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.
File hosting service
Secret key encryption is sometimes referred to as zero knowledge, meaning that only the user has the encryption key needed to decrypt the data. Since data is encrypted using the secret key, identical files encrypted with different keys will be different. To be truly zero knowledge, the file hosting service must not be able to store the user's passwords or see their data even with physical access to the servers. For this reason, secret key encryption is considered the highest level of access security in cloud storage. This form of encryption is rapidly gaining popularity, with companies such as SpiderOak being entirely zero knowledge file storage and sharing.