reverse-engineering-ransomware-encryption-routine

name	reverse-engineering-ransomware-encryption-routine
description	Reverse engineer ransomware encryption routines to identify cryptographic algorithms, key generation flaws, and potential decryption opportunities using static and dynamic analysis.
domain	cybersecurity
subdomain	malware-analysis
tags	["ransomware","encryption","reverse-engineering","cryptanalysis","aes","rsa","decryption","malware-analysis"]
version	1.0
author	mahipal
license	Apache-2.0
d3fend_techniques	["File Metadata Consistency Validation","Content Format Conversion","File Content Analysis","Platform Hardening","File Format Verification"]
nist_csf	["DE.AE-02","RS.AN-03","ID.RA-01","DE.CM-01"]

Reverse Engineering Ransomware Encryption Routine

Overview

Modern ransomware uses hybrid encryption combining symmetric algorithms (AES-256-CBC/CTR, ChaCha20, Salsa20) for file encryption with asymmetric algorithms (RSA-2048/4096, Curve25519) for key protection. The encryption routine typically generates a random symmetric key per file, encrypts file contents, then encrypts the symmetric key with the attacker's embedded public key. Reverse engineering these routines identifies the specific algorithms, key derivation methods, initialization vectors, file targeting patterns, and potential implementation flaws that could enable decryption without paying the ransom. Notable examples include Rhysida (AES-256-CTR + RSA-4096), Qilin.B (AES-256-CTR with AES-NI or ChaCha20 fallback), and Medusa (AES-256 + RSA).

When to Use

• When performing authorized security testing that involves reverse engineering ransomware encryption routine
• When analyzing malware samples or attack artifacts in a controlled environment
• When conducting red team exercises or penetration testing engagements
• When building detection capabilities based on offensive technique understanding

Prerequisites

• IDA Pro or Ghidra for static disassembly
• x64dbg/WinDbg for dynamic debugging
• Python 3.9+ with pycryptodome, pefile
• Understanding of AES, RSA, ChaCha20, Curve25519 algorithms
• Knowledge of Windows CryptoAPI and CNG (BCrypt) functions
• Sandbox environment for safe execution

Key Concepts

Hybrid Encryption Model

Ransomware generates a unique AES key and IV for each file. The file content is encrypted with this symmetric key. The symmetric key is then encrypted with the attacker's RSA public key (embedded in the binary or fetched from C2). The encrypted key is appended or prepended to the encrypted file. Only the attacker holding the RSA private key can decrypt the per-file symmetric keys.

Cryptographic API Identification

Windows ransomware typically uses CryptoAPI (CryptAcquireContext, CryptGenKey, CryptEncrypt) or CNG (BCryptGenerateSymmetricKey, BCryptEncrypt). Some use OpenSSL or custom implementations. Identifying these API calls provides immediate insight into the algorithm, key size, and mode of operation.

Implementation Flaws

Decryption opportunities arise from: hardcoded encryption keys, weak PRNG for key generation (using GetTickCount or time() as seed), reuse of IVs across files, ECB mode usage, keys remaining in memory post-encryption, and race conditions where keys can be captured during encryption.