Electronic Commerce: Digital Cash

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Electronic Commerce Digital Cash

• Guilin Wang
• The School of Computer Science
• 3 March 2008 (L16)

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Outline

• Motivation
• - Motivation
• - Existing of Electronic Money
• - Desired Properties
• On-Line Digital Cash
• - Blind Signatures
• - On-Line Digital Cash
• Off-Line Digital Cash

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1. Motivation

• Money is important for us,
• However, it is a little annoy to carry paper
  cash
• - You could be the target of thieves.
• - Paper cash is also a media for bacteria.

4
1. Motivation

• So, using the existing cryptographic tools and
  security measures, can we replace paper cash with
  digital cash?
• Some forms of money are already in digital
  formats
• Credit or debit cards.
• E-banking.
• Money transfer btw different accounts via
  e-banking or Electronic Funds Transfer (EFT).

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1. Motivation

• Using smart cards to take public transportation
  vehicles or buy drinks from vending machines etc.
  
• - Chipknip in the Netherland
• - The Octopus cards in Hong Kong
• - EZ-link in Singapore (8 million cards in in
  circulation)
• http//en.wikipedia.org/wiki/EZ-Link
• However, these are not digital cash, because they
  fail to meet some essential requirements for
  digital cash.

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1. Motivation

• Three parities in digital cash a customer, a
  merchant, and the bank.
• Desired Security Requirements for Digital Cash
• - Security The digital cash cannot be forged
  and/or reused by a user illegally.
• - Privacy (Untraceability) Nobody, including
  the bank, cannot reveal the relationship btw the
  identities of customers and the digital cash. It
  includes both unlinkability and anonymity.
• - Transferability Digital cash can be
  transferred btw customers without the help from
  the bank.

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1. Motivation

• - Off-Line Payment To verify the validity of
  e-cash, a user does not need to enquire the bank.
• - Divisibility A user can subdivide a piece of
  e-cash into smaller pieces of e-cash in small
  denominations.
• - Independence The use of e-cash does not rely
  on any physical locations and/ or special
  devices.

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2. On-Line Digital Cash

• First scheme
• The bank signs a standard signature as digital
  cash.
• Any weaknesses?

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2. On-Line Digital Cash

• Second scheme
• Using blind signatures to get the digital cash
  from the bank.
• A blind signature scheme allows a signature
  requestor to get the signer's digital signature
  without revealing the signed message to the
  signer.

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2. On-Line Digital Cash

• Basic Idea of Blind Signatures
• Unblind(Sign(Blind(m,r),sk))Sign(m,sk).
• How to implement RSA Blind Signature?

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2. On-Line Digital Cash

• Basic Idea of Blind Signatures
• Unblind(Sign(Blind(m,r),sk))Sign(m,sk).
• How to implement RSA Blind Signature

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2. On-Line Digital Cash

• Any weaknesses in the second scheme?
• Two important issues
• - Double Spending
• - Denomination (three methods)

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3. Off-Line Digital Cash

• A trusted party is needed.
• It could be a tamper-resistant devices (TRD).
• The basic idea Using secret splitting technique
  to escrow users identity.

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3. Off-Line Digital Cash

• Scheme 3 Off-line digital cash.
• Any weaknesses?
• Other issues about e-cash.

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Summary

• This Lecture
• Digital Cash
• Next Lecture
• Firewall