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T
1
Onetime Pad
CS 531, Fall 2007
Copyright © William C. Cheng
i.e., if a cryptanalyst has
c
1
c
2
...c
t
, the cryptanalyst can do
no better than guess at the plaintext
i.e.,
any
t

bit binary strings are equally likely as plaintext
The
onetime pad
can be shown to be
theoretically
unbreakable
the plaintext can be
any
binary string of length
t
It has been proven that to realize an unbreakable system
requires a random key of the same length as the message
this reduces the practicality of onetime pad
Myth or reality?
(until very recently) communication line between Moscow
and Washington was secured by a onetime pad
transport of key done by trusted courier
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View Full Document T
2
The Key Space
CS 531, Fall 2007
Copyright © William C. Cheng
a key is typically a compact way to specify the encryption
transformation
Size of key space is the number of encryption/decryption key
pairs that are available in the cipher system
e.g., a transposition cipher of block length
t
has
t!
encryption functions to choose from, each can be simply
described by a permutation which is called the key
a necessary,
but usually not sufficient
, condition for an
encryption scheme to be secure is that the key space be
large enough to
preclude exhaustive search
It is a great temptation to relate the security of the encryption
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This note was uploaded on 03/05/2008 for the course CSCI 531 taught by Professor Cheng during the Spring '08 term at USC.
 Spring '08
 Cheng

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