The remote host is missing an update to the system as announced in the referenced advisory FreeBSD-SA-05:19.ipsec.asc
Upgrade your system to the appropriate stable release or security branch dated after the correction date https://secure1.securityspace.com/smysecure/catid.html?in=FreeBSD-SA-05:19.ipsec.asc
IPsec is a security protocol for the Internet Protocol networking layer. It provides a combination of encryption and authentication of system, using several possible cryptography algorithms. A programming error in the implementation of the AES-XCBC-MAC algorithm for authentication resulted in a constant key being used instead of the key specified by the system administrator.