In the lesson’s example, XOR encryption can be reversed by applying the same key to the ciphertext.
Answer
true
Explanation
### Why this is right
XOR has the property that applying the same bit pattern twice restores the original bits. Therefore, if ciphertext is plaintext XOR key, XORing that ciphertext with the same key recovers the plaintext.
### The concept
XOR, or exclusive OR, outputs 1 when its two input bits differ and 0 when they match. For example, $0 \text{ XOR } 1 = 1$, and $1 \text{ XOR } 1 = 0$. This reversible property is used in the slide’s stream-cipher illustration. The example repeats an eight-bit key across the letters, but it illustrates the operation rather than establishing a secure real-world key-management method.
### Terms
- **XOR**: Exclusive OR, a bit operation that returns 1 when its input bits differ.
- **Plaintext**: Readable, unencrypted information.
- **Ciphertext**: Information after a cipher has transformed it.
### Remember
XORing a value with the same key twice returns the starting value.
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