Hash Functions, Explained

After reading this you will know what a hash function does, why MD5 and SHA-256 produce the digests they do, why bcrypt gives a different result every time, and which algorithm to reach for in each situation.

What a hash function is

A hash function takes input of any length and returns a fixed-length string of bytes called a digest. Feed it one byte or one gigabyte, and the output size never changes. SHA-256 always returns 256 bits (32 bytes). MD5 always returns 128 bits (16 bytes). The mapping is deterministic: the same input produces the same digest every time, on any machine, in any language.

Here is the hook. The text hello hashed with SHA-256 is:

2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824

Change one character to hallo and the digest becomes:

d3751d33f9cd5049c4af2b462735457e4d3baf130bcbb87f389e349fbaeb20b9

Almost every hexadecimal digit changed. That property, where a tiny input change scrambles roughly half the output bits, is called the avalanche effect. It is what makes a digest useful as a fingerprint.

What the digests are good for

Hashes solve three practical problems.

Integrity checks

Publish the SHA-256 of a download. A reader who rehashes the file and gets the same 64 hex characters knows the bytes arrived intact.

Deduplication and cache keys

Two files with the same digest are almost certainly identical, so you can index them by hash instead of by content.

Password storage (with the right function)

Store a slow, salted hash of a password rather than the password itself. bcrypt is built for this; MD5 and SHA-256 are not.

Why a plain hash is wrong for passwords

SHA-256 is fast. That is the problem. A modern GPU computes billions of SHA-256 hashes per second, so an attacker who steals your hash table can try billions of password guesses per second against it. Speed helps the attacker, not you.

Two other weaknesses make plain hashes worse:

• No salt. Identical passwords produce identical digests, so an attacker sees which users share a password and can attack many accounts at once with a precomputed table.
• No cost control. You cannot make SHA-256 slower as hardware gets faster.

bcrypt fixes all three. It adds a random salt, runs deliberately slowly, and lets you raise the cost as hardware improves.

The bcrypt cost factor

bcrypt has a tunable cost factor (also called work factor or rounds). The number of internal key-setup iterations is 2^{\text{cost}}, so each step up doubles the work.

Here cost is the integer you choose, typically between 10 and 14. At cost 10 bcrypt runs 2^{10} = 1024 iterations. At cost 14 it runs 2^{14} = 16384 iterations, which is 16 times more work than cost 10. If verifying one password takes about 60 milliseconds at cost 12, cost 13 takes about 120 ms and cost 14 about 240 ms.

The goal is to pick the highest cost your server can absorb during a login. A cost that makes one legitimate login take 200 ms makes an attacker's billion-guess campaign take years instead of hours.

Why bcrypt gives a different hash every time

Hash correct horse with bcrypt twice and you get two different outputs. This is not a bug. bcrypt generates a fresh 16-byte random salt on every call and stores it inside the output string. Because the salt differs, the digest differs, even though the password is identical.

A bcrypt hash string carries everything needed to verify it later:

$2b$12$R9h/cIPz0gi.URNNX3kh2OPST9/PgBkqquzi.Ss7KIUgO2t0jWMUW

1. $2b$ is the bcrypt version identifier.
2. 12$ is the cost factor.
3. The next 22 base64 characters (R9h/cIPz0gi.URNNX3kh2O) encode the 16-byte salt.
4. The remaining 31 characters encode the 23-byte hash.

To verify a password, bcrypt reads the cost and salt out of the stored string, rehashes the candidate with those exact values, and compares. There is no separate salt column to manage; it travels with the hash.

A worked example with the demo data

Load the tool with its defaults and hash the sample text. The plain hashes are deterministic, so you can reproduce every value below.

If you need to know the exact byte count that goes into the hash (a hash operates on bytes, not characters), the String Length Inspector shows UTF-8 bytes versus characters. That distinction matters: café is 4 characters but 5 UTF-8 bytes, and the two encodings hash differently.

Reading and interpreting results

A few facts help you read any digest correctly:

• Length is fixed by the algorithm, not the input. A one-character input and a novel both produce a 64-character SHA-256 digest.
• Encoding matters. Hashing the bytes of a file differs from hashing its Base64 text. If your hash does not match a reference, check whether you hashed the raw bytes or an encoded form. The Base64 encoder-decoder helps you see the difference.
• Case and whitespace count. A trailing newline changes every bit of the digest. Many mismatches come from an editor adding a final newline.

Common mistakes

• Using MD5 or SHA-1 for signatures. Both are collision-broken. Use SHA-256 or better.
• Using any fast hash for passwords. Use bcrypt, scrypt or Argon2.
• Treating a hash as encryption. Hashing is one-way. There is no key that reverses SHA-256 back to the input. So-called "MD5 decrypters" are just lookup tables of previously computed hashes.
• Hashing to shorten data safely. Truncating a 64-character hash to 8 characters multiplies collision risk enormously. Keep the full digest unless you have measured the collision math.

Related tools

Hashes rarely travel alone. To decode a token whose signature uses HMAC-SHA256, reach for the JWT Decoder. When you compare a digest length in bits against a file size, the Byte Size Converter keeps KB and KiB straight (a 256-bit digest is 32 bytes, not 256).

Related tools

Base64 / URL / HTML Encoder-Decoder

Encode and decode Base64, hex, URL percent-encoding and HTML entities — with auto-detect, bulk lines and file drop.

JWT Decoder

Decode JWT header and payload, check expiry — locally.

HMAC Calculator

HMAC-SHA256/512/1/MD5 with text or hex inputs, hex + Base64 output, and timing-safe verify.

String Length Inspector

Characters, UTF-8 bytes, UTF-16 units and graphemes of any text.

Byte Size Converter

Convert between KB, KiB, MB, MiB and friends.

CSS Gradient, Shadow & Radius Generator

Visual generators for gradients, box-shadows and border radii.