1. Introduction
The File Transfer Protocol (FTP) provides no protocol security except for a user authentication password which is transmitted in the clear. In addition, the protocol does not protect the file transfer session beyond the original authentication phase.
The Internet Engineering Task Force (IETF) Common Authentication Technology (CAT) Working Group has proposed security extensions to FTP. These extensions allow the protocol to use more flexible security schemes, and in particular allows for various levels of protection for the FTP command and data connections. This document describes a profile for the FTP Security Extensions by which these mechanisms may be provisioned using the Key Exchange Algorithm (KEA) in conjunction with the SKIPJACK symmetric encryption algorithm.
FTP Security Extensions [1] provides:
- user authentication -- augmenting the normal password mechanism;
- server authentication -- normally done in conjunction with user authentication;
- session parameter negotiation -- in particular, encryption keys and attributes;
- command connection protection -- integrity, confidentiality, or both;
- data transfer protection -- same as for command connection protection.
In order to support the above security services, the two FTP entities negotiate a mechanism. This process is open-ended and completes when both entities agree on an acceptable mechanism or when the initiating party (always the client) is unable to suggest an agreeable mechanism. Once the entities agree upon a mechanism, they may commence authentication and/or parameter negotiation.
Authentication and parameter negotiation occur within an unbounded series of exchanges. At the completion of the exchanges, the entities will either be authenticated (unilateral or mutually), and may, additionally, be ready to protect FTP commands and data.
Following the exchanges, the entities negotiate the size of the buffers they will use in protecting the commands and data that follow. This process is accomplished in two steps: the client offers a suggested buffer size and the server may either refuse it, counter it, or accept it.
At this point, the entities may issue protected commands within the bounds of the parameters negotiated through the security exchanges. Protected commands are issued by applying the protection services required to the normal commands and Base64 encoding the results. The encoded results are sent as the data field within a ENC (integrity and confidentiality) command. Base64 is an encoding for mapping binary data onto a textual character set that is able to pass through most 7-bit systems without loss. The server sends back responses in new result codes which allow the identical protections and Base64 encoding to be applied to the results. Protection of the data transfers can be specified via the PROT command which supports the same protections as those afforded the other FTP commands. PROT commands may be sent on a transfer-by-transfer basis, however, the session parameters may not be changed within a session.