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Versions: 00 01                                                         
INTERNET-DRAFT                                        Jonathan Trostle
draft-ietf-cat-kerberos-pk-recovery-01.txt            Cisco Systems
Updates: RFC 1510
expires May 23, 1999

    Public Key Cryptography for KDC Recovery in Kerberos V5

0. Status Of this Memo

   This document is an Internet-Draft.   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 docu-
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   The distribution of  this  memo  is  unlimited.   It  is  filed  as
   draft-ietf-cat-kerberos-pk-recovery-01.txt, and expires May 23,
   1999. Please send comments to the authors.

1. Abstract

   This document defines extensions to the Kerberos protocol
   specification (RFC 1510, "The Kerberos Network Authentication
   Service (V5)", September 1993) to enable the recovery of a
   compromised Kerberos V5 KDC using public key cryptography.
   The document specifies the recovery protocol which uses
   preauthentication data fields and error data fields in Kerberos
   messages to transport recovery data.

2. Motivation

   For both secret key based systems and public key based systems,
   compromise of the security server (KDC in the secret key system and
   CA or certificate authority in the public key system) leads to a
   complete breakdown of the authentication service. The difference
   between the two systems comes when the compromise is detected.
   Assuming that a root key is intact in the public key system, new
   high-level certificates can be signed, any suspicious certificates
   can be revoked, and the system can eventually return to normal
   operation without excessive administrator involvement. For a pure
   secret key based system such as Kerberos V5, the recovery
   operation is very difficult from an administrative point of view,
   since all users must receive new passwords out of band.

   This document describes an extension to Kerberos V5 that can be
   used in conjunction with the protocol in [2]
   (draft-ietf-cat-kerberos-pkinit-07.txt) to allow a KDC to be
   automatically recovered once the administrator has reinstalled
   the operating system and loaded (and certified) the new KDC public
   key. Although the protocols in [2] are a step towards making the KDC
   recovery problem easier, there are still potentially many secret keys
   stored on the KDC. For example, when the user private key is stored
   on the KDC, the user and the KDC share a secret key that is used for
   authentication. The two main issues for recovery are updating the KDC
   public key with all clients, which will happen automatically since we
   assume the KDC public keys are signed as part of a public key
   infrastructure with a revocation capability, and updating the shared
   secret keys that are stored on the KDC.

   We now describe the requirements for the recovery extension:
   (1) Users that use password based keys to authenticate to the KDC
   (as in section 3.4 of [2] will have those keys automatically changed
   by the recovery protocol; the users will not have to change their
   passwords. We will satisfy this requirement by obtaining the secret
   key K2 of section 3.4 of [2] by hashing the key K1 with a salt value
   supplied by the KDC. The update operation during recovery consists
   of changing the salt value. Optionally, the KDC can ask users to
   change their passwords in order to support recovery in an environment
   where users use both recovery capable and non-recovery capable clients.
   (2) The recovery extension requires the KDC public keys to be signed
   in certificates as part of a public key infrastructure that includes
   a revocation capability.
   (3) Recovery capable clients must be pkinit [2] capable.

   We will use the definitions and ASN.1 structures from [2]; we assume
   familiarity on the part of the reader.

3. The Recovery Extension Protocol

   We now briefly overview the proposed recovery extension. When the
   recovery operation is launched, the KDC host operating system(s) for
   the realm along with the KDC database is reloaded from backup media.
   The new KDC public key certificate is placed into the appropriate
   certificate database (if needed), and the old certificate is revoked
   by administrator action. For all principals that have symmetric keys
   in the database, the keys are zeroized.

   To complete recovery, the newly created kdcSalt value (a randomly
   generated 16 byte string) will be sent to user principals to allow
   them to update their shared secrets in the KDC database. This exchange
   allows users to maintain the same passwords. This task is completed
   by the following sequence of messages:

   KDC <-------- AS_REQ message ----------- client

   KDC  -------- KRB_ERROR message -------> client
              (error code KDC_ERR_RECOVERY_USER_NEEDED)
               error data: KDC DH parameters, kdcSalt value,
               optional PA-PK-KEY-REP (encrypted user private keys),
               optional change password flag

   KDC <-------- AS_REQ message  ---------- client
              (with PA-PK-AS-REQ and PA-PK-RECOVERY-DATA (with new user
               secret key K2 encrypted in Diffie-Hellman shared secret
               key) preauthentication fields)

   KDC  -------- AS_REP message  ---------> client
              (with PA-PK-AS-REP preauthentication field)

   This exchange of messages is only necessary between the KDC and each
   user principal that has a shared secret key stored in the KDC

   A recovery capable principal that receives a ticket with an encrypted
   part using a key with an unexpected kvno, should perform a pkinit [2]
   AS exchange with the KDC, including the
   PA-PK-RECOVERY-SET-PRINCKEY-TO-SKEY padata-type to obtain a TGT
   with a ticket session key that will be used as the new principal
   secret key. In this case, the KDC would have previously generated
   the secret key to encrypt the ticket, based on the TGS_REQ from the
   client, and the database bits indicating that the server principal
   should have a valid symmetric key but one does not exist in the
   database. The KDC will always use the symmetric key with the
   appropriate keytype from the database as the ticket session key
   when receiving a pkinit request with the
   PA-PK-RECOVERY-SET-PRINCKEY-TO-SKEY padata-type. The padata-value
   for this padata-type is an empty octet string.

3.1 Definitions

   The proposed extension includes a new algorithm for computing the
   shared key between a user and the KDC. The new algorithm involves
   computing the SHA1 hash of a string (kdcSalt) supplied by the KDC
   concatenated with the RFC 1510 shared key (the key K1 from section
   3.4 of [2]) to obtain a new DES key K2 that is shared between the
   user and the KDC. We propose etype and keytype 16 for this

      DES-recoverable-key                   16

   Similarly, we propose the same definition for 3DES where the key
   K2 or RFC 1510 shared key is a 3DES key:

      3DES-recoverable-key                  17

   If the KDC expects the client to preauthenticate using the key K2
   with a recoverable key keytype, and the client does not
   preauthenticate, then the e-data for the error
   KDC_ERR_PREAUTH_REQUIRED will be present containing the kdcSalt
   value encoded as an OCTET STRING. If the client preauthenticates
   with the key K2 having keytype DES-recoverable-key, the
   preauthentication fails, and the KDC has a key of the same keytype

   in the database, then the e-data for the error KDC_ERR_PREAUTH_FAILED
   will be present containing the kdcSalt value encoded as an OCTET

   As a performance optimization, the kdcSalt can be locally stored on
   the workstation along with the corresponding realm. If the local
   configuration is missing, or incorrect, the above error messages
   allow the client to find out the correct salt. Clients which are
   configured for symmetric key with a recoverable key keytype,
   attempt to preauthenticate with the salt from the local configuration
   as an input into their key, and if the local configuration is not
   present, the client does not use preauthentication.

   The following new preauthentication types are proposed:

      PA-PK-RECOVERY-DATA                   20

   The following new error code is proposed:

      KDC_ERR_RECOVER_USER_NEEDED           67

   We propose the following additional KDC database bits. The first
   database bit applies to all principals to indicate whether a principal
   should have a valid symmetric key in the database. The second bit
   applies to all principals that should have a valid symmetric key
   to indicate if the principal symmetric key is valid.

   The second database bit is cleared when the KDC undergoes a recovery
   operation, and all principal secret keys are zeroized as well. The
   non-human principal keys are then regenerated when a request comes
   in, and the corresponding validity bits are set.

3.2 Protocol Specification

   We now describe the recovery protocol. The recovery operation can be
   set into motion either because a compromise is detected, or as part of
   a periodic preventative operation. The KDC host operating system and
   KDC executable is restored from backup media, and the KDC is loaded
   with a backup private/public key pair. The KDC database is also
   reloaded, and any secret keys are zeroized. The new KDC public key is
   signed by the appropriate authority and placed in the appropriate
   location and any necessary revocation steps are taken for the old

   To complete the recovery process, the KDC will also notify users
   that need to update any shared secrets that are stored in the KDC
   database: a KRB_ERROR message with the error code
   KDC_ERR_RECOVERY_USER_NEEDED is sent in response to these user's
   AS_REQ messages that do not contain the PA-PK-RECOVERY-DATA
   preauthentication type, contain the PA-PK-RECOVERY-USER-SUPPORTED
   preauthentication type, when there is no valid symmetric key in
   the KDC database, but there needs to be one.

   The following ASN.1 structure is encoded
   and placed into the error message e-data field (an OCTET STRING):

     UserRecoveryError ::= SEQUENCE {
           kdcSalt            [0] OCTET STRING,      -- to be hashed
                                                     -- with password
                                                     -- key K1
           kdcPublicValue     [1] SubjectPublicKeyInfo,
                                                     -- DH algorithm
           kdcPubValueId      [2] INTEGER,           -- DH algorithm
           nonce              [3] INTEGER OPTIONAL,  -- copy nonce
                                                     -- from AS_REQ
                                                     -- if paPkKeyRep
                                                     -- is not below
           paPkKeyRep         [4] OCTET STRING OPTIONAL
                                                     -- ASN.1 encoded
                                                     -- PA-PK-KEY-REP
                                                     -- from section
                                                     -- 3.4 of [2]
                                                     -- (encrypted
                                                     -- user private
                                                     -- keys)
           kdcCert            [5] SEQUENCE OF Certificate, OPTIONAL
                                                     -- cert chain
           changePassword     [6] BOOLEAN OPTIONAL,  -- user client
                                                     -- should use
                                                     -- change password
                                                     -- protocol if
                                                     -- present

   The purpose of the kdcPubValueId identifier in the error message is
   to enable the KDC to offload state to the client; the client will then
   send this identifier to the KDC in an AS_REQ message; the identifier
   allows the KDC to look up the Diffie Hellman private value corresponding
   to the identifier. Depending on how often the KDC updates its private
   Diffie Hellman parameters, it will have to store anywhere between one
   and several dozen of these identifiers and their parameters.

   The e-cksum field of the error message is not optional for this error
   code; it will contain the signature of the entire error message (as
   described in [1]: the signature is computed over the ASN.1 encoded
   error message without the e-cksum field, and then the signature is
   placed into the e-cksum field and the message is re-encoded.) The
   KDC will sign using the private half of its new active key pair.

   Upon checking the KRB_ERROR message, the client obtains the user
   password and uses the kdcSalt to compute the new key K2 which is
   computed by SHA1 hashing the concatenation of the kdcSalt and the
   key K1 obtained from the user password. The result of the hash is
   converted into a DES key by truncating the last 12 bytes and fixing
   the parity on each of the first 8 bytes. The client then responds
   with a new AS_REQ message that includes both a PA-PK-RECOVERY-DATA
   padata-type preauthentication field along with a PA-PK-AS-REQ
   preauthentication field (see [2]). The PA-PK-RECOVERY-DATA must
   contain the newUserKey field. If the user's AS_REQ message passes
   the security checks, the KDC will reply with an AS_REP message
   that contains a PA-PK-AS-REP preauthentication field. The client
   will validate this message as described in [2]. (The procedure
   for 3DES needs to be defined).

   We also define the PA-PK-RECOVERY-USER-SUPPORTED preauthentication
   field; it will accompany all AS_REQ messages from clients that
   support the recovery protocol that originate from user principals.
   The padata-value for this padata-type is an empty octet string.

   If the KRB_ERROR message passes the security checks (the nonce should
   match the client AS_REQ nonce if the error message is a reply, and
   the KDC signature validates), the client replies to the KDC with an
   AS_REQ message containing the PA-PK-RECOVERY-DATA padata-type
   preauthentication field along with a PA-PK-AS-REQ preauthentication
   field (see [2]):

           kdcPubValueId      [0] INTEGER,           -- Copied from error
                                                     -- message
           newUserKey         [2] EncryptedData      -- only present in
                                  OPTIONAL           -- reply to
                                                     -- KDC_ERR_RECOVERY_
                                                     -- USER_NEEDED error;
                                                     -- uses DH shared
                                                     -- key to encrypt the
                                                     -- new key K2.
           sigAll             [3] Signature          -- uses shared DH key
                                                     -- computed over
                                                     -- entire encoded
                                                     -- AS_REQ without
                                                     -- this field, then
                                                     -- re-encode message
                                                     -- with this field

   The clientPublicValue field in the AuthPack structure must be filled
   in by the client (in the PA-PK-AS-REQ preauthentication field, since
   Diffie-Hellman is required).

   Upon receiving this message from the client, the KDC then makes the
   normal PA-PK-AS-REQ validation and also checks that the sigAll seal
   is valid after computing the shared Diffie-Hellman key. We note that
   the KDC should use the ctime and cusec fields in the PA-PK-AS-REQ
   message to ensure that the client AS_REQ message is not a replay.
   (The KDC also checks that the kdcPublicKeyKvno is correct (that it
   is the current version), and uses the kdcPubValueId to look up its
   own Diffie-Hellman parameters).

   The KDC now sends an AS_REP message with the PA-PK-AS-REP
   preauthentication fields. The client should validate this message
   (including the normal PA-PK-AS-REP checks).

   If the changePassword flag was present in the KDC error message, the
   client should immediately obtain a change password service ticket
   and use the protocol in [3] to change the user password. This option
   is useful to support an environment where both non-recovery capable
   and recovery capable clients exist. Since multiple keytypes will
   exist on the KDC for a given user, the change password protocol
   password field is the raw user inputted password; the KDC is
   responsible for deriving the appropriate keys from this password.
   In particular, any change password requests should result in
   the recoverable keytypes being derived by the RFC 1510 string
   to key transformation with salt and then hashing as described
   above using the kdcSalt value.

4. Acknowledgement

   This work was previously published as part of draft-ietf-cat-
   kerberos-pkinit-02.txt while the author was employed at Cybersafe
   Corporation, 1605 NW Sammamish Rd., Suite 310, Issaquah, WA 98027.
   Thanks to John Wray, Mark Davis, and the CAT working group for
   some valuable suggestions on how to improve the draft.

5. Bibliography

   [1] J. Kohl, C. Neuman. The Kerberos Network Authentication
   Service (V5). Request for Comments 1510.

   [2] B. Tung, C. Neuman, J. Wray, A. Medvinsky, S. Medvinsky, M. Hur,
   J. Trostle. Public Key Cryptography for Initial Authentication
   in Kerberos. ftp://ds.internic.net/internet-drafts/

   [3] M. Horowitz. Kerberos Change Password Protocol.

6. Expiration Date

This draft expires on May 23, 1999.

7. Authors' Addresses

   Jonathan Trostle
   Cisco Systems
   170 W. Tasman Dr.
   San Jose, CA 95134

   Email: jtrostle@cisco.com, jtrostle@world.std.com