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Interchange:
– Each character is represented using 7 bits
– Total of 128 possibilities
– Often an extra bit is used for parity checking, or used for extended
characters The OneTime Pad
The
The onetime pad is an “unbreakable” cryptosystem developed
by Vernam and Mauborgne in 1918.
The plaintext message is represented using a sequence of bits
The key is a random sequence of 0’s and 1’s as long as the
message.
The ciphertext is the XOR of the plaintext with the key.
Example: (message)
(key) 00101001
⊕ 10101100 (ciphertxt ) 10000101
How to decrypt? Just XOR again with the key! Vernamstyle Ciphers
Vernam
The onetime pad belongs to a more general family of stream
ciphers known that are often referred to as Vernamstyle ciphers
In a Vernam cipher
– The plaintext x is a sequence of bits
– The key sequence k is a sequence of bits
– The ciphertext y is generated by y = x⊕k
The key sequence is typically generated using a (cryptographic)
pseudorandom number generator
We will see many choices for generating k later, for now let us
look at a popular (but weak) method Linear Feedback Shift Registers
Linear
Linea...
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This note was uploaded on 01/07/2013 for the course 332 519 taught by Professor Wadetrappe during the Fall '12 term at Rutgers.
 Fall '12
 WadeTrappe

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