Ransomware is a type of malware that encrypts files on a victim’s computer and demands a ransom payment in order to decrypt the files. Over the past decade, ransomware has become an increasingly lucrative criminal enterprise, with global losses expected to exceed $20 billion in 2021 alone. For individuals and organizations impacted by ransomware, a key question arises: is it possible to decrypt files without paying the ransom? The difficulty of decrypting ransomware varies greatly depending on the specific strain involved.
Factors That Determine Ransomware Decryption Difficulty
There are several key factors that influence how hard it is to decrypt ransomware files without paying the ransom:
- Encryption algorithm – The specific encryption algorithm used by the ransomware impacts decryption difficulty. Older ransomware strains often used simpler symmetric algorithms like AES and RSA that cryptanalysts can sometimes crack. But newer ransomware increasingly uses more complex algorithms that are exponentially harder to break.
- Key length – The bit length of the encryption key also affects difficulty. 64-bit keys are easier to break than 256-bit keys. Ransomware developers now routinely use extremely long key lengths to make decryption virtually impossible.
- Hybrid encryption – Many ransomware variants use a combination of asymmetric and symmetric encryption. The asymmetric public-private key pair encrypts the symmetric key, doubling the complexity.
- Code obfuscation – Advanced techniques like code obfuscation make it harder for security researchers to analyze how the ransomware works, hindering decryption efforts.
- Encryption scope – Full disk encryption ransomware is harder to crack than variants that target specific file types.
- Custom cryptosystems – Completely custom encryption schemes invented by ransomware developers can be especially challenging to break.
In general, ransomware developers have significant advantages over decryption efforts. They can take their time designing encryption algorithms optimized for strength rather than efficiency. In contrast, cryptanalysts must reverse engineer the ransomware rapidly under pressure to help victims.
Ransomware Decryptors from Security Firms
For common ransomware strains, security firms like Kaspersky and Emsisoft have developed decryption tools that allow victims to potentially decrypt their files for free. However, these decryptors have significant limitations:
- Only work for older, less sophisticated ransomware versions susceptible to cryptanalysis.
- Require quick action from victims to run the decryptor before files are overwritten.
- Not effective against newer ransomware families.
- Often only recover partial subsets of encrypted files.
Free decryptors from security companies can provide hope to ransomware victims. But for sophisticated recent ransomware, decryption success rates remain low. Victims cannot count on a decryption tool being available.
Brute Force Attacks on Passwords/Keys
For ransomware variants that encrypt files with a locally generated password or key, one decryption option is brute force guessing of the key. To improve brute force speed, GPUs can be used to parallelize decryption attempts. However, as key lengths increase, brute forcing becomes infeasible:
| Key Length | Brute Force Time Estimate |
|---|---|
| 64-bit | Days to months |
| 128-bit | Hundreds to thousands of years |
| 256-bit | Billions of years |
| 512-bit | Trillions of years |
As this table illustrates, brute forcing decryption keys longer than 128 bits is completely infeasible given current and foreseeable technology. And keys of 256 bits or more are now the norm for ransomware.
Cryptanalysis of Weak Ciphers
In some instances, ransomware developers make implementation mistakes that enable cryptanalysis of the encryption algorithm itself. For example, ransomware might use the same initialization vector (IV) across encryptions. Or the cipher might be susceptible to mathematical weaknesses.
By studying the cipher closely, cryptanalysts can occasionally find theoretical vulnerabilities that allow full or partial decryption. For example, the ransomware Satan used a modified XOR algorithm with critical flaws. This enabled researchers to break the cipher and release free decryption tools.
However, as ransomware developers gain sophistication, fundamental cipher weaknesses become rare. Modern ransomware tends to rely on proven ciphers like AES that have withstood extensive cryptanalysis.
Flaws in Key Management
Outside of the core cipher, there are often opportunities to exploit flaws in how keys are generated, exchanged, or stored by ransomware programs:
- Predictable keys – Weak key generation algorithms can produce predictable keys that enable decryption.
- Hardcoded keys – Ransomware may mistakenly hardcode keys in the executable that can be extracted.
- Intercepted keys – Keys transiting networks may sometimes be intercepted.
- Recoverable keys – Keys saved temporarily to disk or memory may occasionally be recovered by forensic tools.
By identifying mistakes in key management, researchers have occasionally decrypted ransomware variants like NotPetya. However, ransomware developers have grown wise to these pitfalls. Modern ransomware strains tend to use cryptographically secure random number generators and avoid common key management errors.
Exploiting Ransomware Code Flaws
Like all software, ransomware code itself can contain vulnerabilities. By reverse engineering ransomware executables, researchers can sometimes find logic errors enabling decryption. Examples include:
- Failing to encrypt files properly before deleting originals.
- Having secret backdoors or flaws known only to the developers.
- Mistakes configuring encryption parameters like IVs or salts.
Through rigorous code audits, researchers have exploited flaws to decrypt ransomware like CrySis. But as with encryption protocols, ransomware code quality continues to improve. Developers use methods like code obfuscation, anti-debugging, and encryption of code itself to block analysis.
Recovery From Backups
The most reliable way to “decrypt” ransomware is simply to restore encrypted files from clean backups stored offline and inaccessible to the ransomware. With proper backups, the victim can wipe the ransomware infection and restore original files.
However, many organizations fail to implement robust backup plans. And consumer devices like laptops rarely have adequate backups. So while critical for overall resilience, backups provide incomplete protection against ransomware.
Law Enforcement Assistance
In rare cases, law enforcement agencies have been able to secretly obtain decryption keys from ransomware developers and share them with victims. For example, after the disruption of ransomware networks like REvil, keys have occasionally become available.
However, such assistance is unreliable and limited to high-profile incidents. Most ransomware victims cannot expect law enforcement to access keys and decrypt their files.
Payment and Decryption
Across thousands of ransomware strains analyzed to date, researchers collectively have only been able to develop free decryption capabilities for a small percentage. Broadly reliable decryption without paying the ransom remains elusive.
So in the majority of ransomware incidents, the only way for victims to reliably recover encrypted files is to pay the ransom demand. Of course, there is no guarantee hackers will properly decrypt the files even after receiving payment.
Should You Pay the Ransom?
Whether to pay the ransom demand is a complex decision for victims balancing costs, risks, and ethics. But in most cases currently, payment is the only way to ensure decryption. Factors to consider include:
- Cost of business disruption from lost files vs. ransom amount.
- Likelihood hackers will honor decryption after payment.
- Whether payment encourages future ransomware attacks.
- Ability to recover critical operations through other means like backups.
There are arguments on both sides of paying ransoms. But with reliable decryption often impossible otherwise, many victims feel they have no choice but to pay.
Conclusion
With newer generations of ransomware using meticulously engineered cryptography and resilient protocols, reliably decrypting files without paying the ransom has proven extremely challenging. For sophisticated threat actors, the odds currently favor the attackers in this asymmetric battle.
Victims cannot assume free decryption tools will be available. And thorough backups offer the only sure means of recovery. While experts continue developing advanced cryptanalysis techniques, ransomware developers also rapidly iterate and harden their malware.
For now, the difficulty of decrypting files encrypted by most ransomware families without paying the ransom remains high. But cybersecurity researchers have shown incredible skill and dedication in the ongoing fight against ransomware.
