| name | analyzing-ransomware-encryption-mechanisms |
| description | 'Analyzes encryption algorithms, key management, and file encryption routines used by ransomware families to assess decryption feasibility, identify implementation weaknesses, and support recovery efforts. Covers AES, RSA, ChaCha20, and hybrid encryption schemes. Activates for requests involving ransomware cryptanalysis, encryption analysis, key recovery assessment, or ransomware decryption feasibility. ' |
| domain | cybersecurity |
| subdomain | malware-analysis |
| tags | - malware - ransomware - encryption - cryptanalysis - reverse-engineering |
| version | 1.0.0 |
| author | mahipal |
| license | Apache-2.0 |
| nist_csf | - DE.AE-02 - RS.AN-03 - ID.RA-01 - DE.CM-01 |
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:
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()]}")
Common Ransomware Encryption Schemes:
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
AES-256-CBC + RSA-2048: Most common hybrid scheme (LockBit, REvil, Conti)
AES-256-CTR + RSA-4096: Stream cipher mode variant (BlackCat/ALPHV)
ChaCha20 + RSA-4096: Modern stream cipher (Hive, Royal)
Salsa20 + ECDH: Curve25519 key exchange (Babuk)
AES-128-ECB: Weak mode - potential decryption via known-plaintext
XOR-only: Trivial encryption - always recoverable
Custom algorithm: Often contains implementation flaws
Step 2: Analyze Key Generation and Management
Reverse engineer how encryption keys are generated and stored:
Key Management Patterns in Ransomware:
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
1. STRONG (no recovery possible without key):
- Per-file AES key generated with CryptGenRandom
- AES key encrypted with embedded RSA public key
- Encrypted key appended to each file or stored separately
- RSA private key held only by attacker's C2 server
2. WEAK (potential recovery):
- AES key derived from predictable seed (timestamp, PID)
- Same AES key used for all files (single key compromise = full recovery)
- Key transmitted to C2 before encryption starts (PCAP may contain key)
- XOR with short repeating key (brute-forceable)
- PRNG seeded with GetTickCount or time() (limited keyspace)
3. FLAWED IMPLEMENTATION:
- ECB mode (preserves plaintext patterns)
- Initialization vector (IV) reuse across files
- Key stored in plaintext in memory (recoverable from memory dump)
- Partial encryption (only first N bytes encrypted)
Step 3: Examine File Encryption Routine
Reverse engineer the file processing logic:
void encrypt_file(char *filepath) {
if (!is_target_extension(filepath)) return;
BYTE aes_key[32];
CryptGenRandom(hProv, 32, aes_key);
BYTE iv[16];
CryptGenRandom(hProv, 16, iv);
HANDLE hFile = CreateFile(filepath, GENERIC_READ, ...);
BYTE *plaintext = read_entire_file(hFile);
aes_cbc_encrypt(plaintext, file_size, aes_key, iv);
BYTE encrypted_key[256];
rsa_encrypt(aes_key, 32, rsa_pubkey, encrypted_key);
write_file(filepath, encrypted_data, encrypted_key, iv);
rename_file(filepath, strcat(filepath, ".locked"));
}
Step 4: Check for Cryptographic Weaknesses
Test the implementation for exploitable flaws:
from Crypto.Cipher import AES
import os
import struct
def check_key_reuse(file1_enc, file2_enc):
with open(file1_enc, "rb") as f:
data1 = f.read()
with open(file2_enc, "rb") as f:
data2 = f.read()
iv1 = data1[-16:]
iv2 = data2[-16:]
if iv1 == iv2:
print("[!] Same IV detected - key reuse likely")
def brute_force_timestamp_key(encrypted_file, known_header, timestamp_range):
with open(encrypted_file, "rb") as f:
encrypted_data = f.read()
for ts in timestamp_range:
import hashlib
key = hashlib.sha256(str(ts).encode()).digest()
iv = encrypted_data[-16:]
cipher = AES.new(key, AES.MODE_CBC, iv)
decrypted = cipher.decrypt(encrypted_data[:16])
if decrypted[:len(known_header)] == known_header:
print(f"[!] Key found! Timestamp: {ts}")
return key
return None
def check_ecb_mode(encrypted_file):
with open(encrypted_file, "rb") as f:
data = f.read()
blocks = [data[i:i+16] for i in range(0, len(data), 16)]
unique = len(set(blocks))
total = len(blocks)
if unique < total * 0.95:
print(f"[!] ECB mode likely: {total-unique} duplicate blocks out of {total}")
Step 5: Attempt Key Recovery
Use identified weaknesses for key recovery:
def xor_key_recovery(encrypted_file, known_plaintext):
"""Recover XOR key from known plaintext-ciphertext pair"""
with open(encrypted_file, "rb") as f:
ciphertext = f.read()
key = bytes(c ^ p for c, p in zip(ciphertext, known_plaintext))
for key_len in range(1, 256):
candidate = key[:key_len]
if all(key[i] == candidate[i % key_len] for i in range(min(len(key), key_len * 4))):
print(f"XOR key (length {key_len}): {candidate.hex()}")
return candidate
return None
Step 6: Document Encryption Analysis
Compile findings into a structured report:
Analysis should document:
- Algorithm identified (AES, RSA, ChaCha20, custom)
- Key size and mode of operation (CBC, CTR, ECB, GCM)
- Key generation method (CSPRNG, predictable seed, static key)
- Key storage location (appended to file, registry, C2 transmission)
- File modification pattern (full encryption, partial, header-only)
- Targeted file extensions
- Ransom note format and payment infrastructure
- Decryption feasibility assessment (possible/impossible/partial)
- Recommended recovery approach
Key Concepts
| Term | Definition |
|---|
| Hybrid Encryption | Combining symmetric (AES) for fast file encryption with asymmetric (RSA) for secure key wrapping; the standard ransomware approach |
| Key Wrapping | Encrypting the per-file symmetric key with the attacker's RSA public key so only the attacker's private key can decrypt it |
| ECB Mode | Electronic Codebook mode encrypts each block independently; preserves patterns in plaintext, a critical weakness enabling partial recovery |
| Known-Plaintext Attack | Using a known original file and its encrypted version to derive the encryption key; effective against XOR and weak stream ciphers |
| Key Schedule | The expanded form of an AES key in memory; scannable in memory dumps to recover encryption keys before they are erased |
| CSPRNG | Cryptographically Secure Pseudo-Random Number Generator; ransomware using CryptGenRandom produces unpredictable keys |
| Partial Encryption | Some ransomware only encrypts the first N bytes or every Nth block for speed; unencrypted portions may aid recovery |
Tools & Systems
- Ghidra: Reverse engineering suite for analyzing ransomware encryption routines at the assembly level
- PyCryptodome: Python cryptographic library for implementing and testing decryption routines
- NoMoreRansom.org: Free decryption tool repository maintained by Europol and security vendors for known ransomware families
- Volatility: Memory forensics framework for extracting encryption keys from RAM dumps of infected systems
- CryptoTester: Tool for identifying cryptographic algorithms based on constants and code patterns
Common Scenarios
Scenario: Assessing Decryption Feasibility for a Ransomware Incident
Context: An organization is hit with ransomware encrypting file servers. Management needs to know if decryption is possible without paying the ransom before making a recovery decision.
Approach:
- Identify the ransomware family from ransom note, file extension, and sample hash (check ID Ransomware)
- Check NoMoreRansom.org for existing free decryptors for this family
- Reverse engineer the encryption routine in Ghidra to identify the algorithm and key management
- Test for implementation weaknesses (key reuse, predictable seeds, ECB mode)
- Check if PCAP from the incident captured the key transmission to C2 (if key was sent before encryption)
- Scan memory dumps from affected machines for AES key schedules in RAM
- Report findings: decryption possible/impossible with specific technical justification
Pitfalls:
- Testing decryption methods on the only copy of encrypted files (always work on copies)
- Assuming all files use the same key without verifying (some ransomware uses per-file keys)
- Not checking for volume shadow copies (vssadmin) which ransomware may have failed to delete
- Confusing the file encryption algorithm with the key wrapping algorithm in reports
Output Format
RANSOMWARE ENCRYPTION ANALYSIS
================================
Sample: lockbit3.exe
Family: LockBit 3.0 / LockBit Black
SHA-256: abc123def456...
ENCRYPTION SCHEME
File Cipher: AES-256-CTR (per-file unique key)
Key Wrapping: RSA-2048 (public key embedded in binary)
Key Generation: CryptGenRandom (CSPRNG - unpredictable)
IV Generation: Random 16 bytes per file
File Structure: [encrypted_data][rsa_encrypted_key(256B)][iv(16B)][magic(8B)]
TARGETED EXTENSIONS
Total: 412 extensions targeted
Categories: Documents (.doc, .xls, .pdf), Databases (.sql, .mdb),
Archives (.zip, .7z), Source code (.py, .java, .cs)
Excluded: .exe, .dll, .sys, .lnk (system files preserved)
IMPLEMENTATION ANALYSIS
Key Strength: STRONG - per-file random keys, no reuse
Mode Security: STRONG - CTR mode with unique nonces
Key Storage: RSA-encrypted key appended to each file
Shadow Copies: Deleted via vssadmin and WMI
DECRYPTION FEASIBILITY
Without Key: NOT POSSIBLE
- No implementation flaws identified
- RSA-2048 key wrapping prevents brute force
- CSPRNG prevents key prediction
- No existing free decryptor available
RECOVERY OPTIONS
1. Restore from offline backups (recommended)
2. Check for volume shadow copies (low probability - ransomware deletes them)
3. Memory forensics if machine was not rebooted (key may persist in RAM)
4. Negotiate with attacker (last resort - no guarantee of decryption)
