What is a hash value? Unveiling the cryptographic foundation of blockchain

When it comes to blockchain security, everyone mentions hash values. But what exactly is a hash value? How does it ensure the immutability of transactions? Many people easily confuse hash values with encryption. In fact, the process of generating a hash value is one-way and irreversible, which is precisely why it is so powerful in the digital world.

Understanding the Nature of Hash Values Through Irreversible Mapping

Hash, also known as hashing, is a mathematical computer program that takes input data of any length, transforms it through a specific hash algorithm, and ultimately produces a fixed-length output. This output is what we commonly refer to as the hash value.

In simple terms, a hash value is like a “fingerprint” of data—each different input produces a unique fingerprint code. However, this process is completely one-way. You can easily compute the hash value from the original content, but you cannot reverse-engineer the original content from the hash value. It’s like knowing the result is 5—you can’t tell whether it’s 1+4 or 2+3. Hash values are such irreversible mappings.

Strictly speaking, the generation of hash values does not fall under encryption algorithms. Encryption always involves the possibility of decryption—if there is encryption, there must be a way to decrypt. Hash value generation is a one-way cryptographic process, involving only forward computation without any reverse decoding. This is why hash values are an essential part of modern cryptography.

Common Hash Algorithms and Methods for Calculating Hash Values

Currently, the industry commonly uses three categories of algorithms capable of generating hash values: the internationally standardized MD series, the SHA series, and China’s national standard SM3 algorithm.

Among them, SHA-256 is a significant algorithm designed by the US National Security Agency (NSA) and published by the National Institute of Standards and Technology (NIST). Why is it called SHA-256? Because it produces a hash value of 256 bits in length. Due to its extremely high security, SHA-256 has become one of the most reliable methods for protecting digital information and is widely used in blockchain, cryptography, and related fields.

For example, if we apply the SHA-256 algorithm to the string “hello blockchain world, this is yeasy@github,” the resulting hash value will be “db8305d71a9f2f90a3e118a9b49a4c381d2b80cf7bcef81930f30ab1832a3c90.” As long as the original content remains unchanged, this hash value will never change; conversely, even changing one character in the content will produce a completely different hash value.

This demonstrates the power of hash values—the ability to verify data integrity without viewing the original file. If two files have the same SHA-256 hash, you can almost be certain that their contents are identical.

How Hash Values Protect Data Security

In early network communications, information was prone to errors or tampering during transmission. How can the receiver determine whether the received information matches what was sent? Repeated transmission confirmations are inefficient and resource-wasting.

Hash values provide an elegant solution. The sender can compute a hash value of the original message and attach this hash to the message before sending. When the receiver gets the message, they also compute the hash of the original content and compare it with the received hash. If both hash values are identical, it indicates that the message was not tampered with during transmission; if they differ—even by one character—the receiver can immediately detect the alteration. This hash-based integrity verification mechanism has become a standard practice in modern data transmission.

The Critical Role of Hash Values in Blockchain

Hash values are most crucial in blockchain technology. Each block contains the hash value of the previous block, and the next block contains the hash of the current block. This creates a chain linked one after another, forming an irreversible chain.

For example, in Bitcoin, the system uses hash values to generate data summaries for each transaction. If someone attempts to alter historical transaction records, the hash value of that block will change immediately, and all subsequent blocks’ hash values will also change. This chain structure ensures that the entire blockchain’s transaction records are highly trustworthy and secure—any successful tampering would require recalculating the hash values of all subsequent blocks, which is computationally nearly impossible.

This is why blockchain is said to have “immutability”—the underlying support for this immutability is the properties of hash values.

Four Essential Characteristics of a Good Hash Value

Not all hash algorithms are suitable for blockchain or cryptographic applications. A reliable hash algorithm must meet the following four conditions:

Fast in the forward direction—Given the original content and hash algorithm, it must be able to quickly generate the corresponding hash value within limited time and computational resources. This ensures system efficiency.

Pre-image resistance—Given a hash value, it is nearly impossible to reverse-engineer the original content within a limited time. This is the fundamental basis of hash security and the core of “one-wayness.”

Input sensitivity—Even a tiny change in the original input should produce a significantly different hash value. This ensures that any tampering can be immediately detected.

Collision resistance—It is difficult to find two different inputs that produce the same hash value (called a “collision”). Strong collision resistance guarantees the uniqueness of hash values.

Because hash values possess these characteristics, they have become an indispensable foundational tool in modern cryptography and blockchain technology. From data verification to transaction validation, from file integrity checks to the chain structure of blockchains, hash values are everywhere, silently safeguarding the security and trustworthiness of the digital world. Understanding how hash values work also explains why blockchain can maintain security and reliability even without centralized trust institutions.