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æternity glossary


This document is an overview of both æternity-specific and blockchain concepts and terms. It’s intended to help bring newcomers up to speed with both the æternity project, and blockchain concepts in general.


æternity is a decentralized, public blockchain that uses the GHOST consensus protocol, with Proof-of-Work (PoW) for security and Proof-of-Stake (PoS) for governance. æternity is an account-based system, like Ethereum, and does not use Bitcoin-style unspent transaction outputs (UTXO).

Real-world data can interface with smart contracts through decentralized “oracles”. Scalability and trustless Turing-complete state channels set æternity apart from other Blockchain 2.0 projects.


æternity token

The æternity token (AE) is used as payment for any resources that users consume on the platform, e.g. sending payments, using oracles, etc. The distribution of AE in the genesis block will be determined by a smart contract hosted on Ethereum. More AE will be created via mining.

Application-specific integrated circuit (ASIC)

An application-specific integrated circuit (ASIC) /ˈeɪsɪk/, is an integrated circuit (IC) customized for a particular use, rather than intended for general-purpose use. For example, a chip designed to run in a digital voice recorder or a high-efficiency Bitcoin miner is an ASIC. Application-specific standard products (ASSPs) are intermediate between ASICs and industry standard integrated circuits like the 7400 series or the 4000 series.



A block chain is a transaction database shared by all nodes participating in a system based on the Bitcoin protocol. A full copy of a currency’s block chain contains every transaction ever executed in the currency. With this information, one can find out how much value belonged to each address at any point in history.

This ledger of past transactions is called the block chain as it is a chain of blocks. The block chain serves to confirm transactions to the rest of the network as having taken place.

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Cuckoo Cycle

æternity uses Cuckoo Cycle as the ASIC-resistant mining algorithm to avoid centralizing mining power in the hands of a few large mining pools.

Cuckoo Cycle is the first graph-theoretic proof-of-work, and the most memory bound, yet with instant verification.

From this point of view, Cuckoo Cycle is a very simple PoW, requiring hardly any code, time, or memory to verify.

Finding a 42-cycle, on the other hand, is far from trivial, requiring considerable resources, and some luck (for a given header, the odds of its graph having a L-cycle are about 1 in L).

Its large memory requirements make single-chip ASICs economically infeasable for Cuckoo Cycle.

GHOST consensus protocol

GHOST is short for the Greedy Heaviest Observed Subtree chain selection rule which was a proposed modification for the Bitcoin blockchain.

GHOST orignally was a protocol modification, a chain selection rule, that makes use of blocks that are off the main chain to obtain a more secure and scalable system.

With that modification, it is possible to speed up the blockchain to a velocity of up to 1 block per second. The result is a general higher possible transaction rate without compromising the blockchain consensus and security.



When several nodes (usually most nodes on the network) all have the same blocks in their locally-validated best block chain.


Faucet (cryptocurrency)

Bitcoin faucets are a reward system, in the form of a website or app, that dispenses rewards in the form of a satoshi, which is a hundredth of a millionth BTC, for visitors to claim in exchange for completing a captcha or task as described by the website. There are also faucets that dispense alternative cryptocurrencies.

Genesis block

A genesis block is the first block of a block chain. Modern versions of Bitcoin number it as block 0, though very early versions counted it as block 1. The genesis block is almost always hardcoded into the software of the applications that utilize its block chain. It is a special case in that it does not reference a previous block, and for Bitcoin and almost all of its derivatives, it produces an unspendable subsidy.


Hashcash PoW

Hashcash is a proof-of-work algorithm that requires a selectable amount of work to compute, but the proof can be verified efficiently. For email uses, a textual encoding of a hashcash stamp is added to the header of an email to prove the sender has expended a modest amount of CPU time calculating the stamp prior to sending the email. In other words, as the sender has taken a certain amount of time to generate the stamp and send the email, it is unlikely that they are a spammer. The receiver can, at negligible computational cost, verify that the stamp is valid. However, the only known way to find a header with the necessary properties is brute force, trying random values until the answer is found; though testing an individual string is easy, if satisfactory answers are rare enough it will require a substantial number of tries to find the answer.

Merkle tree (hash tree)

In cryptography and computer science, a hash tree or Merkle tree is a tree in which every leaf node is labelled with the hash of a data block and every non-leaf node is labelled with the cryptographic hash of the labels of its child nodes. Hash trees allow efficient and secure verification of the contents of large data structures. Hash trees are a generalization of hash lists and hash chains.

Hash trees can be used to verify any kind of data stored, handled and transferred in and between computers. They can help ensure that data blocks received from other peers in a peer-to-peer network are received undamaged and unaltered, and even to check that the other peers do not lie and send fake blocks.


In cryptocurrency networks, mining is a validation of transactions. For this effort, successful miners obtain new cryptocurrency as a reward. The reward decreases transaction fees by creating a complementary incentive to contribute to the processing power of the network. The rate of generating hashes, which validate any transaction, has been increased by the use of specialized machines such as FPGAs and ASICs running complex hashing algorithms like SHA-256 and Scrypt. This arms race for cheaper-yet-efficient machines has been on since the day the first cryptocurrency, bitcoin, was introduced in 2009. However, with more people venturing into the world of virtual currency, generating hashes for this validation has become far more complex over the years, with miners having to invest large sums of money on employing multiple high performance ASICs. Thus the value of the currency obtained for finding a hash often does not justify the amount of money spent on setting up the machines, the cooling facilities to overcome the enormous amount of heat they produce, and the electricity required to run them.

Bitcoin mining is intentionally designed to be resource-intensive and difficult so that the number of blocks found each day by miners remains steady. Individual blocks must contain a proof of work to be considered valid. This proof of work is verified by other Bitcoin nodes each time they receive a block. Bitcoin uses the hashcash proof-of-work function.

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Nonce (cryptography)

In cryptography, a nonce is an arbitrary number that can only be used once. It is similar in spirit to a nonce word, hence the name. It is often a random or pseudo-random number issued in an authentication protocol to ensure that old communications cannot be reused in replay attacks. They can also be useful as initialization vectors and in cryptographic hash functions.

Nonces are used in proof-of-work systems to vary the input to a cryptographic hash function so as to obtain a hash for a certain input that fulfills certain arbitrary conditions. In doing so, it becomes far more difficult to create a “desirable” hash than to verify it, shifting the burden of work onto one side of a transaction or system. For example, proof of work, using hash functions, was considered as a means to combat email spam by forcing email senders to find a hash value for the email (which included a timestamp to prevent pre-computation of useful hashes for later use) that had an arbitrary number of leading zeroes, by hashing the same input with a large number of nonce values until a “desirable” hash was obtained.



Oracles are trusted entities signing claims about the state of the world - RPCs, gateways to input/output with the physical realm.

They form the ability to refer to values from real-world data.

An oracle is a mechanism that tells the blockchain facts about the real-world we live in (e.g. the weather, the closing price of Apple shares on a particular date, sports events, or human deaths). æternity’s oracle system(21) uses the same governance mechanism as the æternity blockchain itself, it does not require a separate governance layer on top of the æternity main-net (22) (as with Augur on top of Ethereum).

Typed oracles as primitives on the blockchain provide a well-defined way for smart contracts to interface with data from the outside world. Data-feeds from individuals or institutions can directly interface with the blockchain and provide data for smart contracts.

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Proof-of-stake (PoS)

Proof of stake (PoS) is a type of algorithm by which a cryptocurrency blockchain network aims to achieve distributed consensus. In PoS-based cryptocurrencies, the creator of the next block is chosen via various combinations of random selection and wealth or age (i.e., the stake). In contrast, the algorithm of proof-of-work-based cryptocurrencies such as bitcoin uses mining; that is, the solving of computationally intensive puzzles to validate transactions and create new blocks.

Proof-of-stake currencies can be more energy efficient than currencies based on proof-of-work algorithms.


Proof-of-work system (PoW)

A proof-of-work (PoW) system (or protocol, or function) is an economic measure to deter denial of service attacks and other service abuses such as spam on a network by requiring some work from the service requester, usually meaning processing time by a computer.

A key feature of these schemes is their asymmetry: the work must be moderately hard (but feasible) on the requester side but easy to check for the service provider. This idea is also known as a CPU cost function, client puzzle, computational puzzle or CPU pricing function. It is distinct from a CAPTCHA, which is intended for a human to solve quickly, rather than a computer.



Reason lets you write simple, fast and quality type safe code while leveraging both the JavaScript & OCaml ecosystems.


Smart Contract

A smart contract is a computer protocol intended to digitally facilitate, verify, or enforce the negotiation or performance of a contract. Smart contracts allow the performance of credible transactions without third parties. These transactions are trackable and irreversible.

The aim of smart contracts is to provide security that is superior to traditional contract law and to reduce other transaction costs associated with contracting. Various cryptocurrencies have implemented types of smart contracts.

With the present implementations, based on blockchains, “smart contract” is mostly used more specifically in the sense of general purpose computation that takes place on a blockchain or distributed ledger. In this interpretation, used for example by the Ethereum Foundation or IBM, a smart contract is not necessarily related to the classical concept of a contract, but can be any kind of computer program.

A major limitation of smart contracts is that they are unable to communicate with resources external to the blockchain they are secured on. In practice, this means that smart contracts are not able to trigger based on real-life events. This is known as “the oracle problem”, after test oracles. Oracles provide external data and trigger smart contract executions when pre-defined conditions are met, providing connectivity to the outside world.

The overreaching functional goal of Æternity smart contracts is to be able to execute code on the chain. That is, code execution that is verified by a miner and which can alter the state of the chain. A contract runs on a virtual machine. A smart contract is associated with a virtual machine for the execution of that contract. The AEternity blockchain supports four virtual machines.

Further reading (here)( ) and (here)(

Solution-verification PoW

Solution-verification protocols do not assume such a link: as a result the problem must be self-imposed before a solution is sought by the requester, and the provider must check both the problem choice and the found solution. Most such schemes are unbounded probabilistic iterative procedures such as Hashcash.

State channel

State channels enable highly scalable, trustless transactions of value and purely functional, easily verifiable Turing-complete smart contracts.

State channels offer a way to scale “off-chain” by only storing channel opening and closing information on the blockchain. Two parties deposit tokens into a channel when they open it and the sum of two deposits is the only amount of tokens that can be used within that channel. The state of the channel is the current balance of each party, co-signed by both parties. There can be multiple exchanges of state between the parties but only the last undisputed state becomes the final closing state of the channel. Each message in the channel carries an ever-increasing counter and the message with the highest counter value is considered to be the last state.

State channels come from the realization that, for most purposes, only the actors involved in a smart contract are required to know about it. Two or more actors lock a state and a contract on the blockchain and then perform signed transactions between themselves, off of the public network (or off-chain). The final state is then written to the blockchain. If the end result is disputed, the series of signed off-chain transactions can be uploaded to the blockchain for verification or dispute resolution



Solidity is a contract-oriented, high-level language for implementing smart contracts. It was influenced by C++, Python and JavaScript and is designed to target the Ethereum Virtual Machine (EVM).


Sophia (programming language)

An AEternity BlockChain Language - Sophia is a dialect of ReasonML.

Sophia is customized for smart contracts, which can be published to a blockchain (the AEternity BlockChain). Thus some unnecessary features of Reason (and OCaml - since Reason is closely related to OCaml) have been removed, and some blockchain specific primitives, constructions and types have been added.

The Functional Typed Warded Virtual Machine is used to efficiently and safely execute contracts written in the Sophia language.

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Virtual Machine

A virtual machine is a computer program which provides the services usually provided by a computer, but which runs on another computer, which may be of the same type, but as easily may not. Blockchains provide virtual machines in order to enable programs to be run on the blockchain.

The Ethereum Virtual Machine focuses on providing security and executing untrusted code by computers all over the world. To be more specific, this project focuses on preventing Denial-of-service attacks, which have become somewhat common in the cryptocurrency world. Moreover, the EVM ensures programs do not have access to each other’s state, ensuring communication can be established without any potential interference.

To put this into a language everyone can understand, the Ethereum Virtual Machine is designed to serve as a runtime environment for smart contracts based on Ethereum. As most cryptocurrency enthusiasts are well aware of, smart contracts are very popular these days. This technology can be used to automatically conduct transactions or perform specific actions on the Ethereum blockchain. Many people predict smart contracts will help revolutionize finance and other industries over the coming years.

The AEternity blockchain supports four virtual machines.

  • HLM - A high level machine for executing logical formulas and blockchain operations.
  • FTWVM - A Functional Typed Warded Virtual Machine
  • AEVM - A version of the Ethereum VM
  • FAEVM - A fast version of the Ethereum VM

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