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– 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 One-Time Pad
The one-time 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
The ciphertext is the XOR of the plaintext with the key.
⊕ 10101100 (ciphertxt ) 10000101
How to decrypt? Just XOR again with the key! Vernam-style Ciphers
The one-time pad belongs to a more general family of stream
ciphers known that are often referred to as Vernam-style 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)
pseudo-random 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
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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