Horizen launched in May 2017, under the name ZenCash. The project included a sustainable funding model, and building upon zero-knowledge technologies, wanted to incentivize every stakeholder and build an ecosystem around it. Horizen launching a Secure and Super node ecosystem and giving a share of the block reward to them marked an important milestone for Horizen, paving the way for an increase in its node count.
In 2018, Horizen experienced an attempted 51% attack. In response, the team amended Horizen’s consensus mechanism and introduced a penalty for delayed block reporting, which makes it exponentially more difficult and costly for those looking to perform a 51% attack. The solution is being used by other projects, such as Pirl.
The project underwent a brand expansion in August 2018 and emerged under their new name, Horizen. This marked project’s evolution from a cryptocurrency to a privacy platform with a focus on scalability. Horizen’s scaling solution, Zendoo, is an attempt to create a completely decentralized and fully customizable interoperable protocol. The Horizen interoperable blockchain platform was created to enable businesses and developers to affordably and quickly create their own public or private blockchains and decentralized applications on Horizen’s fully distributed, secure, and privacy-preserving architecture. Horizen’s sidechain SDK provides the components needed to build a completely decentralized and fully customizable blockchain or decentralized application on a blockchain.
ZEN, the native currency of Horizen, is a mineable PoW coin available on major crypto exchanges. Users can securely store and transact ZEN using Horizen’s app, Sphere by Horizen, a multifunctional wallet that interfaces with Horizen products and services.
Horizen has a network of more than 40,000 incentivized Secure and Super Nodes, making it one of the largest public node networks in the industry.
ZEN is used as a native currency within the Horizen network. Transactions in Horizen can either be transparent or shielded. Transparent transactions operate similarly to Bitcoin with visible addresses and transaction amounts while shielded transactions operate with hidden addresses and transaction amounts. Parties using shielded transactions have the ability to selectively disclose transaction metadata for the purpose of auditing or regulatory compliance.
ZEN is used for payments, store of value, network fees and will be the fuel for sidechains.
Horizen uses an ammended version of Nakamoto Consensus whereby the valid chain is the longest chain with the most accumulated proof-of-work. Consensus in Horizen, and other systems using Nakamoto Conensus, is probabilistic because there is always a chance that a new, longer competing chain could emerge with more accumulated proof-of-work, that would invalidate the current chain.
Horizen attempts to enhance original Nakamoto Consensus by incorporating a penalty system for delayed block submission to make it much costlier to execute a successful 51% attack. Detailed information can be read in the relevant white paper: https://www.horizen.io/assets/files/A-Penalty-System-for-Delayed-Block-Submission-by-Horizen.pdf
Horizen, the protocol, is a distributed, time-stamped ledger of unspent transaction output (UTXO) transfers stored in an append-only chain of 2MB data blocks. A network of mining and economic nodes maintains this blockchain by validating, propagating, and competing to include pending transactions (mempool) in new blocks. Economic nodes (aka “full nodes”) receive transactions from other network participants, validate them against network consensus rules and double-spend vectors, and propagate the transactions to other full nodes that also validate and propagate. Valid transactions are sent to the network’s mempool waiting for mining nodes to confirm them via inclusion in the next block.
Mining nodes work to empty the mempool usually in a highest-to-lowest fee order by picking transactions to include in the next block and racing against each other to generate a hash less than the target number set by Horizen’s difficulty adjustment algorithm. Horizen uses a Proof-of-Work (PoW) consensus mechanism to establish the chain of blocks with the most accumulated “work” (a.k.a., energy spent on solved hashes) as the valid chain. Other network peers can cheaply verify the chain’s work.
In order to have zero-knowledge privacy in Horizen, the function determining the validity of a transaction according to the network’s consensus rules must return the answer of whether the transaction is valid or not, without revealing any of the information it performed the calculations on. This is done by encoding some of the network’s consensus rules in zk-SNARKs (zero-knowledge succinct non-interactive arguments of knowledge). Zk-SNARKs are specific zero-knowledge proofs whereby one can prove possession of certain information, e.g. a secret key, without revealing that information, and without any interaction between the prover and verifier.
Horizen addresses are either private (z-addresses) or transparent (t-addresses). Z-addresses start with a “zc,” and t-addresses start with a “zn.” The two Horizen address types are interoperable, and funds can be transferred between z-addresses and t-addresses. A Z-to-Z transaction appears on the public blockchain, so it is known to have occurred and that the fees were paid. But the addresses, transaction amount, and the memo field are all encrypted and not publicly visible. Transactions between two transparent addresses (t-addresses) work just like Bitcoin: The sender, receiver, and transaction value are publicly visible. The owner of an address may choose to disclose z-address and transaction details with trusted third parties using view keys and payment disclosure.
Horizen is an open source community. Historically most protocol development is largely handled by Zen Blockchain Foundation, a founding non-profit organization. Zen Blockchain Foundation (ZBF) maintains “zend,” the only production-ready node implementation.
ZBF contracts Horizen Labs for development work and consulting. Horizen Labs is a for-profit company mainly focusing on ZK circuit construction and business support for Horizen’s sidechain implementation – Zendoo. Protocol development is governed by the Horizen Improvement Proposal process (ZenIP) whereby anyone in the open source Horizen community can submit draft ZenIPs. After discussion by the Horizen community, the ZenIP editors, features are selected for the upcoming network upgrade. There are three organizations, Horizen Community Council, Zen Blockchain Foundation, and Horizen Labs Company each have one editor elected for the process. ZenIP Editors will have to make decisions and unanimity cannot be assumed and is not necessary for all kinds of decisions. Majority decision means more than half ( 50%) of the Editors need to agree. Supermajority decision means more than two thirds ( 66.6%) need to agree. Additionally, there are decisions that require unanimity. Decisions from the ZenIP process are written into the Horizen specification, as well as the software that runs the network. Protocol changes are “ratified” on-chain when the majority of the network adopts the upgrade and doesn’t break consensus.
The Zen Blockchain Foundation receives an allocation of the treasury reward every block, which consists of 20% from the block reward. Moving forward Zen Blockchain Foundation plans to further decentralize the Horizen project through building a treasury sidechain governance system, increasing community involvement, and ultimately reducing the influence of itself on the Horizen protocol. Meaningful steps to this end include the creation of Horizen Community Council, a community organization having a representative in ZenIP process, and actively supporting the community and being their voice.