Secure Telnet Working Group Russell Housley (SPYRUS)
Todd Horting (SPYRUS)
Internet-Draft Peter Yee (SPYRUS)
Expires on January 2000 July 1999
Telnet Authentication Using KEA and SKIPJACK
<draft-housley-telnet-auth-keasj-02.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
This document defines a method to authenticate telnet [1,5] using the
Key Exchanage Algorithm (KEA)[4], and encryption of the telnet stream
using SKIPJACK[4]. Two encryption modes are specified; one provides
data integrity and the other does not. It relies on the Telnet
Authentication Option [2].
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1 Introduction
The Telnet protocol [1,5] provides no protocol security. Telnet
servers may require users to login. This is typically a host level
login consisting of a user name and a password, transmitted in the
clear.
The mechanism specified in this document relies on the Telnet
Authentication Option [2].
2 Telnet Security Extensions
Telnet, as a protocol, has no concept of security. Without
negotiated options, it merely passes characters back and forth
between the NVTs represented by the two Telnet processes. In its
most common usage as a protocol for remote terminal access (TCP port
23), Telnet normally connects to a server that requires user-level
authentication through a user name and password in the clear. The
server does not authenticate itself to the user.
The Telnet Authentication Option provides for:
* User authentication -- replacing or augmenting the normal host
password mechanism;
* Server authentication -- normally done in conjunction with user
authentication;
* Session parameter negotiation -- in particular, encryption key
and attributes;
* Session protection -- primarily encryption of the data and
embedded command stream, but the encryption algorithm may also
provide data integrity.
In order to support these security services, the two Telnet entities
must first negotiate their willingness to support the Telnet
Authentication Option and Data Encryption Options. Upon agreeing to
support these options, the parties are then able to perform
suboptions to determine the authentication protocol to be used, and
possibly the remote user name to be used for authorization checking.
Encryption is negotiated along with the type of the authentication.
Authentication and parameter negotiation occur within an unbounded
series of exchanges. The server proposes a preference-ordered list
of authentication types (mechanisms) which it supports. In addition
to listing the mechanisms it supports, the server qualifies each
mechanism with a modifier that specifies whether the authentication
is to be unilateral or mutual, and in which direction the
authentication is to be performed, and if encryption of data is
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desired. The client selects one mechanism from the list and responds
to the server indicating its choice and the first set of
authentication data needed for the selected authentication type. The
client may ignore a request to encrypt data and so indicate, but the
server may also terminate the connection if the client refuses
encryption. The server and the client then proceed through whatever
number of iterations is required to arrive at the requested
authentication.
If encryption is requested, it is started immediately after the
Authentication options are completed. Afterwards either party may
use the Data Encryption Options to turn off encryption, but once this
has been disabled, there is no ability to re-enable encryption
without repeating the complete authentication phase.
3 Use of Key Exchange Algorithm (KEA)
This paper specifies the method in which KEA is used to achieve
Telnet Authentication. KEA (in conjunction with SKIPJACK) [4]
provides authentication, integrity and confidentiality. Figure 1
illustrates the authentication mechanism.
Telnet entities may use KEA to provide mutual authentication and
support for the setup of data encryption keys. A simple token format
and set of exchanges delivers these services.
The nonce used in this exchanged is a 64 bit unsigned integer. The
server generates one, and the client generates another. The nonce
value is selected randomly. The nonce is sent in a big endian form.
The encryption of the nonce will be done with the same mechanism that
the session will use, detailed in the next section.
In figure 1 the two-octet authentication type pair is denoted by
'0?18'. This will be filled in with either hexadecimal '0C18' for
KEA SKIPJACK without integrity mechanism or '0D18' for KEA SKIPJACK
with integrity mechanism.
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---------------------------------------------------------------------
Client (Party A) Server (Party B)
<-- IAC DO AUTHENTICATION
IAC WILL AUTHENTICATION -->
<-- IAC SB AUTHENTICATION SEND
<list of authentication options>
IAC SE
IAC SB AUTHENTICATION
NAME <user name> -->
IAC SB AUTHENTICATION IS
'0?18' '1' CertA||Ra IAC SE -->
<-- IAC SB AUTHENTICATION REPLY
'0?18' '2'
CertB||Rb||IVb||
Encrypt( NonceB )
IAC SE
IAC SB AUTHENTICATION IS
'0?18' '3'
IVa||Encrypt( NonceB Xor '0?18'||NonceA )
IAC SE -->
<-- IAC SB AUTHENTICATION REPLY
'0?18' '4'
Encrypt( NonceA Xor '0?18' )
IAC SE
---------------------------------------------------------------------
Figure 1
On completing these exchanges, the parties have a common SKIPJACK
key. Mutual authentication is provided by verification of the
certificates used to establish the SKIPJACK encryption key and
successful use of the derived SKIPJACK session key. To protect from
an active attacker, encryption will take place after successful
authentication. There will be no way to turn off encryption and
safely turn it back on; repeating the entire authentication is the
only safe way to restart it. If the user does not want to use
encryption, he will have to logoff and logon with the desired
security mechanism.
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3.1 SKIPJACK Modes
There are two distinct modes for encrypting telnet streams; one
provides integrity and the other does not. Because telnet is
normally operated in a character-by-character mode, the KEA SKIPJACK
with stream integrity mechanism requires the transmission of 4 bytes
for every telnet data byte. However, a more simplified mode KEA
SKIPJACK without integrity mechanism will only require the
transmission of one byte for every telnet data byte.
The cryptographic mode for KEA SKIPJACK with stream integrity is
Cipher Feedback on 32 bits of data (CFB-32) and the mode of KEA
SKIPJACK is Cipher Feedback on 8 bits of data (CFB-8).
3.1.1 SKIPJACK without stream integrity
The first and least complicated mode is the SKIPJACK CFB-8. This
mode provides no stream integrity.
For SKIPJACK without stream integrity, the two-octet authentication
type pair (0?18 in Figure 1) is "KEA_SJ CLIENT_TO_SERVER MUTUAL
ENCRYPT_AFTER_EXCHANGE INI_CRED_FWD_OFF". This is encoded as two-
octets: '0C18' in hexadecimal. This indicates that the KEA SKIPJACK
without integrity mechanism will be used for mutual authentication
and telnet stream encryption.
3.1.2 SKIPJACK with stream integrity
SKIPJACK with stream integrity is more complicated. It uses the
SHA-1 [3] on-way hash function to provide integrity of the encryption
stream as follows:
Set H0 to be the SHA-1 hash of a zero length string.
Cn is the nth character in the stream.
Hn = SHA-1( Hn-1||Cn ), where Hn is the hash value
associated with the nth character in the stream.
ICVn is set to the three most significant bytes of Hn.
Transmit Encrypt( Cn||ICVn ).
The ciphertext that is transmitted is the SKIPJACK CFB-32 encryption
of ( Cn||ICVn ). The receiving end of the Telnet link reverses the
process, first decrypting the ciphertext, separating Cn and ICVn,
recalculating Hn, recalculating ICVn, and then comparing the received
ICVn with the recalculated ICVn. Integrity is indicated if the
comparison succeeds, and Cn can then be processed normally as part of
the Telnet stream. Failure of the comparison indicates some loss of
integrity, whether due to active manipulation or loss of
cryptographic synchronization. In either case, the only recourse is
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to drop the telnet connection and start over.
For SKIPJACK with stream integrity, the two-octet authentication type
pair ('0?18' in figure 1) is "KEA_SJ_INTEG CLIENT_TO_SERVER MUTUAL
ENCRYPT_AFTER_EXCHANGE INI_CRED_FWD_OFF". This is encoded as two-
octets: '0D18' in hexadecimal. This indicates that the KEA SKIPJACK
with integrity mechanism will be used for mutual authentication and
telnet stream encryption.
3.1.3 Telnet SYNCH Handling
Telnet supports a "Synch" mechanism to solve the problem of Telnet
control commands from being blocked by network flow control.
Basically, the sender of the Synch is telling the recipient to
discard all incoming data except Telnet commands until the DM (and
end of urgent) is reached. The Synch signal is sent via a TCP Urgent
notification, but does not arrive out of sequence, all data received
will be decrypted and only acted upon if it is a Telnet command.
Since sequence is preserved, no special cryptographic processing is
required.
4.0 Security Considerations
This entire memo is about security mechanisms. For KEA to provide
the authentication discussed, the implementation must protect the
private key from disclosure. Likewise, the SKIPJACK keys must be
protected form disclosure.
By linking the enabling of encryption as a side effect of successful
authentication, protection is provided against an active attacker.
If encryption were enabled as a separate negotiation, it would
provide a window of vulnerability from when the authentication
completes, up to and including the negotiation to turn on encryption.
The only safe way to restart encryption, if it is turned off, is to
repeat the entire authentication process.
5.0 Acknowledgements
We would like to thank William Nace for support during implementation
of this specification.
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6.0 References
[1] - Postel, J, Reynolds, J. "Telnet Protocol Specification".
RFC 854. May 1983.
[2] - T. Ts'o, "Telnet Authentication Option".
<draft-tso-telnet-auth-enc-02.txt>, July 1999.
[3] - Secure Hash Standard. FIPS Pub 180-1. April 17, 1995.
[4] - "Skipjack and KEA Algorithm Specification", Version 2.0,
May 29, 1998. Available from
http://csrc.nist.gov/encryption/skipjack-kea.htm
[5] - Postel, J, Reynolds, J. "Telnet Option Specifications".
RFC 855. May 1983.
7.0 Author's Address
Russell Housley
SPYRUS
381 Elden Street
Suite 1120
Herndon, VA 20170
USA
Email: housley@spyrus.com
Todd Horting
SPYRUS
381 Elden Street
Suite 1120
Herndon, VA 20170
USA
Email: thorthing@spyrus.com
Peter Yee
SPYRUS
5303 Betsy Ross Drive
Santa Clara, CA 95054
USA
Email: yee@spyrus.com
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