---
phase: 12-user-key-input
plan: 02
subsystem: crypto
tags: [argon2id, rpassword, kdf, salt, password-authentication]

# Dependency graph
requires:
  - phase: 12-user-key-input
    plan: 01
    provides: "CLI --key/--key-file key input, KeySource enum, resolve_key()"
provides:
  - "Full --password support with Argon2id KDF and 16-byte random salt"
  - "Salt storage in archive format (flags bit 4, 16 bytes between header and TOC)"
  - "Interactive password prompt via rpassword with confirmation on pack"
  - "resolve_key_for_pack() and resolve_key_for_unpack() two-phase API"
affects: [kotlin-decoder, format-spec]

# Tech tracking
tech-stack:
  added: [argon2 0.5, rpassword 7.4]
  patterns: [two-phase key resolution for password (salt lifecycle), flags-based optional format sections]

key-files:
  created: []
  modified:
    - Cargo.toml
    - src/key.rs
    - src/format.rs
    - src/archive.rs
    - src/main.rs
    - tests/round_trip.rs

key-decisions:
  - "Two-phase key resolution: resolve_key_for_pack() generates salt, resolve_key_for_unpack() reads salt from archive"
  - "Salt stored as 16 plaintext bytes between header (offset 40) and TOC (offset 56) when flags bit 4 set"
  - "Argon2id with default parameters (Argon2::default()) for key derivation"
  - "pack prompts for password confirmation (enter twice), unpack prompts once"

patterns-established:
  - "Flags-based optional format sections: bit 4 signals 16-byte salt between header and TOC"
  - "Two-phase key resolution pattern: pack generates salt, unpack reads salt then derives key"

requirements-completed: [KEY-03, KEY-04, KEY-05, KEY-06]

# Metrics
duration: 5min
completed: 2026-02-26
---

# Phase 12 Plan 02: Password-Based Key Derivation Summary

**Argon2id KDF with 16-byte random salt stored in archive format, completing --password support via rpassword interactive prompt**

## Performance

- **Duration:** 5 min
- **Started:** 2026-02-26T20:56:34Z
- **Completed:** 2026-02-26T21:01:33Z
- **Tasks:** 2
- **Files modified:** 6

## Accomplishments
- Argon2id KDF derives 32-byte key from password + 16-byte random salt using argon2 crate
- Archives created with --password store salt in format (flags bit 4, 16 bytes at offset 40-55, TOC at 56)
- All three key input methods (--key, --key-file, --password) fully functional end-to-end
- Wrong password correctly rejected via HMAC/decryption failure
- All 52 tests pass: 25 unit + 7 golden + 20 integration (5 new password tests added)

## Task Commits

Each task was committed atomically:

1. **Task 1: Implement Argon2id KDF, rpassword prompt, and salt format** - `035879b` (feat)
2. **Task 2: Wire salt into archive pack/unpack, update main.rs, and add tests** - `4077847` (feat)

## Files Created/Modified
- `Cargo.toml` - Added argon2 0.5 and rpassword 7.4 dependencies
- `src/key.rs` - derive_key_from_password(), prompt_password(), resolve_key_for_pack/unpack(), ResolvedKey struct
- `src/format.rs` - FLAG_KDF_SALT, SALT_SIZE constants, read_salt/write_salt functions, relaxed flags validation
- `src/archive.rs` - Pack accepts optional salt, read_archive_metadata returns salt, read_archive_salt() helper
- `src/main.rs` - Two-phase password key resolution for pack/unpack/inspect
- `tests/round_trip.rs` - 5 new tests: password roundtrip, wrong password, salt flag, no-salt flag, directory password

## Decisions Made
- Two-phase key resolution API: resolve_key_for_pack() generates random salt and returns ResolvedKey with key+salt; resolve_key_for_unpack() reads salt from archive before deriving key
- Salt is 16 bytes of plaintext between header and TOC (not encrypted), signaled by flags bit 4 (0x10)
- Argon2id with default parameters (19 MiB memory, 2 iterations, 1 parallelism) for key derivation
- Pack prompts password twice (confirmation), unpack prompts once
- Legacy resolve_key() kept for inspect keyless path (errors on password variant)

## Deviations from Plan

None - plan executed exactly as written.

## Issues Encountered
None

## User Setup Required
None - no external service configuration required.

## Next Phase Readiness
- All three key input methods complete: --key (hex), --key-file (raw bytes), --password (Argon2id)
- Phase 12 is now complete - all user key input requirements fulfilled
- Future work: Kotlin decoder may need password/salt support for interop

## Self-Check: PASSED

All 6 modified files verified present. Both task commits (035879b, 4077847) found in git log.

---
*Phase: 12-user-key-input*
*Completed: 2026-02-26*