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Server produces disposable keys

Data store with automated expiry

When servers produce keys they must have a shared store that can reliably and automatically delete data within a short time window.

Shared caches

TTL expiry is a core feature of cache services such as memcached and REDIS.

Cloud databases?

The AWS DynamodDB offers TTL on data but its reliability over short time windows is unconvincing as of this writing:

Depending on the size and activity level of a table, the actual delete operation of an expired item can vary. Because TTL is meant to be a background process, the nature of the capacity used to expire and delete items via TTL is variable (but free of charge). TTL typically deletes expired items within 48 hours of expiration.

Don't DIY

⚠️ It's tempting to have the server or some batch job clean up the keys but this temptation must be resisted. It is very difficult to ensure reliable clean up in all cases.

Service example

sequenceDiagram
User->>SPA: Clicks form "Send" button
SPA->>Service Instance X: Request key (GET)
Service Instance X->>Service Instance X: Produce temporary key pair            
Service Instance X->>Storage: Save key, expires in N seconds            
Service Instance X->>SPA: Send public key
SPA->>SPA: Encrypt secure data with public key
SPA->>Service Instance Y: Send encrypted data (POST)
Storage->>Service Instance Y: Retrieve key
Service Instance Y->>Service Instance Y: Decrypt message using private key
Service Instance Y->> X-Y-Z: Do X-Y-Z
Service Instance Y->>SPA: Success message
SPA->>User: Confirm card creation
SPA->>SPA: Destroy public key
Time->>Storage: Key deleted after N seconds

Serverless example

sequenceDiagram
User->>SPA: Clicks form "Send" button
SPA->>Lambda X: Request key (GET)
Lambda X->>Lambda X: Produce temporary key pair            
Lambda X->>Storage: Save key, expires in N seconds            
Lambda X->>SPA: Send public key
SPA->>SPA: Encrypt secure data with public key
SPA->>Lambda Y: Send encrypted data (POST)
Storage->>Lambda Y: Retrieve key
Lambda Y->>Lambda Y: Decrypt message using private key
Lambda Y->> X-Y-Z: Do X-Y-Z
Lambda Y->>SPA: Success message
SPA->>User: Confirm card creation
SPA->>SPA: Destroy public key
Time->>Storage: Key deleted after N seconds