<!-- Copyright (c) 2026 defconxt. All rights reserved. -->
<!-- Licensed under AGPL-3.0 — see LICENSE file for details. -->
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---
name: analyzing-ransomware-encryption-mechanisms
description: >-
  Analyze ransomware encryption implementations to identify weaknesses,
  recover keys, and understand cryptographic schemes. Covers symmetric and
  asymmetric encryption analysis, key derivation flaws, IV reuse detection,
  and decryptor development.
domain: cybersecurity
subdomain: malware-analysis
tags:
  - ransomware
  - encryption
  - cryptanalysis
  - key-recovery
  - reverse-engineering
  - incident-response
version: "1.0"
author: defconxt
license: AGPL-3.0
metadata:
  mitre-attack: ["T1486"]
---

# Analyzing Ransomware Encryption Mechanisms

## Overview

Ransomware encrypts victim files using a combination of symmetric and asymmetric
cryptography. Understanding the implementation — key generation, encryption mode,
IV handling, and key storage — is critical for identifying weaknesses that enable
recovery without ransom payment. This skill covers reverse engineering encryption
routines, detecting cryptographic flaws, and building decryptors.

## Prerequisites

| Requirement | Install |
|---|---|
| Ghidra or IDA | Static disassembly and decompilation |
| x64dbg / WinDbg | Dynamic debugging on Windows |
| Python 3.10+ | For agent tooling and decryptor scripts |
| cryptography | `pip install cryptography` |
| pefile | `pip install pefile` — PE file analysis |
| CyberChef | Browser-based crypto analysis |

## Key Concepts

### Common Ransomware Crypto Schemes

| Scheme | Example Families | Weakness Potential |
|---|---|---|
| AES-CBC + RSA | REvil, Conti, LockBit | Low — if implemented correctly |
| AES-CTR + RSA | BlackCat/ALPHV | Nonce reuse enables XOR recovery |
| ChaCha20 + RSA | Hive, Royal | Low — stream cipher with unique nonce |
| AES-ECB | Early ransomware | High — ECB mode leaks patterns |
| XOR-only | Commodity ransomware | Critical — trivially reversible |
| Custom cipher | Jigsaw, various | Variable — implementation bugs common |

### Encryption Flow Analysis

```
Typical ransomware encryption flow:

1. Generate per-session RSA keypair OR embed attacker's RSA public key
2. For each file:
   a. Generate random AES key (32 bytes) and IV (16 bytes)
   b. Encrypt file contents with AES-CBC/CTR
   c. Encrypt AES key with RSA public key
   d. Append encrypted AES key + IV to file (or store in ransom note)
3. Delete original files and shadow copies
4. Drop ransom note with payment instructions
```

### Identifying Crypto APIs in Disassembly

#### Windows CryptoAPI

```
CryptAcquireContext    — Initialize crypto provider
CryptGenKey            — Generate symmetric key
CryptImportKey         — Import RSA public key
CryptEncrypt           — Encrypt data block
CryptExportKey         — Export encrypted session key
BCryptGenerateSymmetricKey — Modern CNG API
BCryptEncrypt          — CNG encryption
```

#### OpenSSL-Linked Malware

```
EVP_EncryptInit_ex     — Initialize cipher context
EVP_EncryptUpdate      — Encrypt data blocks
EVP_EncryptFinal_ex    — Finalize encryption
RSA_public_encrypt     — Encrypt session key
```

### Exploitable Weaknesses

| Weakness | Impact | Detection Method |
|---|---|---|
| Weak PRNG for key generation | Key space reduction | Trace key generation to `rand()`, `time()` |
| IV reuse across files | Known-plaintext attack on CTR mode | Compare IVs across encrypted files |
| Key stored in memory | Recovery from memory dump | Debug or memory forensics |
| ECB mode | Pattern preservation | Visual analysis of encrypted images |
| XOR with short key | Trivial brute force | Frequency analysis on ciphertext |
| Key derived from timestamp | Predictable key space | Check `GetTickCount`, `time()` usage |
| Session key in ransom note | Direct decryption | Parse ransom note metadata |

### Memory Forensics for Key Recovery

```bash
# Dump process memory (live system)
procdump -ma ransomware.exe dump.dmp

# Search for AES key schedules in memory dump
python3 aeskeyfind dump.dmp

# Search for RSA private keys
python3 rsakeyfind dump.dmp

# Volatility 3 — scan for crypto artifacts
vol3 -f memory.raw windows.memmap --pid <pid> --dump
```

### Building a Decryptor

```python
from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
from pathlib import Path


def decrypt_file(
    encrypted_path: str,
    output_path: str,
    key: bytes,
    iv: bytes,
    footer_size: int = 256,
) -> None:
    """Decrypt a file encrypted with AES-CBC.

    Assumes encrypted AES key is appended as footer (common pattern).
    Caller must provide recovered/decrypted AES key and IV.
    """
    data = Path(encrypted_path).read_bytes()
    ciphertext = data[:-footer_size]  # Strip metadata footer

    cipher = Cipher(algorithms.AES(key), modes.CBC(iv))
    decryptor = cipher.decryptor()
    plaintext = decryptor.update(ciphertext) + decryptor.finalize()

    # Remove PKCS7 padding
    pad_len = plaintext[-1]
    plaintext = plaintext[:-pad_len]

    Path(output_path).write_bytes(plaintext)
```

### Encrypted File Structure Analysis

```python
def analyze_encrypted_file(path: str) -> dict:
    """Analyze structure of ransomware-encrypted file."""
    data = Path(path).read_bytes()
    return {
        "size": len(data),
        "entropy": calculate_entropy(data),
        "header_bytes": data[:32].hex(),
        "footer_bytes": data[-64:].hex(),
        "null_ratio": data.count(0) / len(data),
        # High entropy + low null ratio = likely encrypted
        # Repeating patterns in header = possible ECB mode
    }
```

## Workflow

1. **Collect samples** — Encrypted files, ransomware binary, ransom note, memory dump
2. **Identify family** — YARA rules, VT lookup, ransom note analysis
3. **Static analysis** — Disassemble binary, identify crypto APIs and constants
4. **Dynamic analysis** — Debug encryption routine, capture keys in memory
5. **Analyze scheme** — Map full encryption flow, identify key management
6. **Find weaknesses** — Check PRNG, IV reuse, key storage, mode selection
7. **Build decryptor** — Implement decryption using recovered keys or exploited flaws
8. **Validate** — Decrypt test files, verify file integrity

## Detection

```yaml
title: Ransomware Encryption Mechanisms Detection
id: 7d23c06b-7ca7-4155-883b-689138e5076f
status: experimental
description: Detects suspicious activity related to analyzing ransomware encryption mechanisms techniques in malware analysis context
logsource:
  category: process_creation
  product: windows
detection:
  selection:
    CommandLine: "*analyzing*ransomware*"
  condition: selection
level: high
tags:
  - attack.t1486
  - attack.execution
falsepositives:
  - Sandbox detonation environments analyzing submitted samples
```

## Verification

| Check | Method |
|---|---|
| Crypto APIs identified | Cross-reference imports with known crypto functions |
| Encryption mode confirmed | Analyze cipher context initialization parameters |
| Key generation traced | Breakpoint on PRNG / key derivation functions |
| Decryptor works | Decrypt known-plaintext file, compare with original |
| File integrity verified | SHA256 of decrypted file matches pre-encryption hash |

## References

- [MITRE ATT&CK T1486 — Data Encrypted for Impact](https://attack.mitre.org/techniques/T1486/)
- [No More Ransom Project](https://www.nomoreransom.org/)
- [ID Ransomware](https://id-ransomware.malwarehunterteam.com/)
- [aeskeyfind](https://citp.princeton.edu/our-work/memory/code/)
