# pdftract-15cs8: Crypt Filter Implementation Verification

## Task Summary
Implement Crypt filter (identity only, custom rejected with ENCRYPTION_UNSUPPORTED)

## Finding
The Crypt filter implementation was **already complete** in the codebase. No changes were required.

## Implementation Location
- File: `crates/pdftract-core/src/parser/stream.rs`
- Lines: 795-885 (CryptDecoder struct and implementation)
- Registered in get_decoder: line 960
- Orchestrator error handling: lines 1460-1471

## Acceptance Criteria Status

| Criteria | Status | Location |
|----------|--------|----------|
| /Crypt with /Name /Identity passes through unchanged | ✅ PASS | Lines 850-851 |
| /Crypt with /Name /MyCustom returns ENCRYPTION_UNSUPPORTED | ✅ PASS | Lines 853-854 |
| /Crypt with no /DecodeParms defaults to /Identity | ✅ PASS | Lines 822-825 |
| /Crypt with /Identity + FlateDecode works correctly | ✅ PASS | Test line 2678 |
| proptest never panics | ✅ PASS | Tests lines 2879, 2892 |
| INV-8 maintained (no panics) | ✅ PASS | Returns Err for hard errors |

## Test Coverage
The implementation includes comprehensive tests:
- `test_crypt_decode_identity` - Line 2579
- `test_crypt_decode_custom_rejected` - Line 2605
- `test_crypt_decode_no_params` - Line 2632
- `test_crypt_decode_missing_name` - Line 2652
- `test_crypt_identity_then_flate` - Line 2678
- `test_crypt_decoder_invalid_params` - Line 2709
- `test_crypt_decode_bomb_limit` - Line 2755
- `test_crypt_decoder_name` - Line 2778
- `test_crypt_custom_names_rejected` - Line 2784
- `proptest_crypt_decode_no_panic` - Line 2879
- `proptest_crypt_decode_with_params_no_panic` - Line 2892
- `proptest_crypt_decode_bomb_limit_no_panic` - Line 2926

## Implementation Details

### Public API
- `CryptDecoder` implements `StreamDecoder` trait
- `decode()` method checks `/DecodeParms /Name`
- `/Identity` (or missing): pass through unchanged
- Custom name: returns `FilterError::EncryptionUnsupported`

### Orchestrator Integration
The orchestrator catches `FilterError::EncryptionUnsupported` and:
1. Emits `ENCRYPTION_UNSUPPORTED` diagnostic
2. Returns empty bytes for the stream
3. Marks the stream as undecryptable

### INV-8 Compliance
- No panics in the implementation
- Hard errors return `Err(FilterError::EncryptionUnsupported)`
- Corrupt data mid-stream would return `Ok(partial)` with diagnostic (not applicable for Crypt since it's a no-op)

## Conclusion
The Crypt filter is fully implemented per PDF spec 7.4.10 and meets all acceptance criteria. No changes required.