Bit Manipulation in FoxPro for Data Encryption

• Microsoft Visual FoxPro for Windows, version 3.0
• Microsoft FoxPro for MS-DOS, versions 2.0, 2.5, and 2.5a
• Microsoft FoxPro for Windows, versions 2.5 and 2.5a

SUMMARY

Because FoxPro uses characters that have values between 0 and 255 (1 byte), these character values and thus the bit pattern can be manipulated once the characters are converted to numeric values using the ASC() command. This sort of operation could be used for data encryption.

The program below illustrates how the bit pattern of an entire string can be swapped by rotating the bit patterns of each character one at a time and placing the resulting characters in reverse order in the output string.

MORE INFORMATION

   *PROGRAM SHIFTSTR.PRG
   * (RUN BY TYPING 'DO SHIFTSTR.PRG WITH <STRING>')
   PARAMETERS A
   ?A
   A=shift(A)
   ?A

   PROCEDURE shift
   PARAMETERS in_str
   result=[]
   SET DECIMAL TO 0
   lngth=LEN(in_str)
   DIMENSION out_array(lngth)
   FOR char_num=1 TO lngth
      x=ASC(SUBSTR(in_str,char_num,1))
      y=0
      FOR bit=7 TO 0 STEP -1
         IF x>((2^bit)-1)
            x=x-(2^bit)
            y=y+(2^(7-bit))
         ENDIF
      ENDFOR
      out_array(lngth-char_num+1)=CHR(y)
   ENDFOR
   FOR char_num=1 TO lngth
      result=result+out_array(char_num)
   ENDFOR
   RETURN result