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A substitution cipher is a type of encryption where characters or units of text are replaced by others in order to encrypt a text sequence. Substitution ciphers are a part of early cryptography, predating the evolution of computers, and are now relatively obsolete.
In a substitution cipher, a letter such as A or T, is transposed into some other letter, which effectively encrypts the sequence to a human reader. The issue is that simple substitution ciphers do not really encrypt effectively in terms of computer evaluation – with the rise of the personal computer, substitution ciphers became relatively easy for computers to crack. However, some of the ideas behind the substitution cipher continue to live on – some forms of modern encryption might use an extremely large text set and an extremely sophisticated substitution to effectively encrypt information.