Loose Inspiration From The One-time Pad
Stream ciphers can be viewed as approximating the action of a proven unbreakable cipher, the one-time pad (OTP), sometimes known as the Vernam cipher. A one-time pad uses a keystream of completely random digits. The keystream is combined with the plaintext digits one at a time to form the ciphertext. This system was proved to be secure by Claude E. Shannon in 1949. However, the keystream must be (at least) the same length as the plaintext and be generated completely at random. This makes the system very cumbersome to implement in practice, and as a result the one-time pad has not been widely used, except for the most critical applications.
A stream cipher makes use of a much smaller and more convenient key such as 128 bits. Based on this key, it generates a pseudorandom keystream which can be combined with the plaintext digits in a similar fashion to the one-time pad. However, this comes at a cost. The keystream is now pseudorandom and so is not truly random. The proof of security associated with the one-time pad no longer holds. It is quite possible for a stream cipher to be completely insecure.
Read more about this topic: Stream Cipher
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—E.B. (Elwyn Brooks)
“What is called eloquence in the forum is commonly found to be rhetoric in the study. The orator yields to the inspiration of a transient occasion, and speaks to the mob before him, to those who can hear him; but the writer, whose more equable life is his occasion, and who would be distracted by the event and the crowd which inspire the orator, speaks to the intellect and heart of mankind, to all in any age who can understand him.”
—Henry David Thoreau (1817–1862)
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—Theodore Roethke (1908–1963)