1. In ancient times, a codebook was a book where you looked up a word, and replaced with another word according to the substitution table in the book. For example, you may look up the words "attack at dawn" in the book and come up with the words mouse dog cat that you send to your troops. The troops receiving the message would likewise look up these words in their codebooks in order to figure out the original message. Key point: In block-ciphers, the key represents a codebook. In other words, you could use the key to generate a huge book of matching pairs whereby each plaintext block would match to exactly one ciphertext block. Then, you could encrypt messages by looking them up in this table. See also: ECB From Hacking-Lexicon