Block Cipher Modes: Decrypting Ciphertexts

Decrypting Ciphertexts Using Various Block Cipher Modes

In this scenario, we have a simple affine cipher expressed in C programming language, with both the cipher and inverse cipher functions defined. The block size is 8 bits, which is one byte (and one ASCII character). We will be using the fixed key 0x08.

a) Decrypt the ciphertext "262 A350408" using CTR mode

The CTR (Counter) mode involves XORing a stream of values produced by a counter with the plaintext to create the ciphertext. For each block of plaintext, the counter value should be incremented, and the resultant stream is XORed with the plaintext. Apply the inverse cipher function with key 0x08 to decrypt the ciphertext into ASCII characters.

b) Decrypt the ciphertext "EE33495 554" using ECB mode

ECB (Electronic Codebook) mode encrypts each block of plaintext individually using the same key. By applying the inverse cipher function with key 0x08 to each hexadecimal pair, you can decrypt the ciphertext into ASCII characters.

c) Decrypt the ciphertext "3BF03BF4E0" using CFB mode

CFB (Cipher Feedback) mode operates by encrypting the output of the previous encryption and XORing with the plaintext to create the ciphertext. Apply the inverse cipher function with key 0x08 to the ciphertext to obtain the ASCII characters.

d) Decrypt the ciphertext "66555 ABD5B" using CBC mode

CBC (Cipher Block Chaining) mode XORs each block of plaintext with the previous ciphertext block before encryption. Decrypt the provided ciphertext using the inverse cipher function and key 0x08 to reveal the ASCII characters.

e) Decrypt the ciphertext "3ADB652F52" using OFB mode

OFB (Output Feedback) mode creates a stream of data using an initial vector (IV) that is XORed with the plaintext to generate the ciphertext. Apply the inverse cipher function with key 0x08 to decrypt the given ciphertext into ASCII characters.