-Adding support for tx_max_expected_probability, a parameter that defines the maximum expected probability for a single maker to be included in a collaborative transaction. A value of 1 (100%) for this probability corresponds to the previous behavior. A value smaller than 1 allows to prevent a large maker from always being included in a transaction. For a given total amount of fidelity bonds, this mechanism allows to reduce the ability of an attacker to be systematically included in a transaction, and also to be the only entity included as makers in the transaction.

Author: PyGryhapoRbryDer

scripts/tumbler.py

@@ -65,6 +65,18 @@ def main():
     log.info("Using maximum coinjoin fee limits per maker of {:.4%}, {} sat"
              .format(*maxcjfee))
 
+    tx_max_expected_probability = jm_single().config.getfloat("POLICY", "tx_max_expected_probability")
+
+    if tx_max_expected_probability <= 0:
+        jmprint('Error: tx_max_expected_probability must be greater than 0', "error")
+        sys.exit(EXIT_FAILURE)
+
+    elif tx_max_expected_probability > 1:
+        tx_max_expected_probability = 1
+
+    log.info("Using maximum expected probability of selecting a maker of {:.2%}"
+             .format(tx_max_expected_probability))
+
     #Parse options and generate schedule
     #Output information to log files
     jm_single().mincjamount = options['mincjamount']
@@ -185,6 +197,7 @@ def taker_finished(res, fromtx=False, waittime=0.0, txdetails=None):
     taker = Taker(wallet_service,
                   schedule,
                   maxcjfee,
+                  tx_max_expected_probability,
                   order_chooser=options['order_choose_fn'],
                   callbacks=(filter_orders_callback, None, taker_finished),
                   tdestaddrs=destaddrs)

src/jmclient/configure.py

@@ -400,6 +400,8 @@ def jm_single() -> AttributeDict:
 # where x > 1. It is a real number (so written as a decimal).
 bond_value_exponent = 1.3
 
+tx_max_expected_probability = 1.0
+
 ##############################
 # THE FOLLOWING SETTINGS ARE REQUIRED TO DEFEND AGAINST SNOOPERS.
 # DON'T ALTER THEM UNLESS YOU UNDERSTAND THE IMPLICATIONS.

src/jmclient/support.py

@@ -177,7 +177,7 @@ def calc_cj_fee(ordertype, cjfee, cj_amount):
     return real_cjfee
 
 
-def weighted_order_choose(orders, n):
+def weighted_order_choose(orders, n, nrem = None, large_makers_not_chosen_prob = None, tx_max_expected_probability = None):
     """
     Algorithm for choosing the weighting function
     it is an exponential
@@ -208,37 +208,120 @@ def weighted_order_choose(orders, n):
     return orders[chosen_order_index]
 
 
-def random_under_max_order_choose(orders, n):
+def random_under_max_order_choose(orders, n, nrem = None, large_makers_not_chosen_prob = [1], tx_max_expected_probability = None):
     # orders are already pre-filtered for max_cj_fee
+    if tx_max_expected_probability is not None and tx_max_expected_probability<1:
+        log.debug('Remaining probability of not being selected for large makers: ' + str(large_makers_not_chosen_prob[0]) + ' -> ' + str(large_makers_not_chosen_prob[0]*(1-1./len(orders))))
+        large_makers_not_chosen_prob[0] *= (1 - 1./len(orders))
     return random.choice(orders)
 
 
-def cheapest_order_choose(orders, n):
+def cheapest_order_choose(orders, n, nrem = None, large_makers_not_chosen_prob = None, tx_max_expected_probability = None):
     """
     Return the cheapest order from the orders.
     """
     return orders[0]
 
-def fidelity_bond_weighted_order_choose(orders, n):
+def fidelity_bond_weighted_order_choose(orders, n, nrem = None, large_makers_not_chosen_prob = [1], tx_max_expected_probability = None):
     """
     choose orders based on fidelity bond for improved sybil resistance
 
     * with probability `bondless_makers_allowance`: will revert to previous default
       order choose (random_under_max_order_choose)
     * with probability `1 - bondless_makers_allowance`: if there are no bond offerings, revert
-      to previous default as above. If there are, choose randomly from those, with weighting
-      being the fidelity bond values.
+      to previous default as above, or if tx_max_expected_probability is defined and the number
+      of bond offerings is smaller than n/tx_max_expected_probability. If not, choose
+      randomly from those, with weighting being the fidelity bond values.
     """
 
     if random.random() < get_bondless_makers_allowance():
-        return random_under_max_order_choose(orders, n)
+        log.debug('Bondless or bond maker randomly selected')
+        return random_under_max_order_choose(orders, n, large_makers_not_chosen_prob=large_makers_not_chosen_prob, tx_max_expected_probability=tx_max_expected_probability)
     #remove orders without fidelity bonds
-    filtered_orders = list(filter(lambda x: x[0]["fidelity_bond_value"] != 0, orders))
-    if len(filtered_orders) == 0:
-        return random_under_max_order_choose(orders, n)
+    filtered_orders = sorted(list(filter(lambda x: x[0]["fidelity_bond_value"] != 0, orders)), key=lambda x: x[0]["fidelity_bond_value"])
+    nforders = len(filtered_orders)
+
+    if nforders == 0:
+        log.debug('Bondless maker selected because no alternative')
+        return random_under_max_order_choose(orders, n, large_makers_not_chosen_prob=large_makers_not_chosen_prob, tx_max_expected_probability=tx_max_expected_probability)
+
     weights = list(map(lambda x: x[0]["fidelity_bond_value"], filtered_orders))
+    prob = 1 - pow(((1 - tx_max_expected_probability) / large_makers_not_chosen_prob[0]), 1./nrem) if tx_max_expected_probability is not None and tx_max_expected_probability<1. else None
+
+    if prob is not None:
+    
+        #If maximum expected probability target for large makers cannot be achieved using a constant value for prob
+        if prob<=0 or nforders-nrem+1 <= 1. / prob:
+            max_exp_prob = large_makers_not_chosen_prob[0]
+
+            for i in range(nforders-nrem+1, nforders+1):
+                max_exp_prob *= 1 - 1./i
+            max_exp_prob = 1 - max_exp_prob
+
+            #If the probability target cannot be achieved at all
+            if max_exp_prob > tx_max_expected_probability:
+                log.warn('A large maker maximum expected probability target of ' + str(tx_max_expected_probability) + ' cannot be achieved. A probability of ' + str(max_exp_prob) + ' will be targeted instead')
+                #Update prob using the maximum achievable target
+                prob = 1 - pow(((1 - max_exp_prob) / large_makers_not_chosen_prob[0]), 1./nrem)
+
+            else:
+                log.debug('Large maker maximum expected probability target of ' + str(tx_max_expected_probability) + ' achievable using an increasing draw probability due to the limited number of makers')
+            rem_not_chosen_prob = (1 - max_exp_prob) / large_makers_not_chosen_prob[0]
+
+            for i in range(nforders-nrem+1, nforders):
+
+                if 1./i > prob:
+                    rem_not_chosen_prob /= 1 - 1./i
+                    log.debug('Large maker draw probability for draw ' + str(nforders + 1 - i) + ' set to ' + str(1./i))
+                    prob = 1 - pow(rem_not_chosen_prob, 1. / (nforders - i))
+
+                else:
+                    break
+            log.debug('Large maker draw probability for first ' + str(nforders - i) + ' draws set to ' + str(prob) + ' per draw')
+
+        else:
+            log.debug('Large maker draw probability set to ' + str(prob) + ' for each draw')
+
+        normal_bond_value_sum = 0
+        nlargemakers = 0
+        islargemaker = [False] * nforders
+
+        log.debug(str(nforders) + ' remaining makers for the draw')
+        for i, o in enumerate(filtered_orders):
+            #log.debug(o[0])
+            normal_bond_value_sum += weights[i]
+        log.debug('Total value of fidelity bonds: ' + str(normal_bond_value_sum))
+
+        for i, o in enumerate(filtered_orders[::-1]):
+            i = nforders - i - 1
+
+            if prob * (nlargemakers + 1) >= 1:
+                break
+            bvmax = prob * (normal_bond_value_sum - weights[i]) / (1. - prob * (nlargemakers + 1))
+            #log.debug('Maker ' + o[0]['counterparty'] + ' weight ' + str(weights[i]) + ' vs ' + str(bvmax) + ": " + ('normal' if  weights[i] <= bvmax else 'large'))
+
+            if weights[i] <= bvmax:
+                break
+            islargemaker[i] = True
+            normal_bond_value_sum -= weights[i]
+            nlargemakers += 1
+
+        if normal_bond_value_sum <= 0:
+            log.warn('Only large makers are left, selecting a bond maker randomly')
+            return random_under_max_order_choose(filtered_orders, nforders, large_makers_not_chosen_prob=large_makers_not_chosen_prob, tx_max_expected_probability=tx_max_expected_probability)
+
+        log.debug('Remaining probability of not being selected for large makers: ' + str(large_makers_not_chosen_prob[0]) + ' -> ' + str(large_makers_not_chosen_prob[0]*(1-prob)))
+        large_makers_not_chosen_prob[0] *= 1 - prob
+        bvmax = prob * normal_bond_value_sum / (1. - prob * nlargemakers)
+
+        for i, o in enumerate(filtered_orders):
+
+            if islargemaker[i] == True:
+                log.warn('Weight of counterparty ' + o[0]['counterparty'] + ' brought down to ' + str(bvmax) + ' from ' + str(weights[i]))
+                weights[i]=bvmax
+
     weights = [x / sum(weights) for x in weights]
-    return filtered_orders[rand_weighted_choice(len(filtered_orders), weights)]
+    return filtered_orders[rand_weighted_choice(nforders, weights)]
 
 def _get_is_within_max_limits(max_fee_rel, max_fee_abs, cjvalue):
     def check_max_fee(fee):
@@ -249,7 +332,7 @@ def check_max_fee(fee):
 
 def choose_orders(offers, cj_amount, n, chooseOrdersBy, ignored_makers=None,
                   pick=False, allowed_types=["sw0reloffer", "sw0absoffer"],
-                  max_cj_fee=(1, float('inf'))):
+                  max_cj_fee=(1, float('inf')), tx_max_expected_probability=None):
     is_within_max_limits = _get_is_within_max_limits(
         max_cj_fee[0], max_cj_fee[1], cj_amount)
     if ignored_makers is None:
@@ -294,8 +377,10 @@ def choose_orders(offers, cj_amount, n, chooseOrdersBy, ignored_makers=None,
                                                    ]))
     total_cj_fee = 0
     chosen_orders = []
+    large_makers_not_chosen_prob = [1]
     for i in range(n):
-        chosen_order, chosen_fee = chooseOrdersBy(orders_fees, n)
+        chosen_order, chosen_fee = chooseOrdersBy(orders_fees, n, n - i, large_makers_not_chosen_prob, tx_max_expected_probability)
+        log.debug('Choice is ' + str(chosen_order))
         # remove all orders from that same counterparty
         # only needed if offers are manually picked
         orders_fees = [o
@@ -315,7 +400,7 @@ def choose_sweep_orders(offers,
                         chooseOrdersBy,
                         ignored_makers=None,
                         allowed_types=['sw0reloffer', 'sw0absoffer'],
-                        max_cj_fee=(1, float('inf'))):
+                        max_cj_fee=(1, float('inf')), tx_max_expected_probability=None):
     """
     choose an order given that we want to be left with no change
     i.e. sweep an entire group of utxos
@@ -376,13 +461,14 @@ def calc_zero_change_cj_amount(ordercombo):
              if is_within_max_limits(v[1])).values(),
         key=feekey)
     chosen_orders = []
+    large_makers_not_chosen_prob = [1]
     while len(chosen_orders) < n:
         for i in range(n - len(chosen_orders)):
             if len(orders_fees) < n - len(chosen_orders):
                 log.debug('ERROR not enough liquidity in the orderbook')
                 # TODO handle not enough liquidity better, maybe an Exception
                 return None, 0, 0
-            chosen_order, chosen_fee = chooseOrdersBy(orders_fees, n)
+            chosen_order, chosen_fee = chooseOrdersBy(orders_fees, n, n - len(chosen_orders), large_makers_not_chosen_prob, tx_max_expected_probability)
             log.debug('chosen = ' + str(chosen_order))
             # remove all orders from that same counterparty
             orders_fees = [

src/jmclient/taker.py

@@ -48,6 +48,7 @@ def __init__(self,
                  wallet_service,
                  schedule,
                  max_cj_fee,
+                 tx_max_expected_probability,
                  order_chooser=fidelity_bond_weighted_order_choose,
                  callbacks=None,
                  tdestaddrs=None,
@@ -104,6 +105,7 @@ def __init__(self,
         self.schedule = schedule
         self.order_chooser = order_chooser
         self.max_cj_fee = max_cj_fee
+        self.tx_max_expected_probability = tx_max_expected_probability
         self.custom_change_address = custom_change_address
         self.change_label = change_label
 
@@ -290,7 +292,7 @@ def filter_orderbook(self, orderbook, sweep=False):
             self.orderbook, self.total_cj_fee = choose_orders(
                 orderbook, self.cjamount, self.n_counterparties, self.order_chooser,
                 self.ignored_makers, allowed_types=allowed_types,
-                max_cj_fee=self.max_cj_fee)
+                max_cj_fee=self.max_cj_fee, tx_max_expected_probability=self.tx_max_expected_probability)
             if self.orderbook is None:
                 #Failure to get an orderbook means order selection failed
                 #for some reason; no action is taken, we let the stallMonitor
@@ -381,7 +383,7 @@ def prepare_my_bitcoin_data(self):
                 self.orderbook, total_value, self.total_txfee,
                 self.n_counterparties, self.order_chooser,
                 self.ignored_makers, allowed_types=allowed_types,
-                max_cj_fee=self.max_cj_fee)
+                max_cj_fee=self.max_cj_fee, tx_max_expected_probability=self.tx_max_expected_probability)
             if not self.orderbook:
                 self.taker_info_callback("ABORT",
                                 "Could not find orders to complete transaction")

test/jmclient/test_support.py

@@ -76,7 +76,7 @@ def test_choose_orders():
     #test the fidelity bond one
     for i, o in enumerate(orderbook):
         o["fidelity_bond_value"] = i+1
-    orders_fees = choose_orders(orderbook, 100000000, 3, fidelity_bond_weighted_order_choose)
+    orders_fees = choose_orders(orderbook, 100000000, 3, fidelity_bond_weighted_order_choose, tx_max_expected_probability=0.75)
     assert len(orders_fees[0]) == 3
     #test sweep
     result, cjamount, total_fee = choose_sweep_orders(orderbook, 50000000,