Data in use

Data in use is an information technology term referring to active data which is stored in a non-persistent digital state typically in computer random-access memory (RAM), CPU caches, or CPU registers.
Scranton, PA data scientist Daniel Allen in 1996 proposed "Data in use" as a complement to the terms "data in transit" and "data at rest" which together define the three states of digital data.
Alternative definitions.
Data in use refers to data in computer memory. Some cloud software as a service (SaaS) providers refer to data in use as any data currently being processed by applications, as the CPU and memory are utilized.
Concerns.
Because of its nature, data in use is of increasing concern to businesses, government agencies and other institutions. Data in use, or memory, can contain sensitive data including digital certificates, encryption keys, intellectual property (software algorithms, design data), and personally identifiable information. Compromising data in use enables access to encrypted data at rest and data in motion. For example, someone with access to random access memory can parse that memory to locate the encryption key for data at rest. Once they have obtained that encryption key, they can decrypt encrypted data at rest. 
Threats to data in use can come in the form of cold boot attacks, malicious hardware devices, rootkits and bootkits.
Full memory encryption.
Encryption, which prevents data visibility in the event of its unauthorized access or theft, is commonly used to protect Data in Motion and Data at Rest and increasingly recognized as an optimal method for protecting Data in Use.
There have been multiple projects to encrypt memory. Microsoft Xbox systems are designed to provide memory encryption and the company PrivateCore presently has a commercial software product vCage to provide attestation along with full memory encryption for x86 servers. Several papers have been published highlighting the availability of security-enhanced x86 and ARM commodity processors. In that work, an ARM Cortex-A8 processor is used as the substrate on which a full memory encryption solution is built. Process segments (for example, stack, code or heap) can be encrypted individually or in composition. This work marks the first full memory encryption implementation on a mobile general-purpose commodity processor. The system provides both confidentiality and integrity protections of code and data which are encrypted everywhere outside the CPU boundary.
For x86 systems, AMD has a Secure Memory Encryption (SME) feature introduced in 2017 with Epyc. Intel has promised to deliver its Total Memory Encryption (TME) feature in an upcoming CPU.
CPU-based key storage.
Operating system kernel patches such as TRESOR and Loop-Amnesia modify the operating system so that CPU registers can be used to store encryption keys and avoid holding encryption keys in RAM. While this approach is not general purpose and does not protect all data in use, it does protect against cold boot attacks. Encryption keys are held inside the CPU rather than in RAM so that data at rest encryption keys are protected against attacks that might compromise encryption keys in memory.
Enclaves.
Enclaves enable an “enclave” to be secured with encryption in RAM so that enclave data is encrypted while in RAM but available as clear text inside the CPU and CPU cache. Intel Corporation has introduced the concept of “enclaves” as part of its Software Guard Extensions. Intel revealed an architecture combining software and CPU hardware in technical papers published in 2013.
Cryptographic protocols.
Several cryptographic tools, including secure multi-party computation and homomorphic encryption, allow for the private computation of data on untrusted systems. Data in use could be operated upon while encrypted and never exposed to the system doing the processing.