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Presently I am trying to port an encryption library written in C to Delphi. It is almost finished, however the C source code consists of the following instructions that I have not been able to cope with so far.

unsigned char one_block[16];
unsigned char * iv
((unsigned int *)one_block)[3] = ((unsigned int *) iv)[3];

...

I tried to convert it even as

var
  one_block: Array[0..15] of Byte;
  iv : PByte;
begin
  PDWord(one_block[3]):= PDWord(iv[3]);
  ...
end;

but it obviously fails to compile, because the left side of the expression cannot be assigned.

Can anyone with more practice in porting give me a hint on how to solve this problem? Thank you in advance.

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  • You've used one_block in both of your code snippets, with no indication of what it might be. You've also not assigned any value to iv before trying to access iv[3]. It's pretty difficult to try to port code when you haven't included the major parts. Commented Jul 26, 2014 at 15:01
  • Sorry Ken. You absolutely are right. They are mistyping in both cases. 'one_block' should have been 'block' as in the declaration parts. Commented Jul 26, 2014 at 15:08

2 Answers 2

Reset to default
4

Try the following:

{$POINTERMATH ON}
PCardinal(@one_block)[3] := PCardinal(iv)[3]; // fills bytes 12..15 of one_block

As the comments say, I assume that the C code somehow initializes iv, and also sets the other parts of one_block. You did not show that.

Now, if your version of Delphi does not support $POINTERMATH, you'll have to do:

type
  PCardinalArray = ^TCardinalArray;
  TCardinalArray = array[0..3] of Cardinal;

  ...

  PCardinalArray(@one_block)^[3] := PCardinalArray(iv)^[3];
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6 Comments

This one seems to work at first glace but I need to check the whole procedure.
It should work. But if you like it, don't hesitate to upvote it.
Note that, as I commented, PCardinal(@x)^[3] accesses x as if it were an array of Cardinals (or DWORDs), so its sets the third Cardinal, or bytes 12..15 of the array.
FWIW, my obligatory recommendation: Addressing pointers.
My only suggestion is to not use PCardinal, since it is a generic type that could change size in a future release (it already has once before). PDWord or PUint32 is more appropriate and matches the C code.
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1 
PDWord(one_block)[3] := PDWord(iv)[3];

...if Pascal/Delphi can use PDWord as array.
1. Cast (change type of the array).
2. Acces the array (of ints/dwords).

EDIT:

Well, IV and BLOCK should be used as

array[0..3] of Integer (or DWord)

...as it seems you are using AES-128

so, you should declare

TBlock = array[0..3] of Integer/Dword
PBlock = ^TBlock

and cast it this way

PBlock(@block)^[3] = PBlock(@iv)^[3]

or maybe directly

TBlock(block)[3] = TBlock(iv)[3]

...I am sorry but it has been 10 years I was working with Delphi

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3 Comments

Thank you, but I already tried this solution without success. In Delphi, the left side of an expression like this can't be assigned to.
try casting to ^array of integers - see stackoverflow.com/questions/8508783/…
Not quite correct. iv is already a pointer, so should not be referenced: PBlock(@one_block)^[3] := PBlock(iv)^[3];. That was my solution too. <g>

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