(c) Assume a simple reversible crypto system that its users use for both signing and encrypting/decrypting messages. Briefly argue whether using the same key-pair for signing and encrypting/decrypting is more or less secure than using a dedicated signing key-pair and a different, dedicated encryption/decryption key-pair. (5 marks) (d) In actual implementations of crypto systems, it is often recommended to add some random text to the plain text before encrypting it and to remove the random text, without further validation of the random test, after decrypting the cipher text. What attack can be prevented by this technique