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Argon2 Parameters

Using Argon2id is great, but its "cost" parameters determine how expensive each password guess actually is, and you need to get them right:

Memory cost (m) is the amount of memory used by one hash, measured in KiB.
Time cost (t) is the number of passes over that memory.
Parallelism (p) is the number of computational lanes.

Higher costs make offline cracking more expensive, but they also consume more resources on your login server. A password policy that's too cheap helps attackers. One that's too expensive can turn a burst of login attempts into a denial-of-service problem.

Define One Policy

Here's an explicit Argon2id policy based on the minimum configuration in the current OWASP Password Storage Cheat Sheet:

const (
    memoryKiB   = 19 * 1024
    iterations  = 2
    parallelism = 1
)

That's 19 MiB of memory, two iterations, and one degree of parallelism. Real systems should benchmark their authentication workload and choose the strongest policy their hardware and availability requirements can safely support.

Parameters Travel With the Hash

An encoded Argon2id hash records the parameters that created it. Verification reads those stored values, so changing the current policy won't make older hashes unreadable... thank goodness.

The application can compare a stored hash with the current baseline after successful verification. Rehash when the record uses legacy SHA-256 or stale Argon2id parameters, then replace it with a new hash of the already-verified password.

The best time to upgrade is immediately after successful verification, while the application already has the plaintext password.

Assignment

Bearly Secure repeats its Argon2id parameters and never upgrades old hashes. Centralize the policy and upgrade legacy or stale hashes after successful login or MFA recovery.

In internal/auth/passwords/passwords.go, define one set of constants for the current Argon2id policy and use it in Hash.
Implement NeedsRehash(encodedHash string) bool:
- return true for a legacy SHA-256 hash
- return true when a valid Argon2id hash differs from the current version, memory, iteration, parallelism, or derived-key length
- return false for a current or malformed hash
In internal/httpserver/auth.go, rehash and store the password after a successful password login when NeedsRehash returns true.
Apply the same upgrade after successful password verification in the MFA recovery flow in internal/httpserver/auth_advanced.go.
Do not change a hash when password verification fails.

Run and submit the CLI tests from the project root.