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One-Time Pad
- One well-known realization of perfect secrecy is the One-Time Pad (OTP).
- The first OTP system was used by Gilbert Vernam in 1917.
- The OTP is a variant of Vigenere Cipher with the key as long as the plaintext (no key letter is ever used to encrypt more than one plaintext letter).
- Every key should be used with equal probability 1/|K|.
Suppose Alice wants to send the secret message “ATTACK” to Bob. The key is “jdiwpf”. Then the ciphertext will be “kxcxsq”. See the animation to the right for more information.
USA and Britain used one-time pads during the second world war. However today one-time pads have a little usage in practice. These are the reasons:
- One-Time Pad has the problem of transfering securely the key material which is very long (at least as long as the plaintext message).
- The key is also required to be generated in a perfectly random way which is not an easy task.
- State-of-the-art encryption algorithms that we will see in the next chapter suffices for the needs most of the time even though they provide only computational security.
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Animation I.IV-I
One Time Pad demo. Notice that ciphertext and plaintext should be same.
[click to enlarge] |
