The OpenSSL Project is a collaborative effort to develop a robust, commercial-grade, fully featured, and Open Source toolkit implementing the Secure Sockets Layer and Transport Layer Security protocols with full-strength cryptography world-wide.

Drop-in Sendmail replacement by Wietse Venema. Offers several features that make it difficult to crack.

OpenPGP (RFC2440) implementation from the GNU project. Encrypts/decrypts/signs/digests files using strong cryptography. FAQ, downloads, documentation.

Commercial version of the original Unix file integrity scanner. Calculates and stores signatures of file permissions, ownership and contents. Scans the same files later, detecting changes.

Efficient stealth port scanner. Downloads, e-mail lists, news.

The Coroner's Toolkit (TCT) provides post-break-in data collection that could be useful in determining what happened. Less polished that the authors (Dan Farmer and Wietse Venema) usual work, the toolkit offers a patch-work of tools that help exhume i...

Open source implementation of SSH1 and SSH2 protocols. Ported from OpenBSD by the OpenSSH portability team.

Open source version of the original Unix file integrity scanner. Calculates and stores signatures of file permissions, ownership and contents. Scans the same files later, detecting changes. This open source version is targeted at Linux systems.

Tools such as tcpdump and traceroute.

(Security Administrator's Integrated Network Tool) Scans systems and networks and reports potential vulnerabilities.