VX/FPT - Security: Making Code Unmaintainable
It is sometimes necessary to release proprietary or sensitive source code to third parties, so that it can be compiled and run in a new environment. VX/FPT can strip comments, remove formatting and rename symbols so that the code is un-maintainable, but still compiles and runs with its original meaning.
The code shown below is part of an (unclassified) F16 aircraft model (Thanks are due to John Hall, John Burnell and Gordon Dickman at DERA Bedford for its use).
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REAL THTL, THRSTKN
REAL YREFR, ZREFR
REAL YREFT, ZREFT
C
REAL TGEAR
C
SAVE
C
C
C *********************************************************
C *** Start of executable code ***
C *********************************************************
C
C**********************************************************
C*** Engine dynamics ***
C**********************************************************
C
C Initialize the engine power at the initial throttle
C
IF (FPASS .EQ. 1) THEN
ZPOW = TGEAR(THROT)
POW = TGEAR(THROT)
ENDIF
C
C Throttle gearing
C
CPOW = TGEAR(THROT)
C
C Actual power lag
c ...bypass first order lag, if TENLAG = 0
C
IF (TENLAG .GE. 0.5) THEN
CALL PDOT(POW,CPOW,ZPOW)
ELSE
POW = CPOW
ENDIF
C
C Gross thrust look-up table (look up table in metric)
C ...We have no mass flow data, so we can't compute ram drag
C and hence net thrust
C
CALL THRUST(POW,ALT,AMACH,GTHRUST)
C
C**********************************************************
C*** Engine sound variable ***
C**********************************************************
C
C Scale the engine sound variable for a 20.0 idle value
C
IF (POW .LE. 50.0) THEN
RPMSND = POW / 50.0 * 80.0 + 20.0
ELSE
RPMSND = POW + 50.0
ENDIF
C
C**********************************************************
C*** Engine forces ***
C**********************************************************
C
THTL = THTLN
PSTL = PSTLN
C
C Evaluate sines and cosines of thrust line orientation in body axes.
C
CTHTL = COS(THTL)
STHTL = SIN(THTL)
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If the model is to be linked against external libraries, the global names, the names of COMMON blocks and sub-programs, must be preserved. The command to VX/FPT is %hide names:2. The comments are stripped, the formatting is removed and local names are made meaningless:
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REAL A00786,A00787
REAL A00788,A00789
REAL A00790,A00791
REAL TGEAR
SAVE
IF (I00727 .EQ. 1) THEN
A00746=TGEAR(A00730)
A00743=TGEAR(A00730)
ENDIF
A00736=TGEAR(A00730)
IF (A00729 .GE. 0.5) THEN
CALL PDOT(A00743,A00736,A00746)
ELSE
A00743=A00736
ENDIF
CALL THRUST(A00743,A00724,A00725,A00745)
IF (A00743 .LE. 50.0) THEN
A00744=A00743/50.0*80.0+20.0
ELSE
A00744=A00743+50.0
ENDIF
A00786=A00731
A00765=A00728
A00751=COS(A00786)
A00785=SIN(A00786)
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Note that TGEAR and THRUST have survived the name translation.
If the program does not call external routines (other than intrinsic functions) all of the names may be changed. The command is %hide names:3. The result is:
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REAL A00786,A00787
REAL A00788,A00789
REAL A00790,A00791
REAL A00792
SAVE
IF (I00727 .EQ. 1) THEN
A00746=A00792(A00730)
A00743=A00792(A00730)
ENDIF
A00736=A00792(A00730)
IF (A00729 .GE. 0.5) THEN
CALL A00795(A00743,A00736,A00746)
ELSE
A00743=A00736
ENDIF
CALL A00796(A00743,A00724,A00725,A00745)
IF (A00743 .LE. 50.0) THEN
A00744=A00743/50.0*80.0+20.0
ELSE
A00744=A00743+50.0
ENDIF
A00786=A00731
A00765=A00728
A00751=COS(A00786)
A00785=SIN(A00786)
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The modified code still compiles, links and runs in exactly the same way as the original. However the effort required to recover the meaning and to make the code maintainable would be prohibitive.