Instant Messaging (IM) is a useful communication and work collaboration tool between individuals, groups, or enterprises. Unfortunately, most IM systems lack the needed security mechanism capable of ensuring the secure communications of IM client-client and IM client-server. In order to find a solution to secure IM communications, we designed and implemented a Secure Instant Messaging and Presence Protocol (SIMPP) based on elliptic-curve cryptography. The proposed IM service is compatible with the IETF XMPP (eXtensible Messaging and Presence Protocol)/Jabber Standard. Open source jabberd software was revised to create a SIMPP server on the Linux platform, wherein we used C++Builder to create a SIMPP client on the Windows platform. Our IM client and IM server use open source MIRACL cryptographic libraries with iksemel XMPP library.