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What is the role of the S-Box in AES encryption?

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Bob and Alice use a shared secret key ? K (256 bits) to encrypt messages. Alice sends Bob an encrypted message using an algorithm where each character in the message is individually encrypted using AES. Is this encryption protocol secure against a brute-force attack to find the key?

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Given that Alice encrypts each character of the message individually using AES and a 256-bit key, is this encryption protocol secure? What is the size of the domain of all possible ciphertexts?

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How can Alice and Bob improve the security of their encryption protocol, which currently encrypts each character individually using AES?

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Why is the Simple AES algorithm vulnerable to a known plaintext attack, where the attacker knows both the plaintext and the ciphertext?

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A new system was installed at the front gate of a secret organization. When a worker wishes to open the gate, they use an Android application that encrypts the command "Open" using AES with a shared symmetric key K1 The encrypted command (ciphertext) is then sent to the server. What is the problem with this algorithm?

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How can the system be secured against replay attacks where an attacker intercepts and retransmits the same command to open the gate?

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In response to the security issue where an attacker could intercept and retransmit the same encrypted "Open" command to repeatedly open the gate, the developers modified the system by adding a Message Authentication Code (MAC). Now, the application encrypts the "Open" command using AES with the shared symmetric key K1, and generates a MAC using a second key K2 .The encrypted command (ciphertext) and the MAC are sent to the server together Does this modification prevent the attacker from using the intercepted ciphertext to open the gate?

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What is the primary motivation behind using Public Key Encryption instead of Symmetric Encryption?

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In Public Key Encryption, which of the following is true about the keys used by each entity?

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