Username / Password : Request For Comments

RFC Number : 2773

Title : Encryption using KEA and SKIPJACK.

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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RFC 2773 Encryption using KEA and SKIPJACK February 2000

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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RFC 2773 Encryption using KEA and SKIPJACK February 2000

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

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

<-- 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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RFC 2773 Encryption using KEA and SKIPJACK February 2000

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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RFC 2773 Encryption using KEA and SKIPJACK February 2000

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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RFC 2773 Encryption using KEA and SKIPJACK February 2000

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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RFC 2773 Encryption using KEA and SKIPJACK February 2000

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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RFC 2773 Encryption using KEA and SKIPJACK February 2000

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
381 Elden Street
Suite 1120
Herndon, VA 20170

Phone: +1 703 707-0696

Peter Yee
5303 Betsy Ross Drive
Santa Clara, CA 95054

Phone: +1 408 327-1901

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RFC 2773 Encryption using KEA and SKIPJACK February 2000

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

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


Funding for the RFC Editor function is currently provided by the
Internet Society.

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