13.28. DD 28: Deposit Policy Extensions


This is Work-In-Progress.

13.28.1. Summary

We will propose here a plugable mechanism in the exchange to support deposits with associated policy. An exchange can enable support for such policies via configuration.

The inital set of policy extensions that an exchange might provide consists of

Merchant refunds

Merchant can grant customers refundable payments. In this case, the amount of the deposit is put into escrow by the exchange for a certain period until which the customer can claim a refund.

Escrowed payments

A trustor puts coins into escrow with the exchange. It can be claimed by a beneficiary until a certain deadline, when the claim is signed by both, the beneficiary’s and the trustor’s keys.

Brandt-Vickrey auctions

A bidder puts coins into escrow with the exhange in order to participate in an Brandt-Vickrey auction. The deposit confirmation is proof to the seller for the escrow and contains a hash of the auction meta-data and a deadline. After successfull execution of the auction, the seller provides a valid transcript to the exchange from which the exchange learns which bidder(s) won the auction for which prices. It then transfers the amounts from the winners’ coins to the seller. In case of a timeout and for all losing bidders, the coins can be refreshed.

The policies shall be implemented as extensions to the exchange (see DD 06: Extensions for GNU Taler).

13.28.2. Motivation

GNU Taler’s initial set of API’s (withdraw, deposit, refresh) support most payment situations in which customers pay for goods and services within an otherwise unconditioned transaction. (A notable exception from this the ability to provide refunds, which will be re-factored into a policy extension).

However, in many payments depend on additional conditions to be met. GNU Taler already supports payments with age restriction applied, but there are other scenarious that we want to support.

Our aim is to provide an API for extensions of GNU Taler that implement particular policies and policy-handling for payments (also called conditioned payments).

13.28.3. Background and Requirements


13.28.4. Proposed Solution

TODO, explain:

  • C-structs for policy extensions (esp. the handlers)

  • Naming conventions for policy extensions

  • Deadlines and -handling

  • Typical choreography of a deposit with policy and its fulfillment API-Endpoints of the Exchange

TODO Database-schema

TODO: Description

digraph deposit_policies { rankdir = LR; splines = false; fontname="monospace" node [ fontname="monospace" shape=record ] subgraph cluster_deposits { label=<<B>deposits</B>> margin=20 deposits [ label="...|<ref>policy_details_id (null)\l|...|timestamp\l|..." ] } subgraph cluster_policy_details { label=<<B>policy_details</B>> margin=20 policy_details [ label="<id>id\l|<hash>policy_hash_code (unique)\l|deadline\l|commitment (amount)\l|accumulated_total (amount)\l|fee (amount)\l|transferable (amount)\l|fulfillment_state\l|<fid>fulfillment_id (null)\l" ] } subgraph cluster_policy_fulfillments { label=<<B>policy_fulfillments</B>> margin=20 rank=min; policy_fulfillments [ label="<id>id\l|proof\l|timestamp\l|<codes>policy_hash_codes (blob)\l" ] } deposits:ref->policy_details:id [ label="n:1"; fontname="monospace" ]; policy_details:fid->policy_fulfillments:id [label="n:1"; fontname="monospace" ]; }

The field policy_hash_codes in table policy_fulfillments is a binary blob that consists of the concatenation of the sorted policy_details.policy_hash_code entries from all policies that are fulfilled by this proof. Policy Fulfillment States

The fulfillment of a policy can be in one of the following five states:


The policy is funded and ready. The exchange is waiting for a proof of fulfillment to arrive before the deadline.


The policy lacks funding, that is accumulated_total < commitment, but has otherwise been accepted. Funding can be continued by calling /deposit or /batch-deposit with more coins and the same policy details.


The policy is provably fulfilled. The amounts for payout, fees and refresh are transfered/can be claimed. Note that a policy fulfillment handler can change the values for the amounts for payout, fees and refresh.


The policy has timed out. The amounts for payout and refresh are transfered/can be claimed.


The policy is in an failure state. Payouts and refreshes are blocked, timeouts are ignored. Invariants

The following invariants need to be fulfilled and be checked by the auditor:

  • The fulfillment state of a policy is Insufficient IF AND ONLY IF the amount in policy_details.commitment is equal or larger than the amount in policy_details.accumulated_total.

  • The sum of amounts in policy_details.fee and policy_details.transferable MUST be equal or less than the amount in policy_details.accumulated_total.

  • The amount in policy_details.accumulated_total MUST be equal to the total sum of contributions of the individual coins of the deposits that reference this policy.

  • Each hash code encoded in policy_fulfillments.policy_hash_codes MUST refer to an existing policy_details.hash_code AND its .fulfillment_id MUST point to the same policy_fulfillments.id.

  • Conversely: If a policy_details.fulfillment_id points to an entry in policy_fulfillment, the policy_details.policy_hash_code MUST be present in that entry’s .policy_hash_codes. Alternatives

TODO Drawbacks

TODO Discussion / Q&A


(This should be filled in with results from discussions on mailing lists / personal communication.)