read single byte from PHP uploaded file

I am very new to PHP, so forgive me my ignorance.

I have HTML for for upload:

<form action="upload.php" method="post" enctype="multipart/form-data">
<label for="file">Filename:</label>
<input type="file" name="file" id="file" /><br />
<input type="submit" name="submit" value="Submit" />
</form>

My PHP is (It doens't know, but hopefully it shows what I want):

<?php
if ($_FILES["file"]["error"] > 0)
  {
  echo "Error: " . $_FILES["file"]["error"] . "<br />";
  }
else
  {
  $content = fread($_FILES["file"],10);
  echo $content;
  echo "byte10 is: " ???
  }
?>

I want to: From this uploaded file, I want to read a single byte (say byte 10). I want to print out the acscii (HEX) code for this byte. How do I do that? Do I need to save the file to the server?

开发者_如何学Python(The untimate goal is to encrypt the file and send th eencrypted file back to the user. So I want (1) upload file (2) read each individual byte (2) perform encryption on byte level (3) save file and send it back to the user)

Multiple implementations, I'm gonna give you a reusable one, using fseek.

<?php
function getByte($f,$n){
    fseek($f,$n);
    return fread($f,1);
}

$f=fopen('your_file_path');
echo getByte($f,10);
?>

I strongly suggest using move_uploaded_file before using anything $_FILES.

P.S. This can get very very expensive if you're not careful, my suggestion to you, would be to read bigger chunks and then perform operations on them.

Thank you very much for the "ultimate goal" -- it will help us make sure that you don't do something silly by accident.

Encryption is a very complex topic. You should not write your own encryption routine. Use a complete, pre-built solution.

Mcrypt is PHP's best encryption extension. Mcrypt supports multiple common ciphers including AES ("Rijndael"). Encryption is pretty darn easy -- use "CBC" mode for your file.

The Vigenère cipher is a 460-ish year old substitution cipher based on a simple lookup table.

This explains the whole one-character-at-a-time thing.

While you can read the whole file one byte at a time, you might find it more convenient to work with a string. You can read the entire file into a variable (through file_get_contents or whatever), and then use substr or the square bracket substring syntax to read the string one byte at a time. Remember, PHP strings are just simple lists of bytes.

Given $data is the complete data in the file, you can process every byte thusly:

$length = strlen($data);
for($i = 0; $i < $length; $i++) {
    $byte = $data[$i];
// Or
    $byte = substr($data, $i, 1);
// Process the byte here.
}

This is only practical for small files (smaller than a few megabytes), as it requires loading the whole file into memory.

I want to print out the acscii (HEX) code for this byte

You can get the decimal value using ord, then convert that to a hexadecimal string using dechex:

$hex_value = dechex(ord($byte));

Post-substitution, you can reverse it -- use hexdec to go from hex to decimal, then chr to go from decimal to byte:

$converted_byte = chr(hexdec($ciphered_hex));

In PHP, a string is basically an array of bytes, so you can use the fread() function to read some characters in to a string, and then access individual bytes using array notation. Eg.

$content = fread($_FILES["file"],10);
echo $content;
echo "byte10 is: ".$content[9]; // 9 because offset starts from 0