it should respond with a 501 reply code. If the server has not completed an authentication data exchange with the client, it should respond with a 503 reply code. Otherwise, the server should reply with a 200 reply code. If the size provided by the client is too large for the server, it should use a string of the form "PBSZ=number" in the text part of the reply to indicate a smaller buffer size. The client and the server should use the smaller of the two buffer sizes if both buffer sizes are specified. DATA CHANNEL PROTECTION LEVEL (PROT) The argument is a single Telnet character code specifying the data channel protection level. This command indicates to the server what type of data channel protection the client and server will be using. The following codes are assigned: C - Clear S - Safe E - Confidential P - Private The default protection level if no other level is specified is Clear. The Clear protection level indicates that the data channel will carry the raw data of the file transfer, with no security applied. The Safe protection level indicates that the data will be integrity protected. The Confidential protection level indi- cates that the data will be confidentiality protected. The Private protection level indicates that the data will be integrity and confidentiality protected. Expires: September 30, 1995 [Page 7] draft-ietf-cat-ftpsec-06.txtFTP Security Extensions March, 1995 It is reasonable for an authentication mechanism not to provide all data channel protection levels. It is also reasonable for a mechanism to provide more protection at a level than is required (for instance, a mechanism might provide Confidential protection, but include integrity-protection in that encoding, due to API or other considerations). The PROT command must be preceded by a successful protection buffer size negotiation. If the server does not understand the specified protection level, it should respond with reply code 504. If the current authentication mechanism does not support the specified protection level, the server should respond with reply code 536. If the server has not completed a protection buffer size negotia- tion with the client, it should respond with a 503 reply code. The PROT command will be rejected and the server will reply 503 if no previous PBSZ command was issued. If the server is not willing to accept the specified protection level, it should respond with reply code 534. If the server is not able to accept the specified protection level, such as if a required resource is unavailable, it should respond with reply code 431. INTEGRITY PROTECTED COMMAND (MIC) and CONFIDENTIALITY PROTECTED COMMAND (CONF) and PRIVACY PROTECTED COMMAND (ENC) and The argument field of MIC is a Telnet string consisting of a base 64 encoded "safe" message produced by an authentication mechanism specific message integrity procedure. The argument field of CONF is a Telnet string consisting of a base 64 encoded "confidential" message produced by an authentication mechanism specific confiden- tiality procedure. The argument field of ENC is a Telnet string consisting of a base 64 encoded "private" message produced by an authentication mechanism specific message integrity and confiden- tiality procedure. The server will decode and/or verify the encoded message. This command must be preceded by a successful authentication data exchange. Expires: September 30, 1995 [Page 8] draft-ietf-cat-ftpsec-06.txtFTP Security Extensions March, 1995 If the server cannot base 64 decode the argument, it should respond with a 501 reply code. If the server has not completed an authentication data exchange with the client, it should respond with a 503 reply code. If the server rejects the command (if a checksum fails, for instance), it should respond with reply code 535. If the server is not willing to accept the command (if privacy is required by policy, for instance), it should respond with reply code 533. Otherwise, the command will be interpreted as an FTP command. An end-of-line code need not be included, but if one is included, it must be a Telnet end-of-line code, not a local end-of-line code. The server may require that, under some or all circumstances, all commands be protected. In this case, it should make a 533 reply to commands other than MIC, CONF, and ENC. 4. New FTP Replies The new reply codes are divided into two classes. The first class is new replies made necessary by the new FTP Security commands. The second class is a new reply type to indicate protected replies. 4.1. New individual reply codes 232 User logged in, authorized by authentication data exchange. 234 Authentication data exchange complete. 235 [ADAT=base64data] ; This reply indicates that the authentication exchange ; completed successfully. The square brackets are not ; to be included in the reply, but indicate that ; authentication data in the reply is optional. 334 Need more authentication data. 335 [ADAT=base64data] ; This reply indicates that the authentication data is ; acceptable, and more is required to complete the ; authentication data exchange. The square brackets ; are not to be included in the reply, but indicate ; that authentication data in the reply is optional. ; Authentication data ok, need more authentication ; data. Expires: September 30, 1995 [Page 9] draft-ietf-cat-ftpsec-06.txtFTP Security Extensions March, 1995 336 Username okay, need password. Challenge is "...." ; The exact representation of the challenge should be chosen ; by the mechanism to be sensible to the human user of the ; system. 431 Need some unavailable resource to handle authentication. 533 Command protection level denied for policy reasons. 534 Request denied for policy reasons. 535 Failed security check (hash, sequence, etc). 536 Requested PROT level not supported by mechanism. 4.2. Protected replies. One new reply type is introduced: 6yz Protected reply There are three reply codes of this type. The first, reply code 631 indicates an integrity protected reply. The second, reply code 632, indicates a confidentiality and integrity protected reply. the third, reply code 633, indi- cates a confidentiality protected reply. The text part of a 631 reply is a Telnet string consisting of a base 64 encoded "safe" message produced by an authenti- cation mechanism specific message integrity procedure. The text part of a 632 reply is a Telnet string consisting of a base 64 encoded "private" message produced by an authentica- tion mechanism specific message confidentiality and integrity procedure. The text part of a 633 reply is a Tel- net string consisting of a base 64 encoded "confidential" message produced by an authentication mechanism specific message confidentiality procedure. The client will decode and verify the encoded reply. How failures decoding or verifying replies are handled is implementation-specific. An end-of-line code need not be included, but if one is included, it must be a Telnet end- of-line code, not a local end-of-line code. A protected reply may only be sent if an authentication data exchange has succeeded. The 63z reply may be a multiline reply. In this case, the plaintext reply must be broken up into a number of frag- ments. Each fragment must be protected, then base 64 Expires: September 30, 1995 [Page 10] draft-ietf-cat-ftpsec-06.txtFTP Security Extensions March, 1995 encoded in order into a separate line of the multiline reply. There need not be any correspondence between the line breaks in the plaintext reply and the encoded reply. Telnet end-of-line codes must appear in the plaintext of the encoded reply, except for the final end-of-line code, which is optional. The multiline reply must be formatted more strictly than the continuation specification in RFC 959. In particular, each line before the last must be formed by the reply code, fol- lowed immediately by a hyphen, followed by a base 64 encoded fragment of the reply. For example, if the plaintext reply is 123-First line Second line 234 A line beginning with numbers 123 The last line then the resulting protected reply could be any of the fol- lowing (the first example has a line break only to fit within the margins): 631 base64(protect("123-First line\r\nSecond line\r\n 234 A line beginning with numbers\r\n123 The last line\r\n")) 631-base64(protect("123-First line\r\n")) 631-base64(protect("Second line\r\n")) 631-base64(protect(" 234 A line beginning with numbers\r\n")) 631 base64(protect("123 The last line")) 631-base64(protect("123-First line\r\nSecond line\r\n 234 A line b")) 631 base64(protect("eginning with numbers\r\n123 The last line\r\n")) 5. Login Authorization The authentication data exchange may, among other things, establish the identity of the client in a secure way to the server. This iden- tity may be used as one input to the login authorization process. In response to the FTP login commands (AUTH, PASS, ACCT), the server may choose not to follow the sequence of commands and replies speci- fied by RFC 959. There are also some new replies available. If the server is willing to allow the user named by the USER command to log in based on the identity established by the authentication Expires: September 30, 1995 [Page 11] draft-ietf-cat-ftpsec-06.txtFTP Security Extensions March, 1995 data exchange, it should respond with reply code 232. If the authentication mechanism requires a challenge/response pass- word, it should respond to the USER command with reply code 335. The text part of the reply should contain the challenge. The client should be sure to display the challenge to the user before prompting for the password in this case. 6. Data Channel Encapsulation When data transfers are protected between the client and server (in either direction), certain transformations and encapsulations must be performed so that the recipient can properly decode the transmitted file. The sender should apply all protection services after transformations associated with the representation type, file structure, and transfer mode have been performed. The data sent over the data channel is, for the purposes of protection, to be treated as a byte stream. The sender should take the input byte stream, and break it up into blocks such that each block, when encoded using an authentication mechanism specific procedure, will be no larger than the buffer size negotiated by the client with the PBSZ command. Each block should be encoded, then transmitted with the length of the encoded block prepended as a four byte unsigned integer, most significant byte first. When the end of the file is reached, the sender should encode a block of zero bytes, and send this final block to the recipient before closing the data connection. The recipient will read the four byte length, read a block of data that many bytes long, then decode and verify this block with an authentication mechanism specific procedure. This should be repeated until a block encoding a buffer of zero bytes is received. This indicates the end of the encoded byte stream. Any transformations associated with the representation type, file structure, and transfer mode are to be performed by the recipient on the byte stream resulting from the above process. When using block transfer mode, the sender's (cleartext) buffer size is independent of the block size. The server will reply 534 to a STOR, STOU, RETR, LIST, NLST, or APPE command if the current protection level is not at the level dictated Expires: September 30, 1995 [Page 12] draft-ietf-cat-ftpsec-06.txtFTP Security Extensions March, 1995 by the server's security requirements for the particular file transfer. If any data protection services fail at any time during data transfer at the server end (including an attempt to send a buffer size greater than the negotiated maximum), the server will send a 535 reply to the data transfer command (either STOR, STOU, RETR, LIST, NLST, or APPE). 7. Potential policy considerations While there are no restrictions on client and server policy, there are a few recommendations which an implementation should implement. - Once an authentication data exchange takes place, a server should require all commands be protected (with integrity and/or confiden- tiality), and it should protect all replies. Replies should use the same level of protection as the command which produced them. This includes replies which indicate failure of the MIC, CONF, and ENC commands. In particular, it is not meaningful to require that AUTH and ADAT be protected; it is meaningful and useful to require that PROT and PBSZ be protected. - A client should encrypt the PASS command whenever possible. It is reasonable for the server to refuse to accept a non-encrypted PASS command if the server knows encryption is available. - Although no commands are required, it is recommended that an imple- mentation provide all commands which can be implemented, given the mechanisms supported and the policy considerations of the site (export controls, for instance). 8. Declarative specifications These sections are modelled after sections 5.3 and 5.4 of RFC 959, which describe the same information, except for the standard FTP commands and replies. 8.1. FTP Security commands and arguments AUTH ADAT PROT PBSZ MIC CONF Expires: September 30, 1995 [Page 13] draft-ietf-cat-ftpsec-06.txtFTP Security Extensions March, 1995 ENC ::= ::= ; must be formatted as described in section xx ::= C | S | E | P ::= any decimal integer from 1 to (2^32)-1 8.2. Command-Reply sequences Authentication AUTH 234 334 504, 534, 431 500, 501, 421 ADAT 235 335 503, 501, 535 500, 501, 421 Data protection negotiation commands PBSZ 200 503 500, 501, 421, 530 PROT 504, 536, 503, 534, 431 500, 501, 421, 530 Command channel protection commands MIC 535, 533 500, 501, 421 CONF 535, 533 500, 501, 421 ENC 535, 533 500, 501, 421 Security-Enhanced login commands (only new replies listed) USER 232 336 Data channel commands (only new replies listed) STOR 534, 535 STOU Expires: September 30, 1995 [Page 14] draft-ietf-cat-ftpsec-06.txtFTP Security Extensions March, 1995 534, 535 RETR 534, 535 LIST 534, 535 NLST 534, 535 APPE 534, 535 In addition to these reply codes, any command can return 631, 632, 633, or 533. 9. Base 64 Encoding Base 64 encoding is the same as the Printable Encoding described in Section 4.3.2.4 of [RFC-1421], except that no line breaks should be included. This encoding is defined as follows. Proceeding from left to right, the bit string resulting from the mechanism specific protection routine is encoded into characters which are universally representable at all sites, though not neces- sarily with the same bit patterns (e.g., although the character "E" is represented in an ASCII-based system as hexadecimal 45 and as hex- adecimal 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 sub- set of characters is represented identically in IA5 and ASCII.) The character "=" signifies a special processing function used for pad- ding 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 authentication 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 signi- ficance to Telnet (e.g., "", "", 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 Expires: September 30, 1995 [Page 15] draft-ietf-cat-ftpsec-06.txtFTP Security Extensions March, 1995 the right) to form an integral number of 6-bit groups. Output char- acter 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 mul- tiple 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. Implementors should 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 should not be ignored when reading commands and replies contain- ing base 64 encodings. 10. 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. 11. Acknowledgements I would like to thank the members of the CAT WG, as well as all par- ticipants 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, and John Gardiner Myers 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. Expires: September 30, 1995 [Page 16] draft-ietf-cat-ftpsec-06.txtFTP Security Extensions March, 1995 12. References [TELNET-SEC] Borman, D., "Telnet Authentication and Encryption Option", Internet Draft, Cray Research, Inc, April 1993. [RFC-1123] Braden, R., "Requirements for Internet Hosts -- Applica- tion and Support", 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. 13. Author's Address Marc Horowitz Openvision Technologies 1 Main Street Cambridge, MA 02142 Phone: 617 374 2224 Email: marc@cam.ov.com Appendix I: Specification under Kerberos version 4 The authentication mechanism name (for the AUTH command) associated with Kerberos Version 4 is KERBEROS_V4. If the server supports KERBEROS_V4, it will respond with a 334 reply code indicating that an ADAT command is expected next. The client should 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. If the "ftp" principal name is not a registered principal in the Ker- beros 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 should be used, otherwise the "rcmd" principal only should 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 Expires: September 30, 1995 [Page 17] draft-ietf-cat-ftpsec-06.txtFTP Security Extensions March, 1995 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 will 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, con- vert it from network byte order, and pass the result to krb_rd_safe(3). The client should 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 should 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). Appendix II: Specification under the GSSAPI The authentication mechanism name (for the AUTH command) associated with all mechanisms employing the GSSAPI is GSSAPI. If the server supports an authentication mechanism employing the GSSAPI, it will respond with a 334 reply code indicating that an ADAT command is expected next. The client should begin the authentication exchange by calling GSS_Init_Sec_Context, passing in 0 for input_context_handle Expires: September 30, 1995 [Page 18] draft-ietf-cat-ftpsec-06.txtFTP Security Extensions March, 1995 (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 should expect a token to be returned in the reply to the ADAT com- mand. This token should 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 should have been 235. If GSS_Init_Sec_Context returns GSS_S_COMPLETE, then no further tokens should be expected from the server, and the client should 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 should 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 should be 235, and the server should consider the client authenticated. If GSS_Accept_Sec_Context returns GSS_S_CONTINUE_NEEDED, the reply code should be 335. Otherwise, the reply code should be 535, and the text of the reply should contain a descriptive error message. The procedure associated with MIC commands, 631 replies, and Safe file transfers is: GSS_Seal for the sender, with conf_flag == FALSE GSS_Unseal for the receiver The procedure associated with ENC commands, 632 replies, and Private file transfers is: GSS_Seal for the sender, with conf_flag == TRUE GSS_Unseal 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. Expires: September 30, 1995 [Page 19]