In their book, Understanding Cryptography, on page 37, Christof Paar and Jan Pelzel write:
Definition 2.2.2 One-Time Pad (OTP)
A stream cipher for which
- the key stream s0,s1,s2,... is generated by a true random number generator, and
- the key stream is only known to the legitimate communicating parties, and
- every key stream bit s[i] is only used once
I have a problem with the third statement. Perhaps I am confused because of the way it is expressed in the English language.
It says every bit s[i] has to be used once. How come: s[i] is either 0 or 1? So, as soon as I use value 0 for s, does it mean I can't use value 0 anymore?
I am confused as to what the authors mean.
Furthermore, they also say:
Key stream bits cannot be re-used. This implies that we need one key bit for every bit of plaintext
Why does it imply that we need one bit for every plain text ?