What is the SHA-256 Algorithm & How it Works?

SHA-256 (Secure Hashing Algorithm 256-bit) is a hash function created by the NSA and completed by NIST in 2001. It takes data of any length and produces a 256-bit (32-byte) hash, usually displayed as a 64-character hexadecimal string. SHA-256 provides a one-way function, meaning that the data entered cannot be reversed or decrypted. SHA-256 can be used in various digital security applications, including the protection of passwords, verifying the integrity of data, digital signatures, and blockchain. SHA-256 is considered very secure and provides an unlikely chance of collision and pre-image attacks. SHA-256 is a reliable way to handle modern cryptographic systems. In this article, you will learn about the SHA-256 algorithm and its characteristics.

Table of Contents:

What is Hashing?
Why is Hashing Used?
How Does Hashing Work in Cybersecurity?
Types of Hashing Algorithms
What is the SHA-256 Algorithm?
Characteristics of the SHA-256 Algorithm
Steps in the SHA-256 Algorithm
Applications for the SHA-256 Algorithm
Common Misconceptions About Hashing
Security Considerations in Hashing
Conclusion

What is Hashing?

Hashing is the process of transforming inputs of any length into outputs of a fixed length using a mathematical algorithm, also known as a hash function. The output is referred as a hash value or digest, which uniquely identifies the original content of the data. The most important thing to keep in mind about hashing is that even an insignificant change in inputs produces significantly different outputs, and hashing cannot be reversed.

Hashing has many applications in both computer science and cybersecurity, including

Data integrity: Making sure data isn’t tampered with.
Password storage: Storing passwords as hashes instead of plain text.
Digital signatures: Authenticating digital messages or documents.
Blockchain: Binding and securing blocks of data.

Why is Hashing Used?

Due to the speed and security benefits provided by hashing, it is used for many important uses in computing and cybersecurity. Here are some important uses of hashing:

Data Integrity: Hashing assures that data has not been modified in the process of storage or transit by comparing hash values.
Secured Passwords: Passwords are hashed when stored, so even if a database is compromised, the customer’s real passwords are not vulnerable.
Digital Signatures: Hashing is used in the verification of digital documents and messages to help verify the correctness/validity, and integrity of the documentation.
Fast Searching with Hash Tables: Hashing enables fast lookups, insertions, and deletions with table-based data structures.
Blockchain/Cryptocurrency: Hashing secures the data blocks and assures the immutability by connecting them through cryptographically strong hash functions.

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How Does Hashing Work in Cybersecurity?

In cybersecurity, hashing is used to protect data by converting it into a fixed-length hash value using a hash function. This process ensures data hasn’t been tampered with, secures passwords, and validates digital signatures without revealing the original data.

Input Data: Any kind of data (e.g., a password, file, or message) is provided as input.
Hash Function: A cryptographic hash function (like SHA-256) processes the input and generates a unique, fixed-size hash value.
Output (Hash Value): The output is a unique string of characters representing the original data.
One-Way Process: Hashing is irreversible. You cannot get the original input from the hash value.
Verification: To verify integrity or authenticity, the system re-hashes the input and compares it to the stored hash.

Types of Hashing Algorithms

There are several different types of hashing algorithms, each suited to certain use cases in security and managing data.

1. MD5 (Message Digest 5)

Creates a 128-bit hash value.
Inexpensive, but not secure anymore because it is subject to collision attacks.

2. SHA-1 (Secure Hash Algorithm 1)

Creates a 160-bit hash.
Previously widely adopted, but also not secure anymore.

3. SHA-2 (like SHA-256, SHA-512)

More secure than SHA-1.
SHA-256 produces a 256-bit hash widely used in modern cryptographic systems and blockchain.

4. SHA-3

A new algorithm that uses a Keccak-based structure.
Provides strong security and is resistant to attacks.

5. Bcrypt

Designed for password hashing.
Uses a salt and is computationally expensive to mitigate brute-force attacks.

6. Argon2

Argon2 is a key derivation function and password hashing algorithm.
Also very secure and specifically designed for password storage, and resistant to GPU attacks.

These algorithms have different qualities and are best suited for purposes like general hashing, secure password storage, or cryptographic security.

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What is the SHA-256 Algorithm?

The SHA-256 algorithm, or Secure Hash Algorithm 256-bit, is one of the more widely used hashing functions today, and is part of the SHA-2 family developed by the US’s National Security Agency and published by the National Institute of Standards and Technology in 2001. The algorithm takes an arbitrary amount of input data and produces a 256-bit (32-byte) hash, typically represented as a 64-character hexadecimal string.

The SHA-256 algorithm is considered a one-way function, meaning you cannot determine the original input from the hash. The SHA-256 algorithm was designed for speed and security with built-in collision resistance and can be commonly used for:

Integrity of data
Password hashing
Digital signatures
Blockchain technology (e.g., Bitcoin)
Due to the robust nature of the SHA-256 algorithm, it is at the centre of modern cryptographic technology.

Examples of the SHA-256 Algorithm

Converting a string to a SHA-256 Hash.

Input:

hello

SHA-256 Hash:

2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824

Characteristics of the SHA-256 Algorithm

SHA-256 algorithms are designed with specific properties to ensure security and reliability. These properties make the algorithm essential for secure password storage, digital signatures, data integrity checks, and many other cybersecurity applications.

Determined: The same input always produces the same hash output.
Fast Computation: Hashing should be quick and efficient, regardless of the input size.
Pre-image Resistance: It should be computationally impossible to reverse the hash and find the original input.
Second Pre-image Resistance: It should be difficult to find a different input that produces the same hash as a given input. This prevents forgery or substitution attacks.
Collision Resistance: It should be extremely hard to find any two different inputs that produce the same hash output. This ensures uniqueness and integrity.
Avalanche Effect: A slight modification in the input leads to a significantly different hash output, enhancing unpredictability and strengthening data security.
Fixed Output Size: No matter the size of the input, the hash output is always of a fixed length of 256 bits for SHA-256, which is a 64-character hexadecimal.

Steps in the SHA-256 Algorithm

The SHA-256 algorithm processes the input message through a carefully defined series of steps to compute a secure hash.

Step 1: Preprocessing/Padding

First, the input message gets padded with a ‘1’ bit, followed by some number of ‘0’ bits, and finally appended with a 64-bit representation of the original input message (in bits) to get a new message length (in bits) that is a multiple of 512 bits.

Step 2: Parsing the Message

The padded message gets divided into 512-bit blocks.

Step 3: Initialize Hash Variables

SHA-256 initializes eight 32-bit hash values, derived from fractional parts of the square roots of the first eight prime numbers.

Step 4: Prepare Message Schedule

The hashing algorithm expands each 512-bit block into a message schedule of 64 characters (32 bits each) using various logical operations and functions.

Step 5: Compression Function (Main Loop)

For a total of sixty-four steps: iteratively performing a set of bitwise operations, logical functions and using constants defined by SHA-256, the algorithm updates the working variables set from a to h.

Step 6: Update Hash Values

Once the main loop completes for each 512-bit block, it updates the intermediate hash values.

Step 7: Final Output

Once all the 512-bit blocks are processed, the output is formatted by concatenating the eight hash values and produced as the final hash expression, resulting in 256 bits (64 hexadecimal characters).

Applications for the SHA-256 Algorithm

1. Blockchains

SHA-256 secures each block of a blockchain by hashing based on the contents of the block. As each block’s hash changes when the contents of the block change, the SHA-256 values will be unique, breaking the chain link. This immutability is necessary to trust cryptocurrencies like Bitcoin.

2. Password Protection

Instead of storing raw passwords, systems will store the SHA-256 hash of the password. If a hacker gains access to the database of passwords, they will not be able to retrieve the original password from the SHA-256 hash. To further protect the password hash, use the salting technique.

3. Digital Signatures

When signing a message or a document, the SHA-256 hash of the message or document is created before the signing takes place. When the message is received, the receiver then hashes the message and compares it with the hash to determine authenticity and integrity.

4. Data Integrity Checks

Files or messages are hashed by the sender and receiver before and after transmission. If the hashes are the same, the message or file is unchanged; if they are not, then the receiver knows that the data object may have been altered or tampered with during transport.

5. SSL/TLS Certificates

SHA-256 is one component used to hash portions of the digital certificate, which assures that the certificate has not been altered in any way. This adds further security by providing encryption of the web communications while also validating a connection between the user’s browser and the site’s security connection.

Common Misconceptions About Hashing

The difference between encryption and hashing is that encryption is a reversible process where data can be accessed with a decryption key, while hashing is irreversible. When you hash data, there is no way to get back to the original input. The purpose of hashing is not to protect data from access but to validate data.
Hash functions such as SHA-256 are mathematically irreversible and do not have any information about the original input data point. Even a massively powerful computer can’t reverse-engineer the original message from the hash.
Older hashing protocols such as MD5 and SHA-1 are known to be susceptible to vulnerabilities, including collision attacks. The only hash algorithms that can be relied upon for secure applications are modern secure algorithms that have a demonstrated period of reliability: SHA-256, SHA-3, bcrypt, and Argon2.
Collisions happen when a hashing function generates the same output from two different inputs. The chances of finding a collision based on a secure hashing function like SHA-256 are astronomically small (1 in 2^256). It is technically possible but not practically likely.
It is bad practice to use only hashing (for example, SHA-256) to store passwords. Attacks can use rainbow tables or brute-force password hashing techniques. Instead, a secure system will have hashing algorithms that introduce unique data.
Hash functions create an avalanche effect. That is, changing one character in the input creates a completely different hash. This is what allows hashes to be unpredictable and secure for integrity checks.
Hashing does not hide the content of data, it only verifies it. Once someone knows the hash and the hashing algorithm, they can guess potential inputs and see if they match the hash (this is referred to as a dictionary attack), and this is especially true for weak or short inputs.
Hashing is memory-efficient as it processes data in blocks (i.e., SHA-256 processes in 512 bits). It does not load the entire file in memory at once.

Security Considerations in Hashing

When you implement or rely on hashing, especially for sensitive data like passwords, digital signatures, or file integrity, you must follow certain security conditions.

Cryptographic Hash Functions

Use modern and secure hashing algorithms such as SHA-256, SHA-3, bcrypt, and Argon2.
Avoid older algorithms, including MD5 and SHA-1, since they are susceptible to collision attacks.

Preventing Collision Attacks

Collisions are when two different adjacent inputs can produce the same hash.
Ensure collision resistance by using hashing algorithms with strong collision resistance (e.g., SHA-256) to protect your information from alteration or integrity preservation.

Salting Passwords

A salt is a random value that is added to the password before the hash function, this stops pre-computed tables (rainbow tables) from being used for attacks.
Every password must have a unique salt associated with it.

Key Strengthening Password Hashing

Some algorithms, like bcrypt, scrypt, or Argon2, slow down the hashing process and make using brute-force attacks impractical.
A plain hashing algorithm such as SHA-256 is too fast to be useful for password storage.

Mitigating Length Extension Attacks

Some available hashes, such as MD5 and SHA-1, can be vulnerable to this.
Use HMAC (Hash-based Message Authentication Code) if you need authentication or integrity.

Use Secure Channels to Transmit Hashes

While it is true that a hash is not reversible, sending the hash over an insecure channel does allow a predator to replay the hash or compare it to other hashes.

Check Data Type and Size before Hashing

To ensure consistency and mitigate security risks, remember to normalise data types (for example, trim redundant white spaces, standardise encoding).

Monitor for Timing Attacks

Timing differences that occur during a hash comparison can leak information.
Use constant-time comparison techniques to mitigate these types of attacks.

Conclusion

SHA-256 is among the most reliable cryptographic hash functions, and security solutions rely heavily on it. It has a static output size, includes some collision resistance, and provides an avalanche effect. SHA-256 is used for protecting password storage, ensuring data integrity, validating digital signatures, and running blockchain technology. Hashing is distinct from encryption; It is an irreversible process, and is designed to validate rather than retrieve. In addition to hashing and the properties of a hashed value, the security of a hashed value can be essentially doubled, and there are ways to ensure optimal security through standards of best practices, like salting and security in transmission.

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