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..
  This file is part of GNU TALER.
  Copyright (C) 2014-2020 Taler Systems SA

  TALER is free software; you can redistribute it and/or modify it under the
  terms of the GNU General Public License as published by the Free Software
  Foundation; either version 2.1, or (at your option) any later version.

  TALER is distributed in the hope that it will be useful, but WITHOUT ANY
  WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
  A PARTICULAR PURPOSE.  See the GNU Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General Public License along with
  TALER; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>

  @author Christian Grothoff

=============================
The Exchange RESTful JSON API
=============================

The API specified here follows the :ref:`general conventions <http-common>`
for all details not specified in the individual requests.
The `glossary <https://docs.taler.net/glossary.html#glossary>`_
defines all specific terms used in this section.

.. _keys:

-----------------------
Obtaining Exchange Keys
-----------------------

This API is used by wallets and merchants to obtain global information about
the exchange, such as online signing keys, available denominations and the fee
structure.  This is typically the first call any exchange client makes, as it
returns information required to process all of the other interactions with the
exchange.  The returned information is secured by (1) signature(s) from the exchange,
especially the long-term offline signing key of the exchange, which clients should
cache; (2) signature(s) from auditors, and the auditor keys should be
hard-coded into the wallet as they are the trust anchors for Taler; (3)
possibly by using HTTPS.


.. http:get:: /terms

  Get the terms of service of the exchange.
  The exchange will consider the "Accept" and "Accept-Language" and
  "Accept-Encoding" headers when generating a response. Specifically,
  it will try to find a response with an acceptable mime-type, then
  pick the version in the most preferred language of the user, and
  finally apply compression if that is allowed by the client and
  deemed beneficial.

  The exchange will set an "Etag", and subsequent requests of the
  same client should provide the tag in an "If-None-Match" header
  to detect if the terms of service have changed.  If not, a
  "204 Not Modified" response will be returned.

  If the "Etag" is missing, the client should not cache the response and instead prompt the user again at the next opportunity. This is usually only the case if the terms of service were not configured correctly.


.. http:get:: /privacy

  Get the privacy policy of the exchange.
  The exchange will consider the "Accept" and "Accept-Language" and
  "Accept-Encoding" headers when generating a response. Specifically,
  it will try to find a response with an acceptable mime-type, then
  pick the version in the most preferred language of the user, and
  finally apply compression if that is allowed by the client and
  deemed beneficial.

  The exchange will set an "Etag", and subsequent requests of the
  same client should provide the tag in an "If-None-Match" header
  to detect if the privacy policy has changed.  If not, a
  "204 Not Modified" response will be returned.

  If the "Etag" is missing, the client should not cache the response and instead prompt the user again at the next opportunity. This is usually only the case if the privacy policy was not configured correctly.

.. http:get:: /keys

  Get a list of all denomination keys offered by the bank,
  as well as the bank's current online signing key.

  **Request:**

  :query last_issue_date: optional argument specifying the maximum value of any of the "stamp_start" members of the denomination keys of a "/keys" response that is already known to the client. Allows the exchange to only return keys that have changed since that timestamp.  The given value must be an unsigned 64-bit integer representing seconds after 1970.  If the timestamp does not exactly match the "stamp_start" of one of the denomination keys, all keys are returned.

  **Response:**

  :status 200 OK:
    The exchange responds with a `ExchangeKeysResponse` object. This request should
    virtually always be successful.

  **Details:**

  .. ts:def:: ExchangeKeysResponse

    interface ExchangeKeysResponse {
      // libtool-style representation of the Taler protocol version, see
      // https://www.gnu.org/software/libtool/manual/html_node/Versioning.html#Versioning
      // The format is "current:revision:age".
      version: string;

      // EdDSA master public key of the exchange, used to sign entries in 'denoms' and 'signkeys'
      master_public_key: EddsaPublicKey;

      // Relative duration until inactive reserves are closed; not signed, expressed as
      // a string in relative time in microseconds, i.e. "/Delay(1000)/" for 1 second.
      reserve_closing_delay: RelativeTime;

      // Denominations offered by this exchange.
      denoms: Denom[];

      // Denominations for which the exchange currently offers/requests recoup.
      recoup: Recoup[];

      // The date when the denomination keys were last updated.
      list_issue_date: Timestamp;

      // Auditors of the exchange.
      auditors: Auditor[];

      // The exchange's signing keys.
      signkeys: SignKey[];

      // compact EdDSA `signature` (binary-only) over the SHA-512 hash of the
      // concatenation of all SHA-512 hashes of the RSA denomination public keys
      // in ``denoms`` in the same order as they were in ``denoms``.  Note that for
      // hashing, the binary format of the RSA public keys is used, and not their
      // `base32 encoding <base32>`.  Wallets cannot do much with this signature by itself;
      // it is only useful when multiple clients need to establish that the exchange
      // is sabotaging end-user anonymity by giving disjoint denomination keys to
      // different users.  If a exchange were to do this, this signature allows the
      // clients to demonstrate to the public that the exchange is dishonest.
      eddsa_sig: EddsaSignature;

      // Public EdDSA key of the exchange that was used to generate the signature.
      // Should match one of the exchange's signing keys from /keys.  It is given
      // explicitly as the client might otherwise be confused by clock skew as to
      // which signing key was used.
      eddsa_pub: EddsaPublicKey;
    }

  .. ts:def:: Denom

    interface Denom {
      // How much are coins of this denomination worth?
      value: Amount;

      // When does the denomination key become valid?
      stamp_start: Timestamp;

      // When is it no longer possible to deposit coins
      // of this denomination?
      stamp_expire_withdraw: Timestamp;

      // Timestamp indicating by when legal disputes relating to these coins must
      // be settled, as the exchange will afterwards destroy its evidence relating to
      // transactions involving this coin.
      stamp_expire_legal: Timestamp;

      // Public (RSA) key for the denomination.
      denom_pub: RsaPublicKey;

      // Fee charged by the exchange for withdrawing a coin of this denomination
      fee_withdraw: Amount;

      // Fee charged by the exchange for depositing a coin of this denomination
      fee_deposit: Amount;

      // Fee charged by the exchange for refreshing a coin of this denomination
      fee_refresh: Amount;

      // Fee charged by the exchange for refunding a coin of this denomination
      fee_refund: Amount;

      // Signature of `TALER_DenominationKeyValidityPS`
      master_sig: EddsaSignature;
    }

  Fees for any of the operations can be zero, but the fields must still be
  present. The currency of the ``fee_deposit``, ``fee_refresh`` and ``fee_refund`` must match the
  currency of the ``value``.  Theoretically, the ``fee_withdraw`` could be in a
  different currency, but this is not currently supported by the
  implementation.

  .. ts:def:: Recoup

    interface Recoup {
      // hash of the public key of the denomination that is being revoked under
      // emergency protocol (see /recoup).
      h_denom_pub: HashCode;

      // We do not include any signature here, as the primary use-case for
      // this emergency involves the exchange having lost its signing keys,
      // so such a signature here would be pretty worthless.  However, the
      // exchange will not honor /recoup requests unless they are for
      // denomination keys listed here.
    }

  A signing key in the ``signkeys`` list is a JSON object with the following fields:

  .. ts:def:: SignKey

    interface SignKey {
      // The actual exchange's EdDSA signing public key.
      key: EddsaPublicKey;

      // Initial validity date for the signing key.
      stamp_start: Timestamp;

      // Date when the exchange will stop using the signing key, allowed to overlap
      // slightly with the next signing key's validity to allow for clock skew.
      stamp_expire: Timestamp;

      // Date when all signatures made by the signing key expire and should
      // henceforth no longer be considered valid in legal disputes.
      stamp_end: Timestamp;

      // Signature over ``key`` and ``stamp_expire`` by the exchange master key.
      // Must have purpose TALER_SIGNATURE_MASTER_SIGNING_KEY_VALIDITY.
      master_sig: EddsaSignature;
    }

  An entry in the ``auditors`` list is a JSON object with the following fields:

  .. ts:def:: Auditor

    interface Auditor {
      // The auditor's EdDSA signing public key.
      auditor_pub: EddsaPublicKey;

      // The auditor's URL.
      auditor_url: string;

      // An array of denomination keys the auditor affirms with its signature.
      // Note that the message only includes the hash of the public key, while the
      // signature is actually over the expanded information including expiration
      // times and fees.  The exact format is described below.
      denomination_keys: DenominationKey[];
    }

  .. ts:def:: DenominationKey

    interface DenominationKey {
      // hash of the public RSA key used to sign coins of the respective
      // denomination.  Note that the auditor's signature covers more than just
      // the hash, but this other information is already provided in ``denoms`` and
      // thus not repeated here.
      denom_pub_h: HashCode;

      // Signature of `TALER_ExchangeKeyValidityPS`
      auditor_sig: EddsaSignature;
    }

  The same auditor may appear multiple times in the array for different subsets
  of denomination keys, and the same denomination key hash may be listed
  multiple times for the same or different auditors.  The wallet or merchant
  just should check that the denomination keys they use are in the set for at
  least one of the auditors that they accept.

  .. note::

    Both the individual denominations *and* the denomination list is signed,
    allowing customers to prove that they received an inconsistent list.

.. _wire-req:

-----------------------------------
Obtaining wire-transfer information
-----------------------------------

.. http:get:: /wire

  Returns a list of payment methods supported by the exchange.  The idea is that wallets may use this information to instruct users on how to perform wire transfers to top up their wallets.

  **Response:**

  :status 200: The exchange responds with a `WireResponse` object. This request should virtually always be successful.

  **Details:**

  .. ts:def:: WireResponse

    interface WireResponse {

      // Master public key of the exchange, must match the key returned in /keys.
      master_public_key: EddsaPublicKey;

      // Array of wire accounts operated by the exchange for
      // incoming wire transfers.
      accounts: WireAccount[];

      // Object mapping names of wire methods (i.e. "sepa" or "x-taler-bank")
      // to wire fees.
      fees: { method : AggregateTransferFee };
    }

  The specification for the account object is:

  .. ts:def:: WireAccount

    interface WireAccount {
      // payto:// URL identifying the account and wire method
      url: string;

      // Salt value (used when hashing 'url' to verify signature)
      salt: string;

      // Signature using the exchange's offline key
      // with purpose TALER_SIGNATURE_MASTER_WIRE_DETAILS.
      master_sig: EddsaSignature;
    }

  Aggregate wire transfer fees representing the fees the exchange
  charges per wire transfer to a merchant must be specified as an
  array in all wire transfer response objects under ``fees``.  The
  respective array contains objects with the following members:

  .. ts:def:: AggregateTransferFee

    interface AggregateTransferFee {
      // Per transfer wire transfer fee.
      wire_fee: Amount;

      // Per transfer closing fee.
      closing_fee: Amount;

      // What date (inclusive) does this fee go into effect?
      // The different fees must cover the full time period in which
      // any of the denomination keys are valid without overlap.
      start_date: Timestamp;

      // What date (exclusive) does this fee stop going into effect?
      // The different fees must cover the full time period in which
      // any of the denomination keys are valid without overlap.
      end_date: Timestamp;

      // Signature of `TALER_MasterWireFeePS` with purpose TALER_SIGNATURE_MASTER_WIRE_FEES
      sig: EddsaSignature;
    }

----------
Withdrawal
----------

This API is used by the wallet to obtain digital coins.

When transfering money to the exchange such as via SEPA transfers, the exchange creates
a *reserve*, which keeps the money from the customer.  The customer must
specify an EdDSA reserve public key as part of the transfer, and can then
withdraw digital coins using the corresponding private key.  All incoming and
outgoing transactions are recorded under the corresponding public key by the
exchange.

.. note::

   Eventually the exchange will need to advertise a policy for how long it will
   keep transaction histories for inactive or even fully drained reserves.  We
   will therefore need some additional handler similar to ``/keys`` to
   advertise those terms of service.


.. http:get:: /reserves/$RESERVE_PUB

  Request information about a reserve.

  .. note::
    The client currently does not have to demonstrate knowledge of the private
    key of the reserve to make this request, which makes the reserve's public
    key privileged information known only to the client, their bank, and the
    exchange.  In future, we might wish to revisit this decision to improve
    security, such as by having the client EdDSA-sign an ECDHE key to be used
    to derive a symmetric key to encrypt the response.  This would be useful if
    for example HTTPS were not used for communication with the exchange.

  **Request:**

  **Response:**

  :status 200 OK:
    The exchange responds with a `ReserveStatus` object;  the reserve was known to the exchange,
  :status 404 Not Found:
    The reserve key does not belong to a reserve known to the exchange.

  **Details:**

  .. ts:def:: ReserveStatus

    interface ReserveStatus {
      // Balance left in the reserve.
      balance: Amount;

      // Transaction history for this reserve
      history: TransactionHistoryItem[];
    }

  Objects in the transaction history have the following format:

  .. ts:def:: TransactionHistoryItem

    // Union discriminated by the "type" field.
    type ReserveTransaction =
      | ReserveWithdrawTransaction
      | ReserveCreditTransaction
      | ReserveClosingTransaction
      | ReserveRecoupTransaction;

  .. ts:def:: ReserveWithdrawTransaction

    interface ReserveWithdrawTransaction {
      type: "WITHDRAW";

      // Amount withdrawn.
      amount: Amount;

      // Hash of the denomination public key of the coin.
      h_denom_pub: HashCode;

      // Hash of the blinded coin to be signed
      h_coin_envelope: HashCode;

      // Signature of 'TALER_WithdrawRequestPS' created with the reserves's
      // private key.
      reserve_sig: EddsaSignature;

      // Fee that is charged for withdraw.
      withdraw_fee: Amount;
     }


  .. ts:def:: ReserveCreditTransaction

    interface ReserveCreditTransaction {
      type: "CREDIT";

      // Amount deposited.
      amount: Amount;

      // Sender account payto://-URL
      sender_account_url: string;

      // Transfer details uniquely identifying the transfer.
      wire_reference: string;

      // Timestamp of the incoming wire transfer.
      timestamp: Timestamp;
    }


  .. ts:def:: ReserveClosingTransaction

    interface ReserveClosingTransaction {
      type: "CLOSING";

      // Closing balance.
      amount: Amount;

      // Closing fee charged by the exchange.
      closing_fee: Amount;

      // Wire transfer subject.
      wtid: string;

      // payto:// URI of the the wire account into which the funds were returned to.
      receiver_account_details: string;

      // This is a signature over a
      // struct TALER_ReserveCloseConfirmationPS with purpose
      // TALER_SIGNATURE_EXCHANGE_RESERVE_CLOSED.
      exchange_sig: EddsaSignature;

      // Public key used to create exchange_sig.
      exchange_pub: EddsaPublicKey;

      // Time when the reserve was closed.
      timestamp: Timestamp;
    }


  .. ts:def:: ReserveRecoupTransaction

    interface ReserveRecoupTransaction {
      type: "RECOUP";

      // Amount paid back.
      amount: Amount;

      // This is a signature over
      // a struct TALER_PaybackConfirmationPS with purpose
      // TALER_SIGNATURE_EXCHANGE_CONFIRM_PAYBACK.
      exchange_sig: EddsaSignature;

      // Public key used to create exchange_sig.
      exchange_pub: EddsaPublicKey;

      // Time when the funds were paid back into the reserve.
      timestamp: Timestamp;

      // Public key of the coin that was paid back.
      coin_pub: CoinPublicKey;
    }

.. http:post:: /reserves/$RESERVE_PUB/withdraw

  Withdraw a coin of the specified denomination.  Note that the client should
  commit all of the request details, including the private key of the coin and
  the blinding factor, to disk *before* issuing this request, so that it can
  recover the information if necessary in case of transient failures, like
  power outage, network outage, etc.

  **Request:** The request body must be a `WithdrawRequest` object.

  **Response:**

  :status 200 OK:
    The request was succesful, and the response is a `WithdrawResponse`.  Note that repeating exactly the same request
    will again yield the same response, so if the network goes down during the
    transaction or before the client can commit the coin signature to disk, the
    coin is not lost.
  :status 401 Unauthorized: The signature is invalid.
  :status 404 Not Found:
    The denomination key or the reserve are not known to the exchange.  If the
    denomination key is unknown, this suggests a bug in the wallet as the
    wallet should have used current denomination keys from ``/keys``.  If the
    reserve is unknown, the wallet should not report a hard error yet, but
    instead simply wait for up to a day, as the wire transaction might simply
    not yet have completed and might be known to the exchange in the near future.
    In this case, the wallet should repeat the exact same request later again
    using exactly the same blinded coin.
  :status 403 Forbidden:
    The balance of the reserve is not sufficient to withdraw a coin of the indicated denomination.
    The response is `WithdrawError` object.


  **Details:**

  .. ts:def:: WithdrawRequest

    interface WithdrawRequest {
      // Hash of a denomination public key (RSA), specifying the type of coin the client
      // would like the exchange to create.
      denom_pub_hash: HashCode;

      // coin's blinded public key, should be (blindly) signed by the exchange's
      // denomination private key
      coin_ev: CoinEnvelope;

      // Signature of `TALER_WithdrawRequestPS` created with the `reserves's private key <reserve-priv>`
      reserve_sig: EddsaSignature;

    }


  .. ts:def:: WithdrawResponse

    interface WithdrawResponse {
      // The blinded RSA signature over the ``coin_ev``, affirms the coin's
      // validity after unblinding.
      ev_sig: BlindedRsaSignature;

    }

  .. ts:def:: WithdrawError

    interface WithdrawError {
      // Text describing the error
      hint: string;

      // Detailed error code
      code: Integer;

      // Amount left in the reserve
      balance: Amount;

      // History of the reserve's activity, in the same format as returned by /reserve/status.
      history: TransactionHistoryItem[]
    }

.. _deposit-par:

-------
Deposit
-------

Deposit operations are requested by a merchant during a transaction. For the
deposit operation, the merchant has to obtain the deposit permission for a coin
from their customer who owns the coin.  When depositing a coin, the merchant is
credited an amount specified in the deposit permission, possibly a fraction of
the total coin's value, minus the deposit fee as specified by the coin's
denomination.


.. _deposit:

.. http:POST:: /coins/$COIN_PUB/deposit

  Deposit the given coin and ask the exchange to transfer the given :ref:`amount`
  to the merchants bank account.  This API is used by the merchant to redeem
  the digital coins.

  The base URL for "/coins/"-requests may differ from the main base URL of the
  exchange. The exchange MUST return a 307 or 308 redirection to the correct
  base URL if this is the case.

  The request should contain a JSON object with the
  following fields:

  **Request:** The request body must be a `DepositRequest` object.

  **Response:**

  :status 200 Ok:
    The operation succeeded, the exchange confirms that no double-spending took
    place.  The response will include a `DepositSuccess` object.
  :status 401 Unauthorized:
    One of the signatures is invalid.
  :status 403 Forbidden:
    The deposit operation has failed because the coin has insufficient
    residual value; the request should not be repeated again with this coin.
    In this case, the response is a `DepositDoubleSpendError`.
  :status 404 Not Found:
    Either the denomination key is not recognized (expired or invalid) or
    the wire type is not recognized.

  **Details:**

  .. ts:def:: DepositRequest

    interface DepositRequest {
      // Amount to be deposited, can be a fraction of the
      // coin's total value.
      f: Amount;

      // The merchant's account details. This must be a JSON object whose format
      // must correspond to one of the supported wire transfer formats of the exchange.
      // See `wireformats`.
      wire: object;

      // SHA-512 hash of the merchant's payment details from ``wire``.  Although
      // strictly speaking redundant, this helps detect inconsistencies.
      h_wire: HashCode;

      // SHA-512 hash of the contact of the merchant with the customer.  Further
      // details are never disclosed to the exchange.
      h_contract_terms: HashCode;

      // Hash of denomination RSA key with which the coin is signed
      denom_pub_hash: HashCode;

      // exchange's unblinded RSA signature of the coin
      ub_sig: RsaSignature;

      // timestamp when the contract was finalized, must match approximately the
      // current time of the exchange; if the timestamp is too far off, the
      // exchange returns "400 Bad Request" with an error code of
      // "TALER_EC_DEPOSIT_INVALID_TIMESTAMP".
      timestamp: Timestamp;

      // indicative time by which the exchange undertakes to transfer the funds to
      // the merchant, in case of successful payment.
      wire_deadline: Timestamp;

      // EdDSA `public key of the merchant <merchant-pub>`, so that the client can identify the
      // merchant for refund requests.
      merchant_pub: EddsaPublicKey;

      // date until which the merchant can issue a refund to the customer via the
      // exchange, possibly zero if refunds are not allowed.
      refund_deadline: Timestamp;

      // Signature of `TALER_DepositRequestPS`, made by the customer with the
      // `coin's private key <coin-priv>`
      coin_sig: EddsaSignature;
    }

  The deposit operation succeeds if the coin is valid for making a deposit and
  has enough residual value that has not already been deposited or melted.

  .. ts:def:: DepositSuccess

     interface DepositSuccess {
      // Optional base URL of the exchange for looking up wire transfers
      // associated with this transaction.  If not given,
      // the base URL is the same as the one used for this request.
      // Can be used if the base URL for /transactions/ differs from that
      // for /coins/, i.e. for load balancing.  Clients SHOULD
      // respect the transaction_base_url if provided.  Any HTTP server
      // belonging to an exchange MUST generate a 307 or 308 redirection
      // to the correct base URL should a client uses the wrong base
      // URL, or if the base URL has changed since the deposit.
      transaction_base_url?: string;

      // the EdDSA signature of `TALER_DepositConfirmationPS` using a current
      // `signing key of the exchange <sign-key-priv>` affirming the successful
      // deposit and that the exchange will transfer the funds after the refund
      // deadline, or as soon as possible if the refund deadline is zero.
      exchange_sig: EddsaSignature;

      // `public EdDSA key of the exchange <sign-key-pub>` that was used to
      // generate the signature.
      // Should match one of the exchange's signing keys from /keys.  It is given
      // explicitly as the client might otherwise be confused by clock skew as to
      // which signing key was used.
      exchange_pub: EddsaPublicKey;
    }

  .. ts:def:: DepositDoubleSpendError

    interface DepositDoubleSpendError {
      // The string constant "insufficient funds"
      hint: string;

      // Transaction history for the coin that is
      // being double-spended
      history: CoinSpendHistoryItem[];
    }

  .. ts:def:: CoinSpendHistoryItem

    interface CoinSpendHistoryItem {
      // Either "DEPOSIT", "MELT", "REFUND", "RECOUP",
      // "OLD-COIN-RECOUP" or "RECOUP-REFRESH"
      type: string;

      // The total amount of the coin's value absorbed (or restored in the
      // case of a refund) by this transaction.
      // Note that for deposit and melt this means the amount given includes
      // the transaction fee, while for refunds the amount given excludes
      // the transaction fee. The current coin value can thus be computed by
      // subtracting deposit and melt amounts and adding refund amounts from
      // the coin's denomination value.
      amount: Amount;

      // Deposit fee in case of type "DEPOSIT".
      deposit_fee: Amount;

      // public key of the merchant, for "DEPOSIT" operations.
      merchant_pub?: EddsaPublicKey;

      // date when the operation was made.
      // Only for "DEPOSIT", "RECOUP", "OLD-COIN-RECOUP" and
      // "RECOUP-REFRESH" operations.
      timestamp?: Timestamp;

      // date until which the merchant can issue a refund to the customer via the
      // exchange, possibly zero if refunds are not allowed. Only for "DEPOSIT" operations.
      refund_deadline?: Timestamp;

      // Signature by the coin, only present if ``type`` is "DEPOSIT" or "MELT"
      coin_sig?: EddsaSignature;

      // Deposit fee in case of type "MELT".
      melt_fee: Amount;

      // Commitment from the melt operation, only for "MELT".
      rc?: TALER_RefreshCommitmentP;

      // Hash of the bank account from where we received the funds,
      // only present if ``type`` is "DEPOSIT"
      h_wire?: HashCode;

      // Deposit fee in case of type "REFUND".
      refund_fee?: Amount;

      // Hash over the proposal data of the contract that
      // is being paid (if type is "DEPOSIT") or refunded (if
      // ``type`` is "REFUND"); otherwise absent.
      h_contract_terms?: HashCode;

      // Refund transaction ID.  Only present if ``type`` is
      // "REFUND"
      rtransaction_id?: Integer;

      // `EdDSA Signature <eddsa-sig>` authorizing the REFUND. Made with
      // the `public key of the merchant <merchant-pub>`.
      // Only present if ``type`` is "REFUND"
      merchant_sig?: EddsaSignature;

      // public key of the reserve that will receive the funds, for "RECOUP" operations.
      reserve_pub?: EddsaPublicKey;

      // Signature by the exchange, only present if ``type`` is "RECOUP",
      // "OLD-COIN-RECOUP" or "RECOUP-REFRESH".  Signature is
      // of type TALER_SIGNATURE_EXCHANGE_CONFIRM_RECOUP for "RECOUP",
      // and of type TALER_SIGNATURE_EXCHANGE_CONFIRM_RECOUP_REFRESH otherwise.
      exchange_sig?: EddsaSignature;

      // public key used to sign ``exchange_sig``,
      // only present if ``exchange_sig`` present.
      exchange_pub?: EddsaPublicKey;

      // Blinding factor of the revoked new coin,
      // only present if ``type`` is "REFRESH_RECOUP".
      new_coin_blinding_secret: RsaBlindingKeySecret;

      // Blinded public key of the revoked new coin,
      // only present if ``type`` is "REFRESH_RECOUP".
      new_coin_ev: RsaBlindingKeySecret;
    }

----------
Refreshing
----------

Refreshing creates ``n`` new coins from ``m`` old coins, where the sum of
denominations of the new coins must be smaller than the sum of the old coins'
denominations plus melting (refresh) and withdrawal fees charged by the exchange.
The refreshing API can be used by wallets to melt partially spent coins, making
transactions with the freshly exchangeed coins unlinkabe to previous transactions
by anyone except the wallet itself.

However, the new coins are linkable from the private keys of all old coins
using the /refresh/link request.  While /refresh/link must be implemented by
the exchange to achieve taxability, wallets do not really ever need that part of
the API during normal operation.

.. _refresh:
.. http:post:: /coins/$COIN_PUB/melt

  "Melts" a coin.  Invalidates the coins and prepares for exchangeing of fresh
  coins.  Taler uses a global parameter ``kappa`` for the cut-and-choose
  component of the protocol, for which this request is the commitment.  Thus,
  various arguments are given ``kappa``-times in this step.  At present ``kappa``
  is always 3.

  The base URL for "/coins/"-requests may differ from the main base URL of the
  exchange. The exchange MUST return a 307 or 308 redirection to the correct
  base URL if this is the case.

  :status 401 Unauthorized:
    One of the signatures is invalid.
  :status 200 OK:
    The request was succesful.  The response body is `MeltResponse` in this case.
  :status 403 Forbidden:
    The operation is not allowed as at least one of the coins has insufficient funds.  The response
    is `MeltForbiddenResponse` in this case.
  :status 404:
    the exchange does not recognize the denomination key as belonging to the exchange,
    or it has expired

  **Details:**


  .. ts:def:: MeltRequest

    interface MeltRequest {

      // Hash of the denomination public key, to determine total coin value.
      denom_pub_hash: HashCode;

      // Signature over the `coin public key <eddsa-coin-pub>` by the denomination.
      denom_sig: RsaSignature;

      // Signature by the `coin <coin-priv>` over the melt commitment.
      confirm_sig: EddsaSignature;

      // Amount of the value of the coin that should be melted as part of
      // this refresh operation, including melting fee.
      value_with_fee: Amount;

      // Melt commitment.  Hash over the various coins to be withdrawn.
      // See also ``TALER_refresh_get_commitment()``
      rc: TALER_RefreshCommitmentP;

    }

  For details about the HKDF used to derive the new coin private keys and
  the blinding factors from ECDHE between the transfer public keys and
  the private key of the melted coin, please refer to the
  implementation in ``libtalerutil``.

  .. ts:def:: MeltResponse

    interface MeltResponse {
      // Which of the ``kappa`` indices does the client not have to reveal.
      noreveal_index: number;

      // Signature of `TALER_RefreshMeltConfirmationPS` whereby the exchange
      // affirms the successful melt and confirming the ``noreveal_index``
      exchange_sig: EddsaSignature;

      // `public EdDSA key <sign-key-pub>` of the exchange that was used to generate the signature.
      // Should match one of the exchange's signing keys from /keys.  Again given
      // explicitly as the client might otherwise be confused by clock skew as to
      // which signing key was used.
      exchange_pub: EddsaPublicKey;

      // Base URL to use for operations on the refresh context
      // (so the reveal operation).  If not given,
      // the base URL is the same as the one used for this request.
      // Can be used if the base URL for /refreshes/ differs from that
      // for /coins/, i.e. for load balancing.  Clients SHOULD
      // respect the refresh_base_url if provided.  Any HTTP server
      // belonging to an exchange MUST generate a 307 or 308 redirection
      // to the correct base URL should a client uses the wrong base
      // URL, or if the base URL has changed since the melt.
      //
      // When melting the same coin twice (technically allowed
      // as the response might have been lost on the network),
      // the exchange may return different values for the refresh_base_url.
      refresh_base_url?: string;

    }


  .. ts:def:: MeltForbiddenResponse

    interface MeltForbiddenResponse {
      // Text describing the error.
      hint: string;

      // Detailed error code
      code: Integer;

      // public key of a melted coin that had insufficient funds
      coin_pub: EddsaPublicKey;

      // original total value of the coin
      original_value: Amount;

      // remaining value of the coin
      residual_value: Amount;

      // amount of the coin's value that was to be melted
      requested_value: Amount;

      // The transaction list of the respective coin that failed to have sufficient funds left.
      // Note that only the transaction history for one bogus coin is given,
      // even if multiple coins would have failed the check.
      history: CoinSpendHistoryItem[];
    }


.. http:post:: /refreshes/$RCH/reveal

  Reveal previously commited values to the exchange, except for the values
  corresponding to the ``noreveal_index`` returned by the /coins/-melt step.

  The $RCH is the hash over the refresh commitment from the /coins/-melt step
  (note that the value is calculated independently by both sides and has never
  appeared *explicitly* in the protocol before).

  The base URL for "/refreshes/"-requests may differ from the main base URL of
  the exchange. Clients SHOULD respect the "refresh_base_url" returned for the
  coin during melt operations. The exchange MUST return a
  307 or 308 redirection to the correct base URL if the client failed to
  respect the "refresh_base_url" or if the allocation has changed.

  Errors such as failing to do proper arithmetic when it comes to calculating
  the total of the coin values and fees are simply reported as bad requests.
  This includes issues such as melting the same coin twice in the same session,
  which is simply not allowed.  However, theoretically it is possible to melt a
  coin twice, as long as the ``value_with_fee`` of the two melting operations is
  not larger than the total remaining value of the coin before the melting
  operations. Nevertheless, this is not really useful.

  :status 200 OK:
    The transfer private keys matched the commitment and the original request was well-formed.
    The response body is a `RevealResponse`
  :status 409 Conflict:
    There is a problem between the original commitment and the revealed private
    keys.  The returned information is proof of the missmatch, and therefore
    rather verbose, as it includes most of the original /refresh/melt request,
    but of course expected to be primarily used for diagnostics.
    The response body is a `RevealConflictResponse`.

  **Details:**

  Request body contains a JSON object with the following fields:

  .. ts:def:: RevealRequest

    interface RevealRequest {

      // Array of ``n`` new hash codes of denomination public keys to order.
      new_denoms_h: HashCode[];

      // Array of ``n`` entries with blinded coins,
      // matching the respective entries in ``new_denoms``.
      coin_evs: CoinEnvelope[];

      // ``kappa - 1`` transfer private keys (ephemeral ECDHE keys)
      transfer_privs: EddsaPrivateKey[];

      // transfer public key at the ``noreveal_index``.
      transfer_pub: EddsaPublicKey;

      // Array of ``n`` signatures made by the wallet using the old coin's private key,
      // used later to verify the /refresh/link response from the exchange.
      // Signs over a `TALER_CoinLinkSignaturePS`
      link_sigs: EddsaSignature[];

    }


  .. ts:def:: RevealResponse

    interface RevealResponse {
      // List of the exchange's blinded RSA signatures on the new coins.
      ev_sigs : BlindedRsaSignature[];
    }


  .. ts:def:: RevealConflictResponse

    interface RevealConflictResponse {
      // Text describing the error
      hint: string;

      // Detailed error code
      code: Integer;

      // Commitment as calculated by the exchange from the revealed data.
      rc_expected: HashCode;

    }


.. http:get:: /coins/$COIN_PUB/link

  Link the old public key of a melted coin to the coin(s) that were exchangeed during the refresh operation.

  **Request:**

  **Response:**

  :status 200 OK:
    All commitments were revealed successfully.  The exchange returns an array,
    typically consisting of only one element, in which each each element contains
    information about a melting session that the coin was used in.
  :status 404 Not Found:
    The exchange has no linkage data for the given public key, as the coin has not
    yet been involved in a refresh operation.

  **Details:**

  .. ts:def:: LinkResponse

    interface LinkResponse {
      // transfer ECDHE public key corresponding to the ``coin_pub``, used to
      // compute the blinding factor and private key of the fresh coins.
      transfer_pub: EcdhePublicKey;

      // array with (encrypted/blinded) information for each of the coins
      // exchangeed in the refresh operation.
      new_coins: NewCoinInfo[];
    }

  .. ts:def:: NewCoinInfo

    interface NewCoinInfo {
      // RSA public key of the exchangeed coin.
      denom_pub: RsaPublicKey;

      // Exchange's blinded signature over the fresh coin.
      ev_sig: BlindedRsaSignature;

      // Blinded coin.
      coin_ev : CoinEnvelope;

      // Signature made by the old coin over the refresh request.
      // Signs over a `TALER_CoinLinkSignaturePS`
      link_sig: EddsaSignature;
    }


------
Recoup
------

This API is only used if the exchange is either about to go out of
business or has had its private signing keys compromised (so in
either case, the protocol is only used in **abnormal**
situations).  In the above cases, the exchange signals to the
wallets that the emergency cash back protocol has been activated
by putting the affected denomination keys into the cash-back
part of the /keys response.  If and only if this has happened,
coins that were signed with those denomination keys can be cashed
in using this API.

.. http:post:: /coins/$COIN_PUB/recoup

  Demand that a coin be refunded via wire transfer to the original owner.

  The base URL for "/coins/"-requests may differ from the main base URL of the
  exchange. The exchange MUST return a 307 or 308 redirection to the correct
  base URL if this is the case.

  Depending whether ``$COIN_PUB`` is a withdrawn coin or a refreshed coin,
  the remaining amount on the coin will be credited either on the reserve or
  the old coin that ``$COIN_PUB`` was withdrawn/refreshed from.

  Note that the original withdrawal/refresh fees will **not** be recouped.


  **Request:** The request body must be a `RecoupRequest` object.

  **Response:**

  :status 200 OK:
    The request was succesful, and the response is a `RecoupConfirmation`.
    Note that repeating exactly the same request
    will again yield the same response, so if the network goes down during the
    transaction or before the client can commit the coin signature to disk, the
    coin is not lost.
  :status 401 Unauthorized:
    The coin's signature is invalid.
  :status 403 Forbidden:
    The coin was already used for payment.
    The response is a `DepositDoubleSpendError`.
  :status 404 Not Found:
    The denomination key is not in the set of denomination
    keys where emergency pay back is enabled, or the blinded
    coin is not known to have been withdrawn.

  **Details:**

  .. ts:def:: RecoupRequest

    interface RecoupRequest {
      // Hash of denomination public key (RSA), specifying the type of coin the client
      // would like the exchange to pay back.
      denom_pub_hash: HashCode;

      // Signature over the `coin public key <eddsa-coin-pub>` by the denomination.
      denom_sig: RsaSignature;

      // coin's blinding factor
      coin_blind_key_secret: RsaBlindingKeySecret;

      // Signature of `TALER_RecoupRequestPS` created with the `coin's private key <coin-priv>`
      coin_sig: EddsaSignature;

      // Was the coin refreshed (and thus the recoup should go to the old coin)?
      // While this information is technically redundant, it helps the exchange
      // to respond faster.
      // *Optional* (for backwards compatibility); if absent, "false" is assumed
      refreshed?: boolean;
    }


  .. ts:def:: RecoupConfirmation

    type RecoupConfirmation = | RecoupRefreshConfirmation
                              | RecoupWithdrawalConfirmation;

  .. ts:def:: RecoupWithdrawalConfirmation

    interface RecoupWithdrawalConfirmation {
      // Tag to distinguish the RecoupConfirmation response type
      refreshed: false;
      // public key of the reserve that will receive the recoup,
      reserve_pub: EddsaPublicKey;
    }

  .. ts:def:: RecoupRefreshConfirmation

    interface RecoupRefreshConfirmation {
      // Tag to distinguish the RecoupConfirmation response type
      refreshed: true;
      // public key of the old coin that will receive the recoup
      old_coin_pub: EddsaPublicKey;
    }


-----------------------
Tracking wire transfers
-----------------------

This API is used by merchants that need to find out which wire
transfers (from the exchange to the merchant) correspond to which deposit
operations.  Typically, a merchant will receive a wire transfer with a
**wire transfer identifier** and want to know the set of deposit
operations that correspond to this wire transfer.  This is the
preferred query that merchants should make for each wire transfer they
receive.  If a merchant needs to investigate a specific deposit
operation (i.e. because it seems that it was not paid), then the
merchant can also request the wire transfer identifier for a deposit
operation.

Sufficient information is returned to verify that the coin signatures
are correct. This also allows governments to use this API when doing
a tax audit on merchants.

Naturally, the returned information may be sensitive for the merchant.
We do not require the merchant to sign the request, as the same requests
may also be performed by the government auditing a merchant.
However, wire transfer identifiers should have sufficient entropy to
ensure that obtaining a successful reply by brute-force is not practical.
Nevertheless, the merchant should protect the wire transfer identifiers
from his bank statements against unauthorized access, least his income
situation is revealed to an adversary. (This is not a major issue, as
an adversary that has access to the line-items of bank statements can
typically also view the balance.)


.. http:get:: /transfers/$WTID

  Provides deposits associated with a given wire transfer.  The
  wire transfer identifier (WTID) and the base URL for tracking
  the wire transfer are both given in the wire transfer subject.

  **Request:**

  **Response:**

  :status 200 OK:
    The wire transfer is known to the exchange, details about it follow in the body.
    The body of the response is a `TrackTransferResponse`.
  :status 404 Not Found:
    The wire transfer identifier is unknown to the exchange.

  .. ts:def:: TrackTransferResponse

    interface TrackTransferResponse {
      // Total amount transferred
      total: Amount;

      // Applicable wire fee that was charged
      wire_fee: Amount;

      // public key of the merchant (identical for all deposits)
      merchant_pub: EddsaPublicKey;

      // hash of the wire details (identical for all deposits)
      h_wire: HashCode;

      // Time of the execution of the wire transfer by the exchange
      execution_time: Timestamp;

      // details about the deposits
      deposits: TrackTransferDetail[];

      // signature from the exchange made with purpose
      // `TALER_SIGNATURE_EXCHANGE_CONFIRM_WIRE_DEPOSIT`
      exchange_sig: EddsaSignature;

      // public EdDSA key of the exchange that was used to generate the signature.
      // Should match one of the exchange's signing keys from /keys.  Again given
      // explicitly as the client might otherwise be confused by clock skew as to
      // which signing key was used.
      exchange_pub: EddsaSignature;
    }

  .. ts:def:: TrackTransferDetail

    interface TrackTransferDetail {
      // SHA-512 hash of the contact of the merchant with the customer.
      h_contract_terms: HashCode;

      // coin's public key, both ECDHE and EdDSA.
      coin_pub: CoinPublicKey;

      // The total amount the original deposit was worth.
      deposit_value: Amount;

      // applicable fees for the deposit
      deposit_fee: Amount;

    }

.. http:get:: /deposits/$H_WIRE/$MERCHANT_PUB/$H_CONTRACT_TERMS/$COIN_PUB

  Provide the wire transfer identifier associated with an (existing) deposit operation.
  The arguments are the hash of the merchant's payment details (H_WIRE), the
  merchant's public key (EdDSA), the hash of the contract terms that were paid
  (H_CONTRACT_TERMS) and the public key of the coin used for the payment (COIN_PUB).

  **Request:**

  :query merchant_sig: EdDSA signature of the merchant made with purpose ``TALER_SIGNATURE_MERCHANT_TRACK_TRANSACTION`` , affirming that it is really the merchant who requires obtaining the wire transfer identifier.

  **Response:**

  :status 200 OK:
    The deposit has been executed by the exchange and we have a wire transfer identifier.
    The response body is a `TrackTransactionResponse` object.
  :status 202 Accepted:
    The deposit request has been accepted for processing, but was not yet
    executed.  Hence the exchange does not yet have a wire transfer identifier.  The
    merchant should come back later and ask again.
    The response body is a `TrackTransactionAcceptedResponse`.
  :status 401 Unauthorized: The signature is invalid.
  :status 404 Not Found: The deposit operation is unknown to the exchange

  **Details:**

  .. ts:def:: TrackTransactionResponse

    interface TrackTransactionResponse {
      // raw wire transfer identifier of the deposit.
      wtid: Base32;

      // when was the wire transfer given to the bank.
      execution_time: Timestamp;

      // The contribution of this coin to the total (without fees)
      coin_contribution: Amount;

      // Total amount transferred
      total_amount: Amount;

      // binary-only Signature_ for purpose `TALER_SIGNATURE_EXCHANGE_CONFIRM_WIRE`
      // whereby the exchange affirms the successful wire transfer.
      exchange_sig: EddsaSignature;

      // public EdDSA key of the exchange that was used to generate the signature.
      // Should match one of the exchange's signing keys from /keys.  Again given
      // explicitly as the client might otherwise be confused by clock skew as to
      // which signing key was used.
      exchange_pub: EddsaPublicKey;
    }

  .. ts:def:: TrackTransactionAcceptedResponse

    interface TrackTransactionAcceptedResponse {
      // time by which the exchange currently thinks the deposit will be executed.
      execution_time: Timestamp;
    }


-------
Refunds
-------

.. _refund:
.. http:POST:: /coins/$COIN_PUB/refund

  Undo deposit of the given coin, restoring its value.

  **Request:** The request body must be a `RefundRequest` object.

  **Response:**

  :status 200 Ok:
    The operation succeeded, the exchange confirms that the coin can now be refreshed.  The response will include a `RefundSuccess` object.
  :status 401 Unauthorized:
    Merchant signature is invalid.
  :status 404 Not found:
    The refund operation failed as we could not find a matching deposit operation (coin, contract, transaction ID and merchant public key must all match).
  :status 410 Gone:
    It is too late for a refund by the exchange, the money was already sent to the merchant.

  **Details:**

  .. ts:def:: RefundRequest

     interface RefundRequest {

      // Amount to be refunded, can be a fraction of the
      // coin's total deposit value (including deposit fee);
      // must be larger than the refund fee.
      refund_amount: Amount;

      // Refund fee associated with the given coin.
      // must be smaller than the refund amount.
      refund_fee: Amount;

      // SHA-512 hash of the contact of the merchant with the customer.
      h_contract_terms: HashCode;

      // 64-bit transaction id of the refund transaction between merchant and customer
      rtransaction_id: number;

      // EdDSA public key of the merchant.
      merchant_pub: EddsaPublicKey;

      // EdDSA signature of the merchant affirming the refund.
      merchant_sig: EddsaPublicKey;

    }

  .. ts:def:: RefundSuccess

    interface RefundSuccess {

      // the EdDSA :ref:`signature` (binary-only) with purpose
      // `TALER_SIGNATURE_EXCHANGE_CONFIRM_REFUND` using a current signing key of the
      // exchange affirming the successful refund
      exchange_sig: EddsaSignature;

      // public EdDSA key of the exchange that was used to generate the signature.
      // Should match one of the exchange's signing keys from /keys.  It is given
      // explicitly as the client might otherwise be confused by clock skew as to
      // which signing key was used.
      exchange_pub: EddsaPublicKey;
   }