draft-ietf-cat-kerberos-pk-init-00.txt   [plain text]

INTERNET-DRAFT                                         Clifford Neuman
draft-ietf-cat-kerberos-pk-init-00.txt                      Brian Tung
Updates: RFC 1510                                                  ISI
expires September 3, 1995                                    John Wray
                                         Digital Equipment Corporation

    Public Key Cryptography for Initial Authentication in Kerberos

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,
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   The distribution of  this  memo  is  unlimited.   It  is  filed  as
   draft-ietf-cat-kerberos-pk-init-00.txt, and expires August 6, 1995.
   Please send comments to the authors.

1. Abstract

   This document defines extensions to the Kerberos protocol  specifi-
   cation  (RFC  1510,  "The  Kerberos  Network Authentication Service
   (V5)", September 1993) to provide a method for using public key
   cryptography during initial authentication.  The method defined 
   specifies the way in which preauthentication data fields and error
   data fields in Kerberos messages are to be used to transport public
   key data. 

2. Motivation

   Public key cryptography provides a means by which a principal may
   demonstrate possession of a key, without ever having divulged this
   key to anyone else.  In conventional cryptography, the encryption key
   and decryption key are either identical or can easily be derived from
   each other.  In public key cryptography, however, neither key can be
   derived easily from the other; hence, the ability to encrypt a message
   does not imply the ability to decrypt it in turn.  Additionally, each
   key is a full inverse of the other, so that either key can be used
   for encryption or decryption.

   The advantages provided by public key cryptography have produced a
   demand for its integration into the Kerberos authentication protocol.
   There are two important areas where public key cryptography will have
   immediate use: in the initial authentication of users registered with
   the KDC or using public key certificates from outside authorities,
   and to establish inter-realm keys for cross-realm authentication.
   This memo describes a method by which the first of these can be done.
   The second case will be the topic for a separate proposal.

   Some of the ideas on which this proposal is based arose during
   discussions over several years between members of the SAAG, the
   IETF-CAT working group, and the PSRG, regarding integration of
   Kerberos and SPX.  Some ideas are drawn from the DASS system, and
   similar extensions have been discussed for use in DCE.  These changes
   are by no means endorsed by these groups.  This is an attempt to
   revive some of the goals of that group, and the proposal approaches
   those goals primarily from the Kerberos perspective.

   This proposal will allow users with keys already registered for use
   with X.509, PEM, or PGP, to use those keys to obtain Kerberos
   credentials which can then be used for authentication with application
   servers supporting Kerberos.

   Use of public-key will not be a requirement for Kerberos, but if one's
   organization runs a KDC supporting public key, then users may choose
   to be registered with public keys instead of the current secret key.
   The application request and response, between Kerberos clients and
   application servers, will continue to be based on conventional
   cryptography, and the application servers will continue to be
   registered with conventional keys.

3. Initial authentication of users with public keys

   This section proposes extensions to Version 5 of the Kerberos
   protocol that will support the use of public key cryptography 
   by users in the initial request for a ticket granting ticket.

   The advantage of registering public keys with the KDC lies in the
   ease of recovery in case the KDC is compromised.  With Kerberos as it
   currently stands, compromise of the security KDC is disastrous.  All
   keys become known by the attacker and all keys must be changed.  

   If users register public keys, compromise of the KDC does not divulge
   their private key.  Compromise of security on the KDC is still
   disastrous, since the attacker can impersonate any user.  An
   attacker with the private key of the KDC can use it to certify a
   bogus nonce key, and impersonate a user.  Changing this key
   invalidates all bogus certifications.  Legitimate users must
   re-certify their keys with the new KDC key, but users' public
   keys do not have to be changed.  (Users who store their private
   keys in an encrypted form on the KDC do have to change their
   keys, since the encryption key is a symmetric key derived from
   a password (as described below) and hence vulnerable to dictionary
   attacks.  The difference is that, assuming good password policy,
   site policy may allow the use of the old password only for the
   purpose of key change for a time after the compromise, which means
   that users can change their own passwords, rather than forcing the
   administrator to re-key everyone.)

   In the event of compromise of the KDC, recovery is simple since only
   the KDC's key, keys for application servers, and users' private keys
   stored in the KDC (as described above) must be changed.
   Since there are usually fewer servers than users, and since an
   organization usually has better procedures for updating servers,
   changing these keys is much easier than having to individually
   contact every user.

   This proposed extension will not require users to register with
   public keys.  It is intended to allow them to do so, but we recognize
   that there are many reasons, including licensing terms, that users or
   an organization as a whole will choose not to use the public key
   option.  Users registered will public keys will only be able to
   perform initial authentication from a client that support public key,
   and must be registered in a realm that supports public key.  But they
   will be able to use services registered in realms that support only
   conventional Kerberos.  Further, users registered with conventional
   Kerberos keys will be able to use all clients.

   This proposal specifically does not address the registration of
   public keys for services.  The application request and response,
   between Kerberos clients and application servers, will continue to be
   based on conventional cryptography, and the application servers will
   continue to be registered with conventional keys. There are
   performance issues and other reasons that servers may be better off
   using conventional cryptography.  There are also reasons that they
   may want to use public key.  For this proposal, however, we feel that
   80 percent of the benefits of integrating public key with Kerberos
   can be attained for 20 percent of the effort, by addressing only
   initial authentication. This proposal does not preclude separate

   This proposal address two ways in which users may use public key
   cryptography for initial authentication with Kerberos.  In both
   cases, the end result is that the user obtains a conventional ticket
   granting ticket, or conventional server ticket, that may be used for
   subsequent authentication, with such subsequent authentication using
   only conventional cryptography.

   Users may register keys directly with the KDC, or they may present
   certificates by outside certification authorities (or certifications
   by other users) attesting to the association of the public key with
   the named user.  We first consider the case where the user's key is
   registered with the KDC.

3.1 Initial request for user registered with public key on KDC 

   In this scenario it is assumed that the user is registered with a public
   key on the KDC.  The user's private key may be known to the user, or
   may be stored on the KDC, encrypted so that it can not be used by the KDC.

   We consider first the case where the user knows the private key, then
   add support for retrieving the private key from the KDC.

   The initial request to the KDC for a ticket granting ticket proceeds
   according to RFC 1510.  Typically, preauthentication using a secret
   key would not be included, but if included it may be ignored by the
   KDC.  (This may introduce a problem: even if the KDC should ignore
   the preauthentication, an attacker may not, and use an
   intercepted message to guess the password off-line.)
   If the private key is known to the client in advance, the
   response from the KDC would be identical to the response in RFC1510,
   except that instead of being encrypted in the secret key shared by the
   client and the KDC, it is encrypted in a random key freshly generated
   by the KDC.  A preauthentication field (specified below)
   accompanies the response, containing a certificate with the public
   key for the KDC, and a package containing the secret key in which the
   rest of the response is encrypted, itself encrypted in the private key
   of the KDC, and the public key of the user.  This package also contains
   the same nonce used in the rest of the response, in order to prevent
   replays of this part of the message, accompanied by a reconstructed

           encryptPack  EncryptedData -- EncPaPkAsRepPart

   EncPaPkAsRepPart ::= SEQUENCE {
           enc-sess-key INTEGER,
           nonce        INTEGER

   Upon receipt of the response from the KDC, the client will verify the
   public key for the KDC from PA-PK-AS-REP preauthentication data field,
   The certificate must certify the key as belonging to a principal whose
   name can be derived from the realm name.  We solicit discussion on the
   form of the kdc-cert.  If client systems are expected to know (by
   being hard-coded, for example) at least one public key, and to verify
   certificates from that key, then there should be at least some policy
   about which key that is, or alternatively some way to inform the KDC
   which key the client possesses.

   If the certificate checks
   out, the client then extracts the message key for the rest of the
   response by decrypting the field in the PA-PK-AS-REP with the public
   key of the KDC and the private key of the user.  The client then uses
   the message key to decrypt the rest of the response, and continues as
   per RFC1510[1].

3.1.1. Private key held by KDC

   When the user's private key is not carried with the user, the user may
   encrypt the private key using conventional cryptography, and register
   the encrypted private key with the KDC.

   When the user's private key is registered with the KDC, the KDC record
   will also indicate whether preauthentication is required before
   returning the record (we recommend that it be required).  If such
   preauthentication is required, when the initial request is received
   the KDC will respond with a KRB_ERROR message of type
   KDC_ERR_PREAUTH_REQUIRED and with an error data field set to:

	   kdc-cert	OCTET STRING}

   The kdc-cert field is identical to that in the PA-PK-AS-REP
   preauthentication data field returned with the KDC response, and must
   be validated as belonging to the KDC in the same manner.

   Upon receipt of the KRB_ERROR message with a PA-PK-AS-INFO field, the
   client will prompt the user for the password that will be used to
   decrypt the private key when returned, calculate a one way hash H1 of the
   key, and send a request to the KDC, including a timestamp and a
   client-generated nonce secret key that will be used to super-encrypt
   the encrypted private key before it is returned to the client.  This
   information is sent to the KDC in a subsequent AS_REQ message in a
   preauthentication data field:

	   enc-part	EncryptedData -- EncPaPkAsReqPart

   EncPaPkAsReqPart ::= SEQUENCE {
           tstamp       KerberosTime,
           noncekey	INTEGER

   encrypted first with the hash H1, then the public key of the KDC from
   the certificate in the PA-PK-AS-INFO field of the error response.

   Upon receipt of the authentication request with the PA-PK-AS-REQ the
   KDC verifies the hash of the user's DES encryption key by comparing it
   to the hash stored in the users database record.  If valid, it
   generates the AS response as defined in RFC1510, but additionally
   includes a preauthentication field of type PA-PK-USER KEY.  This
   response will also be included in response to the initial request
   without preauthentication if preauthentication is not required for the
   user and the user's encrypted private key is stored on the KDC.  The
   KDC generates a preauthentication data field of type PA-PK-USER-KEY
   which will be returned with the KDC reply (together with the
   PA-PK-AS-REP that is already returned).

           enc-priv-key         OCTET STRING OPTIONAL

   This message contains the encrypted private key that has been
   registered with the KDC by the user, as encrypted by the user,
   super-encrypted with the noncekey from the PA-PK-AS-REQ message if
   preauthentication using that method was provided.  Note that since
   H1 is a one-way hash, it is not possible for one to decrypt the
   message if one possesses H1 but not the noncekey that H1 is derived

3.2. Clients with a public key certified by an outside authority

   In the case where the client is not registered with the current KDC,
   the client is responsible for obtaining the private key on its own.
   The client will request initial tickets from the KDC using the TGS
   exchange, but instead of performing pre-authentication using a
   Kerberos ticket granting ticket, or with the PA-PK-AS-REQ that is used
   when the public key is known to the KDC, the client performs
   preauthentication using the preauthentication data field of type

           user-cert	SEQUENCE OF OCTET STRING,
	   authent	EncryptedData -- PKAuthenticator

   PKAuthenticator ::= SEQUENCE {
	   cksum	Checksum OPTIONAL,
	   cusec	INTEGER,
	   ctime	KerberosTime,

   The fields in the encrypted authenticator are the same as those
   in the Kerberos authenticator.  The structure is itself signed using
   the user's private key corresponding to the public key in the

   The KDC will verify the preauthentication authenticator, and check the
   certification path against its own policy of legitimate certifiers.
   This may be based on a certification hierarchy, or simply a list of
   recognized certifiers in a system like PGP.

   If all checks out, the KDC will issue Kerberos credentials, as in 3.1,
   but with the names of all the certifiers in the certification path
   added to the transited field of the ticket, with a principal name
   taken from the certificate (this might be a long path for X.509, or a
   string like "John Q. Public <jqpublic@company.com>" if the certificate
   was a PGP certificate.  The realm will identify the kind of
   certificate and the final certifier (e.g. PGP:<endorser@company.com>)[2].

4. Compatibility with One-Time Passcodes

   We solicit discussion on how the use of public key crytpgraphy for
   intial authentication will interact with the proposed use of one time
   passwords discussed in Internet Draft

5. Expiration

   This Internet-Draft expires on August 6, 1995.

6. Authors' Addresses

   B. Clifford Neuman
   USC/Information Sciences Institute
   4676 Admiralty Way #1001
   Marina del Rey, CA 90292-6695

   Phone: 310-822-1511
   EMail: bcn@isi.edu

   Brian Tung
   USC/Information Sciences Institute
   4676 Admiralty Way #1001
   Marina del Rey, CA 90292-6695

   Phone: 310-822-1511
   EMail: brian@isi.edu

   John Wray
   Digital Equipment Corporation
   550 King Street, LKG2-2/Z7
   Littleton, MA 01460

   Phone: 508-486-5210
   EMail: wray@tuxedo.enet.dec.com

[1] Note: We have not yet defined the public key encryption method for
encrypting the enc-sess-key field in the PA-PK-AS-REP.

[2] Note: We are soliciting input on the form of the name.  We believe the
name part must be taken without modification from the certificate, but the
realm part is open for discussion.