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Versions: 00 01                                                         
IPSEC Working Group                           Ashar Aziz
INTERNET-DRAFT                                Tom Markson
                                              Hemma Prafullchandra
                                              Sun Microsystems, Inc.

Expires in six months                         December 21, 1995





                    Certificate Discovery Protocol
                     <draft-ietf-ipsec-cdp-00.txt>



Status of this Memo

This document is a submission to the IETF Internet Protocol Security
(IPSEC) Working Group. Comments are solicited and should be addressed to
to the working group mailing list (ipsec@ans.net) or to the authors.

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 draft documents are 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."

To learn the current status of any Internet-Draft, please check the
"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast).

Distribution of this memo is unlimited.









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Abstract

Use of Public key cryptography is becoming widespread on the Internet in
such applications as electronic mail and IP Security (IPSEC).
Currently, however, a common public key certificate infrastructure does
not exist which is interoperable with other systems and ubiquitous.  In
light of this, we describe a protocol which may be used to exchange or
retrieve certificates (essentially signed public keys) with or from
another entity.  The protocol may be used to request certificates from a
directory/name server or from the entity who owns the certificate.






































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                                CONTENTS


    Status of this Memo.................................   1

    Abstract............................................   2

1.  Overview............................................   3

2.  The Certificate Discovery Protocol..................   4

    2.1  Clogging Defense...............................   5

         2.1.1  Cookie Generation   5

3.  Certificate Discovery Packet........................   6

    3.1  Certificate Discovery Header...................   6

    3.2  Name Record....................................   8

    3.3  Certificate Record.............................   9

4.  Assigned Numbers....................................  10

    4.1  Port Number Assignments........................  10

    4.2  Name Type Assignments..........................  10

    4.3  Certificate Type Assignments...................  10

5.  Security Considerations.............................  11

    Acknowledgements....................................  11

    References..........................................  11

    Author's Address(es)................................  12










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1.  Overview

The distribution of authenticated public keys is a fundamental problem
which needs to be resolved with use of public key cryptography.  Many
solutions exist for distributing authenticated public keys.  Two of the
more common distribution methods are the X.500 directory service [1] and
Secure Domain Name System (Secure-DNS) [2]. Each method has a different
encoding format for the entity identity and the public key that belongs
to it. It is expected that many distribution mechanisms will co-exist on
the Internet and hence many "certificate" formats.

We describe a protocol which may be used to exchange certificates on the
Internet.  The protocol has the advantage that it does not require
changes to existing services or deployment of large directory services
in order to be used.  The Certificate Discovery Protocol allows
certificate requests to be made to any arbitrary IP-node. This feature
allows the initiator to send requests to, say, an IP-node which is
acting as a certificate server (and hence would have many certificates
stored in its local certificate database) or to a single IP-node which
only has it's own certificate.

As noted earlier, there are several different types of certificates in
existence: X.509 certificates [11], PGP certificates [3], Secure DNS
resource records and hashed public keys [4].  This protocol is designed
to support all of these and new ones as they emerge.

All certificates have at least two properties:

  (1) it provides for a cryptographic binding to a name/identity of an
      entity, and
  (2) it provides integrity protection of a public key.

The name may be encoded in the certificate (for example, as in X.509 and
PGP certificates) or it may be implicit in the public key itself (i.e.,
the cryptographic hash of the public key).

As with various certificate types, numerous naming conventions exist on
the Internet, for example, IP addresses [5],[6], RFC 822 addresses [7],
DNS names and PGP user names. This protocol attempts to support all of
these and allows for other syntaxes.

Note, that a particular entity may have more than one certificate.  An
entity may have the same public value in different certificate formats,
or have multiple public values each in a separate certificate or have
the same public value certified by different Certifying Authorities, and



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so on. In all these possible certificates the identity of the entity
remains constant.


2.  The Certificate Discovery Protocol

The Initiator is the entity which initiates Certificate Discovery. The
Responder is defined as the entity which receives the Certificate
Discovery initiate message and (optionally) responds with certificates.

The Initiator requests certificates by Name. A Name is defined as a Name
Record consisting of a Name Type, a Name Length and the actual Name of
the entity who the certificate belongs to.  The Name Type specifies the
type of name, for example, a PGP printable string or a SKIP [12] name.
In the case where the Name Type is SKIP, the actual name consists of a
Name Space Identifier (NSID) followed by the Master KeyID (MKID).

The Initiator may optionally include certificates in an initiate
message.  The Responder MUST process any certificate(s) the Initiator
has sent and respond with the requested certificates or an error (for
example, if the requested certificate does not exist or it had problems
processing the certificates it has received).

Note that this protocol allows the initiator to not only request for all
certificates for a particular "Name" but also send in the same message
all the certificates of the Initiator.

Also, in a request, the initiator may simply give the responder
certificates without asking for any in return.  If the responder accepts
these certificates, the response message will return a status of 'OK'.
If the responder does not accept all of these certificates, a bit will
be set indicating this error and an appropriate error status will be
returned.

All certificate discovery protocol communication uses the User Datagram
Protocol (UDP) [8].  The client binds to port cert-initiator and sends a
certificate request to port cert-responder. Port number assignments are
described later.  The responder binds to port cert-responder and sends
the response to port cert-initiator.

Two separate ports are used to allow for multiple certificate requests
to be made without waiting for the response to be received, (that means,
one port is used to simply send requests out and the other port is used
to receive responses).




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2.1  Clogging Defense

The Certificate Discovery Protocol allows an Initiator to both make
certificate discovery requests (i.e. fetch), as well present
certificates (i.e. push). This could lead to a situation where an
Initiator may attempt to clog a certificate server by flooding it with
bogus certificate pushes. The server, when presented with a set of
certificates would at a minimum parse the request and check if it
already has the presented certificates in its local database. It may
also have to verify a signature using a (possibly) expensive  public key
operation.  Rather than discarding certificate pushes when it feels
clogged, the server may request that the Initiator use the optional
cookie exchange mechanism. With this approach, the server may continue
to serve legitimate requests.

If the Responder requires a cookie, it will expect the Initiator to send
the cookie with the expected value.  If the Initiator does not send this
cookie, the Responder SHOULD send a message with the "Cookie Required"
status and the desired cookie in the cookie field.

The protocol also supports a cookie the initiator may set.  The
responder MUST send this cookie back to the initiator in a response.
This cookie may be used for replay protection, clogging defense or as a
means for the client to associate responses with requests.

If either the Initiator or the Responder is feeling clogged it SHOULD
give preference in calculating the shared secrets (i.e. g^ij)
computations to certificates sent to it with magic cookies. (For
example, it could precompute g^ij immediately upon receiving the
certificate and after verifying it.)

2.1.1  Cookie Generation

The cookie generation method is as recommended in the Photuris Internet
Draft [9], i.e., an MD5 [10] hash is applied over the IP Source and IP
Destination Addresses, the UDP source and destination ports, and a
locally generated secret random value.  A subset of this hash is then
used as a cookie.

Note that this is an implementation detail in that the mechanism
employed is purely a local matter, two communicating entities do not
have to use the same mechanism.






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3.  Certificate Discovery Packet

The Certificate Discovery Protocol message consists of three parts:

   1) the certificate discovery header,
   2) zero or more name record(s), and
   3) zero or more certificate record(s).

3.1  Certificate Discovery Header

The following describes a certificate discovery header. All values
are in network order.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | VERS  |I|R|ACT|P|  STATUS     |#-OF-NAME-RECS |#-OF-CERT-RECS |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Initiator Cookie (if I bit is set)               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Responder Cookie (if R bit is set)               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

VERS indicates protocol version number, VERS = 1 for this protocol.

I bit indicates if a 32 bit Initiator Cookie is present (I=1) or not (I=0).
R bit indicates if a 32 bit Responder Cookie is present (R=1) or not (R=0).

ACT indicates the type of message. Possible values are:
1 (Initiate)    - initiate either a request for certificate(s) or simply
                  send certificates.
2 (Response)    - response to a certificate Initiate.

P bit in a response indicates whether the responder has rejected
certificates presented in the initiate message. If the responder has
accepted the certificates or if no certificates were present in the
initiate message, the responder MUST set the P bit to 0. If the responder has
NOT accepted ALL of the certificates present in the initiate message, the P bit
is set to 1 and the status field should indicate the reason for failure.
If the responder has rejected certificates in the initiator's request,
no certificates should be returned to the initiator and #-OF-CERT-RECS
MUST be set to 0.

STATUS is used only in responses (i.e. ACTION = 2). It MUST be ignored
by the responder.  Possible values are:



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0 (OK)                    - The responder has processed the certificates sent
                            or has returned the requested certificates.
1 (Unknown Error)         - An error has occurred.
2 (Unknown Name)          - No certificates for the requested "Name" can
                            be found in the local certificate database or
                            the local domain name server.
3 (Unsupported Name Type) - Name Type in the certificate request is
                            unsupported.
4 (Unsupported CERT-Type) - CERT-Type in the certificate request is
                            unsupported.
5 (Cookie Required)       - Responder requires the initiator to resend this
                            request with a non-zero cookie included.
6 (Request Too Large)     - Request cannot be satisfied in one UDP packet
                            (64K). This may occur, for example, if the
                            initiator has too many names listed in the
                            certificate initiate message. If the initiator
                            receives this response then it SHOULD resend
                            the request with fewer names. The responder,
                            however, SHOULD send as many certificates as
                            it can in the response.

#-OF-NAME-RECS            - The number of Name Records present in the
                            initiate message.  This field is only meaningful
                            in an initiate message.  If the field is set to
                            0 in an initiation, the protocol functions
                            as a way of "pushing" certificates to a
                            remote host.  A response should not have
                            any explicit Name Records, they should be a
                            part of the Certificate Record(s) and hence
                            #-OF-NAME-RECS should be set to 0.

#-OF-CERT-RECS            - The number of Certificate Records present in the
                            initiate or response. If this value is 0 then no
                            Certificate Records are present in the message.
                            If this field is zero in an initiate message, the
                            initiator is requesting certificates without
                            presenting any.   If the field is zero in a
                            response, the R bit and the STATUS field
                            indicate the status of the previous request.

Initiator Cookie          - This is an optional field and is not present
                            when the 'I' bit is set to zero.

                            In an initiate message, this contains the
                            value that the Responder should send back in



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                            the response.

                            In a response message, this field contains the
                            value that was sent by the initiator in this
                            field in the initiate message.  This field
                            should be present in a response message
                            (and the I bit set) if the initiate message
                            has the 'I' bit set to 1.

Responder Cookie          - This is an optional field and is not present
                            when the 'R' bit set to zero.

                            In an initiate message, this contains the value
                            that the Responder previously indicated should
                            be sent in the request. In a response message,
                            if the "Cookie Required" status is set, this
                            contains the value that should be sent in a
                            new request. If the response message does
                            not have the "Cookie Required" status, this
                            value SHOULD be not be present and the 'R'
                            bit should be set to 0.


3.2  Name Record

Following the Certificate Discovery header is one Name record for each
name included in the initiate message.  A correctly formed Certificate
Discovery message MUST contain as many name records as the #-OF-NAME-
RECS field in the Certificate Discovery header field specifies.



















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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Name Type     | Name Length   |  Name                         ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


Name Type    - Identifies the type of the Name. The values of this
               field will be assigned by the Internet Assigned Numbers
               Authority (IANA).

Name Length  - The length of the name in bytes.

Name         - The name of the entity who owns the certificate for
               which the request is being made.


3.3  Certificate Record

Following the Name Record(s) is one certificate record for each
certificate included in the protocol.  A correctly formed Certificate
Discovery message MUST contain as many certificate records as the #-OF-
CERT-RECS field in the Certificate Discovery header field specifies.


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Name Type     | Name Length   |  Name                         ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | CERT-Type     | CERT-Length                   |  Certificate  ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


Name Type, Name Length and Name are the same as in a name record.

CERT-Type     - specifies the certificate type of the certificate that
                is to follow. These values will be assigned by IANA.

CERT-Length   - specifies the length of the certificate in bytes.

Certificate   - the actual certificate.





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4.  Assigned Numbers

4.1  Port Number Assignments

IANA has assigned UDP port 1639 to "cert-initiator".  UDP port 1640 has
been assigned to "cert-responder".

4.2  Name Type Assignments


        Name Type Value         Name Type

                1               SKIP
                2               PGP Printable String
                3               PGP KeyID
                4               DNS name
                5               RFC 822 name
                6               X.509 Distinguished Name


Name Type values 1 through 6 are assigned as is described above.  Name
Type values 7 through 249 inclusive are reserved to IANA for future
allocation as Assigned Numbers.  Such future allocation by IANA will
normally require that a public specification exist for the Name Type
obtaining such allocation.  Name Types in the range 250 through 255
inclusive are reserved for private use among consenting parties. Name
Types in the range 250 through 255 inclusive will hence only have local
uniqueness properties.

4.3  Certificate Type Assignments


        CERT-Type Value         Certificate Type

                1               X.509 [1]
                2               PGP [3]
                3               Secure DNS [2]
                4               MD5 of Unsigned DH Public Value [4]
                5               MD5 of Unsigned Elliptic Curve Public Value
                6               MD5 of Unsigned RSA Public Value


CERT-Type values 1 through 6 are assigned as is described above.  CERT-
Type values 7 through 249 inclusive are reserved to IANA for future
allocation as Assigned Numbers.  Such future allocation by IANA will



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normally require that a public specification exist for the Certificate
Type obtaining such allocation.  CERT-Types in the range 250 through 255
inclusive are reserved for private use among consenting parties.  CERT-
Types in the range 250 through 255 inclusive will hence only have local
uniqueness properties.

The Certificate Discovery Protocol uses two UDP ports to exchange
certificates. A request has been submitted to IANA for these port
assignments.


5.  Security Considerations

A responder should use the cookie exchange mechanism when it feels
clogged.  The Certificate Discovery Protocol uses two ports, we suggest
that these ports be treated as a "by-pass" channel for encryption (i.e.
non encrypted traffic is permitted to be sent on these ports).  As only
certificates fetches/pushes are satisfied on these ports the window for
vulnerability is limited.


Acknowledgements

We would like to thank all of the people who helped make this draft
possible.

Ran Atkinson suggested that this protocol should be independent of other
IPSEC drafts.

Phil Karn and Bill Simpson for their work on the Cookie Exchange scheme
for the Photuris Session Key Management Protocol which influenced the
addition of the Cookie field to this protocol.

Bill Danielson, Marc Dye, Colin Plumb, Rich Skrenta and Ben Stoltz for
reviewing this draft and providing constructive suggestions.


References

[1] CCITT Recommendation X.500 (1988), "The Directory"

[2] Eastlake, D., Kaufman, C., "Domain Name Security Extensions", (I-D
    draft-ietf-dnssec-secext-04.txt), Work In Progress

[3] Atkins, D., Stallings, W., Zimmerman, P., "PGP Message Exchange



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    Formats", (I-D draft-atkins-pgpformats-01.txt), Work In Progress

[4] Aziz, A., Markson, T., Prafullchandra, H., "Encoding of an Unsigned
    Diffie-Hellman Public Value", (I-D draft-ietf-ipsec-skip-udh-
    00.txt), Work In Progress

[5] Postel, J., "Address Mappings", IEN 115, USC/Information Sciences
    Institute, August 1979

[6] Hinden, R., Deering, S., "IP Version 6 Addressing Architecture",
    (I-D draft-ietf-ipngwg-addr-arch-03.txt), Work In Progress

[7] Crocker, D., "Standard for the format of ARPA Internet text
    messages", RFC 822

[8] Postel, J., "User Datagram Protocol", RFC 768

[9] Karn, P., Simpson, W. A., "The Photuris Session Key Management
    Protocol", (I-D draft-ietf-ipsec-photuris-08.txt), Work In Progress

[10] R. Rivest, "The MD5 Message Digest Algorithm", RFC 1321, April 1992

[11] CCITT Recommendation X.509 (1988), "The Directory - Authentication
    Framework"

[12] Aziz, A., Markson, T., Prafullchandra, H., "Simple Key-Management
    for Internet Protocols", (I-D draft-ietf-ipsec-skip-06.txt), Work In
    Progress


Author's Address(es)

















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     Ashar Aziz
     Sun Microsystems, Inc.
     M/S PAL1-550
     2550 Garcia Avenue
     Mountain View, CA 94043

     Email: ashar.aziz@eng.sun.com
     Alternate email address: ashar@incog.com

     Tom Markson
     Sun Microsystems, Inc.
     M/S PAL1-550
     2550 Garcia Avenue
     Mountain View, CA 94043

     Email: markson@incog.com
     Alternate email address: markson@eng.sun.com

     Hemma Prafullchandra
     Sun Microsystems, Inc.
     M/S PAL1-550
     2550 Garcia Avenue
     Mountain View, CA 94043

     Email: hemma@eng.sun.com
     Alternate email address: hemma@incog.com






















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