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2.2.3.1 Secure and Efficient Asynchronous Broadcast Protocols

This part of LibTMCG provides an implementation of reliable broadcast, which is actually based on an optimized variant of Bracha’s double-echo broadcast protocol. It works without further authentication mechanisms (e.g. digital signatures) and thus guarantees the desired properties (i.e. validity, consistency, and totality10) of reliable broadcast only, if the number of faulty or even malicious players t is strictly less than one third of all parties n, i.e. t < n/3. Please note that without further assumptions this condition is rather optimal for asynchronous communication and thus has crucial impact for liveness of the high-level protocols using it.

We describe only those classes, methods, and members that might be of interest for an application programmer.

Class: CachinKursawePetzoldShoupRBC

This class implements the protocol RBC by Cachin, Kursawe, Petzold, and Shoup [CKPS01] for a reliable broadcast in the asynchronous communication model, where t < n/3 holds. Additionally, a FIFO-ordered delivery mechanism based on sequence numbers has been implemented.

Member of CachinKursawePetzoldShoupRBC: size_t n

This is the total number of parties n involved in this protocol.

Member of CachinKursawePetzoldShoupRBC: size_t t

This is the number of possible faulty parties t.

Member of CachinKursawePetzoldShoupRBC: size_t j

This is an uniqe index of the party running this instance.

Constructor on CachinKursawePetzoldShoupRBC: CachinKursawePetzoldShoupRBC (const size_t n_in, const size_t t_in, const size_t j_in, aiounicast* aiou_in, const size_t aio_default_scheduler_in =aiounicast::aio_scheduler_roundrobin, const time_t aio_default_timeout_in =aiounicast::aio_timeout_very_long)

The constructor initializes an instance for a reliable broadcast channel of n parties. This total number of parties is given in the first argument n_in. The number of possible faulty or even malicious parties t (given in the second argument t_in) must not exceed n/3. Otherwise a warning is printed to std::cerr and the liveness of the protocol RBC is not guaranteed. Thus, it is recommended to set t_in to the asynchronous maximum (n-1)/3. The third argument j_in is an index of the party running this instance. Finally, the constructor needs as fourth argument aiou_in a reference to already established point-to-point channels (see Communication Interfaces), which should exclusively11 used for this broadcast channels. The default values for timeout (in seconds) and the deliver scheduler can be modified carefully with respect to the usage scenario.

Method on CachinKursawePetzoldShoupRBC: void setID (const std::string ID_in)

Broadcast channels are parameterized by a tag called ID, that is contained in every message. This method sets the tag to ID_in, which should be equal for all parties for the desired channel.

Method on CachinKursawePetzoldShoupRBC: void unsetID ()

This method unset the current channel tag and returns to the previous value. This is commonly used to return from a channel of a subprotcol to the channel of the calling protocol.

Method on CachinKursawePetzoldShoupRBC: void Broadcast (mpz_srcptr m, const bool simulate_faulty_behaviour =false)

This method broadcasts the integer m to all parties.

Method on CachinKursawePetzoldShoupRBC: bool Deliver (mpz_ptr m, size_t& i_out, size_t scheduler =aiounicast::aio_scheduler_default, time_t timeout =aiounicast::aio_timeout_default)

This method delivers a broadcasted integer m from any party using deliver scheduler scheduler. The index of the sender is returned in i_out. In presence of the fourth argument it waits for at most timeout seconds. Otherwise, the default timeout given to the constructor is applied.

The method returns false, if delivering fails. Only in some critical cases error messages are written to std::cerr.

Method on CachinKursawePetzoldShoupRBC: bool DeliverFrom (mpz_ptr m, const size_t i_in, size_t scheduler =aiounicast::aio_scheduler_default, time_t timeout =aiounicast::aio_timeout_default)

This method delivers a broadcasted integer m from a specified party with index i_in using deliver scheduler scheduler. In presence of the fourth argument it waits at most for timeout seconds. Otherwise, the default timeout given to the constructor is applied.

The method returns false, if delivering fails. Only in some critical cases error messages are written to std::cerr.

Method on CachinKursawePetzoldShoupRBC: bool Sync (time_t timeout =aiounicast::aio_timeout_default, const std::string tag ="")

This method continues the execution of RBC protocol such that the requests of other waiting parties are satisfied. In presence of the first argument it waits approximately for (t + 1) \cdot{} timeout seconds while trying to synchronize all parties based on their corresponding local Unix Epoch time. Otherwise, the default timeout given to the constructor is applied. Each synchronization point is required to be unique. Thus, a string called tag with a description of the synchronization point can be supplied as second argument of this method.

The method returns false, if synchronization is failed.

Destructor on CachinKursawePetzoldShoupRBC: ~CachinKursawePetzoldShoupRBC ()

This destructor releases all occupied resources.


Footnotes

(10)

Totality ensures that all correct parties either deliver a message or don’t. In the literature consistency and totality properties are often combined into a single condition called agreement.

(11)

These channels should be authenticated such that network attacks or errors can be detetcted.


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