INTERNET-DRAFT Matthew Hur Transport Layer Security Working Group Joseph Salowey draft-ietf-tls-kerb-01.txt Cisco Systems Obsoletes: RFC 2712 Ari Medvinsky November 8, 2001 (Expires May 8, 2001) Liberate Kerberos Cipher Suites in Transport Layer Security (TLS) 0. Status Of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as ``work in progress.'' The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html 1. Abstract RFC 2712 [KERBTLS] introduced mechanisms for supporting Kerberos [KERB] authentication within the TLS protocol [TLS]. This document extends RFC 2712 to support delegation of Kerberos credentials. In this way, a TLS server may obtain a Kerberos service ticket on behalf of the TLS client. Thus, a single client identity may be used for authentication within a multi-tier architecture. This draft also proposes a mechanism for a TLS server to indicate Kerberos-specific information to the client within the certificate request message in the initial exchange. 2. Introduction 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. RFC 2712 specified how TLS could be extended to support organizations with heterogeneous security deployments that include authentication systems based on symmetric cryptography. Kerberos, originally developed at MIT, is based on an open standard and is the most widely deployed symmetric key authentication system. Just as other documents specify hybrid asymmetric/symmetric key protocols [PKINIT] [PKCROSS] [PKTAPP], this document specifies how TLS may incorporate both symmetric and asymmetric key crypto systems. This document describes the use of Kerberos authentication within the TLS framework. This achieves mutual authentication and the establishment of a master secret using Kerberos credentials. Additionally, this document specifies support for delegation of Kerberos credentials, which enables end to end authentication within an n-tier architecture. The proposed changes are minimal and, in fact, no different from adding a new public key algorithm to the TLS framework. 3. Kerberos Authentication Option In TLS 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 [TLS]. +-------------------------------------------------------------------+ | 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 RFC 2246. The TLS security context is negotiated in the client and server hello messages. For example: TLS_RSA_WITH_RC4_128_MD5 means the initial authentication will be done using the RSA public key algorithm, RC4 will be used with a 128 bit session key, and MACs will be based on the MD5 algorithm. Thus, to facilitate the Kerberos authentication option, we must start by defining Kerberos cipher suites including (but not limited to): CipherSuite TLS_KRB5_WITH_3DES_EDE_CBC_SHA = { 0x00,0x70 }; CipherSuite TLS_KRB5_WITH_3DES_EDE_CBC_MD5 = { 0x00,0x71 }; CipherSuite TLS_KRB5_WITH_RC4_128_SHA = { 0x00,0x72 }; CipherSuite TLS_KRB5_WITH_RC4_128_MD5 = { 0x00,0x73 }; CipherSuite TLS_KRB5_WITH_DES_CBC_SHA = { 0x00,0x74 }; CipherSuite TLS_KRB5_WITH_DES_CBC_MD5 = { 0x00,0x75 }; CipherSuite TLS_KRB5_WITH_AES_128_CBC_SHA = { 0x00,0x76 }; CipherSuite TLS_KRB5_WITH_AES_256_CBC_SHA = { 0x00,0x77 }; CipherSuite TLS_KRB5_WITH_NULL_SHA = { 0x00,0x78 }; CipherSuite TLS_KRB5_WITH_NULL_MD5 = { 0x00,0x79 }; 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 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. Note that these messages are specified as optional in the TLS protocol; therefore, omitting them is permissible. The Kerberos option affects three of the TLS messages: the CertificateRequest, the client Certificate, and the ClientKeyExchange. However, only the client Certificate and the ClientKeyExchange are required. 3.1. Usage of the CertificateRequest Message If the server accepts a Kerberos-based ciphersuite, then it MUST send the CertificateRequest message to the client. This message conveys Kerberos-specific characteristics such as realm name or attributes such as forwarded ticket. RFC 2246 defines the CertificateRequest message as follows: +-------------------------------------------------------------------+ | | | enum { | | rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4), | | (255) | | } ClientCertificateType; | | | | opaque DistinguishedName<1..2^16-1>; | | | | struct { ClientCertificateType certificate_types<1..2^8-1>; | | DistinguishedName certificate_authorities<3..2^16-1>; | | } CertificateRequest; | | | +-------------------------------------------------------------------+ FIGURE 2: CertificateRequest message from RFC 2246 This specification defines a new ClientCertificateType for a Kerberos certificate. This enables a client to respond to the CertificateRequest message when using Kerberos ciphersuites. The Kerberos ClientCertificateType MUST NOT be included in certificate_types for non-Kerberos ciphersuites. Thus the following change for ClientCertificateType is required (Figure 3). +-------------------------------------------------------------------+ | | | enum { | | rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4), | | kerberos(5), (255) | | } ClientCertificateType; | | | +-------------------------------------------------------------------+ FIGURE 3: New Kerberos ClientCertificateType In the case of a public key based authentication algorithm, the opaque DistinguishedName field is derived from [X509], and it contains the name of an acceptable certification authority (This is as specified in [TLS]). In the case of a Kerberos ClientCertificateType, the DistinguishedName field is defined to represent Kerberos information (KerbInfo) as shown in Figure 4. The srv_tgt attribute type is used by the server to send a TGT that the client presents to the KDC in the case of user-to-user authentication. The KDC uses the session key from this ticket to encrypt a service ticket for the server. In this case the attr_data must be of non-zero length and contain the server's TGT. +-------------------------------------------------------------------+ | | | enum | | { | | srv_tkt(1), fwd_tgt(2), (255) | | } KerbInfoType; | | | | enum | | { | | initial_tkt_required(1), srv_tgt(2), (255) | | } AttrType; /* This may be extended to include attributes */ | | /* such as forwardable or renewable for example */ | | | | struct | | { | | AttrType attr_type; | | opaque attr_data <0..2^16-1>; | | } AttrInfoType | | | | struct | | { | | uint32 length; /* length of this struct */ | | KerbInfoType type; | | opaque sname <0..2^16-1>; | | opaque srealm <0..2^16-1>; | | opaque cname <0..2^16-1>; | | opaque crealm <0..2^16-1>; | | AttrInfoType attr_info <0..2^16-1>; /* sequence of */ | | /* attributes */ | | uint32 etypes <0..2^16-1>; /* list of supported */ | | /* Kerberos etypes */ | | /* for authentication */ | | } TktInfo; | | | | struct | | { | | TktInfo tkt_info <1..2^20-1>; /* MUST have at least */ | | /* 1 TktInfo structs */ | | } KerbInfo | | | +-------------------------------------------------------------------+ FIGURE 4: Kerberos Information for CertificateRequest Message 3.2. Usage of the Client Certificate Message As specified by [TLS], when the client receives the CertificateRequest message, it MUST respond with the client Certificate message. As stated above, this specification defines a Kerberos certificate type. The format for the Kerberos certificate is specified in figure 5 below. This structure consists of a Kerberos AP-REQ message that is used for authenticating the client to he server. It optionally contains a series of Kerberos KRB-CRED messages to convey delegated credentials. Note that the client may determine the type of credentials to send to the server, based on local policy. Part of the input to a client's decision may come from the Kerberos KDC. For example, The client may convey a delegated ticket based on the ok-as-delegate ticket flag set in the service ticket. Also, the session key used to protect a forwarded credential, MUST be of equal or greater strength than the key used to protect the ticket when originally sent to the client (typically, this key is the client principal key, shared with the Kerberos KDC). +-------------------------------------------------------------------+ | | | opaque KrbCred <1..2^16-1>; /* Kerberos-defined KRB-CRED */ | | | | struct | | { | | opaque ap_req <1..2^16-1>; | | KrbCred krb_cred <0..2^20-1>; | | } KerberosCert; | | | +-------------------------------------------------------------------+ FIGURE 5: Kerberos Certificate Type 3.3. Usage of the ClientKeyExchange Message The Kerberos option must be added to the ClientKeyExchange message as shown in Figure 6. +-------------------------------------------------------------------+ | | | struct | | { | | select (KeyExchangeAlgorithm) | | { | | case krb: KerbEncryptedPreMasterSecret; | | case rsa: EncryptedPreMasterSecret; | | case diffie_hellman: ClientDiffieHellmanPublic; | | } Exchange_keys; | | } ClientKeyExchange; | | | | KerbEncryptedPreMasterSecret contains the PreMasterSecret | | encrypted within a Kerberos-defined EncryptedData structure. | | The encryption key is sealed in the ticket sent in the Client | | Certificate message. | | | +-------------------------------------------------------------------+ FIGURE 6: 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. Lastly, the client and server exchange the finished messages to complete the handshake. At this point we have achieved the following: 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. Kerberos fits seamlessly into TLS, without adding any new messages. 4. Naming Conventions: To obtain an appropriate service ticket, the TLS client must determine the principal name of the TLS server. The Kerberos service naming convention is 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. As specified above, in the CertificateRequest message, the server may indicate the appropriate principal name and realm. 5. Summary 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. 6. Security Considerations 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 Kerberos working group of the IETF). The unauthenticated CertificateRequest message, specified above, enables the server to request a particular client principal name as well as a particular service principal name. In the event that a service principal name is specified, there is a risk that the client may be tricked into requesting a ticket for a rogue server. Furthermore, if delegation is requested, the client may be tricked into forwarding its TGT to a rogue server. In order to assure that a service ticket is obtained for the correct server, the client should rely on a combination of its own local policy, local configuration information, and information supplied by the KDC. The client may choose to use only the naming convention specified in section 4. The client may rely on the KDC performing name cannonicalization (this is a matter that is adressed in revisions to RFC 1510). The client must apply its local policy to determine whether or not to forward its credentials. As previously stated, the client should incorporate information from the KDC, in particular the ok-as- delegate ticket flag, in making such a policy decision. The forwarded credential MUST be protected in a key that is at least the same strength as the principal key that originally protected the TGT. A forwarded TGT presents more vulnerabilities in the event of a rogue server or the compromise of the session key. An attacker would be able to impersonate the client to obtain new service tickets. Such an attack may be mitigated by the use of restrictions, such as those described in [Neuman]. It has been shown that 56-bit DES keys are relatively easy to compromise [DESCRACK]; therefore, use of 56-bit DES is discouraged. 7. Acknowledgements We would like to thank the following people for their input for this document: Clifford Neuma - ISI John Brezak and David Mowers - Microsoft 8. References [KERBTLS] A. Medvinsky and M. Hur, "Addition of Kerberos Cipher Suites to Transport Layer Security (TLS)", RFC 2712, October 1999. [KERB] J. Kohl and C. Neuman, "The Kerberos Network Authentication Service (V5)", RFC 1510, September 1993. [TLS] T. Dierks and C. Allen, "The TLS Protocol, Version 1.0", RFC 2246, January 1999. [PKINIT] B. Tung, C. Neuman, M. Hur, A. Medvinsky, S. Medvinsky, J. Wray, J. Trostle. Public Key Cryptography for Initial Authentication in Kerberos. draft-ietf-cat-kerberos-pk-init-14.txt [PKTAPP] A. Medvinsky, M. Hur, S. Medvinsky, C. Neuman. Public Key Utilizing Tickets for Application Servers (PKTAPP). draft-ietf-cat-kerberos-pk-tapp-03.txt [PKCROSS] M. Hur, B. Tung, T. Ryutov, C. Neuman, G. Tsudik, A. Medvinsky, B. Sommerfeld. Public Key Cryptography for Cross-Realm Authentication in Kerberos. draft-ietf-cat-kerberos-pk-cross-07.txt [X509] ITU-T (formerly CCITT) Information technology - Open Systems Interconnection - The Directory: Authentication Framework Recommendation X.509 ISO/IEC 9594-8 [NEUMAN] B.C. Neuman, "Proxy-Based Authorization and Accounting for Distributed Systems". Proceedings of the 13th International Conference on Distributed Computing Systems, May 1993 [DESCRACK] Electronic Frontier Foundation, "Cracking DES: Secrets of Encryption Research, Wiretap Politics, and Chip Design". May 1998, Electronic Frontier Foundation. 9. Authors' Addresses Matthew Hur Cisco Systems 2901 Third Avenue Seattle, WA 98121 Phone: +1 206 256 3197 EMail: mhur@cisco.com http://www.cisco.com Joseph Salowey Cisco Systems 2901 Third Avenue Seattle, WA 98121 Phone: +1 206 256 3380 EMail: jsalowey@cisco.com http://www.cisco.com Ari Medvinsky Liberate 2 Circle Star Way San Carlos, CA 94070-6200 Phone: +1 650 701 4000 EMail: ari@liberate.com http://www.liberate.com 10. Full Copyright Statement Copyright (C) The Internet Society (1999). 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. 11. Appendix Changes from RFC 2712 Added new cipher suites with NULL confidentiality: TLS_KRB5_WITH_NULL_SHA TLS_KRB5_WITH_NULL_MD5 Added new cipher suites to support AES: TLS_KRB5_WITH_AES_128_CBC_SHA TLS_KRB5_WITH_AES_256_CBC_SHA 40 bit ciphers have been removed, and AES ciphers have been added. All of the ciphersuites have been renumbered to avoid conflicts with exisiting implementations of RFC 2712. RFC 2712 utilized only the ClientKeyExchange message for conveying the Kerberos credentials and encrypted premaster-secret. This specification moves the Kerberos credentials to the client certificate message, and it allows the client to pass delegated credentials as well. Additionally, this specification allows the server to specify Kerberos-specific information (realm, delegation required, etc.) in the CertificateRequest message.
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