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 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 Once enabled, coins with age restrictions can be withdrawn by parents/warden who can choose to **commit** the coins to a certain maximum age out of a predefined list of age groups.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 restriction is disabled. We might reconsider this design decision at some point. Main ideas and building blocks ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ The main ideas are simple: #. The exchange defines and publishes M different *age groups* of increasing order: :math:`0 < a_1 < \ldots < a_M` with :math:`a_i \in \mathbb{N}`. #. An **unrestricted** *age commitment* is defined as a vector of length M of pairs of EdDSA public and private keys on Curve25519. In other words: one key pair for each age group: :math:`\bigl\langle (p_1, s_1), \ldots, (p_M, s_M) \bigr\rangle` #. A **restricted** *age commitment* **to age group m** is derived from an unrestricted age commitment by removing all private keys for indices larger than m: :math:`\bigl\langle (p_1, s_1), \ldots, (p_m, s_m), \, (p_{m+1}, \perp), \ldots, (p_M, \perp )\bigr\rangle`. The act of restricting an unrestricted age commitment is performed by the parent/ward. #. An *age commitment* (without prefix) is just the vector of public keys: :math:`\vec{Q} := \langle p_1, \ldots, p_M \rangle`. Note that from just the age commitment one can not deduce if it was originated from an unrestricted or restricted age commitment (and what age). #. An *attestation of age group k* is essentially the signature to any message with the private key for slot k, if the corresponding private key is available in a restricted age commitment. (Unrestricted age commitments can attest for any age group). #. An age commitment is *bound to a particular coin* by incorporating the SHA512 hash value of the age commitment (i.e. the M public keys) into the signature of the coin. So instead of signing :math:`\text{FDH}_N(C_p)` with the RSA private key of a denomination with support for age restriction, we sign :math:`\text{FDH}_N(C_p, h_a)`. Here, :math:`C_p` is the EdDSA public key of a coin and :math:`h_a` is the hash of the age commitment. TODO: Summarize the design based on the five functions ``Commit()``, ``Attest()``, ``Verify()``, ``Derive()``, ``Compare()``, once the paper from Özgür and Christian is published. Changes in the Exchange API ^^^^^^^^^^^^^^^^^^^^^^^^^^^ The necessary changes in the exchange involve * indication of support for age restriction as an extension * modification of the refresh protocol (both, commit and reveal phase) * modification of the deposit protocol Extension for age restriction ----------------------------- The exchange indicates support for age-restriction in response to ``/keys`` by registering the extension ``age_restriction.v1`` with a value type ``ExtensionAgeRestriction``: .. ts:def:: ExtensionAgeRestriction interface ExtensionAgeRestriction { // The field ``critical`` is mandatory for an extension. // Age restriction is not required to be understood by an client, so // ``critical`` will be set to ``false``. critical: false; // Age restriction specific fields. // 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 value is // taken into consideration when signing the denominations in // ``ExchangeKeysResponse.age_restricted_denoms``. age_groups: string; } Registering an extension is defined in :doc:`design document 006 ― Extensions <006-extensions>`. Age restricted denominations ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If age-restriction is registered as an extension under the name ``age_restriction.v1``, as described above, the root-object ``ExchangeKeysResponse`` in response to ``/keys`` MUST be extended by an additional field ``age_restricted_denoms``. This is an *additional* list of denominations that must be used during the modified ``refresh`` and ``deposit`` operations (see below). The data structure for those denominations is the same as for the regular ones 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 ``ExtensionAgeRestriction.age_groups`` from the corresponding extension object (see above). 2. The value of ``TALER_DenominationKeyValidityPS.purpose`` is set to ``TALER_SIGNATURE_MASTER_AGE_RESTRICTED_DENOMINATION_KEY_VALIDITY``. And similar to ``.denoms``, if the query parameter ``last_issue_date`` was provided by the client, the exchange will only return the keys that have changed since the given timestamp. .. ts:def:: ExchangeKeysResponse interface ExchangeKeysResponse { //... // List of denominations that support age-restriction with the age groups // given in age_groups. This is only set **iff** the extension // ``age_restriction.v1`` is registered under ``entensions`` with type // ``ExtensionAgeRestriction``. // // The data structure for each denomination 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 ``ExtensionAgeRestriction.age_groups`` from the // corresponding extension object. // // 2. The value of ``TALER_DenominationKeyValidityPS.purpose`` is set to // ``TALER_SIGNATURE_MASTER_AGE_RESTRICTED_DENOMINATION_KEY_VALIDITY`` // // Similar as for ``.denoms``, if the query parameter ``last_issue_date`` // was provided by the client, the exchange will only return the keys that // have changed since the given timestamp. age_restricted_denoms: Denom[]; //... } SQL changes ----------- The exchange has to mark denominations with support for age restriction as such in the database. Also, during the melting phase of the refresh operation, the exchange will have to persist the SHA512 hash of the age commitment of the original coin. The schema for the exchange is changed as follows: .. sourcecode:: sql -- Everything in one big transaction BEGIN; -- Check patch versioning is in place. SELECT _v.register_patch('exchange-TBD', NULL, NULL); -- Support for age restriction is marked per denomination. ALTER TABLE denominations ADD COLUMN age_restricted BOOLEAN NOT NULL DEFAULT (false); COMMENT ON COLUMN denominations.age_restriced IS 'true if this denomination can be used for age restriction'; -- During the melting phase of the refresh, the wallet has to present the -- hash value of the age commitment (only for denominations with support -- for age restriction). ALTER TABLE refresh_commitments ADD COLUMN age_commitment_h BYTEA CHECK (LENGTH(age_commitment_h)=64); COMMENT ON COLUMN refresh_commitments.age_commitment_h IS 'SHA512 hash of the age commitment of the old coin, iff the corresponding denomimination has support for age restriction, NULL otherwise.'; COMMIT; Note the constraint on ``refresh_commitments.age_commitment_h``: It can be NULL, but only iff the corresponding denomination (indirectly referenced via table ``known_coins``) has ``.age_restricted`` set to true. This constraint can not be expressed reliably with SQL. Protocol changes ---------------- Refresh - melting phase ~~~~~~~~~~~~~~~~~~~~~~~ During the melting phase of the refresh, the wallet has to present the hash value of the age commitment (for denominations with support for age restriction). Therefore, in the ``/coins/$COIN_PUB/melt`` POST request, the ``MeltRequest`` object is extended with an optional field ``age_commitment_hash``: .. ts:def:: MeltRequest interface MeltRequest { ... // SHA512 hash of the age commitment of the coin, IFF the denomination // has age restriction support. MUST be omitted otherwise. age_commitment_hash?: HashCode; ... } The responses to the POST request remain the same. For normal denominations *without* support for age restriction, the calculation for the signature check is as before (borrowing notation from `Florian's thesis `_): .. math:: \text{FDH}_N(C_p)\; \stackrel{?}{=}\; \left(\sigma_C\right)^{e} \;\;\text{mod}\,N Here, :math:`C_p` is the EdDSA public key of a coin, :math:`\sigma_C` is its signature and :math:`\langle e, N \rangle` is the RSA public key of the denomination. For denominations *with* support for age restriction, the exchange takes the hash value ``age_commitment_hash`` (abbreviated as :math:`h_a`) into account when verifying the coin's signature: .. math:: \text{FDH}_N(C_p, h_a)\; \stackrel{?}{=}\; \left(\sigma_C\right)^{e} \;\;\text{mod}N Refresh - reveal phase ~~~~~~~~~~~~~~~~~~~~~~ During the reveal phase -- that is upon POST to ``/refreshes/$RCH/reveal`` -- the client has to provide the original age commitment of the old coin (i.e. the vector of public keys), iff the corresponding denomination had support for age restriction. The size of the vector ist defined by the Exchange implicetly as the amount of age groups defined in the field ``.age_groups`` of the ``ExtensionAgeRestriction``. .. ts:def:: RevealRequest interface RevealRequest { ... // Iff the corresponding denomination has support for age restriction, // the client MUST provide the original age commitment, i.e. the vector // of public keys. // The size of the vector ist defined by the Exchange implicetly as the // amount of age groups defined in the field ``.age_groups`` of the // ``ExtensionAgeRestriction``. old_age_commitment?: EddsaPublicKey[]; ... } TODO: describe how the exchange derives the κ-1 other age-restriction vectors and compares them to the one in ``.old_age_commitment``. Deposit ~~~~~~~ As always, the merchant has to provide the public key of a coin during a POST to ``/coins/$COIN_PUB/deposit``. However, for coins with age restriction, the signature check requires the hash of the age commitment. Therefore the request object ``DepositRequest`` is extended by an optional field ``age_commitment_hash`` which MUST be set (with the SHA512 hash of the age commitment), iff the corresponding denomination had support for age restriction enabled. The merchant has received this value prior from the customer during purchase. .. ts:def:: DepositRequest interface DepositRequest { ... // Iff the corresponding denomination had support for age restriction // enabled, this field MUST contain the SHA512 value of the age commitment that // was provided during the purchase. age_commitment_hash?: HashCode; ... } Again, the exchange can now check the validity of the coin with age restriction by evaluating .. math:: \text{FDH}_N(C_p, h_a)\; \stackrel{?}{=}\; \left(\sigma_C\right)^{e} \;\;\text{mod}N Also again, :math:`C_p` is the EdDSA public key of a coin, :math:`\sigma_C` is its signature, :math:`\langle e, N \rangle` is the RSA public key of the denomination and :math:`h_a` is the value from ``age_commitment_hash``. TODO: maybe rename this field into something more opaque, like ``opaque_signature_salt`` or so? Changes in the Merchant API ^^^^^^^^^^^^^^^^^^^^^^^^^^^ Claiming the order ------------------ If an order requires a minimum age, the merchant MUST express that required minimum age in response to order claim by the wallet, that is, a POST to ``[/instances/$INSTANCE]/orders/$ORDER_ID/claim``. The object ``ContractTerms`` is extended by an optional field ``required_minimum_age`` that can be any integer greater than 0. In reality this value will not be smaller than, say, 8, and not larger than, say, 21. .. ts:def:: ContractTerms interface ContractTerms { ... // If the order requires a minimum age greater than 0, this field is set // to the integer value of that age. In reality this value will not be // smaller than, say, 8, and not larger than, say, 21. required_minimum_age?: Integer; ... } By sending the contract term with the field ``required_minimum_age`` set to an non-zero integer value, the merchant implicetly signals that it understands the extension ``age_restriction.v1`` for age restriction from the exchange. Making the payment ------------------ If the ``ContractTerms`` had a non-zero value in field ``required_minimum_age``, the wallet has to provide evidence of that minimum age by #. *either* using coins which are of denominations that have support for age restriction enabled #. and then ―for each such coin― it has the right private key of the restricted age commitment to the age group into which the required minimum age falls (i.e. a non-empty entry at the right index in vector of EdDSA keys, see above). #. and signs the required minimum age with each coin's private key corresponding to the age group, #. and sends ―for each coin― the complete age commitment and the signature to the merchant. #. *or* using coins which are of denominations that had *no* age support enabled. The object ``CoinPaySig`` used within a ``PayRequest`` during a POST to ``[/instances/$INSTANCE]/orders/$ORDER_ID/pay`` is extended as follows: .. ts:def:: CoinPaySig export interface CoinPaySig { ... // If a minimum age was required by the order and the wallet had coins that // are at least commited to the corresponding age group, this is the // signature of the minimum age as a string, using the private key to the // corresponding age group. minimum_age_sig?: EddsaSignature; // If a minium age was required by the order, this is age commitment bound // to the coin, i.e. the complete vector of EdDSA public keys, one for each // age group (as defined by the exchange). age_commitment?: EddsaPublicKey[]; } The merchant can now verify #. the validity of each (age restricted) coin by evaluating .. math:: \text{FDH}_N(C_p, h_a)\; \stackrel{?}{=}\; \left(\sigma_C\right)^{e} \;\;\text{mod}N Again, :math:`C_p` is the EdDSA public key of a coin, :math:`\sigma_C` is its signature, :math:`\langle e, N \rangle` is the RSA public key of the denomination and :math:`h_a` is the SHA512 hash value of the vector in ``age_commitment``. #. the minimum age requirement by checking the signature in ``minimum_age_sig`` against the public key ``age_commitment[k]`` of the corresponding age group, say, ``k``. (The minimum age must fall into the age group at index ``k`` as defined by the exchange). **Note**: This applies only to coins for denominations that have support for age restriction. Denominations *without* support for age restriction *always* fullfill any minimum age requirement. 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 `__: * `Money with capabilities `_ * `On age-restriction (was: online games in China) `__ * `Age-restriction is about coins, not currencies `__ The upcoming paper on anonymous age-restriction for GNU Taler from Özgür Kesim and Christian Grothoff will be cited here, once it is published.