Merchant API

Before reading the API reference documentation, see the merchant architecture and the payment protocol

The Frontend HTTP API

The merchant frontend API described here describes the minimal set of HTTP requests that a web shop needs to understand in order to support Taler payments. The names proposal_url, pay_url and fulfillment_url are placeholders for the actual URLs that the merchant frontend uses.

Please refer to the glossary for terms like order, proposal, contract, and others.

GET proposal_url

Requesting this URL generates a proposal. Note that the wallet will get properly triggered by the merchant in order to issue this GET request. The merchant will also instruct the wallet whether or not to provide the optional nonce parameter. Payment protocol explains how the wallet is triggered to fetch the proposal.


Query Parameters:
  • nonce – Any string value. This value will be included in the proposal, so that when the wallet receives the proposal it can easily check whether it was the genuine receiver of the proposal it got. This value is needed to avoid proposals’ replications.


Status Codes:
  • 200 OK – The request was successful. The body contains a proposal.
  • 400 Bad Request – Request not understood.
  • 500 Internal Server Error – In most cases, some error occurred while the backend was generating the proposal. For example, it failed to store it into its database.
POST pay_url

Send the deposit permission to the merchant. The client should POST a DepositPermission object. If the payment was processed successfully by the merchant, this URL will set session state that allows the fulfillment URL to show the final product.

interface DepositPermission {
  // a free-form identifier identifying the order that is being payed for
  order_id: string;

  // Public key of the merchant.  Used to identify the merchant instance.
  merchant_pub: EddsaSignature;

  // the chosen exchange's base URL
  exchange: string;

  // the coins used to sign the proposal
  coins: DepositedCoin[];
interface DepositedCoin {
  // the amount this coin is paying for
  amount: Amount;

  // coin's public key
  coin_pub: RsaPublicKey;

  // denomination key
  denom_pub: RsaPublicKey;

  // exchange's signature over this coin's public key
  ub_sig: RsaSignature;

  // Signature of TALER_DepositRequestPS
  coin_sig: EddsaSignature;

Success Response:

Status Codes:
  • 301 Redirection – the merchant should redirect the client to his fullfillment page, where the good outcome of the purchase must be shown to the user.

Failure Responses:

The error codes and data sent to the wallet are a mere copy of those gotten from the exchange when attempting to pay. The section about deposit explains them in detail.

POST fulfillment_url

URL that shows the product after it has been purchased. Going to the a fulfillment URL before the payment was completed must trigger the payment process.

For products that are intended to be purchased only once (such as online news articles), the fulfillment URL should map one-to-one to an article, so that when the user visits the page after they cleared their cookies, the purchase can be replayed.

For purchases that can be repeated, the fulfillment URL should map one-to-one to a proposal, e.g. by including the order id.

Following these rules allows sharing of links and bookmarking to work correctly, and produces nicely looking semantic URLs.


By “replaying” a payment, we mean that the user reuses the same coins he used the first time he/she bought those items, thus not spending new coins (and therefore not spending additional money).

The Merchant Backend HTTP API

The following API are made available by the merchant’s backend to the merchant’s frontend.

POST /proposal

Generate a new proposal, based on the order given in the request. This request is idempotent.


The backend expects an order as input. The order is a `ProposalData`_ object without the fields:

  • exchanges
  • auditors
  • H_wire
  • merchant_pub
  • timestamp

The following fields from `ProposalData`_ are optional and will be filled in by the backend if not present:

  • merchant.instance (default instance will be used)
  • order_id (random alphanumeric identifier will be used)
  • refund_deadline (instance’s default will be used)
  • pay_deadline (instance’s default will be used)


status 200 OK:The backend has successfully created the proposal. It responds with a proposal. On success, the frontend should pass this response verbatim to the wallet.
status 403 Forbidden:
 The frontend used the same order ID with different content in the order.
POST /pay

Asks the backend to execute the transaction with the exchange and deposit the coins.


The frontend passes the deposit permission received from the wallet, and optionally adds a field named wire_transfer_deadline, indicating a deadline by which he would expect to receive the bank transfer for this deal. Note that the wire_transfer_deadline must be after the refund_deadline. The backend calculates the wire_transfer_deadline by adding the wire_transfer_delay value found in the configuration to the current time.


Status Codes:
  • 200 OK – The exchange accepted all of the coins. The body is a PaymentResponse. The frontend should now fullfill the contract.
  • 412 Precondition Failed – The given exchange is not acceptable for this merchant, as it is not in the list of accepted exchanges and not audited by an approved auditor.
  • 403 Forbidden – The exchange rejected the payment because a coin was already spent before. The response will include the coin_pub for which the payment failed, in addition to the response from the exchange to the /deposit request.

The backend will return verbatim the error codes received from the exchange’s deposit API. If the wallet made a mistake, like by double-spending for example, the frontend should pass the reply verbatim to the browser/wallet. This should be the expected case, as the frontend cannot really make mistakes; the only reasonable exception is if the backend is unavailable, in which case the customer might appreciate some reassurance that the merchant is working on getting his systems back online.

interface PaymentResponse {
  // Signature on TALER_PaymentResponsePS with the public
  // key of the instance in the proposal.
  sig: EddsaSignature;

  // Proposal data hash being signed over
  h_proposal_data: HashCode;

  // Proposal, send for convenience so the frontend
  // can do order processing without a second lookup on
  // a successful payment
  proposal: Proposal;
GET /track/transfer

Provides deposits associated with a given wire transfer.


Query Parameters:
  • wtid – raw wire transfer identifier identifying the wire transfer (a base32-encoded value)
  • exchange – base URI of the exchange that made the wire transfer
  • instance – (optional) identificative token of the merchant instance which is being tracked.


Status Codes:
  • 200 OK – The wire transfer is known to the exchange, details about it follow in the body. The body of the response is a MerchantTrackTransferResponse. Note that the similarity to the response given by the exchange for a /track/transfer is completely intended.
  • 404 Not Found – The wire transfer identifier is unknown to the exchange.
  • 424 Failed Dependency – The exchange provided conflicting information about the transfer. Namely, there is at least one deposit among the deposits aggregated by wtid that accounts for a coin whose details don’t match the details stored in merchant’s database about the same keyed coin. The response body contains the TrackTransferConflictDetails.
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_sums: TrackTransferDetail[];

  // signature from the exchange made with purpose
  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;
interface TrackTransferDetail {
  // Business activity associated with the wire tranfered amount
  // `deposit_value`.
  order_id: string;

  // The total amount the exchange paid back for `order_id`.
  deposit_value: Amount;

  // applicable fees for the deposit
  deposit_fee: Amount;


interface TrackTransferConflictDetails {
  // Numerical error code
  code: number;

  // Text describing the issue for humans.
  hint: String;

  // A /deposit response matching `coin_pub` showing that the
  // exchange accepted `coin_pub` for `amount_with_fee`.
  exchange_deposit_proof: DepositSuccess;

  // Offset in the `exchange_transfer_proof` where the
  // exchange's response fails to match the `exchange_deposit_proof`.
  conflict_offset: number;

  // The response from the exchange which tells us when the
  // coin was returned to us, except that it does not match
  // the expected value of the coin.
  exchange_transfer_proof: TrackTransferResponse;

  // Public key of the coin for which we have conflicting information.
  coin_pub: EddsaPublicKey;

  // Merchant transaction in which `coin_pub` was involved for which
  // we have conflicting information.
  transaction_id: number;

  // Expected value of the coin.
  amount_with_fee: Amount;

  // Expected deposit fee of the coin.
  deposit_fee: Amount;

GET /track/order

Provide the wire transfer identifier associated with an (existing) deposit operation.


Query Parameters:
  • id – ID of the transaction we want to trace (an integer)
  • instance – identificative token for the merchant instance which is to be tracked (optional). See This information is needed because the request has to be signed by the merchant, thus we need to pick the instance’s private key.


Status Codes:
  • 200 OK
    The deposit has been executed by the exchange and we have a wire transfer identifier.
    The response body is a JSON array of TransactionWireTransfer objects.
  • 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. Note that the similarity to the response given by the exchange for a /track/order is completely intended.
  • 404 Not Found – The transaction is unknown to the backend.
  • 424 Failed Dependency – The exchange previously claimed that a deposit was not included in a wire transfer, and now claims that it is. This means that the exchange is dishonest. The response contains the cryptographic proof that the exchange is misbehaving in the form of a TransactionConflictProof.


interface TransactionWireTransfer {

  // Responsible exchange
  exchange_uri: string;

  // 32-byte wire transfer identifier
  wtid: Base32;

  // execution time of the wire transfer
  execution_time: Timestamp;

  // Total amount that has been wire transfered
  // to the merchant
  amount: Amount;
interface CoinWireTransfer {
  // public key of the coin that was deposited
  coin_pub: EddsaPublicKey;

  // Amount the coin was worth (including deposit fee)
  amount_with_fee: Amount;

  // Deposit fee retained by the exchange for the coin
  deposit_fee: Amount;
interface TransactionConflictProof {
  // Numerical error code
  code: number;

  // Human-readable error description
  hint: string;

  // A claim by the exchange about the transactions associated
  // with a given wire transfer; it does not list the
  // transaction that `transaction_tracking_claim` says is part
  // of the aggregate.  This is
  // a `/track/transfer` response from the exchange.
  wtid_tracking_claim: TrackTransferResponse;

  // The current claim by the exchange that the given
  // transaction is included in the above WTID.
  // (A response from `/track/order`).
  transaction_tracking_claim: TrackTransactionResponse;

  // Public key of the coin for which we got conflicting information.
  coin_pub: CoinPublicKey;

GET /contract/lookup

Retrieve a proposal, given its transaction ID.


Query Parameters:
  • transaction_id – transaction ID of the proposal to retrieve.


Status Codes:
GET /history

Returns transactions up to some point in the past


Query Parameters:
  • date – only transactions older than this parameter will be returned. It’s a timestamp, given in seconds. Being optional, it defaults to the current time if not given.
  • start – only transactions having row_id less than start will be returned. Being optional, it defaults to the highest row_id contained in the DB (namely, the youngest entry).
  • delta – at most delta entries will be returned. Being optional, it defaults to 20.
  • instance – on behalf of which merchant instance the query should be accomplished.

A typical usage is to firstly call this API without start and date parameter, then fetch the oldest row_id from the results, and then keep calling the API by using the oldest row ID as start parameter. This way we simply “scroll” results from the youngest to the oldest, delta entries at time.


Status Codes:
  • 200 OK – The response is a JSON array of TransactionHistory. The array is sorted such that entry i is younger than entry i+1.
interface TransactionHistory {
  // The serial number this entry has in the merchant's DB.
  row_id: number;

  // order ID of the transaction related to this entry.
  order_id: string;

  // Transaction's timestamp
  timestamp: Timestamp;

  // Total amount associated to this transaction.
  amount: Amount;

The proposal

The proposal is obtained by filling some missing information in the order, and then by signing it. See below.

interface Proposal {
  // The proposal data, effectively the frontend's order with some data filled in
  // by the merchant backend.
  data: ProposalData;

  // Contract's hash, provided as a convenience.  All components that do
  // not fully trust the merchant must verify this field.
  H_proposal: HashCode;

  // Signature over the hashcode of `proposal` made by the merchant.
  merchant_sig: EddsaSignature;


When the proposal is signed by the merchant or the wallet, the signature is made over the hash of the JSON text, as the proposal may be confidential between merchant and customer and should not be exposed to the exchange. The hashcode is generated by hashing the encoding of the proposal’s JSON obtained by using the flags JSON_COMPACT | JSON_PRESERVE_ORDER, as described in the libjansson documentation.

The proposal data must have the following structure:

interface ProposalData {
  // Human-readable description of the whole purchase
  // NOTE: still not implemented
  summary: string;

  // Unique, free-form identifier for the proposal.
  // Must be unique within a merchant instance.
  // For merchants that do not store proposals in their DB
  // before the customer paid for them, the order_id can be used
  // by the frontend to restore a proposal from the information
  // encoded in it (such as a short product identifier and timestamp).
  order_id: string;

  // Total price for the transaction.
  // The exchange will subtract deposit fees from that amount
  // before transfering it to the merchant.
  amount: Amount;

  // The URL where the wallet has to send coins.
  pay_url: string;

  // The URI for this purchase.  Every time is is visited, the merchant
  // will send back to the customer the same proposal.  Clearly, this URL
  // can be bookmarked and shared by users.
  fulfillment_url: string;

  // Maximum total deposit fee accepted by the merchant for this contract
  max_fee: Amount;

  // Maximum wire fee accepted by the merchant (customer share to be
  // divided by the 'wire_fee_amortization' factor, and further reduced
  // if deposit fees are below 'max_fee').  Default if missing is zero.
  max_wire_fee: Amount;

  // Over how many customer transactions does the merchant expect to
  // amortize wire fees on average?  If the exchange's wire fee is
  // above 'max_wire_fee', the difference is divided by this number
  // to compute the expected customer's contribution to the wire fee.
  // The customer's contribution may further be reduced by the difference
  // between the 'max_fee' and the sum of the actual deposit fees.
  // Optional, default value if missing is 1.  0 and negative values are
  // invalid and also interpreted as 1.
  wire_fee_amortization: Integer;

  // List of products that are part of the purchase (see below)
  products: Product[];

  // Time when this contract was generated
  timestamp: Timestamp;

  // After this deadline has passed, no refunds will be accepted.
  refund_deadline: Timestamp;

  // After this deadline, the merchant won't accept payments for the contact
  pay_deadline: Timestamp;

  // Merchant's public key used to sign this proposal; this information
  // is typically added by the backend Note that this can be an ephemeral key.
  merchant_pub: EddsaPublicKey;

  // More info about the merchant, see below
  merchant: Merchant;

  // The hash of the merchant instance's wire details.
  H_wire: HashCode;

  // Wire transfer method identifier for the wire method associated with H_wire.
  // The wallet may only select exchanges via a matching auditor if the
  // exchange also supports this wire method.
  // The wire transfer fees must be added based on this wire transfer method.
  wire_method: string;

  // Any exchanges audited by these auditors are accepted by the merchant.
  auditors: Auditor[];

  // Exchanges that the merchant accepts even if it does not accept any auditors that audit them.
  exchanges: Exchange[];

  // Map from labels to locations
  locations: { [label: string]: [location: Location], ... };

  // Nonce generated by the wallet and echoed by the merchant
  // in this field when the proposal is generated.
  nonce: string;

  // Extra data that is only interpreted by the merchant frontend.
  // Useful when the merchant needs to store extra information on a
  // contract without storing it separately in their database.
  extra?: any;

The wallet must select a exchange that either the mechant accepts directly by listing it in the exchanges arry, or for which the merchant accepts an auditor that audits that exchange by listing it in the auditors array.

The product object describes the product being purchased from the merchant. It has the following structure:

interface Product {
  // Human-readable product description.
  description: string;

  // The quantity of the product to deliver to the customer (optional, if applicable)
  quantity?: number;

  // The price of the product; this is the total price for the amount specified by `quantity`
  price: Amount;

  // merchant's 53-bit internal identification number for the product (optional)
  product_id?: number;

  // a list of objects indicating a `taxname` and its amount. Again, italics denotes the object field's name.
  taxes?: any[];

  // time indicating when this product should be delivered
  delivery_date: Timestamp;

  // where to deliver this product. This may be an URI for online delivery
  // (i.e. `` or ``),
  // or a location label defined inside the proposition's `locations`.
  // The presence of a colon (`:`) indicates the use of an URL.
  delivery_location: string;
interface Merchant {
  // label for a location with the business address of the merchant
  address: string;

  // the merchant's legal name of business
  name: string;

  // label for a location that denotes the jurisdiction for disputes.
  // Some of the typical fields for a location (such as a street address) may be absent.
  jurisdiction: string;

  // Which instance is working this proposal.
  // See `Merchant Instances <>`_.
  // This field is optional, as the "default" instance is not forced to provide any
  // `instance` identificator.
  instance: string;
interface Location {
  country?: string;
  city?: string;
  state?: string;
  region?: string;
  province?: string;
  zip_code?: string;
  street?: string;
  street_number?: string;
interface Auditor {
  // official name
  name: string;

  // Auditor's public key
  auditor_pub: EddsaPublicKey;

  // Base URL of the auditor
  url: string;
interface Exchange {
  // the exchange's base URL
  url: string;

  // master public key of the exchange
  master_pub: EddsaPublicKey;