The term “cryptograph” first appeared in the nineteenth century, in Edgar Allan Poe’s “The Gold-Bug.” The primary focus of cryptography has been on encryption, which converts plaintext into incomprehensible ciphertext, and decryption, which reverses the process. A cipher, often known as a cypher, is a pair of algorithms that perform encryption and decryption.
The key is a secret known only to the communicants that is required to decrypt the ciphertext. A cryptosystem is an ordered list of finite potential plain texts, cypher texts, keys, and the encryption and decryption techniques that correspond to each key. Keys are crucial both formally and practically, because ciphers with fixed keys are easily broken........
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