NAME
Crypt::Nash - Implementation of Nash Cryptosystem
DESCRIPTION
This is a self-synchronizing cipher feedback stream cipher proposed by
John Nash in 1950, just recently declassified.
NOTES
-- Nash doesn't say anything about the initial state of the bits in the
system; here we allow an initial state as part of the key It would be
reasonable and interesting to consider other possibilities, such as
having a fixed initial state (all zeros), or running the system with
"0"'s as input for a while to arrive at an initial state, or ... ??
-- We implement the example given in his note. There is one arrow
missing a label; we assume here the missing label is a "+". We also
choose an arbitrary starting state as part of the key.
-- There are many interesting open questions about this system; here are
some as ``food for thought'': (a) Are there ``weak keys''? (Keys that
shouldn't be used?) (b) If the system receives periodic input, it will
exhibit periodic output. (E.g. input 001001001001001...) What can be
said about the periodicities? (c) How do different guesses about what
Nash intended for the starting state affect security? (d) How long can a
given bit circulate internally? (e) Can you figure out the permutations
and bit-flips if you are allowed to specify inputs to the system, and to
reset it to the initial state whenever you like? (Effectively, a chosen
ciphertext attack) (f) Is the output of the system balanced (equal
number of 0's and 1's) or unbalanced (biased somehow)?
METHODS
new , , , , ,
n - number of state bits (not counting D, P entry point, or output bit)
red permutation - specifies the red permutation: redp[i] says where bit
i comes from, in the red permutation
red bits - 1 = complement, 0 = no complement
blue permutation - blue permutation
blue bits - same as for redbits
initial permuatation - initial state P[0...n] and P[n+1]=output bit.
P[0] is entry point
encrypt
Encrypt bitstring, return ciphertext string
decrypt
Decrypt bitstring, return ciphertext string
AUTHOR
Python Implementation by Ronald L. Rivest (2/17/2012)
Available here http://courses.csail.mit.edu/6.857/2012/files/nash.py
Perl port by Simon Wistow