Bug 29203: Provide ReducedCircuitPadding torrc and consensus params

doc/tor.1.txt

@@ -963,6 +963,12 @@ The following options are useful only for clients (that is, if
     support (unlike ConnectionPadding, CircuitPadding cannot be force-enabled).
     (Default: 1)
 
+[[ReducedCircuitPadding]] **ReducedCircuitPadding** **0**|**1**::
+    If set to 1, Tor will only use circuit padding algorithms that have low
+    overhead. Only clients may set this option. This option should be offered
+    via the UI to mobile users for use where bandwidth may be expensive.
+    (Default: 0)
+
 [[ExcludeNodes]] **ExcludeNodes** __node__,__node__,__...__::
     A list of identity fingerprints, country codes, and address
     patterns of nodes to avoid when building a circuit. Country codes are

src/app/config/config.c

@@ -597,6 +597,7 @@ static config_var_t option_vars_[] = {
   V(ConnectionPadding,           AUTOBOOL, "auto"),
   V(RefuseUnknownExits,          AUTOBOOL, "auto"),
   V(CircuitPadding,              BOOL,     "1"),
+  V(ReducedCircuitPadding,       BOOL,     "0"),
   V(RejectPlaintextPorts,        CSV,      ""),
   V(RelayBandwidthBurst,         MEMUNIT,  "0"),
   V(RelayBandwidthRate,          MEMUNIT,  "0"),
@@ -3746,6 +3747,10 @@ options_validate(or_options_t *old_options, or_options_t *options,
     REJECT("Relays cannot set CircuitPadding to 0. ");
   }
 
+  if (server_mode(options) && options->ReducedCircuitPadding == 1) {
+    REJECT("Relays cannot set ReducedCircuitPadding. ");
+  }
+
   if (options->BridgeDistribution) {
     if (!options->BridgeRelay) {
       REJECT("You set BridgeDistribution, but you didn't set BridgeRelay!");

src/app/config/or_options_st.h

@@ -253,6 +253,12 @@ struct or_options_t {
    * disabled. */
   int CircuitPadding;
 
+  /** Boolean: if true, then this client will only use circuit padding
+   * algorithms that are known to use a low amount of overhead. If false,
+   * we will use all available circuit padding algorithms.
+   */
+  int ReducedCircuitPadding;
+
   /** To what authority types do we publish our descriptor? Choices are
    * "v1", "v2", "v3", "bridge", or "". */
   struct smartlist_t *PublishServerDescriptor;

src/core/or/circuitpadding.c

@@ -82,6 +82,7 @@ static double circpad_distribution_sample(circpad_distribution_t dist);
 
 /** Cached consensus params */
 static uint8_t circpad_padding_disabled;
+static uint8_t circpad_padding_reduced;
 static uint8_t circpad_global_max_padding_percent;
 static uint16_t circpad_global_allowed_cells;
 static uint16_t circpad_max_circ_queued_cells;
@@ -1086,6 +1087,10 @@ circpad_new_consensus_params(const networkstatus_t *ns)
       networkstatus_get_param(ns, "circpad_padding_disabled",
          0, 0, 1);
 
+  circpad_padding_reduced =
+      networkstatus_get_param(ns, "circpad_padding_reduced",
+         0, 0, 1);
+
   circpad_global_allowed_cells =
       networkstatus_get_param(ns, "circpad_global_allowed_cells",
          0, 0, UINT16_MAX-1);
@@ -1662,6 +1667,14 @@ circpad_machine_conditions_met(origin_circuit_t *circ,
   if (circpad_padding_disabled || !get_options()->CircuitPadding)
     return 0;
 
+  /* If the consensus or our torrc has selected reduced connection padding,
+   * then only allow this machine if it is flagged as acceptable under
+   * reduced padding conditions */
+  if (circpad_padding_reduced || get_options()->ReducedCircuitPadding) {
+    if (!machine->conditions.reduced_padding_ok)
+      return 0;
+  }
+
   if (!(circpad_circ_purpose_to_mask(TO_CIRCUIT(circ)->purpose)
       & machine->conditions.purpose_mask))
     return 0;

src/core/or/circuitpadding.h

@@ -152,6 +152,17 @@ typedef struct circpad_machine_conditions_t {
   /** Only apply the machine *if* vanguards are enabled */
   unsigned requires_vanguards : 1;
 
+  /**
+   * This machine is ok to use if reduced padding is set in consensus
+   * or torrc. This machine will still be applied even if reduced padding
+   * is not set; this flag only acts to exclude machines that don't have
+   * it set when reduced padding is requested. Therefore, reduced padding
+   * machines should appear at the lowest priority in the padding machine
+   * lists (aka first in the list), so that non-reduced padding machines
+   * for the same purpose are given a chance to apply when reduced padding
+   * is not requested. */
+  unsigned reduced_padding_ok : 1;
+
   /** Only apply the machine *if* the circuit's state matches any of
    *  the bits set in this bitmask. */
   circpad_circuit_state_t state_mask;