Problems when processing bytes in perl - string

Question

I am working on a script that will encrypt a file using RSA. It reads and encrypts 1 byte at a time. Encrypting and decrypting normal .txt files works well, but when encrypting and then decrypting binary files (e.g. .gif) they come out corrupted.
This is done by the encryptFile and decryptFile sub.
I am using the IO::File module to open files and add binmode(":raw") so that when a byte is read it is interpreted as a byte and not text, so this can't be the problem.
When encrypting the bytes I first use my bytesToBigint() sub to translate the byte into an integer. Then I use my rsa::encrypt() sub to encrypt the integer. Now the encrypted integer will be much larger than 1 byte so I have to represent it in multiple bytes.
I do this in the sub bigintToBytes() which basically splits an integer into multiple bytes and stores them in a string. The string is returned and then written to the file.
For example: if bigintToBytes(16739) is called then the string 'Ac' is returned because
Starting with 16739 (dec)
→ 0100000101100011 (binary)
→ 01000001|01100011
→ 65|99 (dec)
and chr(65) = 'A' and chr(99) = 'c' → 'Ac'.
This sub may be the cause, but why? is it because I am storing bytes in a string? When calling the read($buf, $bufsize) sub on open files, using the IO::File module, it also stores bytes in a string, which works.
I would be really thankfull if you point out what I am missing.
Here is the bigintToBytes sub
sub bigintToBytes {
use bignum;
use bytes;
my $bigint = shift;
my $bits_in_int = length_in_bytes($bigint)*8;
my $bytes = '';
my $new_byte = 0;
my $count = 0;
while ($bits_in_int > 0){
# add first bit in bigint to the new byte
$new_byte = ($new_byte << 1)|($bigint >> ($bits_in_int-1));
# remove the first bit in bigint
$bigint = $bigint & (2**($bits_in_int)-1);
$bits_in_int--;
$count++;
if ($count == 8){
$bytes = $bytes.chr($new_byte);
$new_byte = 0;
$count = 0;
}
}
return $bytes;
}
I have also added use bytes; to the encryptFile and decryptFile subs if it matter.
I don't feel it is necessary for me to post the bytesToBigint, encryptFile and decryptFile subs, because bytesToBigint basically does the reverse and encryptFile and decryptFile just read the bytes and process them using these function.
Edit:
Here is the code snippet in encryptFile sub. This is where bytesToBigint and encryptFile are used and written to the file.
my $bufsize = 1;
my $buf = '';
while ( $file->read($buf, $bufsize)){
my $msg = myMath::bytesToBigint($buf);
my $enc_msg = rsa::encrypt($n, $enc_key, $msg);
my $enc_msg_as_chars = myMath::bigintToBytes($enc_msg);
#in case the encrypted unit is to small
while (length($enc_msg_as_chars) < $min_bytes){
$enc_msg_as_chars = chr(0).$enc_msg_as_chars;
}
$newFile->print($enc_msg_as_chars);
}

Answer 1

The problem in bigintToBytes is that it returns an empty string when input integer is zero. So I added
if($x == 0){
return chr(0);
}
and the problem is solved!