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IPSEC Working Group Ashar Aziz
INTERNET-DRAFT Tom Markson
Hemma Prafullchandra
Rich Skrenta
Sun Microsystems, Inc.
Germano Caronni
Swiss Federal Institute of
Technology
Expires in six months June 6, 1996
Certificate Discovery Protocol
<draft-ietf-ipsec-cdp-01.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
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ftp.isi.edu (US West Coast).
Distribution of this memo is unlimited.
draft-ietf-ipsec-cdp-01.txt [Page 1]
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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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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.
A Certificate has 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. Overview of the Certificate Discovery Protocol (CDP)
The Certificate Discovery protocol is a request/response protcocol used
by two parties to transfer certificates. The Requester initiates
Certificate Discovery. The Responder receives the Certificate Discovery
message and responds.
All certificate discovery protocol communication uses the User Datagram
Protocol (UDP) [8]. The requester binds to a random port and sends a
request to port cert-responder. The port assignment is described later.
The responder has bound to port cert-responder and sends the response to
the random port on which the requester sent the request.
The CDP consists of two parts: a Certificate Discovery Header and zero
or more CDP records. The CDP header contains global status and the
number of CDP records present in the message. A CDP header MUST be
present in a CDP. One CDP record is present for each "question" or
"answer", if present.
The simplest example of CDP is the requester asking for a particular
certificate from the responder. The responder replies with that
certificate. If the responder does not have the certificate the
requester asked for, it will set the error status and not return a
certificate. In this example, one CDP header and one CDP record would
be sent in both the request and the response.
A CDP message may be requests for multiple certificates. The requester
will produce one CDP record for each certificate being requested. The
responder will reply with a set of CDP records containing certificates
or errors. It is important to note that if one CDP question generated
an error, the responder SHOULD still process all of the other CDP
questions. Errors are generally handled in the CDP record per-
certificate level.
It is also important to note that questions and answers may not
necessarily map one to one. For instance, the requester may ask the
responder for a certificate and receive multiple certificates as a
response. The request might contain one CDP record, but the response
would contain one CDP record for EACH certificate returned.
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The use of the term requester is a bit confusing. Not only may the
requester request certificates from the responder, but it may also PUT
certificates to the responder. A PUT is an unsolicited presentation of
certificates from requester to responder. The responder will reply
with a status indicating whether the PUT was successful.
The protocol supports a mixing of GETs and PUTs. The requester may
PUT it's certificate in the same CDP that he GETs the responders.
The responder MUST either indicate an error in the STATUS field of the
CDP header or generate at least one CDP record for each CDP record
present in a request. The response to each request CDP record MUST have
the same name fields (name type, name len, Name) as the request. The
response MAY simply be an error if the responder chooses not to process
the request.
For example, if a requester asks for 5 certificates from the responder,
the request packet will contain the CDP header and 5 CDP records. In
the absence of an error in the header STATUS field, if the responder has
5 certificates to return, the response packet will also contain 5 CDP
packets. If the responder only had 2 of the 5 certificates, it would
still contain 5 CDP records. Three of the CDP records would indicate an
error.
2.1 Clogging Defense
The Certificate Discovery Protocol allows a requester to both make
certificate discovery requests (i.e. GET), as well present certificates
(i.e. push). This could lead to a situation where a requester 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 requester 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 requester to send
the cookie with the expected value. If the requester does not send this
cookie, the Responder SHOULD send a message with the "Cookie Required"
status and the desired cookie in the cookie field.
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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 requester 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 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 may be 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.
3. Certificate Discovery Packet
The Certificate Discovery Protocol message consists of two parts:
1) the certificate discovery Header (CDP header),
2) zero or more CDP record(s).
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3.1 CDP 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 | ACTION/STATUS | #-OF-RECS | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Requester Cookie |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Responder Cookie |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
VERS indicates protocol version number, VERS = 1 for this protocol.
Action/Status indicates either a request or the status of a response.
It MUST be set to the value REQUEST by the initiator. The responder
MUST set the field to one of the values RESPONSE, COOKIE_REQUIRED
or REQUEST_TOO_LARGE.
0 (REQUEST) - This Certificate discovery packet is
initiating CDP.
1 (RESPONSE) - This Certificate discovery packet is a
response to a previous CDP initiate.
2 (COOKIE_REQUIRED) - Responder requires the initiator to resend this
request with a non-zero responder cookie.
This cookie is present in the Responder Cookie
field.
3 (REQUEST_TOO_LARGE) - Request cannot be satisfied in one UDP packet
(64K). This may occur, for example, if the
initiator has asked for too many certificates
in a request. If the initiator receives this
response then it SHOULD resend the request
with fewer queries. The responder,
however, SHOULD send as many certificates as
it can in the response.
#-OF-RECS - The number of CDP Records present. If this
value is 0 then no CDP Records are present in
the message.
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Reserved - This field is current reserved. It should be
set to zero by the sender and ignored by the
receiver.
Requester Cookie - In a request message, this contains a
value that the Responder should send back
in the Requester Cookie field of the response.
In a response message, this field MUST contain
the value that was sent by the requester in this
field in the request.
Responder Cookie - In a request, 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 MUST be sent in a
new request. If the requester has not received
a cookie from the responder, the requester MUST
set this field to be zero.
3.2 CDP Record
Following the Certificate Discovery header is one CDP record for each
name or certificate included in the request message. A correctly formed
Certificate Discovery message MUST contain as many CDP records as the
#-OF-RECS field in the CDP header 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Action/Status | Name Type | Name Length | Name ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| CERT-Type | CERT-Length | Certificate ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Action/Status - is used to indicate either the action that is requested
in a particular CDP record or the status of a response.
The initiator MUST treat this field as an action field.
The responder MUST treat this field as a status field.
Actions values are as follows:
1 GET
2 PUT
Status values are as follows:
100 GET SUCCEEDED
101 GET FAILED
102 PUT SUCCEEDED
103 PUT FAILED
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. This field must be
the size as specified in Name Length.
Cert-Type - specifies the certificate type of the certificate that
is to follow. If no certificate is present, this field
is set to zero. These values will be assigned by IANA.
Cert-Length - specifies the length of the certificate in bytes. If no
certificate is present, this field is set to 0.
Certificate - the certificate. This field must be the size that is
specified in the Cert-Length field.
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4. Assigned Numbers
4.1 Port Number Assignments
IANA has assigned cert-responder UDP port 1640.
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
7 X509 Certificate Revocation List
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.
5. Security Considerations
A responder should use the cookie exchange mechanism when it feels
clogged.
We suggest that the UDP ports used by the Certificate Discovery Protocol
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
GETs/PUTs 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.
Bill Danielson, Marc Dye and Ben Stoltz for reviewing this draft and
providing constructive suggestions.
Special thanks to Colin Plumb for his valuable suggestions and
contributions to this protocol.
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.
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., Zimmermann, P., "PGP Message Exchange
draft-ietf-ipsec-cdp-01.txt [Page 11]
INTERNET-DRAFT CDP June 12, 1996
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)
draft-ietf-ipsec-cdp-01.txt [Page 12]
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Ashar Aziz
Sun Microsystems, Inc.
M/S PAL1-550
2550 Garcia Avenue
Mountain View, CA 94043
ashar.aziz@eng.sun.com
Tom Markson
Sun Microsystems, Inc.
M/S PAL1-550
2550 Garcia Avenue
Mountain View, CA 94043
markson@eng.sun.com
Hemma Prafullchandra
Sun Microsystems, Inc.
M/S PAL1-550
2550 Garcia Avenue
Mountain View, CA 94043
hemma@eng.sun.com
Rich Skrenta
Sun Microsystems, Inc.
M/S PAL1-550
2550 Garcia Avenue
Mountain View, CA 94043
skrenta@eng.sun.com
Germano Caronni
Computer Engineering and Networks Laboratory
Swiss Federal Institute of Technology (ETH)
ETH Zentrum
CH-8092 Zurich
caronni@tik.ee.ethz.ch
draft-ietf-ipsec-cdp-01.txt [Page 13]
CONTENTS
Status of this Memo................................. 1
Abstract............................................ 2
1. Overview............................................ 3
2. Overview of the Certificate Discovery Protocol
(CDP)............................................... 4
2.1 Clogging Defense............................... 5
2.1.1 Cookie Generation 6
3. Certificate Discovery Packet........................ 6
3.1 CDP Header..................................... 7
3.2 CDP Record..................................... 8
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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