I am designing a service which identifies records with public keys. All other fields are encrypted with that public key and protected by a HMAC. For authentication, I am thinking about signing requests to the service with the same (RSA?) private key which is also used for decryption (a timestamp will also be included in every request to avoid replay attacks). Since every record only contains a small number of fields and every record has its own key pair, I would like to keep the overhead reasonably low, meaning that I would like to avoid storing an additional key pair for signing for each record and instead re-use the en-/decryption key pair for signing. Which security implications could this lead to?