Question 1

What is SHA-256?

Accepted Answer

SHA-256 (Secure Hash Algorithm 256-bit) is a cryptographic hash function from the SHA-2 family, standardized by NIST. It produces a fixed 256-bit (32-byte) output, displayed as a 64-character hexadecimal string. SHA-256 is used in SSL/TLS certificates, code signing, blockchain and Bitcoin proof-of-work, password hashing schemes (as the underlying primitive in PBKDF2, bcrypt derivations, and others), and digital signatures.

Question 2

Is SHA-256 reversible?

Accepted Answer

No. SHA-256 is a one-way function: given a hash output, it is computationally infeasible to find the original input. This property is called preimage resistance. You cannot decrypt a SHA-256 hash. If you need to verify a value, you hash the candidate input and compare the result to the stored hash. This is why password systems store hashes rather than plain text.

Question 3

What is the difference between SHA-256 and SHA-512?

Accepted Answer

Both are members of the SHA-2 family. SHA-256 produces a 256-bit (64 hex characters) output while SHA-512 produces a 512-bit (128 hex characters) output. SHA-512 uses 64-bit word arithmetic and can be faster than SHA-256 on 64-bit CPUs despite its larger output. SHA-256 is more widely supported in constrained environments, embedded systems, and hardware accelerators. For most applications SHA-256 provides sufficient security.

Question 4

What is HMAC-SHA256?

Accepted Answer

HMAC (Hash-based Message Authentication Code) combines a secret key with the hash function to produce a keyed hash. HMAC-SHA256 uses SHA-256 as the underlying function. It provides both data integrity and authentication: only someone who knows the secret key can produce or verify the HMAC. It is used in API request signing (AWS Signature Version 4, Stripe webhooks), JWT signature verification (HS256 algorithm), and secure cookie signing.

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