| Abstract: |
One of the most remarkable of all ciphers is the
one-time-pad where the
plaintext message is added bit by bit (or in general, character by
character) to a random sequence of the same length. The remarkable fact
about the one-time-pad is its perfect security, but the disadvantage is
the
unlimited length of key.The appealing feature of the one-time-pad
suggested
building synchronous stream ciphers which encrypt the plaintext by use
of a
pseudo-random sequence. This removes the requirement of an unlimited
key.
The pseudo-random sequence is controlled by a secret key that is
generated
by a deterministic algorithm called the keystream generator. One
method of
generating the keystream is to combine a fixed number of linear
feedback
shift register's (LFSR) outputs by means of a nonlinear function. Using
the
correlation between the keystream sequence and the output of LFSRs, it
is
possible to determine the secret key (initial state of LFSRs). This
cryptanalytic attack is called fast correlation attack. In a fast
correlation
attack, the cryptographic key is recovered from the keystream sequence
using error control coding techniques. In this seminar, we
present an
improved fast correlation attack on stream ciphers. The proposed
technique
is based on constructing an unequal error protecting low-density
parity-check
code (LDPC) from the LFSR output sequence. The unequal error
protection
allows achieving lower bit-error probability for recovering the secret
key
compared to the previous fast correlation attacks.
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| Biography: |
Maneli Noorkami attended Sharif University of Technology in Tehran, Iran
where she earned her B.S. in Electrical Engineering in 2001. She
received
her M.S. in Electrical Engineering from School of Electical
and Computer Engineering at Georgia Institute of Technology in Summer
2003.
There, she conducted resesarch on cryptography. Currently, she is
working
towards her Ph.D. in watermarking and information hiding.
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