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							Two Stories of Ring Signatures


               Yoshikazu Hanatani *
                  Kazuo Ohta *

    *The University of Electro-Communications1
     Ring Signature : Present
• In 2001, a ring signature scheme was
  proposed by Rivest, Shamir, Tauman.

• The signature scheme convinces a verifier
  that a document has been signed by one
  of n independent signers.




                                          2
Ring Signature : Present
     • A signer can connect the head and
       tail of the series of values by using
       own secret key.
     • A verifier computes series of
       values from the message and
       members’ public keys, and checks
       that a signature has a ring
       structure.

 Anyone cannot distinguish a part of the
 signature which is used secret key.
          Anyone cannot distinguish
          the actual signer.             3
          Ring Signature : Old
• Once upon a time, there was a signature scheme
  like a ring signature scheme in Japan.




                                                                      4
                      http://www.nihonkoenmura.jp/theme3/takarabito07.htm
Background
• In 1756 (in the middle of
  Edo period), a signature was
  generated as a proof of
  solidarity when farmers in a
  certain village resisted
  their ruler.
• A purpose of the shape of
  this signature is to hide
  their leader.

                            5
Reason for a shape of ring
      • If members simply signed,
        the first signer would be
        suspected of the leader.


       • Members sequentially signed
         like a ring, try to hide the
         order of signers.
       • The members who
         participate in the signature
         take equal responsibility.
                                    6
               Derivation
• The signature scheme is called
  “KARAKASARENPAN”.
   KARAKASARENPAN

An umbrella      Joint signatures

  •Because the shape of the signature looks like
  Japanese traditional umbrella.




                                                   7
   Various “KARAKASARENPAN”




http://www.archives.pref.fukui.jp/fukui/07/zusetsu/C26/C261.htm       http://asao20.hp.infoseek.co.jp/karakasa.htm




                                         http://www.pref.iwate.jp/~hp0910/korenaani/h/032.html                       8
                     Comparison
                   Ring signature   “KARAKASARENPAN”

  Shape              Ring                Ring
The number
of signers             1                  n
Other’s
cooperation         Unnecessary        Necessary
A signer to
shift the blame     Possible           Impossible
to members

 Purpose          Hide an actual    Hide a members’
                  signer            leader.
                                                       9
     Ring Signature : Future
• Ring Signature schemes which have an
  additional functionality, with which the
  involved members of the ring can deny
  the signature, are proposed.
CRYPTO2002
    Deniable ring authentication.
                   M. Naor
In preparing
    Toward the Fair Anonymous Signatures:
    Deniable Ring Signatures
               Yuichi Komano, Kazuo Ohta,
               Atsushi Shimbo, Shinichi Kawamura 10