In the lecture, we discussed codebook ciphers and that they are subject to statistical attacks. To avoid such attacks, people use additives to make identical plaintext words mapped to different codewords. The full procedure of encryption is as follows: 1. Find the codewords for plaintext words, 2. Generate a random number sequence that has the same length as the number of codewords. This random number sequence is known as the additives. 3. Align the codewords and additives. 4. Add the additives to the corresponding codewords to yield the final ciphertext.