GnuTLS is a secure communications library implementing the SSL, TLS, and DTLS protocols and technologies around them. It provides a simple C language application programming interface (API) to access the secure communications protocols, as well as APIs to parse and write X.509, PKCS #12, OpenPGP, and other required structures. It is intended to be portable and efficient with a focus on security and interoperability.
Libmcrypt is a library which provides a uniform interface to several symmetric encryption algorithms. It is intended to have a simple interface to access encryption algorithms in ofb, cbc, cfb, and ecb modes. The algorithms it supports are DES, 3DES, RIJNDAEL, Twofish, IDEA, GOST, CAST-256, ARCFOUR, SERPENT, SAFER+, and more. The algorithms and modes are also modular so you can add and remove them on the fly without recompiling the library.
mcrypt is a program for encrypting files or streams. It is intended to be a replacement for the old UNIX crypt. It uses well-known and well-tested algorithms like BLOWFISH, AES, ARCFOUR, CAST-128, and more in several modes of operation. It also has a compatibility mode with the old UNIX crypt program and the des program found in Solaris.
mhash is a thread-safe hash library implemented in C, and provides a uniform interface to a large number of hash algorithms (MD5, SHA-1, HAVAL, RIPEMD160, TIGER, GOST). These algorithms can be used to compute checksums, message digests, and other signatures. The HMAC support implements the basics for message authentication, following RFC 2104.
shash is a program which produces message digests for files, and checks whether the digest remains the same (i.e., whether the files changed). Since anyone can generate the message digest, it may not be suitable for some security-related applications. Because of this, shash also supports HMAC (rfc2104), which is a mechanism for message authentication using cryptographic hash functions. shash can use a key with a hash algorithm to produce hashes that can only be verified with the same key. This way, you can securely check whether files in a filesystem were altered.
Hydra is a high performance multi-threaded HTTP server. Unlike traditional multi-threaded Web servers, it uses a constant, configurable pool of threads, and each thread can handle several connections by multiplexing the connections. This is even better than non-blocking servers, since Hydra will use every available CPU in a multi-CPU system. It also has features such as host-based virtual hosting, HTTP 1.1 features, CGI 1.1, SSL 3.0, TLS 1.0, and more. Hydra was based on the Boa Web server.
OpenConnect server (ocserv) is an SSL VPN GNU/Linux server. Its purpose is to be a secure, small, fast, and configurable VPN server which depends on standard protocols like TLS 1.2 and Datagram TLS. It implements the AnyConnect SSL VPN protocol and is compatible with the OpenConnect VPN client (compatibility with other Anyconnect SSL VPN clients is experimental).