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

Expires in six months                           August 1, 1996





          Encoding of an Unsigned Diffie-Hellman Public Value
                   <draft-ietf-ipsec-skip-udh-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
"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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INTERNET-DRAFT            SKIP-UDH            August 1, 1996



Abstract

It is useful to be able to communicate public keys in the absence of a
certificate hierarchy and a signature infrastructure.  This document
describes a method by which certificates which communicate Diffie-
Hellman public values and parameters may be encoded and securely named.










































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1.  Unsigned Public Keys

In public key cryptography, certificates provide a binding between an
entity's name and their public key.  The signature on the certificate
provides this binding.  However, certificates tend to be difficult to
implement and usually require infrastructure to verify signatures.  This
infrastructure and certificates, in general, are not in wide use on the
Internet.  Instead of explicitly binding a name to a public value using
a signature, the name may be derived directly from the public key. This
can be done by defining the name of the certificate to be the message
digest of the public key.

Although the public value is distributed in an unsigned manner, there is
still a strong binding between a name and the public value, given the
collision resistance properties of a message digest. The entity's names
need to be securely distributed out of band.

This distribution of keys has a number of advantages over conventional
signed certificates:  no infrastructure is required to use Unsigned
Public Keys.  No signature algorithm needs to be supported. No complex
encoding of certificates is required.

A disadvantage of this method is that the name must be securely (but not
secretly) communicated to anyone using the key.  Since the name is the
hash value of the public key, it is a cryptic string of hexadecimal
digits which is not user-friendly.

The encoding does not specify the hash algorithm used to generate the
name.  The hash algorithm must be transferred out of band.  This may be
done by creating a "certificate type" that includes this information.
One valid certificate type is "MD5 of Hashed DH Public Key".


2.  Encoding of an Unsigned DH public value

This encoding scheme is used to authenticate/distribute a DH public
value, for cases where the entity's name is the message digest of the
public value.










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The following is how the public value is encoded for purposes
of message digest computation and distribution in the network.
All values are in network order. All variable-length fields
must begin with a non-zero byte.

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                   Not Valid Before                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                   Not Valid After                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            PrimeLen           | Prime (p)   (variable length) ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~  Prime (p)  (variable length) |            GenLen             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Generator (g) (variable length)                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        PublicValueLen         | Public Value (variable length)~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~  Public Value (g^i mod p)    (variable length)                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


"Not Valid Before" is the time at which the public value becomes valid.
It is in NTP time format [3] (the Integer portion). "Not Valid After" is
the time at which the public value expires. It is in NTP [3] time format
(the Integer portion).

PrimeLen is Length of the DH Prime (p) in bytes.  Prime contains the
binary representation of the DH prime with most significant byte first.
GenLen is the length of the Generator (g) in bytes. Generator is the
binary representation of generator with most significant byte first.
PublicValueLen is the Length of the Public Value (g^i mod p) in bytes.
PublicValue is the binary representation of the DH public value with
most significant byte first.

The Name associated with the public key and parameters is the
cryptographic hash of the above encoding.









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3.  Verification of the Unsigned Public Value

Verification of the Encoding in this instance means verifying that the
message digest of the entire encoding (as specified above) is the same
as the (securely known) name of the entity. When using this instead of
signed certificates, certificate verification MUST be done by performing
the message digest computation.


4.  Security Considerations

The unsigned DH public value can ONLY be used when entities are named
using the message digest of their DH public value, AND these names are
securely communicated. Unsigned DH public values MUST NOT be used
instead of signed DH certificates when entities are named using
something other than the message digest of their public value, since
this opens up the possibility of an intruder-in-the-middle attack
described in [1]. In order to use other naming schemes, signed
certificates such as X.509, Secure DNS, PGP, etc.  should be used.


Acknowledgements

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

Jeff Schiller originally suggested using the hash of the public key as
the Entity's name.

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


References

[1] Aziz, A., "Simple Key Management for Internet Protocols", (I-D
    draft-ietf-ipsec-skip-06.txt), Work in Progress

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

[3] Mills, D.,"Network Time Protocol", RFC 1305, March 1992







draft-ietf-ipsec-skip-udh-01.txt                    [Page 5]


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Author's Address(es)

     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




















draft-ietf-ipsec-skip-udh-01.txt                    [Page 6]

                                CONTENTS


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

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

1.  Unsigned Public Keys.................................  3

2.  Encoding of an Unsigned DH public value..............  3

3.  Verification of the Unsigned Public Value............  5

4.  Security Considerations..............................  5

    Acknowledgements.....................................  5

    References...........................................  5

    Author's Address(es).................................  6




























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