What Is SHA-256 Algorithm & How It Works

A common specification of all SSL certificates, irrespective of brand, price, and type, is the SHA-256 algorithm. You will see a mention of it in the certificate’s details or the features listed on the product’s information. For non-techies, SHA-256 is usually a complete mystery.

In this article, we want to show you what is SHA-256 and how it works, without going too much into technical jargon and the math behind it.

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Table of Contents

1. What Is Hashing?
2. What Is the SHA-256 Algorithm?
3. SHA-256 Explained
4. SHA-256 Algorithm’s Key Features
5. Is SHA-256 Secure?
6. How Does SHA-256 Work?
7. What Is SHA-256 Used For?
8. SHA Algorithms History

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To understand the SHA-256 algorithm, we need to explain hashing first. 

What Is Hashing?

Hashing is the process of transforming any given information into another value. Essentially, it contains blocks of data, which are transformed into a short fixed-length key or value from the original string. It involves running the data through a mathematical function that converts it into a unique and complex series of characters.

Hashing provides a secure and efficient way to verify data integrity. When you hash a piece of data, you can compare the resulting hash to a known or expected value to ensure the data has not been tampered with or corrupted. If the hash values match, you can be confident that the data has not been altered or modified since it was originally hashed.

One of the benefits of hashing is that it’s irreversible. You can’t use the hash value to retrieve the original data. This property makes hashing useful for password storage, as it allows you to store a hash of a password rather than the password itself. When a user enters their password, the system can hash and compare it to the stored hash value to verify that it’s correct without needing the actual password.

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What Is the SHA-256 Algorithm?

SHA-256 is a cryptographic hash function that generates a fixed-size, 256-bit (32-byte) hash value. The purpose of the SHA-256 algorithm is to create a unique digital fingerprint of a piece of data, such as a message or a file.

SHA-256 refers to a cryptographic hashing algorithm where the input data is processed through a sophisticated mathematical function, resulting in a distinct output hash. This hash acts as a digital fingerprint, uniquely representing the original data.

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SHA-256 Explained

SHA-256, an acronym for Secure Hash Algorithm 256-bit, is a member of the SHA-2 cryptographic hash functions designed by the NSA. It is commonly used in various security applications (such as digital signatures, password authentication, and blockchain technology) and protocols, including TLS and SSL, PGP, SSH, IPsec, and more.

SHA-256 is designed to be a one-way function, meaning that it’s practically impossible to reverse-engineer the original input from its hash output. This attribute makes it an essential tool for verifying data integrity, as any change to the input data, even a single character, results in a dramatically different hash value.

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How Does SHA-256 Work?

SHA-256 operates by taking an input (often a string of text or data) and processing it through a series of mathematical operations. The algorithm divides the input into blocks of 512 bits, each processed in 64 rounds of operations. These operations include bitwise logical functions such as AND, OR, XOR, as well as addition and bitwise rotation.

Let’s see a real example of how hashing works. Say you write the message “I love apples” and apply an SHA-256 hash function to it. Here’s what you’ll get:

40b7df43f24bea395b2c0c3c9d48a3db4db631fa396dd0dd8fe7dc64c9de6f6d

Now, let’s add an exclamation mark at the end of your message so it looks like this: “I love apples!” and generate an output. The result may surprise you:

1751c183f35ed15c2977e5ae7e439fdca79eeae28527ece6efc1a24e4388096f

As you can see, with just one additional character, the output is completely different but the length remains the same. Whether you write a word or an essay, the hash value will be identical, hiding the size of the original input.

If you were to send this message to a friend, you would provide the hash value and specify the algorithm. Your friend would generate the hash on their end, and if it matches, they’ll know that the message is genuine.

Now let’s look at the technical side.

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SHA-256 Algorithm Key Features

The key features of the SHA-256 algorithm are message length, digest length, and irreversibility.

• Message Length: The length of the plaintext (the readable text before it is encrypted) should be less than 264 bits.
• Digest Length: The hash digest (the result of applying a cryptographic hash function to data) length should be 256 bits. When installing an SSL certificate on your server, you may select SHA-512 and bigger digests. While SHA-512 is more secure, it’s not recommended for most systems, as it requires more calculations and computer power.
• Irreversibility: All hash functions such as the SHA-256 are irreversible by design. For each input, you have exactly one output, but not the other way around. Multiple inputs produce the same output. The output has a fixed size, but the input doesn’t have size restrictions.

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Is SHA-256 Secure?

While no cryptographic algorithm is immune to attacks, the SHA-256 algorithm has passed extensive analysis and the test of time to remain an essential element in securing digital data.

SHA-256 security uses a combination of complex mathematical and bitwise operations to generate the hash value. 

This design makes it extremely difficult to find two inputs that produce the same hash. However, the security of SHA-256 depends on the correct implementation and strength of the security protocols used in the application that utilizes it.

One weakness of the SHA-256 algorithm is the possibility of a collision attack. It occurs when two different inputs produce the same hash value. While this is highly unlikely with SHA-256, as it generates a 256-bit hash value (i.e., multiple possible outputs), it is still theoretically possible. If a collision attack were to be successful, it could compromise the application that relies on the SHA-256 hash values to verify data integrity. 

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What Are the Steps in SHA-256 Algorithm?

As you already know, SHA-256 takes the input data and processes it through a series of mathematical operations called rounds. Each round consists of several steps, including data preprocessing, message expansion, and message compression function.

Here are the main steps in the SHA-256 algorithm:

1. Data Preprocessing: The input data is padded and extended to ensure it’s a multiple of 512 bits in length by adding a one-bit followed by zeros and then appending the length of the original message in bits.
2. Message Expansion: The 512-bit input block is divided into 16 32-bit words, and then expanded into 64 32-bit words through a series of logical operations.
3. Message Compression: The expanded 64-word message block is then processed through a series of 64 rounds, each of which involves several steps:
   – Determine the round constant: Each round has a unique 32-bit constant value generated based on the position in the sequence.
   – Calculate the message schedule: A 64-entry message schedule is generated based on the 64-word message block and the round constant.
   – Update the working variables: The working variables, which are 8 32-bit words that store intermediate values during the hashing process, are updated based on the message schedule and a set of logical operations.
   – Calculate the hash value: After all 64 rounds have been completed, the final values of the working variables are combined to produce the 256-bit hash value.

The output of the SHA-256 hash algorithm is a 256-bit final hash value that serves as a digital fingerprint of the input data.

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What Is SHA-256 Used For?

SHA-256 is the standard hashing algorithm for digital signature verification, SSL handshake, password protection, and a host of other security-related operations.

1. Digital signature verification. A digital signature is a type of electronic signature used to validate the authenticity and integrity of a message (e.g., an email, a credit card transaction, or a digital document). It’s created by hashing the file and using PKI (Public Key Infrastructure) to encrypt it. 
   
   The SHA-256 hash algorithm’s role in the whole process is to ensure the integrity of the digital signature. The recipient’s client checks the hashing algorithm on its end and uses the public key to decrypt the message. If it matches, the data is authentic and unchanged.
2. SSL handshake. The SSL handshake is a crucial element of web browsing sessions, and it relies on SHA-256 compatibility and functions. Communications over SS/TLS always begin with the SSL handshake which is asymmetric cryptography that allows the browser to verify the web server, get the public key, and establish a secure connection before the beginning of the data transfer.
3. Password protection. Websites store user passwords in a hashed format. As already discussed, secure password hashing turns them into a short string of letters and/or numbers using an encryption algorithm. If a website is hacked, cyber attackers don’t get access to hashed passwords.
4. Blockchain transactions verification. The SHA-256 hash algorithm is the first algorithm that was used with a cryptocurrency when Bitcoin was created. Block headers are an essential element of blockchains, as they help to chain/connect one block of transactions to the next in a specific order. SHA-256 hash ensures that no previous blocks are changed without tampering with the new block’s header.

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SHA Algorithms History

Secure cryptographic hash algorithms are the creation of the National Security Agency (NSA). The United States Government patented the technology and then released it under a royalty-free license for everyone to use. 

The first SHA-0 algorithm dates back to 1993. Its successor, SHA-1, arrived in 1995, and despite being cracked, it’s still in use today on older servers and clients. Six years later, in 2001, the NSA published the SHA -2 family of hashing algorithms which includes SHA-256 and other five hash functions:

• SHA 224
• SHA 384
• SHA 512
• SHA 512/224
• SHA 512/256

On August 5, 2015, the NIST (National Institute of Standards and Technology) released SHA -3, the latest secure hash algorithm with a different internal algorithm design. While NIST does not currently plan to withdraw the SHA-2 algorithm, SHA-3 can replace it in current applications if necessary.

According to SSL Pulse, the global dashboard for monitoring the quality of SSL / TLS support, based on Alexa’s list of the most popular sites in the world, 97.2% use the SHA-256 algorithm.

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Final Words

The SHA-256 hash algorithm is integral to data protection on the Web. In this article, we scratched only its surface without diving deeper into the math and the functions behind it. Even so, you now have a general understanding of the algorithm’s purpose and use. 

Until we build a quantum computer with enough power to crack complex hash functions, SHA-256 will remain the industry-standard hashing algorithm for data and file integrity. We’re still a few decades away from such a scenario, so for the time being, SHA-256 will be a part of encryption and authentication protocols.

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