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DataStructuresandtheAnatomyofaFile·Chapter7 197 process, which usually requires some key piece of information, is called decryption, which means reversing the encryption process and converting the data back to its original form. Encryption has been around since the time ideas were first exchanged, and it is an ultimate provider of privacy and protector of sensitive information. In wartime, a cipher can be employed by one side to keep the other from getting a usable understanding of the contents of transmissions. Simple ciphers include the substitution of letters for numbers, the rotation of letters in the alphabet, and the mangling of voice signals by inverting sideband frequencies. Today, most ciphers use hardened and sophisticated computer algorithms that digitize data bits into seemingly random signals. To recover the contents of an encrypted signal, the correct decryption key is required unless you are attempting to use brute force to decrypt an object. The key is an algorithm that reverses the work of the encryption algorithm. A simple rule of thumb is that the more complex the encryption algorithm, the more difficult it becomes to listen in on the communication without knowing the key. Windows includes the following mechanisms to encrypt single files or entire volumes, which render them difficult to use in discovery without the associated decryption file or key: The Encrypting File System, or EFS EFS allows files to be transparently encrypted on NTFS file systems to protect confidential data from attackers with physical access to the computer. EFS uses a symmetric encryption algorithm to encrypt files. The key to decrypt the files is ultimately based on the encrypting user's account password, meaning strong passwords secure the encrypted file more than weaker, shorter passwords. BitLocker New to Windows Vista and Windows Server 2008, BitLocker is a full disk