Network Working Group R. Housley
Request for Comments: 2951 T. Horting
Category: Informational P. Yee
SPYRUS
September 2000
TELNET Authentication Using KEA and SKIPJACK
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This document defines a method to authenticate TELNET using the Key
Exchange Algorithm (KEA), and encryption of the TELNET stream using
SKIPJACK. Two encryption modes are specified; one provides data
integrity and the other does not. The method relies on the TELNET
Authentication Option.
1. Command Names and Codes
AUTHENTICATION 37
Authentication Commands:
IS 0
SEND 1
REPLY 2
NAME 3
Authentication Types:
KEA_SJ 12
KEA_SJ_INTEG 13
Modifiers:
AUTH_WHO_MASK 1
AUTH_CLIENT_TO_SERVER 0
AUTH_SERVER_TO CLIENT 1
AUTH_HOW_MASK 2
AUTH_HOW_ONE_WAY 0
AUTH_HOW_MUTUAL 2
ENCRYPT_MASK 20
ENCRYPT_OFF 0
ENCRYPT_USING_TELOPT 4
ENCRYPT_AFTER_EXCHANGE 16
ENCRYPT_RESERVED 20
INI_CRED_FWD_MASK 8
INI_CRED_FWD_OFF 0
INI_CRED_FWD_ON 8
Sub-option Commands:
KEA_CERTA_RA 1
KEA_CERTB_RB_IVB_NONCEB 2
KEA_IVA_RESPONSEB_NONCEA 3
KEA_RESPONSEA 4
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. Upon agreeing to support this option, the
parties are then able to perform sub-option negotiations 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) that it supports. In addition
to listing the mechanisms it supports, the server qualifies each
mechanism with a modifier that specifies whether encryption of data
is 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.
Encryption is started immediately after the Authentication Option is
completed.
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 and confidentiality. Integrity may also be
provided.
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.
NonceA and NonceB used in this exchange are 64-bit bit strings. The
client generates NonceA, and the server generates NonceB. 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.
Ra and Rb used in this exchange are 1024 bit strings and are defined
by the KEA Algorithm [4].
The IVa and IVb are 24 byte Initialization Vectors. They are
composed of "THIS IS NOT LEAF" followed by 8 random bytes.
CertA is the client's certificate. CertB is the server's
certificate. Both certificates are X.509 certificates [6] that
contain KEA public keys [7]. The client must validate the server's
certificate before using the KEA public key it contains. Likewise,
the server must validate the client's certificate before using the
KEA public key it contains.
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
against active attacks, 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 may disable encryption after the session is
established.
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 SKIPJACK with
stream integrity mechanism requires the transmission of 4 bytes for
every TELNET data byte. However, a simplified mode SKIPJACK without
integrity mechanism will only require the transmission of one byte
for every TELNET data byte.
The cryptographic mode for SKIPJACK with stream integrity is Cipher
Feedback on 32 bits of data (CFB-32) and the mode of SKIPJACK is
Cipher Feedback on 8 bits of data (CFB-8).
3.1.1. SKIPJACK without stream integrity
The first and least complicated mode uses SKIPJACK CFB-8. This mode
provides no stream integrity.
For SKIPJACK without stream integrity, the two-octet authentication
type pair is KEA_SJ AUTH_CLIENT_TO_SERVER AUTH_HOW_MUTUAL
ENCRYPT_AFTER_EXCHANGE INI_CRED_FWD_OFF. This indicates that the
SKIPJACK without integrity mechanism will be used for mutual
authentication and TELNET stream encryption. Figure 1 illustrates
the authentication mechanism of KEA followed by SKIPJACK without
stream integrity.
---------------------------------------------------------------------
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
KEA_SJ
AUTH_CLIENT_TO_SERVER
AUTH_HOW_MUTUAL
ENCRYPT_AFTER_EXCHANGE
INI_CRED_FWD_OFF
KEA_CERTA_RA
CertARa IAC SE -->
<-- IAC SB AUTHENTICATION REPLY
KEA_SJ
AUTH_CLIENT_TO_SERVER
AUTH_HOW_MUTUAL
ENCRYPT_AFTER_EXCHANGE
INI_CRED_FWD_OFF
IVA_RESPONSEB_NONCEA
KEA_CERTB_RB_IVB_NONCEB
CertBRbIVb
Encrypt( NonceB )
IAC SE
IAC SB AUTHENTICATION IS
KEA_SJ
AUTH_CLIENT_TO_SERVER
AUTH_HOW_MUTUAL
ENCRYPT_AFTER_EXCHANGE
INI_CRED_FWD_OFF
KEA_IVA_RESPONSEB_NONCEA
IVaEncrypt( (NonceB XOR 0x0C12)NonceA )
IAC SE -->
Client (Party A) Server (Party B)
<client begins encryption>
<-- IAC SB AUTHENTICATION REPLY
KEA_SJ
AUTH_CLIENT_TO_SERVER
AUTH_HOW_MUTUAL
ENCRYPT_AFTER_EXCHANGE
INI_CRED_FWD_OFF
KEA_RESPONSEA
Encrypt( NonceA XOR 0x0C12 )
IAC SE
<server begins encryption>
---------------------------------------------------------------------
Figure 1.
3.1.2. SKIPJACK with stream integrity
SKIPJACK with stream integrity is more complicated. It uses the
SHA-1 [3] one-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 TELNET stream.
Hn = SHA-1( Hn-1Cn ), 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( CnICVn ).
The ciphertext that is transmitted is the SKIPJACK CFB-32 encryption
of ( CnICVn ). 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
to drop the TELNET connection and start over.
For SKIPJACK with stream integrity, the two-octet authentication type
pair is KEA_SJ_INTEG AUTH_CLIENT_TO_SERVER AUTH_HOW_MUTUAL
ENCRYPT_AFTER_EXCHANGE INI_CRED_FWD_OFF. This indicates that the
KEA SKIPJACK with integrity mechanism will be used for mutual
authentication and TELNET stream encryption. Figure 2 illustrates
the authentication mechanism of KEA SKIPJACK with stream integrity.
---------------------------------------------------------------------
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
KEA_SJ_INTEG
AUTH_CLIENT_TO_SERVER
AUTH_HOW_MUTUAL
ENCRYPT_AFTER_EXCHANGE
INI_CRED_FWD_OFF
KEA_CERTA_RA
CertARa IAC SE -->
<-- IAC SB AUTHENTICATION REPLY
KEA_SJ_INTEG
AUTH_CLIENT_TO_SERVER
AUTH_HOW_MUTUAL
ENCRYPT_AFTER_EXCHANGE
INI_CRED_FWD_OFF
IVA_RESPONSEB_NONCEA
KEA_CERTB_RB_IVB_NONCEB
CertBRbIVb
Encrypt( NonceB )
IAC SE
IAC SB AUTHENTICATION IS
KEA_SJ_INTEG
AUTH_CLIENT_TO_SERVER
AUTH_HOW_MUTUAL
ENCRYPT_AFTER_EXCHANGE
INI_CRED_FWD_OFF
KEA_IVA_RESPONSEB_NONCEA
IVaEncrypt( (NonceB XOR 0x0D12)NonceA )
IAC SE -->
Client (Party A) Server (Party B)
<client begins encryption>
<-- IAC SB AUTHENTICATION REPLY
KEA_SJ_INTEG
AUTH_CLIENT_TO_SERVER
AUTH_HOW_MUTUAL
ENCRYPT_AFTER_EXCHANGE
INI_CRED_FWD_OFF
KEA_RESPONSEA
Encrypt( NonceA XOR 0x0D12 )
IAC SE
<server begins encryption>
---------------------------------------------------------------------
Figure 2
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 from disclosure.
Implementations must randomly generate KEA private keys,
initialization vectors (IVs), and nonces. The use of inadequate
pseudo-random number generators (PRNGs) to generate cryptographic
keys can result in little or no security. An attacker may find it
much easier to reproduce the PRNG environment that produced the keys,
searching the resulting small set of possibilities, rather than brute
force searching the whole key space. The generation of quality
random numbers is difficult. RFC1750 [8] offers important guidance
in this area, and Appendix 3 of FIPS Pub 186 [9] provides one quality
PRNG technique.
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. IANA Considerations
The authentication types KEA_SJ and KEA_SJ_INTEG and their associated
suboption values are registered with IANA. Any suboption values used
to extend the protocol as described in this document must be
registered with IANA before use. IANA is instructed not to issue new
suboption values without submission of documentation of their use.
6.0. Acknowledgements
We would like to thank William Nace for support during implementation
of this specification.
7.0. References
[1] Postel, J. and J. Reynolds, "TELNET Protocol Specification", ASTD
8, RFC854, May 1983.
[2] Ts'o, T. and J. Altman, "Telnet Authentication Option", RFC2941,
September 2000.
[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. and J. Reynolds, "TELNET Option Specifications", STD
8, RFC855, May 1983.
[6] Housley, R., Ford, W., Polk, W. and D. Solo, "Internet X.509
Public Key Infrastructure: X.509 Certificate and CRL Profile",
RFC2459, January 1999.
[7] Housley, R. and W. Polk, "Internet X.509 Public Key
Infrastructure - Representation of Key Exchange Algorithm (KEA)
Keys in Internet X.509 Public Key Infrastructure Certificates",
RFC2528, March 1999.
[8] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
Recommendations for Security", RFC1750, December 1994.
[9) National Institute of Standards and Technology. FIPS Pub 186:
Digital Signature Standard. 19 May 1994.
8.0. Authors' Addresses
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: thorting@spyrus.com
Peter Yee
SPYRUS
5303 Betsy Ross Drive
Santa Clara, CA 95054
USA
EMail: yee@spyrus.com
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