Question 1

What is a hash function?

Accepted Answer

A hash function is a mathematical algorithm that converts any input data into a fixed-length string of characters called a hash or digest. The same input always produces the same output, but even a tiny change to the input produces a completely different hash. Hash functions are one-way — you cannot recover the original data from the hash.

Question 2

What is the difference between MD5 and SHA-256?

Accepted Answer

MD5 produces a 128-bit (32-character hex) digest and is fast but cryptographically broken — it is vulnerable to collision attacks. SHA-256 produces a 256-bit (64-character hex) digest and is part of the SHA-2 family, which remains cryptographically secure. SHA-256 is recommended for security-sensitive applications; MD5 is only suitable for non-security checksums.

Question 3

Is MD5 still secure?

Accepted Answer

No, MD5 is not considered secure for cryptographic purposes. Researchers demonstrated collision attacks against MD5 in 2004, meaning two different inputs can produce the same MD5 hash. MD5 should not be used for password hashing, digital signatures, or certificate validation. It is still acceptable for non-security uses like detecting accidental file corruption.

Question 4

Can a hash be reversed (decrypted)?

Accepted Answer

No, a cryptographic hash cannot be reversed or decrypted by design — hash functions are one-way. However, attackers can use precomputed lookup tables called rainbow tables to look up common inputs that match a given hash. This is why passwords should always be hashed with a salted, slow hash function (bcrypt, Argon2) rather than MD5 or SHA-256 directly.

Question 5

What is SHA-256 used for?

Accepted Answer

SHA-256 is widely used in security-critical applications including TLS/SSL certificates, digital signatures, Bitcoin's proof-of-work algorithm, code signing, and password hashing (when combined with a salt). It is the de facto standard hash algorithm recommended by NIST for most cryptographic use cases.

Algorithm	Hash	Copy
MD5	Enter text above to generate hash	—
SHA-1	Enter text above to generate hash	—
SHA-256	Enter text above to generate hash	—
SHA-384	Enter text above to generate hash	—
SHA-512	Enter text above to generate hash	—
HMAC-SHA256	Enter text above to generate hash	—

Algorithm	Hex chars	Bits	Notes
MD5	32	128	Deprecated for security; fast checksums only
SHA-1	40	160	Deprecated; collision attacks exist
SHA-256	64	256	Standard for software distribution
SHA-384	96	384	TLS certificates, extra margin
SHA-512	128	512	Maximum strength, larger output

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