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| − | The job of the consensus layer of the IC is to order transaction requests so that all replicas in a subnet will process
| + | Please see [https://internetcomputer.org/how-it-works/consensus/ How Consensus Layer works] |
| − | such requests in the same order.
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| − | There are many protocols in the literature for this problem.
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| − | The IC uses a new consensus protocol, which is described here at a high level.
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| − | For more details, see the paper https://eprint.iacr.org/2021/1330 (in particular, Protocol ICC1 in that paper).
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| − | Any secure consensus protocol should guarantee two properties, which (roughly stated) are:
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| − | * <b>safety</b>: all replicas in fact agree on the same ordering of transaction requests, and
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| − | * <b>liveness</b>: all replicas should make steady progress.
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| − | The paper https://eprint.iacr.org/2021/1330 proves that the IC consensus protocol satisfies both of these properties
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| − | The IC consensus protocol is design to be
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| − | * extremely simple, and
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| − | * robust (performance degrades gracefully when some replicas are malicious).
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| − | | |
| − | As discussed [[The Internet Computer for Computer Scientists|in the introduction]], we assume
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| − | * a subnet of <math>n</math> replicas, and
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| − | * at most <math>f < n/3</math> of the replicas are faulty.
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| − | Faulty replicas may exhibit arbitrary, malicious (i.e., Byzantine) behavior.
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| − | To simplify the presentation, we assume here that <math>n=3f+1</math>.
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| − | We assume that communication is <b>asynchronous</b>, with no <i>a priori</i>
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| − | bound on the delay of messages sent between replicas.
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| − | In fact, the scheduling of message delivery may be completely under adversarial control.
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| − | The IC consensus protocol guarantees safety under this very weak communication assumption.
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| − | However, to guarantee liveness, we need to assume a form of <b>partial synchrony</b>,
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| − | which (roughly stated) says that the network will be periodically <b>timely</b> .
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| − | Somewhat more precisely, there exists a bound <math>\Delta</math> such that
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| − | periodically all undelivered
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| − | messages will be delivered within time <math>\Delta</math>.
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| − | The bound <math>\Delta</math> does not have to be known in advance
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| − | (the protocol is initialized with a bound, but will dynamically
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| − | adapt and increase this bound if it is too small).
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| − | Regardless of whether we are assuming an asynchronous
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| − | or a partially synchronous network,
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| − | we assume that every message sent from one honest
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| − | party to another will <i>eventually</i> be delivered.
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| − | | |
| − | Like a number of consensus protocols,
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| − | the IC consensus protocol is based in a [[wikipedia:blockchain|blockchain]].
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| − | As the protocol progresses, a tree of blocks is grown,
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| − | starting from a special <i>genesis block</i> that is the root of the tree.
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| − | Each non-genesis block in the tree contains (among other things)
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| − | a <i>payload</i>, consisting of a sequence of transaction requests,
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| − | and a hash of the block's parent in the tree.
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| − | The honest replicas have a consistent view of this tree:
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| − | while each replica may have a different, partial view
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| − | of this tree, all the replicas have a view of the <i>same</i> tree.
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| − | In addition, as the protocol progresses,
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| − | there is always a path of <i>finalized</i> blocks in this tree.
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| − | Again, the honest replicas have a consistent view of this path:
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| − | while each replica may have a different, partial view
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| − | of this path, all the parties have a view of the <i>same</i> path.
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| − | The transaction requests in the payloads of the blocks along this
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| − | path are the ordered transaction requests will be processed by the [[IC execution layer|execution layer]]
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| − | of the Internet Computer.
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| − | The protocol proceeds in <b>rounds</b>.
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| − | In round <math>h</math> of the protocol,
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| − | one or more blocks of <b>height</b> <math>h</math> are added to the tree.
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| − | That is, the blocks added in round <math>h</math> are always at a distance
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| − | of exactly <math>h</math> from the root.
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| − | In each round, a pseudo-random process is used to assign
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| − | each replica a unique <b>rank</b>, which is an
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| − | integer in the range <math>0,\ldots,h-1</math>.
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| − | This is pseudo-random process is implemented using a <b>random beacon</b> (see below).
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| − | The party of lowest rank is the <b>leader</b> of that round.
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| − | When the leader is honest and the network is timely,
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| − | the leader will propose a block, which will be added to the tree;
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| − | moreover, this will be the <i>only</i> block added to the tree
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| − | in this round and it will extend the finalized path.
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| − | If the leader is not honest or the network is not timely,
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| − | some other parties of higher rank may also propose blocks,
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| − | and also have their blocks added to the tree.
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| − | In any case, the logic of the protocol gives highest priority
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| − | to the leader's proposed block
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| − | and <i>some</i> block or blocks will be added to this tree in this round.
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| − | When the network is timely,
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| − | the message complexity in a given round of the IC consensus protocol is <math>O(n^2)</math>
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| − | with overwhelming probability.
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| − | The protocol also enjoys the property known as <b>optimistic responsiveness</b>,
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| − | which means that
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| − | so long as the leader is honest, the protocol will proceed at the pace
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| − | of the actual network delay, rather than some upper bound on
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| − | the network delay.
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| − | To implement the protocol, each replica is associated with a
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| − | public key for the [[wikipedia:BLS_digital_signature|BLS signature scheme]], and each replica also holds
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| − | the corresponding secret signing key.
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| − | The association of replicas to public keys is maintained by the
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| − | [[IC network nervous system (NNS)|network nervous system (NNS) of the Internet Computer]].
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| − | These BLS signatures will be used to authenticate messages, also called <b>artifacts</b>,
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| − | sent by replicas.
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| − | The protocol also uses the <b>signature aggregation</b> feature of BLS signatures,
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| − | which allows many signatures on the same message to be aggregated into
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| − | a compact multi-signature.
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| − | FIXME: citation, discussion of rogue key attack mitigation via PoPs.
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| − | <b>Random beacon.</b>
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| − | In addition to BLS signatures and multi-signatures as discussed above,
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| − | the protocol makes use of a BLS [[wikipedia:Threshold_cryptosystem|threshold signature scheme]] to implement the
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| − | above-mentioned <b>random beacon</b>.
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| − | The random beacon for height <math>h</math> is a <math>(f+1)</math>-threshold signature
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| − | on a message unique to height <math>h</math>.
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| − | In each round of the protocol, each replica broadcasts its share of the beacon for
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| − | the next round, so that when the next round begins, all replicas should have enough shares
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| − | to reconstruct the beacon for that round.
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| − | As discussed above, the random beacon at height <math>h</math> is used to
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| − | assign a pseudo-random rank to each replica that will be used in round <math>h</math> of the protocol.
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| − | Because of the security properties of the threshold signature,
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| − | an adversary will not be able to predict the ranking of the replicas more than one round in advance,
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| − | and these rankings will effectively be as good as random.
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| − | To set up such a threshold signature scheme, the Internet Computer runs a
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| − | [[wikipedia:Distributed_key_generation|distributed key generation (DKG)]] protocol.
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| − | There are many DKG protocols in the literature.
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| − | The Internet Computer implements a new [[non-interactive DKG (NiDKG) protocol]].
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| − | <b>Block making.</b>
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| − | Each replica may at different points in time play the role of a <b>block maker</b>.
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| − | As a block maker in round <math>h</math>,
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| − | the replica proposes a block <math>B</math> of height <math>h</math>
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| − | that to be child of a block <math>B'</math> of height <math>h-1</math>
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| − | in the tree of blocks.
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| − | To do this, the block maker first gathers together a <b>payload</b>
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| − | consisting of all transaction requests it knows about
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| − | (but not including those already included in payloads in blocks in the
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| − | path through the tree ending at <math>B'</math>).
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| − | The block <math>B</math> consists of
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| − | * the payload,
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| − | * the hash of <math>B'</math>,
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| − | * the rank of the block maker,
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| − | * the height <math>h</math> of the block.
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| − | After forming the block <math>B</math>, the block maker forms
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| − | a <b>block proposal</b>, consisting of
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| − | * the block <math>B</math>,
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| − | * the block maker's identity <math>\textit{BMID}</math>, and
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| − | * the block maker's signature on <math>B</math>.
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| − | A block maker will broadcast its block proposal to all other replicas.
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| − | <b>Notarization.</b>
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| − | A block is only added to the tree of blocks when it becomes <b>notarized</b>.
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| − | For a block to become notarized, it must be notarized by <math>2f+1</math>
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| − | distinct replicas.
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| − | To notarize a proposed block <math>B</math> at height <math>h</math>,
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| − | a replica will verify a number of conditions,
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| − | and if these tests pass, the replica will notarize the block
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| − | by broadcasting a <b>notarization share</b> for <math>B</math>,
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| − | consisting of
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| − | * the hash of <math>B</math>,
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| − | * the height <math>h</math> of <math>B</math>,
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| − | * the identity <math>\textit{NID}</math> of the notarizing replica, and
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| − | * the notarizing replica's signature on a message comprising the hash of <math>B</math> and the height <math>h</math>.
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| − | Any set of <math>2f+1</math> notarization shares on <math>B</math>
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| − | may be aggregated together to form a <b>notarization</b> for <math>B</math>,
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| − | consisting of
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| − | * the hash of <math>B</math>,
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| − | * the height <math>h</math> of <math>B</math>,
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| − | * a set of <math>2f+1</math> distinct identities <math>NIDSet</math> of the notarizing replicas, and
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| − | * an aggregate signature of the notarizing replicas in <math>NIDSet</math> on a message comprising the hash of <math>B</math> and the height <math>h</math>.
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| − | Each replica will finish round <math>h</math>
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| − | as soon as it obtains a notarized block of height <math>h</math>,
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| − | and will not subsequently issue any notarization shares at height <math>h</math>.
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| − | <b>Finalization.</b>
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| − | There may be more than one notarized block at a given height <math>h</math>.
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| − | However, if a block is <b>finalized</b>, then we can be sure that there
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| − | is no other notarized block at height <math>h</math>.
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| − | Let us call this the <b>finalization invariant</b>.
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| − | For a block to become finalized, it must be finalized by <math>2f+1</math>
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| − | distinct replicas.
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| − | Recall that round <math>h</math> ends for a replica when
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| − | it obtains a notarized block <math>B</math> of height <math>h</math>.
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| − | At that point in time, such a replica will finalize <math>B</math>
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| − | by broadcasting a
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| − | <b>finalization share</b> for <math>B</math> if it has not previously
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| − | issued notarized any block other than <math>B</math>.
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| − | A finalization share has exactly the same format as a notarization
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| − | (but are tagged in such a way notarization shares
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| − | and finalization shares cannot be confused with one another).
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| − | Any set of <math>2f+1</math> finalization shares on <math>B</math>
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| − | may be aggregated together to form a <b>finalization</b> for <math>B</math>,
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| − | which has exactly the same format as a notarization.
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| − | One can easily see that the logic for finalizing blocks
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| − | implies that the finalization invariant must hold.
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| − | Here is a proof:
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| − | # Suppose that the number of corrupt parties is exactly <math>f^* \le f < n/3</math>.
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| − | # If a block <math>B</math> is finalized, then it has been finalized by a set <math>S</math> of at least <math>n-f-f^*</math> honest replicas (by the security property for aggregate signatures).
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| − | # If another block <math>B' \ne B</math> were notarized, then it has been notarized by a set <math>S'</math> of at least <math>n-f-f^*</math> honest replicas (again, by the security property for aggregate signatures).
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| − | # The sets <math>S</math> and <math>S'</math> are disjoint (by the finalization logic).
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| − | # Therefore, <math>n-f^* \ge |S| + |S'| \ge 2 (n-f-f^*)</math>, which implies <math>n \le 3f</math>, a contradiction.
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| − | One consequence of the finalization invariant is the following.
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| − | Suppose one replica sees a block <math>B</math> at height <math>h</math>
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| − | along with a corresponding finalization for <math>B</math>,
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| − | and that another replica sees a block
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| − | <math>B'</math> at height <math>h' \le h</math>
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| − | along with a corresponding finalization for <math>B'</math>.
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| − | Then the finalization invariant implies that the path in the tree of notarized blocks
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| − | ending at <math>B'</math> is a prefix of the path ending at <math>B</math>
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| − | (if not, then there would be two notarized blocks at height <math>h'</math>,
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| − | contradicting the finalization invariant).
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| − | Thus, whenever replica sees a finalization for a block <math>B</math>,
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| − | it may view all ancestors of <math>B</math> as being
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| − | <b>implicitly finalized</b>, and because of the
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| − | finalization invariant, the <b>safety property</b> is
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| − | guaranteed to hold for these (explicitly and implicitly) finalized blocks.
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