Optimized enveloping via key reuse

In a public key encryption system, a method for selecting a current secret key to be used to encrypt a message, the method comprising:

• determining whether a new secret key is required, wherein determining whether a new secret key is required further comprises:
• determining whether a previous message has been sent to a recipient;
• if a previous message has not been sent to the recipient, determining that a new secret key is required, and
• if a previous message has been sent to the recipient:
• retrieving counter data from a local data store;
• and
• comparing the counter data to a reuse criterion, wherein the reuse criterion comprises a maximum number of bytes of message data and the counter data comprises a cumulative number of bytes of message data previously sent using an associated reusable secret key;
• if the counter data satisfies the reuse criterion, determining that a new secret key is not required;
• and
• if the counter data fails to satisfy the reuse criterion, determining that a new secret key is required;
• if a new secret key is required;
• generating the new secret key;
• generating a new encrypted secret key by encrypting the new secret key using a public key associated with the recipient of the message;
• storing in the local data store the new secret key as a reusable secret key, the new encrypted secret key as a corresponding reusable encrypted secret key, and counter data associated with the reusable secret key;
• and
• selecting as the current secret key the new secret key;
• and
• if a new secret key is not required:
• retrieving from the local data store a reusable secret key and the corresponding reusable encrypted secret key;
• updating the counter data associated with the reusable secret key in the local data store;
• selecting as the current secret key the reusable secret key;
• encrypting the message using the current secret key;
• and
• sending the encrypted message and the encrypted secret key.