Simone Tonelli Relatore
Simone Tonelli is a cybersecurity professional and independent security researcher focused on malware analysis, reverse engineering, threat hunting, incident response, and offensive security. His work centers on understanding how real-world malware behaves at implementation level, with particular attention to loaders, ransomware, Windows internals, and cryptographic misuse. He holds a BSc in Computer Engineering and an MSc in Telecommunications Engineering from Politecnico di Milano, and currently works in cybersecurity consulting, where he has contributed to threat hunting, purple teaming, incident response, and malware-analysis activities. Outside client work, he develops independent research and tooling for reverse engineering and recovery, with an emphasis on reproducibility rather than black-box results. His recent research on the r543t/ok75 ransomware lineage reconstructed a CryptoAPI key-derivation flaw that reduces the practical recovery problem from AES-256 to a 32-bit handle space, leading to a standalone decryptor validated against multiple ransomware generations.
Lista Talk
- 256 Bits on Paper, 32 Bits in Practice: Breaking r543t/ok75 Ransomware Through a CryptoAPI Implementation Flaw AES-256 should make ransomware-encrypted files practically unrecoverable without the key. But what happens when a subtle implementation mistake reduces the effective recovery problem from 256 bits to just 32? This talk presents an original reverse-engineering investigation of the r543t/ok75 ransomware lineage, observed in samples from 2023 and 2025. By recovering the embedded AutoIt source code and comparing both generations, I reconstructed the ransomware's partial-encryption format, its key-derivation process, and a critical flaw in its use of the Windows CryptoAPI. Although the malware initially derives a valid AES-256 session key, an unexpected interaction between CryptoAPI and the AutoIt encryption wrapper undermines the security of the entire encryption scheme. In the analyzed 32-bit implementations, this mistake reduces the effective recovery search space to at most 2^32 candidates. The presentation will uncover the vulnerable code path, explain the cryptographic consequences, and demonstrate the development of a standalone decryptor. It will also cover candidate-key validation techniques and an alternative recovery path that can eliminate brute force when a ransomware artifact survives. Validated against both ransomware generations, the decryptor successfully recovered encrypted files byte-for-byte, matching their original cryptographic hashes. This case demonstrates how a single API-level mistake can turn apparently strong cryptography into a practical reverse-engineering and recovery problem. - 14:30/15:15, 21 Nov 2026