A somewhat simplistic cipher known as the XOR cipher uses binary 8-bit keys to encode strings. The idea is to first convert each letter in a plaintext string to their character code in 8-bit binary. So the letter A whose character code is 65, is converted into 1000001. A key string, say the letter K, is similarly converted to an 8-bit binary representation. Then each letter in the plaintext is encoded by performing a bit XOR operation on the plaintext letter and the key, both using their 8-bit binary representation. The resulting ciphertext could remain in binary or it could be converted back from character codes to encoded text, the ciphertext.

Create an XOR cipher and encode a plaintext string to produce a ciphertext. See Exercise 6, Section 2.2 for information on converting a string to its binary representation. As an aside, the XOR cipher is a terribly insecure cipher as simply reversing the encoding operations makes it easy to recover the key (Churchhouse 2001).