11.2 What cryptographic guarantees does encryption provide?

On a more fundamental level, the attacks described in the above examples work because Alice and Bob, as illustrated in Figure 11.1, can only use encryption e_k.

Intuitively, it might seem as if encryption protects Alice’s and Bob’s messages against manipulation by Mallory because the ciphertext hides the plaintext message and Mallory cannot know how to manipulate the encrypted message in a meaningful way. But this is completely wrong! Encryption provides no guarantees for message integrity or authenticity.

We can convince ourselves that this is indeed the case by taking a closer look at the one-time pad encryption scheme from Chapter 4, Encryption and Decryption.

Recall that the one-time pad encrypts a message m under the key k as:

where ⊕ denotes a bit-wise exclusive OR (XOR) operation. If you take two bits b₀,b₁ and apply the XOR operation to them, b₀ ⊕ b₁ will yield zero whenever both...