Q: You need to store highly sensitive customer data (like Social Security Numbers) in a database. Explain the "Envelope Encryption" architecture using AWS KMS.

Cryptographically encrypting gigabytes of data directly via an AWS KMS API call is incredibly slow, expensive, and subject to strict network payload limits (4KB max).

• The application asks AWS KMS to generate a Data Encryption Key (DEK). KMS returns two versions of this DEK: one in plainly usable text, and one encrypted by the highly secure KMS Master Key (Customer Managed Key).
• The application uses the *plaintext* DEK to locally and rapidly encrypt the massive payload using a fast symmetric algorithm like AES-GCM.
• The application then immediately deletes the plaintext DEK from RAM.

Envelope Encryption solves this by using two tiers of keys: To decrypt, the application reads the encrypted envelope from the database, sends it to KMS to be decrypted, gets the plaintext DEK back, decrypts the payload locally, and discards the DEK again.

• The application stores the newly encrypted payload *alongside* the KMS-encrypted version of the DEK in the database (the DEK acts as an "envelope" for the payload).