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Substitution cipher: comparing double and single encryption

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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?

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Solution gives a mathematical formulation to justify the answer and also gives a detailed example for better grasp.

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Resultant double encryption will not be any different than the given single-encryption method. It is same as other two ...

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