daemon, wallet: new pay for RPC use system

Daemons intended for public use can be set up to require payment
in the form of hashes in exchange for RPC service. This enables
public daemons to receive payment for their work over a large
number of calls. This system behaves similarly to a pool, so
payment takes the form of valid blocks every so often, yielding
a large one off payment, rather than constant micropayments.

This system can also be used by third parties as a "paywall"
layer, where users of a service can pay for use by mining Monero
to the service provider's address. An example of this for web
site access is Primo, a Monero mining based website "paywall":
https://github.com/selene-kovri/primo

This has some advantages:
 - incentive to run a node providing RPC services, thereby promoting the availability of third party nodes for those who can't run their own
 - incentive to run your own node instead of using a third party's, thereby promoting decentralization
 - decentralized: payment is done between a client and server, with no third party needed
 - private: since the system is "pay as you go", you don't need to identify yourself to claim a long lived balance
 - no payment occurs on the blockchain, so there is no extra transactional load
 - one may mine with a beefy server, and use those credits from a phone, by reusing the client ID (at the cost of some privacy)
 - no barrier to entry: anyone may run a RPC node, and your expected revenue depends on how much work you do
 - Sybil resistant: if you run 1000 idle RPC nodes, you don't magically get more revenue
 - no large credit balance maintained on servers, so they have no incentive to exit scam
 - you can use any/many node(s), since there's little cost in switching servers
 - market based prices: competition between servers to lower costs
 - incentive for a distributed third party node system: if some public nodes are overused/slow, traffic can move to others
 - increases network security
 - helps counteract mining pools' share of the network hash rate
 - zero incentive for a payer to "double spend" since a reorg does not give any money back to the miner

And some disadvantages:
 - low power clients will have difficulty mining (but one can optionally mine in advance and/or with a faster machine)
 - payment is "random", so a server might go a long time without a block before getting one
 - a public node's overall expected payment may be small

Public nodes are expected to compete to find a suitable level for
cost of service.

The daemon can be set up this way to require payment for RPC services:

  monerod --rpc-payment-address 4xxxxxx \
    --rpc-payment-credits 250 --rpc-payment-difficulty 1000

These values are an example only.

The --rpc-payment-difficulty switch selects how hard each "share" should
be, similar to a mining pool. The higher the difficulty, the fewer
shares a client will find.
The --rpc-payment-credits switch selects how many credits are awarded
for each share a client finds.
Considering both options, clients will be awarded credits/difficulty
credits for every hash they calculate. For example, in the command line
above, 0.25 credits per hash. A client mining at 100 H/s will therefore
get an average of 25 credits per second.
For reference, in the current implementation, a credit is enough to
sync 20 blocks, so a 100 H/s client that's just starting to use Monero
and uses this daemon will be able to sync 500 blocks per second.

The wallet can be set to automatically mine if connected to a daemon
which requires payment for RPC usage. It will try to keep a balance
of 50000 credits, stopping mining when it's at this level, and starting
again as credits are spent. With the example above, a new client will
mine this much credits in about half an hour, and this target is enough
to sync 500000 blocks (currently about a third of the monero blockchain).

There are three new settings in the wallet:

 - credits-target: this is the amount of credits a wallet will try to
reach before stopping mining. The default of 0 means 50000 credits.

 - auto-mine-for-rpc-payment-threshold: this controls the minimum
credit rate which the wallet considers worth mining for. If the
daemon credits less than this ratio, the wallet will consider mining
to be not worth it. In the example above, the rate is 0.25

 - persistent-rpc-client-id: if set, this allows the wallet to reuse
a client id across runs. This means a public node can tell a wallet
that's connecting is the same as one that connected previously, but
allows a wallet to keep their credit balance from one run to the
other. Since the wallet only mines to keep a small credit balance,
this is not normally worth doing. However, someone may want to mine
on a fast server, and use that credit balance on a low power device
such as a phone. If left unset, a new client ID is generated at
each wallet start, for privacy reasons.

To mine and use a credit balance on two different devices, you can
use the --rpc-client-secret-key switch. A wallet's client secret key
can be found using the new rpc_payments command in the wallet.
Note: anyone knowing your RPC client secret key is able to use your
credit balance.

The wallet has a few new commands too:

 - start_mining_for_rpc: start mining to acquire more credits,
regardless of the auto mining settings
 - stop_mining_for_rpc: stop mining to acquire more credits
 - rpc_payments: display information about current credits with
the currently selected daemon

The node has an extra command:

 - rpc_payments: display information about clients and their
balances

The node will forget about any balance for clients which have
been inactive for 6 months. Balances carry over on node restart.

1 files changed, 375 insertions, 7 deletions

diff --git a/src/simplewallet/simplewallet.cpp b/src/simplewallet/simplewallet.cpp
index 6a54c24fb..d203f905d 100644
--- a/src/simplewallet/simplewallet.cpp
+++ b/src/simplewallet/simplewallet.cpp
@@ -58,6 +58,7 @@
 #include "cryptonote_basic/cryptonote_format_utils.h"
 #include "storages/http_abstract_invoke.h"
 #include "rpc/core_rpc_server_commands_defs.h"
+#include "rpc/rpc_payment_signature.h"
 #include "crypto/crypto.h"  // for crypto::secret_key definition
 #include "mnemonics/electrum-words.h"
 #include "rapidjson/document.h"
@@ -99,12 +100,17 @@ typedef cryptonote::simple_wallet sw;
 #define LOCK_IDLE_SCOPE() \
   bool auto_refresh_enabled = m_auto_refresh_enabled.load(std::memory_order_relaxed); \
   m_auto_refresh_enabled.store(false, std::memory_order_relaxed); \
-  /* stop any background refresh, and take over */ \
+  /* stop any background refresh and other processes, and take over */ \
+  m_suspend_rpc_payment_mining.store(true, std::memory_order_relaxed); \
   m_wallet->stop(); \
   boost::unique_lock<boost::mutex> lock(m_idle_mutex); \
   m_idle_cond.notify_all(); \
   epee::misc_utils::auto_scope_leave_caller scope_exit_handler = epee::misc_utils::create_scope_leave_handler([&](){ \
+    /* m_idle_mutex is still locked here */ \
     m_auto_refresh_enabled.store(auto_refresh_enabled, std::memory_order_relaxed); \
+    m_suspend_rpc_payment_mining.store(false, std::memory_order_relaxed);; \
+    m_rpc_payment_checker.trigger(); \
+    m_idle_cond.notify_one(); \
   })
 
 #define SCOPED_WALLET_UNLOCK_ON_BAD_PASSWORD(code) \
@@ -125,15 +131,24 @@ typedef cryptonote::simple_wallet sw;
     return true; \
   } while(0)
 
+#define REFRESH_PERIOD 90 // seconds
+
+#define CREDITS_TARGET 50000
+#define MAX_PAYMENT_DIFF 10000
+#define MIN_PAYMENT_RATE 0.01f // per hash
+
 enum TransferType {
   Transfer,
   TransferLocked,
 };
 
+static std::string get_human_readable_timespan(std::chrono::seconds seconds);
+
 namespace
 {
   const std::array<const char* const, 5> allowed_priority_strings = {{"default", "unimportant", "normal", "elevated", "priority"}};
   const auto arg_wallet_file = wallet_args::arg_wallet_file();
+  const auto arg_rpc_client_secret_key = wallet_args::arg_rpc_client_secret_key();
   const command_line::arg_descriptor<std::string> arg_generate_new_wallet = {"generate-new-wallet", sw::tr("Generate new wallet and save it to <arg>"), ""};
   const command_line::arg_descriptor<std::string> arg_generate_from_device = {"generate-from-device", sw::tr("Generate new wallet from device and save it to <arg>"), ""};
   const command_line::arg_descriptor<std::string> arg_generate_from_view_key = {"generate-from-view-key", sw::tr("Generate incoming-only wallet from view key"), ""};
@@ -248,6 +263,9 @@ namespace
   const char* USAGE_LOCK("lock");
   const char* USAGE_NET_STATS("net_stats");
   const char* USAGE_WELCOME("welcome");
+  const char* USAGE_RPC_PAYMENT_INFO("rpc_payment_info");
+  const char* USAGE_START_MINING_FOR_RPC("start_mining_for_rpc");
+  const char* USAGE_STOP_MINING_FOR_RPC("stop_mining_for_rpc");
   const char* USAGE_VERSION("version");
   const char* USAGE_HELP("help [<command>]");
 
@@ -490,22 +508,28 @@ namespace
       fail_msg_writer() << sw::tr("invalid format for subaddress lookahead; must be <major>:<minor>");
     return r;
   }
+}
 
-  void handle_transfer_exception(const std::exception_ptr &e, bool trusted_daemon)
-  {
+void simple_wallet::handle_transfer_exception(const std::exception_ptr &e, bool trusted_daemon)
+{
     bool warn_of_possible_attack = !trusted_daemon;
     try
     {
       std::rethrow_exception(e);
     }
-    catch (const tools::error::daemon_busy&)
+    catch (const tools::error::payment_required&)
     {
-      fail_msg_writer() << sw::tr("daemon is busy. Please try again later.");
+      fail_msg_writer() << tr("Payment required, see the 'rpc_payment_info' command");
+      m_need_payment = true;
     }
     catch (const tools::error::no_connection_to_daemon&)
     {
       fail_msg_writer() << sw::tr("no connection to daemon. Please make sure daemon is running.");
     }
+    catch (const tools::error::daemon_busy&)
+    {
+      fail_msg_writer() << tr("daemon is busy. Please try again later.");
+    }
     catch (const tools::error::wallet_rpc_error& e)
     {
       LOG_ERROR("RPC error: " << e.to_string());
@@ -602,8 +626,10 @@ namespace
 
     if (warn_of_possible_attack)
       fail_msg_writer() << sw::tr("There was an error, which could mean the node may be trying to get you to retry creating a transaction, and zero in on which outputs you own. Or it could be a bona fide error. It may be prudent to disconnect from this node, and not try to send a transaction immediately. Alternatively, connect to another node so the original node cannot correlate information.");
-  }
+}
 
+namespace
+{
   bool check_file_overwrite(const std::string &filename)
   {
     boost::system::error_code errcode;
@@ -1908,6 +1934,77 @@ bool simple_wallet::unset_ring(const std::vector<std::string> &args)
   return true;
 }
 
+bool simple_wallet::rpc_payment_info(const std::vector<std::string> &args)
+{
+  if (!try_connect_to_daemon())
+    return true;
+
+  LOCK_IDLE_SCOPE();
+
+  try
+  {
+    bool payment_required;
+    uint64_t credits, diff, credits_per_hash_found, height, seed_height;
+    uint32_t cookie;
+    std::string hashing_blob;
+    crypto::hash seed_hash, next_seed_hash;
+    crypto::public_key pkey;
+    crypto::secret_key_to_public_key(m_wallet->get_rpc_client_secret_key(), pkey);
+    message_writer() << tr("RPC client ID: ") << pkey;
+    message_writer() << tr("RPC client secret key: ") << m_wallet->get_rpc_client_secret_key();
+    if (!m_wallet->get_rpc_payment_info(false, payment_required, credits, diff, credits_per_hash_found, hashing_blob, height, seed_height, seed_hash, next_seed_hash, cookie))
+    {
+      fail_msg_writer() << tr("Failed to query daemon");
+      return true;
+    }
+    if (payment_required)
+    {
+      uint64_t target = m_wallet->credits_target();
+      if (target == 0)
+        target = CREDITS_TARGET;
+      message_writer() << tr("Using daemon: ") << m_wallet->get_daemon_address();
+      message_writer() << tr("Payments required for node use, current credits: ") << credits;
+      message_writer() << tr("Credits target: ") << target;
+      uint64_t expected, discrepancy;
+      m_wallet->credit_report(expected, discrepancy);
+      message_writer() << tr("Credits spent this session: ") << expected;
+      if (expected)
+        message_writer() << tr("Credit discrepancy this session: ") << discrepancy << " (" << 100.0f * discrepancy / expected << "%)";
+      float cph = credits_per_hash_found / (float)diff;
+      message_writer() << tr("Difficulty: ") << diff << ", " << credits_per_hash_found << " " << tr("credits per hash found, ") << cph << " " << tr("credits/hash");;
+      const boost::posix_time::ptime now = boost::posix_time::microsec_clock::universal_time();
+      bool mining = (now - m_last_rpc_payment_mining_time).total_microseconds() < 1000000;
+      if (mining)
+      {
+        float hash_rate = m_rpc_payment_hash_rate;
+        if (hash_rate > 0)
+        {
+          message_writer() << (boost::format(tr("Mining for payment at %.1f H/s")) % hash_rate).str();
+          if (credits < target)
+          {
+            std::chrono::seconds seconds((unsigned)((target - credits) / cph / hash_rate));
+            std::string target_string = get_human_readable_timespan(seconds);
+            message_writer() << (boost::format(tr("Estimated time till %u credits target mined: %s")) % target % target_string).str();
+          }
+        }
+        else
+          message_writer() << tr("Mining for payment");
+      }
+      else
+        message_writer() << tr("Not mining");
+    }
+    else
+      message_writer() << tr("No payment needed for node use");
+  }
+  catch (const std::exception& e)
+  {
+    LOG_ERROR("unexpected error: " << e.what());
+    fail_msg_writer() << tr("unexpected error: ") << e.what();
+  }
+
+  return true;
+}
+
 bool simple_wallet::blackball(const std::vector<std::string> &args)
 {
   uint64_t amount = std::numeric_limits<uint64_t>::max(), offset, num_offsets;
@@ -2214,6 +2311,50 @@ bool simple_wallet::cold_sign_tx(const std::vector<tools::wallet2::pending_tx>&
   return m_wallet->import_key_images(exported_txs, 0, true);
 }
 
+bool simple_wallet::start_mining_for_rpc(const std::vector<std::string> &args)
+{
+  if (!try_connect_to_daemon())
+    return true;
+
+  LOCK_IDLE_SCOPE();
+
+  bool payment_required;
+  uint64_t credits, diff, credits_per_hash_found, height, seed_height;
+  uint32_t cookie;
+  std::string hashing_blob;
+  crypto::hash seed_hash, next_seed_hash;
+  if (!m_wallet->get_rpc_payment_info(true, payment_required, credits, diff, credits_per_hash_found, hashing_blob, height, seed_height, seed_hash, next_seed_hash, cookie))
+  {
+    fail_msg_writer() << tr("Failed to query daemon");
+    return true;
+  }
+  if (!payment_required)
+  {
+    fail_msg_writer() << tr("Daemon does not require payment for RPC access");
+    return true;
+  }
+
+  m_rpc_payment_mining_requested = true;
+  m_rpc_payment_checker.trigger();
+  const float cph = credits_per_hash_found / (float)diff;
+  bool low = (diff > MAX_PAYMENT_DIFF || cph < MIN_PAYMENT_RATE);
+  success_msg_writer() << (boost::format(tr("Starting mining for RPC access: diff %llu, %f credits/hash%s")) % diff % cph % (low ? " - this is low" : "")).str();
+  success_msg_writer() << tr("Run stop_mining_for_rpc to stop");
+  return true;
+}
+
+bool simple_wallet::stop_mining_for_rpc(const std::vector<std::string> &args)
+{
+  if (!try_connect_to_daemon())
+    return true;
+
+  LOCK_IDLE_SCOPE();
+  m_rpc_payment_mining_requested = false;
+  m_last_rpc_payment_mining_time = boost::posix_time::ptime(boost::gregorian::date(1970, 1, 1));
+  m_rpc_payment_hash_rate = -1.0f;
+  return true;
+}
+
 bool simple_wallet::set_always_confirm_transfers(const std::vector<std::string> &args/* = std::vector<std::string>()*/)
 {
   const auto pwd_container = get_and_verify_password();
@@ -2602,6 +2743,53 @@ bool simple_wallet::set_segregate_pre_fork_outputs(const std::vector<std::string
   return true;
 }
 
+bool simple_wallet::set_persistent_rpc_client_id(const std::vector<std::string> &args/* = std::vector<std::string>()*/)
+{
+  const auto pwd_container = get_and_verify_password();
+  if (pwd_container)
+  {
+    parse_bool_and_use(args[1], [&](bool r) {
+      m_wallet->persistent_rpc_client_id(r);
+      m_wallet->rewrite(m_wallet_file, pwd_container->password());
+    });
+  }
+  return true;
+}
+
+bool simple_wallet::set_auto_mine_for_rpc_payment_threshold(const std::vector<std::string> &args/* = std::vector<std::string>()*/)
+{
+  const auto pwd_container = get_and_verify_password();
+  if (pwd_container)
+  {
+    float threshold;
+    if (!epee::string_tools::get_xtype_from_string(threshold, args[1]) || threshold < 0.0f)
+    {
+      fail_msg_writer() << tr("Invalid threshold");
+      return true;
+    }
+    m_wallet->auto_mine_for_rpc_payment_threshold(threshold);
+    m_wallet->rewrite(m_wallet_file, pwd_container->password());
+  }
+  return true;
+}
+
+bool simple_wallet::set_credits_target(const std::vector<std::string> &args/* = std::vector<std::string>()*/)
+{
+  const auto pwd_container = get_and_verify_password();
+  if (pwd_container)
+  {
+    uint64_t target;
+    if (!epee::string_tools::get_xtype_from_string(target, args[1]))
+    {
+      fail_msg_writer() << tr("Invalid target");
+      return true;
+    }
+    m_wallet->credits_target(target);
+    m_wallet->rewrite(m_wallet_file, pwd_container->password());
+  }
+  return true;
+}
+
 bool simple_wallet::set_key_reuse_mitigation2(const std::vector<std::string> &args/* = std::vector<std::string>()*/)
 {
   const auto pwd_container = get_and_verify_password();
@@ -2844,6 +3032,12 @@ simple_wallet::simple_wallet()
   , m_last_activity_time(time(NULL))
   , m_locked(false)
   , m_in_command(false)
+  , m_need_payment(false)
+  , m_rpc_payment_mining_requested(false)
+  , m_last_rpc_payment_mining_time(boost::gregorian::date(1970, 1, 1))
+  , m_daemon_rpc_payment_message_displayed(false)
+  , m_rpc_payment_hash_rate(-1.0f)
+  , m_suspend_rpc_payment_mining(false)
 {
   m_cmd_binder.set_handler("start_mining",
                            boost::bind(&simple_wallet::on_command, this, &simple_wallet::start_mining, _1),
@@ -3022,7 +3216,13 @@ simple_wallet::simple_wallet()
                                   "device-name <device_name[:device_spec]>\n "
                                   "  Device name for hardware wallet.\n "
                                   "export-format <\"binary\"|\"ascii\">\n "
-                                  "  Save all exported files as binary (cannot be copied and pasted) or ascii (can be).\n "));
+                                  "  Save all exported files as binary (cannot be copied and pasted) or ascii (can be).\n "
+                                  "persistent-client-id <1|0>\n "
+                                  "  Whether to keep using the same client id for RPC payment over wallet restarts.\n"
+                                  "auto-mine-for-rpc-payment-threshold <float>\n "
+                                  "  Whether to automatically start mining for RPC payment if the daemon requires it.\n"
+                                  "credits-target <unsigned int>\n"
+                                  "  The RPC payment credits balance to target (0 for default)."));
   m_cmd_binder.set_handler("encrypted_seed",
                            boost::bind(&simple_wallet::on_command, this, &simple_wallet::encrypted_seed, _1),
                            tr("Display the encrypted Electrum-style mnemonic seed."));
@@ -3333,6 +3533,18 @@ simple_wallet::simple_wallet()
                            boost::bind(&simple_wallet::on_command, this, &simple_wallet::version, _1),
                            tr(USAGE_VERSION),
                            tr("Returns version information"));
+  m_cmd_binder.set_handler("rpc_payment_info",
+                           boost::bind(&simple_wallet::rpc_payment_info, this, _1),
+                           tr(USAGE_RPC_PAYMENT_INFO),
+                           tr("Get info about RPC payments to current node"));
+  m_cmd_binder.set_handler("start_mining_for_rpc",
+                           boost::bind(&simple_wallet::start_mining_for_rpc, this, _1),
+                           tr(USAGE_START_MINING_FOR_RPC),
+                           tr("Start mining to pay for RPC access"));
+  m_cmd_binder.set_handler("stop_mining_for_rpc",
+                           boost::bind(&simple_wallet::stop_mining_for_rpc, this, _1),
+                           tr(USAGE_STOP_MINING_FOR_RPC),
+                           tr("Stop mining to pay for RPC access"));
   m_cmd_binder.set_handler("help",
                            boost::bind(&simple_wallet::on_command, this, &simple_wallet::help, _1),
                            tr(USAGE_HELP),
@@ -3402,6 +3614,9 @@ bool simple_wallet::set_variable(const std::vector<std::string> &args)
         << " (disabled on Windows)"
 #endif
         ;
+    success_msg_writer() << "persistent-rpc-client-id = " << m_wallet->persistent_rpc_client_id();
+    success_msg_writer() << "auto-mine-for-rpc-payment-threshold = " << m_wallet->auto_mine_for_rpc_payment_threshold();
+    success_msg_writer() << "credits-target = " << m_wallet->credits_target();
     return true;
   }
   else
@@ -3463,6 +3678,9 @@ bool simple_wallet::set_variable(const std::vector<std::string> &args)
     CHECK_SIMPLE_VARIABLE("setup-background-mining", set_setup_background_mining, tr("1/yes or 0/no"));
     CHECK_SIMPLE_VARIABLE("device-name", set_device_name, tr("<device_name[:device_spec]>"));
     CHECK_SIMPLE_VARIABLE("export-format", set_export_format, tr("\"binary\" or \"ascii\""));
+    CHECK_SIMPLE_VARIABLE("persistent-rpc-client-id", set_persistent_rpc_client_id, tr("0 or 1"));
+    CHECK_SIMPLE_VARIABLE("auto-mine-for-rpc-payment-threshold", set_auto_mine_for_rpc_payment_threshold, tr("floating point >= 0"));
+    CHECK_SIMPLE_VARIABLE("credits-target", set_credits_target, tr("unsigned integer"));
   }
   fail_msg_writer() << tr("set: unrecognized argument(s)");
   return true;
@@ -4227,6 +4445,17 @@ bool simple_wallet::init(const boost::program_options::variables_map& vm)
     return false;
   }
 
+  if (!command_line::is_arg_defaulted(vm, arg_rpc_client_secret_key))
+  {
+    crypto::secret_key rpc_client_secret_key;
+    if (!epee::string_tools::hex_to_pod(command_line::get_arg(vm, arg_rpc_client_secret_key), rpc_client_secret_key))
+    {
+      fail_msg_writer() << tr("RPC client secret key should be 32 byte in hex format");
+      return false;
+    }
+    m_wallet->set_rpc_client_secret_key(rpc_client_secret_key);
+  }
+
   if (!m_wallet->is_trusted_daemon())
   {
     message_writer(console_color_red, true) << (boost::format(tr("Warning: using an untrusted daemon at %s")) % m_wallet->get_daemon_address()).str();
@@ -4742,6 +4971,7 @@ bool simple_wallet::close_wallet()
   if (m_idle_run.load(std::memory_order_relaxed))
   {
     m_idle_run.store(false, std::memory_order_relaxed);
+    m_suspend_rpc_payment_mining.store(true, std::memory_order_relaxed);
     m_wallet->stop();
     {
       boost::unique_lock<boost::mutex> lock(m_idle_mutex);
@@ -5009,6 +5239,57 @@ bool simple_wallet::stop_mining(const std::vector<std::string>& args)
   return true;
 }
 //----------------------------------------------------------------------------------------------------
+bool simple_wallet::check_daemon_rpc_prices(const std::string &daemon_url, uint32_t &actual_cph, uint32_t &claimed_cph)
+{
+  try
+  {
+    auto i = m_claimed_cph.find(daemon_url);
+    if (i == m_claimed_cph.end())
+      return false;
+
+    claimed_cph = m_claimed_cph[daemon_url];
+    bool payment_required;
+    uint64_t credits, diff, credits_per_hash_found, height, seed_height;
+    uint32_t cookie;
+    cryptonote::blobdata hashing_blob;
+    crypto::hash seed_hash, next_seed_hash;
+    if (m_wallet->get_rpc_payment_info(false, payment_required, credits, diff, credits_per_hash_found, hashing_blob, height, seed_height, seed_hash, next_seed_hash, cookie) && payment_required)
+    {
+      actual_cph = RPC_CREDITS_PER_HASH_SCALE * (credits_per_hash_found / (float)diff);
+      return true;
+    }
+    else
+    {
+      const std::string host = node.host + ":" + std::to_string(node.rpc_port);
+      if (host == daemon_url)
+      {
+        claimed_cph = node.rpc_credits_per_hash;
+        bool payment_required;
+        uint64_t credits, diff, credits_per_hash_found, height;
+        uint32_t cookie;
+        cryptonote::blobdata hashing_blob;
+        if (m_wallet->get_rpc_payment_info(false, payment_required, credits, diff, credits_per_hash_found, hashing_blob, height, cookie) && payment_required)
+        {
+          actual_cph = RPC_CREDITS_PER_HASH_SCALE * (credits_per_hash_found / (float)diff);
+          return true;
+        }
+        else
+        {
+          fail_msg_writer() << tr("Error checking daemon RPC access prices");
+        }
+      }
+    }
+  }
+  catch (const std::exception &e)
+  {
+    // can't check
+    fail_msg_writer() << tr("Error checking daemon RPC access prices: ") << e.what();
+    return false;
+  }
+  // no record found for this daemon
+  return false;
+}
+//----------------------------------------------------------------------------------------------------
 bool simple_wallet::set_daemon(const std::vector<std::string>& args)
 {
   std::string daemon_url;
@@ -5066,6 +5347,8 @@ bool simple_wallet::set_daemon(const std::vector<std::string>& args)
       catch (const std::exception &e) { }
     }
     success_msg_writer() << boost::format("Daemon set to %s, %s") % daemon_url % (m_wallet->is_trusted_daemon() ? tr("trusted") : tr("untrusted"));
+
+    m_daemon_rpc_payment_message_displayed = false;
   } else {
     fail_msg_writer() << tr("This does not seem to be a valid daemon URL.");
   }
@@ -5304,6 +5587,11 @@ bool simple_wallet::refresh_main(uint64_t start_height, enum ResetType reset, bo
   {
     ss << tr("no connection to daemon. Please make sure daemon is running.");
   }
+  catch (const tools::error::payment_required&)
+  {
+    ss << tr("payment required.");
+    m_need_payment = true;
+  }
   catch (const tools::error::wallet_rpc_error& e)
   {
     LOG_ERROR("RPC error: " << e.to_string());
@@ -5630,6 +5918,11 @@ bool simple_wallet::rescan_spent(const std::vector<std::string> &args)
   {
     fail_msg_writer() << tr("no connection to daemon. Please make sure daemon is running.");
   }
+  catch (const tools::error::payment_required&)
+  {
+    fail_msg_writer() << tr("payment required.");
+    m_need_payment = true;
+  }
   catch (const tools::error::is_key_image_spent_error&)
   {
     fail_msg_writer() << tr("failed to get spent status");
@@ -5733,6 +6026,7 @@ bool simple_wallet::print_ring_members(const std::vector<tools::wallet2::pending
         req.outputs[j].index = absolute_offsets[j];
       }
       COMMAND_RPC_GET_OUTPUTS_BIN::response res = AUTO_VAL_INIT(res);
+      req.client = cryptonote::make_rpc_payment_signature(m_wallet->get_rpc_client_secret_key());
       bool r = m_wallet->invoke_http_bin("/get_outs.bin", req, res);
       err = interpret_rpc_response(r, res.status);
       if (!err.empty())
@@ -8474,6 +8768,7 @@ void simple_wallet::wallet_idle_thread()
 #endif
     m_refresh_checker.do_call(boost::bind(&simple_wallet::check_refresh, this));
     m_mms_checker.do_call(boost::bind(&simple_wallet::check_mms, this));
+    m_rpc_payment_checker.do_call(boost::bind(&simple_wallet::check_rpc_payment, this));
 
     if (!m_idle_run.load(std::memory_order_relaxed))
       break;
@@ -8527,6 +8822,78 @@ bool simple_wallet::check_mms()
     return true;
 }
 //----------------------------------------------------------------------------------------------------
+bool simple_wallet::check_rpc_payment()
+{
+  if (!m_rpc_payment_mining_requested && m_wallet->auto_mine_for_rpc_payment_threshold() == 0.0f)
+    return true;
+
+  uint64_t target = m_wallet->credits_target();
+  if (target == 0)
+    target = CREDITS_TARGET;
+  if (m_rpc_payment_mining_requested)
+    target = std::numeric_limits<uint64_t>::max();
+  bool need_payment = m_need_payment || m_rpc_payment_mining_requested || (m_wallet->credits() < target && m_wallet->daemon_requires_payment());
+  if (need_payment)
+  {
+    const boost::posix_time::ptime start_time = boost::posix_time::microsec_clock::universal_time();
+    auto startfunc = [this](uint64_t diff, uint64_t credits_per_hash_found)
+    {
+      const float cph = credits_per_hash_found / (float)diff;
+      bool low = (diff > MAX_PAYMENT_DIFF || cph < MIN_PAYMENT_RATE);
+      if (credits_per_hash_found > 0 && cph >= m_wallet->auto_mine_for_rpc_payment_threshold())
+      {
+        MINFO(std::to_string(cph) << " credits per hash is >= our threshold (" << m_wallet->auto_mine_for_rpc_payment_threshold() << "), starting mining");
+        return true;
+      }
+      else if (m_rpc_payment_mining_requested)
+      {
+        MINFO("Mining for RPC payment was requested, starting mining");
+        return true;
+      }
+      else
+      {
+        if (!m_daemon_rpc_payment_message_displayed)
+        {
+          success_msg_writer() << boost::format(tr("Daemon requests payment at diff %llu, with %f credits/hash%s. Run start_mining_for_rpc to start mining to pay for RPC access, or use another daemon")) %
+              diff % cph % (low ? " - this is low" : "");
+          m_cmd_binder.print_prompt();
+          m_daemon_rpc_payment_message_displayed = true;
+        }
+        return false;
+      }
+    };
+    auto contfunc = [&,this](unsigned n_hashes)
+    {
+      if (m_suspend_rpc_payment_mining.load(std::memory_order_relaxed))
+        return false;
+      const boost::posix_time::ptime now = boost::posix_time::microsec_clock::universal_time();
+      m_last_rpc_payment_mining_time = now;
+      if ((now - start_time).total_microseconds() >= 2 * 1000000)
+        m_rpc_payment_hash_rate = n_hashes / (float)((now - start_time).total_seconds());
+      if ((now - start_time).total_microseconds() >= REFRESH_PERIOD * 1000000)
+        return false;
+      return true;
+    };
+    auto foundfunc = [this, target](uint64_t credits)
+    {
+      m_need_payment = false;
+      return credits < target;
+    };
+    auto errorfunc = [this](const std::string &error)
+    {
+      fail_msg_writer() << tr("Error mining to daemon: ") << error;
+      m_cmd_binder.print_prompt();
+    };
+    bool ret = m_wallet->search_for_rpc_payment(target, startfunc, contfunc, foundfunc, errorfunc);
+    if (!ret)
+    {
+      fail_msg_writer() << tr("Failed to start mining for RPC payment");
+      m_cmd_binder.print_prompt();
+    }
+  }
+  return true;
+}
+//----------------------------------------------------------------------------------------------------
 std::string simple_wallet::get_prompt() const
 {
   if (m_locked)
@@ -9728,6 +10095,7 @@ int main(int argc, char* argv[])
   command_line::add_arg(desc_params, arg_create_address_file);
   command_line::add_arg(desc_params, arg_subaddress_lookahead);
   command_line::add_arg(desc_params, arg_use_english_language_names);
+  command_line::add_arg(desc_params, arg_rpc_client_secret_key);
 
   po::positional_options_description positional_options;
   positional_options.add(arg_command.name, -1);