Question 1

Is it possible to reverse or decrypt a SHA-256 hash back to the original text?

Accepted Answer

No, it is mathematically impossible to reverse a SHA-256 hash. Hashing is a one-way mathematical compression function, not a two-way encryption cipher. When large amounts of data are processed, the vast majority of the original information is permanently discarded. The only way an attacker can find the original text is by guessing millions of possible inputs, hashing them one by one, and checking if the resulting output matches the target hash—a process known as a brute-force attack.

Question 2

Are there any known collisions in SHA-256?

Accepted Answer

As of today, no one has ever found a collision in SHA-256. A collision occurs when two completely different inputs produce the exact same 64-character hash output. While the mathematics of infinity dictate that collisions must exist (since there are infinite possible inputs but only a finite $2^{256}$ possible outputs), the number of possible outputs is so astronomically large that finding a collision by chance or by brute force is practically impossible, even if you utilized all the computing power on Earth for millions of years.

Question 3

What is the maximum file size that SHA-256 can hash?

Accepted Answer

The SHA-256 algorithm has a strictly defined maximum input size, which is $2^{64} - 1$ bits. In practical terms, this equals roughly 2.3 million terabytes (or 2.3 exabytes) of data. Given that the largest commercially available single hard drives currently hold around 30 terabytes, it is practically impossible for any individual user or standard enterprise application to encounter a file that exceeds the maximum input size limitation of the SHA-256 algorithm.

Question 4

Why is the output always exactly 64 characters long?

Accepted Answer

The SHA-256 algorithm outputs data at a fixed length of exactly 256 bits. In computer science, bits are almost always represented in hexadecimal (base-16) format to make them readable by humans. In the hexadecimal system, each character (0-9 and a-f) represents exactly 4 bits of data. Therefore, dividing the 256 total bits by 4 bits per character results in exactly 64 hexadecimal characters. Regardless of whether you hash a single letter or a massive database, the mathematical rules of the algorithm force the final output into this exact 256-bit, 64-character format.

Question 5

Is SHA-256 safe to use for storing user passwords?

Accepted Answer

Using raw, unadulterated SHA-256 to store passwords is a major security risk and is heavily discouraged by cybersecurity experts. Because SHA-256 is designed to be highly efficient, hackers using modern graphics cards can guess billions of passwords per second. To safely store passwords, you must use a specialized, intentionally slow algorithm like Argon2, bcrypt, or PBKDF2. If you are strictly required to use SHA-256, you must implement PBKDF2 with SHA-256, utilizing a unique random salt for every user and looping the hash function hundreds of thousands of times to thwart brute-force attacks.

SHA-256 Hash Generator

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