Cryptographic hash function A function that maps a bit string of arbitrary length to a fixed-length bit string. Approved hash functions satisfy the following properties: 1. (Preimage resistant) It is computationally infeasible to compute the correct input value given some output value (the hash function is ‘one way’). 2. (Second preimage resistant) One cannot find an input that hashes to a specific output. 3. (Collision resistant) It is computationally infeasible to find any two distinct inputs that map to the same output. See the NIST SP 800-175B Guideline for Using Cryptographic Standards in the Federal Government: Cryptographic Mechanisms, http://dx.doi.org/10.6028/NIST.SP.800-175B. Cryptographic nonce An arbitrary number that is used once.

Decentralized network A network configuration where there are multiple authorities that serve as a centralized hub for a subsection of participants. Since some participants are behind a centralized hub, the loss of that hub will prevent those participants from communicating. Digest See hash digest Digital asset Any asset that is purely digital, or is a digital representation of a physical asset Digital signature A cryptographic technique that utilizes asymmetric-keys to determine authenticity (i.e., users can verify that the message was signed with a private key corresponding to the specified public key), non-repudiation (a user cannot deny having sent a message) and integrity (that the message was not altered during transmission).

Distributed network A network configuration where every participant can communicate with one another without going through a centralized point. Since there are multiple pathways for communication, the loss of any participant will not prevent communication. This is also known as a peer-to-peer network. NISTIR 8202 BLOCKCHAIN TECHNOLOGY OVERVIEW 52 This publication is available free of charge from: https://doi.org/10.6028/NIST.IR.8202 Double spend (problem) Transacting with the same set of digital assets more than once. This is a problem which has plagued many digital money systems, and a problem that most blockchain networks are designed to prevent. Double spend (attack)

An attack where a blockchain network user attempts to explicitly double spend a digital asset. Fault tolerance A property of a system that allows proper operation even if components fail. Fork A change to blockchain network’s software (usually the consensus algorithm). The changes may be backwards compatible - see Soft Fork, or the changes may not be backwards compatible - see Hard Fork. Full node A blockchain node that stores the blockchain data, passes along the data to other nodes, and ensures that newly added blocks are valid and authentic. Genesis block The first block of a blockchain network; it records the initial state of the system. Hard fork A change to a blockchain implementation that is not backwards compatible. Non-updated nodes cannot continue to transact with updated nodes. Hash chain An append-only data structure where data is bundled into data blocks that include a hash of the previous data block’s data within the newest data block.

This data structure provides evidence of tampering because any modification to a data block will change the hash digest recorded by the following data block. Hash digest The output of a hash function (e.g., hash(data) = digest). Also known as a message digest, digest or hash value. Hash rate The number of cryptographic hash functions a processor can calculate in a given time, usually denominated as hashes per second. Hash value See Hash digest.

Hashing A method of calculating a relatively unique output (called a hash digest) for an input of nearly any size (a file, text, image, etc.) by applying a cryptographic hash function to the input data. Immutable Data that can only be written, not modified or deleted. Incentive mechanism See Reward system Ledger A record of transactions. Lightweight node A blockchain node that does not need to store a full copy of the blockchain and often passes its data to full nodes to be processed.

Appendix A – Glossary of Terms (Continued)

Merkle Tree

A hierarchical data structure in which transaction data is hashed and repeatedly combined until a single root hash is produced. This root hash represents the integrity of the entire data structure.

Mining

The act of solving a computational puzzle within a Proof of Work consensus model in order to earn the right to publish the next block.

Pending Transaction Pool

A distributed queue where candidate transactions wait until they are validated and added to the blockchain.

Also known as the memory pool or mempool.

Publishing Node

A node that, in addition to performing the responsibilities of a full node, is tasked with extending the blockchain by creating and publishing new blocks.

Also known as a:

Mining node
Committing node
Minting node

Node

An individual computing system that participates in a blockchain network.

Nonce

See Cryptographic Nonce.

Orphan Block

A block that is not included in the main blockchain after a temporary ledger conflict or fork has been resolved.

Permissioned

A blockchain system in which every node and user must be granted explicit permission to participate. Permissions are typically assigned by an administrator or consortium.

Permissionless

A blockchain system in which all users have equal permissions, and access is not controlled by any central authority or consortium.

Permissions

Authorized user actions within a system, such as reading, writing, or executing transactions.

Proof of Stake (PoS) Consensus Model

A consensus model in which network security is maintained by participants locking (staking) a quantity of cryptocurrency within the blockchain network.

Participants with greater stake are generally given more influence in consensus decisions, as they are economically incentivized to act honestly to protect their investment.

Proof of Work (PoW) Consensus Model

A consensus model in which a publishing node earns the right to publish the next block by expending computational effort, time, and energy to solve a difficult-to-solve but easy-to-verify cryptographic puzzle.

Typically, this involves finding a nonce that, when combined with block data, produces a hash output matching specific criteria.

Public-Key Cryptography

See Asymmetric-Key Cryptography.

Reward System

A mechanism used to incentivize blockchain network users by providing compensation for participating in network activities, most commonly for successfully publishing new blocks.

Also known as an Incentive System.

Round Robin Consensus Model

A consensus model commonly used in permissioned blockchain networks in which nodes are pseudo-randomly selected to create blocks.

Once selected, a node must wait several block-creation cycles before being eligible to create another block. This approach prevents any single participant from producing the majority of blocks.

This model typically:

Does not require cryptographic puzzles
Has low computational and energy requirements
Uses a straightforward selection mechanism

Smart Contract

A collection of code and associated data (often referred to as functions and state) deployed to a blockchain network via cryptographically signed transactions.

Smart contracts are executed by nodes within the blockchain network. All nodes must produce identical results from execution, and the results are recorded on the blockchain.

Soft Fork

A change to a blockchain protocol that is backward compatible. Nodes that have not updated their software can continue to transact with updated nodes under the revised rules.

Tamper Evident

A characteristic of a system that makes any alteration to stored data easily detectable.

Tamper Resistant

A characteristic of a system that makes data alteration difficult, costly, or both.

Transaction

A recorded event on a blockchain, such as:

The transfer of digital assets between parties
The transfer of inventory units
The creation of new digital assets

Transaction Fee

An amount of cryptocurrency charged to process a blockchain transaction. Transaction fees are typically awarded to publishing nodes as compensation for including transactions in a block.

Turing Complete

A property of a system (such as a programming language or computational environment) indicating that it can execute any algorithm, regardless of complexity, provided sufficient resources are available.