Safely erase (Delete) Files

[Theo Gottwald]

As we all know, Files can be reconstructed if you simply use the PB "KILL" command.
There are mostly two standards to safely erase files,
1. The US DoD 5220.22-M standard (3-pass and 7-pass)
2. the Gutmann method, which overwrites the file with random data 35 times.
File-Buffering should be taken into account anyway.

I have implemented two versions, one fast version for files in the range up to several hundred MB (32-bit).
And one version that is really slow - for very large files.
Now maybe somebody want to improve or give a comment on it.

Code Select Expand


'----------------------------------------------------------------------------

'----------------------------------------------------------------------------
' Überschreibt Datei 9 mal mit blinkenden Bits und Zufallszeichen
' löscht Datei dann
'
FUNCTION Kill_DoD(BYVAL U01 AS STRING) AS LONG
' These are the necessary Declarations for the Variables (MACROS)
G_REG
G_T01
G_S03
G_B03
LOCAL Q01,Q02 AS QUAD
' This function fills a string with Random Bytes, here is the RANDOMIZE for it
S03=RandBytes$(0)
IF zero(ISFILE(U01)) THEN R01=0:GOTO enx
' These are changing Bits (blinking bits)
B01=&B10101010:S01=CHR$(B01)
B02=&B01010101:S02=CHR$(B02)
' Länge der Datei holen
Q01=LOF(T01)
FOR R02=1 TO 35
S03=STRING$(Q01,S01)
R01=Overwrite_File(U01,S03)
IF R01=0 THEN GOTO ero
S03=STRING$(Q01,S02)
R01=Overwrite_File(U01,S03)
IF R01=0 THEN GOTO ero
' This function fills a string with Random Bytes, here is the RANDOMIZE for it
S03=RandBytes$(Q01)
R01=Overwrite_File(U01,S03)
IF R01=0 THEN GOTO ero
NEXT
' Jetzt Datei löschen
KILL U01
R01=1
enx:
  FUNCTION=R01
EXIT FUNCTION
ero:
R01=0:GOTO enx
END FUNCTION
'----------------------------------------------------------------------------

'----------------------------------------------------------------------------
' Überschreibt Datei U01 mit Muster U02, leert Puffer
'
FUNCTION Overwrite_File(BYVAL U01 AS STRING, BYREF U02 AS STRING) AS LONG
' These are the necessary Declarations for the Variables (MACROS)
G_REG
G_S01
LOCAL T01 AS DWORD
ON ERROR GOTO ero
T01=LEN(U02)
IF zero(T01) THEN GOTO ero
R01=FREEFILE
OPEN U01 FOR BINARY ACCESS READ WRITE LOCK READ WRITE AS R01
SEEK #R01,1
PUT$ #R01,U02
FLUSH #R01
CLOSE R01
OPEN U01 FOR BINARY ACCESS READ WRITE LOCK READ WRITE AS R01
GET$ #R01,T01,S01
FLUSH #R01
CLOSE R01
IF S01<>U02 THEN R02=0 ELSE R02=1
enx:
  FUNCTION=R02
EXIT FUNCTION
ero:
CLOSE R01
R02=0
GOTO enx
END FUNCTION
'----------------------------------------------------------------------------

'----------------------------------------------------------------------------
' Überschreibt große Datei
' 40 mal mit Zufallszeichen
' löscht Datei dann
'
FUNCTION Kill_large(BYVAL U01 AS STRING) AS LONG
' These are the necessary Declarations for the Variables (MACROS)
G_REG
G_S01
LOCAL Q01,Q02 AS QUAD
G_T01
ON ERROR GOTO ero
FOR T01=0 TO 40
R01=FREEFILE
OPEN U01 FOR BINARY ACCESS READ WRITE LOCK READ WRITE AS R01
Q01=LOF(R01)
FOR Q02=1 TO Q01
    SEEK #R01,Q02
    PUT$ #R01,CHR$(RND(0,255))
NEXT
FLUSH #R01
CLOSE R01
NEXT
KILL U01
R01=1
enx:
  FUNCTION=R02
EXIT FUNCTION
ero:
CLOSE R01
R02=0
GOTO enx
END FUNCTION
'----------------------------------------------------------------------------

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