OpenEuphoria

Data Encryption

• Posted by Bryan Watts <mr_bungle at COMPUSERVE.COM> May 14, 1998
• 767 views

To all you Euphorians out there,

I have recently gotten into data encryption algorithms, and just to
see, I have developed a *very* basic one, implemented into
the program at the end of this letter.  I can get a sentence
encrypted and decrypted fine, and I can also encrypt a file
as well.  My problem is that when I try to decrypt the file,
sometimes the marker that shows which bit to switch in
the character ("ch" in the "decrypt_file" procedure)
sometimes reads in a very strange value, and causes a
subscript error.  It happens randomly, sometimes not at all,
and it has me completely baffled.  I think this would be a
very cool program, and useful too, if I could get it to run!!
Any help as to what may be causing this weird error would
be greatly appreciated!!  Thanks a lot.

Regards,
  Bryan Watts
  (Oh yeah, I'm a junior in high school, and while it may
   not seem like much to you guys, my team came in 4th place
   out of 40 teams in the SouthWest US in a computer
   programming contest at UCF.  Yay me!)

-----------   Encrypt.ex   starts here   ----------------

include get.e
with trace

function ch_to_bits(integer ch)  --changes a character to bits
  integer num2
  sequence bits
  bits =3D repeat(0,8)
  num2 =3D 128
  for i =3D 1 to 8 do
    if ch - num2 < 0 then
      bits[i] =3D '0'
    else
      bits[i] =3D '1'
    end if
    if bits[i] =3D '1' then
      ch =3D ch - num2
    end if
    if i !=3D 8 then
      num2 =3D num2 / 2
    end if
  end for
  return bits
end function

function bits_to_ch(sequence bits)  --changes bits to a character
  integer num2,final
  num2 =3D 128
  final =3D 0
  for i =3D 1 to 8 do
    if bits[i] =3D '1' then
      final =3D final + num2
    end if
    if i !=3D 8 then
      num2 =3D num2 / 2
    end if
  end for
  return final
end function

function encrypt(integer ch)  =

  integer x
  sequence bits
  x =3D rand(8)  --select a random number...
  bits =3D ch_to_bits(ch)
  bits[x] =3D (1 - (bits[x] - '0')) + '0'  --...and switch that element's=
 bit
  ch =3D bits_to_ch(bits)
  return {x,ch}  --return {element changed,new random character}
end function

function decrypt(sequence encrypted) --a sequence of {bit switched,new
character}
  integer ch
  sequence bits
  bits =3D ch_to_bits(encrypted[2])
  bits[encrypted[1]] =3D (1-(bits[encrypted[1]] - '0')) + '0' --switch
original bit back
  ch =3D bits_to_ch(bits)
  return ch
end function

procedure encrypt_file()  --self-explanatory
  integer ch,filenum,done,filenum2
  sequence file,dummy
  done =3D 0
  puts(1,"Enter the file name now: ")
  file =3D gets(0)
  position(17,30)
  file =3D file[1..length(file)-1]
  filenum =3D open(file,"r")
  if filenum =3D -1 then
    printf(1,"Cannot open \"%s\"",{file})
    ch =3D wait_key()
  else
    filenum2 =3D open(file,"w")
    while not done do
      ch =3D getc(filenum)
      if ch =3D -1 then
        done =3D 1
      elsif ch =3D '\n' then
        puts(filenum2,ch)
      else
        dummy =3D encrypt(ch)
        printf(filenum2,"%s%s",{dummy[1],dummy[2]})
      end if
    end while
    close(filenum)
    close(filenum2)
    puts(1,"Your file has been encrypted.")
    ch =3D wait_key()
  end if
end procedure
    =

procedure decrypt_file()
  sequence file,dummy2
  integer done,filenum,filenum2,dummy,ch2,ch
  done =3D 0
  puts(1,"Enter the filename now: ")
  file =3D gets(0)
  position(17,30)
  dummy2 =3D ""
  file =3D file[1..length(file)-1]
  filenum =3D open(file,"r")
  if filenum =3D -1 then
    printf(1,"Cannot open \"%s\"",{file})
  else
    filenum2 =3D open(file,"w")
    --trace(1)
    while not done do
      ch =3D getc(filenum)  --here is where the error occurs.  It should
always be in [1..8]
      if ch =3D -1 then
        done =3D 1
      elsif ch =3D '\n' then
        puts(filenum2,ch)
      else
        ch2 =3D getc(filenum)
        dummy =3D decrypt({ch,ch2})
        dummy2 =3D dummy2 & dummy  --debug help, ignore
        puts(filenum2,dummy)
      end if
    end while
    close(filenum)
    close(filenum2)
    puts(1,"Your file has been decrypted.")
    done =3D wait_key()
  end if
end procedure

procedure main()
  sequence choices
  integer ch,done
  done =3D 0
  choices =3D {"Encrypt a file","Decrypt a file","Quit"}
  while not done do
    clear_screen()
    for i =3D 1 to 3 do
      position(10+i,30)
      printf(1,"%d: %s",{i,choices[i]})
    end for
    ch =3D wait_key()
    position(15,30)
    if ch =3D '1' then
      encrypt_file()
    elsif ch =3D '2' then
      decrypt_file()
    elsif ch =3D '3' then
      done =3D 1
    end if
  end while  =

end procedure

main()