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 it is and how it works, without going too much into technical jargon and the math behind it.

What is the SHA 256 algorithm?

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

SHA 256 is a part of the SHA 2 family of algorithms or hash functions, where SHA is an acronym for Secure Hash Algorithm. The 256 value in the name stands for the final hash digest value, i.e. regardless of the size of plaintext, the hash value will always be 256 bits.

In a nutshell, SHA 256 protects data such as messages and files from being intercepted or altered by converting them into an indistinguishable string of characters with a fixed length of 256 bits. It’s the industry strand used by pretty much everyone, including government agencies and innovative technologies like blockchain.

SHA 256 Algorithm’s 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.

SHA 256 in practice

Ok, enough with boring definitions. 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:


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:


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.

SHA algorithms history

Secure 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 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.

Where is SHA 256 used?

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

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 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.

SSL Handshake

The SSL handshake is a crucial element of web browsing sessions, and it relies on SHA 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.

Password Protection

Websites store user passwords in a hashed format. As already discussed, hashing turns passwords 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.

Blockchain Transactions Verification

The SHA-256 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.

Final Words

The SHA 256 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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