# ADR-0018: Password Hashing Algorithm

## Status
Accepted

## Context
The platform needs to securely store user passwords. Requirements:
- Resist brute-force attacks
- Resist rainbow table attacks
- Future-proof against advances in computing
- Reasonable performance (not too slow)

Options considered:
1. **bcrypt** - Battle-tested, widely used
2. **argon2id** - Modern, memory-hard, recommended by OWASP
3. **scrypt** - Memory-hard, good alternative
4. **PBKDF2** - Older standard, less secure

## Decision
Use **argon2id** for password hashing with recommended parameters:

- **Algorithm**: argon2id (variant)
- **Memory**: 64 MB (65536 KB)
- **Iterations**: 3 (time cost)
- **Parallelism**: 4 (number of threads)
- **Salt length**: 16 bytes (random, unique per password)

**Rationale:**
- Recommended by OWASP for new applications
- Memory-hard algorithm (resistant to GPU/ASIC attacks)
- Good balance of security and performance
- Future-proof design
- Standard library support in Go 1.23+

## Consequences

### Positive
- Strong security guarantees
- Memory-hard (resistant to hardware attacks)
- OWASP recommended
- Standard library support

### Negative
- Slightly slower than bcrypt (acceptable trade-off)
- Requires tuning parameters for production

### Implementation Notes
- Use `golang.org/x/crypto/argon2` package
- Store hash in format: `$argon2id$v=19$m=65536,t=3,p=4$salt$hash`
- Use `crypto/rand` for salt generation
- Verify passwords with `argon2.CompareHashAndPassword()`
- Consider increasing parameters for high-security environments

