Introduction:
Blockchains are tamper evident and tamper resistant digital ledgers implemented in a distributed fashion (i.e., without a central repository) and usually without a central authority (i.e., a bank, company or government). At their basic level, they enable a community of users to record transactions in a shared ledger within that community, such that under normal operation of the blockchain network no transaction can be changed once published. In 2008, the blockchain idea was combined with several other technologies and computing concepts to create modern cryptocurrencies: electronic cash protected through cryptographic mechanisms instead of a central repository or authority. This technology became widely known in 2009 with the launch of the Bitcoin network, the first of many modern cryptocurrencies. In Bitcoin, and similar systems, the transfer of digital information that represents electronic cash takes place in a distributed system.
Bitcoin users can digitally sign and transfer their rights to that information to another user and the Bitcoin blockchain records this transfer publicly, allowing all participants of the network to independently verify the validity of the transactions. The Bitcoin blockchain is independently maintained and managed by a distributed group of participants.
This, along with cryptographic mechanisms, makes the blockchain resilient to attempts to alter the ledger later (modifying blocks or forging transactions). Blockchain technology has enabled the development of many cryptocurrency systems such as Bitcoin and Ethereum1 . Because of this, blockchain technology is often viewed as bound to Bitcoin or possibly cryptocurrency solutions in general. However, the technology is available for a broader variety of applications and is being investigated for a variety of sectors.
The numerous components of blockchain technology along with its reliance on cryptographic primitives and distributed systems can make it challenging to understand. However, each component can be described simply and used as a building block to understand the larger complex system. Blockchains can be informally defined as: Blockchains are distributed digital ledgers of cryptographically signed transactions that are grouped into blocks. Each block is cryptographically linked to the previous one (making it tamper evident) after validation and undergoing a consensus decision. As new blocks are added, older blocks become more difficult to modify (creating tamper resistance). New blocks are replicated across copies of the ledger
Background and History:
The core ideas behind blockchain technology emerged in the late 1980s and early 1990s. In 1989, Leslie Lamport developed the Paxos protocol, and in 1990 submitted the paper The PartTime Parliament [2] to ACM Transactions on Computer Systems; the paper was finally published in a 1998 issue. The paper describes a consensus model for reaching agreement on a result in a network of computers where the computers or network itself may be unreliable. In 1991, a signed chain of information was used as an electronic ledger for digitally signing documents in a way that could easily show none of the signed documents in the collection had been changed [3]. These concepts were combined and applied to electronic cash in 2008 and described in the paper, Bitcoin: A Peer to Peer Electronic Cash System [4], which was published pseudonymously by Satoshi Nakamoto, and then later in 2009 with the establishment of the Bitcoin cryptocurrency blockchain network. Nakamoto’s paper contained the blueprint that most modern cryptocurrency schemes follow (although with variations and modifications). Bitcoin was just the first of many blockchain applications. Many electronic cash schemes existed prior to Bitcoin (e.g., ecash and NetCash), but none of them achieved widespread use. The use of a blockchain enabled Bitcoin to be implemented in a distributed fashion such that no single user controlled the electronic cash and no single point of failure existed; this promoted its use. Its primary benefit was to enable direct transactions between users without the need for a trusted third party.
It also enabled the issuance of new cryptocurrency in a defined manner to those users who manage to publish new blocks and maintain copies of the ledger; such users are called miners in Bitcoin. The automated payment of the miners enabled distributed administration of the system without the need to organize. By using a blockchain and consensus-based maintenance, a self-policing mechanism was created that ensured that only valid transactions and blocks were added to the blockchain. In Bitcoin, the blockchain enabled users to be pseudonymous. This means that users are anonymous, but their account identifiers are not; additionally, all transactions are publicly visible. This has effectively enabled Bitcoin to offer pseudo-anonymity because accounts can be created without any identification or authorization process (such processes are typically required by Know-Your-Customer (KYC) laws). Since Bitcoin was pseudonymous,
it was essential to have mechanisms to create trust in an environment where users could not be easily identified. Prior to the use of blockchain technology, this trust was typically delivered through intermediaries trusted by both parties. Without trusted intermediaries, the needed trust within a blockchain network is enabled by four key characteristics of blockchain technology, described below: • Ledger – the technology uses an append only ledger to provide full transactional history. Unlike traditional databases, transactions and values in a blockchain are not overridden.
• Secure – blockchains are cryptographically secure, ensuring that the data contained within the ledger has not been tampered with, and that the data within the ledger is attestable. • Shared – the ledger is shared amongst multiple participants. This provides transparency across the node participants in the blockchain network.
1. Purpose and Scope:
1.1 Overview
This document provides a high-level technical overview of blockchain technology. It examines various categories of blockchain implementation approaches and outlines the fundamental components that comprise blockchain systems.
Where applicable, diagrams and examples are referenced to clarify key concepts.
1.2 Coverage
This document includes discussion of:
Core components of blockchain technology
High-level descriptions of consensus models used in blockchain networks
The concept of blockchain network changes (forking) and their impact
The evolution of blockchain technology beyond attestable transactions to support attestable application logic (smart contracts)
Limitations and common misconceptions surrounding blockchain technology
Key considerations for organizations evaluating blockchain adoption
1.3 Intended Audience
This document is intended to help readers understand the foundational technologies that comprise blockchain networks. It is designed for:
Technical professionals
Organizational decision-makers
Researchers
Technology evaluators
2. Notes on Terminology
Blockchain terminology varies across different implementations. For consistency, this document uses generic terms defined below.
2.1 Core Terms
Blockchain
The distributed ledger that records transactions in a structured, append-only format.Blockchain Technology
A general term describing the underlying technologies and mechanisms that enable blockchain systems.Blockchain Network
The network environment in which a blockchain operates, consisting of interconnected nodes.Blockchain Implementation
A specific instance or deployment of blockchain technology.Blockchain Network User
Any person, organization, entity, business, or government that utilizes a blockchain network.
2.2 Node Classifications
A Node is an individual system participating in a blockchain network.
2.2.1 Full Node
A full node:
Stores a complete copy of the blockchain
Independently validates transactions and blocks
Enforces network rules
Publishing Node (Block-Producing Node)
A publishing node is a type of full node that:
Creates and broadcasts new blocks
Participates directly in the consensus process
2.2.2 Lightweight Node
A lightweight node:
Does not store a full copy of the blockchain
Relies on full nodes for transaction validation
Forwards transactions to full nodes for inclusion in blocks
Blockchain Categorization:
Blockchain networks can be categorized based on their permission model, which determines who can maintain them (e.g., publish blocks). If anyone can publish a new block, it is permissionless. If only particular users can publish blocks, it is permissioned. In simple terms, a permissioned blockchain network is like a corporate intranet that is controlled, while a permissionless blockchain network is like the public internet, where anyone can participate. Permissioned blockchain networks are often deployed for a group of organizations and individuals, typically referred to as a consortium. This distinction is necessary to understand as it impacts some of the blockchain components discussed later in this document.
Permissionless:
Permissionless blockchain networks are decentralized ledger platforms open to anyone publishing blocks, without needing permission from any authority. Permissionless blockchain platforms are often open source software, freely available to anyone who wishes to download them. Since anyone has the right to publish blocks, this results in the property that anyone can read the blockchain as well as issue transactions on the blockchain (through including those transactions within published blocks). Any blockchain network user within a permissionless blockchain network can read and write to the ledger. Since permissionless blockchain networks are open to all to participate, malicious users may attempt to publish blocks in a way that subverts the system (discussed in detail later). To prevent this, permissionless blockchain networks often utilize a multiparty agreement or ‘consensus’ system (see Section 4) that requires users to expend or maintain resources when attempting to publish blocks. This prevents malicious users from easily subverting the system. Examples of such consensus models include proof of work (see Section 4.1) and proof of stake (see Section 4.2) methods. The consensus systems in permissionless blockchain networks usually promote non-malicious behavior through rewarding the publishers of protocol-conforming blocks with a native cryptocurrency.
Permissioned :
Permissioned blockchain networks are ones where users publishing blocks must be authorized by some authority (be it centralized or decentralized). Since only authorized users are maintaining the blockchain, it is possible to restrict read access and to restrict who can issue transactions. Permissioned blockchain networks may thus allow anyone to read the blockchain or they may restrict read access to authorized individuals. They also may allow anyone to submit transactions to be included in the blockchain or, again, they may restrict this access only to authorized individuals. Permissioned blockchain networks may be instantiated and maintained using open source or closed source software. Permissioned blockchain networks can have the same traceability of digital assets as they pass through the blockchain, as well as the same distributed, resilient, and redundant data storage system as a permissionless blockchain networks. They also use consensus models for publishing blocks, but these methods often do not require the expense or maintenance of resources (as is the case with current permissionless blockchain networks). This is because the establishment of one’s identity is required to participate as a member of the permissioned blockchain network; those maintaining the blockchain have a level of trust with each other,
authorized to publish blocks and since their authorization can be revoked if they misbehave. Consensus models in permissioned blockchain networks are then usually faster and less computationally expensive. Permissioned blockchain networks may also be used by organizations that need to more tightly control and protect their blockchain. However, if a single entity controls who can publish blocks, the users of the blockchain will need to have trust in that entity. Permissioned blockchain networks may also be used by organizations that wish to work together but may not fully trust one another. They can establish a permissioned blockchain network and invite business partners to record their transactions on a shared distributed ledger. These organizations can determine the consensus model to be used, based on how much they trust one another. Beyond trust, permissioned blockchain networks provide transparency and insight that may help better inform business decisions and hold misbehaving parties accountable. This can explicitly include auditing and oversight entities making audits a constant occurrence versus a periodic event.
Some permissioned blockchain networks support the ability to selectively reveal transaction information based on a blockchain network users identity or credentials. With this feature, some degree of privacy in transactions may be obtained. For example, it could be that the blockchain records that a transaction between two blockchain network users took place, but the actual contents of transactions is only accessible to the involved parties. Some permissioned blockchain networks require all users to be authorized to send and receive transactions (they are not anonymous, or even pseudo-anonymous). In such systems parties work together to achieve a shared business process with natural disincentives to commit fraud or otherwise behave as a bad actor (since they can be identified). If bad behavior were to occur, it is well known where the organizations are incorporated, what legal remedies are available and how to pursue those remedies in the relevant judicial system.
2. Cryptographic Hash Functions
2.1 Overview
An essential component of blockchain technology is the use of cryptographic hash functions. These functions are used extensively for ensuring data integrity, securing transactions, and linking blocks within a blockchain.
Hashing is the process of applying a cryptographic hash function to input data of arbitrary size (e.g., text, files, images) to produce a fixed-length output known as a message digest (or simply, digest).
A key property of hashing is determinism:
Given identical input data, independent parties will compute the same output digest. This property enables verification that data has not been altered.
Even the smallest modification to the input (e.g., a single-bit change) produces a completely different output digest. This sensitivity ensures strong data integrity guarantees.
2.2 Security Properties of Cryptographic Hash Functions
Cryptographic hash functions are designed to satisfy the following fundamental security properties:
2.2.1 Preimage Resistance
A cryptographic hash function is preimage resistant, meaning it is computationally infeasible to determine the original input given only the output digest.
Formally:
Given a digest d, it is computationally infeasible to find an input x such that:
hash(x) = d
This property ensures the function operates as a one-way function.
2.2.2 Second Preimage Resistance
A cryptographic hash function is second preimage resistant, meaning it is computationally infeasible to find a different input that produces the same hash as a given input.
Formally:
Given a specific input x, it is computationally infeasible to find a different input y such that:
hash(x) = hash(y)
The only theoretical method for finding such a value is exhaustive search across the input space, which is computationally infeasible for secure hash functions.
2.2.3 Collision Resistance
A cryptographic hash function is collision resistant, meaning it is computationally infeasible to find any two distinct inputs that produce the same output digest.
Formally:
It is computationally infeasible to find inputs x and y, where x ≠ y, such that:
hash(x) = hash(y)
Collision resistance is critical for maintaining the integrity and immutability of blockchain data structures.
2.3 SHA-256 in Blockchain Implementations
A widely used cryptographic hash function in blockchain implementations is the Secure Hash Algorithm with a 256-bit output, commonly referred to as SHA-256.
2.3.1 Output Characteristics
Output length: 256 bits
Equivalent to: 32 bytes
Common representation: 64-character hexadecimal string
Since:
1 byte = 8 bits
32 bytes = 256 bits
The total number of possible SHA-256 digest values is:
2^256 ≈ 10^77
More precisely:
2^256 = 115,792,089,237,316,195,423,570,985,008,687,907,
853,269,984,665,640,564,039,457,584,007,913,129,639,936
This extremely large output space makes brute-force attacks computationally infeasible with current technology.
2.4 Standards and Specifications
The Secure Hash Algorithm (SHA-256), along with other SHA variants, is formally specified in:
Federal Information Processing Standard (FIPS) 180-4
FIPS 180-4 defines the Secure Hash Standard (SHS) and specifies approved hash algorithms.
Additional documentation and approved hashing standards are published by:
The National Institute of Standards and Technology (NIST)
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