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Electronic Commerce: Digital Cash. Guilin Wang The School of Computer Science 3 March 2008 ( L16 ). Outline. Motivation - Motivation - Existing of Electronic Money - Desired Properties On-Line Digital Cash - Blind Signatures - On-Line Digital Cash
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Electronic Commerce: Digital Cash Guilin Wang The School of Computer Science 3 March 2008 (L16)
Outline • Motivation - Motivation - Existing of Electronic Money - Desired Properties • On-Line Digital Cash - Blind Signatures - On-Line Digital Cash • Off-Line Digital Cash
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.
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).
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.
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.
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.
2. On-Line Digital Cash First scheme: The bank signs a standard signature as digital cash. Any weaknesses?
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.
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?
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:
2. On-Line Digital Cash Any weaknesses in the second scheme? Two important issues: - Double Spending - Denomination (three methods)
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 user’s identity.
3. Off-Line Digital Cash Scheme 3: Off-line digital cash. Any weaknesses? Other issues about e-cash.
Summary This Lecture: ■ Digital Cash Next Lecture: ■ Firewall
