Secure communication
Encryption is a method in which data is rendered hard to read by an unauthorized party. Since encryption methods are created to extremely hard to break, many communication methods either use deliberately weaker encryption than possible, or have backdoors inserted to permit rapid decryption. In some cases government authorities have required backdoors be installed in secret. Many methods of encryption are also subject to "man in the middle" attack whereby a third party who can 'see' the establishment of the secure communication is made privy to the encryption method, this would apply for example to interception of computer use at an ISP. Provided it is correctly programmed, sufficiently powerful, and the keys not intercepted, encryption would usually be considered secure. The article on key size examines the key requirements for certain degrees of encryption security.
Mcrypt
mcrypt is a replacement for the popular Unix crypt command.
crypt was a file encryption tool that used an algorithm very close to the
World War II Enigma cipher. Mcrypt provides the same functionality but uses several modern algorithms such as AES. Libmcrypt, Mcrypt's companion, is a library of code which contains the actual encryption functions and provides an easy method for use. The last update to libmcrypt was in 2007, despite years of unmerged patches. These facts have led security experts to declare mcrypt abandonware and discourage its use in new development. Prefer ccrypt, libressl, and others.