From a mathematical view, cryptography consists of two basic principles: substitution and transposition. Substitution consists of permuting the letters of the alphabet, in other words the letters in plaintext are replaced by other letters or symbols. Transposition consists of jumbling the letters in plaintext; their normal order is disarranged (Kahn). Modular arithmetic is a mathematical tool used for cryptography. The concept of congruency is essential since it is a way to mathematically describe simple rotations of ciphers. The alphabet is viewed as a loop, such that when it reaches the end, it starts all over.
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