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What is Hashcash? Explaining the Bitcoin predecessor

source-logo  crypto.news 03 April 2024 16:49, UTC

Want to know what is Hashcash? It’s the original PoW system, inspiring Bitcoin. Dive in and learn how it laid the groundwork for the number one crypto.

With Bitcoin (BTC) surging to new all-time highs, fueled in part by the recent approval of spot Bitcoin exchange-traded funds (ETFs) by the U.S. Securities and Exchange Commission (SEC), the cryptocurrency is once again dominating headlines and capturing the attention of investors worldwide. Anticipation surrounding the upcoming halving has further contributed to its growing prominence.

However, amidst the enthusiasm surrounding its price movements and its potential future trajectory, many people may not be aware of the foundational protocols and technologies that paved the way for Bitcoin’s creation. One such crucial element is Hashcash, a concept that played a pivotal role in the development of Bitcoin.

As discussions surrounding Bitcoin’s future intensify, delving into the history and significance of Hashcash may offer invaluable insights into the origins of the world’s leading cryptocurrency.

Table of Contents

What is Hashcash?

Hashcash is a cryptographic protocol and proof-of-work (PoW) system developed by British cryptographer Dr Adam Back to combat two big problems: email spam and denial-of-service (DoS) attacks.

The main idea behind the protocol is to require an email sender or anyone requesting a service to solve a mathematical puzzle before being able to send the email or access the service. It proves you’re not just a bot or a spammer trying to flood the system.

While the computational requirement posed minimal inconvenience to regular email users, it made it costly for spammers to send out tons of emails by forcing them to solve these puzzles.

Here’s how Back explained it: “To you as a normal user, with an entry-level desktop or laptop class machine, the CPU overhead per mail is negligible because you don’t send that many mails; at worst, your mail is delayed a few seconds before being sent on slow old hardware. However, to spammers, this is a show-stopper: they want to send 10,000+ emails per minute down a DSL line bought with a stolen credit card quickly before the account gets canceled.”

How does Hashcash work?

As stated earlier, in Hashcash, the person sending an email or requesting a service must solve a computational puzzle that involves taking a piece of data related to the email and feeding it into a hashing algorithm.

Hashcash employed the Secure Hash Algorithm 1 (SHA-1) to generate a unique stamp for each email. This stamp acted as a verification measure for recipients, helping them distinguish legitimate emails from unsolicited spam.

In simpler terms, the sender would perform a mathematical calculation on a piece of data tied to the recipient’s email address. This data point could be the sender address, recipient address, or email timestamp.

Upon receiving the email, the recipient could then perform the reverse operation to verify the stamp’s authenticity. This process ensured that the stamp was indeed created specifically for the recipient’s email address and not a generic one used for mass spam campaigns.

However, not just any scrambled data qualified as a valid Hashcash stamp, the resulting hash needed to start with a specific number of zeros. Finding this particular hash was like searching for a specific grain of sand on a beach. The sender had to keep experimenting with different random numbers until they stumbled upon the one that creates a hash with the required string of zeros at the beginning. This process is what came to be known as proof-of-work.

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How does Hashcash prevent spam and DoS attacks?

As explained previously, Hashcash prevents spam and denial-of-service attacks by making sending emails or requesting services costly for bad actors without impacting regular users. So, how does it achieve this?

First, it creates a speed bump by presenting the person sending an email or requesting a service with a computational puzzle to solve. To do that, the sender or requester must use computational resources, such as CPU cycles or electricity, meaning they’ll have to put in time and effort.

By adding this cost to sending emails or accessing services, it reduces abuse by bad actors, making online communication and services safer and more secure.

Hashcash vs. other proof-of-work systems

From its early days of deterring email spammers, the proof-of-work system developed by Hashcash has now become synonymous with cryptocurrency mining. But how alike or different are the modern-day iterations to their older sibling? Let’s take a look.


Both Hashcash and crypto-based PoW systems share a core principle: expending computational power. Participants in both systems solve complex puzzles derived from cryptographic functions, with the “work” acting as a hurdle to stop bad actors from spamming emails or disrupting networks. Additionally, the solution to the puzzle also serves as proof that the work was indeed done.

Like Hashcash, Bitcoin and other PoW cryptocurrencies operate on decentralized networks, with no single entity controlling the show. In Hashcash, this means email servers can independently verify PoW to filter spam. In crypto, miners compete on a decentralized network to secure the blockchain.


The core differences between Hashcash and other proof-of-work systems lie in purpose and application. Hashcash was designed to combat email spam by making it computationally expensive to send mass emails.

On the other hand, PoW systems in cryptocurrencies serve as a consensus mechanism, an agreement on the state of the network and the validity of transactions.

Additionally, while both systems utilize cryptographic puzzles, the specifics differ. Unlike Hashcash, Bitcoin and similar PoW cryptos employ the SHA-256 hashing algorithm and dynamically adjust the difficulty to maintain a steady block production rate.

Furthermore, when it comes to rewarding the work, PoW cryptocurrencies incentivize participation by giving successful miners new coins and transaction fees. However, there is no Hashcash crypto to be mined. It offers no such direct reward. Solving the puzzle only grants access to the desired service, like sending an email.

Finally, in crypto PoW systems, solving the puzzle plays a crucial role in achieving network consensus. Hashcash, on the other hand, focuses solely on preventing email abuse; the proof-of-work doesn’t determine network consensus.

Connection between Bitcoin and Hashcash

While Hashcash never became the ultimate spam slayer, it served as a foundational concept for the PoW system implemented in Bitcoin mining. It also showed that scarcity, a fundamental principle of money, could exist in the digital space without relying on a central authority.

Bitcoin’s anonymous creator, Satoshi Nakamoto, acknowledged the influence of Back’s system on the cryptocurrency’s core mining function in the Bitcoin whitepaper published in 2008.

In the document, Nakamoto proposed using “a proof-of-work system similar to Adam Back’s Hashcash” to maintain a decentralized timestamp server for Bitcoin’s distributed ledger.

In the Bitcoin network, miners compete to solve a mathematical puzzle based on the network’s hashing algorithm, SHA-256. It transforms transaction data into a unique string of characters, typically 64 characters long, called a Bitcoin hash. This process requires significant computational effort, effectively serving as proof-of-work.

Think of it as a giant guessing game. Miners use their computing power to churn out countless Bitcoin hash solutions until one falls below the target. Once a miner finds a valid solution to the puzzle, they broadcast the new block to the network. Other participants then verify the validity of the block and its transactions, accepting it into the blockchain if it meets the consensus rules.

The connection between Bitcoin and Hashcash is thus rooted in the adoption of Hashcash’s proof-of-work concept as the basis for Bitcoin’s consensus mechanism. While Bitcoin expanded upon Hashcash’s original purpose by applying it to decentralized currency and blockchain technology, the core idea of requiring computational effort to achieve consensus remains a fundamental aspect shared by both systems.

Read more: Why we want to uncover Satoshi’s identity, and what changes if we do