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DD 024: Anonymous Age Restriction Extension for GNU Taler
#########################################################
Summary
=======
This document presents and discusses an extension to GNU Taler that provides
anonymous age-restriction.
Motivation
==========
Merchants are legally obliged to perform age verification of customers when
they buy certain goods and services. Current mechanisms for age verification
are either ID-based or require the usage of credit/debit cards. In all cases
sensitive private information is disclosed.
We want to offer a better mechanism for age-restriction with GNU Taler that
* ensures anonymity and unlinkability of purchases
* can be set to particular age groups by parents/wardens at withdrawal
* is bound to particular coins/tokens
* can be verified by the merchant at purchase time
* persists even after refresh
The mechanism is presented as an 'extension' to GNU Taler, that is, as an
optional feature that can be switched on by the exchange operator.
Requirements
============
TODO
* legal requirements for merchants must allow for this kind of mechanism
Proposed Solution
=================
We propose an extension to GNU Taler for age-restriction that can be enabled
optionally by an Exchange¹).
Once enabled, coins with age restrictions can be withdrawn by parents/warden
who can choose to **commit** the coins to a certain maximum age out of a
predefined list of age groups.
The minors/wards receive those coins and can now **attest** a required minimum
age (provided that age is less or equal to the committed age of the coins) to
merchants, who can **verify** the minimum age.
For the rest values (change) after an transaction, the minor/ward can
**derive** new age-restricted coins. The exchange can **compare** the equality
of the age-restriction of the old coin with the new coin (in a zero-knowledge
protocol, that gives the minor/ward a 1/κ chance to raise the minimum age for
the new coin).
The proposed solution maintains the guarantees of GNU Taler with respect to
anonymity and unlinkability. (TODO: refer to the paper, once published)
¹) Once the feature is enabled and the age groups are defined, the exchange has
to stick to that decision until the support for age groups are disabled. We
might reconsider this design decision at some point.
Building Blocks
^^^^^^^^^^^^^^^
TODO: Summarize the design based on the five functions ``Commit()``,
``Attest()``, ``Verify()``, ``Derive()``, ``Compare()``.
Changes in the Exchange
^^^^^^^^^^^^^^^^^^^^^^^
The necessary changes in the exchange involve
* indication of support for age restriction
* modification of the refresh protocol (both, commit and reveal phase)
* modification of the deposit protocol
Support for Age Restriction
---------------------------
The exchange indicates support for age-restriction in response to ``/keys`` in
an optional field ``'age_restriction`` with new type ``AgeRestriction``:
.. ts:def:: AgeRestriction
interface AgeRestriction {
// Representation of the age groups as comma separated edges: Increasing
// from left to right, the values mark the begining of an age group up
// to, but not including the next value. The initial age group starts at
// 0 and is not listed. Example: "8:10:12:14:16:18:21".
// This field is mandatory and binding in the sense that its hash value
// is taken into consideration when signing the denominations below.
age_groups: string;
// List of denominations that support age-restriction with the age groups
// given in ``age_groups``. The data structure is the same as for the
// denominations in ``ExchangeKeysResponse.denoms``.
// **However**, the following differences apply for each denomination in
// the list:
//
// 1. The value of ``TALER_DenominationKeyValidityPS.denom_hash``
// is taken over the public key of the denomination __and__ the
// string in ``age_groups``.
//
// 2. The value of ``TALER_DenominationKeyValidityPS.purpose`` is set to
// TALER_SIGNATURE_MASTER_AGE_RESTRICTED_DENOMINATION_KEY_VALIDITY
denoms: Denom[];
// Same role as ``ExchangeKeysResponse.eddsa_sig``, but only for the
// denominations listed in ``denoms`` above for age restriction. The public
// EdDSA key of the exchange that was used to generate the signature is the
// same as ``ExchangeKeysResponse.eddsa_pub``.
eddsa_sig: EddsaSignature;
}
Refresh
-------
TODO: Extension of the cut'n-choose-protocol.
Deposit
-------
TODO: Add opaque hash value of the commitments to the protocol
Changes in the Merchant
^^^^^^^^^^^^^^^^^^^^^^^
TODO
* Spending protocol
Changes in the Wallet
^^^^^^^^^^^^^^^^^^^^^
TODO.
* choosing age-restriction during withdrawal coins from denominations with
support for age restriction.
* Define protocol to pass denominations to child/ward.
Alternatives
============
TODO.
* ID-based systems
* credit/debit card based systems
Drawbacks
=========
TODO.
* age groups, once defined, are set permanently
* age restricted coins are basically shared between ward and warden.
Also discuss:
* storage overhead
* computational overhead
* bandwidth overhead
* legal issues?
Discussion / Q&A
================
We had some very engaged discussions on the GNU Taler mailing list
`<mailto:taler@gnu.org>`:
* Money with capabilities
`<https://lists.gnu.org/archive/html/taler/2021-08/msg00005.html>`_
* On age-restriction (was: online games in China)
`<https://lists.gnu.org/archive/html/taler/2021-09/msg00006.html>`_
* Age-restriction is about coins, not currencies
`<https://lists.gnu.org/archive/html/taler/2021-09/msg00021.html>`_
The upcoming paper on anonymous age-restriction for GNU Taler from Özgür Kesim
and Christian Grothoff will be cited here, once it is published.
|
