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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