From: tad.bochan@...
Date: 2004-09-10T23:37:33+09:00
Subject: Réf. : ASCII, RSA AND BIGNUM


Hi Barry,

Being aruby-nuby myself, and having next to no experience of C/C++,
Perl,Python or Unix or Windows programming, I found it difficult to grasp
the strategy of your code.
Please don't take this as criticism!

If I were doing this, I would start with some way of taking an ascii
message,
splitting it into a series of bitstrings, and use each bitstring as a
number.
The size of each bitstring would be a function of the size of the primepair
product used in the RSA algorithm.

Each number would be encrypted using the RSA algorithm, and then the array
of encrypted numbers  would be re-assembled as a string.
I'm sure your code follows this approach.

The difficult  bit is getting Ruby to cast a String into a Bignum and
back again.
That is, given the string "ABCDEFGH" , return 0x4142434445464648
and given 0x4142434445464648, return "ABCDEFGH".
It's a shame that there is no standard method for doing that,
and I'm sure Matz could add the methods quicker than you could blink.

Anyway,
Here's my version of this process,
(hope it's useful, and if I'm on the wrong track, any comments would be appreciated)

# CONVERT A NUMBER INTO ITS EQUIVALENT BIT-STRING
def n2s(num,width)
    s  = "x" * width
    (width-1).downto(0) { |k| s[k] = (num & 0xff); num=num>>8}
    return s
end

# CONVERT AN ARRAY OF NUMBERS INTO THE EQUIVALENT STRING OF BITS
def a2s(a,width)
    (a.collect {|x| n2s(x,width)}).join
end


# SPLIT STRG INTO SEGMENTS OF N-BYTES EACH, AND CONVERT EACH TO A NUMBER
def s2a(strg,width)
    a=strg.unpack("a#{width}" * ((strg.size + width - 1)/width))
    a.collect do |s|
        big=0
        s.each_byte {|k| big = ((big << 8) | k)}
        big = big << ((width-s.length)*8)
    end
end

def encrypt(n,primepair)
    # the clever stuff
    n
end

# Example :-
prime1,prime2=104729,104743
primepair=prime1 * prime2
blksize=primepair.size # bytes per msg block

puts msgoriginal="RSA encryption in Ruby"
msgarray=s2a(msgoriginal,blksize)
puts msgarray.inspect
msgarray.collect! {|m| encrypt(m,primepair)}
msgoriginal=a2s(msgarray,blksize)
puts msgoriginal









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