xtea encryption

[Ron Marcus]

OK, here it is. These subs will take an eight byte block of code, and encrypt it, then decrypt it. The key must be exactly the same for encrypt and decrypt, and don't mess with "Delta". This is only for the PIC18s and PBPL(2.50a)

;**********************************************************************
; XTEA VARIABLES  *****************************************************
;**********************************************************************
roundin     con     32            ;number of loops
Delta       con    $9E3779B9       ;5/4/2 * 2power32

round       var     byte
Sumin       var     long
vy          var     Long
text_byte   var     byte[8]     ;text coming in, going out.
vz          var     Long
tb          var     byte
Sum         var     long
temp        var     long
tmp0         var    temp.byte0
tmp1         var    temp.byte1
tmp2         var    temp.byte2
tmp3         var    temp.byte3
key         var     long[4]
ka         var      key[0]

kb         var     key[1]
kc         var     key[2]
kd         var     key[3]
limit       var     long
block       var     long[2]
index       var     word
key[0] =    $31323334        ;128 bit key
key[1]  =   $93D8E9AF       ;don't use mine
key[2]  =   $275FC2BA       ;must be
key[3]  =   $95E20110       ;random
        

;**********************************************************************



main:
           
           
                       
            hserout [str text_byte\8,13]    ;the unchanged block of 8 bytes
              gosub beep   
            ;pause 1000
               GOsub TEAenc                    ;scramble well
            hserout [str text_byte\8,13]    ;show mess
             gosub teadec                       ;unscramble
            hserout [str text_byte\8,13]    ;No s--t, it works!
               pause 100    
             stop   
TEAenc:     for tb = 0 to 3  ; convert string into code
             vy.byte0[tb] = text_byte[tb]
             vz.byte0[tb] = text_byte[tb+4] 
             next tb                                                            
            sum = 0
            Limit = Delta * roundin 
             while Sum <> limit
             vy = vy + (((vz<<4) ^ (vz >>5))^(Sum + Key[sum & 3]))
             sum = sum + Delta
             vz = vz + (((vy<<4) ^ (vy >>5))^(Sum + Key[sum>>11 & 3]))
                wend
                for tb = 0 to 3
               text_byte[tb]= vy.byte0[tb]
               text_byte[tb+4] = vz.byte0[tb]
             next tb
               return 
          
;***************************************************************************            
TEAdec:       ;Convert code into string
            for tb = 0 to 3
             vy.byte0[tb] = text_byte[tb]
             vz.byte0[tb] = text_byte[tb+4] 
             next tb
                         
                   sum = Delta * roundin
             while sum <> 0      
               vz = vz - (((vy<<4) ^ (vy >>5))^(Sum + Key[(sum>>11) & 3]))
                sum = sum - Delta
               vy = vy - (((vz<<4) ^ (vz >>5))^(Sum + Key[sum & 3]))
                 wend
                 for tb = 0 to 3
               text_byte[tb]= vy.byte0[tb]
               text_byte[tb+4] = vz.byte0[tb]
             next tb
               return

[mister_e]

Looks good to me at first glance. Sure some will use it.