Block Cipher Basics: Encryption and Decryption Overview

This section explains the encryption and decryption process using a simple system with an 8-bit block size. Key operations involve XOR functions applied to plaintext and keys, demonstrating foundational cryptographic principles.

• Consider a simple system with an 8-bit block size.
• Assume the encryption (and decryption) to be as follows: If plaintext is LT||RT and the key is LK||RK, where LC, RC, LT, and RT are each 4 bits, then ciphertext= LC||RC where LC=LK XOR RT; and RC = RK XOR LT; Plaintext and ciphertext are each 8 bits.
• Similarly, to decrypt the ciphertext, we perform exactly the reverse operation where LT=RC XOR RK and RT = LC XOR LK. You are given the following 16-bit input D7F1 (in Hexa).
• You are provided IV as: A9 (hexadecimal).
• For CTR assume the stream of bits to be used for a counter to be starting from 0001 and
• incremented by 1 every time; so the stream would be 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111 . . .
• The 8-bit key to be used (where appropriate) is C5 (hexadecimal).
• Compute the encrypted output with (i) ECB, (ii) CBC, (iii) OFB, (iv) CFB, and (v) CTR (with IV=01012).
• Express the output as 4 hexadecimal characters so it is easy to read.