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diff --git a/src/lib/exchange_api_common.h b/src/lib/exchange_api_common.h
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+++ b/src/lib/exchange_api_common.h
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+/*
+  This file is part of TALER
+  Copyright (C) 2015-2022 Taler Systems SA
+
+  TALER is free software; you can redistribute it and/or modify it under the
+  terms of the GNU General Public License as published by the Free Software
+  Foundation; either version 3, 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 General Public License for more details.
+
+  You should have received a copy of the GNU General Public License along with
+  TALER; see the file COPYING.  If not, see
+  <http://www.gnu.org/licenses/>
+*/
+/**
+ * @file lib/exchange_api_common.h
+ * @brief common functions for the exchange API
+ * @author Christian Grothoff
+ */
+#ifndef EXCHANGE_API_COMMON_H
+#define EXCHANGE_API_COMMON_H
+
+#include "taler_json_lib.h"
+#include "taler_exchange_service.h"
+
+
+/**
+ * Check proof of a purse creation conflict.
+ *
+ * @param cpurse_sig conflicting signature (must
+ *        not match the signature from the proof)
+ * @param purse_pub the public key (must match
+ *        the signature from the proof)
+ * @param proof the proof to check
+ * @return #GNUNET_OK if the @a proof is OK for @a purse_pub and conflicts with @a cpurse_sig
+ */
+enum GNUNET_GenericReturnValue
+TALER_EXCHANGE_check_purse_create_conflict_ (
+  const struct TALER_PurseContractSignatureP *cpurse_sig,
+  const struct TALER_PurseContractPublicKeyP *purse_pub,
+  const json_t *proof);
+
+
+/**
+ * Check proof of a purse merge conflict.
+ *
+ * @param cmerge_sig conflicting signature (must
+ *        not match the signature from the proof)
+ * @param merge_pub the public key (must match
+ *        the signature from the proof)
+ * @param purse_pub the public key of the purse
+ * @param exchange_url the base URL of this exchange
+ * @param proof the proof to check
+ * @return #GNUNET_OK if the @a proof is OK for @a purse_pub and @a merge_pub and conflicts with @a cmerge_sig
+ */
+enum GNUNET_GenericReturnValue
+TALER_EXCHANGE_check_purse_merge_conflict_ (
+  const struct TALER_PurseMergeSignatureP *cmerge_sig,
+  const struct TALER_PurseMergePublicKeyP *merge_pub,
+  const struct TALER_PurseContractPublicKeyP *purse_pub,
+  const char *exchange_url,
+  const json_t *proof);
+
+
+/**
+ * Check @a proof that claims this coin was spend
+ * differently on the same purse already. Note that
+ * the caller must still check that @a coin_pub is
+ * in the list of coins that were used, and that
+ * @a coin_sig is different from the signature the
+ * caller used.
+ *
+ * @param purse_pub the public key of the purse
+ * @param exchange_url base URL of our exchange
+ * @param proof the proof to check
+ * @param[out] h_denom_pub hash of the coin's denomination
+ * @param[out] phac age commitment hash of the coin
+ * @param[out] coin_pub set to the conflicting coin
+ * @param[out] coin_sig set to the conflicting signature
+ * @return #GNUNET_OK if the @a proof is OK for @a purse_pub and showing that @a coin_pub was spent using @a coin_sig.
+ */
+enum GNUNET_GenericReturnValue
+TALER_EXCHANGE_check_purse_coin_conflict_ (
+  const struct TALER_PurseContractPublicKeyP *purse_pub,
+  const char *exchange_url,
+  const json_t *proof,
+  struct TALER_DenominationHashP *h_denom_pub,
+  struct TALER_AgeCommitmentHash *phac,
+  struct TALER_CoinSpendPublicKeyP *coin_pub,
+  struct TALER_CoinSpendSignatureP *coin_sig);
+
+
+/**
+ * Check proof of a contract conflict.
+ *
+ * @param ccontract_sig conflicting signature (must
+ *        not match the signature from the proof)
+ * @param purse_pub public key of the purse
+ * @param proof the proof to check
+ * @return #GNUNET_OK if the @a proof is OK for @a purse_pub and conflicts with @a ccontract_sig
+ */
+enum GNUNET_GenericReturnValue
+TALER_EXCHANGE_check_purse_econtract_conflict_ (
+  const struct TALER_PurseContractSignatureP *ccontract_sig,
+  const struct TALER_PurseContractPublicKeyP *purse_pub,
+  const json_t *proof);
+
+
+/**
+ * Check proof of a coin spend value conflict.
+ *
+ * @param keys exchange /keys structure
+ * @param proof the proof to check
+ * @param[out] coin_pub set to the public key of the
+ *        coin that is claimed to have an insufficient
+ *        balance
+ * @param[out] remaining set to the remaining balance
+ *        of the coin as provided by the proof
+ * @return #GNUNET_OK if the @a proof is OK for @a purse_pub demonstrating that @a coin_pub has only @a remaining balance.
+ */
+enum GNUNET_GenericReturnValue
+TALER_EXCHANGE_check_coin_amount_conflict_ (
+  const struct TALER_EXCHANGE_Keys *keys,
+  const json_t *proof,
+  struct TALER_CoinSpendPublicKeyP *coin_pub,
+  struct TALER_Amount *remaining);
+
+
+/**
+ * Verify that @a proof contains a coin history that demonstrates that @a
+ * coin_pub was previously used with a denomination key that is different from
+ * @a ch_denom_pub.  Note that the coin history MUST have been checked before
+ * using #TALER_EXCHANGE_check_coin_amount_conflict_().
+ *
+ * @param proof a proof to check
+ * @param ch_denom_pub hash of the conflicting denomination
+ * @return #GNUNET_OK if @a ch_denom_pub differs from the
+ *         denomination hash given by the history of the coin
+ */
+enum GNUNET_GenericReturnValue
+TALER_EXCHANGE_check_coin_denomination_conflict_ (
+  const json_t *proof,
+  const struct TALER_DenominationHashP *ch_denom_pub);
+
+
+/**
+ * Find the smallest denomination amount in @e keys.
+ *
+ * @param keys keys to search
+ * @param[out] min set to the smallest amount
+ * @return #GNUNET_SYSERR if there are no denominations in @a keys
+ */
+enum GNUNET_GenericReturnValue
+TALER_EXCHANGE_get_min_denomination_ (
+  const struct TALER_EXCHANGE_Keys *keys,
+  struct TALER_Amount *min);
+
+
+/**
+ * Verify signature information about the deposit.
+ *
+ * @param dcd contract details
+ * @param ech hashed policy (passed to avoid recomputation)
+ * @param h_wire hashed wire details (passed to avoid recomputation)
+ * @param cdd coin-specific details
+ * @param dki denomination of the coin
+ * @return #GNUNET_OK if signatures are OK, #GNUNET_SYSERR if not
+ */
+enum GNUNET_GenericReturnValue
+TALER_EXCHANGE_verify_deposit_signature_ (
+  const struct TALER_EXCHANGE_DepositContractDetail *dcd,
+  const struct TALER_ExtensionPolicyHashP *ech,
+  const struct TALER_MerchantWireHashP *h_wire,
+  const struct TALER_EXCHANGE_CoinDepositDetail *cdd,
+  const struct TALER_EXCHANGE_DenomPublicKey *dki);
+
+
+#endif