A New Approach of Signing Documents with Symmetric Cryptosystems and an Arbitrator

1
A New Approach of Signing Documents with
Symmetric Cryptosystems and an Arbitrator

• Nol Premasathian
• inolhian_at_kmutt.ac.th
• Faculty of Science
• King Mongkuts University of Technology Thonburi
• Bangkok, Thailand

2
Signing Documents

• Public Key
• Private Key

3
Signing Documents (Public Key)

• No trusted third party needed
• Problems with speed

4
Signing Documents (Private Key)

• A trusted third party needed
• Disputes dissolved by the trusted third party

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Existing Method

• The sender shares a private key with the
  arbitrator (senders key).
• The arbitrator shares a private key with the
  receiver (receivers key).

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Existing Method (continue)

• The sender encrypts the message with the senders
  key and sends it to the arbitrator.
• The arbitrator decrypts the message, encrypts it
  with the receivers key and sends it to the
  receiver.

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Existing Method (continue)

• The receiver decrypts the message and sends an
  acknowledgment encrypted with the receivers key
  to the arbitrator.
• The arbitrator decrypts the acknowledgment,
  encrypts it with the senders key and sends it to
  the sender.
• The arbitrator keeps a record for the message and
  its acknowledgment.

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Existing Method (limitation)

• The arbitrator must keep a record of every sent
  message.
• The arbitrator perceives the content of the
  message.

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Proposed Method

• There are three secret keys in the protocol
• Private key between the sender and the receiver
  (Km)
• Private key between the sender and the arbitrator
  (Ks)
• Private key between the receiver and the
  arbitrator (Kr)

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Three transmissions

• From the sender to the arbitrator
• From the arbitrator to the receiver
• From the receiver to the sender

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The protocol

• The sender sends Ks(H(M)) and Km(M) to the
  arbitrator.
• From Ks(H(M)), the arbitrator can produce
  Kr(Kr(H(M))) and Ks(H(Kr(Kr(H(M))))H(M)).
• The arbitrator sends Km(M), Ks(H(M)),
  Kr(Kr(H(M))), and Ks(H(Kr(Kr(H(M))))H(M)) to the
  receiver.

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The protocol (continue)

• The receiver verifies H(M) and sends Kr(Kr(H(M)))
  and Ks(H(Kr(Kr(H(M))))H(M)) to the sender to
  acknowledge the message
• The sender verify H(M) in Ks(H(Kr(Kr(H(M))))H(M))
  with the original one and verify the validity of
  the acknowledgment.

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Signature and Acknowledgment

• Ks(H(M)) is the signature of the message.
• Kr(Kr(H(M))) is the acknowledgment of the message

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Conclusions

• The proposed method reduces the number of
  transmissions from four to three.
• The arbitrator does not perceive the content of
  the message.
• The arbitrator needs not keep a record.