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Glossary

A

Accumulator Root Hash

Access path

  • An access path specifies the location of a resource or Move module within a specific account.
  • In a state of the Libra Blockchain, an account is represented as a map of access paths to values. The Move VM deserializes this representation into modules and resources.
  • Clients can use access paths to request a resource or a specific piece of data stored inside a resource.

Account

  • An account in the Libra Blockchain is a container for an arbitrary number of Move modules and Move resources. This essentially means that the state of each account is comprised of both code and data.
  • The account is identified by an account address.

Account Address

  • The address of a Libra account is a 256-bit value.
  • Users can create an address by generating a cryptographic key-pair.
  • The account address is a cryptographic hash of a user's public verification key.
  • There is no limit on the number of addresses a Libra user can create.

Admission Control (AC)

  • In Libra Core, admission control is the sole external interface to the validator. Any incoming request (transaction submission or queries) from a client goes through admission control. A client does not have the ability to access the storage, or any other component in the system, without going through AC. This filters requests and protects the system.

  • AC is a validator's entry point for all client interactions. It performs basic validity checks on a submitted transaction. After completing validity checks, it passes the transaction to mempool.

  • A client will use AC for submitting transactions and performing queries (reads).

Authentication Key

  • An authentication key is used to authenticate the cryptographic key used to sign a transaction.
  • It is a piece of data stored in the user's account on the blockchain.
  • Users can rotate their signing key by rotating their authentication key.

B

Block

  • A block is an ordered list of one or more transactions. It is used by validators to reach consensus on the ordering and execution results of the transactions.
  • Blocks are an internal implementation concept in the Libra Blockchain, i.e., they are not visible to the client. All transactions that are committed to the Libra ledger were part of a block at some point in time, however the blockchain is represented as a sequence of transactions.

Blockchain

  • A blockchain is a distributed public ledger.
  • The Libra Blockchain is formed with approved transactions and the execution results of those transactions.

Byzantine (Validator)

  • A validator that does not follow the specification of the consensus protocol, and wishes to compromise the correct execution of the protocol.
  • BFT algorithms traditionally support up to one-third of the algorithm's voting power being held by Byzantine validators.

Byzantine Fault Tolerance (BFT)

  • Byzantine Fault Tolerance (BFT) is the ability of a distributed system to provide safety and liveness guarantees in the presence of faulty, or “Byzantine,” members below a certain threshold.
  • The Libra Blockchain uses LibraBFT, a consensus protocol based on HotStuff.
  • BFT algorithms typically operate with a number of entities, collectively holding N votes (which are called “validators” in the Libra application of the system).
  • N is chosen to withstand some number of validators holding f votes, which might be malicious.
  • In this configuration, N is typically set to 3f+1. Validators holding up to f votes will be allowed to be faulty — offline, malicious, slow, etc. As long as 2f+1 votes are held by honest validators, they will be able to reach consensus on consistent decisions.
  • This implies that BFT consensus protocols can function correctly, even if up to one-third of the voting power is held by validator nodes that are compromised, or fail.

C

Client

A client is a piece of software that has the capability to interact with the Libra Blockchain.

  • It can allow the user to construct, sign, and submit new transactions to the admission control component of a validator node.
  • It can issue queries to the Libra Blockchain and request the status of a transaction or account.
  • A client can be run by the end-user or on behalf of the end user (for example, for a custodial wallet).

Consensus

  • Consensus is a component of a validator node.
  • The consensus component is responsible for coordination and agreement amongst all validators on the block of transactions to be executed, their order, and the execution results.
  • The Libra Blockchain is formed with these agreed-upon transactions and their corresponding execution results.

Consensus Protocol

  • A consensus protocol is collectively executed by n validator nodes to accept or reject a transaction and to agree on the ordering of transactions and execution results.
  • See BFT

Custodial Wallet

  • In a custodial wallet model, the wallet product takes custody of customers' funds and private keys.

D

Digital currency

  • Alternate name: Cryptocurrency
  • The type of money Libra is.

E

Ed25519

  • Ed25519 is our supported digital signature scheme.
  • More specifically, Libra uses the PureEdDSA scheme over the Ed25519 curve, as defined in RFC 8032.

Epoch

  • An epoch is a period of time during which an instance of the consensus protocol runs with a fixed set of validators and voting rights.
  • To change the set of validators and/or their voting rights, the current epoch ends with the commit of a special/administrative smart contract transaction and a new one is started.

Event

  • An event is the user-facing representation of the effects of executing a transaction.
  • A transaction may be designed to emit any number of events as a list. For example, a peer-to-peer payment transaction emits a SentPaymentEvent for the sender account and a ReceivedPaymentEvent for the recipient account.
  • In the Libra protocol, events provide evidence that the successful execution of a transaction resulted in a specific effect. The ReceivedPaymentEvent (in the above example) allows the recipient to confirm that a payment was received into their account.
  • Events are persisted on the blockchain and are used to answer queries by clients.

Execution Result

  • Execution result of a transaction is a combination of:
    • The new state of the set of accounts affected by the transaction.
    • The events emitted by executing the transaction.
    • The exit code, which indicates either success or a specific error.
    • The number of gas units consumed while executing the transaction.

Expiration Time

A transaction ceases to be valid after its expiration time. If it is assumed that:

  • Time_C is the current time that is agreed upon between validators (Time_C is not the local time of the client);
  • Time_E is the expiration time of a transaction T_N; and
  • Time_C > Time_E and transaction T_N has not been included in the blockchain,

then there is a guarantee that T_N will never be included in the blockchain.

F

Faucet

  • Faucet is the way to create Libra currency with no real world value, only on our testnet.
  • The Faucet is a service running along with the testnet. This service only exists to facilitate minting coins for the testnet.
  • You can use the Faucet by sending a request to create coins and transfer them into a given account on your behalf.

G

Gas

  • Gas is a way to pay for computation and storage on a blockchain network. All transactions on the Libra network cost a certain amount of gas.
  • The gas required for a transaction depends on the size of the transaction, the computational cost of executing the transaction, and the amount of additional global state created by the transaction (e.g., if new accounts are created).
  • The purpose of gas is regulating demand for the limited computational and storage resources of the validators, including preventing denial of service (DoS) attacks.

Gas Price

  • Each transaction specifies the gas price (in microlibra/gas units) it is willing to pay.
  • The price of gas required for a transaction depends on the current demand for usage of the network.
  • The gas cost (denominated in gas units) is fixed at a point in time.

H

Honest (Validator)

  • A validator that faithfully executes the consensus protocol and is not Byzantine.

HotStuff

  • HotStuff is a recent proposal for a BFT consensus protocol.
  • LibraBFT, Libra's consensus algorithm, is based on HotStuff.
  • It simplifies the reasoning about safety, and it addresses some performance limitations of previous consensus protocols.

L

LBR

  • LBR is the abbreviation for Libra currency.

Leader

  • A leader is a validator node that proposes a block of transactions for the consensus protocol.
  • In leader-based protocols, nodes must agree on a leader to make progress.
  • Leaders are selected by a function that takes the current round number as input.

Libra (The Currency)

  • Libra is a global digital currency.
  • It is stored on the Libra Blockchain.
  • It is backed by a reserve of assets.
  • It is governed by the independent Libra Association.

Libra Association

  • The Libra Association is an independent, not-for-profit membership organization, headquartered in Geneva, Switzerland. The association's purpose is to coordinate and provide a framework for governance of the network and reserve.
  • The association is created by the validator nodes who will run on the Libra network.
  • Refer to the Libra white paper for the a description of the mission, vision, and purview of the Libra Association.

Libra Association Council

  • Libra Association Council is the governing body of the Libra Association.
  • Libra Association Council is part of the Libra Association.

LibraBFT

  • LibraBFT is the Libra protocol's BFT consensus algorithm.
  • LibraBFT is based on HotStuff.

Libra Blockchain

  • The Libra Blockchain is a ledger of immutable transactions agreed upon by the validator nodes on the Libra network (the network of validator nodes).

Libra Core

  • Libra Core is the official name for the open-source implementation of the Libra protocol published by the Libra Association.
  • This software is the first implementation of the Libra protocol and the Move language.
  • Libra Core includes both validator and client functionalities.

Libra Protocol

  • Libra protocol is the specification of how transactions are submitted, ordered, executed, and recorded within the Libra ecosystem.

Libra Reserve

  • Libra reserve is the total monetary holdings that back Libra.
  • To be a validator node for the Libra Association, it is a requirement to invest in the reserve.

LibraAccount.T

  • A LibraAccount.T is a Move resource that holds all the administrative data associated with an account, such as sequence number, balance, and authentication key.
  • A LibraAccount.T is the only resource that every account is guaranteed to contain.

LibraAccount module

  • The LibraAccount module is a Move module that contains the code for manipulating the administrative data held in a particular LibraAccount.T resource.
  • Code for checking or incrementing sequence numbers, withdrawing or depositing currency, and extracting gas deposits is included in the LibraAccount module.

Libra testnet

M

mainnet

  • The Libra mainnet is the main network of the Libra Blockchain with a digital currency known as Libra.
  • The Libra currency on mainnet will be backed by a reserve of assets.
  • Mainnet will be governed by the independent Libra Association tasked with evolving the ecosystem.

Maximum Gas Amount

  • The Maximum Gas Amount of a transaction is the maximum amount of gas the sender is ready to pay for the transaction.
  • The gas charged is equal to the gas price multiplied by units of work required to process this transaction. If the result is less than the max gas amount, the transaction has been successfully executed.
  • If the transaction runs out of gas while it is being executed or the account runs out of balance during execution, then the sender will be charged for gas used and the transaction will fail.

Mempool

  • Mempool is one of the components of the validator node. It holds an in-memory buffer of transactions that have been submitted but not yet agreed upon and executed. Mempool receives transactions from admission control.
  • Transactions in the mempool of a validator are added from the admission control (AC) of the current validator and from the mempool of other validators.
  • When the current validator is the leader, its consensus pulls the transactions from its mempool and proposes the order of the transactions that form a block. The validator quorum then votes on the proposal.

Merkle Trees

  • Merkle tree is a type of authenticated data structure that allows for efficient verification of data integrity and updates.
  • Libra network treats the entire blockchain as a single data structure that records the history of transactions and states over time.
  • The Merkle tree implementation simplifies the work of apps accessing the blockchain. It allows apps to:
    • Read any data from any point in time.
    • Verify the integrity of the data using a unified framework.

Merkle Accumulator

  • The Merkle Accumulator is an append-only Merkle tree that the Libra Blockchain uses to store the ledger.
  • Merkle accumulators can provide proofs that a transaction was included in the chain (“proof of inclusion”).
  • They are also called history trees in literature.

Move

Move Bytecode

  • Move programs are compiled into Move bytecode.
  • Move bytecode is used to express transaction scripts and Move modules.

Move Module

  • A Move module defines the rules for updating the global state of the Libra Blockchain.
  • In the Libra protocol, a Move module is a smart contract.
  • Each user-submitted transaction includes a transaction script. The transaction script invokes procedures of one or more Move modules to update the global state of the blockchain according to the rules.

Move Resources

  • Move resources contain data that can be accessed according to the procedures declared in a Move module.
  • Move resources can never be copied, reused, or lost. This protects Move programmers from accidentally or intentionally losing track of a resource.

Move Virtual Machine (MVM)

N

Node

  • A node is a peer entity of the Libra ecosystem that tracks the state of the Libra Blockchain.
  • A Libra node comprises of logical components. Mempool, consensus, and virtual machine are examples of node components.

O

Open Source Community

  • Open source community is a term used for a group of developers who work on open-source software. If you're reading this glossary, then you're part of the Libra developer community.

P

Permissioned vs. Permissionless

  • Permissioned and permissionless are attributes of the way by which nodes join the set of validators in a blockchain.
  • If only the nodes chosen by a single entity or organization are allowed to join the committee, it's a permissioned system.
  • If any node can join the committee, it's a permissionless system.
  • Libra starts as a permissioned system and will transition to permissionless.

Proof

  • A proof is a way to verify the accuracy of data in the blockchain.
  • Every operation in the Libra Blockchain can be verified cryptographically that it is indeed correct and that data has not been omitted.
  • For example, if a user queries the information within a particular executed transaction, they will be provided with a cryptographic proof that the data returned to them is indeed correct.

R

Round

  • A round consists of achieving consensus on a block of transactions and their execution results.

Round Number

  • A round number is a shared counter used to select leaders during an epoch of the consensus protocol.

S

Sequence Number

  • The sequence number for an account indicates the number of transactions that have been sent from that account. It is incremented every time a transaction sent from that account is executed and stored in the blockchain.
  • A transaction is executed only if it matches the current sequence number for the sender account. This helps sequence multiple transactions from the same sender and prevents replay attacks.
  • If the current sequence number of an account A is X, then a transaction T on account A will only be executed if T's sequence number is X.
  • These transactions will be held in mempool until they are the next sequence number for that account (or until they expire).
  • When the transaction is applied, the sequence number of the account will become X+1. The account has a strictly increasing sequence number.

Sender

  • Alternate name: Sender address.
  • Sender is the address of the originator account for a transaction. A transaction must be signed by the originator.

Smart Contract

State

  • A state in the Libra protocol is a snapshot of the distributed database.
  • A transaction modifies the database and produces a new and updated state.

State Root Hash

  • State root hash is a Merkle hash over all keys and values the state of the Libra Blockchain at a given version.

T

testnet

  • The testnet is a live demonstration of an early prototype of the Libra Blockchain software, also known as Libra Core.
  • The Libra testnet is comprised of test validator nodes running Libra Core, the software which maintains the Libra cryptocurrency.
  • The testnet is built for experimenting with new ideas without disturbing or breaking the main cryptocurrency software.
  • testnet is the predecessor to the Libra mainnet, but testnet has a digital currency with no real world value.

Transaction

Transaction script

  • Each transaction submitted by a user includes a transaction script.
  • It represents the operation a client submits to a validator node.
  • The operation could be a request to move coins from user A to user B, or it could involve interactions with published Move modules/smart contracts.
  • The transaction script is an arbitrary program that interacts with resources published in the global storage of the Libra Blockchain by calling the procedures of a module. It encodes the logic for a transaction.
  • A single transaction script can send funds to multiple recipients and invoke procedures from several different modules.
  • A transaction script is not stored in the global state and cannot be invoked by other transaction scripts. It is a single-use program.

V

Validator Node

  • Alternate name: Validators.
  • A validator is an entity of the Libra ecosystem that validates the Libra Blockchain. It receives requests from clients and runs consensus, execution, and storage.
  • A validator maintains the history of all the transactions on the blockchain.
  • Internally, a validator node needs to keep the current state, to execute transactions and to calculate the next state.

Version

  • A version is also called “height” in blockchain literature.
  • The Libra Blockchain doesn't have an explicit notion of a block — it only uses blocks for batching and executing transactions.
  • A transaction at height 0 is the first transaction (genesis transaction), and a transaction at height 100 is the 101th transaction in the transaction store.

W

Well Formed Transaction

A Libra transaction is well formed if each of the following conditions are true for the transaction:

  • The transaction has a valid signature.
  • An account exists at the sender address.
  • It includes a public key, and the hash of the public key matches the sender account's authentication key.
  • The sequence number of the transaction matches the sender account's sequence number.
  • The sender account's balance is greater than the maximum gas amount.
  • The expiration time of the transaction has not passed.
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