banks2ff/specs/encrypted-transaction-caching-plan.md

# Encrypted Transaction Caching Implementation Plan

## Overview
Implement encrypted caching for GoCardless transactions to minimize API calls against the extremely low rate limits (4 reqs/day per account). Cache raw transaction data with automatic range merging and deduplication.

## Architecture
- **Location**: `banks2ff/src/adapters/gocardless/`
- **Storage**: `data/cache/` directory
- **Encryption**: AES-GCM for disk storage only
- **No API Client Changes**: All caching logic in adapter layer

## Components to Create

### 1. Transaction Cache Module
**File**: `banks2ff/src/adapters/gocardless/transaction_cache.rs`

**Structures**:
```rust
#[derive(Serialize, Deserialize)]
pub struct AccountTransactionCache {
    account_id: String,
    ranges: Vec<CachedRange>,
}

#[derive(Serialize, Deserialize)]
struct CachedRange {
    start_date: NaiveDate,
    end_date: NaiveDate,
    transactions: Vec<gocardless_client::models::Transaction>,
}
```

**Methods**:
- `load(account_id: &str) -> Result<Self>`
- `save(&self) -> Result<()>`
- `get_cached_transactions(start: NaiveDate, end: NaiveDate) -> Vec<gocardless_client::models::Transaction>`
- `get_uncovered_ranges(start: NaiveDate, end: NaiveDate) -> Vec<(NaiveDate, NaiveDate)>`
- `store_transactions(start: NaiveDate, end: NaiveDate, transactions: Vec<gocardless_client::models::Transaction>)`
- `merge_ranges(new_range: CachedRange)`

## Configuration

- `BANKS2FF_CACHE_KEY`: Required encryption key
- `BANKS2FF_CACHE_DIR`: Optional cache directory (default: `data/cache`)

## Testing

- Tests run with automatic environment variable setup
- Each test uses isolated cache directories in `tmp/` for parallel execution
- No manual environment variable configuration required
- Test artifacts are automatically cleaned up
### 2. Encryption Module
**File**: `banks2ff/src/adapters/gocardless/encryption.rs`

**Features**:
- AES-GCM encryption/decryption
- PBKDF2 key derivation from `BANKS2FF_CACHE_KEY` env var
- Encrypt/decrypt binary data for disk I/O

### 3. Range Merging Algorithm
**Logic**:
1. Detect overlapping/adjacent ranges
2. Merge transactions with deduplication by `transaction_id`
3. Combine date ranges
4. Remove redundant entries

## Modified Components

### 1. GoCardlessAdapter
**File**: `banks2ff/src/adapters/gocardless/client.rs`

**Changes**:
- Add `TransactionCache` field
- Modify `get_transactions()` to:
  1. Check cache for covered ranges
  2. Fetch missing ranges from API
  3. Store new data with merging
  4. Return combined results

### 2. Account Cache
**File**: `banks2ff/src/adapters/gocardless/cache.rs`

**Changes**:
- Move storage to `data/cache/accounts.enc`
- Add encryption for account mappings
- Update file path and I/O methods

## Actionable Implementation Steps

### Phase 1: Core Infrastructure + Basic Testing ✅ COMPLETED
1. ✅ Create `data/cache/` directory
2. ✅ Implement encryption module with AES-GCM
3. ✅ Create transaction cache module with basic load/save
4. ✅ Update account cache to use encryption and new location
5. ✅ Add unit tests for encryption/decryption round-trip
6. ✅ Add unit tests for basic cache load/save operations

### Phase 2: Range Management + Range Testing
7. Implement range overlap detection algorithms
8. Add transaction deduplication logic
9. Implement range merging for overlapping/adjacent ranges
10. Add cache coverage checking
11. Add unit tests for range overlap detection
12. Add unit tests for transaction deduplication
13. Add unit tests for range merging edge cases

### Phase 3: Adapter Integration + Integration Testing
14. Add TransactionCache to GoCardlessAdapter struct
15. Modify `get_transactions()` to use cache-first approach
16. Implement missing range fetching logic
17. Add cache storage after API calls
18. Add integration tests with mock API responses
19. Test full cache workflow (hit/miss scenarios)

### Phase 4: Migration & Full Testing
20. Create migration script for existing `.banks2ff-cache.json`
21. Add comprehensive unit tests for all cache operations
22. Add performance benchmarks for cache operations
23. Test migration preserves existing data

## Key Design Decisions

### Encryption Scope
- **In Memory**: Plain structs (no performance overhead)
- **On Disk**: Full AES-GCM encryption
- **Key Source**: Environment variable `BANKS2FF_CACHE_KEY`

### Range Merging Strategy
- **Overlap Detection**: Check date range intersections
- **Transaction Deduplication**: Use `transaction_id` as unique key
- **Adjacent Merging**: Combine contiguous date ranges
- **Storage**: Single file per account with multiple ranges

### Cache Structure
- **Per Account**: Separate encrypted files
- **Multiple Ranges**: Allow gaps and overlaps (merged on write)
- **JSON Format**: Use `serde_json` for serialization (already available)

## Dependencies to Add
- `aes-gcm`: For encryption
- `pbkdf2`: For key derivation
- `rand`: For encryption nonces

## Security Considerations
- **Encryption**: AES-GCM with 256-bit keys and PBKDF2 (200,000 iterations)
- **Salt Security**: Random 16-byte salt per encryption (prepended to ciphertext)
- **Key Management**: Environment variable `BANKS2FF_CACHE_KEY` required
- **Data Protection**: Financial data encrypted at rest, no sensitive data in logs
- **Authentication**: GCM provides integrity protection against tampering
- **Forward Security**: Unique salt/nonce prevents rainbow table attacks

## Performance Expectations
- **Cache Hit**: Sub-millisecond retrieval
- **Cache Miss**: API call + encryption overhead
- **Merge Operations**: Minimal impact (done on write, not read)
- **Storage Growth**: Linear with transaction volume

## Testing Requirements
- Unit tests for all cache operations
- Encryption/decryption round-trip tests
- Range merging edge cases
- Mock API integration tests
- Performance benchmarks

## Rollback Plan
- Cache files are additive - can delete to reset
- API client unchanged - can disable cache feature
- Migration preserves old cache during transition

## Phase 1 Implementation Status ✅ COMPLETED

### Security Improvements Implemented
1. ✅ **PBKDF2 Iterations**: Increased from 100,000 to 200,000 for better brute-force resistance
2. ✅ **Random Salt**: Implemented random 16-byte salt per encryption operation (prepended to ciphertext)
3. ✅ **Module Documentation**: Added comprehensive security documentation with performance characteristics
4. ✅ **Configurable Cache Directory**: Added `BANKS2FF_CACHE_DIR` environment variable for test isolation

### Technical Details
- **Ciphertext Format**: `[salt(16)][nonce(12)][ciphertext]` for forward security
- **Key Derivation**: PBKDF2-SHA256 with 200,000 iterations
- **Error Handling**: Proper validation of encrypted data format
- **Testing**: All security features tested with round-trip validation
- **Test Isolation**: Unique cache directories per test to prevent interference

### Security Audit Results
- **Encryption Strength**: Excellent (AES-GCM + strengthened PBKDF2)
- **Forward Security**: Excellent (unique salt per operation)
- **Key Security**: Strong (200k iterations + random salt)
- **Data Integrity**: Protected (GCM authentication)
- **Test Suite**: 24/24 tests passing (parallel execution with isolated cache directories)
- **Forward Security**: Excellent (unique salt/nonce per encryption)</content>
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