update pay logic

4 files changed, 1356 insertions, 694 deletions

diff --git a/src/include/taler_merchant_service.h b/src/include/taler_merchant_service.h
index 39509de9..d5edcff5 100644
--- a/src/include/taler_merchant_service.h
+++ b/src/include/taler_merchant_service.h
@@ -1508,8 +1508,8 @@ struct TALER_MERCHANT_PaidCoin
  *
  * @param ctx execution context
  * @param merchant_url base URL of the merchant
- * @param merchant_pub public key of the merchant
  * @param order_id which order should be paid
+ * @param session_id session to pay for, or NULL for none
  * @param num_coins length of the @a coins array
  * @param coins array of coins to pay with
  * @param pay_cb the callback to call when a reply for this request is available
@@ -1520,8 +1520,8 @@ struct TALER_MERCHANT_OrderPayHandle *
 TALER_MERCHANT_order_pay_frontend (
   struct GNUNET_CURL_Context *ctx,
   const char *merchant_url,
-  const struct TALER_MerchantPublicKeyP *merchant_pub,
   const char *order_id,
+  const char *session_id,
   unsigned int num_coins,
   const struct TALER_MERCHANT_PaidCoin coins[],
   TALER_MERCHANT_OrderPayCallback pay_cb,
@@ -1584,6 +1584,7 @@ struct TALER_MERCHANT_PayCoin
  *
  * @param ctx execution context
  * @param merchant_url base URL of the merchant
+ * @param session_id session to pay for, or NULL for none
  * @param h_wire hash of the merchant’s account details
  * @param h_contract hash of the contact of the merchant with the customer
  * @param transaction_id transaction id for the transaction between merchant and customer
@@ -1602,23 +1603,23 @@ struct TALER_MERCHANT_PayCoin
  * @return a handle for this request
  */
 struct TALER_MERCHANT_OrderPayHandle *
-TALER_MERCHANT_order_pay (
-  struct GNUNET_CURL_Context *ctx,
-  const char *merchant_url,
-  const struct GNUNET_HashCode *h_contract,
-  const struct TALER_Amount *amount,
-  const struct TALER_Amount *max_fee,
-  const struct TALER_MerchantPublicKeyP *merchant_pub,
-  const struct TALER_MerchantSignatureP *merchant_sig,
-  struct GNUNET_TIME_Absolute timestamp,
-  struct GNUNET_TIME_Absolute refund_deadline,
-  struct GNUNET_TIME_Absolute pay_deadline,
-  const struct GNUNET_HashCode *h_wire,
-  const char *order_id,
-  unsigned int num_coins,
-  const struct TALER_MERCHANT_PayCoin coins[],
-  TALER_MERCHANT_OrderPayCallback pay_cb,
-  void *pay_cb_cls);
+TALER_MERCHANT_order_pay (struct GNUNET_CURL_Context *ctx,
+                          const char *merchant_url,
+                          const char *session_id,
+                          const struct GNUNET_HashCode *h_contract,
+                          const struct TALER_Amount *amount,
+                          const struct TALER_Amount *max_fee,
+                          const struct TALER_MerchantPublicKeyP *merchant_pub,
+                          const struct TALER_MerchantSignatureP *merchant_sig,
+                          struct GNUNET_TIME_Absolute timestamp,
+                          struct GNUNET_TIME_Absolute refund_deadline,
+                          struct GNUNET_TIME_Absolute pay_deadline,
+                          const struct GNUNET_HashCode *h_wire,
+                          const char *order_id,
+                          unsigned int num_coins,
+                          const struct TALER_MERCHANT_PayCoin coins[],
+                          TALER_MERCHANT_OrderPayCallback pay_cb,
+                          void *pay_cb_cls);
 
 
 /**
diff --git a/src/lib/Makefile.am b/src/lib/Makefile.am
index b4d9386d..02e8b229 100644
--- a/src/lib/Makefile.am
+++ b/src/lib/Makefile.am
@@ -30,6 +30,7 @@ libtalermerchant_la_SOURCES = \
   merchant_api_post_products.c \
   merchant_api_post_orders.c \
   merchant_api_post_order_claim.c \
+  merchant_api_post_order_pay.c \
  \
   merchant_api_check_payment.c \
   merchant_api_history.c \
diff --git a/src/lib/merchant_api_post_order_abort.c b/src/lib/merchant_api_post_order_abort.c
new file mode 100644
index 00000000..f860f3a8
--- /dev/null
+++ b/src/lib/merchant_api_post_order_abort.c
@@ -0,0 +1,1067 @@
+/*
+  This file is part of TALER
+  Copyright (C) 2014, 2015, 2016, 2017, 2020 Taler Systems SA
+
+  TALER is free software; you can redistribute it and/or modify
+  it under the terms of the GNU Lesser 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.LGPL.
+  If not, see <http://www.gnu.org/licenses/>
+*/
+/**
+ * @file lib/merchant_api_pay.c
+ * @brief Implementation of the /pay request
+ *        of the merchant's HTTP API
+ * @author Christian Grothoff
+ * @author Marcello Stanisci
+ */
+#include "platform.h"
+#include <curl/curl.h>
+#include <jansson.h>
+#include <microhttpd.h> /* just for HTTP status codes */
+#include <gnunet/gnunet_util_lib.h>
+#include <gnunet/gnunet_curl_lib.h>
+#include "taler_merchant_service.h"
+#include <taler/taler_json_lib.h>
+#include <taler/taler_signatures.h>
+#include <taler/taler_exchange_service.h>
+#include <taler/taler_curl_lib.h>
+
+
+/**
+ * @brief A Pay Handle
+ */
+struct TALER_MERCHANT_Pay
+{
+
+  /**
+   * The url for this request.
+   */
+  char *url;
+
+  /**
+   * Handle for the request.
+   */
+  struct GNUNET_CURL_Job *job;
+
+  /**
+   * Function to call with the result in "pay" @e mode.
+   */
+  TALER_MERCHANT_OrderPayCallback pay_cb;
+
+  /**
+   * Closure for @a pay_cb.
+   */
+  void *pay_cb_cls;
+
+  /**
+   * Function to call with the result in "abort-refund" @e mode.
+   */
+  TALER_MERCHANT_PayRefundCallback abort_cb;
+
+  /**
+   * Closure for @a abort_cb.
+   */
+  void *abort_cb_cls;
+
+  /**
+   * Operational mode, either "pay" or "abort-refund".
+   */
+  const char *mode;
+
+  /**
+   * Reference to the execution context.
+   */
+  struct GNUNET_CURL_Context *ctx;
+
+  /**
+   * Minor context that holds body and headers.
+   */
+  struct TALER_CURL_PostContext post_ctx;
+
+  /**
+   * The coins we are paying with.
+   */
+  struct TALER_MERCHANT_PaidCoin *coins;
+
+  /**
+   * Number of @e coins we are paying with.
+   */
+  unsigned int num_coins;
+
+  /**
+   * Hash of the contract, only available in "abort-refund" mode.
+   */
+  struct GNUNET_HashCode h_contract_terms;
+
+};
+
+
+/**
+ * Check that the response for a /pay refund is well-formed,
+ * and call the application callback with the result if it is
+ * OK. Otherwise returns #GNUNET_SYSERR.
+ *
+ * @param ph handle to operation that created the reply
+ * @param json the reply to parse
+ * @return #GNUNET_OK on success
+ */
+static int
+check_abort_refund (struct TALER_MERCHANT_Pay *ph,
+                    const json_t *json)
+{
+  json_t *refunds;
+  unsigned int num_refunds;
+  struct TALER_MerchantPublicKeyP merchant_pub;
+  struct GNUNET_JSON_Specification spec[] = {
+    GNUNET_JSON_spec_json ("refund_permissions", &refunds),
+    GNUNET_JSON_spec_fixed_auto ("merchant_pub", &merchant_pub),
+    GNUNET_JSON_spec_end ()
+  };
+
+  if (GNUNET_OK !=
+      GNUNET_JSON_parse (json,
+                         spec,
+                         NULL, NULL))
+  {
+    GNUNET_break_op (0);
+    return GNUNET_SYSERR;
+  }
+  num_refunds = json_array_size (refunds);
+  {
+    struct TALER_MERCHANT_RefundEntry res[GNUNET_NZL (num_refunds)];
+
+    for (unsigned int i = 0; i<num_refunds; i++)
+    {
+      struct TALER_MerchantSignatureP *sig = &res[i].merchant_sig;
+      json_t *refund = json_array_get (refunds, i);
+      struct GNUNET_JSON_Specification spec_detail[] = {
+        GNUNET_JSON_spec_fixed_auto ("merchant_sig",
+                                     sig),
+        GNUNET_JSON_spec_fixed_auto ("coin_pub",
+                                     &res[i].coin_pub),
+        GNUNET_JSON_spec_uint64 ("rtransaction_id",
+                                 &res[i].rtransaction_id),
+        GNUNET_JSON_spec_end ()
+      };
+      int found;
+
+      if (GNUNET_OK !=
+          GNUNET_JSON_parse (refund,
+                             spec_detail,
+                             NULL, NULL))
+      {
+        GNUNET_break_op (0);
+        GNUNET_JSON_parse_free (spec);
+        return GNUNET_SYSERR;
+      }
+
+      found = -1;
+      for (unsigned int j = 0; j<ph->num_coins; j++)
+      {
+        if (0 == memcmp (&ph->coins[j].coin_pub,
+                         &res[i].coin_pub,
+                         sizeof
+                         (struct TALER_CoinSpendPublicKeyP)))
+        {
+          found = j;
+          break;
+        }
+      }
+      if (-1 == found)
+      {
+        GNUNET_break_op (0);
+        GNUNET_JSON_parse_free (spec);
+        return GNUNET_SYSERR;
+      }
+
+      {
+        struct TALER_RefundRequestPS rr = {
+          .purpose.purpose = htonl (TALER_SIGNATURE_MERCHANT_REFUND),
+          .purpose.size = htonl (sizeof (struct TALER_RefundRequestPS)),
+          .h_contract_terms = ph->h_contract_terms,
+          .coin_pub = res[i].coin_pub,
+          .merchant = merchant_pub,
+          .rtransaction_id = GNUNET_htonll (res[i].rtransaction_id)
+        };
+
+        TALER_amount_hton (&rr.refund_amount,
+                           &ph->coins[found].amount_with_fee);
+        TALER_amount_hton (&rr.refund_fee,
+                           &ph->coins[found].refund_fee);
+        if (GNUNET_OK !=
+            GNUNET_CRYPTO_eddsa_verify (TALER_SIGNATURE_MERCHANT_REFUND,
+                                        &rr,
+                                        &sig->eddsa_sig,
+                                        &merchant_pub.eddsa_pub))
+        {
+          GNUNET_break_op (0);
+          GNUNET_JSON_parse_free (spec);
+          return GNUNET_SYSERR;
+        }
+      }
+    }
+    {
+      struct TALER_MERCHANT_HttpResponse hr = {
+        .reply = json,
+        .http_status = MHD_HTTP_OK
+      };
+
+      ph->abort_cb (ph->abort_cb_cls,
+                    &hr,
+                    &merchant_pub,
+                    &ph->h_contract_terms,
+                    num_refunds,
+                    res);
+    }
+    ph->abort_cb = NULL;
+  }
+  GNUNET_JSON_parse_free (spec);
+  return GNUNET_OK;
+}
+
+
+/**
+ * We got a 403 response back from the exchange (or the merchant).
+ * Now we need to check the provided cryptographic proof that the
+ * coin was actually already spent!
+ *
+ * @param pc handle of the original coin we paid with
+ * @param json cryptographic proof of coin's transaction
+ *        history as was returned by the exchange/merchant
+ * @return #GNUNET_OK if proof checks out
+ */
+static int
+check_coin_history (const struct TALER_MERCHANT_PaidCoin *pc,
+                    json_t *json)
+{
+  struct TALER_Amount spent;
+  struct TALER_Amount spent_plus_contrib;
+
+  if (GNUNET_OK !=
+      TALER_EXCHANGE_verify_coin_history (NULL, /* do not verify fees */
+                                          pc->amount_with_fee.currency,
+                                          &pc->coin_pub,
+                                          json,
+                                          &spent))
+  {
+    /* Exchange's history fails to verify */
+    GNUNET_break_op (0);
+    return GNUNET_SYSERR;
+  }
+  if (0 >
+      TALER_amount_add (&spent_plus_contrib,
+                        &spent,
+                        &pc->amount_with_fee))
+  {
+    /* We got an integer overflow? Bad application! */
+    GNUNET_break (0);
+    return GNUNET_SYSERR;
+  }
+  if (-1 != TALER_amount_cmp (&pc->denom_value,
+                              &spent_plus_contrib))
+  {
+    /* according to our calculations, the transaction should
+       have still worked, exchange error! */
+    GNUNET_break_op (0);
+    return GNUNET_SYSERR;
+  }
+  GNUNET_log (GNUNET_ERROR_TYPE_INFO,
+              "Accepting proof of double-spending\n");
+  return GNUNET_OK;
+}
+
+
+/**
+ * We got a 409 response back from the exchange (or the merchant).
+ * Now we need to check the provided cryptographic proof that the
+ * coin was actually already spent!
+ *
+ * @param ph handle of the original pay operation
+ * @param json cryptographic proof returned by the
+ *        exchange/merchant
+ * @return #GNUNET_OK if proof checks out
+ */
+static int
+check_conflict (struct TALER_MERCHANT_Pay *ph,
+                const json_t *json)
+{
+  json_t *history;
+  json_t *ereply;
+  struct TALER_CoinSpendPublicKeyP coin_pub;
+  struct GNUNET_JSON_Specification spec[] = {
+    GNUNET_JSON_spec_json ("exchange_reply", &ereply),
+    GNUNET_JSON_spec_fixed_auto ("coin_pub", &coin_pub),
+    GNUNET_JSON_spec_end ()
+  };
+  struct GNUNET_JSON_Specification hspec[] = {
+    GNUNET_JSON_spec_json ("history", &history),
+    GNUNET_JSON_spec_end ()
+  };
+
+  if (GNUNET_OK !=
+      GNUNET_JSON_parse (json,
+                         spec,
+                         NULL, NULL))
+  {
+    GNUNET_break_op (0);
+    return GNUNET_SYSERR;
+  }
+  if (GNUNET_OK !=
+      GNUNET_JSON_parse (ereply,
+                         hspec,
+                         NULL, NULL))
+  {
+    GNUNET_break_op (0);
+    GNUNET_JSON_parse_free (spec);
+    return GNUNET_SYSERR;
+  }
+  GNUNET_JSON_parse_free (spec);
+
+  for (unsigned int i = 0; i<ph->num_coins; i++)
+  {
+    if (0 == memcmp (&ph->coins[i].coin_pub,
+                     &coin_pub,
+                     sizeof (struct TALER_CoinSpendPublicKeyP)))
+    {
+      int ret;
+
+      ret = check_coin_history (&ph->coins[i],
+                                history);
+      GNUNET_JSON_parse_free (hspec);
+      return ret;
+    }
+  }
+  GNUNET_break_op (0); /* complaint is not about any of the coins
+                          that we actually paid with... */
+  GNUNET_JSON_parse_free (hspec);
+  return GNUNET_SYSERR;
+}
+
+
+/**
+ * Function called when we're done processing the
+ * HTTP /pay request.
+ *
+ * @param cls the `struct TALER_MERCHANT_Pay`
+ * @param response_code HTTP response code, 0 on error
+ * @param json response body, NULL if not in JSON
+ */
+static void
+handle_pay_finished (void *cls,
+                     long response_code,
+                     const void *response)
+{
+  struct TALER_MERCHANT_Pay *ph = cls;
+  const json_t *json = response;
+  struct TALER_MERCHANT_HttpResponse hr = {
+    .http_status = (unsigned int) response_code,
+    .reply = json
+  };
+
+  ph->job = NULL;
+  GNUNET_log (GNUNET_ERROR_TYPE_INFO,
+              "/pay completed with response code %u\n",
+              (unsigned int) response_code);
+  if (0 == strcasecmp (ph->mode,
+                       "pay"))
+  {
+    switch (response_code)
+    {
+    case 0:
+      hr.ec = TALER_EC_INVALID_RESPONSE;
+      break;
+    case MHD_HTTP_OK:
+      break;
+    /* Tolerating Not Acceptable because sometimes
+     * - especially in tests - we might want to POST
+     * coins one at a time.  */
+    case MHD_HTTP_NOT_ACCEPTABLE:
+      hr.ec = TALER_JSON_get_error_code (json);
+      hr.hint = TALER_JSON_get_error_hint (json);
+      break;
+    case MHD_HTTP_BAD_REQUEST:
+      hr.ec = TALER_JSON_get_error_code (json);
+      hr.hint = TALER_JSON_get_error_hint (json);
+      /* This should never happen, either us
+       * or the merchant is buggy (or API version conflict);
+       * just pass JSON reply to the application */
+      break;
+    case MHD_HTTP_FORBIDDEN:
+      hr.ec = TALER_JSON_get_error_code (json);
+      hr.hint = TALER_JSON_get_error_hint (json);
+      /* Nothing really to verify, merchant says we tried to abort the payment
+       * after it was successful. We should pass the JSON reply to the
+       * application */
+      break;
+    case MHD_HTTP_NOT_FOUND:
+      hr.ec = TALER_JSON_get_error_code (json);
+      hr.hint = TALER_JSON_get_error_hint (json);
+      /* Nothing really to verify, this should never
+         happen, we should pass the JSON reply to the
+         application */
+      break;
+    case MHD_HTTP_PRECONDITION_FAILED:
+      TALER_MERCHANT_parse_error_details_ (json,
+                                           response_code,
+                                           &hr);
+      /* Nothing really to verify, the merchant is blaming us for failing to
+         satisfy some constraint (likely it does not like our exchange because
+         of some disagreement on the PKI).  We should pass the JSON reply to the
+         application */
+      break;
+    case MHD_HTTP_REQUEST_TIMEOUT:
+      hr.ec = TALER_JSON_get_error_code (json);
+      hr.hint = TALER_JSON_get_error_hint (json);
+      /* The merchant couldn't generate a timely response, likely because
+         it itself waited too long on the exchange.
+         Pass on to application. */
+      break;
+    case MHD_HTTP_CONFLICT:
+      hr.ec = TALER_JSON_get_error_code (json);
+      hr.hint = TALER_JSON_get_error_hint (json);
+      if (GNUNET_OK != check_conflict (ph,
+                                       json))
+      {
+        GNUNET_break_op (0);
+        response_code = 0;
+      }
+      break;
+    case MHD_HTTP_GONE:
+      hr.ec = TALER_JSON_get_error_code (json);
+      hr.hint = TALER_JSON_get_error_hint (json);
+      /* The merchant says we are too late, the offer has expired or some
+         denomination key of a coin involved has expired.
+         Might be a disagreement in timestamps? Still, pass on to application. */
+      break;
+    case MHD_HTTP_FAILED_DEPENDENCY:
+      TALER_MERCHANT_parse_error_details_ (json,
+                                           response_code,
+                                           &hr);
+      /* Nothing really to verify, the merchant is blaming the exchange.
+         We should pass the JSON reply to the application */
+      break;
+    case MHD_HTTP_INTERNAL_SERVER_ERROR:
+      hr.ec = TALER_JSON_get_error_code (json);
+      hr.hint = TALER_JSON_get_error_hint (json);
+      /* Server had an internal issue; we should retry,
+         but this API leaves this to the application */
+      break;
+    case MHD_HTTP_SERVICE_UNAVAILABLE:
+      TALER_MERCHANT_parse_error_details_ (json,
+                                           response_code,
+                                           &hr);
+      /* Exchange couldn't respond properly; the retry is
+         left to the application */
+      break;
+    default:
+      TALER_MERCHANT_parse_error_details_ (json,
+                                           response_code,
+                                           &hr);
+      /* unexpected response code */
+      GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
+                  "Unexpected response code %u/%d\n",
+                  (unsigned int) response_code,
+                  (int) hr.ec);
+      GNUNET_break_op (0);
+      break;
+    }
+    ph->pay_cb (ph->pay_cb_cls,
+                &hr);
+  }
+  else
+  {
+    GNUNET_assert (0 == strcasecmp (ph->mode,
+                                    "abort-refund"));
+
+    switch (response_code)
+    {
+    case 0:
+      hr.ec = TALER_EC_INVALID_RESPONSE;
+      break;
+    case MHD_HTTP_OK:
+      if (GNUNET_OK ==
+          check_abort_refund (ph,
+                              json))
+      {
+        TALER_MERCHANT_pay_cancel (ph);
+        return;
+      }
+      hr.http_status = 0;
+      hr.ec = TALER_EC_PAY_MERCHANT_INVALID_RESPONSE;
+      break;
+    case MHD_HTTP_BAD_REQUEST:
+      hr.ec = TALER_JSON_get_error_code (json);
+      hr.hint = TALER_JSON_get_error_hint (json);
+      /* This should never happen, either us or the
+         merchant is buggy (or API version conflict); just
+         pass JSON reply to the application */
+      break;
+    case MHD_HTTP_CONFLICT:
+      hr.ec = TALER_JSON_get_error_code (json);
+      hr.hint = TALER_JSON_get_error_hint (json);
+      break;
+    case MHD_HTTP_FORBIDDEN:
+      hr.ec = TALER_JSON_get_error_code (json);
+      hr.hint = TALER_JSON_get_error_hint (json);
+      /* Nothing really to verify, merchant says one of
+         the signatures is invalid; as we checked them,
+         this should never happen, we should pass the JSON
+         reply to the application */
+      break;
+    case MHD_HTTP_NOT_FOUND:
+      hr.ec = TALER_JSON_get_error_code (json);
+      hr.hint = TALER_JSON_get_error_hint (json);
+      /* Nothing really to verify, this should never
+   happen, we should pass the JSON reply to the
+         application */
+      break;
+    case MHD_HTTP_FAILED_DEPENDENCY:
+      TALER_MERCHANT_parse_error_details_ (json,
+                                           response_code,
+                                           &hr);
+      /* Nothing really to verify, the merchant is blaming the exchange.
+         We should pass the JSON reply to the application */
+      break;
+    case MHD_HTTP_INTERNAL_SERVER_ERROR:
+      hr.ec = TALER_JSON_get_error_code (json);
+      hr.hint = TALER_JSON_get_error_hint (json);
+      /* Server had an internal issue; we should retry,
+         but this API leaves this to the application */
+      break;
+    default:
+      /* unexpected response code */
+      TALER_MERCHANT_parse_error_details_ (json,
+                                           response_code,
+                                           &hr);
+      GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
+                  "Unexpected response code %u/%d\n",
+                  (unsigned int) response_code,
+                  (int) hr.ec);
+      GNUNET_break_op (0);
+      break;
+    }
+    ph->abort_cb (ph->abort_cb_cls,
+                  &hr,
+                  NULL,
+                  NULL,
+                  0,
+                  NULL);
+  }
+  TALER_MERCHANT_pay_cancel (ph);
+}
+
+
+/**
+ * Issue /pay request. Generic version for the various
+ * variants of the API.
+ *
+ * @param ctx the execution loop context
+ * @param merchant_url base URL of the merchant's backend
+ * @param merchant_pub public key of the merchant
+ * @param num_coins number of coins used to pay
+ * @param coins array of coins we use to pay
+ * @param mode mode string to use ("pay" or "abort-refund").
+ * @param pay_cb the callback to call when a reply for this
+ *        request is available
+ * @param pay_cb_cls closure for @a pay_cb
+ * @param abort_cb callback to call for the abort-refund variant
+ * @param abort_cb_cls closure for @a abort_cb
+ * @return a handle for this request
+ */
+static struct TALER_MERCHANT_Pay *
+request_pay_generic (
+  struct GNUNET_CURL_Context *ctx,
+  const char *merchant_url,
+  const struct TALER_MerchantPublicKeyP *merchant_pub,
+  const char *order_id,
+  unsigned int num_coins,
+  const struct TALER_MERCHANT_PaidCoin *coins,
+  const char *mode,
+  TALER_MERCHANT_OrderPayCallback pay_cb,
+  void *pay_cb_cls,
+  TALER_MERCHANT_PayRefundCallback abort_cb,
+  void *abort_cb_cls)
+{
+  struct TALER_MERCHANT_Pay *ph;
+  json_t *pay_obj;
+  json_t *j_coins;
+  CURL *eh;
+  struct TALER_Amount total_fee;
+  struct TALER_Amount total_amount;
+
+  if (0 == num_coins)
+  {
+    GNUNET_break (0);
+    return NULL;
+  }
+  j_coins = json_array ();
+  for (unsigned int i = 0; i<num_coins; i++)
+  {
+    json_t *j_coin;
+    const struct TALER_MERCHANT_PaidCoin *pc = &coins[i];
+    struct TALER_Amount fee;
+
+    if (0 >
+        TALER_amount_subtract (&fee,
+                               &pc->amount_with_fee,
+                               &pc->amount_without_fee))
+    {
+      /* Integer underflow, fee larger than total amount?
+         This should not happen (client violated API!) */
+      GNUNET_break (0);
+      json_decref (j_coins);
+      return NULL;
+    }
+    if (0 == i)
+    {
+      total_fee = fee;
+      total_amount = pc->amount_with_fee;
+    }
+    else
+    {
+      if ( (0 >
+            TALER_amount_add (&total_fee,
+                              &total_fee,
+                              &fee)) ||
+           (0 >
+            TALER_amount_add (&total_amount,
+                              &total_amount,
+                              &pc->amount_with_fee)) )
+      {
+        /* integer overflow */
+        GNUNET_break (0);
+        json_decref (j_coins);
+        return NULL;
+      }
+    }
+
+    /* create JSON for this coin */
+    j_coin = json_pack (
+      "{s:o, s:o,"                   /* contribution/coin_pub */
+      " s:s, s:o,"          /* exchange_url / denom_pub */
+      " s:o, s:o}",          /* ub_sig / coin_sig */
+      "contribution",
+      TALER_JSON_from_amount (&pc->amount_with_fee),
+      "coin_pub",
+      GNUNET_JSON_from_data_auto (&pc->coin_pub),
+      "exchange_url",
+      pc->exchange_url,
+      "denom_pub",
+      GNUNET_JSON_from_rsa_public_key (pc->denom_pub.rsa_public_key),
+      "ub_sig",
+      GNUNET_JSON_from_rsa_signature (pc->denom_sig.rsa_signature),
+      "coin_sig",
+      GNUNET_JSON_from_data_auto (&pc->coin_sig));
+    if (0 !=
+        json_array_append_new (j_coins,
+                               j_coin))
+    {
+      GNUNET_break (0);
+      json_decref (j_coins);
+      return NULL;
+    }
+  }
+
+  pay_obj = json_pack ("{"
+                       " s:s," /* mode */
+                       " s:o," /* coins */
+                       " s:s," /* order_id */
+                       " s:o," /* merchant_pub */
+                       "}",
+                       "mode",
+                       mode,
+                       "coins",
+                       j_coins, /* reference consumed! */
+                       "order_id",
+                       order_id,
+                       "merchant_pub",
+                       GNUNET_JSON_from_data_auto (merchant_pub));
+  if (NULL == pay_obj)
+  {
+    GNUNET_break (0);
+    return NULL;
+  }
+  ph = GNUNET_new (struct TALER_MERCHANT_Pay);
+  ph->ctx = ctx;
+  ph->mode = mode;
+  ph->abort_cb = abort_cb;
+  ph->abort_cb_cls = abort_cb_cls;
+  ph->pay_cb = pay_cb;
+  ph->pay_cb_cls = pay_cb_cls;
+  ph->url = TALER_url_join (merchant_url, "pay", NULL);
+  if (NULL == ph->url)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
+                "Could not construct request URL.\n");
+    json_decref (pay_obj);
+    GNUNET_free (ph);
+    return NULL;
+  }
+  ph->num_coins = num_coins;
+  ph->coins = GNUNET_new_array (num_coins,
+                                struct TALER_MERCHANT_PaidCoin);
+  memcpy (ph->coins,
+          coins,
+          num_coins * sizeof (struct TALER_MERCHANT_PaidCoin));
+
+  eh = curl_easy_init ();
+  if (GNUNET_OK != TALER_curl_easy_post (&ph->post_ctx,
+                                         eh,
+                                         pay_obj))
+  {
+    GNUNET_break (0);
+    json_decref (pay_obj);
+    GNUNET_free (ph);
+    return NULL;
+  }
+
+  json_decref (pay_obj);
+  GNUNET_assert (CURLE_OK == curl_easy_setopt (eh,
+                                               CURLOPT_URL,
+                                               ph->url));
+  ph->job = GNUNET_CURL_job_add2 (ctx,
+                                  eh,
+                                  ph->post_ctx.headers,
+                                  &handle_pay_finished,
+                                  ph);
+  return ph;
+}
+
+
+/**
+ * Pay a merchant.  API for wallets that have the coin's private
+ * keys.
+ * _NOTE_: this function does NOT calculate each coin amount in
+ * order to match the contract total price.  This calculation is
+ * to be made by the logic using this library.
+ *
+ * @param ctx the execution loop context
+ * @param merchant_url base URL of the merchant's backend
+ * @param h_contract_terms hashcode of the proposal being paid
+ * @param amount total value of the contract to be paid to the
+ *        merchant
+ * @param max_fee maximum fee covered by the merchant
+ *        (according to the contract)
+ * @param merchant_pub the public key of the merchant
+ *        (used to identify the merchant for refund requests)
+ * @param merchant_sig signature from the merchant over the
+ *        original contract
+ * @param timestamp timestamp when the contract was finalized,
+ *        must match approximately the current time of the merchant
+ * @param refund_deadline date until which the merchant can issue
+ *        a refund to the customer via the merchant (can be zero
+ *        if refunds are not allowed)
+ * @param pay_deadline maximum time limit to pay for this contract
+ * @param h_wire hash of the merchant’s account details
+ * @param order_id order id of the proposal being paid
+ * @param num_coins number of coins used to pay
+ * @param coins array of coins we use to pay
+ * @param pay_cb the callback to call when a reply for this
+ *        request is available
+ * @param pay_cb_cls closure for @a pay_cb
+ * @return a handle for this request
+ */
+static struct TALER_MERCHANT_Pay *
+prepare_pay_generic (struct GNUNET_CURL_Context *ctx,
+                     const char *merchant_url,
+                     const struct GNUNET_HashCode *h_contract_terms,
+                     const struct TALER_Amount *amount,
+                     const struct TALER_Amount *max_fee,
+                     const struct TALER_MerchantPublicKeyP *merchant_pub,
+                     const struct TALER_MerchantSignatureP *merchant_sig,
+                     struct GNUNET_TIME_Absolute timestamp,
+                     struct GNUNET_TIME_Absolute refund_deadline,
+                     struct GNUNET_TIME_Absolute pay_deadline,
+                     const struct GNUNET_HashCode *h_wire,
+                     const char *order_id,
+                     unsigned int num_coins,
+                     const struct TALER_MERCHANT_PayCoin *coins,
+                     const char *mode,
+                     TALER_MERCHANT_PayCallback pay_cb,
+                     void *pay_cb_cls,
+                     TALER_MERCHANT_PayRefundCallback abort_cb,
+                     void *abort_cb_cls)
+{
+  struct TALER_DepositRequestPS dr;
+  struct TALER_MERCHANT_PaidCoin pc[num_coins];
+
+  (void) GNUNET_TIME_round_abs (&timestamp);
+  (void) GNUNET_TIME_round_abs (&pay_deadline);
+  (void) GNUNET_TIME_round_abs (&refund_deadline);
+
+  if (GNUNET_YES !=
+      TALER_amount_cmp_currency (amount,
+                                 max_fee))
+  {
+    GNUNET_break (0);
+    return NULL;
+  }
+
+  dr.purpose.purpose = htonl (TALER_SIGNATURE_WALLET_COIN_DEPOSIT);
+  dr.purpose.size = htonl (sizeof (struct TALER_DepositRequestPS));
+  dr.h_contract_terms = *h_contract_terms;
+  dr.h_wire = *h_wire;
+  dr.timestamp = GNUNET_TIME_absolute_hton (timestamp);
+  dr.refund_deadline = GNUNET_TIME_absolute_hton (refund_deadline);
+  dr.merchant = *merchant_pub;
+
+  for (unsigned int i = 0; i<num_coins; i++)
+  {
+    const struct TALER_MERCHANT_PayCoin *coin = &coins[i]; // coin priv.
+    struct TALER_MERCHANT_PaidCoin *p = &pc[i]; // coin pub.
+    struct TALER_Amount fee;
+
+    /* prepare 'dr' for this coin to generate coin signature */
+    GNUNET_CRYPTO_eddsa_key_get_public (&coin->coin_priv.eddsa_priv,
+                                        &dr.coin_pub.eddsa_pub);
+    TALER_amount_hton (&dr.amount_with_fee,
+                       &coin->amount_with_fee);
+    if (0 >
+        TALER_amount_subtract (&fee,
+                               &coin->amount_with_fee,
+                               &coin->amount_without_fee))
+    {
+      /* Integer underflow, fee larger than total amount?
+         This should not happen (client violated API!) */
+      GNUNET_break (0);
+      return NULL;
+    }
+    TALER_amount_hton (&dr.deposit_fee,
+                       &fee);
+    {
+      TALER_LOG_DEBUG ("... amount_with_fee was %s\n",
+                       TALER_amount2s (&coin->amount_with_fee));
+      TALER_LOG_DEBUG ("... fee was %s\n",
+                       TALER_amount2s (&fee));
+    }
+
+    GNUNET_CRYPTO_eddsa_sign (&coin->coin_priv.eddsa_priv,
+                              &dr,
+                              &p->coin_sig.eddsa_signature);
+    p->denom_pub = coin->denom_pub;
+    p->denom_sig = coin->denom_sig;
+    p->denom_value = coin->denom_value;
+    p->coin_pub = dr.coin_pub;
+    p->amount_with_fee = coin->amount_with_fee;
+    p->amount_without_fee = coin->amount_without_fee;
+    p->refund_fee = coin->refund_fee;
+    p->exchange_url = coin->exchange_url;
+  }
+  return request_pay_generic (ctx,
+                              merchant_url,
+                              merchant_pub,
+                              order_id,
+                              num_coins,
+                              pc,
+                              mode,
+                              pay_cb,
+                              pay_cb_cls,
+                              abort_cb,
+                              abort_cb_cls);
+}
+
+
+/**
+ * Pay a merchant.  API for wallets that have the coin's private keys.
+ * _NOTE_: this function does NOT calculate each coin amount in order
+ * to match the contract total price.  This calculation is to be made
+ * by the logic using this library.
+ *
+ * @param ctx the execution loop context
+ * @param merchant_url base URL of the merchant's backend
+ * @param h_contract_terms hashcode of the proposal being paid
+ * @param amount total value of the contract to be paid to the merchant
+ * @param max_fee maximum fee covered by the merchant (according to the contract)
+ * @param merchant_pub the public key of the merchant (used to identify the merchant for refund requests)
+ * @param merchant_sig signature from the merchant over the original contract
+ * @param timestamp timestamp when the contract was finalized, must match approximately the current time of the merchant
+ * @param refund_deadline date until which the merchant can issue a refund to the customer via the merchant (can be zero if refunds are not allowed)
+ * @param pay_deadline maximum time limit to pay for this contract
+ * @param h_wire hash of the merchant’s account details
+ * @param order_id order id of the proposal being paid
+ * @param num_coins number of coins used to pay
+ * @param coins array of coins we use to pay
+ * @param pay_cb the callback to call when a reply for this request is available
+ * @param pay_cb_cls closure for @a pay_cb
+ * @return a handle for this request
+ */
+struct TALER_MERCHANT_Pay *
+TALER_MERCHANT_pay_wallet (struct GNUNET_CURL_Context *ctx,
+                           const char *merchant_url,
+                           const struct GNUNET_HashCode *h_contract_terms,
+                           const struct TALER_Amount *amount,
+                           const struct TALER_Amount *max_fee,
+                           const struct TALER_MerchantPublicKeyP *merchant_pub,
+                           const struct TALER_MerchantSignatureP *merchant_sig,
+                           struct GNUNET_TIME_Absolute timestamp,
+                           struct GNUNET_TIME_Absolute refund_deadline,
+                           struct GNUNET_TIME_Absolute pay_deadline,
+                           const struct GNUNET_HashCode *h_wire,
+                           const char *order_id,
+                           unsigned int num_coins,
+                           const struct TALER_MERCHANT_PayCoin *coins,
+                           TALER_MERCHANT_PayCallback pay_cb,
+                           void *pay_cb_cls)
+{
+  return prepare_pay_generic (ctx,
+                              merchant_url,
+                              h_contract_terms,
+                              amount,
+                              max_fee,
+                              merchant_pub,
+                              merchant_sig,
+                              timestamp,
+                              refund_deadline,
+                              pay_deadline,
+                              h_wire,
+                              order_id,
+                              num_coins,
+                              coins,
+                              "pay",
+                              pay_cb,
+                              pay_cb_cls,
+                              NULL,
+                              NULL);
+}
+
+
+/**
+ * Run a payment abort operation, asking for refunds for coins
+ * that were previously spend on a /pay that failed to go through.
+ *
+ * @param ctx execution context
+ * @param merchant_url base URL of the merchant
+ * @param h_wire hash of the merchant’s account details
+ * @param h_contract hash of the contact of the merchant with the customer
+ * @param transaction_id transaction id for the transaction between merchant and customer
+ * @param amount total value of the contract to be paid to the merchant
+ * @param max_fee maximum fee covered by the merchant (according to the contract)
+ * @param merchant_pub the public key of the merchant (used to identify the merchant for refund requests)
+ * @param merchant_sig signature from the merchant over the original contract
+ * @param timestamp timestamp when the contract was finalized, must match approximately the current time of the merchant
+ * @param refund_deadline date until which the merchant can issue a refund to the customer via the merchant (can be zero if refunds are not allowed)
+ * @param pay_deadline maximum time limit to pay for this contract
+ * @param num_coins number of coins used to pay
+ * @param coins array of coins we use to pay
+ * @param coin_sig the signature made with purpose #TALER_SIGNATURE_WALLET_COIN_DEPOSIT made by the customer with the coin’s private key.
+ * @param payref_cb the callback to call when a reply for this request is available
+ * @param payref_cb_cls closure for @a pay_cb
+ * @return a handle for this request
+ */
+struct TALER_MERCHANT_Pay *
+TALER_MERCHANT_pay_abort (struct GNUNET_CURL_Context *ctx,
+                          const char *merchant_url,
+                          const struct GNUNET_HashCode *h_contract,
+                          const struct TALER_Amount *amount,
+                          const struct TALER_Amount *max_fee,
+                          const struct TALER_MerchantPublicKeyP *merchant_pub,
+                          const struct TALER_MerchantSignatureP *merchant_sig,
+                          struct GNUNET_TIME_Absolute timestamp,
+                          struct GNUNET_TIME_Absolute refund_deadline,
+                          struct GNUNET_TIME_Absolute pay_deadline,
+                          const struct GNUNET_HashCode *h_wire,
+                          const char *order_id,
+                          unsigned int num_coins,
+                          const struct TALER_MERCHANT_PayCoin *coins,
+                          TALER_MERCHANT_PayRefundCallback payref_cb,
+                          void *payref_cb_cls)
+{
+  struct TALER_MERCHANT_Pay *ph;
+
+  ph = prepare_pay_generic (ctx,
+                            merchant_url,
+                            h_contract,
+                            amount,
+                            max_fee,
+                            merchant_pub,
+                            merchant_sig,
+                            timestamp,
+                            refund_deadline,
+                            pay_deadline,
+                            h_wire,
+                            order_id,
+                            num_coins,
+                            coins,
+                            "abort-refund",
+                            NULL,
+                            NULL,
+                            payref_cb,
+                            payref_cb_cls);
+  if (NULL == ph)
+    return NULL;
+  ph->h_contract_terms = *h_contract;
+  return ph;
+}
+
+
+/**
+ * PAY a merchant.  API for frontends talking to backends. Here,
+ * the frontend does not have the coin's private keys, but just
+ * the public keys and signatures.  Note the subtle difference
+ * in the type of @a coins compared to #TALER_MERCHANT_pay().
+ *
+ * @param ctx the execution loop context
+ * @param merchant_url base URL of the merchant's backend
+ * @param merchant_pub public key of the merchant
+ * @param num_coins number of coins used to pay
+ * @param coins array of coins we use to pay
+ * @param pay_cb the callback to call when a reply for this request is available
+ * @param pay_cb_cls closure for @a pay_cb
+ * @return a handle for this request
+ */
+struct TALER_MERCHANT_Pay *
+TALER_MERCHANT_pay_frontend (
+  struct GNUNET_CURL_Context *ctx,
+  const char *merchant_url,
+  const struct TALER_MerchantPublicKeyP *merchant_pub,
+  const char *order_id,
+  unsigned int num_coins,
+  const struct TALER_MERCHANT_PaidCoin *coins,
+  TALER_MERCHANT_PayCallback pay_cb,
+  void *pay_cb_cls)
+{
+  return request_pay_generic (ctx,
+                              merchant_url,
+                              merchant_pub,
+                              order_id,
+                              num_coins,
+                              coins,
+                              "pay",
+                              pay_cb,
+                              pay_cb_cls,
+                              NULL,
+                              NULL);
+}
+
+
+/**
+ * Cancel a pay permission request.  This function cannot be used
+ * on a request handle if a response is already served for it.
+ *
+ * @param pay the pay permission request handle
+ */
+void
+TALER_MERCHANT_pay_cancel (struct TALER_MERCHANT_Pay *pay)
+{
+  if (NULL != pay->job)
+  {
+    GNUNET_CURL_job_cancel (pay->job);
+    pay->job = NULL;
+  }
+  TALER_curl_easy_post_finished (&pay->post_ctx);
+  GNUNET_free (pay->coins);
+  GNUNET_free (pay->url);
+  GNUNET_free (pay);
+}
+
+
+/* end of merchant_api_pay.c */
diff --git a/src/lib/merchant_api_post_order_pay.c b/src/lib/merchant_api_post_order_pay.c
index f860f3a8..3cef8043 100644
--- a/src/lib/merchant_api_post_order_pay.c
+++ b/src/lib/merchant_api_post_order_pay.c
@@ -17,8 +17,8 @@
   If not, see <http://www.gnu.org/licenses/>
 */
 /**
- * @file lib/merchant_api_pay.c
- * @brief Implementation of the /pay request
+ * @file lib/merchant_api_post_order_pay.c
+ * @brief Implementation of the POST /order/$ID/pay request
  *        of the merchant's HTTP API
  * @author Christian Grothoff
  * @author Marcello Stanisci
@@ -39,7 +39,7 @@
 /**
  * @brief A Pay Handle
  */
-struct TALER_MERCHANT_Pay
+struct TALER_MERCHANT_OrderPayHandle
 {
 
   /**
@@ -63,21 +63,6 @@ struct TALER_MERCHANT_Pay
   void *pay_cb_cls;
 
   /**
-   * Function to call with the result in "abort-refund" @e mode.
-   */
-  TALER_MERCHANT_PayRefundCallback abort_cb;
-
-  /**
-   * Closure for @a abort_cb.
-   */
-  void *abort_cb_cls;
-
-  /**
-   * Operational mode, either "pay" or "abort-refund".
-   */
-  const char *mode;
-
-  /**
    * Reference to the execution context.
    */
   struct GNUNET_CURL_Context *ctx;
@@ -97,145 +82,16 @@ struct TALER_MERCHANT_Pay
    */
   unsigned int num_coins;
 
-  /**
-   * Hash of the contract, only available in "abort-refund" mode.
-   */
-  struct GNUNET_HashCode h_contract_terms;
-
 };
 
 
 /**
- * Check that the response for a /pay refund is well-formed,
- * and call the application callback with the result if it is
- * OK. Otherwise returns #GNUNET_SYSERR.
- *
- * @param ph handle to operation that created the reply
- * @param json the reply to parse
- * @return #GNUNET_OK on success
- */
-static int
-check_abort_refund (struct TALER_MERCHANT_Pay *ph,
-                    const json_t *json)
-{
-  json_t *refunds;
-  unsigned int num_refunds;
-  struct TALER_MerchantPublicKeyP merchant_pub;
-  struct GNUNET_JSON_Specification spec[] = {
-    GNUNET_JSON_spec_json ("refund_permissions", &refunds),
-    GNUNET_JSON_spec_fixed_auto ("merchant_pub", &merchant_pub),
-    GNUNET_JSON_spec_end ()
-  };
-
-  if (GNUNET_OK !=
-      GNUNET_JSON_parse (json,
-                         spec,
-                         NULL, NULL))
-  {
-    GNUNET_break_op (0);
-    return GNUNET_SYSERR;
-  }
-  num_refunds = json_array_size (refunds);
-  {
-    struct TALER_MERCHANT_RefundEntry res[GNUNET_NZL (num_refunds)];
-
-    for (unsigned int i = 0; i<num_refunds; i++)
-    {
-      struct TALER_MerchantSignatureP *sig = &res[i].merchant_sig;
-      json_t *refund = json_array_get (refunds, i);
-      struct GNUNET_JSON_Specification spec_detail[] = {
-        GNUNET_JSON_spec_fixed_auto ("merchant_sig",
-                                     sig),
-        GNUNET_JSON_spec_fixed_auto ("coin_pub",
-                                     &res[i].coin_pub),
-        GNUNET_JSON_spec_uint64 ("rtransaction_id",
-                                 &res[i].rtransaction_id),
-        GNUNET_JSON_spec_end ()
-      };
-      int found;
-
-      if (GNUNET_OK !=
-          GNUNET_JSON_parse (refund,
-                             spec_detail,
-                             NULL, NULL))
-      {
-        GNUNET_break_op (0);
-        GNUNET_JSON_parse_free (spec);
-        return GNUNET_SYSERR;
-      }
-
-      found = -1;
-      for (unsigned int j = 0; j<ph->num_coins; j++)
-      {
-        if (0 == memcmp (&ph->coins[j].coin_pub,
-                         &res[i].coin_pub,
-                         sizeof
-                         (struct TALER_CoinSpendPublicKeyP)))
-        {
-          found = j;
-          break;
-        }
-      }
-      if (-1 == found)
-      {
-        GNUNET_break_op (0);
-        GNUNET_JSON_parse_free (spec);
-        return GNUNET_SYSERR;
-      }
-
-      {
-        struct TALER_RefundRequestPS rr = {
-          .purpose.purpose = htonl (TALER_SIGNATURE_MERCHANT_REFUND),
-          .purpose.size = htonl (sizeof (struct TALER_RefundRequestPS)),
-          .h_contract_terms = ph->h_contract_terms,
-          .coin_pub = res[i].coin_pub,
-          .merchant = merchant_pub,
-          .rtransaction_id = GNUNET_htonll (res[i].rtransaction_id)
-        };
-
-        TALER_amount_hton (&rr.refund_amount,
-                           &ph->coins[found].amount_with_fee);
-        TALER_amount_hton (&rr.refund_fee,
-                           &ph->coins[found].refund_fee);
-        if (GNUNET_OK !=
-            GNUNET_CRYPTO_eddsa_verify (TALER_SIGNATURE_MERCHANT_REFUND,
-                                        &rr,
-                                        &sig->eddsa_sig,
-                                        &merchant_pub.eddsa_pub))
-        {
-          GNUNET_break_op (0);
-          GNUNET_JSON_parse_free (spec);
-          return GNUNET_SYSERR;
-        }
-      }
-    }
-    {
-      struct TALER_MERCHANT_HttpResponse hr = {
-        .reply = json,
-        .http_status = MHD_HTTP_OK
-      };
-
-      ph->abort_cb (ph->abort_cb_cls,
-                    &hr,
-                    &merchant_pub,
-                    &ph->h_contract_terms,
-                    num_refunds,
-                    res);
-    }
-    ph->abort_cb = NULL;
-  }
-  GNUNET_JSON_parse_free (spec);
-  return GNUNET_OK;
-}
-
-
-/**
  * We got a 403 response back from the exchange (or the merchant).
- * Now we need to check the provided cryptographic proof that the
+ * Now we need to check the provided cryptograophic proof that the
  * coin was actually already spent!
  *
  * @param pc handle of the original coin we paid with
- * @param json cryptographic proof of coin's transaction
+ * @param json cryptograophic proof of coin's transaction
  *        history as was returned by the exchange/merchant
  * @return #GNUNET_OK if proof checks out
  */
@@ -282,16 +138,16 @@ check_coin_history (const struct TALER_MERCHANT_PaidCoin *pc,
 
 /**
  * We got a 409 response back from the exchange (or the merchant).
- * Now we need to check the provided cryptographic proof that the
+ * Now we need to check the provided cryptograophic proof that the
  * coin was actually already spent!
  *
- * @param ph handle of the original pay operation
- * @param json cryptographic proof returned by the
+ * @param oph handle of the original pay operation
+ * @param json cryptograophic proof returned by the
  *        exchange/merchant
  * @return #GNUNET_OK if proof checks out
  */
 static int
-check_conflict (struct TALER_MERCHANT_Pay *ph,
+check_conflict (struct TALER_MERCHANT_OrderPayHandle *oph,
                 const json_t *json)
 {
   json_t *history;
@@ -326,15 +182,15 @@ check_conflict (struct TALER_MERCHANT_Pay *ph,
   }
   GNUNET_JSON_parse_free (spec);
 
-  for (unsigned int i = 0; i<ph->num_coins; i++)
+  for (unsigned int i = 0; i<oph->num_coins; i++)
   {
-    if (0 == memcmp (&ph->coins[i].coin_pub,
+    if (0 == memcmp (&oph->coins[i].coin_pub,
                      &coin_pub,
                      sizeof (struct TALER_CoinSpendPublicKeyP)))
     {
       int ret;
 
-      ret = check_coin_history (&ph->coins[i],
+      ret = check_coin_history (&oph->coins[i],
                                 history);
       GNUNET_JSON_parse_free (hspec);
       return ret;
@@ -360,238 +216,154 @@ handle_pay_finished (void *cls,
                      long response_code,
                      const void *response)
 {
-  struct TALER_MERCHANT_Pay *ph = cls;
+  struct TALER_MERCHANT_OrderPayHandle *oph = cls;
   const json_t *json = response;
   struct TALER_MERCHANT_HttpResponse hr = {
     .http_status = (unsigned int) response_code,
     .reply = json
   };
 
-  ph->job = NULL;
+  oph->job = NULL;
   GNUNET_log (GNUNET_ERROR_TYPE_INFO,
               "/pay completed with response code %u\n",
               (unsigned int) response_code);
-  if (0 == strcasecmp (ph->mode,
-                       "pay"))
-  {
-    switch (response_code)
-    {
-    case 0:
-      hr.ec = TALER_EC_INVALID_RESPONSE;
-      break;
-    case MHD_HTTP_OK:
-      break;
-    /* Tolerating Not Acceptable because sometimes
-     * - especially in tests - we might want to POST
-     * coins one at a time.  */
-    case MHD_HTTP_NOT_ACCEPTABLE:
-      hr.ec = TALER_JSON_get_error_code (json);
-      hr.hint = TALER_JSON_get_error_hint (json);
-      break;
-    case MHD_HTTP_BAD_REQUEST:
-      hr.ec = TALER_JSON_get_error_code (json);
-      hr.hint = TALER_JSON_get_error_hint (json);
-      /* This should never happen, either us
-       * or the merchant is buggy (or API version conflict);
-       * just pass JSON reply to the application */
-      break;
-    case MHD_HTTP_FORBIDDEN:
-      hr.ec = TALER_JSON_get_error_code (json);
-      hr.hint = TALER_JSON_get_error_hint (json);
-      /* Nothing really to verify, merchant says we tried to abort the payment
-       * after it was successful. We should pass the JSON reply to the
-       * application */
-      break;
-    case MHD_HTTP_NOT_FOUND:
-      hr.ec = TALER_JSON_get_error_code (json);
-      hr.hint = TALER_JSON_get_error_hint (json);
-      /* Nothing really to verify, this should never
-         happen, we should pass the JSON reply to the
-         application */
-      break;
-    case MHD_HTTP_PRECONDITION_FAILED:
-      TALER_MERCHANT_parse_error_details_ (json,
-                                           response_code,
-                                           &hr);
-      /* Nothing really to verify, the merchant is blaming us for failing to
-         satisfy some constraint (likely it does not like our exchange because
-         of some disagreement on the PKI).  We should pass the JSON reply to the
-         application */
-      break;
-    case MHD_HTTP_REQUEST_TIMEOUT:
-      hr.ec = TALER_JSON_get_error_code (json);
-      hr.hint = TALER_JSON_get_error_hint (json);
-      /* The merchant couldn't generate a timely response, likely because
-         it itself waited too long on the exchange.
-         Pass on to application. */
-      break;
-    case MHD_HTTP_CONFLICT:
-      hr.ec = TALER_JSON_get_error_code (json);
-      hr.hint = TALER_JSON_get_error_hint (json);
-      if (GNUNET_OK != check_conflict (ph,
-                                       json))
-      {
-        GNUNET_break_op (0);
-        response_code = 0;
-      }
-      break;
-    case MHD_HTTP_GONE:
-      hr.ec = TALER_JSON_get_error_code (json);
-      hr.hint = TALER_JSON_get_error_hint (json);
-      /* The merchant says we are too late, the offer has expired or some
-         denomination key of a coin involved has expired.
-         Might be a disagreement in timestamps? Still, pass on to application. */
-      break;
-    case MHD_HTTP_FAILED_DEPENDENCY:
-      TALER_MERCHANT_parse_error_details_ (json,
-                                           response_code,
-                                           &hr);
-      /* Nothing really to verify, the merchant is blaming the exchange.
-         We should pass the JSON reply to the application */
-      break;
-    case MHD_HTTP_INTERNAL_SERVER_ERROR:
-      hr.ec = TALER_JSON_get_error_code (json);
-      hr.hint = TALER_JSON_get_error_hint (json);
-      /* Server had an internal issue; we should retry,
-         but this API leaves this to the application */
-      break;
-    case MHD_HTTP_SERVICE_UNAVAILABLE:
-      TALER_MERCHANT_parse_error_details_ (json,
-                                           response_code,
-                                           &hr);
-      /* Exchange couldn't respond properly; the retry is
-         left to the application */
-      break;
-    default:
-      TALER_MERCHANT_parse_error_details_ (json,
-                                           response_code,
-                                           &hr);
-      /* unexpected response code */
-      GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
-                  "Unexpected response code %u/%d\n",
-                  (unsigned int) response_code,
-                  (int) hr.ec);
-      GNUNET_break_op (0);
-      break;
-    }
-    ph->pay_cb (ph->pay_cb_cls,
-                &hr);
-  }
-  else
+  // FIXME: check response codes here match documentation match what service generates!
+  switch (response_code)
   {
-    GNUNET_assert (0 == strcasecmp (ph->mode,
-                                    "abort-refund"));
-
-    switch (response_code)
+  case 0:
+    hr.ec = TALER_EC_INVALID_RESPONSE;
+    break;
+  case MHD_HTTP_OK:
+    // FIXME: should verify 'sig' from merchant here!
+    // FIXME: probably should return merchant signature to callback!
+    break;
+  /* Tolerating Not Acceptable because sometimes
+   * - especially in tests - we might want to POST
+   * coins one at a time.  */
+  case MHD_HTTP_NOT_ACCEPTABLE:
+    hr.ec = TALER_JSON_get_error_code (json);
+    hr.hint = TALER_JSON_get_error_hint (json);
+    break;
+  case MHD_HTTP_BAD_REQUEST:
+    hr.ec = TALER_JSON_get_error_code (json);
+    hr.hint = TALER_JSON_get_error_hint (json);
+    /* This should never happen, either us
+     * or the merchant is buggy (or API version conflict);
+     * just pass JSON reply to the application */
+    break;
+  case MHD_HTTP_FORBIDDEN:
+    hr.ec = TALER_JSON_get_error_code (json);
+    hr.hint = TALER_JSON_get_error_hint (json);
+    /* Nothing really to verify, merchant says we tried to abort the payment
+     * after it was successful. We should pass the JSON reply to the
+     * application */
+    break;
+  case MHD_HTTP_NOT_FOUND:
+    hr.ec = TALER_JSON_get_error_code (json);
+    hr.hint = TALER_JSON_get_error_hint (json);
+    /* Nothing really to verify, this should never
+       happen, we should pass the JSON reply to the
+       application */
+    break;
+  case MHD_HTTP_PRECONDITION_FAILED:
+    TALER_MERCHANT_parse_error_details_ (json,
+                                         response_code,
+                                         &hr);
+    /* Nothing really to verify, the merchant is blaming us for failing to
+       satisfy some constraint (likely it does not like our exchange because
+       of some disagreement on the PKI).  We should pass the JSON reply to the
+       application */
+    break;
+  case MHD_HTTP_REQUEST_TIMEOUT:
+    hr.ec = TALER_JSON_get_error_code (json);
+    hr.hint = TALER_JSON_get_error_hint (json);
+    /* The merchant couldn't generate a timely response, likely because
+       it itself waited too long on the exchange.
+       Pass on to application. */
+    break;
+  case MHD_HTTP_CONFLICT:
+    hr.ec = TALER_JSON_get_error_code (json);
+    hr.hint = TALER_JSON_get_error_hint (json);
+    if (GNUNET_OK != check_conflict (oph,
+                                     json))
     {
-    case 0:
-      hr.ec = TALER_EC_INVALID_RESPONSE;
-      break;
-    case MHD_HTTP_OK:
-      if (GNUNET_OK ==
-          check_abort_refund (ph,
-                              json))
-      {
-        TALER_MERCHANT_pay_cancel (ph);
-        return;
-      }
-      hr.http_status = 0;
-      hr.ec = TALER_EC_PAY_MERCHANT_INVALID_RESPONSE;
-      break;
-    case MHD_HTTP_BAD_REQUEST:
-      hr.ec = TALER_JSON_get_error_code (json);
-      hr.hint = TALER_JSON_get_error_hint (json);
-      /* This should never happen, either us or the
-         merchant is buggy (or API version conflict); just
-         pass JSON reply to the application */
-      break;
-    case MHD_HTTP_CONFLICT:
-      hr.ec = TALER_JSON_get_error_code (json);
-      hr.hint = TALER_JSON_get_error_hint (json);
-      break;
-    case MHD_HTTP_FORBIDDEN:
-      hr.ec = TALER_JSON_get_error_code (json);
-      hr.hint = TALER_JSON_get_error_hint (json);
-      /* Nothing really to verify, merchant says one of
-         the signatures is invalid; as we checked them,
-         this should never happen, we should pass the JSON
-         reply to the application */
-      break;
-    case MHD_HTTP_NOT_FOUND:
-      hr.ec = TALER_JSON_get_error_code (json);
-      hr.hint = TALER_JSON_get_error_hint (json);
-      /* Nothing really to verify, this should never
-   happen, we should pass the JSON reply to the
-         application */
-      break;
-    case MHD_HTTP_FAILED_DEPENDENCY:
-      TALER_MERCHANT_parse_error_details_ (json,
-                                           response_code,
-                                           &hr);
-      /* Nothing really to verify, the merchant is blaming the exchange.
-         We should pass the JSON reply to the application */
-      break;
-    case MHD_HTTP_INTERNAL_SERVER_ERROR:
-      hr.ec = TALER_JSON_get_error_code (json);
-      hr.hint = TALER_JSON_get_error_hint (json);
-      /* Server had an internal issue; we should retry,
-         but this API leaves this to the application */
-      break;
-    default:
-      /* unexpected response code */
-      TALER_MERCHANT_parse_error_details_ (json,
-                                           response_code,
-                                           &hr);
-      GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
-                  "Unexpected response code %u/%d\n",
-                  (unsigned int) response_code,
-                  (int) hr.ec);
       GNUNET_break_op (0);
-      break;
+      response_code = 0;
     }
-    ph->abort_cb (ph->abort_cb_cls,
-                  &hr,
-                  NULL,
-                  NULL,
-                  0,
-                  NULL);
+    break;
+  case MHD_HTTP_GONE:
+    hr.ec = TALER_JSON_get_error_code (json);
+    hr.hint = TALER_JSON_get_error_hint (json);
+    /* The merchant says we are too late, the offer has expired or some
+       denomination key of a coin involved has expired.
+       Might be a disagreement in timestamps? Still, pass on to application. */
+    break;
+  case MHD_HTTP_FAILED_DEPENDENCY:
+    TALER_MERCHANT_parse_error_details_ (json,
+                                         response_code,
+                                         &hr);
+    /* Nothing really to verify, the merchant is blaming the exchange.
+       We should pass the JSON reply to the application */
+    break;
+  case MHD_HTTP_INTERNAL_SERVER_ERROR:
+    hr.ec = TALER_JSON_get_error_code (json);
+    hr.hint = TALER_JSON_get_error_hint (json);
+    /* Server had an internal issue; we should retry,
+       but this API leaves this to the application */
+    break;
+  case MHD_HTTP_SERVICE_UNAVAILABLE:
+    TALER_MERCHANT_parse_error_details_ (json,
+                                         response_code,
+                                         &hr);
+    /* Exchange couldn't respond properly; the retry is
+       left to the application */
+    break;
+  default:
+    TALER_MERCHANT_parse_error_details_ (json,
+                                         response_code,
+                                         &hr);
+    /* unexpected response code */
+    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
+                "Unexpected response code %u/%d\n",
+                (unsigned int) response_code,
+                (int) hr.ec);
+    GNUNET_break_op (0);
+    break;
   }
-  TALER_MERCHANT_pay_cancel (ph);
+  oph->pay_cb (oph->pay_cb_cls,
+               &hr);
+  TALER_MERCHANT_order_pay_cancel (oph);
 }
 
 
 /**
- * Issue /pay request. Generic version for the various
- * variants of the API.
+ * PAY a merchant.  API for frontends talking to backends. Here,
+ * the frontend does not have the coin's private keys, but just
+ * the public keys and signatures.  Note the subtle difference
+ * in the type of @a coins compared to #TALER_MERCHANT_pay().
  *
  * @param ctx the execution loop context
  * @param merchant_url base URL of the merchant's backend
- * @param merchant_pub public key of the merchant
+ * @param order_id order to pay
+ * @param session_id session to pay for, or NULL for none
  * @param num_coins number of coins used to pay
  * @param coins array of coins we use to pay
- * @param mode mode string to use ("pay" or "abort-refund").
- * @param pay_cb the callback to call when a reply for this
- *        request is available
+ * @param pay_cb the callback to call when a reply for this request is available
  * @param pay_cb_cls closure for @a pay_cb
- * @param abort_cb callback to call for the abort-refund variant
- * @param abort_cb_cls closure for @a abort_cb
  * @return a handle for this request
  */
-static struct TALER_MERCHANT_Pay *
-request_pay_generic (
+struct TALER_MERCHANT_OrderPayHandle *
+TALER_MERCHANT_order_pay_frontend (
   struct GNUNET_CURL_Context *ctx,
   const char *merchant_url,
-  const struct TALER_MerchantPublicKeyP *merchant_pub,
   const char *order_id,
+  const char *session_id,
   unsigned int num_coins,
-  const struct TALER_MERCHANT_PaidCoin *coins,
-  const char *mode,
+  const struct TALER_MERCHANT_PaidCoin coins[],
   TALER_MERCHANT_OrderPayCallback pay_cb,
-  void *pay_cb_cls,
-  TALER_MERCHANT_PayRefundCallback abort_cb,
-  void *abort_cb_cls)
+  void *pay_cb_cls)
 {
-  struct TALER_MERCHANT_Pay *ph;
+  struct TALER_MERCHANT_OrderPayHandle *oph;
   json_t *pay_obj;
   json_t *j_coins;
   CURL *eh;
@@ -609,6 +381,7 @@ request_pay_generic (
     json_t *j_coin;
     const struct TALER_MERCHANT_PaidCoin *pc = &coins[i];
     struct TALER_Amount fee;
+    struct GNUNET_HashCode denom_hash;
 
     if (0 >
         TALER_amount_subtract (&fee,
@@ -644,6 +417,8 @@ request_pay_generic (
       }
     }
 
+    GNUNET_CRYPTO_rsa_public_key_hash (pc->denom_pub.rsa_public_key,
+                                       &denom_hash);
     /* create JSON for this coin */
     j_coin = json_pack (
       "{s:o, s:o,"                   /* contribution/coin_pub */
@@ -655,8 +430,8 @@ request_pay_generic (
       GNUNET_JSON_from_data_auto (&pc->coin_pub),
       "exchange_url",
       pc->exchange_url,
-      "denom_pub",
-      GNUNET_JSON_from_rsa_public_key (pc->denom_pub.rsa_public_key),
+      "h_denom",
+      GNUNET_JSON_from_data_auto (&denom_hash),
       "ub_sig",
       GNUNET_JSON_from_rsa_signature (pc->denom_sig.rsa_signature),
       "coin_sig",
@@ -671,202 +446,79 @@ request_pay_generic (
     }
   }
 
-  pay_obj = json_pack ("{"
-                       " s:s," /* mode */
-                       " s:o," /* coins */
-                       " s:s," /* order_id */
-                       " s:o," /* merchant_pub */
-                       "}",
-                       "mode",
-                       mode,
+  pay_obj = json_pack ("{ s:o }",
                        "coins",
-                       j_coins, /* reference consumed! */
-                       "order_id",
-                       order_id,
-                       "merchant_pub",
-                       GNUNET_JSON_from_data_auto (merchant_pub));
+                       j_coins);
   if (NULL == pay_obj)
   {
     GNUNET_break (0);
     return NULL;
   }
-  ph = GNUNET_new (struct TALER_MERCHANT_Pay);
-  ph->ctx = ctx;
-  ph->mode = mode;
-  ph->abort_cb = abort_cb;
-  ph->abort_cb_cls = abort_cb_cls;
-  ph->pay_cb = pay_cb;
-  ph->pay_cb_cls = pay_cb_cls;
-  ph->url = TALER_url_join (merchant_url, "pay", NULL);
-  if (NULL == ph->url)
+  if (NULL != session_id)
+  {
+    if (0 != json_object_set (pay_obj,
+                              "session_id",
+                              json_string (session_id)))
+    {
+      GNUNET_break (0);
+      json_decref (pay_obj);
+      return NULL;
+    }
+  }
+
+  oph = GNUNET_new (struct TALER_MERCHANT_OrderPayHandle);
+  oph->ctx = ctx;
+  oph->pay_cb = pay_cb;
+  oph->pay_cb_cls = pay_cb_cls;
+  {
+    char *path;
+
+    GNUNET_asprintf (&path,
+                     "orders/%s/pay",
+                     order_id);
+    oph->url = TALER_url_join (merchant_url,
+                               path,
+                               NULL);
+    GNUNET_free (path);
+  }
+  if (NULL == oph->url)
   {
     GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                 "Could not construct request URL.\n");
     json_decref (pay_obj);
-    GNUNET_free (ph);
+    GNUNET_free (oph);
     return NULL;
   }
-  ph->num_coins = num_coins;
-  ph->coins = GNUNET_new_array (num_coins,
-                                struct TALER_MERCHANT_PaidCoin);
-  memcpy (ph->coins,
+  oph->num_coins = num_coins;
+  oph->coins = GNUNET_new_array (num_coins,
+                                 struct TALER_MERCHANT_PaidCoin);
+  memcpy (oph->coins,
           coins,
           num_coins * sizeof (struct TALER_MERCHANT_PaidCoin));
 
   eh = curl_easy_init ();
-  if (GNUNET_OK != TALER_curl_easy_post (&ph->post_ctx,
-                                         eh,
-                                         pay_obj))
+  if (GNUNET_OK !=
+      TALER_curl_easy_post (&oph->post_ctx,
+                            eh,
+                            pay_obj))
   {
     GNUNET_break (0);
     json_decref (pay_obj);
-    GNUNET_free (ph);
+    GNUNET_free (oph);
     return NULL;
   }
 
   json_decref (pay_obj);
-  GNUNET_assert (CURLE_OK == curl_easy_setopt (eh,
-                                               CURLOPT_URL,
-                                               ph->url));
-  ph->job = GNUNET_CURL_job_add2 (ctx,
-                                  eh,
-                                  ph->post_ctx.headers,
-                                  &handle_pay_finished,
-                                  ph);
-  return ph;
-}
-
-
-/**
- * Pay a merchant.  API for wallets that have the coin's private
- * keys.
- * _NOTE_: this function does NOT calculate each coin amount in
- * order to match the contract total price.  This calculation is
- * to be made by the logic using this library.
- *
- * @param ctx the execution loop context
- * @param merchant_url base URL of the merchant's backend
- * @param h_contract_terms hashcode of the proposal being paid
- * @param amount total value of the contract to be paid to the
- *        merchant
- * @param max_fee maximum fee covered by the merchant
- *        (according to the contract)
- * @param merchant_pub the public key of the merchant
- *        (used to identify the merchant for refund requests)
- * @param merchant_sig signature from the merchant over the
- *        original contract
- * @param timestamp timestamp when the contract was finalized,
- *        must match approximately the current time of the merchant
- * @param refund_deadline date until which the merchant can issue
- *        a refund to the customer via the merchant (can be zero
- *        if refunds are not allowed)
- * @param pay_deadline maximum time limit to pay for this contract
- * @param h_wire hash of the merchant’s account details
- * @param order_id order id of the proposal being paid
- * @param num_coins number of coins used to pay
- * @param coins array of coins we use to pay
- * @param pay_cb the callback to call when a reply for this
- *        request is available
- * @param pay_cb_cls closure for @a pay_cb
- * @return a handle for this request
- */
-static struct TALER_MERCHANT_Pay *
-prepare_pay_generic (struct GNUNET_CURL_Context *ctx,
-                     const char *merchant_url,
-                     const struct GNUNET_HashCode *h_contract_terms,
-                     const struct TALER_Amount *amount,
-                     const struct TALER_Amount *max_fee,
-                     const struct TALER_MerchantPublicKeyP *merchant_pub,
-                     const struct TALER_MerchantSignatureP *merchant_sig,
-                     struct GNUNET_TIME_Absolute timestamp,
-                     struct GNUNET_TIME_Absolute refund_deadline,
-                     struct GNUNET_TIME_Absolute pay_deadline,
-                     const struct GNUNET_HashCode *h_wire,
-                     const char *order_id,
-                     unsigned int num_coins,
-                     const struct TALER_MERCHANT_PayCoin *coins,
-                     const char *mode,
-                     TALER_MERCHANT_PayCallback pay_cb,
-                     void *pay_cb_cls,
-                     TALER_MERCHANT_PayRefundCallback abort_cb,
-                     void *abort_cb_cls)
-{
-  struct TALER_DepositRequestPS dr;
-  struct TALER_MERCHANT_PaidCoin pc[num_coins];
-
-  (void) GNUNET_TIME_round_abs (&timestamp);
-  (void) GNUNET_TIME_round_abs (&pay_deadline);
-  (void) GNUNET_TIME_round_abs (&refund_deadline);
-
-  if (GNUNET_YES !=
-      TALER_amount_cmp_currency (amount,
-                                 max_fee))
-  {
-    GNUNET_break (0);
-    return NULL;
-  }
-
-  dr.purpose.purpose = htonl (TALER_SIGNATURE_WALLET_COIN_DEPOSIT);
-  dr.purpose.size = htonl (sizeof (struct TALER_DepositRequestPS));
-  dr.h_contract_terms = *h_contract_terms;
-  dr.h_wire = *h_wire;
-  dr.timestamp = GNUNET_TIME_absolute_hton (timestamp);
-  dr.refund_deadline = GNUNET_TIME_absolute_hton (refund_deadline);
-  dr.merchant = *merchant_pub;
-
-  for (unsigned int i = 0; i<num_coins; i++)
-  {
-    const struct TALER_MERCHANT_PayCoin *coin = &coins[i]; // coin priv.
-    struct TALER_MERCHANT_PaidCoin *p = &pc[i]; // coin pub.
-    struct TALER_Amount fee;
-
-    /* prepare 'dr' for this coin to generate coin signature */
-    GNUNET_CRYPTO_eddsa_key_get_public (&coin->coin_priv.eddsa_priv,
-                                        &dr.coin_pub.eddsa_pub);
-    TALER_amount_hton (&dr.amount_with_fee,
-                       &coin->amount_with_fee);
-    if (0 >
-        TALER_amount_subtract (&fee,
-                               &coin->amount_with_fee,
-                               &coin->amount_without_fee))
-    {
-      /* Integer underflow, fee larger than total amount?
-         This should not happen (client violated API!) */
-      GNUNET_break (0);
-      return NULL;
-    }
-    TALER_amount_hton (&dr.deposit_fee,
-                       &fee);
-    {
-      TALER_LOG_DEBUG ("... amount_with_fee was %s\n",
-                       TALER_amount2s (&coin->amount_with_fee));
-      TALER_LOG_DEBUG ("... fee was %s\n",
-                       TALER_amount2s (&fee));
-    }
-
-    GNUNET_CRYPTO_eddsa_sign (&coin->coin_priv.eddsa_priv,
-                              &dr,
-                              &p->coin_sig.eddsa_signature);
-    p->denom_pub = coin->denom_pub;
-    p->denom_sig = coin->denom_sig;
-    p->denom_value = coin->denom_value;
-    p->coin_pub = dr.coin_pub;
-    p->amount_with_fee = coin->amount_with_fee;
-    p->amount_without_fee = coin->amount_without_fee;
-    p->refund_fee = coin->refund_fee;
-    p->exchange_url = coin->exchange_url;
-  }
-  return request_pay_generic (ctx,
-                              merchant_url,
-                              merchant_pub,
-                              order_id,
-                              num_coins,
-                              pc,
-                              mode,
-                              pay_cb,
-                              pay_cb_cls,
-                              abort_cb,
-                              abort_cb_cls);
+  GNUNET_assert (CURLE_OK ==
+                 curl_easy_setopt (eh,
+                                   CURLOPT_URL,
+                                   oph->url));
+  oph->job = GNUNET_CURL_job_add2 (ctx,
+                                   eh,
+                                   oph->post_ctx.headers,
+                                   &handle_pay_finished,
+                                   oph);
+  return oph;
 }
 
 
@@ -878,6 +530,7 @@ prepare_pay_generic (struct GNUNET_CURL_Context *ctx,
  *
  * @param ctx the execution loop context
  * @param merchant_url base URL of the merchant's backend
+ * @param session_id session to pay for, or NULL for none
  * @param h_contract_terms hashcode of the proposal being paid
  * @param amount total value of the contract to be paid to the merchant
  * @param max_fee maximum fee covered by the merchant (according to the contract)
@@ -894,73 +547,11 @@ prepare_pay_generic (struct GNUNET_CURL_Context *ctx,
  * @param pay_cb_cls closure for @a pay_cb
  * @return a handle for this request
  */
-struct TALER_MERCHANT_Pay *
-TALER_MERCHANT_pay_wallet (struct GNUNET_CURL_Context *ctx,
-                           const char *merchant_url,
-                           const struct GNUNET_HashCode *h_contract_terms,
-                           const struct TALER_Amount *amount,
-                           const struct TALER_Amount *max_fee,
-                           const struct TALER_MerchantPublicKeyP *merchant_pub,
-                           const struct TALER_MerchantSignatureP *merchant_sig,
-                           struct GNUNET_TIME_Absolute timestamp,
-                           struct GNUNET_TIME_Absolute refund_deadline,
-                           struct GNUNET_TIME_Absolute pay_deadline,
-                           const struct GNUNET_HashCode *h_wire,
-                           const char *order_id,
-                           unsigned int num_coins,
-                           const struct TALER_MERCHANT_PayCoin *coins,
-                           TALER_MERCHANT_PayCallback pay_cb,
-                           void *pay_cb_cls)
-{
-  return prepare_pay_generic (ctx,
-                              merchant_url,
-                              h_contract_terms,
-                              amount,
-                              max_fee,
-                              merchant_pub,
-                              merchant_sig,
-                              timestamp,
-                              refund_deadline,
-                              pay_deadline,
-                              h_wire,
-                              order_id,
-                              num_coins,
-                              coins,
-                              "pay",
-                              pay_cb,
-                              pay_cb_cls,
-                              NULL,
-                              NULL);
-}
-
-
-/**
- * Run a payment abort operation, asking for refunds for coins
- * that were previously spend on a /pay that failed to go through.
- *
- * @param ctx execution context
- * @param merchant_url base URL of the merchant
- * @param h_wire hash of the merchant’s account details
- * @param h_contract hash of the contact of the merchant with the customer
- * @param transaction_id transaction id for the transaction between merchant and customer
- * @param amount total value of the contract to be paid to the merchant
- * @param max_fee maximum fee covered by the merchant (according to the contract)
- * @param merchant_pub the public key of the merchant (used to identify the merchant for refund requests)
- * @param merchant_sig signature from the merchant over the original contract
- * @param timestamp timestamp when the contract was finalized, must match approximately the current time of the merchant
- * @param refund_deadline date until which the merchant can issue a refund to the customer via the merchant (can be zero if refunds are not allowed)
- * @param pay_deadline maximum time limit to pay for this contract
- * @param num_coins number of coins used to pay
- * @param coins array of coins we use to pay
- * @param coin_sig the signature made with purpose #TALER_SIGNATURE_WALLET_COIN_DEPOSIT made by the customer with the coin’s private key.
- * @param payref_cb the callback to call when a reply for this request is available
- * @param payref_cb_cls closure for @a pay_cb
- * @return a handle for this request
- */
-struct TALER_MERCHANT_Pay *
-TALER_MERCHANT_pay_abort (struct GNUNET_CURL_Context *ctx,
+struct TALER_MERCHANT_OrderPayHandle *
+TALER_MERCHANT_order_pay (struct GNUNET_CURL_Context *ctx,
                           const char *merchant_url,
-                          const struct GNUNET_HashCode *h_contract,
+                          const char *session_id,
+                          const struct GNUNET_HashCode *h_contract_terms,
                           const struct TALER_Amount *amount,
                           const struct TALER_Amount *max_fee,
                           const struct TALER_MerchantPublicKeyP *merchant_pub,
@@ -971,97 +562,99 @@ TALER_MERCHANT_pay_abort (struct GNUNET_CURL_Context *ctx,
                           const struct GNUNET_HashCode *h_wire,
                           const char *order_id,
                           unsigned int num_coins,
-                          const struct TALER_MERCHANT_PayCoin *coins,
-                          TALER_MERCHANT_PayRefundCallback payref_cb,
-                          void *payref_cb_cls)
+                          const struct TALER_MERCHANT_PayCoin coins[],
+                          TALER_MERCHANT_OrderPayCallback pay_cb,
+                          void *pay_cb_cls)
 {
-  struct TALER_MERCHANT_Pay *ph;
-
-  ph = prepare_pay_generic (ctx,
-                            merchant_url,
-                            h_contract,
-                            amount,
-                            max_fee,
-                            merchant_pub,
-                            merchant_sig,
-                            timestamp,
-                            refund_deadline,
-                            pay_deadline,
-                            h_wire,
-                            order_id,
-                            num_coins,
-                            coins,
-                            "abort-refund",
-                            NULL,
-                            NULL,
-                            payref_cb,
-                            payref_cb_cls);
-  if (NULL == ph)
+  (void) GNUNET_TIME_round_abs (&timestamp);
+  (void) GNUNET_TIME_round_abs (&pay_deadline);
+  (void) GNUNET_TIME_round_abs (&refund_deadline);
+  if (GNUNET_YES !=
+      TALER_amount_cmp_currency (amount,
+                                 max_fee))
+  {
+    GNUNET_break (0);
     return NULL;
-  ph->h_contract_terms = *h_contract;
-  return ph;
-}
-
+  }
 
-/**
- * PAY a merchant.  API for frontends talking to backends. Here,
- * the frontend does not have the coin's private keys, but just
- * the public keys and signatures.  Note the subtle difference
- * in the type of @a coins compared to #TALER_MERCHANT_pay().
- *
- * @param ctx the execution loop context
- * @param merchant_url base URL of the merchant's backend
- * @param merchant_pub public key of the merchant
- * @param num_coins number of coins used to pay
- * @param coins array of coins we use to pay
- * @param pay_cb the callback to call when a reply for this request is available
- * @param pay_cb_cls closure for @a pay_cb
- * @return a handle for this request
- */
-struct TALER_MERCHANT_Pay *
-TALER_MERCHANT_pay_frontend (
-  struct GNUNET_CURL_Context *ctx,
-  const char *merchant_url,
-  const struct TALER_MerchantPublicKeyP *merchant_pub,
-  const char *order_id,
-  unsigned int num_coins,
-  const struct TALER_MERCHANT_PaidCoin *coins,
-  TALER_MERCHANT_PayCallback pay_cb,
-  void *pay_cb_cls)
-{
-  return request_pay_generic (ctx,
-                              merchant_url,
-                              merchant_pub,
-                              order_id,
-                              num_coins,
-                              coins,
-                              "pay",
-                              pay_cb,
-                              pay_cb_cls,
-                              NULL,
-                              NULL);
+  {
+    struct TALER_MERCHANT_PaidCoin pc[num_coins];
+    struct TALER_DepositRequestPS dr = {
+      .purpose.purpose = htonl (TALER_SIGNATURE_WALLET_COIN_DEPOSIT),
+      .purpose.size = htonl (sizeof (dr)),
+      .h_contract_terms = *h_contract_terms,
+      .h_wire = *h_wire,
+      .timestamp = GNUNET_TIME_absolute_hton (timestamp),
+      .refund_deadline = GNUNET_TIME_absolute_hton (refund_deadline),
+      .merchant = *merchant_pub
+    };
+
+    for (unsigned int i = 0; i<num_coins; i++)
+    {
+      const struct TALER_MERCHANT_PayCoin *coin = &coins[i]; // coin priv.
+      struct TALER_MERCHANT_PaidCoin *p = &pc[i]; // coin pub.
+      struct TALER_Amount fee;
+
+      /* prepare 'dr' for this coin to generate coin signature */
+      GNUNET_CRYPTO_eddsa_key_get_public (&coin->coin_priv.eddsa_priv,
+                                          &dr.coin_pub.eddsa_pub);
+      TALER_amount_hton (&dr.amount_with_fee,
+                         &coin->amount_with_fee);
+      if (0 >
+          TALER_amount_subtract (&fee,
+                                 &coin->amount_with_fee,
+                                 &coin->amount_without_fee))
+      {
+        /* Integer underflow, fee larger than total amount?
+           This should not happen (client violated API!) */
+        GNUNET_break (0);
+        return NULL;
+      }
+      TALER_amount_hton (&dr.deposit_fee,
+                         &fee);
+      GNUNET_CRYPTO_eddsa_sign (&coin->coin_priv.eddsa_priv,
+                                &dr,
+                                &p->coin_sig.eddsa_signature);
+      p->denom_pub = coin->denom_pub;
+      p->denom_sig = coin->denom_sig;
+      p->denom_value = coin->denom_value;
+      p->coin_pub = dr.coin_pub;
+      p->amount_with_fee = coin->amount_with_fee;
+      p->amount_without_fee = coin->amount_without_fee;
+      p->refund_fee = coin->refund_fee;
+      p->exchange_url = coin->exchange_url;
+    }
+    return TALER_MERCHANT_order_pay_frontend (ctx,
+                                              merchant_url,
+                                              order_id,
+                                              session_id,
+                                              num_coins,
+                                              pc,
+                                              pay_cb,
+                                              pay_cb_cls);
+  }
 }
 
 
 /**
- * Cancel a pay permission request.  This function cannot be used
- * on a request handle if a response is already served for it.
+ * Cancel a pay request.  This function cannot be used on a request handle if
+ * a response is already served for it.
  *
- * @param pay the pay permission request handle
+ * @param oph the pay request handle
  */
 void
-TALER_MERCHANT_pay_cancel (struct TALER_MERCHANT_Pay *pay)
+TALER_MERCHANT_order_pay_cancel (struct TALER_MERCHANT_OrderPayHandle *oph)
 {
-  if (NULL != pay->job)
+  if (NULL != oph->job)
   {
-    GNUNET_CURL_job_cancel (pay->job);
-    pay->job = NULL;
+    GNUNET_CURL_job_cancel (oph->job);
+    oph->job = NULL;
   }
-  TALER_curl_easy_post_finished (&pay->post_ctx);
-  GNUNET_free (pay->coins);
-  GNUNET_free (pay->url);
-  GNUNET_free (pay);
+  TALER_curl_easy_post_finished (&oph->post_ctx);
+  GNUNET_free (oph->coins);
+  GNUNET_free (oph->url);
+  GNUNET_free (oph);
 }
 
 
-/* end of merchant_api_pay.c */
+/* end of merchant_api_post_order_pay.c */