Data is encrypted before being exfiltrated in order to hide the information that is being exfiltrated from detection or to make the exfiltration less conspicuous upon inspection by a defender. The encryption is performed by a utility, programming library, or custom algorithm on the data itself and is considered separate from any encryption performed by the command and control or file transfer protocol. Common file archive formats that can encrypt files are RAR and zip.
This type of attack technique cannot be easily mitigated with preventive controls since it is based on the abuse of system features.
Epic encrypts collected data using a public key framework before sending it over the C2 channel. Some variants encrypt the collected data with AES and encode it with base64 before transmitting it to the C2 server.
Lazarus Group malware IndiaIndia saves information gathered about the victim to a file that is compressed with Zlib, encrypted, and uploaded to a C2 server. Lazarus Group malware RomeoDelta archives specified directories in .zip format, encrypts the .zip file, and uploads it to its C2 server. A Lazarus Group malware sample encrypts data using a simple byte based XOR operation prior to exfiltration.
Encryption software and encrypted files can be detected in many ways. Common utilities that may be present on the system or brought in by an adversary may be detectable through process monitoring and monitoring for command-line arguments for known encryption utilities. This may yield a significant amount of benign events, depending on how systems in the environment are typically used. Often the encryption key is stated within command-line invocation of the software.
A process that loads the Windows DLL crypt32.dll may be used to perform encryption, decryption, or verification of file signatures.
Network traffic may also be analyzed for entropy to determine if encrypted data is being transmitted.  If the communications channel is unencrypted, encrypted files of known file types can be detected in transit during exfiltration with a network intrusion detection or data loss prevention system analyzing file headers. 
- ESET. (2016, October). En Route with Sednit - Part 2: Observing the Comings and Goings. Retrieved November 21, 2016.
- Brumaghin, E., et al. (2018, October 15). Old dog, new tricks - Analysing new RTF-based campaign distributing Agent Tesla, Loki with PyREbox. Retrieved November 5, 2018.
- Shevchenko, S.. (2008, November 30). Agent.btz - A Threat That Hit Pentagon. Retrieved April 8, 2016.
- Dumont, R. (2019, March 20). Fake or Fake: Keeping up with OceanLotus decoys. Retrieved April 1, 2019.
- Dumont, R.. (2019, April 9). OceanLotus: macOS malware update. Retrieved April 15, 2019.
- Symantec Security Response. (2014, July 7). Dragonfly: Cyberespionage Attacks Against Energy Suppliers. Retrieved April 8, 2016.
- Counter Threat Unit Research Team. (2017, October 12). BRONZE BUTLER Targets Japanese Enterprises. Retrieved January 4, 2018.
- Minerva Labs LTD and ClearSky Cyber Security. (2015, November 23). CopyKittens Attack Group. Retrieved September 11, 2017.
- FireEye. (2018, February 20). APT37 (Reaper): The Overlooked North Korean Actor. Retrieved March 1, 2018.
- DiMaggio, J. (2016, April 28). Tick cyberespionage group zeros in on Japan. Retrieved July 16, 2018.
- Symantec Security Response. (2011, November). W32.Duqu: The precursor to the next Stuxnet. Retrieved September 17, 2015.
- Xiaopeng Zhang. (2017, May 3). Deep Analysis of New Emotet Variant – Part 1. Retrieved April 1, 2019.
- Kaspersky Lab's Global Research and Analysis Team. (2014, August 7). The Epic Turla Operation: Solving some of the mysteries of Snake/Uroburos. Retrieved December 11, 2014.
- Kaspersky Lab's Global Research & Analysis Team. (2014, August 06). The Epic Turla Operation: Solving some of the mysteries of Snake/Uroboros. Retrieved November 7, 2018.
- Patil, S. (2018, June 26). Microsoft Office Vulnerabilities Used to Distribute FELIXROOT Backdoor in Recent Campaign. Retrieved July 31, 2018.
- FireEye Threat Intelligence. (2016, April). Follow the Money: Dissecting the Operations of the Cyber Crime Group FIN6. Retrieved June 1, 2016.
- FireEye Labs. (2015, April). APT30 AND THE MECHANICS OF A LONG-RUNNING CYBER ESPIONAGE OPERATION. Retrieved May 1, 2015.
- Sherstobitoff, R., Saavedra-Morales, J. (2018, February 02). Gold Dragon Widens Olympics Malware Attacks, Gains Permanent Presence on Victims’ Systems. Retrieved June 6, 2018.
- Patil, S. and Williams, M.. (2019, June 5). Government Sector in Central Asia Targeted With New HAWKBALL Backdoor Delivered via Microsoft Office Vulnerabilities. Retrieved June 20, 2019.
- Sherstobitoff, R. (2018, March 02). McAfee Uncovers Operation Honeybee, a Malicious Document Campaign Targeting Humanitarian Aid Groups. Retrieved May 16, 2018.
- Hromcová, Z. (2018, June 07). InvisiMole: Surprisingly equipped spyware, undercover since 2013. Retrieved July 10, 2018.
- Villeneuve, N., Bennett, J. T., Moran, N., Haq, T., Scott, M., & Geers, K. (2014). OPERATION “KE3CHANG”: Targeted Attacks Against Ministries of Foreign Affairs. Retrieved November 12, 2014.
- Novetta Threat Research Group. (2016, February 24). Operation Blockbuster: Unraveling the Long Thread of the Sony Attack. Retrieved February 25, 2016.
- Novetta Threat Research Group. (2016, February 24). Operation Blockbuster: Loaders, Installers and Uninstallers Report. Retrieved March 2, 2016.
- Novetta Threat Research Group. (2016, February 24). Operation Blockbuster: Remote Administration Tools & Content Staging Malware Report. Retrieved March 16, 2016.
- Sherstobitoff, R. (2018, February 12). Lazarus Resurfaces, Targets Global Banks and Bitcoin Users. Retrieved February 19, 2018.
- Faou, M. (2019, May). Turla LightNeuron: One email away from remote code execution. Retrieved June 24, 2019.
- United States District Court Southern District of New York (USDC SDNY) . (2018, December 17). United States of America v. Zhu Hua and Zhang Shilong. Retrieved April 17, 2019.
- Horejsi, J. (2018, April 04). New MacOS Backdoor Linked to OceanLotus Found. Retrieved November 13, 2018.
- Dell SecureWorks Counter Threat Unit Threat Intelligence. (2015, August 5). Threat Group-3390 Targets Organizations for Cyberespionage. Retrieved August 18, 2018.
- Lunghi, D., et al. (2017, December). Untangling the Patchwork Cyberespionage Group. Retrieved July 10, 2018.
- Cherepanov, A.. (2016, May 17). Operation Groundbait: Analysis of a surveillance toolkit. Retrieved May 18, 2016.
- TrendLabs Security Intelligence Blog. (2015, April). RawPOS Technical Brief. Retrieved October 4, 2017.
- Bromiley, M. and Lewis, P. (2016, October 7). Attacking the Hospitality and Gaming Industries: Tracking an Attacker Around the World in 7 Years. Retrieved October 6, 2017.
- Visa. (2015, March). Visa Security Alert: "RawPOS" Malware Targeting Lodging Merchants. Retrieved October 6, 2017.
- Grunzweig, J. and Miller-Osborn, J. (2017, November 10). New Malware with Ties to SunOrcal Discovered. Retrieved November 16, 2017.
- Legezo, D. (2019, January 30). Chafer used Remexi malware to spy on Iran-based foreign diplomatic entities. Retrieved April 17, 2019.
- Falcone, R. (2018, January 25). OilRig uses RGDoor IIS Backdoor on Targets in the Middle East. Retrieved July 6, 2018.
- Cybereason Nocturnus. (2019, June 25). Operation Soft Cell: A Worldwide Campaign Against Telecommunications Providers. Retrieved July 18, 2019.
- Grunzweig, J. and Miller-Osborn, J.. (2016, February 4). T9000: Advanced Modular Backdoor Uses Complex Anti-Analysis Techniques. Retrieved April 15, 2016.
- Counter Threat Unit Research Team. (2017, June 27). BRONZE UNION Cyberespionage Persists Despite Disclosures. Retrieved July 13, 2017.
- Symantec DeepSight Adversary Intelligence Team. (2019, June 20). Waterbug: Espionage Group Rolls Out Brand-New Toolset in Attacks Against Governments. Retrieved July 8, 2019.
- Lancaster, T., Cortes, J. (2018, January 29). VERMIN: Quasar RAT and Custom Malware Used In Ukraine. Retrieved July 5, 2018.
- Kaspersky Lab's Global Research & Analysis Team. (2018, February 20). A Slice of 2017 Sofacy Activity. Retrieved November 27, 2018.
- ESET. (2018, November 20). Sednit: What’s going on with Zebrocy?. Retrieved February 12, 2019.
- Zhang, H., Papadopoulos, C., & Massey, D. (2013, April). Detecting encrypted botnet traffic. Retrieved August 19, 2015.
- Wikipedia. (2016, March 31). List of file signatures. Retrieved April 22, 2016.