analyzing-ransomware-encryption-mechanisms

Analyzing Ransomware Encryption Mechanisms

When to Use

• A ransomware infection has occurred and recovery requires understanding the encryption scheme used
• Assessing whether decryption is possible without paying the ransom (implementation flaws, known decryptors)
• Reverse engineering ransomware to identify the encryption algorithm, key derivation, and key storage mechanism
• Developing a decryptor tool when a weakness in the ransomware's cryptographic implementation is identified
• Classifying a ransomware sample by its encryption approach to attribute it to a known family

Do not use for production data recovery operations without first verifying the decryption method on test copies of encrypted files.

Prerequisites

• Ghidra or IDA Pro for reverse engineering the ransomware binary
• Python 3.8+ with pycryptodome library for testing encryption/decryption routines
• Sample encrypted files and their corresponding plaintext originals (known-plaintext pairs)
• Access to the ransomware binary (unpacked if applicable)
• Familiarity with symmetric (AES, ChaCha20) and asymmetric (RSA) cryptographic algorithms
• NoMoreRansom.org database for checking existing free decryptors

Workflow

Step 1: Identify the Encryption Algorithm

Determine which cryptographic algorithm the ransomware uses:

# Check for Windows Crypto API usage in imports
import pefile

pe = pefile.PE("ransomware.exe")

crypto_apis = {
    "CryptAcquireContextA": "Windows CryptoAPI",
    "CryptAcquireContextW": "Windows CryptoAPI",
    "CryptGenKey": "Windows CryptoAPI key generation",
    "CryptEncrypt": "Windows CryptoAPI encryption",
    "CryptImportKey": "Windows CryptoAPI key import",
    "BCryptOpenAlgorithmProvider": "Windows CNG (modern crypto)",
    "BCryptEncrypt": "Windows CNG encryption",
    "BCryptGenerateKeyPair": "Windows CNG asymmetric key gen",
}

print("Crypto API Imports:")
for entry in pe.DIRECTORY_ENTRY_IMPORT:
    for imp in entry.imports:
        if imp.name and imp.name.decode() in crypto_apis:
            print(f"  {entry.dll.decode()} -> {imp.name.decode()}: {crypto_apis[imp.name.decode()]}")