 For example, 64-bit block ciphers like DES can be used to generate a keystream in output feedback (OFB) mode.However, when not using full feedback, the resulting stream has a period of around 232 blocks on average; for many applications, the period is far too low.For example, if encryption is being performed at a rate of 8 megabytes per second, a stream of period 232 blocks will repeat after about a half an hour.