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A PDA Implementation of an Off-line e-Cash Protocol

A PDA Implementation of an Off-line e-Cash Protocol. E-cash basic definitions. Banknotes: Provides anonymity They are valid virtually everywhere The legitimate owner is the one who carries them Portability of great amounts of money is difficult/risky

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A PDA Implementation of an Off-line e-Cash Protocol

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  1. A PDA Implementation of an Off-line e-CashProtocol

  2. E-cash basic definitions

  3. Banknotes: Provides anonymity They are valid virtually everywhere The legitimate owner is the one who carries them Portability of great amounts of money is difficult/risky All transactions must be performed personally Features of the Standard Cash

  4. Credit/Debit Cards: transactions of big quantities of money are possible They are portable and secure →People trust them. Electronic transactions are possible They can be used for getting banknotes A legitimate owner must authenticate him/herself They do not provide anonymity The Bank must authorize the transaction via electronic connection → on-line protocols are required Credit/Debit Cards Features

  5. Goals To substitute standard banknotes To provide more flexibility than credit/debit cards Requirements anonymity Non-traceable protocols fairness divisibility transferability Off-line protocols accountability Alternative: E-cash

  6. In 1982, David Chaum proposed a way to make electronic payments anonymously, introducing the concept of e-cash. However the main drawback of the e-cash concept is that electronic money could be copied and reused (double spending problem). In ‘88, Chaum, Fiat and Naor proposed an off-line protocol. In ’91, Okamoto and Otha proposed that an ideal e-cash system should have the following properties: independence, security, privacy, off-line payment, transferability, divisibility. E-cash: Previous Works (1/2)

  7. In 1993, S. Brands proposed a new protocol, whose security lies in the Schnorr digital signatures and prime finite field arithmetic. In 1996, Frankel, Tsiounnis and Yung [11], [12] proposed the concept of Fair Off-line e-Cash. There, an entity called the Authority was used to guarantee the anonymity of a purchaser as long as he/she makes legal transactions. If a purchaser tries to commit fraud, the Bank could request the tracing of a coin or the tracing of the owner of a coin. Many other systems have been proposed recently… E-cash: Previous Works (2/2)

  8. Comparison Table of e-cash Protocols

  9. Protocol Description

  10. E-cash Model

  11. Our system consists of four entities, namely, The Bank. The Purchaser; The Store; The Authority And the scheme consists of five sub-protocols: Initialization Process Withdrawing protocol Payment/purchasing protocol Deposit/collection protocol Owner/coin tracing protocol Model and Protocols

  12. Initialization Process

  13. Coin Generation • A coin is represented as a six-tuple: • {A, B, z, a, b, r}, Where: • A, B contain user information (encrypted) • z, a, b contain coin information required • for verification (encrypted) • r Bank signature (under the Schnorr scheme)

  14. Coin Signature

  15. Design and Implementation

  16. The Mobile e-cash system

  17. Architectural Design

  18. PDA Specification

  19. Withdraw protocol using a key of 128 bits

  20. Cryptographic Operations per Protocol

  21. Withdraw protocol using a key of 128 bits

  22. Withdraw protocol using a key of 256 bits

  23. Withdraw protocol using a key of 512 bits

  24. We present the implementation of a fair e-cash protocol especially designed for mobile wireless environments, with the following features: Our protocol attempts to offer a reasonable balance between anonymity; and the possibility of revoking that anonymity under special circumstances. For that, our system considers two protocols especially designed for tracing purposes: a coin tracing and an owner tracing protocol; Our system was written in Java and it was implemented in a wireless environment with PDA mobile devices. Conclusion

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