diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -319,12 +319,15 @@
 	threadinterrupt.cpp
 	uint256.cpp
 	util/bytevectorhash.cpp
+	util/error.cpp
 	util/moneystr.cpp
 	util/settings.cpp
 	util/strencodings.cpp
 	util/system.cpp
 	util/threadnames.cpp
 	util/time.cpp
+	util/url.cpp
+	util/validation.cpp
 
 	# obj/build.h
 	"${BUILD_HEADER}"
diff --git a/src/Makefile.am b/src/Makefile.am
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -234,12 +234,15 @@
   util/bitmanip.h \
   util/bytevectorhash.h \
   util/check.h \
+  util/error.h \
   util/macros.h \
   util/moneystr.h \
   util/system.h \
   util/settings.h \
   util/threadnames.h \
   util/time.h \
+  util/url.h \
+  util/validation.h \
   validation.h \
   validationinterface.h \
   versionbits.h \
@@ -525,12 +528,15 @@
   threadinterrupt.cpp \
   uint256.cpp \
   uint256.h \
+  util/error.cpp \
   util/system.cpp \
   util/moneystr.cpp \
   util/settings.cpp \
   util/strencodings.cpp \
   util/threadnames.cpp \
   util/time.cpp \
+  util/url.cpp \
+  util/validation.cpp \
   util/bytevectorhash.cpp \
   $(BITCOIN_CORE_H)
 
diff --git a/src/init.cpp b/src/init.cpp
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -51,6 +51,7 @@
 #include <util/moneystr.h>
 #include <util/system.h>
 #include <util/threadnames.h>
+#include <util/validation.h>
 #include <validation.h>
 #include <validationinterface.h>
 #include <walletinitinterface.h>
diff --git a/src/miner.cpp b/src/miner.cpp
--- a/src/miner.cpp
+++ b/src/miner.cpp
@@ -26,6 +26,7 @@
 #include <txmempool.h>
 #include <util/moneystr.h>
 #include <util/system.h>
+#include <util/validation.h>
 #include <validation.h>
 #include <validationinterface.h>
 
diff --git a/src/net_processing.cpp b/src/net_processing.cpp
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -33,6 +33,7 @@
 #include <util/moneystr.h>
 #include <util/strencodings.h>
 #include <util/system.h>
+#include <util/validation.h>
 #include <validation.h>
 #include <validationinterface.h>
 
diff --git a/src/node/transaction.h b/src/node/transaction.h
--- a/src/node/transaction.h
+++ b/src/node/transaction.h
@@ -6,30 +6,11 @@
 #define BITCOIN_NODE_TRANSACTION_H
 
 #include <primitives/transaction.h>
+#include <util/error.h>
 
 class Config;
 struct TxId;
 
-enum class TransactionError {
-    OK = 0,
-    UNKNOWN_ERROR,
-
-    MISSING_INPUTS,
-    ALREADY_IN_CHAIN,
-    P2P_DISABLED,
-    MEMPOOL_REJECTED,
-    MEMPOOL_ERROR,
-    INVALID_PSBT,
-    PSBT_MISMATCH,
-    SIGHASH_MISMATCH,
-
-    ERROR_COUNT
-};
-
-#define TRANSACTION_ERR_LAST TransactionError::ERROR_COUNT
-
-const char *TransactionErrorString(const TransactionError error);
-
 /**
  * Broadcast a transaction
  *
diff --git a/src/node/transaction.cpp b/src/node/transaction.cpp
--- a/src/node/transaction.cpp
+++ b/src/node/transaction.cpp
@@ -10,39 +10,12 @@
 #include <net.h>
 #include <primitives/txid.h>
 #include <txmempool.h>
+#include <util/validation.h>
 #include <validation.h>
 #include <validationinterface.h>
 
 #include <future>
 
-const char *TransactionErrorString(const TransactionError err) {
-    switch (err) {
-        case TransactionError::OK:
-            return "No error";
-        case TransactionError::MISSING_INPUTS:
-            return "Missing inputs";
-        case TransactionError::ALREADY_IN_CHAIN:
-            return "Transaction already in block chain";
-        case TransactionError::P2P_DISABLED:
-            return "Peer-to-peer functionality missing or disabled";
-        case TransactionError::MEMPOOL_REJECTED:
-            return "Transaction rejected by AcceptToMemoryPool";
-        case TransactionError::MEMPOOL_ERROR:
-            return "AcceptToMemoryPool failed";
-        case TransactionError::INVALID_PSBT:
-            return "PSBT is not sane";
-        case TransactionError::PSBT_MISMATCH:
-            return "PSBTs not compatible (different transactions)";
-        case TransactionError::SIGHASH_MISMATCH:
-            return "Specified sighash value does not match existing value";
-
-        case TransactionError::UNKNOWN_ERROR:
-        default:
-            break;
-    }
-    return "Unknown error";
-}
-
 bool BroadcastTransaction(const Config &config, const CTransactionRef tx,
                           TxId &txid, TransactionError &error,
                           std::string &err_string, const bool allowhighfees) {
diff --git a/src/policy/policy.cpp b/src/policy/policy.cpp
--- a/src/policy/policy.cpp
+++ b/src/policy/policy.cpp
@@ -6,14 +6,13 @@
 // NOTE: This file is intended to be customised by the end user, and includes
 // only local node policy logic
 
+#include <coins.h>
 #include <policy/policy.h>
-
 #include <policy/settings.h>
 #include <script/interpreter.h>
 #include <tinyformat.h>
 #include <util/strencodings.h>
 #include <util/system.h>
-#include <validation.h>
 
 Amount GetDustThreshold(const CTxOut &txout, const CFeeRate &dustRelayFeeIn) {
     /**
diff --git a/src/qt/signverifymessagedialog.cpp b/src/qt/signverifymessagedialog.cpp
--- a/src/qt/signverifymessagedialog.cpp
+++ b/src/qt/signverifymessagedialog.cpp
@@ -10,7 +10,7 @@
 #include <qt/guiutil.h>
 #include <qt/platformstyle.h>
 #include <qt/walletmodel.h>
-#include <validation.h> // For strMessageMagic
+#include <util/validation.h> // For strMessageMagic
 #include <wallet/wallet.h>
 
 #include <QClipboard>
diff --git a/src/rpc/blockchain.cpp b/src/rpc/blockchain.cpp
--- a/src/rpc/blockchain.cpp
+++ b/src/rpc/blockchain.cpp
@@ -27,6 +27,7 @@
 #include <txmempool.h>
 #include <util/strencodings.h>
 #include <util/system.h>
+#include <util/validation.h>
 #include <validation.h>
 #include <validationinterface.h>
 #include <versionbitsinfo.h> // For VersionBitsDeploymentInfo
diff --git a/src/rpc/mining.cpp b/src/rpc/mining.cpp
--- a/src/rpc/mining.cpp
+++ b/src/rpc/mining.cpp
@@ -25,6 +25,7 @@
 #include <txmempool.h>
 #include <util/strencodings.h>
 #include <util/system.h>
+#include <util/validation.h>
 #include <validation.h>
 #include <validationinterface.h>
 #include <warnings.h>
diff --git a/src/rpc/misc.cpp b/src/rpc/misc.cpp
--- a/src/rpc/misc.cpp
+++ b/src/rpc/misc.cpp
@@ -20,6 +20,7 @@
 #include <util/check.h>
 #include <util/strencodings.h>
 #include <util/system.h>
+#include <util/validation.h>
 #include <validation.h>
 #include <warnings.h>
 
diff --git a/src/test/setup_common.cpp b/src/test/setup_common.cpp
--- a/src/test/setup_common.cpp
+++ b/src/test/setup_common.cpp
@@ -30,6 +30,7 @@
 #include <txmempool.h>
 #include <util/strencodings.h>
 #include <util/time.h>
+#include <util/validation.h>
 #include <validation.h>
 #include <validationinterface.h>
 
diff --git a/src/ui_interface.h b/src/ui_interface.h
--- a/src/ui_interface.h
+++ b/src/ui_interface.h
@@ -145,11 +145,6 @@
 /** Show error message **/
 bool InitError(const std::string &str);
 
-std::string AmountHighWarn(const std::string &optname);
-
-std::string AmountErrMsg(const char *const optname,
-                         const std::string &strValue);
-
 extern CClientUIInterface uiInterface;
 
 #endif // BITCOIN_UI_INTERFACE_H
diff --git a/src/ui_interface.cpp b/src/ui_interface.cpp
--- a/src/ui_interface.cpp
+++ b/src/ui_interface.cpp
@@ -106,13 +106,3 @@
 void InitWarning(const std::string &str) {
     uiInterface.ThreadSafeMessageBox(str, "", CClientUIInterface::MSG_WARNING);
 }
-
-std::string AmountHighWarn(const std::string &optname) {
-    return strprintf(_("%s is set very high!"), optname);
-}
-
-std::string AmountErrMsg(const char *const optname,
-                         const std::string &strValue) {
-    return strprintf(_("Invalid amount for -%s=<amount>: '%s'"), optname,
-                     strValue);
-}
diff --git a/src/util/error.h b/src/util/error.h
new file mode 100644
--- /dev/null
+++ b/src/util/error.h
@@ -0,0 +1,44 @@
+// Copyright (c) 2010-2018 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_UTIL_ERROR_H
+#define BITCOIN_UTIL_ERROR_H
+
+/**
+ * util/error.h is a common place for definitions of simple error types and
+ * string functions. Types and functions defined here should not require any
+ * outside dependencies.
+ *
+ * Error types defined here can be used in different parts of the bitcoin
+ * codebase, to avoid the need to write boilerplate code catching and
+ * translating errors passed across wallet/node/rpc/gui code boundaries.
+ */
+
+#include <string>
+
+enum class TransactionError {
+    OK = 0,
+
+    MISSING_INPUTS,
+    ALREADY_IN_CHAIN,
+    P2P_DISABLED,
+    MEMPOOL_REJECTED,
+    MEMPOOL_ERROR,
+    INVALID_PSBT,
+    PSBT_MISMATCH,
+    SIGHASH_MISMATCH,
+
+    ERROR_COUNT
+};
+
+#define TRANSACTION_ERR_LAST TransactionError::ERROR_COUNT
+
+const char *TransactionErrorString(const TransactionError error);
+
+std::string AmountHighWarn(const std::string &optname);
+
+std::string AmountErrMsg(const char *const optname,
+                         const std::string &strValue);
+
+#endif // BITCOIN_UTIL_ERROR_H
diff --git a/src/util/error.cpp b/src/util/error.cpp
new file mode 100644
--- /dev/null
+++ b/src/util/error.cpp
@@ -0,0 +1,44 @@
+// Copyright (c) 2010-2018 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <util/error.h>
+
+#include <util/system.h>
+
+const char *TransactionErrorString(const TransactionError err) {
+    switch (err) {
+        case TransactionError::OK:
+            return "No error";
+        case TransactionError::MISSING_INPUTS:
+            return "Missing inputs";
+        case TransactionError::ALREADY_IN_CHAIN:
+            return "Transaction already in block chain";
+        case TransactionError::P2P_DISABLED:
+            return "Peer-to-peer functionality missing or disabled";
+        case TransactionError::MEMPOOL_REJECTED:
+            return "Transaction rejected by AcceptToMemoryPool";
+        case TransactionError::MEMPOOL_ERROR:
+            return "AcceptToMemoryPool failed";
+        case TransactionError::INVALID_PSBT:
+            return "PSBT is not sane";
+        case TransactionError::PSBT_MISMATCH:
+            return "PSBTs not compatible (different transactions)";
+        case TransactionError::SIGHASH_MISMATCH:
+            return "Specified sighash value does not match existing value";
+
+        default:
+            break;
+    }
+    return "Unknown error";
+}
+
+std::string AmountHighWarn(const std::string &optname) {
+    return strprintf(_("%s is set very high!"), optname);
+}
+
+std::string AmountErrMsg(const char *const optname,
+                         const std::string &strValue) {
+    return strprintf(_("Invalid amount for -%s=<amount>: '%s'"), optname,
+                     strValue);
+}
diff --git a/src/util/url.h b/src/util/url.h
new file mode 100644
--- /dev/null
+++ b/src/util/url.h
@@ -0,0 +1,12 @@
+// Copyright (c) 2015-2018 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_UTIL_URL_H
+#define BITCOIN_UTIL_URL_H
+
+#include <string>
+
+std::string urlDecode(const std::string &urlEncoded);
+
+#endif // BITCOIN_UTIL_URL_H
diff --git a/src/util/url.cpp b/src/util/url.cpp
new file mode 100644
--- /dev/null
+++ b/src/util/url.cpp
@@ -0,0 +1,21 @@
+// Copyright (c) 2015-2018 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <util/url.h>
+
+#include <cstdlib>
+#include <event2/http.h>
+#include <string>
+
+std::string urlDecode(const std::string &urlEncoded) {
+    std::string res;
+    if (!urlEncoded.empty()) {
+        char *decoded = evhttp_uridecode(urlEncoded.c_str(), false, nullptr);
+        if (decoded) {
+            res = std::string(decoded);
+            free(decoded);
+        }
+    }
+    return res;
+}
diff --git a/src/util/validation.h b/src/util/validation.h
new file mode 100644
--- /dev/null
+++ b/src/util/validation.h
@@ -0,0 +1,18 @@
+// Copyright (c) 2009-2010 Satoshi Nakamoto
+// Copyright (c) 2009-2019 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_UTIL_VALIDATION_H
+#define BITCOIN_UTIL_VALIDATION_H
+
+#include <string>
+
+class CValidationState;
+
+/** Convert CValidationState to a human-readable message for logging */
+std::string FormatStateMessage(const CValidationState &state);
+
+extern const std::string strMessageMagic;
+
+#endif // BITCOIN_UTIL_VALIDATION_H
diff --git a/src/util/validation.cpp b/src/util/validation.cpp
new file mode 100644
--- /dev/null
+++ b/src/util/validation.cpp
@@ -0,0 +1,19 @@
+// Copyright (c) 2009-2010 Satoshi Nakamoto
+// Copyright (c) 2009-2019 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <util/validation.h>
+
+#include <consensus/validation.h>
+#include <tinyformat.h>
+
+/** Convert CValidationState to a human-readable message for logging */
+std::string FormatStateMessage(const CValidationState &state) {
+    return strprintf(
+        "%s%s (code %i)", state.GetRejectReason(),
+        state.GetDebugMessage().empty() ? "" : ", " + state.GetDebugMessage(),
+        state.GetRejectCode());
+}
+
+const std::string strMessageMagic = "Bitcoin Signed Message:\n";
diff --git a/src/validation.h b/src/validation.h
--- a/src/validation.h
+++ b/src/validation.h
@@ -177,7 +177,6 @@
 extern CScript COINBASE_FLAGS;
 extern RecursiveMutex cs_main;
 extern CTxMemPool g_mempool;
-extern const std::string strMessageMagic;
 extern Mutex g_best_block_mutex;
 extern std::condition_variable g_best_block_cv;
 extern uint256 g_best_block;
@@ -439,9 +438,6 @@
                         const Amount nAbsurdFee, bool test_accept = false)
     EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
-/** Convert CValidationState to a human-readable message for logging */
-std::string FormatStateMessage(const CValidationState &state);
-
 /**
  * Simple class for regulating resource usage during CheckInputs (and
  * CScriptCheck), atomic so as to be compatible with parallel validation.
diff --git a/src/validation.cpp b/src/validation.cpp
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -48,6 +48,7 @@
 #include <util/moneystr.h>
 #include <util/strencodings.h>
 #include <util/system.h>
+#include <util/validation.h>
 #include <validationinterface.h>
 #include <warnings.h>
 
@@ -283,8 +284,6 @@
 /** Constant stuff for coinbase transactions we create: */
 CScript COINBASE_FLAGS;
 
-const std::string strMessageMagic = "Bitcoin Signed Message:\n";
-
 // Internal stuff
 namespace {
 CBlockIndex *&pindexBestInvalid = g_chainstate.pindexBestInvalid;
@@ -453,14 +452,6 @@
     return EvaluateSequenceLocks(index, lockPair);
 }
 
-/** Convert CValidationState to a human-readable message for logging */
-std::string FormatStateMessage(const CValidationState &state) {
-    return strprintf(
-        "%s%s (code %i)", state.GetRejectReason(),
-        state.GetDebugMessage().empty() ? "" : ", " + state.GetDebugMessage(),
-        state.GetRejectCode());
-}
-
 // Command-line argument "-replayprotectionactivationtime=<timestamp>" will
 // cause the node to switch to replay protected SigHash ForkID value when the
 // median timestamp of the previous 11 blocks is greater than or equal to
diff --git a/src/wallet/init.cpp b/src/wallet/init.cpp
--- a/src/wallet/init.cpp
+++ b/src/wallet/init.cpp
@@ -9,6 +9,7 @@
 #include <interfaces/chain.h>
 #include <net.h>
 #include <scheduler.h>
+#include <util/error.h>
 #include <util/moneystr.h>
 #include <util/system.h>
 #include <validation.h>
diff --git a/src/wallet/rpcwallet.cpp b/src/wallet/rpcwallet.cpp
--- a/src/wallet/rpcwallet.cpp
+++ b/src/wallet/rpcwallet.cpp
@@ -25,6 +25,8 @@
 #include <timedata.h>
 #include <util/moneystr.h>
 #include <util/system.h>
+#include <util/url.h>
+#include <util/validation.h>
 #include <validation.h>
 #include <wallet/coincontrol.h>
 #include <wallet/psbtwallet.h>
@@ -41,18 +43,6 @@
 
 static const std::string WALLET_ENDPOINT_BASE = "/wallet/";
 
-static std::string urlDecode(const std::string &urlEncoded) {
-    std::string res;
-    if (!urlEncoded.empty()) {
-        char *decoded = evhttp_uridecode(urlEncoded.c_str(), false, nullptr);
-        if (decoded) {
-            res = std::string(decoded);
-            free(decoded);
-        }
-    }
-    return res;
-}
-
 bool GetWalletNameFromJSONRPCRequest(const JSONRPCRequest &request,
                                      std::string &wallet_name) {
     if (request.URI.substr(0, WALLET_ENDPOINT_BASE.size()) ==
diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -29,8 +29,10 @@
 #include <timedata.h>
 #include <txmempool.h>
 #include <ui_interface.h>
+#include <util/error.h>
 #include <util/moneystr.h>
 #include <util/system.h>
+#include <util/validation.h>
 #include <validation.h>
 #include <wallet/coincontrol.h>
 #include <wallet/coinselection.h>
diff --git a/test/lint/lint-circular-dependencies.sh b/test/lint/lint-circular-dependencies.sh
--- a/test/lint/lint-circular-dependencies.sh
+++ b/test/lint/lint-circular-dependencies.sh
@@ -10,7 +10,6 @@
 
 EXPECTED_CIRCULAR_DEPENDENCIES=(
     "index/txindex -> validation -> index/txindex"
-    "policy/policy -> validation -> policy/policy"
     "policy/policy -> policy/settings -> policy/policy"
     "qt/addresstablemodel -> qt/walletmodel -> qt/addresstablemodel"
     "qt/bantablemodel -> qt/clientmodel -> qt/bantablemodel"