Encryption and Byte[] manipulation.

encoding - decoding routine written in GAMBAS

Published under the Creative Commons Attribution-ShareAlike (CC-BY-SA) 3.0 license.

PRIVATE SUB encodedecode(whichFileIn AS String, whichFileOut AS String, whichCode AS Integer, OPTIONAL howmanybits AS Integer = 8, OPTIONAL frombit AS Integer = 0) AS Integer
'only deals with files of size up to 2.147.483.647-(whichcode+howmanybits+1) bytes
'change all declarations to "Long" if you want bigger files or "codes"
'And is memory depended because it loads all of the file to memory.
'(No check is done here)
'There is a cryptographic "leak" to consider:
'In every group of "howmanybits" bytes the first byte
'is decreased in every turn.
DIM PRERROROPENINFILE AS Integer = -1
DIM PRERRORRESIZINGARRAY AS Integer = -2
DIM PRERRORREADINFILE AS Integer = -3
DIM PRERRORKILLINGOUTFILE AS Integer = -4
DIM PRERROROPENOUTFILE AS Integer = -5
DIM PRERRORWRITEOUTFILE AS Integer = -6
DIM PRERROROPT1 AS Integer = -7
DIM PRERROROPT2 AS Integer = -8
DIM PRERROROPT1OR2 AS Integer = -9

  DIM xcounter AS Integer 'using a temporary counter for small "FOR..NEXT"s
  DIM prhFileIn AS File 'holds "file in" handle
  DIM prhFileOut AS File 'holds "file out" handle
  DIM prhBytesIn AS NEW Byte[] 'holds content of file in handle
  DIM prhBitCollection AS NEW Integer[] 'a "collection" of bit potisions
  DIM prhCode AS Integer 'holds a number for decrementing "code"
  DIM prhTemp8 AS Integer 'holds a temporary index to "collection" of bit positions
  DIM prhBitToXOR AS Integer 'holds the actual bit position to "XOR"
  'We can use prhCode and find which byte to change
  'but I think that will decrease the speed
  DIM prhByteToChangeCounter AS Integer

  IF howmanybits > 8 OR howmanybits < 1 THEN 
    'debuging:Message.Error("Bits must be from 1 to 8")
    RETURN PRERROROPT1
  END IF
  IF frombit < 0 THEN 
    'debuging:Message.Error("frombits must be >=0")
    RETURN PRERROROPT2
  END IF
  IF howmanybits + frombit > 8 THEN 
    'debuging:Message.Error("howmanybits + frombit must be <=8")
    RETURN PRERROROPT1OR2
  END IF
  
  TRY prhFileIn = OPEN whichFileIn FOR READ
  IF ERROR THEN 
    'debuging:Message.Error("Unable to open the 'IN' file")
    RETURN PRERROROPENINFILE
  END IF
  'resize the byte array which will hold the in values
  TRY prhBytesIn.Resize(Lof(prhFileIn))
  IF ERROR THEN 
    'debuging:Message.Error("Unable to resize the array. Maybe not enough memory.") 
    RETURN PRERRORRESIZINGARRAY
  END IF
 'this actually reads all bytes in a byte array
  TRY prhBytesIn.Read(prhFileIn)
  IF ERROR THEN 
    'debuging:Message.Error("Unable to read the 'IN' file")
    RETURN PRERRORREADINFILE
  END IF
  CLOSE prhFileIn
  'create a "code" based on byte array size and number given from user
  prhCode = whichCode + prhBytesIn.Max
  'add bits of one "howmanybits" plus one so that, at the end,
  'we allways have a number greater then howmanybits 
  'even if user gave as code 0 (zero)
  prhCode += howmanybits + 1
  'add as many bytes as groups of "howmanybits" bytes exists in source
  prhCode += Int(prhCode / howmanybits) + 1 'added
  'set the counter of bytes changed to zero
  prhByteToChangeCounter = 0
  'initialize the "collection" of bit positions
  FOR xcounter = 0 TO howmanybits - 1
    prhBitCollection.Add(xcounter)
  NEXT
  DO
    'if we have changed all bytes then exit loop
    IF prhByteToChangeCounter > prhBytesIn.Max THEN BREAK 
    'find which of the remaining "positions" of bits we must change
    prhTemp8 = prhCode MOD (prhBitCollection.Max + 1)
    'find which bit to xor from the "collection"
    prhBitToXOR = prhBitCollection[prhTemp8]
    'debuging: 
    TextArea1.Text &= prhBitToXOR
    'remove that position from the collection
    prhBitCollection.Remove(prhTemp8)
    '"xor" the n'th position (=prhBitToXOR)
    prhBytesIn[prhByteToChangeCounter] = BChg(prhBytesIn[prhByteToChangeCounter], prhBitToXOR + frombit)
    'decrease "code"
    DEC prhCode
    IF prhBitCollection.Count < 1 THEN
    '"reinitialize" the "collection" of bits
    'debuging:TextArea1.Text &= "\n"
      FOR xcounter = 0 TO howmanybits - 1
        prhBitCollection.Add(xcounter)
      NEXT
      DEC prhCode 'added
    END IF
    'increease the counter of bytes changed 
    INC prhByteToChangeCounter
  LOOP

  IF Exist(whichFileOut) THEN 
    TRY KILL whichFileOut
    IF ERROR THEN 
      'debuging:Message.Error("Unable to kill the "OUT" file")
      RETURN PRERRORKILLINGOUTFILE
    END IF
  END IF
  TRY prhFileOut = OPEN whichFileOut FOR CREATE 'WRITE 'CREATE 
  IF ERROR THEN 
    'debuging:Message.Error("Unable to open the "OUT" file")
    RETURN PRERROROPENOUTFILE
  END IF
  
  TRY prhBytesIn.Write(prhFileOut)
  IF ERROR THEN 
    'debuging:Message.Error("Unable to write to the "OUT" file")
    RETURN PRERRORWRITEOUTFILE
  END IF
  CLOSE prhFileOut
  RETURN prhBytesIn.Count
END

PUBLIC SUB sampleusageENcodefile()
  DIM prhReturnedVar as Integer
  prhReturnedVar=encodedecode("original.jpg", "encoded.notajpg", 12345)
  IF prhReturnedVar<0 THEN
    'an error occured
    'do something here
    'debuging:Message.Error("Unable to write to the "OUT" file")
  ELSE
    'no error occured
    '
  END IF
END

PUBLIC SUB sampleusageDEcodefile()
  DIM prhReturnedVar as Integer
  prhReturnedVar=encodedecode( "encoded.notajpg", "decoded.jpg", 12345)
END