From f8c01f4b46a8391f3196889e5df7f8ebdfb6475d Mon Sep 17 00:00:00 2001
From: Christian Grothoff <christian@grothoff.org>
Date: Tue, 23 Jun 2020 11:00:23 +0200
Subject: implement signature verification on pay response

---
 src/lib/merchant_api_post_order_pay.c | 85 ++++++++++++++++++++++++++++++-----
 1 file changed, 75 insertions(+), 10 deletions(-)

(limited to 'src/lib/merchant_api_post_order_pay.c')

diff --git a/src/lib/merchant_api_post_order_pay.c b/src/lib/merchant_api_post_order_pay.c
index 650211c9..477c3a31 100644
--- a/src/lib/merchant_api_post_order_pay.c
+++ b/src/lib/merchant_api_post_order_pay.c
@@ -77,11 +77,29 @@ struct TALER_MERCHANT_OrderPayHandle
    */
   struct TALER_MERCHANT_PaidCoin *coins;
 
+  /**
+   * Hash of the contract we are paying, set
+   * if @e am_wallet is true.
+   */
+  struct GNUNET_HashCode h_contract_terms;
+
+  /**
+   * Public key of the merchant (instance) being paid, set
+   * if @e am_wallet is true.
+   */
+  struct TALER_MerchantPublicKeyP merchant_pub;
+
   /**
    * Number of @e coins we are paying with.
    */
   unsigned int num_coins;
 
+  /**
+   * Set to true if this is the wallet API and we have
+   * initialized @e h_contract_terms and @e merchant_pub.
+   */
+  bool am_wallet;
+
 };
 
 
@@ -234,8 +252,45 @@ handle_pay_finished (void *cls,
     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!
+    if (oph->am_wallet)
+    {
+      /* Here we can (and should) verify the merchant's signature */
+      struct PaymentResponsePS pr = {
+        .purpose.purpose = htonl (TALER_SIGNATURE_MERCHANT_PAYMENT_OK),
+        .purpose.size = htonl (sizeof (pr)),
+        .h_contract_terms = oph->h_contract_terms
+      };
+      struct TALER_MerchantSignatureP merchant_sig;
+      struct GNUNET_JSON_Specification spec[] = {
+        GNUNET_JSON_spec_fixed_auto ("sig",
+                                     &merchant_sig),
+        GNUNET_JSON_spec_end ()
+      };
+
+      if (GNUNET_OK !=
+          GNUNET_JSON_parse (json,
+                             spec,
+                             NULL, NULL))
+      {
+        GNUNET_break_op (0);
+        hr.ec = TALER_EC_INVALID_RESPONSE;
+        hr.http_status = 0;
+        hr.hint = "sig field missing in response";
+        break;
+      }
+
+      if (GNUNET_OK !=
+          GNUNET_CRYPTO_eddsa_verify (TALER_SIGNATURE_MERCHANT_PAYMENT_OK,
+                                      &pr,
+                                      &merchant_sig.eddsa_sig,
+                                      &oph->merchant_pub.eddsa_pub))
+      {
+        GNUNET_break_op (0);
+        hr.ec = TALER_EC_INVALID_RESPONSE;
+        hr.http_status = 0;
+        hr.hint = "signature invalid";
+      }
+    }
     break;
   /* Tolerating Not Acceptable because sometimes
    * - especially in tests - we might want to POST
@@ -623,14 +678,24 @@ TALER_MERCHANT_order_pay (struct GNUNET_CURL_Context *ctx,
       p->amount_without_fee = coin->amount_without_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);
+    {
+      struct TALER_MERCHANT_OrderPayHandle *oph;
+
+      oph = TALER_MERCHANT_order_pay_frontend (ctx,
+                                               merchant_url,
+                                               order_id,
+                                               session_id,
+                                               num_coins,
+                                               pc,
+                                               pay_cb,
+                                               pay_cb_cls);
+      if (NULL == oph)
+        return NULL;
+      oph->h_contract_terms = *h_contract_terms;
+      oph->merchant_pub = *merchant_pub;
+      oph->am_wallet = true;
+      return oph;
+    }
   }
 }
 
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