Currently, Cisco PIX Firewall and ASA does not support RSA-encrypted nonces. But it is supported in Cisco routers.
The Multiple RSA Key Pair Support feature allows a user to configure a Cisco IOS router to have multiple Rivest, Shamir, and Adelman (RSA) key pairs.
For information on how to encrypt a pre-shared key on a router, refer to Encrypt Pre-shared Keys in Cisco IOS Router Configuration Example.
How to (General Information)
ASA Hardware & Software