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Computer Science IGCSEPDF 18
142 pagesrevision 46930
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7. Encryption
Purpose
● Prevents unauthorized access during transmission.
● Plaintext (readable) → encrypted → Ciphertext (unreadable).
Symmetric Encryption
● Same key used for encryption and decryption.
● Simple but less secure (key must be transmitted to receiver).
Asymmetric Encryption
● Uses two keys:
○ Public key (shared openly).
○ Private key (kept secret by receiver).
● Sender encrypts with public key → only receiver’s private key can decrypt.
● More secure because private key is never transmitted.
Automatic Repeat Requests (ARQ)
● Ensures reliable communication.
● Uses:
○ Acknowledgements (ACK/NACK): Confirm receipt or request resend.
○ Timeouts: If no ACK in a set time, data resent.
● Often used in mobile phone networks.
● Can work with Cyclic Redundancy Check (CRC) , parity, or checksum for
error detection.