What are the three states of data, and which one is the hardest to protect?

Data at rest, data in transit, and data in use — and "in use" is the hard, unsolved gap.

We have mature, widely deployed protection for two states:

State	Meaning	Typical protection
At rest	Stored on disk/DB	Encryption (AES), access control
In transit	Moving over a network	TLS/HTTPS, VPNs
In use	Being processed in memory/CPU	The gap

The problem: to compute on data, software traditionally has to decrypt it first — so during processing it sits in plaintext in memory, exposed to a compromised host, malicious insider, or co-tenant in the cloud.

Closing this "data-in-use" gap is exactly what MPC, Confidential Computing, and Homomorphic Encryption set out to do.