The Essence of a Block Cipher

[xcoder]

print "###########"
print " 32 bit integers as sub keys"
global half32
half32 = 2^32/2

randomize 2654435769

for i = 0 to 4
   x = int(rnd(1)* 2^32)
   K(i) = x
   print K(i)
next i
print:print

a = 4222333444   'this represents a 32 bit integer data block
cipher = Encrypt(a)
print cipher
input "Press Enter to Decrypt: ";entry$
print Decrypt(cipher)
print:print
print "End of Code"
End



function Encrypt(a)
   for j = 0 to 0                'simulates block of data
   '  recycled
      for i = 0 to 4             'cycle rounds equals same as sub keys
         for k = 1 to 4          'sets amout of bit rotation ...
            a = rotateLeft(a)    'for substitution box *******
            print dec2Bin$(a)
         next k
         a = a xor K(i)          'allows final value outside of loop
         print a
      next i
     'cycles back
      print ""
      Encrypt = a
   next j
end function



function Decrypt(a)
   for j = 0 to 0  'block of data
   '  recycled
      for i = 4 to 0  step -1     'reverse key order for Decrypt
         a = a xor K(i)           'allows final value outside of loop
         for k = 1 to 4
            a = rotateRight(a)
            print dec2Bin$(a)
         next k
         print a
      next i
     'cycles back
     print ""
     Decrypt = a
   next j
end function

function rotateLeft(b)
   rotateLeft = (( 2 *b) mod 2^32) or (b> 2^31-1)
end function

function rotateRight(b)
   rotateRight = (half32*( b and 1)) or int(b/2)
end function

function dec2Bin$(num)
   n =num
   dec2Bin$ =""
   while (num >0)
       dec2Bin$ =str$( num mod 2) +dec2Bin$
       num =int(num /2)
   wend
   dec2Bin$ =right$( "00000000" +dec2Bin$, 32)
end function



edit: code tags added by Rod. Given the double spacing I think xcoder tried to get the tags right. The forum is abit confusing really with this Preview/BBCode difference. I would love some explanitory words in addition to the code and perhaps a simple demo of using the functions.

[B+]

xcoder, So you are into encryption but can't manage code tags?

justbasiccom.proboards.com/thread/633/stream-cipher-random-initial-vector

[Rod]

I edited the tags. I think xcoder did have an attempt at the tags but a few more practices required

[B+]

+1 To Rod!

I too would like some more description if English is first language.