Consider two substitution ciphers. One adds a value of i to the ASCII code of the plaintext character. The other adds a value of j to the plaintext character. All additions are modulo 256.
Now consider a double-encryption method that adds i to each plaintext character and then adds j to the resulting ciphertext character to get another ciphertext character. Again, all calculations are modulo 256.
How much more secure is this double encryption when compared with either single-encryption method?
Resultant double encryption will not be any different than the given single-encryption method. It is same as other two ...
Solution gives a mathematical formulation to justify the answer and also gives a detailed example for better grasp.