Kerberos V5 Reply Keys From TLS
draft-josefsson-krb5starttls-bootstrap-01

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Abstract

This document describes how the Kerberos V5 over TLS protocol together with a users' long term shared secret can be used to 1) allow clients to securely learn a realm's KDC X.509 certificate, 2) distribute the X.509 trust anchors used by the KDC, and 3) make it possible for clients to use Kerberos V5 over TLS without having to validate the server certificates.

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 user and the KDC by instead using TLS user authentication.

This goals are achieved by introducing two new Kerberos V5 pre-authentication mechanisms that modify how the Kerberos V5 reply key is derived.

Table of Contents

1.  Introduction and Background
2.  TLS Exporter Function
3.  Reply Key Strengthening
4.  Avoiding Use Of Long-Term Shared Key
5.  IANA Considerations
6.  Acknowledgements
7.  Security Considerations
8.  References
    8.1.  Normative References
    8.2.  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 mechanism that uses Kerberos V5 over TLS (Josefsson, S., “Using Kerberos V5 over the Transport Layer Security (TLS) protocol,” January 2010.) [I‑D.josefsson‑kerberos5‑starttls] to achieve:

• 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.
• The ability to use Kerberos V5 over TLS without having to validate the server certificates.

These goals are achieved by having the client connect to a KDC, take a note of the server's certificate, and verify them as belonging to the KDC the user trusts by properly decrypting the Kerberos V5 response using the user's password. Only the correct KDC will be able to generate a Kerberos V5 response using the user's password and the secrets derived from the TLS channel.

The mechanism to achieve the above goals is for the KDC to strengthen the Kerberos V5 reply key using keying material derived from the TLS channel (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.) [RFC5246] using the algorithm specified in Keying Material Exporters for Transport Layer Security (TLS) (Rescorla, E., “Keying Material Exporters for Transport Layer Security (TLS),” September 2009.) [I‑D.ietf‑tls‑extractor].

The document also describes a pre-authentication mechanism that can be used to achieve:

• Allow use of Kerberos V5 without a long-term shared secret between the user 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,” January 2010.) [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.  TLS Exporter Function

The following function Krb5KeyFromTLS is used elsewhere to derive keys from a TLS session.

   Krb5KeyFromTLS (inkey, inkey_len, tlscb, tlscb_len, length, label)

   Input:          inkey      encryption key, an octet string
                   inkey_len  length of encryption key,
                              a positive integer
                   tlscb      channel binding data, an octet string,
                   tlscb_len  length of channel binding data,
                              a positive integer
                   length     number of bytes to derive,
                              a positive integer
                   label      the TLS PRF label to use,
                              a IANA registered string

   Output:         outkey   derived key, an "length"-octet string

   Steps:

      1. Perform the TLS Exporter step:

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

         The "context_value" should be the concatenation of "inkey"
         followed by "tlscb".

         Consequently, the length of "context_value" (which used to
         derived "context_value_length") will be the sum of
         "inkey_len" and "tlscb_len".

         The values of "length" and "label" are as the inputs to this
         function.

      3. Output the derived key "outkey".

3.  Reply Key Strengthening

If the client do not (yet) have trust anchors for the KDC, it should delay verification of the server certificate.

To signal that the client wishes the KDC to strengthen the reply key using keying material derived from the TLS session, it sends a pre-authentication mechanism called "pa-krb5starttls-strengthen". It has a pdata-type integer value of #TBD.

The pre-authentication structure is defined in RFC 4120 as:

  PA-DATA         ::= SEQUENCE {
          -- NOTE: first tag is [1], not [0]
          padata-type     [1] Int32,
          padata-value    [2] OCTET STRING -- might be encoded AP-REQ
  }

The content of the padata-value should be the DER encoding of the empty string.

When receiving the request to use the "pa-krb5starttls-strengthen" pre-authentication message, the KDC needs to decide whether to honor it or not. This is a policy decision that can depend on several reasons, including the content of the request. If the KDC decides that it does not wish to honor the "pa-krb5starttls-strengthen" request, the KDC MUST fail the request by returning KDC_ERR_PREAUTH_FAILED.

When the KDC decides to honor the client's request, it will process the incoming request as usual except that the KDC-REP reply key is post processed. The post processing uses Keying Material Exporters for Transport Layer Security (TLS) (Rescorla, E., “Keying Material Exporters for Transport Layer Security (TLS),” September 2009.) [I‑D.ietf‑tls‑extractor], by invoking the Krb5KeyFromTLS function with the following inputs:

   inkey: user's long term shared secret
   inkey_len: length of "inkey"
   tlscb: the client's TLS Finished message data,
          as described in the "tls-unique" channel binding
          registration.
   tlscb_len: length of "tlscb".
   length: same as "inkey_len"
   label: "Kerberos V5 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 KDC server certificate and/or trust anchors transferred during the TLS handshake, to be used during future Kerberos V5 over TLS connections.

The client MAY skip using this protocol for future connections, and instead rely on the standard Kerberos V5 over TLS protocol with proper validation of server certificate.

4.  Avoiding Use Of Long-Term Shared Key

The ETYPE-INFO-TLS pre-authentication type is sent by the KDC in a KRB-ERROR indicating a requirement for additional pre-authentication before sending a reply protected using a key derived only from the TLS session. It is used to notify a client of which encryption type to use for the encryption of an encrypted timestamp for the purposes of sending a PA-ENC-TIMESTAMP pre-authentication value using an encryption key derived from the TLS session.

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

The client choses a supported encryption type and re-send the request with a PA-ENC-TIMESTAMP encrypted using a key derived from the TLS session by using Krb5KeyFromTLS with the following input:

   inkey: empty string
   inkey_len: 0
   tlscb: the client's TLS Finished message data,
          as described in the "tls-unique" channel binding
          registration.
   tlscb_len: length of "tlscb".
   length: key length of the chosen encryption type
   label: "Kerberos V5 pre-auth key"

The KDC verifies the PA-ENC-TIMESTAMP and if successful it knows it is talking to the authenticated user and can send a response encrypted using the same encryption type as the client selected but with a key derived using Krb5KeyFromTLS with the same inputs except for:

    label: "Kerberos V5 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.)

5.  IANA Considerations

The IANA is requested to allocate the string "kerberos V5 reply key" in the TLS Exporter label registry.

6.  Acknowledgements

Nicolas Williams mentioned the advantages in <http://permalink.gmane.org/gmane.ietf.krb-wg/5016>, and also suggested the use of PA-ENC-TIMESTAMP.

7.  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,” January 2010.) [I‑D.josefsson‑kerberos5‑starttls] are inherited.

By using ETYPE-INFO-TLS the long-term shared key of the user is no longer involved in deriving the 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.

8.  References

8.1. Normative References

8.2. Informative References

Author's Address