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GNU Taler Details

links: AC2 TOC GNU Taler - Index

Withdrawing coins

Taler Withdraw Coins.png

Payment

Taler Payment.png

Double Spending Problem

A lot of logic is needed to ensure double spending does not happen. (Multiple keys, key rotation etc.)

Also, the exchange needs a database to detect double spending.

Giving change

It would be inefficient to pay EUR 100 with 1 cent coins!

  • Denomination key represents value of a coin.
  • Exchange may offer various denominations for coins.
  • Wallet may not have exact change!
  • Usability requires ability to pay given sufficient total funds.

Key goals:

  • maintain unlinkability
  • maintain taxability of transactions

Method:

  • Contract can specify to only pay partial value of a coin.
  • Exchange allows wallet to obtain unlinkable change for remaining coin value.

Refresh protocol

Cut-and-choose zero-knowledge proof is needed here.

  • Customer asks exchange to convert old coin to new coin
  • Protocol ensures new coins can be recovered from old coin
    • New coins are owned by the same entity!

Thus, the refresh protocol allows:

  • To give unlinkable change.
  • To give refunds to an anonymous customer.
  • To expire old keys and migrate coins to new ones.
  • To handle protocol aborts.

links: AC2 TOC GNU Taler - Index