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The message is decoded by reversing the substitution rule. If the encryption changed S to Y, the decryption changes Y back to S, applying the inverse mapping to all characters.

Conditions: The receiver knows the substitution mapping for each character.; The example uses direct reversal, such as Y back to S.

The message is decoded by reversing the substitution rule. If the encryption changed S to Y, the decryption changes Y back to S, applying the inverse mapping to all characters.

Conditions: The receiver knows the substitution mapping for each character.; The example uses direct reversal, such as Y back to S.