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RFC 2773

Encryption using KEA and SKIPJACK

Pages: 9
Updates:  0959

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Network Working Group                                        R. Housley
Request for Comments: 2773                                       P. Yee
Updates: 959                                                     SPYRUS
Category: Experimental                                          W. Nace
                                                          February 2000

                   Encryption using KEA and SKIPJACK

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.


This document defines a method to encrypt a file transfer using the FTP specification STD 9, RFC 959, "File Transfer Protocol (FTP)", (October 1985) [3] and RFC 2228, "FTP Security Extensions" (October 1997) [1]. This method will use the Key Exchange Algorithm (KEA) to give mutual authentication and establish the data encryption keys. SKIPJACK is used to encrypt file data and the FTP command channel.

1.0 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.
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   FTP Security Extensions [1] provides:

      *  user authentication -- augmenting the normal password

      *  server authentication -- normally done in conjunction with user

      *  session parameter negotiation -- in particular, encryption keys
         and attributes;

      *  command connection protection -- integrity, confidentiality, or

      *  data transfer protection -- same as for command connection

   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
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   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.

2.0 Key Exchange Algorithm (KEA) Profile

This paper profiles KEA with SKIPJACK to achieve certain security services when used in conjunction with the FTP Security Extensions framework. FTP entities may use KEA to give mutual authentication and establish data encryption keys. We specify a simple token format and set of exchanges to deliver these services. Functions that may be performed by the Fortezza Crypto Card. The reader should be familiar with the extensions in order to understand the protocol steps that follow. In the context of the FTP Security Extensions, we suggest the usage of KEA with SKIPJACK for authentication, integrity, and confidentiality. A client may mutually authenticate with a server. What follows are the protocol steps necessary to perform KEA authentication under the FTP Security Extensions framework. Where failure modes are encountered, the return codes follow those specified in the Extensions. They are not enumerated in this document as they are invariant among the mechanisms used. The certificates are ASN.1 encoded. The exchanges detailed below presume a working knowledge of the FTP Security Extensions. The notation for concatenation is " || ". Decryption of encrypted data and certification path validation is implicitly assumed, but is not shown. --------------------------------------------------------------------- Client Server AUTH KEA-SKIPJACK --> <-- 334 ADAT=Base64( Certb || Rb ) ADAT Base64( Certa || Ra || WMEK || IV || Encrypt( Label-Type || Label-Length || Label-List || pad || ICV ) ) --> <-- 235 ADAT=Base64( IV ) --------------------------------------------------------------------- Figure 1 The server and client certificates contain KEA public keys. The client and server use KEA to generate a shared SKIPJACK symmetric key, called the TEK. The client uses the random number generator to create a second SKIPJACK key, called the MEK. The MEK is wrapped in
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   the TEK for transfer to the server.  An initialization vector (IV)
   associated with the MEK is generated by the client and transferred to
   the server.  A list of security labels that the client wants to use
   for this FTP session may be transferred to the server encrypted in
   the MEK.  As shown in Figure 2, the security label data is formatted
   as a one octet type value, a four octet label length, the security
   label list, padding, followed by an eight octet integrity check value
   (ICV).  Figure 3 lists the label types.  If the label type is absent
   (value of zero length), then the label size must be zero.

   In order to ensure that the length of the plain text is a multiple of
   the cryptographic block size, padding shall be performed as follows.
   The input to the SKIPJACK CBC encryption process shall be padded to a
   multiple of 8 octets.  Let n be the length in octets of the input.
   Pad the input by appending 8 - (n mod 8) octets to the end of the
   message, each having the value 8 - (n mod 8), the number of octets
   being added.  In hexadecimal, he possible pad strings are: 01, 0202,
   030303, 04040404, 0505050505, 060606060606, 07070707070707, and
   0808080808080808.  All input is padded with 1 to 8 octets to produce
   a multiple of 8 octets in length.  This pad technique is used
   whenever SKIPJACK CBC encryption is performed.

   An ICV is calculated over the plaintext security label and padding.
   The ICV algorithm used is the 32-bit one's complement addition of
   each 32-bit block followed by 32 zero bits.  This ICV technique is
   used in conjunction with SKIPJACK CBC encryption to provide data

                 Label Type                1 Octet
                 Label Length              4 octets
                 Label List                variable length
                 Pad                       1 to 8 octets
                 ICV                       8 octets
                                Figure 2

              Label Type   Label Syntax                Reference
              0            Absent                      Not applicable
              1            MSP                         SDN.701[2]
              2-255        Reserved for Future Use     To Be Determined

                                Figure 3
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   FTP command channel operations are now confidentiality protected.  To
   provide integrity, the command sequence number, padding, and ICV are
   appended to each command prior to encryption.

   Sequence integrity is provided by using a 16-bit sequence number
   which is incremented for each command.  The sequence number is
   initialized with the least significant 16-bits of Ra.  The server
   response will include the same sequence number as the client command.

   An ICV is calculated over the individual commands (including the
   carriage return and line feed required to terminate commands), the
   sequence number, and pad.

     Client                             Server

     ENC Base64(Encrypt("PBSZ 65535"
         || SEQ || pad || ICV ))     -->
                                        <-- 632  Base64(Encrypt("200" ||
                                                   SEQ || pad || ICV))
     ENC Base64(Encrypt("USER yee"
           || SEQ || pad || ICV))    -->
                                        <-- 632  Base64(Encrypt("331" ||
                                                   SEQ || pad || ICV))
     ENC Base64(Encrypt("PASS
           fortezza" || SEQ ||
           pad || ICV))              -->
                                        <-- 631  Base64(Sign("230"))
                                Figure 4

   After decryption, both parties verifying the integrity of the PBSZ
   commands by checking for the expected sequence number and correct
   ICV.  The correct SKIPJACK key calculation, ICV checking, and the
   validation of the certificates containing the KEA public keys
   provides mutual identification and authentication.

     Client                          Server

     ENC Base64(Encrypt("PROT P" ||
             SEQ || pad || ICV))  -->
                                     <-- 632 Base64(Encrypt("200" || SEQ
                                                    ||  pad || ICV))
                                Figure 5
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   At this point, files may be sent or received with encryption and
   integrity services in use.  If encryption is used, then the first
   buffer will contain the token followed by enough encrypted file
   octets to completely fill the buffer (unless the file is too short to
   fill the buffer).  Subsequent buffers contain only encrypted file
   octets.  All buffers are completely full except the final buffer.

     Client                         Server

     ENC Base64(Encrypt(
        ("RETR") ||
        SEQ || pad || ICV))    -->
                                    <-- 632 Base64(Encrypt("150" ||
                                                SEQ || pad || ICV))
                                Figure 6

   The next figure shows the header information and the file data.

                Plaintext Token IV    24 octets
                WMEK                  12 octets
                Hashvalue             20 octets
                IV                    24 octets
                Label Type            1 octets
                Label Length          4 octets
                Label                 Label Length octets
                Pad                   1 to 8 octets
                ICV                   8 octets
                                Figure 7

2.1 Pre-encrypted File Support

In order to support both on-the-fly encryption and pre-encrypted files, a token is defined for carrying a file encryption key (FEK). To prevent truncation and ensure file integrity, the token also includes a hash computed on the complete file. The token also contains the security label associate with the file. This FEK is wrapped in the session TEK. The token is encrypted in the session TEK using SKIPJACK CBC mode. The token contains a 12 octet wrapped FEK, a 20 octet file hash, a 24 octet file IV, a 1 octet label type, a 4 octet label length, a variable length label value, a one to 8 octet pad, and an 8 octet ICV. The first 24 octets of the token are the plaintext IV used to encrypt the remainder of the token. The token requires its own encryption IV because it is transmitted across the data channel, not the command channel, and ordering between the
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   channels cannot be guaranteed.  Storage of precomputed keys and
   hashes for files in the file system is a local implementation matter;
   however, it is suggested that if a file is pre-encrypted, then the
   FEK be wrapped in a local storage key.  When the file is needed, the
   FEK is unwrapped using the local storage key, and then rewrapped in
   the session TEK.  Figure 8 shows the assembled token.

               Plaintext Token IV            24 octets
               Wrapped FEK                   12 octets
               Hashvalue                     20 octets
               IV                            24 octets
               Label Type                    1 octet
               Label Length                  4 octets
               Label                         Label Length octets
               Pad                           1 to 8 octets
               ICV                           8 octets
                                 Figure 8

3.0 Table of Key Terminology

In order to clarify the usage of various keys in this protocol, Figure 9 summarizes key types and their usage: --------------------------------------------------------------------- Key Type Usage TEK Encryption of token at the beginning of each file, also wraps the MEK and the FEK MEK Encryption of command channel FEK Encryption of the file itself (may be done out of scope of FTP) --------------------------------------------------------------------- Figure 9

4.0 Security Considerations

This entire memo is about security mechanisms. For KEA to provide the authentication and key management discussed, the implementation must protect the private key from disclosure. For SKIPJACK to provide the confidentiality discussed, the implementation must protect the shared symmetric keys from disclosure.

5.0 Acknowledgements

We would like to thank Todd Horting for insights gained during implementation of this specification.
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6.0 References

[1] Horowitz, M. and S. Lunt, "FTP Security Extensions", RFC 2228, October 1997. [2] Message Security Protocol 4.0 (MSP), Revision A. Secure Data Network System (SDNS) Specification, SDN.701, February 6, 1997. [3] Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9, RFC 959, October 1985.

7.0 Authors' Addresses

Russell Housley SPYRUS 381 Elden Street Suite 1120 Herndon, VA 20170 USA Phone: +1 703 707-0696 EMail: Peter Yee SPYRUS 5303 Betsy Ross Drive Santa Clara, CA 95054 USA Phone: +1 408 327-1901 EMail:
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8.0 Full Copyright Statement

Copyright (C) The Internet Society (2000). 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 implementation may be prepared, copied, published and 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. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society.