# GCM Mode Issues - Development Handoff

**Date:** 2025-12-06 15:07 UTC  
**From:** Test Agent  
**To:** Development Agent  
**Priority:** 🔴 CRITICAL (P0)  
**Status:** 3 Tests Failing

---

## 🚨 Problem Summary

The Python SM-BC implementation has **3 failing tests** in GCM (Galois/Counter Mode), all related to MAC (Message Authentication Code) verification:

1. ❌ `test_with_aad` - AAD (Additional Authenticated Data) handling fails
2. ❌ `test_tampered_tag_rejected` - Tampered authentication tag detection fails
3. ❌ `test_tampered_ciphertext_rejected` - Tampered ciphertext detection fails

**Current Test Results:**
- ✅ 543 tests passing (99.3%)
- ❌ 3 tests failing (GCM mode)
- ⏱️ Test execution time: 3.64 seconds

---

## 📍 Failure Location

```
File: src/sm_bc/crypto/modes/gcm_block_cipher.py
Line: 407
Method: _decrypt_do_final()
Error: InvalidCipherTextException: mac check in GCM failed
```

### Stack Trace

```python
tests\test_gcm_mode.py:190: in test_with_aad
    dec_cipher2.do_final(decrypted2, dec_len2)
src\sm_bc\crypto\modes\gcm_block_cipher.py:216: in do_final
    return self._decrypt_do_final(output, out_off)
src\sm_bc\crypto\modes\gcm_block_cipher.py:407: in _decrypt_do_final
    raise InvalidCipherTextException("mac check in GCM failed")
```

---

## 🔬 Root Cause Analysis

### Issue 1: AAD Not Properly Integrated into MAC (`test_with_aad`)

**What's Happening:**
- Test encrypts data WITH additional authenticated data (AAD)
- Decryption fails MAC verification
- This suggests AAD bytes are not being correctly included in GHASH calculation

**Likely Causes:**
1. `process_aad_bytes()` not correctly feeding AAD into GHASH
2. AAD length encoding incorrect in final MAC calculation
3. AAD processing state not properly managed between init/update/final

**Expected Behavior:**
```python
# Encryption with AAD
cipher_enc.init(True, params)
cipher_enc.process_aad_bytes(aad, 0, len(aad))  # Process AAD
cipher_enc.process_bytes(plaintext, 0, len(plaintext), ciphertext, 0)
cipher_enc.do_final(ciphertext, enc_len)  # Appends auth tag

# Decryption with AAD should succeed
cipher_dec.init(False, params)
cipher_dec.process_aad_bytes(aad, 0, len(aad))  # Same AAD
cipher_dec.process_bytes(ciphertext, 0, ct_len, decrypted, 0)
cipher_dec.do_final(decrypted, dec_len)  # Should verify MAC successfully ✅
```

### Issue 2 & 3: MAC Calculation Issues (Tampering Tests)

**What's Happening:**
- Tests intentionally tamper with ciphertext/tag to verify detection
- However, MAC verification is failing even for correct data
- This indicates the MAC calculation itself is incorrect

**Likely Causes:**
1. GHASH function implementation error
2. Incorrect field multiplication in GF(2^128)
3. Final MAC assembly (S = GHASH_H(A||C) ⊕ E_K(J_0)) has issues
4. Tag length handling incorrect

---

## 🔧 Files to Review

### Primary Files (Must Fix)

```
src/sm_bc/crypto/modes/gcm_block_cipher.py
├── _decrypt_do_final() [line ~390-410]  ⚠️ MAC verification fails here
├── process_aad_bytes() [line ~?]         ⚠️ AAD not properly processed
├── _calculate_mac() or similar           ⚠️ MAC calculation logic
└── _g_hash() or GHASH implementation     ⚠️ Core crypto primitive

src/sm_bc/crypto/modes/gcm_multiplier.py
├── multiply_h() method                   ⚠️ GF(2^128) multiplication
└── GHASH algorithm implementation        ⚠️ Core authentication function
```

### Test Files (Reference)

```
tests/test_gcm_mode.py
├── test_with_aad [line 190]              ❌ Failing
├── test_tampered_tag_rejected [line 288]  ❌ Failing
└── test_tampered_ciphertext_rejected [line 314]  ❌ Failing
```

---

## 🐛 Debugging Strategy

### Step 1: Add Detailed Logging

Add logging to see what's happening:

```python
def _decrypt_do_final(self, output, out_off):
    """Finalize decryption and verify MAC"""
    
    # Extract received tag from end of buffer
    received_tag = self.buf_block[-self.mac_size:]
    print(f"[DEBUG] Received tag: {received_tag.hex()}")
    
    # Calculate expected tag
    calculated_tag = self._calculate_mac()
    print(f"[DEBUG] Calculated tag: {calculated_tag.hex()}")
    
    # Log AAD info
    print(f"[DEBUG] AAD length: {self.aad_length}")
    print(f"[DEBUG] Ciphertext length: {self.buf_off}")
    
    # Compare
    if not Arrays.constant_time_are_equal(received_tag, calculated_tag):
        print(f"[DEBUG] MAC MISMATCH!")
        raise InvalidCipherTextException("mac check in GCM failed")
    
    return decrypted_length
```

### Step 2: Test with Known Vectors

Use NIST SP 800-38D test vectors to validate GHASH:

```python
def test_ghash_nist_vector():
    """Test GHASH with NIST test vector"""
    # From NIST SP 800-38D Appendix B
    h = bytes.fromhex("66e94bd4ef8a2c3b884cfa59ca342b2e")
    x = bytes.fromhex("0388dace60b6a392f328c2b971b2fe78")
    expected = bytes.fromhex("5e2ec746917062882c85b0685353deb7")
    
    # Test GHASH function
    result = ghash(h, x)
    assert result == expected, f"GHASH mismatch: {result.hex()} != {expected.hex()}"
```

### Step 3: Compare with Reference Implementation

Look at Bouncy Castle Java implementation:

```java
// org.bouncycastle.crypto.modes.GCMBlockCipher.java

private byte[] calculateMac() {
    byte[] output = new byte[BLOCK_SIZE];
    
    // Process any buffered AAD
    if (atLength > 0) {
        gHASH(atBlock, 0, atLength);
    }
    
    // Process buffered ciphertext
    if (bufOff > 0) {
        gHASH(bufBlock, 0, bufOff);
    }
    
    // Process lengths: len(A) || len(C)
    byte[] lengths = new byte[BLOCK_SIZE];
    Pack.longToBigEndian(atLength * 8L, lengths, 0);
    Pack.longToBigEndian(totalLength * 8L, lengths, 8);
    gHASH(lengths, 0, BLOCK_SIZE);
    
    // XOR with encrypted counter
    cipher.processBlock(J0, 0, output, 0);
    GCMUtil.xor(output, S);
    
    return output;
}
```

### Step 4: Verify GF(2^128) Multiplication

The GHASH function relies on multiplication in GF(2^128):

```python
def test_gf128_multiplication():
    """Test Galois Field multiplication"""
    # Test with simple values
    a = bytes.fromhex("00000000000000000000000000000001")  # 1
    b = bytes.fromhex("00000000000000000000000000000002")  # 2
    expected = bytes.fromhex("00000000000000000000000000000002")  # 1*2=2
    
    result = gf128_multiply(a, b)
    assert result == expected
```

---

## 📚 Reference Materials

### NIST Standards
- **NIST SP 800-38D**: Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM)
  - URL: https://csrc.nist.gov/publications/detail/sp/800-38d/final
  - Contains test vectors and detailed algorithm description

### Bouncy Castle Implementations
- **Java BC GCMBlockCipher**: 
  - https://github.com/bcgit/bc-java/blob/main/core/src/main/java/org/bouncycastle/crypto/modes/GCMBlockCipher.java
- **JavaScript sm-js-bc**:
  - `sm-js-bc/src/crypto/modes/gcm_block_cipher.js`
  - Already tested and working

### GCM Algorithm Overview

```
GCM Encryption:
1. Generate IV → Counter block J₀
2. Increment counter for each block
3. Encrypt: C = P ⊕ E_K(CTR)
4. Calculate: S = GHASH_H(A || C || len(A) || len(C))
5. Calculate: T = S ⊕ E_K(J₀)
6. Output: C || T

GCM Decryption:
1. Parse C || T from input
2. Calculate: S' = GHASH_H(A || C || len(A) || len(C))
3. Calculate: T' = S' ⊕ E_K(J₀)
4. Verify: T == T' (constant-time comparison!)
5. If valid: Decrypt P = C ⊕ E_K(CTR)
```

---

## ✅ Acceptance Criteria

The fix is complete when:

```bash
# All GCM tests pass
python -m pytest tests/test_gcm_mode.py -v

# Expected output:
# test_with_aad - PASSED ✅
# test_tampered_tag_rejected - PASSED ✅  
# test_tampered_ciphertext_rejected - PASSED ✅
# ... (all other tests passing)

# Full test suite passes
python -m pytest tests/ -v

# Expected: 547/547 tests passing (100%)
```

### Specific Test Cases

1. **test_with_aad**: 
   - Encrypt with AAD, decrypt with same AAD → SUCCESS
   - Decrypt with different AAD → FAILURE (MAC mismatch)

2. **test_tampered_tag_rejected**:
   - Valid ciphertext + tampered tag → EXCEPTION before decryption

3. **test_tampered_ciphertext_rejected**:
   - Tampered ciphertext + valid tag → EXCEPTION before decryption

---

## ⏱️ Estimated Effort

- **Complexity**: High (crypto algorithm debugging)
- **Estimated Time**: 4-8 hours
  - Understanding GCM/GHASH: 2 hours
  - Debugging and root cause: 2-4 hours
  - Implementation and testing: 2 hours

---

## 🔄 Next Steps

1. **Developer Agent**:
   - Review this document
   - Add debug logging to GCM implementation
   - Compare with Java BC line-by-line
   - Fix AAD handling and MAC calculation
   - Verify with NIST test vectors
   - Run tests and confirm all pass

2. **Test Agent** (after fix):
   - Verify all 547 tests pass (100%)
   - Update test alignment tracker
   - Continue with GraalVM integration tests
   - Generate final test report

---

## 📝 Current Test Status

```
Total: 547 tests
✅ Passed: 543 (99.3%)
❌ Failed: 3 (0.5%)
⚠️ Skipped: 1 (0.2%)
⏱️ Time: 3.64s
```

**Only these 3 GCM tests are blocking 100% pass rate!**

---

**Document Owner:** Test Agent  
**Last Updated:** 2025-12-06 15:07 UTC  
**Status:** Awaiting Developer Agent
