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/*! @header CommonCryptor.h @abstract Generic interface for symmetric encryption. @discussion This interface provides access to a number of symmetric encryption algorithms. Symmetric encryption algorithms come in two "flavors" - block ciphers, and stream ciphers. Block ciphers process data (while both encrypting and decrypting) in discrete chunks of data called blocks; stream ciphers operate on arbitrary sized data. The object declared in this interface, CCCryptor, provides access to both block ciphers and stream ciphers with the same API; however some options are available for block ciphers that do not apply to stream ciphers. The general operation of a CCCryptor is: initialize it with raw key data and other optional fields with CCCryptorCreate(); process input data via one or more calls to CCCryptorUpdate(), each of which may result in output data being written to caller-supplied memory; and obtain possible remaining output data with CCCryptorFinal(). The CCCryptor is disposed of via CCCryptorRelease(), or it can be reused (with the same key data as provided to CCCryptorCreate()) by calling CCCryptorReset(). CCCryptors can be dynamically allocated by this module, or their memory can be allocated by the caller. See discussion for CCCryptorCreate() and CCCryptorCreateFromData() for information on CCCryptor allocation. One option for block ciphers is padding, as defined in PKCS7; when padding is enabled, the total amount of data encrypted does not have to be an even multiple of the block size, and the actual length of plaintext is calculated during decryption. Another option for block ciphers is Cipher Block Chaining, known as CBC mode. When using CBC mode, an Initialization Vector (IV) is provided along with the key when starting an encrypt or decrypt operation. If CBC mode is selected and no IV is provided, an IV of all zeroes will be used. CCCryptor also implements block bufferring, so that individual calls to CCCryptorUpdate() do not have to provide data whose length is aligned to the block size. (If padding is disabled, encrypting with block ciphers does require that the *total* length of data input to CCCryptorUpdate() call(s) be aligned to the block size.) A given CCCryptor can only be used by one thread at a time; multiple threads can use safely different CCCryptors at the same time. */