By Aleksandar Milenkoski & Jim Walter
We observe a new trend on the ransomware scene – intermittent encryption, or partial encryption of victims’ files. This encryption method helps ransomware operators to evade detection systems and encrypt victims’ files faster. We observe that ransomware developers are increasingly adopting the feature and intensively advertising intermittent encryption to attract buyers or affiliates.
Intermittent encryption is important to ransomware operators from two perspectives:
- Speed: Encryption can be a time-intensive process and time is crucial to ransomware operators – the faster they encrypt the victims’ files, the less likely they are to be detected and stopped in the process. Intermittent encryption does irretrievable damage in a very short time frame.
- Evasion: Ransomware detection systems may use statistical analysis to detect ransomware operation. Such an analysis may evaluate the intensity of file IO operations or the similarity between a known version of a file, which has not been affected by ransomware, and a suspected modified, encrypted version of the file. In contrast to full encryption, intermittent encryption helps to evade such analyses by exhibiting a significantly lower intensity of file IO operations and much higher similarity between non-encrypted and encrypted versions of a given file.
In mid-2021, the LockFile ransomware was one of the first major ransomware families to use intermittent encryption for evading detection mechanisms, encrypting every other 16 bytes of a file. Since then an increasing number of ransomware operations have joined the trend.
In this post, we review several recent ransomware families that feature intermittent encryption in an attempt to evade detection and prevention: Qyick, Agenda, BlackCat (ALPHV), PLAY, and Black Basta.
Qyick Ransomware
At the end of August 2022, we observed a user named lucrostm advertising a new commercial ransomware called Qyick in a popular TOR-based crime forum. We track the same user as an established vendor of other malicious tools including remote access tools and malware loaders.
The Qyick ransomware offering is a one-time purchase, as opposed to the more common subscription model. The price ranges from .2 BTC to approximately 1.5 BTC, depending on the level of customization the buyer requires. The buyer receives a compiled executable with a guarantee: if the ransomware is detected by security software within 6 months of purchase, the author will provide a new sample with a discount between 60% and 80% of the original price.
Qyick is written in Go and features intermittent encryption. lucrostm claims the apparent speed of the Qyick ransomware is achieved through the use of intermittent encryption and the ransomware’s implementation in Go, hinting at the current trend of intermittent encryption in the ransomware threat scene.
The exact manner in which Qyick conducts intermittent encryption is open to investigation as samples become available.
The current version of Qyick does not have data exfiltration capabilities. However, lucrostm has announced that future versions will feature execution of arbitrary executable code, meant primarily for the execution of data exfiltration capabilities.
Agenda Ransomware
Agenda ransomware, first spotted in August 2022, is written in Go and has been used primarily to target healthcare and education organizations in Africa and Asia. The ransomware has some customization options, which include changing the filename extensions of encrypted files and the list of processes and services to terminate.
Agenda ransomware supports several encryption modes that the ransomware operator can configure through the encryption setting. The ‘help’ screen displays the different encryption modes available: skip-step, percent, and fast.
Our analysis of Agenda revealed the following information about each mode.
| Encryption mode | Description |
| skip-step [skip: N, step: Y] | Encrypt every Y MB of the file, skipping N MB. |
| fast [f: N] | Encrypt the first N MB of the file. |
| percent [n: N; p:P] | Encrypt every N MB of the file, skipping P MB, where P equals P% of the total file size. |
BlackCat (ALPHV), the First Rust Ransomware-As-A-Service
The BlackCat (or ALPHV) ransomware came to prominence in late 2021 and is the first known ransomware to be written in the Rust programming language. The developers behind BlackCat were first spotted advertising its services in early December 2021 on a Russian underground forum.
The ALPHV threat group runs a ransomware-as-a-service (RaaS) program and shares ransom payments with affiliates. ALPHV uses bulletproof hosting to host their web sites and a Bitcoin mixer to anonymize transactions.
The ALPHV threat group is an early adopter of extortion schemes such as threatening victims with DDoS attacks, leaking exfiltrated data online as well as intimidating employees and customers of victim organizations should they not pay ransom. Major organizations and businesses have been the target of the BlackCat ransomware globally. For example, in September 2022, the BlackCat ransomware targeted Italy’s state-owned energy services firm GSE.
SentinelLabs researcher Aleksandar Milenkoski has reverse-engineered BlackCat ransomware samples and outlined the different encryption modes that BlackCat supports, the majority of which implement intermittent encryption. The table below lists these encryption modes.
| Encryption mode | Description |
| Full | Encrypt all file content. |
| HeadOnly [N] | Encrypt the first N bytes of the file. |
| DotPattern [N,Y] | Encrypt every N bytes of the file with a step of Y bytes. |
| SmartPattern [N,P] | Encrypt the first N bytes of the file. BlackCat divides the rest of the file into equal-sized blocks, such that each block is 10% of the rest of the file in size. BlackCat encrypts P% of the bytes of each block. |
| AdvancedSmartPattern [N,P,B] | Encrypt the first N bytes of the file. BlackCat divides the rest of the file into B equal-sized blocks. BlackCat encrypts P% of the bytes of each block. |
| Auto | Combinatory file encryption mode. Encrypt the content of the file according to one of the file encryption modes Full, DotPattern [N,Y], and AdvancedSmartPattern [N,P,B]. BlackCat selects and parametrizes a file encryption mode based on the filename extension and the size of the file. |
An evaluation study subjecting files of varying sizes (50 MB, 500 MB, 5 GB, and 50 GB) to the BlackCat ransomware revealed that using intermittent encryption can be of significant benefit to threat actors. For example, in contrast to full encryption, encrypting files using the Auto file encryption mode resulted in noticeably reduced wallclock processing time starting at 5 GB file size (8.65 seconds) and a maximum reduction in wallclock processing time of 1.95 minutes at 50 GB file size. Wallclock processing time is the total wallclock time (in seconds) that the ransomware spends on processing a file, which includes reading, encrypting, and writing file content. The full results of this study will be presented at the VirusBulletin Conference 2022.
We also note that BlackCat includes some internal logic for maximizing encryption speed. The ransomware encrypts files using the Advanced Encryption Standard (AES) encryption algorithm if the victim’s platform implements AES hardware acceleration. If not, the ransomware falls back to the ChaCha20 algorithm that is fully implemented in software.
PLAY Ransomware
PLAY ransomware is a new entrant in the ransomware scene and was first spotted at the end of June 2022. The ransomware has recently victimized high profile targets, such as the Court of Córdoba in Argentina in August 2022. PLAY’s ransom note consists of a single word – PLAY – and a contact email address.
In contrast to Agenda and BlackCat, PLAY ransomware does not feature encryption modes that can be configured by the operator. PLAY orchestrates intermittent encryption based on the size of the file under encryption, encrypting chunks (file portions) of 0x100000 bytes. For example, previous research states that under certain conditions, the PLAY ransomware encrypts:
- 2 chunks, if the file size is less than or equal to 0x3fffffff bytes;
- 3 chunks, if the file size is less than or equal to 0x27fffffff bytes;
- 5 chunks, if the file size is greater than 0x280000000 bytes.
In our analysis, we observed that a sample encrypted every other 0x100000 byte chunk until the end of the file. The file consisted only of null characters, which effectively makes the encrypted and non-encrypted chunks visually distinguishable.
Black Basta Ransomware
Black Basta is a RaaS program that emerged in April 2022 with ransomware samples dating back to February 2022. Current intelligence indicates that Black Basta emerged from the crumbled ashes of the Conti operation. The ransomware is written in the C++ programming language and supports Windows and Linux operating systems. Black Basta operators use the double extortion scheme threatening victim organizations with leaking exfiltrated data on the threat group’s TOR-based web site Basta News should the victims not pay ransom.
Black Basta is rapidly gaining ground on the ransomware scene and targets major organizations globally – the ransomware operation reported more than 20 victim organizations on Basta News within the first two weeks of its existence. Targeting, especially early on, was primarily focused on utilities, technology, financial, and manufacturing industries. For example, the major German building materials manufacturer Knauf suffered an attack conducted by Black Basta affiliates at the end of June 2022.
Like PLAY ransomware, Black Basta does not feature encryption modes that can be configured by the ransomware operator, but orchestrates intermittent encryption based on the size of the file under encryption. Black Basta encrypts:
- all file content, if the file size is less than 704 bytes;
- every 64 bytes, starting from the beginning of the file, skipping 192 bytes, if the file size is less than 4 KB;
- every 64 bytes, starting from the beginning of the file, skipping 128 bytes, if the file size is greater than 4 KB.
Our analysis showed that for a file with a size greater than 4 KB, the Black Basta ransomware encrypted 64 byte portions with an interval of 128 bytes between each, until the end of the file. In similar fashion to PLAY ransomware, the file consisted only of null characters, making the encrypted and non-encrypted chunks visually distinguishable.
Conclusion
Intermittent encryption is a very useful tool to ransomware operators. This encryption method helps to evade some ransomware detection mechanisms and encrypt victims’ files faster. Given the significant benefits to threat actors while also being practical to implement, we estimate that intermittent encryption will continue to be adopted by more ransomware families.
SentinelOne Singularity fully detects these ransomware samples.
Ransomware Samples
| Family | SHA1 |
| Agenda | 5f99214d68883e91f586e85d8db96deda5ca54af |
| BlackCat | 8917af3878fa49fe4ec930230b881ff0ae8d19c9 |
| PLAY | 14177730443c70aefeeda3162b324fdedf9cf9e0 |
| Black Basta | a996ccd0d58125bf299e89f4c03ff37afdab33fc |
