One of the drawbacks of the previous CPA-secure encryption scheme is that its ciphertexts are \(\lambda\) bits longer than its plaintexts. In the common case that we are using a block cipher with blocklength blen \(=\lambda\), this means that ciphertexts are twice as long as plaintexts. Is there any way to encrypt data (especially lots of it) without requiring such a significant overhead?
A block cipher mode refers to a way to use a block cipher to efficiently encrypt a large amount of data (more than a single block). In this chapter, we will see the most common modes for CPA-secure encryption of long plaintexts.