counter mode

A block cipher mode that enhances ECB mode by ensuring that each encrypted block is different from every other block encrypted under the same key. [SP38A] (See: block cipher.)

Tutorial: This mode operates by first encrypting a generated sequence of blocks, called "counters", that are separate from the input sequence of plaintext blocks which the mode is intended to protect. The resulting sequence of encrypted counters is exclusive-ORed with the sequence of plaintext blocks to produce the final ciphertext output blocks. The sequence of counters must have the property that each counter is different from every other counter for all of the plain text that is encrypted under the same key.