ITPEC FE Subject A October 2024 Question 28
ITPEC FE Subject A October 2024 — Question 28 of 60
Stream cipher & XOR — why XOR is the only self-inverse bitwise operation.
XOR is self-inverse: A XOR K = C, and C XOR K = A.
This means the same operation encrypts and decrypts — perfect for stream ciphers.
Why not others:
- (a) AND — not reversible: 1 AND 0 = 0, but you can't recover the original bit
- (b) NAND — not self-inverse, not used in symmetric encryption
- (c) OR — not reversible: 1 OR 0 = 1, original bit lost
Key rule: XOR is the only standard bitwise op where applying it twice with the same key restores the original value.
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