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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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Presentation Transcript
features of the standard cash

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
credit debit cards features
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
alternative e cash

To substitute standard banknotes

To provide more flexibility than credit/debit cards



Non-traceable protocols




Off-line protocols


Alternative: E-cash
e cash previous works 1 2
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)
e cash previous works 2 2
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)
model and protocols
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
coin generation
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)
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.