Question 1

What is a hash function?

Accepted Answer

A hash function converts input data of any length into a fixed-length string. The same input always produces the same output, but it's mathematically impossible to reverse the hash to recover the original input.

Question 2

Should I use MD5, SHA-1, or SHA-256?

Accepted Answer

Use SHA-256 or higher for any security purpose. MD5 was broken in 1996 and SHA-1 in 2005 — both have known collision vulnerabilities. SHA-256 or SHA-512 is recommended for checksums and digital signatures.

Question 3

Can the original text be recovered from a hash?

Accepted Answer

No. Hash functions are one-way — there is no mathematical inverse. No algorithm exists to recover the original input from a SHA-256 hash.

Question 4

What is a rainbow table attack?

Accepted Answer

A precomputed table mapping common passwords to their hash values. If a hash is stolen, it can be reversed by looking it up. Adding a random salt to each password before hashing defeats rainbow table attacks.

Question 5

Does the same text always produce the same hash?

Accepted Answer

Yes. Hash functions are deterministic. Even a single character difference — a capital letter, a space — produces a completely different hash. This property is used to verify file integrity.

Question 6

Can I use SHA-256 to store passwords in a database?

Accepted Answer

Not recommended. SHA-256 is fast and without salting is vulnerable to rainbow table attacks. For password storage, use bcrypt, argon2, or scrypt, which are slow by design and include salting.

Question 7

How do I use a hash to verify file integrity?

Accepted Answer

After downloading a file, compute its SHA-256 hash and compare it against the official hash. If the values match, the file has not been tampered with.

Question 8

What is HMAC and how is it different from a regular hash?

Accepted Answer

HMAC (Hash-based Message Authentication Code) combines a hash function with a secret key to provide both integrity and authentication. Used in JWT signing (HS256), it prevents forgery without the key.

Question 9

Is the output length of SHA-256 always the same?

Accepted Answer

Yes. SHA-256 always produces 256 bits (64 hex characters) regardless of input size. SHA-512 produces 512 bits (128 chars), and MD5 produces 128 bits (32 chars).

Question 10

How are hash functions used in blockchain?

Accepted Answer

Each block contains the hash of the previous block. Changing data in any block alters its hash, invalidating all subsequent blocks. This chained structure makes blockchain tamper-evident.

Question 11

What is a hash collision?

Accepted Answer

A collision occurs when two different inputs produce the same hash. MD5 and SHA-1 have demonstrated collisions. SHA-256 has no known collisions and remains secure.

Question 12

Is my input text sent to a server?

Accepted Answer

No. All hashing is performed using your browser's Web Crypto API (crypto.subtle). Nothing is transmitted to any server.

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