--- name: analyzing-ransomware-encryption-mechanisms-9b0ad1aeba93 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 mitre_attack: - T1486 - T1573.001 - T1573.002 - T1027 mitre_f3: version: '1.1' tactics: - monetization - positioning techniques: - id: F1018 name: Convert to Cryptocurrency tactic: monetization source: f3 - id: F1047 name: Transfer of funds tactic: monetization source: f3 - id: T1219 name: Remote Access Tools tactic: positioning source: attack --- # 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: ```python # 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()]}") ``` ``` 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: ```c // Typical ransomware file encryption flow (decompiled pseudo-code from Ghidra): void encrypt_file(char *filepath) { // 1. Check file extension against target list if (!is_target_extension(filepath)) return; // 2. Generate per-file AES key (32 bytes for AES-256) BYTE aes_key[32]; CryptGenRandom(hProv, 32, aes_key); // 3. Generate random IV (16 bytes) BYTE iv[16]; CryptGenRandom(hProv, 16, iv); // 4. Read file contents HANDLE hFile = CreateFile(filepath, GENERIC_READ, ...); BYTE *plaintext = read_entire_file(hFile); // 5. Encrypt with AES-256-CBC aes_cbc_encrypt(plaintext, file_size, aes_key, iv); // 6. Encrypt AES key with RSA public key BYTE encrypted_key[256]; // RSA-2048 output rsa_encrypt(aes_key, 32, rsa_pubkey, encrypted_key); // 7. Write: encrypted_data + encrypted_key + IV to file write_file(filepath, encrypted_data, encrypted_key, iv); // 8. Rename file with ransomware extension rename_file(filepath, strcat(filepath, ".locked")); } ``` ### Step 4: Check for Cryptographic Weaknesses Test the implementation for exploitable flaws: ```python from Crypto.Cipher import AES import os import struct # Test 1: Check if same key is used for multiple files # Compare encrypted versions of known files 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() # Extract IVs (location depends on ransomware family) # If IVs are same and files share encrypted blocks -> same key iv1 = data1[-16:] # Example: IV at end iv2 = data2[-16:] if iv1 == iv2: print("[!] Same IV detected - key reuse likely") # Test 2: Check for predictable key derivation # If key is derived from timestamp, iterate possible values 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: # Derive key the same way ransomware does 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 # Test 3: Check for ECB mode (pattern preservation) def check_ecb_mode(encrypted_file): with open(encrypted_file, "rb") as f: data = f.read() # ECB produces identical ciphertext for identical plaintext blocks 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: ```python # Recovery Method 1: Extract key from memory dump # Volatility plugin to scan for AES key schedules # vol3 -f memory.dmp windows.yarascan --yara-rule "aes_key_schedule" # Recovery Method 2: Known-plaintext attack (weak algorithms) 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)) # Find repeating key length 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 # Recovery Method 3: Check NoMoreRansom for existing decryptors # https://www.nomoreransom.org/en/decryption-tools.html ``` ### 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**: 1. Identify the ransomware family from ransom note, file extension, and sample hash (check ID Ransomware) 2. Check NoMoreRansom.org for existing free decryptors for this family 3. Reverse engineer the encryption routine in Ghidra to identify the algorithm and key management 4. Test for implementation weaknesses (key reuse, predictable seeds, ECB mode) 5. Check if PCAP from the incident captured the key transmission to C2 (if key was sent before encryption) 6. Scan memory dumps from affected machines for AES key schedules in RAM 7. 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) ``` --- Source attribution: hironow Original project: https://github.com/hironow/cyber/blob/20efb3a6a2a9e7ebbde38ed32325979866110540/.agents/skills/analyzing-ransomware-encryption-mechanisms/SKILL.md License: Apache-2.0 License information: https://www.apache.org/licenses/LICENSE-2.0 Original upstream instructions; downloaded from Mega AI Hub. ## Upstream license notice Apache License Version 2.0, January 2004 http://www.apache.org/licenses/ TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION 1. Definitions. "License" shall mean the terms and conditions for use, reproduction, and distribution as defined by Sections 1 through 9 of this document. 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See the License for the specific language governing permissions and limitations under the License. # Mega AI Hub resource metadata Creator / publisher: hironow Original project: https://github.com/hironow/cyber/blob/20efb3a6a2a9e7ebbde38ed32325979866110540/.agents/skills/analyzing-ransomware-encryption-mechanisms/SKILL.md Attribution basis: Source repository record Fields of work: Data & Analytics, Development & Engineering Roles: Data Scientist, ML / AI Engineer, Security Engineer Specialties: API, Testing Tools: Ghidra, Pycryptodome, Ida Pro Country applicability: Country not specified Classification basis: Source catalog labels Instruction SHA-256: 8e9ce0c6de56b8499856bbbd0f98ebe90b9b9732c25063be6eaf47a958e9d227