Question

I need to know if any kind of file can be signed digitally, using RSA, a certificate, and that stuff, or if only certain kind of files can be signed. All this, using PHP.

In example: Can a plain text file be signed digitally?, What about images (png, jpeg, bmp)?

I don’t need to “attach” an image with a graphic signature.

Thank you for your help.

Answer 1

Using phpseclib, a pure PHP RSA implementation (updated here):

<?php
include('Crypt/RSA.php');

$rsa = new Crypt_RSA();
extract($rsa->createKey());

$plaintext = 'terrafrost';

$rsa->loadKey($privatekey);
$signature = $rsa->sign($plaintext);

$rsa->loadKey($publickey);
echo $rsa->verify($plaintext, $signature) ? 'verified' : 'unverified';
?>

The analog to that with the CLI openssl is as follows:

openssl dgst -sha1 -sigopt rsa_padding_mode:pss -sigopt rsa_pss_saltlen:-1 -out signature.txt -sign privatekey.txt plaintext.txt 
openssl dgst -sha1 -sigopt rsa_padding_mode:pss -sigopt rsa_pss_saltlen:-1 -prverify privatekey.txt -signature signature.txt plaintext.txt