morees
asked on
xor hex and decimal
Hi there,
I need to write a small function that will xor a hex value with a decimal value.
can someone please help with a small example.
Thanks
I need to write a small function that will xor a hex value with a decimal value.
can someone please help with a small example.
Thanks
ASKER
Thanks,
better explain this a bit more.
I need to read the char from a file, convert that char to hex an then xor it with an integer value read from a file.
My program boms out at the xor with invalid variant type conversion.
How can I cast the hex value to "1F" to 0x1F so that the xor will work.
better explain this a bit more.
I need to read the char from a file, convert that char to hex an then xor it with an integer value read from a file.
My program boms out at the xor with invalid variant type conversion.
How can I cast the hex value to "1F" to 0x1F so that the xor will work.
ASKER CERTIFIED SOLUTION
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> need to read the char from a file
Do you mean you have a text file with a text (ASCII) representation of a number in it?
Do you mean you have a text file with a text (ASCII) representation of a number in it?
ASKER
the one file is a binary file that was xored with a key and the key file just contains the key.
the key file look like this
14159265358979323846264338
a byte at position x in the source file must be xored with the byte at position x in the key file.
that is what I want to do
the key file look like this
14159265358979323846264338
a byte at position x in the source file must be xored with the byte at position x in the key file.
that is what I want to do
The key file looks like ASCII.
> 14159265358979323846264338
Is the 2nd byte '4' as in 0x34?
> 14159265358979323846264338
Is the 2nd byte '4' as in 0x34?
ASKER
i think it should be..
Hi morees,
> the key file look like this
> 14159265358979323846264338
>
> a byte at position x in the source file must be xored with the byte at
> position x in the key file.
To understand you correctly: As the key is in hex-ascii the key file is twice as long as the encoded file, right?
For the hex->int conversion, you *could* use the strtol() function:
int hexbyte=strtol(hexstring,N
(hexstring MUST be \0 at the third byte)
This won't be terribly efficient, though probably better by eons than a fscanf() solution. You could convert 4 bytes (or even 8 using strtoll) at once, but then you'll skid into endianess issues.
Recommendation: Use a lookup table. Here's a Perl program to create one:
my $i;
my @tab;
for $i (0 .. 0xff){
my ($hi,$lo) = unpack("C2",sprintf "%02x",$i);
my $arridx=($hi << 8) + $lo;
$tab[$arridx]=$i;
($hi,$lo) = unpack("C2",sprintf "%02X",$i);
$arridx=($hi << 8) + $lo;
$tab[$arridx]=$i;
}
print "#define ERR -1\nshort lookup[65536]={";
for $i (0 .. 0xffff){
print "\n\t" if ($i & 0xf) == 0;
if(defined $tab[$i]){
printf "0x%02x,",$tab[$i];
} else {
print " ERR,";
}
}
print "\n};\n";
The lookup logic is: "94" -> ASCII 0x3934 -> lookup[0x3934] -> 0x94
The nice side effect is that you'll have both upper & lower hex values and you can detect errors without major extra effort -> faaaast.
Cheers,
Stefan
> the key file look like this
> 14159265358979323846264338
>
> a byte at position x in the source file must be xored with the byte at
> position x in the key file.
To understand you correctly: As the key is in hex-ascii the key file is twice as long as the encoded file, right?
For the hex->int conversion, you *could* use the strtol() function:
int hexbyte=strtol(hexstring,N
(hexstring MUST be \0 at the third byte)
This won't be terribly efficient, though probably better by eons than a fscanf() solution. You could convert 4 bytes (or even 8 using strtoll) at once, but then you'll skid into endianess issues.
Recommendation: Use a lookup table. Here's a Perl program to create one:
my $i;
my @tab;
for $i (0 .. 0xff){
my ($hi,$lo) = unpack("C2",sprintf "%02x",$i);
my $arridx=($hi << 8) + $lo;
$tab[$arridx]=$i;
($hi,$lo) = unpack("C2",sprintf "%02X",$i);
$arridx=($hi << 8) + $lo;
$tab[$arridx]=$i;
}
print "#define ERR -1\nshort lookup[65536]={";
for $i (0 .. 0xffff){
print "\n\t" if ($i & 0xf) == 0;
if(defined $tab[$i]){
printf "0x%02x,",$tab[$i];
} else {
print " ERR,";
}
}
print "\n};\n";
The lookup logic is: "94" -> ASCII 0x3934 -> lookup[0x3934] -> 0x94
The nice side effect is that you'll have both upper & lower hex values and you can detect errors without major extra effort -> faaaast.
Cheers,
Stefan
In case you ask "why is this table defined as short?": All 256 possible values of (unsigned) char are valid, so you need a larger data type to store the error condition.
Stefan
Stefan
Do something like the following:
#include <fstream>
#include <iostream>
using std::fstream;
using std::ios_base;
using std::cout;
using std::endl;
...
// open binary file
fstream f("my file.bin", ios_base::binary | ios_base::in);
unsigned short decimal_val = 12;
while (!f.eof())
{
char c;
// read char
f.read(&c, 1);
// do what you like, xor the value with our decimal and print it
cout << (c ^ decimal_val) << endl;
}
Stefan73, you assume morees knows perl ? ....
#include <fstream>
#include <iostream>
using std::fstream;
using std::ios_base;
using std::cout;
using std::endl;
...
// open binary file
fstream f("my file.bin", ios_base::binary | ios_base::in);
unsigned short decimal_val = 12;
while (!f.eof())
{
char c;
// read char
f.read(&c, 1);
// do what you like, xor the value with our decimal and print it
cout << (c ^ decimal_val) << endl;
}
Stefan73, you assume morees knows perl ? ....
hex or decimal does not matter as long as you use the xor operator correctly
x = 56 ^ 0xE4; is perfectly valid though a bit unintuitive
Sunnycoder