Network Working Group A. Medvinsky
Request for Comments: 2712 Excite
Category: Standards Track M. Hur
CyberSafe Corporation
October 1999
Addition of Kerberos Cipher Suites to Transport Layer Security (TLS)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
IESG Note:
The 40-bit ciphersuites defined in this memo are included only for
the purpose of documenting the fact that those ciphersuite codes have
already been assigned. 40-bit ciphersuites were designed to comply
with US-centric, and now obsolete, export restrictions. They were
never secure, and nowadays are inadequate even for casual
applications. Implementation and use of the 40-bit ciphersuites
defined in this document, and elsewhere, is strongly discouraged.
This document proposes the addition of new cipher suites to the TLS
protocol [1] to support Kerberos-based authentication. Kerberos
credentials are used to achieve mutual authentication and to
establish a master secret which is subsequently used to secure
client-server communication.
Flexibility is one of the main strengths of the TLS protocol.
Clients and servers can negotiate cipher suites to meet specific
security and administrative policies. However, to date,
authentication in TLS is limited only to public key solutions. As a
result, TLS does not fully support organizations with heterogeneous
security deployments that include authentication systems based on
symmetric cryptography. Kerberos, originally developed at MIT, is
Medvinsky & Hur Standards Track [Page 1]
RFC 2712 Addition of Kerberos Cipher Suites to TLS October 1999
based on an open standard [2] and is the most widely deployed
symmetric key authentication system. This document proposes a new
option for negotiating Kerberos authentication within the TLS
framework. This achieves mutual authentication and the establishment
of a master secret using Kerberos credentials. The proposed changes
are minimal and, in fact, no different from adding a new public key
algorithm to the TLS framework.
This section describes the addition of the Kerberos authentication
option to the TLS protocol. Throughout this document, we refer to
the basic SSL handshake shown in Figure 1. For a review of the TLS
handshake see [1].
CLIENT SERVER
------ ------
ClientHello
-------------------------------->
ServerHello
Certificate *
ServerKeyExchange*
CertificateRequest*
ServerHelloDone
<-------------------------------
Certificate*
ClientKeyExchange
CertificateVerify*
change cipher spec
Finished
| -------------------------------->
| change cipher spec
| Finished
| |
| |
Application Data <------------------------------->Application Data
FIGURE 1: The TLS protocol. All messages followed by a star are
optional. Note: This figure was taken from an IETF document
[1].
The TLS security context is negotiated in the client and server hello
messages. For example: TLS_RSA_WITH_RC4_MD5 means the initial
authentication will be done using the RSA public key algorithm, RC4
will be used for the session key, and MACs will be based on the MD5
algorithm. Thus, to facilitate the Kerberos authentication option,
we must start by defining new cipher suites including (but not
limited to):
Medvinsky & Hur Standards Track [Page 2]
RFC 2712 Addition of Kerberos Cipher Suites to TLS October 1999
CipherSuite TLS_KRB5_WITH_DES_CBC_SHA = { 0x00,0x1E };
CipherSuite TLS_KRB5_WITH_3DES_EDE_CBC_SHA = { 0x00,0x1F };
CipherSuite TLS_KRB5_WITH_RC4_128_SHA = { 0x00,0x20 };
CipherSuite TLS_KRB5_WITH_IDEA_CBC_SHA = { 0x00,0x21 };
CipherSuite TLS_KRB5_WITH_DES_CBC_MD5 = { 0x00,0x22 };
CipherSuite TLS_KRB5_WITH_3DES_EDE_CBC_MD5 = { 0x00,0x23 };
CipherSuite TLS_KRB5_WITH_RC4_128_MD5 = { 0x00,0x24 };
CipherSuite TLS_KRB5_WITH_IDEA_CBC_MD5 = { 0x00,0x25 };
CipherSuite TLS_KRB5_EXPORT_WITH_DES_CBC_40_SHA = { 0x00,0x26 };
CipherSuite TLS_KRB5_EXPORT_WITH_RC2_CBC_40_SHA = { 0x00,0x27 };
CipherSuite TLS_KRB5_EXPORT_WITH_RC4_40_SHA = { 0x00,0x28 };
CipherSuite TLS_KRB5_EXPORT_WITH_DES_CBC_40_MD5 = { 0x00,0x29 };
CipherSuite TLS_KRB5_EXPORT_WITH_RC2_CBC_40_MD5 = { 0x00,0x2A };
CipherSuite TLS_KRB5_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x2B };
To establish a Kerberos-based security context, one or more of the
above cipher suites must be specified in the client hello message.
If the TLS server supports the Kerberos authentication option, the
server hello message, sent to the client, will confirm the Kerberos
cipher suite selected by the server. The server's certificate, the
client
CertificateRequest, and the ServerKeyExchange shown in Figure 1 will
be omitted since authentication and the establishment of a master
secret will be done using the client's Kerberos credentials for the
TLS server. The client's certificate will be omitted for the same
reason. Note that these messages are specified as optional in the
TLS protocol; therefore, omitting them is permissible.
The Kerberos option must be added to the ClientKeyExchange message as
shown in Figure 2.
Medvinsky & Hur Standards Track [Page 3]
RFC 2712 Addition of Kerberos Cipher Suites to TLS October 1999
struct
{
select (KeyExchangeAlgorithm)
{
case krb5: KerberosWrapper; /* new addition */
case rsa: EncryptedPreMasterSecret;
case diffie_hellman: ClientDiffieHellmanPublic;
} Exchange_keys;
} ClientKeyExchange;
struct
{
opaque Ticket;
opaque authenticator; /* optional */
opaque EncryptedPreMasterSecret; /* encrypted with the session key
which is sealed in the ticket */
} KerberosWrapper; /* new addition */
FIGURE 2: The Kerberos option in the ClientKeyExchange.
To use the Kerberos authentication option, the TLS client must obtain
a service ticket for the TLS server. In TLS, the ClientKeyExchange
message is used to pass a random 48-byte pre-master secret to the
server.
The client and server then use the pre-master secret to independently
derive the master secret, which in turn is used for generating
session keys and for MAC computations. Thus, if the Kerberos option
is selected, the pre-master secret structure is the same as that used
in the RSA case; it is encrypted under the Kerberos session key and
sent to the TLS server along with the Kerberos credentials (see
Figure 2). The ticket and authenticator are encoded per RFC 1510
(ASN.1 encoding). Once the ClientKeyExchange message is received,
the server's secret key is used to unwrap the credentials and extract
the pre-master secret.
Note that a Kerberos authenticator is not required, since the master
secret derived by the client and server is seeded with a random value
passed in the server hello message, thus foiling replay attacks.
However, the authenticator may still prove useful for passing
authorization information and is thus allotted an optional field (see
Figure 2).
Lastly, the client and server exchange the finished messages to
complete the handshake. At this point we have achieved the
following:
Medvinsky & Hur Standards Track [Page 4]
RFC 2712 Addition of Kerberos Cipher Suites to TLS October 1999
1) A master secret, used to protect all subsequent communication, is
securely established.
2) Mutual client-server authentication is achieved, since the TLS
server proves knowledge of the master secret in the finished
message.
Note that the Kerberos option fits in seamlessly, without adding any
new messages.
To obtain an appropriate service ticket, the TLS client must
determine the principal name of the TLS server. The Kerberos service
naming convention is used for this purpose, as follows:
host/MachineName@Realm
where:
- The literal, "host", follows the Kerberos convention when not
concerned about the protection domain on a particular machine.
- "MachineName" is the particular instance of the service.
- The Kerberos "Realm" is the domain name of the machine.
The proposed Kerberos authentication option is added in exactly the
same manner as a new public key algorithm would be added to TLS.
Furthermore, it establishes the master secret in exactly the same
manner.
Kerberos ciphersuites are subject to the same security considerations
as the TLS protocol. In addition, just as a public key
implementation must take care to protect the private key (for example
the PIN for a smartcard), a Kerberos implementation must take care to
protect the long lived secret that is shared between the principal
and the KDC. In particular, a weak password may be subject to a
dictionary attack. In order to strengthen the initial authentication
to a KDC, an implementor may choose to utilize secondary
authentication via a token card, or one may utilize initial
authentication to the KDC based on public key cryptography (commonly
known as PKINIT - a product of the Common Authentication Technology
working group of the IETF).
Medvinsky & Hur Standards Track [Page 5]
RFC 2712 Addition of Kerberos Cipher Suites to TLS October 1999
[1] Dierks, T. and C. Allen, "The TLS Protocol, Version 1.0", RFC
2246, January 1999.
[2] Kohl J. and C. Neuman, "The Kerberos Network Authentication
Service (V5)", RFC 1510, September 1993.
Copyright (C) The Internet Society (1999). All Rights Reserved.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Medvinsky & Hur Standards Track [Page 7]