Deriving Keys From TLS for Kerberos V5
draft-josefsson-krb5starttls-bootstrap-03

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Abstract

This document describes how clients can use the Kerberos V5 over TLS protocol together with its long term key to 1) avoid having to validate the server certificate, 2) securely learn a KDC's server certificate, and 3) learn the trust anchors used by the KDC.

We also describe how the Kerberos V5 over TLS protocol can be used to 4) avoid the need for a long term shared key between the client and the KDC by instead using TLS client authentication.

These goals are achieved by introducing a new Kerberos V5 pre-authentication type that modify how the Kerberos V5 reply key is derived.

Table of Contents

1.  Introduction and Background
2.  The Krb5KeyFromTLS Function
3.  The PA-TLS Pre-Authentication Type
4.  Reply Key Strengthening
5.  Avoiding Use Of Long-Term Shared Key
6.  IANA Considerations
7.  Acknowledgements
8.  Security Considerations
9.  References
    9.1.  Normative References
    9.2.  Informative References
§  Author's Address

1.  Introduction and Background

This document describes a Kerberos V5 (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.) [RFC4120] pre-authentication type that uses Kerberos V5 over TLS (Josefsson, S., “Using Kerberos V5 over the Transport Layer Security (TLS) protocol,” December 2008.) [I‑D.josefsson‑kerberos5‑starttls] to achieve:

• The ability to use Kerberos V5 over TLS without having to validate the server certificates.
• Allow Kerberos V5 clients to securely learn a Kerberos V5 realm's Key Distribution Center (KDC) certificates.
• Securely distribute the trust anchors used by the Key Distribution Center (KDC) in a Kerberos V5 realm.

These goals are achieved by having the client connect to a KDC without verifying the server certificates, take a note of the server certificate and the certificate chain, and verify them as belonging to the KDC the client trusts by properly decrypting the Kerberos V5 response using the clients long term key. Only the correct KDC will be able to generate a Kerberos V5 response using the clients long-term key and the secrets derived from the TLS channel (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.) [RFC5246].

The document also describes a mechanism to achieve:

• Allow use of Kerberos V5 without a long-term shared secret between the client and the KDC.

This goal is achieved by having the client authenticate itself using TLS, and having the KDC request that the client send a PA-ENC-TIMESTAMP pre-authentication data encrypted using a key derived from the TLS channel. If successful, the KDC will encrypt the response using a reply key derived only from the TLS channel.

This document requires that both the client and the KDC MUST support Kerberos V5 over TLS (Josefsson, S., “Using Kerberos V5 over the Transport Layer Security (TLS) protocol,” December 2008.) [I‑D.josefsson‑kerberos5‑starttls].

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].

2.  The Krb5KeyFromTLS Function

The following function Krb5KeyFromTLS is used to derive keys from a TLS session. This builds on the Keying Material Exporters for Transport Layer Security (TLS) (Rescorla, E., “Keying Material Exporters for Transport Layer Security (TLS),” February 2009.) [I‑D.ietf‑tls‑extractor] framework and uses functions defined in Encryption and Checksum Specifications for Kerberos 5 (Raeburn, K., “Encryption and Checksum Specifications for Kerberos 5,” February 2005.) [RFC3961].

   Krb5KeyFromTLS (ltkey, ltkey_len,
                   tlscb, tlscb_len,
                   enctype,
                   label)

   Input:          ltkey      long term key,
                              an octet string
                   ltkey_len  length of long term key,
                              an integer larger or equal to 0
                   tlscb      channel binding data,
                              an octet string
                   tlscb_len  length of channel binding data,
                              a positive integer
                   etype      number assigned for an encryption type
                   label      the TLS PRF label to use,
                              a IANA registered string

   Output:         protkey    derived protocol-key

   Steps:

      1. Set "length" to the key-generation seed length, K, for the
         encryption type "enctype" as per RFC 3961.

      2. Set "context_value" to the concatenation of "ltkey" followed
         by "tlscb".  Note that "ltkey" may be empty.

      3. Derive the value for "context_value_length" from the sum of
         "ltkey_len" and "tlscb_len".

      4. Perform the TLS Exporter step:

          outkey = PRF(master_secret, label,
                       SecurityParameters.client_random +
                       SecurityParameters.server_random +
                       context_value_length + context_value
                      )[length]

      5. Output random-to-key(outkey).  The random-to-key function is
         defined in RFC 3961.

3.  The PA-TLS Pre-Authentication Type

The PA-TLS pre-authentication type is sent by the client to a KDC. It requests that the server uses a different Kerberos V5 reply key. If the "only-tls" flag is true, the reply key will be derived from only the TLS session. If the "only-tls" flag is false, the key will be derived from both the TLS session and the the client long-term key. The exact semantic is described in sub-sequent sections.

The syntax of PA-TLS is defined as follows.

   PA-TLS        ::= EncryptedData -- PA-TLS-ENC

   PA-TLS-ENC           ::= SEQUENCE {
           patimestamp     [0] KerberosTime -- client's time --,
           pausec          [1] Microseconds OPTIONAL,
           only-tls        [2] BOOLEAN
   }

The client choses the encryption type to use. Kerberos V5 (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.) [RFC4120] mandates support for AES256-CTS-HMAC-SHA1-96 (Raeburn, K., “Advanced Encryption Standard (AES) Encryption for Kerberos 5,” February 2005.) [RFC3962]. If the client do not have out of band information to use another encryption type, clients MUST use AES256-CTS-HMAC-SHA1-96.

The key used to encrypt the PA-TLS-ENC is derived using Krb5KeyFromTLS with the following input:

   ltkey: empty string
   ltkey_len: 0
   tlscb: the client's TLS Finished message data,
          as described in the "tls-unique" channel binding
          registration.
   tlscb_len: length of "tlscb".
   etype: the encryption type number chosen by the client
   label: "EXPORTER Kerberos pre-auth key"

The server process an PA-TLS by verifying that the encryption type is acceptable. If this fails, the server MAY respond with a PA-ETYPE-INFO-TLS as defined below. The server proceed and derive the keys and decrypt the PA-TLS. If this fails, the server MUST respond with a KDC_ERR_PREAUTH_FAILED error.

When the PA-TLS is successfully decrypted, the KDC needs to decide whether to honor the request or not. This is a policy decision that can depend on several reasons, including the content of the request.

When the "tls-only" flag is true, the server MUST verify that TLS has authenticated the client (e.g., by a X.509 client certificate or OpenPGP key or Secure Remote Password (SRP) password). The KDC may perform policy checks whether a particular client should be allowed to use this pre-authentication type.

If for any reason the server decides that it does not wish to accept the PA-TLS request, the server MUST fail the request by returning KDC_ERR_PREAUTH_FAILED.

An PA-ETYPE-INFO-TLS message is used by the KDC to demand that a client sends a PA-TLS. The PA-ETYPE-INFO-TLS contains, by the KDC, acceptable encryption types. The PA-ETYPE-INFO-TLS message can be used by a KDC to require that clients uses PA-TLS, or to require that clients send a PA-TLS using some particular encryption types.

The PA-ETYPE-INFO-TLS is used as follows. The KDC sends a KRB-ERROR packet with the KDC_ERR_PREAUTH_REQUIRED error-code and store a METHOD-DATA containing an PA-ETYPE-INFO-TLS in the e-data field.

PA-ETYPE-INFO-TLS          ::= SEQUENCE OF Int32 -- EncryptionType
                                    -- in preference order --,

The client responds by sending a PA-TLS encrypted using one of the indicated types, or fail for policy reasons (e.g., none of the proposed encryption types are acceptable).

4.  Reply Key Strengthening

If the client do not have the required information needed to verify a server certificate, it will delay verification of the server certificate. The server MUST include a root certificate in the TLS certificate_list.

The client sends a PA-TLS type with the "tls-only" flag set to FALSE. The server process the PA-TLS as described earlier. On success, the server process the incoming requests as usual except that any KDC-REP reply key is post processed using the Krb5KeyFromTLS function with the following inputs:

   ltkey: client long term key in protocol-key format
   ltkey_len: length of "ltkey"
   tlscb: the client's TLS Finished message data,
          as described in the "tls-unique" channel binding
          registration.
   tlscb_len: length of "tlscb".
   etype: encryption type number of client long-term key
   label: "EXPORTER Kerberos strengthen key"

The client will strengthen its local KDC-REP reply key using the same procedure. On successful decryption of the KDC-REP, the clients is certain that it is talking to a KDC that knows the client's shared key without any man-in-the-middle. The client can then remember the server certificate and/or trust anchors transferred during the TLS handshake, to be used during future Kerberos V5 over TLS connections.

If the client can securely store the information required to validate the server in the future, the client MAY skip using the PA-TLS for future connections, and instead rely on the standard Kerberos V5 over TLS protocol with proper validation of server certificate.

5.  Avoiding Use Of Long-Term Shared Key

The client can use TLS to authenticate it, and then ask the KDC to use the TLS authentication to authenticate the Kerberos request. The latter step is performed by sending a PA-TLS type with "only-tls" set to TRUE.

The server process the PA-TLS as described earlier. On success, the server process the incoming Kerberos requests as usual except that the KDC-REP reply key will be generated by Krb5KeyFromTLS with the following inputs:

   ltkey: empty string
   ltkey_len: 0
   tlscb: the client's TLS Finished message data,
          as described in the "tls-unique" channel binding
          registration.
   tlscb_len: length of "tlscb".
   etype: encryption type used for the PA-TLS
   label: "EXPORTER Kerberos derive key"

The client derives the key the same way, and will be able to decrypt the response.

Note that this means the long term shared key will not be involved in deriving the reply that protects the Kerberos V5 response.

(The reason for encrypting the response is because Kerberos V5 does not have any null encryption scheme.)

6.  IANA Considerations

The IANA is requested to allocate the strings "EXPORTER Kerberos pre-auth key", "EXPORTER Kerberos strengthen key", and "EXPORTER Kerberos derive key" in the TLS Exporter label registry.

7.  Acknowledgements

Nicolas Williams mentioned the advantages in <http://permalink.gmane.org/gmane.ietf.krb-wg/5016>, and suggested the use of (what became) PA-TLS.

8.  Security Considerations

The security considerations in Kerberos V5 (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.) [RFC4120], TLS (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.) [RFC5246], Kerberos V5 TCP extension (Josefsson, S., “Extended Kerberos Version 5 Key Distribution Center (KDC) Exchanges over TCP,” August 2007.) [RFC5021], and Kerberos V5 over TLS (Josefsson, S., “Using Kerberos V5 over the Transport Layer Security (TLS) protocol,” December 2008.) [I‑D.josefsson‑kerberos5‑starttls] are inherited.

When a client requests PA-TLS with "only-tls" set to TRUE, the client's long-term key is no longer involved in deriving the encrypted Kerberos V5 ticket. Instead only the authentication from the TLS channel is used. This changes the cryptographic model of Kerberos V5 significantly, and makes it possible to operate Kerberos V5 without even having a long term shared key for a particular user. This changes how a Kerberos V5 security analysis should be made, so be aware of this model change when reading other literature.

9.  References

9.1. Normative References

9.2. Informative References

Author's Address