Normally when we send encrypted messages, we prefer to do so using ephemeral message keys that are erased when communication is complete. This means that a message can’t be decrypted later using the same key it was encrypted with; while this is good for security, it means the burden of figuring out how to store messages (if that is a requirement) falls on the system. Some systems, such as Pond (https://pond.imperialviolet.org/), enforce a week lifetime for messages. This forced erasure of messages is considered the social norm; such factors will have to play into decisions about how to store decrypted messages and how long to store them.

There’s also the transition between data storage and message traffic: message traffic is encrypted with ephemeral keys that are never stored, while stored data needs to be encrypted with a long-term key. The system architecture should account for these different types of cryptography, and ensure that stored data is protected appropriately.

References

• how-does-tls-work

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