MD5 vs SHA: Understanding Cryptographic Hash Functions

BY TOOLS.FUN · MARCH 28, 2026 · 6 min read

Cryptographic hash functions are a cornerstone of modern security — used in digital signatures, TLS certificates, password storage, file integrity verification, and blockchains. Yet many developers still reach for MD5 or SHA-1 out of habit, unaware that both have been cryptographically broken for years. This guide explains what makes a hash function secure, where each algorithm fits, and what you should be using today.

Related tools: use our Base64 Decoder to decode hashed tokens, Hex Converter to read hash outputs in different formats, and Password Generator to create strong inputs for hashing.

What is a Hash Function?

A cryptographic hash function takes an input of any length and produces a fixed-size output (the hash, digest, or checksum). Four properties define a secure hash function:

Deterministic — the same input always produces the same output.
Fast to compute — the hash of any input should be quick to calculate.
Pre-image resistant — given a hash, it should be computationally infeasible to find an input that produces it.
Collision resistant — it should be computationally infeasible to find two different inputs that produce the same hash.

A "broken" hash function is one where researchers have found practical attacks against properties 3 or 4.

Key point: All hash functions are technically breakable with enough time — the question is whether a break is computationally feasible with current hardware. MD5 and SHA-1 have known feasible attacks.

MD5: Fast but Broken

MD5 (Message Digest 5) was designed in 1991 and produces a 128-bit (32 hex character) hash. It was widely used for file checksums, password storage, and digital signatures for over a decade.

MD5("hello") = 5d41402abc4b2a76b9719d911017c592

MD5 has been considered cryptographically broken since 2004. Researchers can generate collision attacks in seconds on modern hardware — meaning two different inputs can be crafted to produce the same MD5 hash. This breaks its use in digital signatures and certificates.

MD5 is still acceptable for non-cryptographic purposes like checksumming files for accidental corruption detection (not malicious tampering), or as a fast hash in hash tables. For anything security-related, retire it immediately.

SHA-1: Also Deprecated

SHA-1 (Secure Hash Algorithm 1) produces a 160-bit (40 hex character) hash. It was widely used in SSL/TLS certificates, Git commit IDs, and code signing until it was theoretically broken in 2005. In 2017, Google's SHAttered attack produced the first practical SHA-1 collision, generating two different PDFs with the same SHA-1 hash.

Major browsers stopped accepting SHA-1 TLS certificates in 2017. Git still uses SHA-1 internally for object IDs but is transitioning to SHA-256. Do not use SHA-1 for new security-sensitive applications.

SHA-256 and SHA-512: The Modern Standard

SHA-256 and SHA-512 are part of the SHA-2 family, standardised in 2001. No practical attacks are known against either. SHA-256 produces a 256-bit (64 hex character) hash; SHA-512 produces a 512-bit (128 hex character) hash.

SHA-256("hello") =
2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824

SHA-256 is the workhorse of modern cryptography: used in TLS 1.3, Bitcoin, AWS request signing, JWT HMAC signatures, and file integrity in package managers. Use SHA-256 as your default for integrity checks and digital signatures.

Key point: SHA-3 (Keccak) is the newest NIST-standardised hash family and is structurally different from SHA-2. It's not faster, but it provides algorithmic diversity — valuable if a break in SHA-2's design is ever discovered.

HMAC: Keyed Hashing

HMAC (Hash-based Message Authentication Code) combines a hash function with a secret key to produce a message authentication code. Unlike a plain hash, an HMAC can only be verified by someone who knows the secret key, providing both integrity and authenticity.

HMAC-SHA256(key="secret", message="Hello") =
88aab3ede8d3adf94d26ab90d3bafd4a2083070c3bcce9c014ee04a443847c0b

HMAC-SHA256 is used in JWT signature verification (HS256), AWS request signing (SigV4), webhook signature verification, and API authentication tokens.

Hashing Passwords: bcrypt and Argon2

For password storage, fast hash functions like SHA-256 are a security liability. A modern GPU can compute billions of SHA-256 hashes per second, making brute-force attacks trivially fast. Password hashing requires a deliberately slow function:

bcrypt — designed in 1999, still the most widely deployed. Has a configurable work factor (cost). bcrypt(password, cost=12) takes ~250ms — fast enough for login, too slow for brute force.
Argon2id — winner of the 2015 Password Hashing Competition. Memory-hard (resists GPU attacks), configurable time and memory cost. Preferred for new systems.
scrypt — also memory-hard, used by many cryptocurrency systems.

All three automatically handle salting internally. Never use MD5, SHA-1, or SHA-256 directly for password storage.

Hash Collisions and Rainbow Tables

A hash collision is when two different inputs produce the same hash. Collisions are inevitable in theory (infinite inputs, finite outputs), but a secure hash makes them computationally infeasible to find intentionally.

A rainbow table is a precomputed lookup table mapping common passwords and their hash values. An attacker who steals your password database can look up hashes in seconds. Salting defeats rainbow tables: a random salt is appended to each password before hashing, so identical passwords produce different hashes, and precomputed tables are useless.

Key point: bcrypt and Argon2 generate a unique salt automatically and store it in the output hash string. You don't need to manage salts separately — the library handles it.

Which Hash Should You Use?

Use the Hash Generator at Tools.Fun to generate MD5, SHA-1, SHA-256, and SHA-512 hashes for any input — useful for verifying file checksums, generating test vectors, and understanding hash outputs. For production systems: SHA-256 for integrity, HMAC-SHA256 for authentication, and bcrypt/Argon2id for passwords.

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