We have Salesforce Shield for encryption at rest. Have you implemented your archiving solution with Shield before?

Salesforce Shield does. It support Big Object or outside data store ( AWS cloud). So, we built our own encryption tool as part of the product offerings using AES 256 algorithm with key rotation. This solution has been aligned and validated by Salesforce ISV tech team. This tool supports both big object and outside storage out of the box.

Can I encrypt archived data?

Fields of archived data can be encrypted at rest for standard/custom objects. It uses the highest standard of encryption technology and also supports key rotation policy.

Can the archived data be encrypted?

Yes, the archived data can always be encrypted.