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
Famous quotes containing the words loose, inspiration and/or pad:
“Cry “havoc!” and let loose the dogs of war,
That this foul deed shall smell above the earth
With carrion men, groaning for burial.”
—William Shakespeare (1564–1616)
“As one knows the poet by his fine music, so one can recognise the liar by his rich rhythmic utterance, and in neither case will the casual inspiration of the moment suffice. Here, as elsewhere, practice must precede perfection.”
—Oscar Wilde (1854–1900)
“I have known the inexorable sadness of pencils,
Neat in their boxes, dolor of pad and paper-weight,
All the misery of manilla folders and mucilage,
Desolation in immaculate public places,”
—Theodore Roethke (1908–1963)