Choosing a Hash Algorithm -- SHA-256, SHA-1, MD5

Choosing a Hash Algorithm — SHA-256, SHA-1, MD5

A cryptographic hash function maps an arbitrary input to a fixed-length output (the "digest" or "fingerprint"). Good hash functions are deterministic, fast to compute, and — critically — hard to reverse or collide intentionally.

Algorithm comparison

Algorithm	Output size	Status	Use today?
MD5	128 bits (32 hex)	Broken	Legacy checksums only
SHA-1	160 bits (40 hex)	Weak	Avoid for new designs
SHA-256	256 bits (64 hex)	Strong	Yes — default choice
SHA-384	384 bits (96 hex)	Strong	High-security contexts
SHA-512	512 bits (128 hex)	Strong	High-security / large data
SHA-3 (256)	256 bits	Strong	Yes — alternate when SHA-2 is mandated out

When to use SHA-256 or SHA-512

• File integrity verification: checksums for downloads, backups, build artifacts
• Digital signatures: the hash of a document is what gets signed (ECDSA, RSA)
• HMAC: message authentication codes in APIs and webhooks
• Blockchain / Merkle trees: Bitcoin uses SHA-256 twice (SHA-256d)
• TLS certificates: SHA-256 is the standard in modern X.509 certificates

SHA-512 is preferred when processing large volumes of data on 64-bit hardware (it is actually faster than SHA-256 there per byte) or when extra collision resistance margin is desired.

When SHA-1 and MD5 are still found

Both are broken for collision resistance — an attacker can craft two different documents with the same hash. They still appear in:

• Git object IDs: Git uses SHA-1 for commit, tree, and blob hashes (transitioning to SHA-256)
• Legacy checksums: Many mirror sites publish MD5 sums for historical compatibility
• Non-security contexts: Detecting accidental corruption, not malicious manipulation

Do not use either for new security-sensitive designs. A SHA-1 or MD5 digest proves nothing if an adversary controls the data.

Passwords are fundamentally different

Hashing passwords with SHA-256 (or any fast hash) is insecure. An attacker with the hash database can test billions of passwords per second on commodity hardware.

Password storage requires a slow key-derivation function (KDF) with a per-user random salt:

• Argon2id — current recommended choice (winner of the Password Hashing Competition)
• bcrypt — proven, widely supported, work factor is adjustable
• scrypt — memory-hard, good when Argon2 is unavailable

Never roll your own hashing for passwords — use the KDF implementations in your language's standard library or a reputable security library.

Experiment with SHA-256, SHA-512, and MD5 digests on any input text with the Hash Generator — all computation runs locally in your browser.