Breaking Down Hash Functions

Hash functions compress arbitrary data into fixed-size digests. The security depends on three core properties: preimage resistance, second preimage resistance, and collision resistance.

Key properties

• Preimage resistance: given a digest, it should be infeasible to recover the input.
• Second preimage resistance: given one input, it should be hard to find another with the same digest.
• Collision resistance: it should be hard to find any two distinct inputs with the same digest.

Common mistakes

• Using fast hashes like MD5 or SHA1 for passwords.
• Skipping a unique salt per user.
• Reusing a single hash for both integrity and authentication.

Safer patterns

• Password storage: Argon2id or bcrypt with strong parameters.
• Integrity only: SHA-256 or SHA-3 with a trusted channel.
• Authentication: use HMAC with a secret key.

Next up: a quick lab for benchmarking hashes and comparing cost factors.