6.4. Threshold Coin

class aleph.crypto.threshold_coin.ThresholdCoin(dealer_id, process_id, n_processes, threshold, secret_key, verification_key)

Implements dual threshold coin described in the whitepaper.

Parameters

• dealer_id (int) – identification number of a process dealing this coin, from 0 to n-1

• process_id (int) – identification number of a process using this coin

• n_processes (int) – number of processes

• threshold (int) – number of shares required to toss the coin, has to satisfy n_processes//3 < threshold <= n_processes

• verification_key (VerifyKey) – key for combining shares

• secret_key (SigningKey) – key for generating a share of a coin toss

check_validity()

Checks if this threshold coin is valid.

combine_coin_shares(shares, nonce)

Combines the coin shares by forming a threshold signature and taking its 1st bit, subsequently it verifies the result. NOTE: combining shares should always succeed except when some of the shares were invalid or the dealer was dishonest, in which case the toss might be biased and should ideally be discarded

Parameters

• shares (dict) – keys are processes ids, values are shares (group G1 elements)

• nonce (string) – the nonce the shares were created for – necessary for verifying the result of combining

Returns

pair (int, bool) : (coin toss in {0,1}) , (whether combining shares was succesful)

create_coin_share(nonce)

Parameters

nonce (int) – nonce for the coin share

Returns

coin share for the nonce

verify_coin_share(coin_share, process_id, nonce)

Parameters

• coin_share (CoinShare) – coin_share which validity is checked

• process_id (int) – identification number of a process that generated the coin_share

• nonce (int) – nonce for which the coin_share was generated

Returns

True if coin_share is valid and False otherwise